WORK DONE IN COOPERATION WITH U.S. DEPARTMENT OF HOUSING AND URBAN DEVELOPMENT. OFFICE OF POLICY DEVELOPMENT AND RESEARCH GEOLOGICAL SURVEYPIIOFBSSIONAL PAPER 944 Relative Slope Stability And Land-use Planning In The San Francisco Bay Region, California By TOR H. NILSEN and ROBERT H. WRIGHT, U.S. GEOLOGICAL SURVEY, and THOMAS C. VLASIC and WILLIAM E. SPANGLE, WILLIAM SPANGLE AND ASSOCIATES, CITY AND REGIONAL PLANNERS GEOLOGICAL SURVEY PROFESSIONAL PAPER 944 jointly supported by the US. Geological Survey and the Department of Housing anaI Urban Development) Office of Policy Development and Research as a part ofa program to develop anal apply earth-science information in support of land-use planning and decisionmaking UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress catalog-card N0. 79-600059 For sale by the Superintendent of Documents, U. S. Government Printing Office Washington, D. C. 20402 Stock Number 024-001-03165-5 FOREWORD This report is a product of the San Francisco Bay Region Environment and Resources Planning Study, an experimental study designed to facilitate the use of earth-science information in regional planning and decisionmaking. The study is jointly supported by the US. Geological Survey and the Office of Policy Devel- opment and Research, Department of Housing and Urban Development. The Association of Bay Area Governments participates in the study and provides liaison with other regional planning agencies and with county and local governments. Although the study focuses on the nine-county, 7,400-square-mile (19,100 kmz) San Francisco Bay re- gion, it bears on a complex issue that is of national concern: how best to accommodate orderly develop- ment and growth while conserving our natural re— source base, insuring public health and safety, and minimizing degradation of our natural and manmade environment. The complexity of the problem can be greatly reduced if we understand the natural charac- teristics of the land, the processes that shape it, its re- source potential, and its natural hazards. These subjects are chiefly within the domain of the earth sci- ences: geology, geophysics, hydrology, and the soil sci- ences. Appropriate earth—science information, if available, can be rationally applied in guiding growth and development, but the existence of the information does not assure its effective use in the day-to-day de- cisions that shape development. Planners, elected of- ficials, and the public rarely have the training or experience needed to recognize the significance of ba- sic earth-science information, and many of the con- ventional methods of communicating earth-science information are ill suited to their needs. The study is intended to aid the planning and deci- sionmaking community by (1) identifying important problems that are rooted in the earth sciences and re- lated to growth and development in the bay region; (2) providing the earth-science information that is need- ed to solve these problems; (3) interpreting and pub— lishing findings in forms understandable to and usable by nonscientists; (4) establishing new avenues of communication between scientists and users, and (5) exploring alternate ways of applying earth-science information in planning and decisionmaking. Since the study was started in 1970, more than one hundred reports and maps have been completed. These cover a wide range of topics: flood and earth- quake hazards, unstable slopes, engineering charac- teristics of hillside and lowland areas, mineral and water resources management, solid and liquid waste disposal, erosion and sedimentation problems, bay water circulation patterns, and others. The methods used in the study and the results that have been pro— duced have elicited great interest and have been widely applied by planners, government officials, in- dustry, universities, and by the general public. In this report, the results of several years of research on problems of slope stability are interpreted and summarized. Some of these results, derived chiefly from research and experience in the San Francisco Bay region, will be useful wherever the threat of slope failure complicates decisions on land use. For exam- ple, the report describes a method of evaluating slope stability. Based on a knowledge of geology, slope, and the incidence of landslide deposits, this method can help planners, elected officials, and developers antici- pate and avoid problems where development is immi- nent. Maps that accompany the report illustrate the method as it has been used in the San Francisco Bay region. The maps also show a relation that is particu- larly important in planning for land use: slope stabil- ity varies throughout the region, but some large areas are relatively stable and others, equally large, are po- tentially unstable. Finally, the report discusses how a regionwide knowledge of relative slope stability may be used to improve both planning and day-to-day de- cisions on land use. The maps that accompany the report are at a scale of 1:125,000 (1 inch = about 2 miles). This scale is a compromise between the need for abundant detail and precision, which are attainable on maps at large scales, and the need for regionwide coverage on map sheets of manageable size. Furthermore, at this scale, the maps provide uniform coverage of the entire nine- county region. They show that all nine counties and many of the 91 cities in the region contain potentially unstable slopes and that most slope-stability prob- lems are not confined by political boundaries. The nonpolitical nature of landslides and other kinds of slope failure suggest a need for coordinated planning, whether it be regionwide or by the joint efforts of ju- risdictions with common boundaries or agencies with overlapping responsibilities. ”Maw. Robert D. Brown, Jr. Project Director San Francisco Bay Region Study III CONTENTS Page Foreword ______________________________ III Definitions of terms ________________________ VII Abstract _______________________________ 1 Introduction by T. H. Nilsen, T. C. Vlasic, and W. E. Spangle 1 Planning for slope stability—an overview ________ 2 The landslide problem _________________ 3 Costs of landslide disasters _______________ 9 Actions to reduce “risk” _________________ 10 Land-use planning and regulation in the San Francisco Bay region ______________ 11 Federallevel ____________ ________ 11 State level _____________________ 11 Regional level ____________________ 12 Local level ______________________ 13 Slope stability considerations in land-use planning _ _ _ 13 Relative slope stability of the San Francisco Bay region, by T. H. Nilsen and R. H. Wright ___________ 16 Previous work ________________________ 16 Landslides __________________________ 20 General classification __________________ 20 Falls ____________________________ 21 Slides ___________________________ 22 Lateral spreading ____________________ 24 Flows ___________________________ 26 Soil slips _________________________ 28 Complex landslides ___________________ 28 Creep ___________________________ 28 Summary ________________________ 30 Factors causing landslides _________________ 30 Photointerpretive mapping of landslides _________ 33 Slope-stability maps _____________________ 34 Preparation of slope stability maps of the San Francisco Bay region __________________________ 37 Slope maps __________________________ 37 Maps of landslide deposits __________________ 37 Surficial and bedrock geology maps ____________ 39 Derivation of the slope-stability maps __________ 41 Explanation of slope-stability categories __________ 41 Category 1 _______________________ 42 Category 1A ______________________ 43 Category 2 _______________________ 43 Category 3 _______________________ 43 Category 4 _______________________ 43 Category 5 _______________________ 53 Uses and limitations of maps ________________ 53 Suggestions for future research ______________ 54 Use of slope stability information in land-use planning, By T. C. Vlasic and W. E. Spangle __________ 55 Relevance of the bay region slope stability information to land-use planning __________________ 55 Responsibility of the user ________________ 56 Interpretation of slope stability categories _____ 56 Relevance to land-use planning ____________ 56 Regional agencies __________________ 58 City and county agencies _____________ 58 Application to land-use planning _____________ 60 I“ Page Use of slope stability information in land-use planning—Con. Application to land-use planning—Continued Basic guidelines _____________________ 60 Application in plan formulation ___________ 60 Federal level ____________________ 61 Required land-use element of the Compre- hensive Planning Assistance Program _ _ 61 HUD Housing production and mortgage cre- dit/minimum property standards _____ 61 Federal disaster-assistance program _ _ _ _ 61 State level _____________________ 62 Office of Planning and Research ______ 62 California Resources Agency _ _, _______ 62 State Lands Commission ___________ 63 Business and transportation agencies _ _ _ _ 63 Regional level ____________________ 63 Association of Bay Area Governments _ _ _ 63 Metropolitan Transportation Commission _ 65 San Francisco Bay Conservation and Devel- opment Commission ____________ 65 California Coastal Zone Conservation Com- mission ____________________ 66 City and county general plans __________ 67 Portola Valley general plan ___________ 68 Hayward general plan _______________ 69 Sonoma County general plan ___________ 71 Land-capability studies ______________ 75 Application in plan implementation __________ 79 Early warning system _______________ 79 Regulations _____________________ 79 Zoning ordinance _______________ 79 Slope-density provisions ________ 80 Resource management zoning _____ 81 Cluster zoning ______________ 82 Subdivision ordinance ____________ 82 Site development ordinance _________ 83 Building codes _________________ 83 Public policies and programs ___________ 83 Project review ____________________ 85 Local project review ______________ 85 Initial review _______________ 85 Detailed study ______________ 85 Acceptance of detailed study and project approval ________________ 85 A—95 review __________________ 85 Environmental impact assessment _____ 85 Roles of professionals _________________ 86 Land-use planner __________________ 86 Civil engineer ___________________ . 86 Engineering geologist _______________ 87 Interrelations ____________________ 87 Summary and conclusions, by T. H. Nilsen, T. C. Vlasic, and W. E. Spangle _____________________ -_ 87 Mapping of relative slope stability _____________ 87 Application of slope—stability information to land-use plan- ning _____________________________ 88 V 3*.) as. VI CONTENTS Page 1 Page Summary and conclusions—Continued Summary and conclusions—Continued Recommendations for improving the process of planning Conclusions __________________________ 89 for slope stability _ __________________ 88 References _____________________________ 89 Slope-stability information _______________ 88 Planning for slope stability _______________ 88 ILLUSTRATIONS [Plates are in pocket] Plates 1—3. Regional slope-stability map of the San Francisco Bay region, California. 1. Northwestern section. 2. Northern section. 3. Southern section. Page FIGURE 1. Map showing landslide severity in the United States _____________________________________ 3 2. Generalized map showing relative amounts of landslides in California ___________________________ 5 3. Index map of San Francisco Bay region ____________________________________________ 6 4. Maps showing landslide damage and landslide deposits, San Jose Highlands, San Jose, Calif. _____________ 8 5. Diagram of the land-use planning process ___________________________________________ 14 6. Index map of US. Geological Survey quadrangles, San Francisco Bay region _______________________ 17 7. Map showing overall setting of the San Francisco Bay region ________________________________ 18 8—17. Photographs of landslide damage: 8. Road north of Cloverdale in Sonoma County _________________________________________ 19 9. Coastal road near Thornton Beach, San Mateo County ___________________________________ 19 10. Eastmoor Drive in Daly City ____________________________________________________ 20 11. Highway 24 between Oakland and Orinda ___________________________________________ 21 12. Interstate Highway 80 near Pinole _______________________________________________ 21 13. Private homes in Redwood City _________________________________________________ 21 14. Private home in Oakland _____________________________________________________ 21 15. Private homes in Redwood City _________________________________________________ 22 16. Private homes on London Road, Oakland ___________________________________________ 23 17. Private homes on Van Cleave Way, Oakland __________________________________________ 23 18. Diagram showing shape, nomenclature, and appearance of a landslide ___________________________ 24 19. Diagram of relative speeds of landslide movements ______________________________________ 24 20. Diagram of a fall ___________________________________ ‘ _______________________ 25 21. Photograph of small rockfall in the northern part of the bay region ____________________________ 25 22. Diagram of a slide _________________________________________________________ 25 23. Photograph of small rockslide west of Pleasanton _______________________________________ 25 24. Diagram of a slump ________________________________________________________ 26 25. Photograph of a large slump block east of San Gregorio ____________________________________ 26 26. Photograph of a slump along Interstate Highway 280 in Woodside _____________________________ 26 27. Photograph of a slump resulting from lateral spreading at a very low slope angle on Brewer Island __________ 26 , 28. Diagram of a flow _________________________________________________________ 27 29—33. Photographs of: 29. Debris flow near Dublin ______________________________________________________ 27 30. Thin soil flow near Healdsburg _________________________________________________ 27 31. Soil flow near Half Moon Bay __________________________________________________ 27 32. Large flow south of Gilroy ____________________________________________________ 27 33. Damage resulting from a debris flow near Monterey _____________________________________ 28 34. Cross section of a complex landslide _ _ _ _ _ _ _ _ _' _____________________________________ 28 35. Diagram showing creep and its effects _____________________________________________ 29 36. Photograph showing fence posts toppled by creep along Highland Road __________________________ 29 37. Diagram showing four ways by which stable cut slope may be made unstable _______________________ 33 38. Diagram showing landslide developed in a syncline ______________________________________ 33 39. Index map showing sources of landslide mapping _______________________________________ 38 40. Index map showing sources of bedrock mapping _______________________________________ 40 41. Index map showing sources of previous slope-stability studies _______________________________ 42 42—50. Maps of northern Contra Costa and southern Solano Counties, Calif., showing: 42. Topography _____________________________________________________________ 44 CONTENTS VII Page FIGURE 43. Slopes ________________________________________________________________ 45 44. Generalized slopes _________________________________________________________ 46 45. Landslide deposits _________________________________________________________ 47 46. Generalized landslide deposits __________________________________________________ 48 47. Geologic units ___________________________________________________________ 49 48. Distribution of bedrock and surficial units that form relatively unstable slopes ______________________ 50 49. Preliminary relative slope stability _______________________________________________ 51 50. Relative slope stability ______________________________________________________ 52 51. Map of significant factors and probable environment impacts, Hayward, Calif ______________________ 72 52. Map of Sonoma County, Calif __________________________________________________ 74 53. Grid cell scoring and interpretation in hypothetical land-capability study _________________________ 76 54. Diagram of the planning-regulation-development process __________________________________ 80 TABLES Page TABLE 1. Distribution, frequency, and losses by landslide type in the United States _________________________ 4 2. Losses from landslides in 1968—69 and 1972—73 in the San Francisco Bay region _____________________ 7 3. Number and distribution of landslides that occurred during the 1968—69 and 1972-73 rainy seasons in the San Francisco Bay region ___________________________________________________________ 7 4. Processes leading to landslides __________________________________________________ 32 5. Geologic units susceptible to landsliding ____________________________________________ 35 6. Characteristics of relative slope-stability categories and relative level of risk to life and property ____________ 57 7. Slope-stability categories for land-use planning ________________________________________ 59 8. Description of categories shown on “movement potential of undisturbed ground map,” Portola Valley, Calif _____ 69 9. Weighted capability factors ____________________________________________________ 75 10. Costs associated with landslide potential ____________________________________________ 78 11. Criteria for permissible land use in Portola Valley _______________________________________ 84 DEFINITIONS OF TERMS Q Alluvium. Unconsolidated clay, sand, or gravel deposited by run- ning water. . . Argillaceous. Rocks or sediments largely composed of clay. Basalt. A fine grained, compact, dark-colored volcanic rock. Colluvium. A loose mass of soil or rock fragments deposited largely by the force of gravity at the base of a steep slope or cliff. Conglomerate. Pebbles, cobbles, and boulders larger than 2 mm in diameter set in a fine-grained matrix of sand, silt, or other cementing material. The rocks may vary in composition and size but they are usually rounded from transportation by water or waves. Cretaceous. A period of geologic time extending from about 136 million years ago to 65 million years ago. Diabase. A dark-gray to black fine-textured crystalline rock that was solidified from molten or partly molten rock material at depth in the Earth’s crust. Eocene. An epoch of geologic time extending from about 53 mil- lion years ago to about 38 million years ago. Evapotranspiration. Loss of water from a land area through tran- spiration of plants and evaporation from the soil. Expansive soils. Soils that increase in volume according to the amount of water they absorb. Facies. A distinguishable part of a single geologic unit that differs from other parts in some general aspect such as appearance or composition. The term implies physical closeness and genetic re- lation or connection between the parts. Franciscan rocks. A complex assortment of sandstone, shale, chert, volcanic rocks such as basalt and pillow lavas, and intru- sive coarse-grained crystalline rocks such as gabbro and serpen- tine. Many of the Franciscan rocks have been intensely sheared. The rocks are Jurassic to Eocene in age and crop out in western California. Geotrophic. A type of growth in which an organism turns or curves in response to gravity. Glauconite. A dull-green earthy or granular mineral of the mica group found in marine sedimentary rocks. It indicates very slow sedimentation. Graywacke. A very hard dark-gray or greenish-gray clayey im- pure sandstone generally formed in an environment in which erosion, transportation, deposition, and burial are rapid. Gener- ally of marine origin. Infrared photography. A type of aerial photography using a film more sensitive to infrared than to visible light rays, that is, to wavelengths just beyond the red end of the visual spectrum. Isopleth map. A map that shows the distribution of a variable quantity by means of lines of equal value. For example, a map that shows the thickness of a rock unit throughout a geographic area. Jurassic. A period of geologic time extending from about 190 mil- lion years ago to 136 million years ago. Lithified. Changed from an unconsolidated sediment into a solid rock through such processes as cementation, crystallization, and compression. Loess. A widespread unconsolidated blanket deposit, buff to light VIII yellow, consisting largely of silt with lesser amounts of clay and sand. Generally believed to be windblown dust of Pleistocene age. Melange. A heterogeneous mixture of rock materials consisting of a fine-grained sheared matrix thoroughly mixed with angular fragments, blocks, or slabs of diverse origin and age. Metagraywacke. A graywaclre that has been somewhat altered, or metamorphosed. Metamorphic rocks. Rocks derived from preexisting rocks. Through changes in temperature, pressure, shearing stress, and chemicals, the original rocks have been wholly or partly trans- formed mineralogically, chemically, and structurally. Many metamorphic rocks contain prominent well—formed crystals set in a finer matrix. Most metamorphic rocks are characterized by well-marked foliation—thin, leaflike layers or laminae. The rocks tend to split along parallel planes or surfaces determined by the foliation. Miocene. An epoch of geologic time extending from about 26 mil- lion years ago to 5 million years ago. Oligocene. An epoch of geologic time extending from about 37 million years ago to 26 million years ago. Paleocene. An epoch of geologic time extending from 65 million CONTENTS years ago to about 53 million years ago. Phototrophic. A plant that is nourished entirely from its own or- gans. Quaternary. A period of geologic time extending from 2 or 3 mil— lion years ago to the present. Seismicity. The amount or degree of earthquake activity. Serpentine. A green, greenish-yellow, or greenish-gray rock that is formed by alteration of other minerals. They are found in both igneous and metamorphic rocks. Their presence may indicate re— gional rock metamorphism. Siltstone. A sedimentary rock composed of detrital particles smaller than very fine sand grains and larger than coarse clay. The particles, mechanically formed fragments of older rock, were transported from their source, deposited in water or from air, and consolidated to form the rock. Syncline. Rock layers folded concave upward. The folding is usu- ally produced by deformation, generally compression, and re- sults in an undulating land surface. Tectonics. A branch of geology dealing with the structural or de- formational features of the upper part of the Earth’s crust. Tertiary. A period of geologic time extending from 65 million years ago to 2 or 3 million years ago. RELATIVE SLOPE STABILITY AND LAND-USE PLANNING IN THE SAN FRANCISCO BAY REGION, CALIFORNIA By TOR H. NILSEN, ROBERT H. WRIGHT,1 THOMAS C. VLASIC, AND WILLIAM SPANGLE ABSTRACT Landslides and associated types of slope failure such as acceler— ated soil and rock creep have become a major geologic hazard in the San Francisco Bay region. As increasing development of hillside areas has taken place since the mid-1940’s, the costs of damage from slope failures have steadily increased. More than $1 million in losses was documented from a single hillside development in the city of San Jose. For the entire San Francisco Bay region, more than $25 million of damage was caused by landslides during the rainy season of 1968—69 and more than $10 million in 1972—73. These losses can be greatly reduced by: (1) using geologic informa- tion to recognize, evaluate, and map those areas and slopes that are potentially unstable, and (2) applying this information in plan- ning, designing, and organizing the use of hillside areas. For this report, we have prepared the first standardized relative slope-sta- bility maps (scale 1:125,000) of the entire San Francisco Bay re- gion, and we discuss the implications and uses of these maps in the regional land-use planning process. We have divided the land area of the bay region into five categor- ies and one subcategory of relative slope stability ranging from un- stable to stable. The categories have been derived by analyses of the steepness of slope angles, the distribution of ancient landslide deposits, and the relative strength of bedrock and surficial geologic units. Previous studies have shown that most landslides in a given year occur on slopes greater than 15 percent (8°), in areas where landsliding has previously taken place, and in areas underlain by particular landslide-prone geologic units. Other secondary and re— lated factors such as rainfall distribution, active seismicity, active faults, soil thickness and strength, and various effects of urbaniza- tion have not been specifically included in our analysis. However, most of these factors have already been incorporated in our analy— sis through the combined effects of slope, ancient landslide depos- its, and landslide-prone geologic units. The relative slope stability maps indicate that much of the San Francisco Bay region is relatively unstable and susceptible to natu- ral slope failures. Unstable uplands are common in the Coast Ranges north of San Francisco Bay and in the Diablo Range east and southeast of San Francisco Bay, where steep slopes, abundant ancient landslide deposits, and weak, structurally deformed rocks of the Franciscan assemblage and Great Valley sequence and nu- merous poorly consolidated younger Tertiary siltstones and shales are very susceptible to landsliding. Large parts of the Santa Cruz Mountains southwest of San Francisco Bay, underlain by Tertiary sandstones and shales, are also highly unstable. More stable areas are located in interior valleys and along the gently sloping foothills ‘Wright, Robert 1-1., Earth Sciences Associates, 701 Welch Road, Palo Alto, CA 94303. of these upland areas. However, lowlands along the margins of San Francisco, San Pablo, Suisun, and Grizzly Bays and in the Sacra- mento-San Joaquin delta region, underlain by soft, moist, uncon- solidated muds, are unstable and susceptible to lateral flowage, particularly during earthquakes. . The relative slope stability maps have a variety of potential uses in long-range regional land-use planning for purposes such as transportation and communication networks, nuclear reactor sites, open space, and urban growth. However, because of their regional scale, they are not intended to be used for specific site investiga- tions; these should be undertaken by qualified engineering geolo- gists and soils engineers. The maps are designed so that in future years, as more detailed and useful data are obtained for making more sophisticated slope—stability maps (perhaps in part using computer-based technologic improvements), they will form a data base to be incorporated in the next generation of maps. For land—use planning purposes, the six relative slope stability categories and subcategories have been subdivided into three risk groups—low, moderate, and high. Each group suggests specific ac- tions and data requirements. These actions and data needs have been examined for three different levels of governmental concern: (1) regional, (2) county and city, and (3) specific sites. Regional slope-stability analyses such as those described herein must be supplemented by more detailed information at levels (2) and (3). At all levels of government, effective planning and land-use deci- sions require a continuing exchange between earth scientists, plan- ners, and engineers. INTRODUCTION By T. H. NILSEN, T. C. VLASIC, and W. E. SPANGLE The recognition of landslide hazards in urban areas is essential if safe living environments are to be pro- vided. Planners and earth scientists need to work to- gether to achieve such safety. The earth scientist prepares data on slope stability that can be used by the land-use planner in formulating policy to reduce landslide hazards. This study focuses on landslide conditions in the San Francisco Bay region and the procedures associ— ated with collecting slope-stability information and applying it to land-use planning. The methods and ex- amples that are described are also relevant to manage- 1 2 RELATIVE SLOPE STABILITY AND LAND~USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. ment of lands in other hillside areas where planning and governmental processes are similar. Slope failures have caused millions of dollars worth of damage and losses in the San Francisco Bay region alone. The delineation of unstable areas and the pre- diction of landslide possibilities can mitigate the dam- age suffered by local communities as well as the adverse effects on terrain used for nonurban purposes, such as watershed, agricultural, and forest lands. In fact, land-use planning to reduce this risk to life and property is mandated, in one way or another, by Fed- eral and State legislation. Landslides are a local phenomenon, and slope sta- bility varies from area to area. Consequently, detailed guides for planning agencies to follow in acquiring and applying slope-stability data are not presented in this study. General guidelines and some examples are pro— vided, however, to assist jurisdictions in determining ways to reduce risk from landslide hazards. In making this determination, it is necessary to balance the costs of acquiring and interpreting adequate earth-science data against the benefits to be gained by reducing losses. Another important objective of this report is com- munication between the earth scientist and the land- use planner. Therefore, we describe the activities and products of the two disciplines and their interrela— tions. Communication is essential if land-use planners and earth scientists are to be responsive to each oth- er’s ideas. The changing requirements of the planner need to be made clear to the earth scientist, so that earth scientists can prepare products that can be read— ily incorporated into the planning and decision- making process. The first section of this report serves as a general introduction to planning for slope stability and in- cludes a general description of the nationwide poten- tial for landsliding. A description is also given of the losses resulting from landsliding in the major urban regions of California. In addition, the concept of “risk analysis” is described together with the relation of slope stability to land-use planning. The second section presents a discussion of the rela- tive slope stability of the San Francisco Bay region. A logical method for preparing regionwide slope-stabil- ity information is described in detail. The third section provides a description of how slope-stability information can be applied to mitigate potential hazards and reduce risk to life and property. Ways of applying the relative slope—stability map of the bay region to planning at the regional and local level are discussed. In addition, Federal, State, and re- gional involvements are outlined, and basic guide- lines, techniques, and examples are described. The fourth section is a summary of major findings of the study. Recommendations are offered both for improving slope-stability mapping and for applying slope-stability data in planning and decisionmaking. In the pocket of this report are three slope—stability maps that cover the entire San Francisco Bay region at a scale of 1:125,000. These maps divide the land area of the region into several categories and subcate- gories of relative slope stability on the basis of geologic analyses. The maps, which are a result of more than 5 years of data collection, assimilation, and analysis, present the major results of this study. PLANNING FOR SLOPE STABILITY— AN OVERVIEW Slope instability is, perhaps, potentially the most dangerous and damaging geologic hazard threatening residents of hillside areas. Experience has shown that failure to recognize slope-stability hazards during planning and development can result in catastrophic destruction. At the same time, geologists can deter- mine the potential for landsliding through study of such factors as bedrock and soil conditions, slope of the land surface, earlier landslide deposits, and amount of rainfall. In addition, it has been found that in most cases, through sound planning and engineer- ing, landslides can be controlled or avoided. Thus it is important for planners and geologists to work togeth- er to inform the general public and decisionmakers of ways to reduce problems and cost of slope instability. Earth-science information from the geologist, such as is described later in this report, can be of great im- portance to the planner (as advisor to decisionmakers on appropriate actions in preparing, adopting, and implementing comprehensive plans) to ensure accept- able levels of risk to life and property. The land-use planner, by profession a generalist and coordinator, plays a key role in seeing that slope stability is consid- ered as well as all other physical, social, and economic conditions that affect a region or community. The planner must also know what roles other planning agencies and governmental bodies, from the local to the Federal level, play in land-use planning. To put planning for slope stability in context, the magnitude of the landslide problem, particularly as it exists in California, is described below. In addition, some general procedures for reducing landslide risk through sound planning and decisionmaking are dis- cussed. To provide perspective on government in- volvement in planning for slope stability, land-use planning in the San Francisco Bay region is used as an example. INTRODUCTION 3 THE LANDSLIDE PROBLEM Several studies have been made of the historic dis- tribution and potential occurrence of landslides for all of the United States (Sorensen and others, 1975). Fig- ure 1, showing landslide severity of the United States, was prepared by Baker and Chieruzzi (1958) using a regional concept of landslide occurrence based on physiographic divisions of the United States. Rad- bruch-Hall and others (1976) produced a preliminary landslide overview map of the conterminous US. A chart (table 1) contained in a report of the Federal Of— fice of Emergency Preparedness (1972) relates types of landslides to major physiographic areas of the Unit- ed States and describes their severity in terms of lives and property losses. Although these studies are of limited usefulness in land—use planning because they are so generalized, they clearly indicate that in many areas throughout the United States, landslides are a risk to life and property. The severity of the landslide hazards can be judged from a review of slope failures that have oc- curred in California. Figure 2 is a generalized map of the State showing relative “severity zones” ranging from “least” to “most” landslides. Because of the scale of the map, the amount of detail is limited. Thus, the units shown on the map cannot be used to define local landslide conditions. Landsliding in California causes damage to struc- tures as well as loss of usefulness of the land itself (measured by cost of remedial measures). Past dam- age to urban areas in the State has been calculated in terms of millions of dollars (California Div. Mines and Geology, 1971). Although individual landslides may affect only a few houses and the amount of movement may be slight on many landslides, landslides are so numerous that the total annual loss is great. Landslides that have occurred on “urbanized” hill- sides of the State’s two major populated areas—the Medium severity I: Landslide problem nonexistent NOTE: Severity measured by size and frequency of occurrence relative to Engineering Works Engineering Experiment Station The Ohio State University R. F. Baker and R. Chieruzzi 1957 FIGURE 1.—Landslide severity in the United States (Baker and Chieruzzi, 1958). 4 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. TABLE 1.-——Distribution, frequency, and losses by landslide type in the United States [Office of Emergen cy Preparedness, 1972] Approximate Frequency Estimated property damage Number of per 100 per 40,000 (million S historical miles miles adjusted to Recorded Type of slide Major areas slides (260 km?) (103,600 km’) 1971 values) deaths . White, Blue Ridge, Several hundred ——— 1 per 10 yr 30 42 Rockglldekf 11 Great Smoky, Rocky an we a Mtns. and Appala— chian Plateau Rockslump Widespread in central Several thousand 10 per yr hill 100 per yr. hill 325 188 and rockfall and west U.S.; preva- areas; 1 per yr areas; 10 per lent in Colo. Plateau, plateaus yr. plateaus Wyo., Mont., south- ern Calif, Greg, and Wash. _ Appalachian Plateau Several thousand 1 per 10 yr 70 per yr 350 20 (mainly in high- way and rail- road damage) Calif. Coast Ranges, Several hundred 1 per 10 yr 10 per yr 30 - - Northern Rocky Mtns. Slump Maine, Conn. River About 70 1 per 100 yr 1 per yr 140 103 Valley, Hudson Val- ley, Chicago, Red River, Puget Sound, Mont. glacial lakes, Alaska Long Island, Md., Va., Several hundred 1 per 50 yr 1 per yr 30 (mainly to — — Ala., S. Dak., Wyo., highways and Mont, Colo. foundations Miss. and Mo. River Several hundred 1 per 10 yr 1 per yr 2 — — valleys, eastern Wash., southern Ida- ho Appalachian Piedmont About 100 — — — 1 per yr less than 1 — — Debris flow White (N.H.), Adiron- Several hundred 1 group slides, 1 group slides, 100 89 and mudflow dack, and Appala- 10+ per group, 10+ per group, chian Mountains per 100 yr 1n per 15 yr White Mtns. and North Carolina Los Angeles and San Francisco Bay regions—are of special importance to Californians. Before World War II, hillside subdivisions were not uncommon; however, they were considerably different from postwar subdi- visions in nature of development, scale, and amount of grading. Most of the earlier structures were individ- ually built single-family houses without much grad- ing. The postwar population migration into California with its accompanying demand for housing, particu- larly on View sites, resulted in increased development of hillsides, especially in the Los Angeles and San Francisco Bay areas. Mass grading operations were made possible by the use of heavy excavation equip- ment developed during the war, and initially, very few controls were placed on the operations. Grading was , often done without adequate compaction, erosion con- trol, or provision for drainage. As a result, major and minor landslides occurred subsequently, and homes were destroyed. An unusually wet winter in 1951—52 caused erosion, settlement, subsidence, and major landsliding in many parts of Los Angeles; as a result losses were heavy (Yelverton, 1971). Consequently, in 1952 the first grading ordinance in Los Angeles was adopted, placing some control and supervision on all grading activities. However, despite these grading controls, losses due to landslides continued, and when such losses were combined with termination of landslide insurance by the insurance industry in the late 1950’s, many hillside residents reached a state of “semi-hys- INTRODUCTION EXPLANATION SEVERITY ZONES [N = NillL = Low I M = Moderate [H =Hfl] Least landslides ———> Most landslides NOTE: Units do not show which areas are safe or unsafe for construc- tion, only the estimated relative amounts of landslides. The areas having the most landslides contain many stable localities; conversely, many landslides occur locally within the “Nil” and “Low” severity areas Map generalized after Radbruch and Crowder (1973). LOW severity corresponds to their units 2 and 3; MODERATE severity correspnds to their units 4 and 5 ; HIGH severity corresponds to their unit 6. (NIL , severity corresponds to their unit 1 .) o 50 100 MILES . ‘ A. L .1 o 50 100 KILOMETERS § '- ‘ _,.z' FIGURE 2.—Generalized map showing relative amounts of landsliding in California (from Alfors and others, 1973, as modified from Radbruch and Crowder, 1973). 6 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. teria” (Yelverton, 1971). During a heavy storm in the winter season of 1961—62, approximately 1,700 of the 60,000 hillside homes in Los Angeles were damaged (Gill, 1967; Alfors and others, 1973). The estimated cost of repairs ranged from $50 to over $100,000 per site. The total estimated cost was about $5,440,000, or an average of $3,200 for each of the 1,700 sites. After the disasters that occurred in the rainy season of 1961—62, Los Angeles amended the 1952 grading or- dinance, making it more stringent. Slope angles were regulated, and both soils and geologic reports were re- quired, where necessary, before issuance of permits. All grading was to be supervised by engineering geolo- gists and soils engineers. The San Francisco Bay region (fig. 3) has also had its share of damaging landslides, and many counties and cities have adopted grading ordinances similar to those in Los Angeles. The history of landsliding in the bay region is discussed in more detail later. However, to provide insight to the slope-instability problems that geologists, planners, and decisionmakers in the San Francisco Bay region must face, data describing recent costs of landslides in the region are presented in table 2. YOLO Sigramento l \ \ O \K NAPA )V"-’1 r | N k‘ \ I..- /(SACRA- Santa Rosa 0 Napa " \ SONOMA \ o P ‘ Fairfield I MENTO 3 O Ix— 1 ‘ v1 J _ J SOLANO / ’. SAN & (JOAQUIN Martinez ‘ \ CONTRA COSTA z / _ ’I SAN FRANCISC K, , , / I I ALAMEDA I / / ‘ f - - —- r 7 San Jose \ o I SANTA CLARA R —‘ \ \ SANTA\ \ \( CRUZ Santa \ ( Cru . 2 {Etng — J 0 1O 20 30 MILES 0 1O 20 30 40 KILOMETERS FIGURE 3,—Index map of San Francisco Bay region. An indication of the magnitude of the landslide problem in the bay region can be obtained from the reports by Taylor and Brabb (1972) and Taylor, Nil- sen, and Dean (1975). These reports present the loca- tions of all recorded landslides and the public and private costs of these landslides for the entire region during the rainy seasons of 1968—69 and 1972—73, re- spectively. In a recent study of the natural conditions that control landsliding in the bay region, Nilsen, Taylor, and Dean (1976) compared the data from the 1968—69 and the 1972—73 rainy seasons. The purpose of the analysis was to compare modern landslides in the bay region to ancient landslide deposits, slope, bedrock geology, and the temporal distribution of pre- cipitation. The landslide information reported in the study is summarized in tables 2 and 3. The study also showed that large numbers of landslides were trig- gered during storm periods with more than 6—8 inches (15—20 cm) of rain in areas where 10—15 inches (25—38 cm) of rain had previously fallen during the season. One of the most important observations of Nilsen, Taylor, and Dean (1976) was that human activity in the hills marginal to San Francisco Bay has been a prime force in creating or adding to problems of slope stability. They also observed, however, that careful geologic mapping and slope-stability analysis (consid- ering ancient landslide deposits, slope, bedrock geol- ogy, and rainfall patterns) can provide fairly reliable information about areas that are susceptible to slope failure. Use of such information in land-use planning can help minimize future landslide damage that would otherwise result from human activities. Other conclusions were drawn on frequency of high rainfall and the nature of landslide damage that can be expected during unusually wet winters. Winters of heavy rainfall may occur every five to ten years. Most damage from landsliding triggered by rainfall during the wet winters will probably be to roads and private homes, with lesser damage to utilities, public build- ings, parklands, dams, and other structures. Public and private costs will not necessarily be proportional to the number of landslides reported for any specific area, but costs will be related more directly to the type and location of landslide activity. Precautions can be taken to reduce potential landslide damage during es- pecially wet winters, including installation of special drainage systems both in developed areas and as part of new development, and addition of vegetation to help stabilize slopes. More will be said about mitigat- ing these hazards in later sections. The hazard that landsliding represents to man and his works in the bay region is more specifically de- scribed in a study of the San Jose Highlands hillside development in the northeastern part of the city of INTRODUCTION 7 TABLE 2.—Losses from landslides in 1968—69 and 1972—73 in the San Francisco Bay region Contra San Santa Costs Alameda Costa Marin Napa Francisco San Mateo Clara Solano Sonoma Totals 1968—69 RAINY SEASON‘ Public: State _____________ 5 $ 53,000 $1,970,000 $ 164,000 $ 48,000 $ 33,000 $ 735,000 $ 148,000 $ ‘ $1,844,800 $ 4,995,800 County: Roads and purchases_ _ _ _ 390,000 1,682,190 678,950 380,000 — 448,500 904,758 4,000 688,750 5,177,148 Tax loss ___________ —— — — — — 12,000 — — , — 12,000 Private: Property depreciation _ _ _ _ 3,942,900 1,295,070 — 800,000 — 583,056 484,520 — — 7,105,546 Other _____________ 986,800 145,000 82,000 — 100,000 662,462 7,000 —— — 1,983,262 Miscellaneous __________ 24,000 90,000 130,000 250,000 — 1,158,000 355,000 — ”3,900,200 5,907,200 Total ___________ 5,396,700 5,182,260 1,054,950 1,478,000 133,000 3,599,018 1,899,278 34,000 “6,433,750 125,180,956 1972—73 RAINY SEASON5 Public: State ______________ $191,000 $ 40,243 $ 340,000 $ 87,000 $400,000 $2,182,500 $ 41,000 $ — $ 195,000 $ 3,476,743 County ____________ 20,000 901,400 630,570 42,000 see “City” 50,000 ? 8,750 '? 1,652,720 City ______________ 57,500 —— 967,150 — 90,000 49,000 30,543 200 1,000 1,195,393 Parks _____________ — 10,845 — 300 — — 4,000 — 4,250 19,395 Tax loss ____________ 2,345 22,140 32,820 — — 29,810 — — ? 87,115 Private ______________ 88,400 712,550 1,093,950 2,000 —— 1,284,000 74,518 19,500 10,000 3,284,918 Total ___________ 359,245 1,687,178 3,064,490 131,300 490,000 3,595,310 150,061 28,450 210,250 9,716,284 COST BREAKDOWN, 1968—69 AND 1972—736 Population” ___________ 1,073,184 555,805 206,038 79,140 715,674 556,234 1,064,714 171,989 204,885 4,627,663 Cost per capita: 1968—69 ————————————— $ 5.03 $ 9.32 $ 5.12 $ $ 18.68 $ 0.19 $ 6.47 $ 1.78 $ 0.02 $ 12.37 1972-73 0.33 3.04 14.87 1.66 0.68 6.46 0.14 0.17 1.03 Average 2.68 6.18 10.00 10.17 0.44 6.47 0.90 0.10 6.70 avg. 4.85 Dwelling units 365,000 173,000 68,000 25,000 295,000 185,000 323,000 51,000 68,000 1,553,000 Cost per unit: 1968—69 _____________ $ 14.79 $ 29.96 $ 15.51 $ 59.12 $ 0.45 $ 19.45 $ 5.88 $ 0.08 $ 37.26 1972—73 _____________ 0.98 9.75 45.07 5.25 1.66 19.45 0.46 0.56 3.09 Average _________ 7.89 19.86 30.29 32.19 1.06 19.45 3.17 0.32 12.06 avg. 1403 Area of urban land (sq mi) _ _ 162 102 40 10 39 90 184 27 26 680 Cost per square mile: 1968—69 _____________ $33,313 $50,806 $ 26,374 $147,800 $ 3,410 $ 39,989 $ 10,322 $ 148 $ 97,452 1972—73 _____________ 2,218 16,541 76,612 13,130 12,564 39,948 816 1,054 8,087 Average _________ 17,766 33,674 51,493 80,465 7,987 39,969 5,569 601 52,770 avg. 32,254.89 ' From Taylor and Brabb (1972). 2 Costs attributed to the Warm Springs Dam totaled $3,900,000 in 1968—69, but no costs were reported in 1972—73. This cost is anomalous and has been omitted from this comparison. “ These counties did not report a considerable part of their costs, hence these values will be lower than the actual amount. ‘ Total should include $213,000 damage reported by Pacific Gas and Electric for the entire region. ‘ From Taylor, Nilsen, and Dean (1975). " From Nilsen, Taylor and Dean (1976). " US. Census, 1970. TABLE 3.—Number and distribution Of landslides that 06- San Jose (Nilsen and Brabb, 1972) Landslide depos- curred during the 1968—69 and 1972—73 rainy seasons in the San Francisco Bay region [From Nilsen, Taylor, and Dean, 1976] its in the area were mapped, and damage from land- sliding to roads, curbs, utilities, and homes was noted (fig. 4). Nilsen and Brabb (1972) found the dollar loss 1968—69 1972-73 as a result of development on these landslide deposits Number of landslides reported 335 411 to be as follows: Landslides that t00k place The economic loss as a consequence of development on these 31112111332335? 32%eggsflf an landslide deposits is already large, will continue to grow, and will (percent) __________ 55 69 probably become significantly greater if additional development is Landslides that took place on permitted without thorough engineering geology investigations of slopes steeper than 15 per the area. The estimated 1969—70 loss in market value for all houses cent (percent) ______ ._ _ 74 80 in San Jose Highlands, for example, was $228,000, the loss for lots Landlslides fhfit t00k plhace 6nd was $195,000, and the loss in valuation for specific landslide dam‘ 031:1: gzgioéizfiigitggedgraily- age to certain houses was $61,520—a total loss of $484,520 (Santa considered to be highly sus- Clara County Assessor’s Office, written commun., 9/22/71). The ceptible to slope failure, as cost data tabulated below, provided by the San Jose Department of shown on plates 1—3 (per Public Works (written commun., 9/28/71), reveal the variety and cent) ___________ 61 65 magnitude of expenses to a municipality when landslide activity takes place within a subdivision area: 8 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. SAN JOSE HIGHLANDS CALIFORNIA DEPARTMENT OF \VATER RESOURCES SOUTH BAY AQUEDUCT TERMINAL IZESEIZVOID. HALT ADRDN AROUND T K AS 9 a w / / DIE/”SET SZVZEAL INCA/ES / Provosro LOCATION FOP, SAN JOSE WATER VéORKS SANTA CLARA coumv FLOOD \éflffigl‘ ILA E counter. 9 WATER ms‘rnlcr VATEII TREATMENT PLANT l/T/L/TV LINES ABOVE 6201/li17\ \J o_\2ma BADLY DAMAGED 90“) ~.\ zoooeazr A Curb broken or offset at joints I W Road damaged _/ .. r.—/\ / 0 I000 “' I . I . . l I . / Approxmn: scALL I21 House damaged The damage observed appeared to be. or in some cases definitely is. related to landslide movement. No comparable damage was observed outside of the area mapped as land- slides. Several of the roads, curbs, and houses within the landslide areas were checked and had no apparent damage. but a more thorough survey must be made before the extent of the damage can be fully assessed. Observations of damage were made during 2 days of field checks in July and September 1 9 7 1 . Damage repaired before those dates is not shown on the-map. FIGURE 4.—Map showing damage observed in the San Jose Highlands area in northeastern San Jose, Calif., in 1971 and preliminary photointerpretation map of landslide and other surficial deposits in the same area (from Nilsen and Brabb, 1972). ACTIONS TAKEN BY AND FINANCED BY 1967 - 0 - THE CITY OF SAN JOSE Winter and spring road maintenance 1968 9,000 IN THE SAN JOSE HIGHLANDS AREA, 1968—71 to remove ground swells and 1969 30,000 increasing grade due to downward 1970 32,000 creep ' 1971 30,000 Soils study and consultant fees _ _ _ _ 1968 $10,000 Total $760,500 Soils study and consultant fees _ _ _ _ 1969 10,000 Estimated value of city streets in San Jose High- Consultant for new road ________ 1970 30,000 lands (exclusive of new access road) _ _ _ _ $750,000 Construct 1,400’ gravel-fill interception Estimated value of city utilities (street lights ditch (no water was apparently re- and sewers) in San Jose Highlands _____ $300,000 moved) 1969 15,000 Landslide damage to gas lines in San Jose Clean Hydraughers several times ___ —— 3,000 Highlands totaled $20,000 by late 1970 Construct de-watering wells (deactivated (Pacific Gas and Electric Co., written com- after 1 year, no apparent help) _ _ _ 1969 25,000 mun., 11/18/70). Landslide damage to wa- Above—ground flexible aluminum sani- ter lines has become progressively worse tary sewer _______________ 1968 4,500 according to the following figures provided Sewer photo survey ___________ 1971 3,000 by the San Jose Highlands Water Company Replace sanitary sewer _________ 1971 7,000 (written commun., 11/3/71): Aerial photography —— 2,000 1967—438 (1 repair) _________________ $ 215 Abandon 600’ of only access road and 1968—69 (5 repairs) ________________ $1,570 build 4,000’ of new access around land- 1969—70 (7 repairs) ________________ $1,660 slide area ________________ — — 550,000 1970—71 (20 repairs) _________________ $5,816 INTRODUCTION SCALE 1: 24,000 CONTOUR INTERVAL 40 FEET DOTTED LINES REPRESENT 10 FOOT CONTOURS DATUM IS MEAN SEA LEVEL Qal Alluvial deposits Qaf Artificial fill only one shown Colluvial deposits and small alluvial fan deposits Bedrock (Queried where identification uncertain ) // Landslide deposits (Arrows indicate general direction of downslope m ovemen ts ,' q ueried uncertain) wh ere iden tifica tion is FIGURE 4.—Continued. No information was obtained on the cost of landslide dam- age in the map area outside of the San Jose Highlands, but landslides were a substantial and presumably costly problem during and after construction of terminal facilities for the South Bay aqueduct. An important aspect of this example is that some landslide deposits were shown on a map published by the California Division of Mines and Geology as early as 1951, well before the land was developed (Critten- den, 1951). This is a case where some basic earth-sci- ence data were available but were not effectively incorporated into land-use decisions. The result of this failure was costly to San Jose and catastrophic for those individuals who lost their homes. COSTS OF LANDSLIDE DISASTERS It is evident that landsliding in an urban area often results in substantial costs that are borne by both public agencies and private landowners. Dollar costs include replacement or reconstruction of public facili— ties or utilities, loss or reconstruction of private struc- tures, decrease in land values, and public acquisition of damaged land with corresponding loss of tax in- come. Although complete dollar cost figures for each landslide disaster are not always immediately avail- able, they are fairly easy to estimate because they can be based on prevailing construction costs, land costs, and costs of materials. In their study of landsliding in the San Francisco Bay region during the 1968—69 rainy season, Taylor and Brabb (1972) determined dollar costs as follows: Two categories of costs are reported—public and private. Public costs are dollars spent or lost by gov- 10 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. ernmental agencies, costs ultimately paid by the tax- payer. Public landslide costs include such emergency ex- penses as salaries for firemen, policemen, and others responsible for protecting health and safety. These costs are rarely available and are not included in this report. Most of the public landslide cost consists of the direct expense of repairing, restoring, or relocating roads. This figure includes expenses readily attribut— ed to specific large landslides and an estimate for clearing up smaller slides (included in budgets for rou- tine road maintenance and repair). Some expense for damage to sewer lines, street lighting, sidewalks, and other publicly owned facilities is included, but this is a small part of the total cost. To protect public property or to repair existing landslides, it sometimes becomes necessary for a pub- lic agency to obtain title to privately owned land. In addition to the original cost of procurement, the agency assumes costs for erosion and weed control and minor repairs. It sometimes becomes more economical to obtain title to property and have it vacated than to attempt to maintain services that are continually dis- rupted by an active landslide. Litigation results in further public costs. No figures were obtained on costs of preparing and conducting court proceedings, and only limited data were avail— able on settlements of civil suits resulting from land- slide damage. Another public cost is lost tax revenue when land is transferred from private to public ownership and thereby removed from the tax rolls. Revenue loss also results from devaluation of private property because of landslide damage and a subsequent lowering of the tax. Private costs are those resulting from loss of real property, improvements, and possessions. Of these three, the last two can be replaced if an individual is financially able. The first, real property, may be ren- dered unusable. In addition to the direct costs of re- pairs, property that has been damaged by landsliding often depreciates in value. A reappraisal by the tax as- sessor’s office that shows a difference between the fair market values before and after a landslide represents a loss to the property owners. No attempt was made to put a dollar value on in- conveniences, such as time lost taking detours. Nor were costs explored that resulted from evacuating a home—for example, the cost of food and lodging. Some costs could not be classified under state, county, or private categories, and were grouped as “miscellaneous.” These were costs that might be spe- cifically for one county, slide damage where responsi- bility is disputed, litigation costs not specifically attributed to a governing agency, and costs to the Fed- eral Government, cities, utility companies, sanitation districts, and water districts. More difficult to calculate than the costs outlined above are the socio-psychological “costs” of a land- slide disaster, yet such costs may have significant and lasting effects. These costs can range from emotional shock brought about from living with the landslide threat to actual loss of home or life. There are many documented cases from various parts of the world of loss of life from landslide disasters. For example, on May 4, 1971, a landslide at St. Jean-Vianney in the Lake St. John district of Quebec, Canada, destroyed 40 homes in a residential development and was re- sponsible for 31 deaths (Legget, 1973, p. 427). ACTIONS TO REDUCE “RISK” Thus, in general, landsliding can be a major threat to man and his works. Damage and casualties from landslides and demands for government relief will cer- tainly recur because of ill-advised developments that have already been built on unstable slopes and future development in unstable areas where an adequate evaluation of geologic hazards has not been made. This potential for disaster creates a “risk” which, sim- ply defined, is a chance of damage or injury to life and property occurring over a period of time. By incorporating information on relative stability of slopes in land-use planning, the public agencies charged with regulating use of land can formulate and implement effective strategies to significantly reduce the risk to life and property. For example, the applica- tion of modern grading techniques in the city of Los Angeles, which require grading to be done in compli- ance with the professional analysis of information on soils and geology, has reduced slope failure damage from $330 per site developed prior to 1952, to $7 per site developed after 1963 (Slosson, 1969). From slope- stability information prepared by geologists, landslide risks associated with any potentital or existing plan- ning program, project, or structure can be defined. Through comparative analyses, these risks can be evaluated against risks of alternatives, planning deci- sions can be made, and measures can be implemented to reduce the risk. Thus potential costs, both public and private, can be reduced over a given period of time. Critical to such risk analysis is the determination on the part of the governmental jurisdiction of the point at which a risk becomes acceptable. Generally, accept- able risk will be defined primarily on awareness of the range of risk associated with various activities and conditions and by the level of risk the majority of citi- zens will accept without asking for governmental ac- tion to provide protection. INTRODUCTION 1 1 Often risks from landsliding can be effectively miti- gated through techniques such as special grading, in- stallation of drainage devices, and landscaping for slope stabilization. Of course, it will be necessary to consider the total cost for such risk mitigation, such as costs for detailed slope-stability studies, cost to miti- gate identified hazards in conjuction with land devel- opment, environmental-impact costs (for example, visual impact of mass grading), and public-safety costs. At some point, the landslide hazard becomes so great that the cost of mitigation clearly overshadows the benefits of development, or the hazard will be so great that it cannot be practically contained. In any case, sufficient earth-science information should be available so that decisionmakers will be in- formed about the effects of their action (or inaction) on risk to life, damage to public and private property, and risk of economic or social dislocation. In addition, whenever a risk has been defined, the public agency should assume the responsiblity to make each individ- ual aware of the risk. Mader (1974) provides the fol- lowing insight on risk and community responsibility. ***Where does the responsiblity lie for protecting people and property? An often-heard argument is that if an individual wants to take the risk of building in a hazardous area, he should be al- lowed to do so. The argument goes on that only he will suffer in the event of a failure. In an isolated location, this position might be acceptable. But in urban and suburban settings, land failure on an individual property usually has intense repercussions on the sur- rounding area. Decreased property values, possible fire hazards, costly public assistance, and possible physical impact on adjacent land are frequent major results. Similarly, a developer often says he is willing to accept the risk in an unstable area. In the end, of course, that risk is passed on to purchasers in the development and to the public agency that as- sumes responsiblity for streets and other public improvements, for the developer is usually out of the picture by the time a failure occurs. Thus the burden is unfairly shifted to all the taxpayers in the community. It becomes clear that geologic hazards are not private matters, but concern the public in general. It is therefore incumbent upon government to protect the public interest. Alfors, Burnett, and Gay (1973) have identified statewide risk in terms of dollar loss due to the 10 greatest geologic hazards. Their report projects the to- tal dollar loss of property and life in the State of Cali- fornia from 1970 to the year 2,000 at $55 billion, of which $10 billion will be the result of landsliding. Of greatest significance is their finding that $38 billion of the total estimated losses could be prevented. Thus the risk to life and property could be reduced by nearly 70 percent by applying the most advanced loss- prevention measures. LAND-USE PLANNING AND REGULATION IN THE SAN FRANCISCO BAY REGION In the bay region and throughout California, plan- ning and regulation carried out at the local (city and county) level have had the greatest effect on actual distribution of land use. There is increasing aware- ness, however, that local planning decisions often have broader impact. Local powers and functions are more and more affected by the actions of other levels of government. Therefore, while slope-stability prob- lems are largely local in nature, requirements or direc— tives concerning them may well be initiated from other levels of government. Higher level agencies of- ten preempt or affect the decisionmaking of lower level agencies through regulating planning programs and program funding, the content of local planning, standards of air and water quality, and through shared responsibility for specific functions such as transportation, air quality, and geologic mapping. FEDERAL LEVEL The Federal Government exerts its most significant influence on planning and regulation for slope stabil- ity through its funding requirements. The following Department of Housing and Urban Development (HUD) programs and requirements pertain to the problem: 1. Required Land Use Element of the Compre- hensive Planning Assistance Program (HUD 701). 2. HUD Housing Production and Mortgage Cre- dit/Minimum Property Standards. 3. Federal Disaster Assistance Administration (FDAA). , These programs and requirements are discussed in detail in later sections of this report. Other agencies besides HUD that have significant interest in slope stability for land-use planning pur- poses are the US. Geological Survey (USGS) of the Department of the Interior and the Soil Conservation Service (SCS) of the Department of Agriculture. The USGS provides technical information on landsliding and the relative stability of slopes but has no powers other than review of Federal projects. The SCS has responsibility for developing and carrying out a na- tional soil and water conservation program and, as a part of this program, provides information on soil sta- bility. STATE LEVEL The primary influence exerted by the State of Cali— fornia regarding planning for slope stability is through the State law requiring open space, seismic safety, and safety elements of general (comprehensive) plans. Zoning and subdivision regulations must be consistent with such plans. The required open-space element provides for pres- ervation as open space of highly hazardous areas such 12 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. as active landslides, which cannot be effectively con- trolled. The required seismic-safety element is to identify seismic hazards including appraisal of mud- slides, landslides, and‘slopeistabilty as necessary geo- logic hazards that must be cbnsidered simultaneously with other hazards such as possible surface ruptures from faulting, ground shaking, ground failure, and seismically induced waves.I The required safety ele- ment is to provide for protection of the community from fires and geologic hazards.2 The nature of these elements and related general-plan requirements and their relation to planning Jfor slope stability are dis- cussed in detail in a later section of this report. Sig- nificant State influence on the local planning process in California has also resulted from the adoption of the California Environmental Quality Act of 1970 by the State Legislature. This act requires that any “pro- ject,” unless categorically exempt, requiring discre- tionary action by a government agency must be evaluated for its enviromental impact; a “project” thus includes almost all local land-use decisions and any action on a local general plan. The Resources Agency of California has published guidelines (section 15000 of the Calfornia Administrative Code) for inter- pretation of the act which, in part, require that a full environmental-impact investigation and report, meet- ing specified requirements, be completed for any pro- ject that could expose people or structures to major geologic hazards such as landsliding. The investiga- tion and report must be approved by the local agency with project approval authority; the report findings are to guide the actions of the local decisionmakers. REGIONAL LEVEL Regional land-use planning and decisionmaking is a complex process involving interconnected responsibil- ities and functions of a bewildering array of public agencies and units of government. Agencies with re- gional jurisdiction (including at least parts of more than one county) attempt to solve problems of region- al significance with powers either voluntarily ceded to them by city and county governments or conferred di- rectly by Federal or State legislation. Decisionmaking authority at the regional level is diffused among more than 20 agencies with disparate responsiblities and ju— risdictional boundaries. Five major agencies have limited approval and(or) regulatory authority over projects related to slope stability; the responsiblities of these agencies are briefly discussed below. The im- pact of slope stability on their land-use planning acti- vites is discussed later. 2 Because of the relation between the seismic-safety and safety elements, several bay region communities have chosen to combine the elements into one “seismicsafety/ safety” element. ABAG (the Association of Bay Area Governments) is a council of governments, established in 1961 to “meet regional problems through the cooperative ac- tion of its member cities and counties.” At present, 86 of 92 cities and 8 of 9 counties in the bay area are members. ABAG is the areawide comprehensive plan- ning agency for the bay area, and its approved region- al plan provides a policy framework for regional planning of a variety of issues, including safety from geologic hazards. A key function of ABAG is formulat- ing criteria for evaluating the regional significance of developments and activities or of special land areas having critical environmental concerns. ABAG imple- ments its plans and policies primarily through project review and joint memorandums of agreement with other agencies for pursuit of planning objectives. The Federal Government has designated ABAG as the “clearing house agency” for the bay region. In this ca- pacity, ABAG reviews requests for Federal funds available under more than 150 Federal programs. ABAG also reviews and comments on Federal devel- opment projects in the bay area and on environmental impact statements, required by Federal and State law for projects in the region. The MTC (Metropolitan Transportation Commis- sion) was established to coordinate development of re- gional transportation facilities. Its planning and project-review responsibilities and other duties are normally coordinated with ABAG. MTC is charged with preparing and adopting a Regional Transporta- tion Plan, including proposals for major highways, mass transit, transbay bridges, airports, and harbors. It must also develop a transportation improvement program and a financial program for carrying out that program. MTC’s approval is required for all applica- tions from local governments or districts for State or Federal funds for any kind of transportation facility and certain applications from other government agen- cies. In addition to reviewing projects, MTC adminis- ters the public transit funds acquired from State and local sales taxes on gasoline. MTC needs slope-stabil- ity information in planning the location of transporta- tion facilities and reviewing transportation proposals. The BCDC (Bay Conservation and Development Commission) initially was authorized by the State Legislature to prepare a comprehensive plan for San Francisco Bay and its shores and to control develop- ment within its area of jurisdiction. The plan was sub- sequently adopted by the State Legislature, and BCDC became a permanent agency charged with car- rying out the plan. The adopted plan has legal status and serves as a guide in the review of projects. BCDC shares jurisdiction over land-use decisions with the cities and counties, which retain normal land-use and INTRODUCTION 13 building—permit controls. However, with certain mi- nor exceptions, a permit from BCDC is required for all projects within its jurisdiction. An important consid- eration in BCDC’s planning and regulatory activities is the effect of unstable bay muds on land-use propos- als. The CCZCC (California Coastal Zone Conservation Commission), working with six regional commissions, was created by initiative and was charged with prepar- ing a plan for the future of the California coastal zone. While the plan was being prepared, the commissions controlled all development, through a permit process, to insure consistency with the objectives of the legisla- tion and the emerging plan policies. The plan was pre- sented to the Governor and Legislature in December, 1975 for adoption and implementation. In September 1976 the California Coastal Act of 1976 was enacted, establishing the California Coastal Commission and six regional commissions as successors to the previous commissions. Under the terms of the Act, the six re- gional commissions will expire 30 days after the last required local coastal program has been certified, but no later than January 1, 1981. Coastal areas of the bay region are represented by two regional commissions: Central (San Mateo County) and North Central (San Francisco, Marin, and Sonoma Counties). The Coastal Plan stresses the importance of considering natural earth processes, particularly landslides, in planning for conservation and development of the land within the coastal zone. LOCAL LEVEL Although California cities and counties are parts of the State, they exercise broad authority over most 10- cal concerns. However, the scope of local land-use planning is mandated to a large degree by State re- quirements for general plans, consistency of zoning and subdivision ordinances with general plans, and environmental-impact assessment. In addition to State-mandated responsibilities for land-use planning and regulation, local jurisdictions find themselves responsible for such specific hazards as landsliding. The Sheffet decision (Los Angeles Su- perior Court Case No. 32487) declared that a public entity is liable for damages to adjacent property re- sulting from improvements planned, specified, or au- thorized by the public entity in the exercise of its governmental power. Also, the Los Angeles County Superior Court (Case No. 684595 and consolidated cases) found the county liable for damages which may have resulted from road work and the placement of fill by the county. This case concerned the Portuguese Bend landslide, in the Palos Verdes Hills in Los Ange— les. As a direct result of these and similar cases, coun- sels to local government have advised local decisionmakers to give special attention to problems of slope stability Although State requirements establish the frame- work for local planning, local agencies have some dis- cretion in how the requirements are carried out. Many local agencies in California have been able to adapt the requirements (which have evolved in a piecemeal manner over a number of years, most often in re- sponse to crisis situations) into more comprehensive and creative planning strategies for decisionmaking. SLOPE-STABILITY CONSIDERATIONS IN LAND-USE PLANNING If landslide risk is to be reduced, it is essential that planning for mitigation of geologic hazards take place throughout the land-use planning process. Land-use planning is that part of comprehensive planning which deals with all aspects of the future growth and development of an area and requires the proper bal- ance of economic, political, social, and physical fac- tors. Land-use planning is concerned with the arrangement and types of land uses, their impact on the landscape, their relation to transportation and other community facilities and utilities, and the changes in these conditions and relations over time. Although the form and content of land-use plans and implementing strategies vary across the United States and from jurisdiction to jurisdiction there is a strong similarity in the planning process. The plan- ning process is composed of six conceptually distinct yet functionally related phases. Although these phases are generally followed in sequence, there is a great deal of “recycling” or interplay between them. The phases are: (1) issue identification and definition of objectives; (2) data collection and interpretation; (3) policy review and plan formulation; (4) impact evaluation; (5) plan review and adoption; and (6) plan implementation. The planning process is shown in schematic form in figure 5. As shown by the arrows, each phase of the process is interrelated with all the others, and the se- quence, while logical, often varies, especially in re- sponse to crises, political opportunities, or legal requirements. Interaction among the phases, however, usually is continuous. For example, plan formulation often indicates the need for additional information; additional information may alter the concept of the objectives and problems; and plan implementation may reveal the need for additional information or modification of the plan. Public initiative and response are key parts of every phase of land-use planning. “Public” may refer to 14 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. Public initiative and response Identify problems and define goals and objectives Collect and interpret data Formulate plans Review and adopt plans Evaluate impacts Implement plans \J Feedback for review and revision FIGURE 5,—The land-use planning process. elective political bodies, special—interest groups, or in- terested individuals. Elected officials have final re- sponsibility for most key policy decisions, although persons in nonelective positions actually make many important day-to-day decisons. Decisions range from the decision to engage in a planning effort, to the final approval of a plan and adoption of implementing reg- ulations, programs, and procedures. The phases of the land-use planning process are ex- plained below with a brief description of how they re- late to planning for slope stability: 1. Issue identification and definition of objec- tives.—Land-use concerns are identified and re- viewed in relation to any existing land-use plans and policies, projected growth trends, and anticipated changes; a tentative set of goals and priorities is de- fined. Some issues are obvious because of their con- tinuing impact on many people, for example the need for conveniently located quality low-cost housing; others are less obvious. However, earth-science con— cerns such as slope stability frequently do not become apparent until a disaster has occured. At the time an issue is identified, therefore, it is important that the planner assemble and review the available earth-sci- ence information so that appropriate objectives and priorities can be defined. A regional slope-stability map such as is contained in this report could be an essential part of this phase of land-use planning. 2. Data collection and interpretation—A pro- gram for utilizing available data and compiling new data is developed in connection with the goals and priorities established in phase 1. The planner, to- gether with the earth scientist, needs to determine what earth-science data are available and the most ef- fective manner of using existing data and collecting and interpreting new data. Interpretive maps and text should be close in scale and detail to other basic plan- ning information. The planner estimates the future demand for land, considering projections of popula- tion growth and distribution, economic activity, social and cultural needs, and transportation requirements. Preparation of land-capability maps, showing poten- tial uses of specific areas, can be a significant part of this phase. The slope-stability map in this report would be use- ful in helping define land capability at the regional level. In addition, it provides information needed to evaluate the regional significance of the slope-stabil- ity problem. The regional slope-stability map will pro- vide the city or county planner with some indication of the local problem, but it will be necessary to work closely with an earth scientist to determine the re- quirements for additional slope-stability information to serve specific local needs. 3. Policy review and plan formulation—On the basis of a land-capability study, appropriate projec- tions, and environmental, economic, social, and politi- cal analyses, local or regional land-use strategies are considered. Alternative land uses can be eval- uated, and the best uses and the best ways of guiding growth and managing land use can be selected. A land-use plan is then prepared, incorporating the pol- icy and proposals necessary to serve as an effective ba- sis for decisionmaking. In formulating plans for pro— tection from slope failure, the slope-stability map will help to determine potential risk to life and property INTRODUCTION 15 from landsliding for each alternative use. Risk should be considered not only in terms of harm to the individ— ual who occupies a particular area identified as unsta- ble, but also in terms of impact on the public interest if damage should occur, including damage to adjoining public and private property. In addition, acceptable risk must meet Federal and State requirements, par- ticularly when Federal or State funds are involved. Plans should consider all potential methods of imple- mentation. 4. Impact evaluation—Federal and State legis- lation in the late 1960’s and early 1970’s have focused considerable attention on “environmental impact evaluation,” with the result that impact evaluation has become Virtually a separate step in the planning process. Realistically, however, judging the effects of each alternative plan and land-use strategy is an inte- gral part of plan formulation. As indicated in the dis- cussion on plan formulation, development of land-use alternatives and management strategies is based on analysis of the environmental, economic, social, and political consequences of the various alternatives— that is, the impact evaluation of these various factors. In addition, impact evaluation is critical to analysis during implementation and particularly during re- view of land-development proposals. 5. Plan review and adoption—The land-use plan, either separate or as part of a comprehensive plan, is prepared as a statement of city, county, or re- gional policy and as a commitment to a future course of action. The plan might be a series of policy state- ments establishing criteria for urban growth and land use and development, or it may take the form of a text containing policy and proposals accompanied by dia- grams showing the desired or expected spatial distri- bution of land uses in the future. It is essential that policy—makers understand thoroughly the content, implications, and use in decisionmaking of any plan they adopt. They should also understand that the plan is a document that will change as new informa- tion becomes available. Most governing bodies are genuinely concerned about their constituency, the public, understanding the content and implications of plans prior to official adoption. It is highly desirable, therefore, that adequate information on the plans be made available as a part of the review and adoption process. At the time of plan review, information should be available to provide background on how the plan was formulated. This information might include a descrip- tion of data used to develop the plan proposals, among which might be a relative slope stability map and text. In addition, methods of implementation should be summarized, noting possible changes in regulations and implied expenditure of funds, environmental and economic impacts described, and social consequences analyzed. Public review of plan proposals may bring recom- mendations for changing the plan. If this is the case, it will be necessary to repeat some of the earlier steps in the planning process. 6. Plan implementation—After a plan is adop- ted, land-use regulations (for example, zoning, subdi- vision, and land development ordinances) and pro- grams for land acquisition and capital improvement are prepared and adopted. Methods to implement slope-stability proposals can include partial or full ac- quisition of hazardous lands, open-space zoning of areas of great hazard, and establishment of special regulations to guide development in areas where un- stable slopes require some limits on land use. Also, guidelines and procedures for conducting the earth- science studies needed to evaluate proposals should be established. Procedures should be developed and staff provided for reviewing soils and geology reports, envi- ronmental impact assessments, and project proposals. The planning process is not finished with the com- pletion of the six steps summarized above; it is an on- going process that continually receives public input. Governments usually find that, by design or by cir- cumstance, they are routinely revising their statement of goals, collecting and analyzing new information about their jurisdiction, revising statement of policy, updating plans, and enacting new strategies to imple- ment their plans. The foregoing generalized model of the planning process is necessarily idealized and simplified. Actual practices vary widely depending on the responsibility, authority, and financial position of the planning agency, the diversity of the planning area, the scope of the planning effort, and availability of data. For ex- ample, planning by regional councils of government is likely to emphasize the development of objectives, policies, and criteria for use in reviewing projects and plans, because the councils’ Federally mandated pow- er is that of review. Local planning, on the other hand, is more likely to emphasize the development of objec— tives, policies, and criteria to serve as a basis for public projects and land-use and development regulations—— the latter traditionally a local responsibility. In addi- tion, planning practices are not static. Planning is in a state of flux, with planners, legislators, and citizens searching for new ways to make the process more ef- fective. The scope of planning is expanding and its role changing, fresh approaches are being tried, and new relationships—local, metropolitan-regional, State, and Federal—are emerging. 16 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. RELATIVE SLOPE STABILITY OF THE SAN FRANCISCO BAY REGION By T. H. NILSEN and R. H. WRIGHT One purpose of this report is to present a method of classifying the land surface of the San Francisco Bay region in terms of relative slope stability, or the rela- tive susceptibility of the land surface to landsliding. The relative slope stability is portrayed in three maps that were prepared to indicate broad regional vari— ations in relative slope stability at a scale of 1:125,000 (pls. 1, 2, and 3). The area is divided into five categor— ies and one subcategory, ranging from stable areas, where landslides are highly unlikely to occur, to un- stable areas, where landsliding is very likely to occur. The maps were prepared from an analysis of three of the more important factors that contribute to and control the generation of landslides—the nature of the underlying bedrock, the angle of slope of the land sur- face, and the presence or absence of earlier landslide deposits in the area. Numerous studies and maps of relative slope stabil- ity have been made for different parts of the bay re- gion at various scales and using different techniques. The entire region is covered in this report, and we have attempted to incorporate as many as possible of the previous concepts, ideas, and maps of slope stabil- ity in the bay region. However, as better data become available in the future, the maps presented herein should be revised and superseded by newer maps. The nine counties bordering San Francisco Bay that make up the San Francisco Bay region cover a total land area of about 7,400 square miles (19,200 km?) and include all or parts of 162 US. Geological Survey topographic quadrangle maps (fig. 6). The pre- sentlpopulation is about five million. The region is extremely varied in topography, vege- tation, relief, population density, geology, and local climate. It lies primarily within the central and north- ern Coast Ranges but includes part of the San Joaquin and Sacramento Valleys (Scott, 1959). The region is characterized by large flat areas that surround San Francisco Bay and extend into adjacent interior val- leys (fig. 7). These valleys abut on rugged highlands that reach elevations of over 4,000 ft (1,200 m). Population growth historically has been confined primarily to flat areas such as interior valleys and the margins of San Francisco Bay (Scott, 1959); however, in recent years, development has spread rapidly into upland areas, where slope-stability problems have be- come increasingly common. Though the damage from slope failures in the San Francisco Bay region may not be quite so destructive or so widespread as in some other parts of the world, for example, damage has been more severe in regions such as Calabria, Italy (Burton, 1970; Guida and others, 1974), urban centers such as Hong Kong (Lumb, 1975) and Rio de Janeiro (Jones, 1973), and along major highway networks in eastern Tennessee (Royster, 1973), Ohio (Marshall, 1969), and West Virginia (Long and Stinnet, 1969), nonetheless, landsliding is one of the major geologic problems and hazards in the bay region. The geology of the bay region is very complex (Schlocker, 1968, 1970). Many different types of rocks and numerous active faults are present (Brown, 1970), and the structural and tectonic history has been com- plex. Local climates within the region are highly vari- able; the rainy season commences in October or November and ends in March or April. The total sea- sonal rainfall can be more than 40 in. (100 cm) in the redwood forests along the Pacific coast and less than 10 in. (25 cm) in the drier oak and grassland areas of the interior. All the primary conditions responsible for land- slides are present in the bay region: (1) steep, irregular slopes; (2) abundant and seasonally intense rainfall; (3) extensive human activity, including logging and the grading and cutting of slopes; (4) many weak and unconsolidated rock units that form unstable slopes, including extensively crushed and sheared Franciscan sedimentary complexes and unlithified upper Terti- ~ ary to Holocene sediments; (5) thick unconsolidated colluvial deposits and thick weathered zones on steep slopes; (6) many expansive clay soils; and (7) frequent and occasionally strong seismic activity. Because these and other factors are present, landsliding is a very costly problem at present and will continue to be one during the future growth of the region (Harding, 1969). Figures 8 through 17 illustrate the types of damage caused by landsliding in the bay area. Damage is closely related to the type of landslide involved. A thin mudslide of low velocity will cause less damage than a large debris flow of high velocity or a debris slide or slump involving large blocks of mate- rial. Also, a landslide that falls on a road is usually far less expensive (involving only cleanup of debris) than a landslide that undermines a roadbed (requiring ex- tensive work preparing a new foundation and road- bed). Mud and debris flows represent the greatest hazard to human life inasmuch as they occur rapidly and commonly without warning. PREVIOUS WORK Many studies of engineering geology, slope stability, and landsliding have been made in the bay region. 1'7 3 a \ s u m , PB\\\\\\ 0 x 5 fi\ mm Nn é H 5 EB afimfiw‘ M amemv \\MW 6 n» s @VP_I_\ (5*! A‘, % é .v @ — eé*Vov G 0.5%an wuvkuv R cwq N $er 9.79 as Vim Q9» 6% ....I. n P§0R8 Q 55 _ R9 RP] I5 I A1 LSw - L9 we. *0“? e». m. Wha’ \ +s meeee as kovvuw MI 3 #6 *6 e xfmw A¢~v® 0t &¢8 w‘e H $6 xsmw m Rarl\\ h§no H Afivax av \ Q '1 I _ fl \ r. S as _ s s — I/III & Q Aw s e e a N _ x e m _ o v e+ év x A a” « am a A: $ 3”er $3.3! §§$l\ +§~9I|£quq\lmk¢uw\ _ «3, m Us SM ex 8* 00% .l as (so 2., 0W sew «w H...” BM w , wfl . Mm. _ Mm .Tm Mm 0m 0 . as a, « ”rescues _ MM w“ w 35%? m s¥a$v mflhfi 3&W¢ Mr &§%§% édflv T Br v 3e Q x « av _ x c H. ex SIJJ at é e V» s e M _ r _ V N s _ as a“ u e _ La _ _ _ 9 Q 4. w 9 §. W& a» «mi w em» :3 xx 50A 6a, . seas Yea, You as. D é & e w. E w $1L a o G o e x a v n ¢ 9 r. \ ¢ 5 % kvx F m sows set u 0&0 8&3 R goosanA 9.1..“ es“ mi aosw _ $3ch %1 Mu . Wu V muchAT a in. 0w ' Mm \ Em x a Nm 0 @qu wwvéeor. A $.99 W arcs E 58 a.» M W wfiy §Q\ V ’ $.v L éwfim §\ \ _ s _ _ _ n 0V énv G ¢ _ _ fl 9&& £W D as 3‘ s\m1 m “Mmhlyn v w. R,| é , arammer yflV»? wwéy8 M &§\ 8 w*é®x .@ 3 a _ 3 we 6 _ w\\\li 9 3r _ # wé é 3H9 @wxé‘ A awash» See V o 0.? 03w... m AA or m WXI 2 m \06 0 so 9+9“. L. I HOPLAND... ‘96!) RELATIVE SLOPE STABILITY OF THE SAN FRANCISCO BAY REGION ORNEAUN VALLEV 1960 J w as V?» ers, 1939), a general study of landslides in the central Publications in the 1950’s dealt largely with par- ticular aspects of the factors that controlled landslid— ing such as slope exposure (Beaty, 1956), landslides that resulted from the 1957 San Francisco earthquake Coast Ranges (Thomas, 1939), a discussion of soil slips causes of landslides in the bay region and ways of pre- by Kesseli (1943), and a general discussion of the venting them by Forbes (1947). FIGURE 6..—Index map of US Geological Survey quadrangle maps in the San Francisco Bay region. neering geologists and is unpublished. Some early pa- pers and research studies of major interest that relate to present-day slope-stability problems include the analysis of landslides triggered during the 1906 San Francisco earthquake (Lawson, 1908; Anderson, 1908), an analysis of an induced landslide on Lone Mountain in San Francisco by Cogen (1936), a study of a major landslide near Gilroy (Krauskopf and oth- Most of this work has been done by consulting engi- 18 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. Point Arena Point Reyes Farfllon islands (S‘AN FRANCISCO co: Pillar Point Pigeon Point FIGURE 7,—Overall setting of the San Francisco Bay region. (Bonilla, 1959), specific studies of individual land- slides (Woods, 1958), and studies of slope stability at particular sites of development (Kachadoorian, 1956, 1959). Several geologic maps published during this decade incorporated much engineering geologic data and were forerunners of recent types of engineering geologic maps (Radbruch, 1957; Schlocker and others, 1958). Numerous studies were completed in the 1960’s, in— cluding an analysis of landslides in the San Francisco South quadrangle (Bonilla, 1960), a study of land- slides in the Orinda Formation (Radbruch and Weiler, RELATIVE SLOPE STABILITY OF THE SAN FRANCISCO BAY REGION 19 1963), engineering geologic mapping and landslide studies in the Oakland area (Radbruch and Case, 1967), slope stability studies in the town of Portola Valley (Johnson and Ellen, 1968; Johnson and Lobo- Guerrero, 1968) and in the Nicasio Valley (Twiss and others, 1970), a study of the mechanics of creep and rates of creep (Kojan, 1968), a study of landslides at Point Reyes National Seashore (Clague, 1969), and a _, summary of the environmental aspects of landsliding in the bay region (Harding, 1969). The 1970’s have seen a great increase in slope-sta- bility studies in the bay region. The US. Geological Survey undertook extensive regional mapping of land-
W E’iI ‘ij'w 3-‘1 "Mill“ fl IMAM ,A :flé‘é‘rJ" FIGURE 39.—Sources of landslide mapping. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. RELATIVE SLOPE STABILITY OF THE SAN FRANCISCO BAY REGION 39 Frizzell (1974) Blake and others (1971) Edgar H. Bailey, unpublished data, l:62,500 Carl M. Wentworth, unpublished data, 1:24,000 Fox and others (1973) Huffman (1972) Huffman (1973) Douglas M. Morton, unpublished data, l:24,000 Huffman (1971) John A. Bartow, unpublished data, l:24,000 Gladys Louke, unpublished data, l:24,000 Rice and Strand (1972) Virgil A. Frizzell, Jr., unpublished data, l:24,000 Blake and others (1974) Wright and Reid (1975) Dave Wagner, unpublished data, 1212,000 Kenneth F. Fox, Jr., unpublished data, 1:24,000 Sims and others (1973) Frizzell and others (1974) Sims and Nilsen (1972) John T. Alfors, unpublished data, 1:24,000 Nilsen (1973b) Nilsen (1971) Nilsen (1972b) Julius Schlocker, unpublished data, 1:24,000 Schlocker and others (1958) Schlocker (1974) Bonilla (1971) Nilsen (1973a) Nilsen (1972c) Brabb and Pampeyan (1972b) Nilsen (1972a) Earl E. Brabb, unpublished data, l:62,500 Rogers (1971) Rogers and Armstrong (1973) Nilsen (l972d) FIGURE 39.-—Continued. of one of these maps. Many small landslides that are shown as enclosed areas on the original larger scale maps were reduced to dots at the smaller scale. From the source maps, we incorporated all the landslide de- posits shown, including those mapped with queries or other degrees of uncertainty. As a result, the maxi- mum possible number of landslide deposits shown by the authors was incorporated in our maps. The maps of landslide deposits were generally far too detailed and complex for us to use easily in the slope-stability analysis. Consequently, as was done for the slope maps, we prepared generalized or simplified maps of the landslide deposits. These generalized maps were made primarily by grouping large and small landslide deposits that were located close to one another as larger areas underlain by many closely spaced landslide deposits. Figure 46 is an example of a generalized map of landslide deposits. These generalized maps were prepared manually by enclosing areas within which the mapped landslide deposits were spaced less than LOGO—1,500 feet (300— 460 m) apart. Thus, areas with numerous closely spaced, small landslide deposits or with closely spaced small, medium, and large landslide deposits are en- closed as zones, belts, strips, and irregularly shaped areas. All areas more than about LOGO—1,500 feet (300—460 m) wide that do not contain landslide depos— its but may be surrounded by closely spaced landslide deposits are delineated on the maps. The generalizing process results in the inclusion of many areas less than LOGO—1,500 feet (300—460 m) wide that are not cov- ered by landslide deposits within the enclosed areas of landslide deposits. Thus, as a result of the generaliz- ing process, narrow areas unaffected by landslide pro- cesses are included within the areas affected by landslide processes. Solitary medium and large land- slide deposits are delineated separately and not grouped with other landslide deposits more than LOGO—1,500 feet (300—460 m) away. Solitary small landslide deposits are shown separately. The general topography and direction of slope were also used to delineate the landslide deposits. Land- slides on the same continuous slope, creek bank, ridge top, or cliff have been grouped together because they are presumably generically related. SURFICIAL AND BEDROCK GEOLOGY MAPS Geologic maps of the San Francisco Bay region were prepared at a scale of 1:125,000 from the published and unpublished sources shown in figure 40 (index 40 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. map). These maps show the distribution of the geo- logic units that underlie the region; the units are di- vided according to their age and rock type (fig. 47). From discussions with geologists of the US. Geo— logical Survey in Menlo Park, Calif, who have done mapping of or research on the physical properties of hillside materials, and from our own working exper- ience in the bay region, we outlined on the maps those ‘3, § '3: \ URNBAUN MU) SHEET 1 FIGURE 40.—Sources of bedrock mapping. geologic units generally considered to be especially susceptible to slope failures (fig. 48). Each of these units has had a history of extensive landsliding and generally forms relatively unstable slopes. The names of the bedrock units judged to be susceptible to slope failure are listed in table 5 according to their age and the areas where they occur. . The muds along the margins of San Francisco Bay 0150M ‘ H41 p, RUM/w I llib‘ MT a} LROY HUI SPRINGS "lhlllmflr. o M .5" v '3" N LUMQND ,n ‘01,!) AW PASS G1. ,k 1" 0-1;)" 37“ CE 1915 :w RELATIVE SLOPE STABILITY OF THE SAN FRANCISCO BAY REGION and the San Joaquin-Sacramento River delta, which generally form tidal marshes, swamps, and lagoons, are also susceptible to failure, even when nearly flat lying. These wet, unconsolidated, soft muds tend to flow laterally into cuts and are particularly suscept- ible to movement during earthquakes. These deposits had to be mapped separately for slope-stability pur- poses because of their unique properties and were out- lined as a separate category on the geologic map using the previous mapping of Nichols and Wright (1971) (fig. 48). DERIVATION OF THE SLOPE-STABILITY MAPS After completing the steps described above, we combined each generalized slope map with the corre— sponding generalized landslide deposits map and geo- logic map. These three maps, all at the scale of 1:125,000, were combined in two stages to produce the completed slope—stability maps of plates, 1, 2, and 3. The first stage of this procedure was the combina- tion of the generalized slope maps (fig. 44) and the 1. Blake and others (1971) 2. Fox and others (1973) 3. Blake and others (1974) 4. Sims and others (1973) 5. Earl E. Brabb, R. Wagner, and H. S. Sonneman, unpublished data 1:24,000 6. R. Wagner and Earl E. Brabb, unpublished data, 1:24.000 7. Brabb and others (1971) 8. Brabb and Pampeyan (1972a) 9. Earl E. Brahb, unpublished data, 1:24,000 10. Brabb (1970) 11. Dibblee (1972a, Milpitas quadrangle) 12. Dibblee (1972a, Calaveras Reservoir quadrangle) 13. Dibblee (1972b, San Jose East quadrangle) 14. Dibblee (1972c, Lick observatory quadrangle) 15. Cotton (1972) 16. McLaughlin and others (1971) 17. Dibblee (1973a, Morgan Hill quadrangle) 18. Dibblee (1973b, Mt. Sizer quadrangle) 19. Dibblee (1973c, Mt. Madonna quadrangle) 20. Dibblee (1973d, Gilroy quadrangle) 21. Dibblee (l973e, Gilroy Hot Springs quadrangle) FIGURE 40.—Continued. 41 generalized maps of landslide deposits (fig. 46). This stage was accomplished by overlaying the slope maps on the maps of landslide deposits and transcribing the generalized areas of landslide deposits onto the slope maps (fig. 49). By this procedure, preliminary relative slope stability maps were produced that had four cate- gories: (1) areas of 0—5 percent (0—3°) slope, (2) areas of 5—15 percent (3—8.5°) slope, (3) areas greater than 15 percent (8.5°) slope, and (4) areas underlain by landslide deposits. In the final stage, the preliminary relative slope sta- bilty maps (fig. 49) were combined with the modified geologic maps showing the distribution of bedrock and surficial deposits considered to be especially sus- ceptible to slope failures (fig. 48). This stage was ac- complished by superimposing the preliminary slope- stability maps on the modified geologic maps and transferring to the slope-stability maps the bound— aries of all geologic units considered to be especially susceptible to slope failure (fig. 50). The bedrock units were transferred only in areas underlain by slopes greater than 15 percent (8.5°); where gentler slopes were present, the units were not transferred. However, the moist, unconsolidated muds surrounding the bay were placed in a separate category because they are exclusively in areas of 0—5 percent (0—3°) slope. Thus, the final relative slope stability maps show the San Francisco Bay region divided into five cate- gories and one subcategory of slope stability: (1) 0—5 percent (0—3°) slope, (1A) 0—5 percent (0—3°) slope underlain by moist unconsolidated bay muds, (2) 5—15 percent (3—8.5°) slope, (3) greater than 15 percent (8.5°) slope, (4) greater than 15 percent slope under- lain by bedrock geologic units considered to be espe- cially susceptible to slope failure, and (5) areas underlain by individual or closely spaced landslide de- posits. These five categories and one subcategory ef- fectively divide the map into areas ranging from relatively stable to relatively unstable. EXPLANATION OF SLOPE-STABILITY CATEGORIES Each of the areas shown on the relative slope stabil- ity maps (pl. 1, 2, and 3) is underlain by a different combination of slope angle, type of bedrock unit, type of surficial unit, or number of landslide deposits; the areas are thus separable into distinctive categories in terms of relative slope stability. However, because of the scale used and the extent of generalization used to prepare the working maps, there may be many small areas within each mapped category with higher or low- er slope—stability characteristics. These areas are too small to show at the scale used. 42 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. CATEGORY 1 Category 1 consists of areas of 0—5 percent (0—3°) slope that are not underlain by landslide deposits or other surficial deposits that are highly susceptible to slope failures. They may be underlain by bedrock units that are susceptible to slope failures on steeper slopes but are generally stable at these low slopes. The areas within category 1 are generally underlain by floodplain alluvium, alluvial terrace deposits, marine terrace deposits, and gently sloping alluvial fan depos- its; but they may also form the flat, gently sloping summit areas of some ridge crests and mountains. They may locally be susceptible to flooding and to de- position of debris flows derived from surrounding up- lands during periods of heavy rainfall. However, ORNBAUN Ax PMLPAM ,4" rm $51.“ SHEET 1 i U 155535?— ,‘r hiit ‘ Miggfi FIGURE 41.—Areas of previous slope stability studies. RELATIVE SLOPE STABILITY OF THE SAN FRANCISCO BAY REGION 43 within the category 1 areas the slopes are generally stable. Exceptions may include some small areas of steeper slopes adjacent to roads, creeks, rivers, and coastal margins. These areas may include riverbanks, coastal cliffs, and edges of terraces; they are generally too small or narrow to be shown at this scale and common- ly have low relief. Riverbanks may be particularly haz- ardous during periods of flooding and the coastal areas particularly hazardous during severe storms. In addition to these exceptions, the areas in category 1 may be underlain by bedrock types that are locally un— stable at slopes of 0—5 percent (0—3°) and therefore susceptible to landsliding. CATEGORYIA Category 1A consists of areas of 0—5 percent (0—3°) slope that are underlain by moist unconsolidated sedi- ments surrounding San Francisco, San Pablo, Suisun, and Grizzly Bays and in the confluent Sacramento and San Joaquin delta. These areas are generally tidal flats, marshes or swamps, unless modified by artificial fill, so they are susceptible to flowage, lateral move- ment and liquefaction at slopes of less than 1° (Ni- chols and Wright, 1971; Youd, 1973; Youd and others, 1975). During earthquakes, they are particularly sus- ceptible to ground failure, and structures built on arti— ficial fill placed over the muds may be damaged. The margins of tidal channels are especially subject to fail- ure when undercut, excavated, or subjected to differ- ential loading. 1. Huffman (1972) 2. U. S. Army Corps of Engineers (1967) 3. Huffman (1971) 4. Huffman (1973) 5. Rice and Strand (1972) 6. Radbruch and Wentworth (1971) 7. Twiss and others (1970) 8. Burnett (1972) 9. Brabb, Pampeyan, and Bonilla (1972) 10. Rogers (1971) 11. Rogers and Armstrong (1973) 12. Frame (1974) 13. Wright and Nilsen (1974) FWGURE4lr—Confinued CATTKHDRY'2 Category 2 consists of areas of 5—15 percent (3—8.5°) slope that are not underlain by landslide deposits or other deposits that are highly susceptible to slope fail- ures. They may be underlain by bedrock units that are susceptible to slope failures at steeper slopes but are generally stable at slopes of 5—15 percent (3—8.5°). The areas within category 2 are generally underlain by colluvial deposits, alluvial fans, tilted alluvial flood plains, and marine and alluvial terraces that common- ly form gently sloping areas at the bases of upland areas. These areas are generally relatively stable but may include locally steeper slopes along roads, creeks, riv- ers, or the coast that may be more susceptible to landsliding but are too small or narrow to be shown at this scale. In addition, some areas within category 2 may be underlain by bedrock types that are locally un- stable at slopes of 5—15 percent (3—8.5°) and therefore susceptible to landsliding. CATTKHDRY'3 Category 3 consists of areas of greater than 15 per- cent (8.5°) slope that are underlain neither by land- slide deposits nor by bedrock units that are susceptible to landsliding. This category generally comprises hillside and upland areas that are common- ly underlain by bedrock rather than surficial deposits, although colluvial deposits may be present on the low- er parts of the slopes and in ravines or canyons. These areas are generally reasonably stable but may include some small areas that are locally unstable for various reasons, such as the failure of areas above or below that are underlain by bedrock types susceptible to landsliding or by landslide deposits; proximity to areas of active erosion along creeks, rivers and coastal areas; slopes saturated with water adjacent to lakes and reservoirs; proximity to active landslides that may be enlarging; and man’s activities such as logging, cutting and filling, construction, and adding moisture to slopes. These areas may also include small landslide deposits not large enough to be shown at this scale or to have been mapped by geologists. CATEGORY4 Category 4 consists of areas of greater than 15 per- cent (8.5°) slope that are underlain by bedrock units that are highly susceptible to landsliding but are not underlain by landslide deposits. This category com- prises hillside and upland areas that are commonly underlain by bedrock rather than surficial deposits, although colluvial deposits may be present on the low- er parts of the slopes or in canyons and ravines. 44 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. SHEET DIAGRAM u;- TATE AnggglANAGEMENT 90 Rfifi COUNTY KEY A. MARIN B. SONOMA C. NAPA D. SOLANO E. CONTRA COSTA F. ALAMEDA G. SANTA CLARA H. SANTA CRUZ I. SAN MATEO J. SAN FRANCISCO LEGEND Primary highway _______________ .___ Primary highway under construction_ _ __-_ Secondary highway _____________ .— Other all-weather road or street _____ _ Dirt road _____ ._ ___________________ _ Village or locality ______________ O TRUE NORTH DECLINATION AT CENTER OF SHEET 037°52'30" 121 45' 1 V2 0 1 2 3 4 5 6 7 8 9 10 MILES H H F 4. 1 L. . 4. }______. . L J 5000 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 FEET H H H if . {—7 ' , . . 1 1 V2 0 1 2 3 4 5 6 7 8 9 10 KILOMETERS :1 r L r i u . r . 4L 4} J CONTOUR INTERVAL 200 FEET * DOTTED LINES REPRESENT 40-FOOT CONTOURS ' DATUM IS MEAN SEA LEVEL DEPTH CURVES AND SOUNDINGS IN FEET—DATUM IS MEAN LOWER LOW WATER SHORELINE SHOWN REPRESENTS THE APPROXIMATE LINE OF MEAN HIGH WATER THE MEAN RANGE OF TIDE IS APPROXIMATELY 2 TO 5 FEET FIGURE 42.—Topographic map of part of northern Contra Costa and southern Solano Counties, Calif. (from US. Geol. Survey, 1970; Sheet 2). RELATIVE SLOPE STABILITY OF THE SAN FRANCISCO BAY REGION 2 050 000 EET PERCENT OF SLOPE mm 0—5 FIGURE 43.— Slope map of part of northern Contra Costa and southern Solano Counties, Calif. (from US. Geol. Survey, 1972, Sheet 2). 46 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. 121° 45' 37° 52'30" o v STAT? , 5 J no 03}? goGAM§° ,ANAGEMENT * 0'50 EXPLANATION Percent of slope DI- 0-5 5-15 >15 38°oo' 37° 52' so" |2l° 45' FIGURE 44.— Generalized slope map of part of northern Contra Costa and southern Solano Counties, Calif. RELATIVE SLOPE STABILITY OF THE SAN FRANCISCO BAY REGION |2|° 45' ; I D _ TATE V s H ANAGEMENT , .- fiw l ‘ 38°oo' — ‘ ~ .7 we; 37°52'30" -- A - ' - ‘ ° ( b 37°52'30" I2I°45' EXPLANATION Large landslide deposit larger than 500 Small landslide deposit approximately feet in longest dimension 200-500 feet in longest dimension FIGURE 45.—Photointerpretive map of landslide deposits in part of northern Contra Costa and southern Solano Counties, Calif. (modified from Nilsen, 1971 and Sims and Nilsen, 1972). 47 48 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. I2I°45' 3e°oo'— ‘ V_ , g c = ~3e°od E \ I \ h: g... \ .— ) _ ‘b g F M \ _.i~_ 5%: ¥ 37°52 30 . 37 52 3O 121°45' EXPLANATION - . Areas underlain by single landslide Single isolated small landslide deposit deposit or group of closely spaced large and small landslide deposits FIGURE 46.— Generalized photointerpretive map of landslide deposits in part of northern Contra Costa and southern Solano Counties, Calif. RELATIVE SLOPE STABILITY OF THE SAN FRANCISCO BAY REGION 49 These areas are susceptible to future landsliding even though landsliding has not occurred there in the past. The underlying bedrock units possess physical characteristics, such as extensive shearing or jointing, poor consolidation, and structurally weak components that make them susceptible to slope failures and have caused slope failures in adjacent areas. The exact con- ditions required for future landsliding in these areas are not known, but under the effects of high rainfall, seismic activity, the influence of man, and other fac- I2I°45' 38°00'— 37°52'30" L38°oo' ' Mm; 37°52' 30" 121°45' FIGURE 47.— Geologic map of part of northern Contra Costa and southern Solano Counties (from Brabb and others, 1971 and Sims and others, 1973). 50 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. tors mentioned previously, these areas are likely to be mentioned under category 3. Conversely, local areas unstable. Category 4 may include some small areas within category 4 may be more stable than the average within it that are locally more unstable for the reasons because of variations in the local character of the bed- I2I°45' 38°00'— —38°00‘ 37°52'30" 37°52‘30” 121°45' EXPLANATION Bedrock units Bay mud FIGURE 48.—— Distribution of bedrock and surficial geologic units considered to be especially susceptible to slope failures in part of northern Contra Costa and southern Solano Counties (modified from Brabb and others, 1971, Nichols and Wright, 1971, and Sims and others, 1973). RELATIVE SLOPE STABILITY OF THE SAN FRANCISCO BAY REGION 18°45 JJ' ”:9an , A i r , \ ANAGEMENT \ OpGAMH 0030 “ 4,571 38°00' 37°52'30" EXPLANATION I , m in.“ Areas of 5-15 percent slope and no landslide deposits D Areas of less than 5 percent slope and no landslide deposits Areas of greater than 15 percent slope and no landslide deposits Single isolated small landslide deposits 38°00“ 37°52'30" I2I°45' Areas underlain by sin— gle landslide deposit or group of closely spaced large and small landslide deposits FIGURE 49.— Preliminary relative slope stability map of part of northern Contra Costa and southern Solano Counties derived by combining the generalized slope map (fig. 44) and generalized map of landslide deposits (fig. 46). 51 52 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. 121°45' \ ”NJ/(73” s TATE ‘3 Q on 0G gpcmé‘ glANAGEMENT ' 'Co 0 \ “ a 38000' 38° 00I 37°52'30" 37°52'30" |2|° 45' . EXPLANATION Stable Subject to Generally Moderately Moderately Unstable liquefaction stable stable unstable 0 Single isolated small landslide deposits FIGURE 50.——~Relative slope stability map of part of northern Contra Costa and southern Solano Counties derived by combining the preliminary relative slope stability map (fig. 49) and map of bedrock and surficial geologic units considered to be especially susceptible to slope failures (fig. 48). RELATIVE SLOPE STABILITY OF THE SAN FRANCISCO BAY REGION 5 53 rock units, which may include small areas where the rocks are different from those shown on the geologic maps we used. CATEGORY 5 Category 5 consists of areas underlain by or imme- diately adjacent to landslide deposits. They range in slope from 0 to 90° and may be underlain by any bed- rock type but they are underlain most commonly by bedrock or surficial deposits that are highly suscept- ible to landsliding. This category comprises a wide va- riety of topographic situations, commonly hillsides steeper than 15 percent (8.5°) and steep slopes adja- cent to coastal areas and river banks. They are com- monly underlain by bedrock units, but substantial areas are underlain by surficial deposits such as allu- vial, marine terrace, and colluvial deposits. Many areas are in places where the slopes have been modi- fied by construction, logging or cutting and filling of the ground. The areas of category 5 have undergone landsliding in the past and are generally very susceptible to future landsliding, especially if the slopes are cut and filled. They do, however, include many small or narrow areas less than LOGO—5,000 feet (300—1500 m) across that are not underlain by either landslide deposits or bed- rock units highly susceptible to landsliding; however, they are too small or too narrow to be shown at this scale and at the level of generalization that we used. USES AND LIMITATIONS OF MAPS These maps provide a generalized regional repre— sentation of the relative stability of slopes in the San Francisco Bay region. They are based on more data and are at a larger scale (1:125,000) than an earlier map at a scale of 1:500,000 showing the estimated rela- tive abundance of landslides in the San Francisco Bay region (Radbruch and Wentworth, 1971). However, the slope stability maps are of smaller scale and are not based on as much information as previously pub- lished, more detailed relative slope stability maps that cover smaller areas, such as that by Brabb, Pampeyan, and Bonilla (1972) at a scale of 1:62,500 for San Mateo County, Frame (1974) for the Mount Sizer area in Santa Clara County at a scale of 1:12,000, Rogers (1971) and Rogers and Armstrong (1973) for part of the Santa Cruz Mountains in Santa Clara County at a scale of 1:12,000, Rice and Strand (1972) and.Huff- man (1971) at a scale of 124,000 for parts of the San Francisco Bay region, and others shown on the slope stability index map (fig. 41). The scale of the present maps is suitable for a variety of regionally oriented purposes, especially those that require a uniform and consistent evaluation of slope stability and one that is independent of jurisdictional boundaries. At the present time, many land-use and regional planning decisions in the San Francisco Bay region are being made without the necessary background of earth-science information. Except for the detailed maps of small areas and the San Mateo County map (Brabb and others, 1972), general maps of relative slope stability that cover large areas have not been available for regional land-use and planning studies in the bay area. Preliminary evaluations by Kockelman (1975, 1976) indicate widespread use of almost all re- cent U.S. Geological Survey publications related to landsliding and slope stability. Several maps have formed the basis for land-use planning decisions. The present maps have a variety of potential uses for long-range regional land-use planning for such purposes as: transportation and communication net- works; nuclear reactor sites or other large power plants; major research facilities that require large areas with stable foundations; national defense estab- lishments; urban development and growth; pumping plants and pipeline locations for the movement of wa- ter, natural gas, or petroleum; large industrial sites; open spaces such as regional park systems, wildlife areas, and golf courses; and development and utiliza- tion of coastal areas and flood basins where landslid- ing may be an important constraint. The maps may also serve as a guide for planning future slope stability studies within the bay region by indicating those areas where severe problems may be expected and by show- ing the extent of our present knowledge throughout the region. The regional trends in relative slope stabil- ity may be used in a variety of studies of the physical environment by demonstrating relationships between slope stability and other natural or manmade phe- nomena such as, for example, seismic activity (Bor- cherdt and others, 1975). The maps may aid in the preparation of general plans for various communities, especially as part of the seismic safety and open-space elements. The maps have been greatly generalized and simpli- fied in order to present a broad picture of the vari- ations in slope stability. The original landslide mapping, bedrock mapping, and slope mapping have all been generalized and simplified. Because of this generalizing, the maps should not be used to interpret the stability of specific or local areas—such use is un- warranted and unintended. The maps have shortcomings that limit some of their uses, and these must be clearly pointed out to the user. As has already been stated, the maps are based on an analysis of only three factors that affect slope stability—previous landslide activity, general 54 RELATIVE SLOPE STABILITY AND LAND—USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. nature of the underlying bedrock, and angle of slope of the land surface. The maps are thus primarily oriented toward the study of natural slope stability. The stability of cut slopes, as for excavations, is a sep- arate area of study involving more detailed analyses of the engineering characteristics of bedrock units. Many other factors that influence slope stability, some of which were mentioned in the introduction, have not been used in tliis analysis but may be of im- portance locally. Computer analysis of the factors contributing to landsliding, which might be feasible for this region (Adams, 1975), has not been utilized. The maps rely on the landslide and bedrock map- ping done by a variety of workers, who mapped with differing techniques, skills, and philosophies, yielding maps that vary considerably in character. We have adopted their mapping and used it in our analysis without attempting to weigh its accuracy, quality, or veracity. The mapping of landslide deposits, done largely by photointerpretation, has produced particu— larly variable interpretations of what constitutes a mappable landslide deposit. Where we have had over- lapping coverage, we have shown all of the landslide deposits mapped by all of the workers, so as to maxi- mize the size of the areas underlain by landslide de— posits. In addition, we have shown as unquestioned those landslide deposits mapped as questionable or possible in those areas that the worker was not certain whether the topographic feature seen in the aerial photographs was a true landslide or not. The map user is encouraged to go back to the original source map if more detail is needed. The authors have conducted limited field studies in the map areas and have no detailed information re- garding the chemical, physical, and engineering char- acteristics of the bedrock materials, although some of this information is presently being collated. Most im- portantly, few data were available regarding the his- tory of movement of the mapped landslide deposits or their nature—whether they are flows, slides, falls, or slumps, and whether they are thin surficial landslide deposits or thick deposits extending deep into the un- derlying bedrock. Confident statements regarding slope stability are difficult to make without such data. The maps are largely based on the presence of land- slide deposits, but the deposits of some types of land- slides are not preserved. These types include many soil slips, mudflows, and debris flows that form during periods of heavy rainfall; the moving material gener- ally is transported rapidly downslope along short, steep drainage channels that are tributary to larger streams (Campbell, 1975). Debris is commonly depos- ited downstream on alluvial fans or possibly alluvial plains and is not recognizable as a landslide deposit on aerial photographs. These types of slope failure are commonly the most dangerous in terms of hazards to life, because they move rapidly and occur very sud- denly. Areas where large alluvial fans have developed at the base of steep slopes should be regarded as po- tentially very hazardous; because of the procedures used to make this map, these areas are generally not included within unstable areas. Some other types of landslide deposits may be modified very quickly by natural or man-made pro- cesses, so they are not easily recognized. Many such areas have been mapped (see Wright and Reid, 1975, for example) as being underlain by anomalous topo- graphic configurations the origin of which may be from landsliding but which could also be due to other reasons. These areas are also not necessarily included within unstable areas on the slope-stability maps. No detailed analysis was done on the stability char- acteristics of the bay muds of category 1A. For more information pertaining to their hazard potential, see Youd (1973), Nichols and Wright (1971), and the re- ferences cited in these papers. Specific problems regarding slope stability, as por- trayed on our maps, may be present in certain small areas. For example, a specific problem exists in the western part of plate 2 (northeastern bay region), where resistant volcanic rocks of the Sonoma Volcan- ics are in contact with less resistant sedimentary rocks of the Petaluma, Huichica and Glen Ellen Formations (Fox and others, 1973). Commonly the resistant vol- canic rocks underlie relatively steep slopes above gentler lower slopes that are typically more unstable than the higher slopes because of the difference in un- derlying rock types (K. F. Fox, Jr., written commun., 14 May 1974); thus in some parts of the map area, be— cause of the geologic situation and the method used to prepare the map, the relative stability of lower and higher slopes may be reversed. The role the geologist plays in urban development, major construction, re- gional planning, site investigations, and other activi- ties, will increase in the future, particularly as the geologist obtains better information and prepares more useful maps (Price, 1972; Taylor, 1972; Rawl- ings, 1972). Our maps should be regarded as an initial attempt to predict regional slope stability, to be superseded in future years as better data and techniques become available. SUGGESTIONS FOR FUTURE RESEARCH In order to prepare more detailed slope-stability maps in the San Francisco Bay region, research and data collection must be expanded. Future work should USE OF SLOPE-STABILITY INFORMATION IN LAND-USE PLANNING 55 be oriented toward obtaining standardized data cov- ering the entire map area. In this section we mention some of the types of studies that seem important to us for future slope-stability studies. Mapping of landsliding deposits should be more uniform and clearly defined with regard to purpose, objective, and minimum size of deposits. A group of well-trained photogeologists with reasonably similar skills will be required to obtain uniform and compati- ble maps. Many comparisons should be made between photointerpretation and ground observations to im- prove the uniformity of mapping. As the present maps clearly indicate, nonuniform mapping poses serious problems in the preparation of regional maps. Mapping of bedrock and surficial geology should also be more uniform. Changes in stratigraphic names across quadrangle boundaries and complex stratigra- phic facies changes must be clearly indicated. How- ever, for slope-stability analyses, a standard geologic map is not the most suitable tool. An engineering geo- logic map that groups different mapped units in terms of similar engineering and physical characteristics rather than age would be more suitable for slope- stability analyses. Surficial deposits should also be mapped in this manner, to provide uniform coverage of the entire area. Much research and mapping of other factors that contribute to landsliding should be undertaken. Map- ping of soil types and soil characteristics such as strength, thickness, and physical and chemical changes under the influence of water will be required in order to incorporate soils data into slope-stability mapping. Another important factor about which little is known is the effect of rainfall on slope stability: how much rainfall and what sequence of storms are re- quired to generate landslides in different areas? is there any correlation between mean annual rainfall and landslide frequency? Some information about these relationships is available, but further studies are required. The effect of geologic structure on landslides has been examined in a few local areas in the bay region, but no generally applicable and regionally useful stud- ies have been completed. The relations between vege- tation and slope stability are poorly known—locally, grass-covered slopes seem to be more prone to land- sliding, although in other areas tree—covered slopes are more susceptible. Areas covered with Chaparral ap- pear to be unusually stable in many parts of the re- gion. However, the effect of vegetation on soils and hillsides in terms of slope stability is poorly under- stood. Despite having reasonably good maps of vege- tation cover in the map area, we were unsure about how these maps should be used and whether the influ- ence of vegetation was comparable in any way with the influence of bedrock, slope, and previous history of landsliding on slope stability. Another important factor that was not treated in our study is the effect of seismic shaking on slope sta— bility. We know, of course, that seismic shaking con- tributes to slope instability in general, and that landslides are commonly generated in the bay region during or after major earthquakes (Nilsen and Brabb, 1975). However, we have little information about the effects of seismic waves passing through landslide de- posits, or marginally stable slopes, and thus are not able to make predictions about the specific effects of certain types and magnitudes of earthquakes in the bay region. Finally, more data are required concerning the ef- fects of development on natural slopes. Many studies have clearly shown that cutting and loading of slopes have contributed to landsliding in specific areas, but the regional effects of major development over broad areas have not been studied and are not well known. What types of development and how extensive must it be in different areas to contribute to extensive land- sliding? In summary, we have just begun to study natural slope stability in the San Francisco Bay region, and much work must be done before more detailed and more useful regional and local slope-stability maps can be prepared. This assessment is also applicable to the United States as a whole, as pointed out by Soren- son, Ericksen, and Mileti (1975). USE OF SLOPE-STABILITY INFORMATION IN LAND-USE PLANNING By T. C. VLASIC and W. E. SPANGLE RELEVANCE OF THE BAY REGION SLOPE-STABILITY INFORMATION TO LAND-USE PLANNING The preceding section identifies the range of slope- stability conditions that need to be considered for land use and development in the San Francisco Bay region. Within limits, earth-science information can be applied in land—use planning to help define con- straints on land use. The relative slope stability map, although unsuitable for individual site studies, can be applied to regional analysis of policy for areawide land use, inventory of potential open space (in response to State—mandated open-space planning), and initial evaluation of regionally significant projects. In addi- 56 tion, this information is useful to planning agencies throughout the bay region for evaluating the impact of land-use proposals and determining the need for de- tailed studies. The real relevance of the slope-stability informa- tion will, of course, hinge on its actual use by regional, county, and city planning agencies. To assist these agencies, the interpretive tables in this section have been prepared. The tables describe the range of land- use conclusions that earth scientists agree can reason- ably be drawn from the 1:125,000-scale relative slope- stability map. RESPONSIBILITY OF THE USER Almost every jurisdiction in the bay region has some hillside areas with slopes of questionable stability. Slope stability, therefore, is important in land-use planning by the local jurisdictions, as well as by the ABAG (Association of Bay Area Governments) and other regional agencies with land-use management re- sponsibilty. Planners and decisionmakers at local and regional levels must assume the responsibility of see- ing that slope stability is given consideration. In fact, State planning requirements make it mandatory that local planners and decisionmakers assume this re- sponsibility. The land—use planning process described in the in- troduction of this report stated that issues need to be identified, objectives set, and critical data collected and interpreted. The relative slope-stability map of the bay region can provide an important input for re- gional, county, and city agencies. It is the responsibil- ity of each planning agency to determine what information is available and how it relates to their planning program. If the relative slope stability map of the region indi- cates the possibility of a significant landslide hazard within a local jurisdiction (particularly for lands that are developed or in the immediate path of develop- ment), the planning agency will need to take steps to determine how it may limit the land use. More de- tailed mapping may be needed. If the hazards are sig- nificant, the jurisdiction should take measures to reduce risk to an acceptable level. Some jurisdictions have already established procedures whereby detailed slope-stability information prepared by a professional engineering geologist is incorporated into their land- use planning and decisionmaking. Examples of such procedures are described later in this report. INTERPRETATION OF SLOPE STABILITY CATEGORIES Tables 6 and 7 were prepared to show the range of potential risk to life and property represented by the RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. relative slope stability categories shown on the map and described on pages 41 through 53. Table 6 summarizes the characteristics of the sta- bility categories, including slope and general qualities of stability. In addition, the table provides a rating of the categories in terms of relative risk to life and prop— erty from low to moderate to high risk. The ratings are stated in general terms in keeping with the scale of the relative slope-stability map. The risk ratings are based on evaluation of the factors contributing to slope stability used in deriving the map categories. Level of risk is an important factor in evaluating capa— bility of lands to accommodate urban development. The risk ratings are based on a rational and consis- tent analysis of the factors used to develop the relative slope-stability information for land-use planning pur- poses. In using this risk information, the earth scientists advise that: the relative slope-stability map of the San Fran- cisco Bay region be regarded by the map user as an initial attempt to predict regional slope stability, to be superseded in future years as more and better data and techniques become available. Because of the method of map preparation and resulting generalization of basic information, the map is not appropriate nor intended to be used to interpret the stability of specific local areas. It is simply a generalized regional representation of the relative stability of slopes in the San Francisco Bay region and should be considered as a framework for more detailed studies of smaller areas, where more specific geologic and engineering data and information are re- quired. With this qualification, then, the risk to life and property that may be encountered in any particular mapped area can be found by referring to table 7. RELEVANCE TO LAND-USE PLANNING Perhaps the most important function of the map is that it establishes a consistent regionwide description of relative slope stability. It can be used to determine the relative risk from slope failure in any jurisdiction and to label areas where particular attention must be paid to landslide hazards. It also provides a frame- work for more detailed slope-stability studies. The map shows that in many areas such studies will be necessary both for formulating and implementing lo— cal plans and for reviewing regional projects. Further- more, regional slope-stability information will help define the future refinements needed to evaluate slope-stability conditions for land-use planning. The discussion of the actual relevance of the rela- tive slope stability map of the San Francisco Bay re- gion to bay area planning agencies has been divided USE OF SLOPE-STABILITY INFORMATION IN LAND-USE PLANNING 57 TABLE 6.——~Characteristics of relative slope stability categories and relative level of risk to life and property Category Slope Risk to Life (percent) Stability and property Comments 1 2* 3* 4* 5* 0—5 Generally stable (slopes are not under- Low lain by landslide deposits or other surficial deposits that are highly sus— ceptible to slope failure). 5—15* do* Low* >15 (some hillsides Reasonably stable (slopes are not un- Moderate* as steep as 90°) derlain by either landslides deposits or bedrock units that are susceptible to landsliding). >15 (some hillsides Susceptible to future landsliding *Moderate-high* as steep as 90°) (slopes are underlain by bedrock units that are highly susceptible to landsliding but are not underlain by landslide deposits). 0—100* Highly susceptible to landsliding *High* (slopes underlain by bedrock or sur- ficial deposits that have slid and are highly susceptible to future land- sliding). 3. H . Locally bedrock may be unstable and therefore susceptible to landsliding. . Limited areas along creeks, rivers, coastal cliffs, and edges of terraces have steeper slopes than those gener- ally found in this category. They are generally too small or narrow to be shown at this scale and commonly have low relief. Riverbanks may be particu- larly hazardous during periods of flooding and the coastal areas during periods of storms. Some deposits (alluvial terrace, marine terrace, alluvial fan) may be locally susceptible to flooding and debris flows from surrounding uplands during periods of intense rainfall. . Some areas may be underlain by bed- rock types that are locally unstable and therefore susceptible to landsliding. . Limited areas along creeks, rivers, or coastal margins have steeper slopes and may be susceptible to landsliding but are too small or narrow to be shown at this scale. . Small areas are locally unstable owing to various reasons including: a. Failure of areas above or below that are underlain by bedrock types sus- ceptible to landsliding or by land- slide deposits; b. Proximity to areas of active erosion along creeks, rivers, and coastal areas; c. Saturated slopes adjacent to lakes and reservoirs; d. Proximity to active landslides that may be enlarging; e. Activities such as logging, cutting, and filling, construction and adding moisture to slopes. . This category may include small land- slide deposits not large enough to be shown at this scale or to have been mapped. . Local areas may be more stable than the average. The bedrock units with high suscepti- bility to slope failure may include some small areas within them that are locally more unstable for the same reasons as mentioned for category 3. . Exact conditions required for future landsliding are not known, but under the effects of high rainfall, seismic ac- tivity, human activity, and other fac- tors, the bedrock units within this category that are highly susceptible to slope failure may become unstable. . Many areas are not underlain by either landslide deposits or bedrock units highly susceptible to landsliding but are too small or too narrow to show at this scale and level of generalization. . Areas that have undergone landsliding in the past and are generally very sus- ceptible to future landsliding, espe- cially if the slopes are cut and filled. 58 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. TABLE 6.—Characteristics of relative slope stability categories and relative level of risk to life and property—Continued Slope Risk to Life Category (percent) Stability and property Comments 1 O—5* Highly unstable (slopes underlain by *High* 1. Areas susceptible to flowage, lateral 1A moist unconsolidated muds). movement, and liquefaction at slopes less than 1°. 2. During earthquakes, these areas are particularly susceptible to ground fail- ure and may cause damage to struc— tures built on artificial fill placed over the muds. 3. Such areas may be particularly sus— ceptible to failure along the margins of tidal channels and when cut into, exca- vated, or subjected to differential loa- ding. ‘Special category of instability. For specific description of differences between category 1A and other categories, see pages 41 through 53. into two parts—its relevance to regional agencies and its relevance to city and county agencies. REGIONAL AGENCIES As at any level of planning, the relative slope stabil- ity map is only one item of basic data to be incorpor- ated into regional planning and decisionmaking. The map will be of greatest use in assessing the nature and general extent of slope stability conditions. This as- sessment could lead to appropriate general policy for use of lands in the several risk categories. The data can also be used in formulating policies for avoiding or mitigating identified hazards. More specifically, the map and interpretive information should be useful in land-capability studies. ABAG (Association of Bay Area Governments) will find the map useful in identifying critical areas and developing regional policies, standards, and criteria for project review. The map was used in the agency’s evaluation of the general capabilities of land to ac- commodate urban growth and development (Laird and others, 1978). Those areas of greatest relative in- stability can be identified from the map. The planner can also readily identify those areas with the fewest limitations to land use. The areas of greatest relative stability can then be considered as the location for more intensive future urban development. Other limi- tations to development in areas identified as relatively stable (for instance, flood-prone areas and areas with seismic hazards and other geologic problems) can be assessed in region-wide land-capability studies. The map could also be used by ABAG in selecting projects in moderate-to high-risk areas for more detailed slope-stability evaluation. The MTC (Metropolitan Transportation Commis- sion) should find the map useful in developing an en- vironmental impact analysis of the regional transportation plan, specific transportation projects, and changes within a transportation corridor. The map can be effectively used by MTC to identify those areas where additional study will be necessary before locating, designing, and constructing specific facili- ties. This information can be incorporated into re- gional policies, objectives, and proposals as well as project-review criteria. The BCDC (Bay Conservation and Development Commission) will probably find little use for the map. Instead, BCDC will refer to the map on bay muds (Ni- chols and Wright, 1971), or the extensive information that was prepared specifically for the BCDC San Francisco Bay Plan adopted by the State legislature in 1969. The CCC (California Coastal Commission) could find the map useful as background for identifying is- sues. However, the Central and North Central Region— al Commissions would be likely to refer to more detailed slope and landslide maps for their own areas. CITY AND COUNTY AGENCIES City and county agencies may use the map as a basis for identifying issues. For local land-capability analy- sis, for designating specific local comprehensive land- use plans, and for developing land-use strategies and regulations, the map would not be adequate. Far more detailed data on slope stability for the agencies’ area of responsibility would be necessary for preparing and implementing a plan. The exact nature of the detailed data necessary should be determined in conjunction with the more detailed maps from which this map was prepared and with a professional geologist serving the jurisdiction (either a staff geologist or a consultant). The relative slope-stability map does indicate how extensive slope-stability problems will probably be and the most effective approaches that can be taken in USE OF SLOPE-STABILITY INFORMATION IN LAND-USE PLANNING 59 TABLE 7.—Slope-stability categories for land-use planning Low risk Moderate risk High risk Overall land-use potential Generally very few limitations to land use imposed by slope instability. The most intensive urban growth and develop- ment will be located in low risk areas. Local limitations may be imposed by soil conditions, susceptibility to flooding, and seismic hazards. Limitations to urban-type land use are present. However, much of the area can support urban growth and development if appropriate measures are taken to minimize risk to life and property. Local areas may be unsuitable for urban devel- opment without extensive grading and filling, or structures to ensure stability. Urban development is usually inappropri- ate. These areas should be assigned low- est priority for urban growth and development. These areas may be desig- nated as permanent open space for pub- lic health and safety or as regional parks. Unstable bay muds may be of value as wildlife refuges. Some areas may be suit- able for low-density residential develop- ment making use of clustering tech.- niques, on slopes of adequate stability. 1. No further slope-stability studies neces- sary for development of regional poli- cies, standards, and criteria. 2. Slope stability is not critical factor in regional land-capability analysis. 3. Regional planning policies and criteria should indicate need for more de- tailed studies of local bedrock geolo- gy, soils, flood-prone areas, and areas of seismic hazards and the impact of these factors on local slope stability. Regional 1. No further slope-stability study neces— sary for development of regional poli- cies, standards, and criteria. 2. Regional land-capability analysis must recognize that slope stability may be critical in local areas and plan on higher costs for studying and reduc- ing hazards. 3. Regional planning criteria and stan- dards reflect lower priority for urban land uses, particularly critical facili- ties serving the region, as a result of potential slope instability. 4. More slope-stability data may be re- quired to evaluate impact of specific projects of regional significance. 1. No further slope-stability study neces- sary for development of regional poli- cies, standards, and criteria. 2. Regional land-capability analysis should reflect possible limits to urban land use imposed by slope instability throughout high-risk areas and costs of studying and reducing hazards. 3. Avoid locating critical facilities in high- risk areas, and consider designating such areas as regional open space. 4. More slope-stability data will be neces- sary to evaluate impact of specific projects with regional significance. County or city comprehensive plan and implementation regulations . More detailed data on local conditions, particularly stabilily of bedrock, should be obtained for preparing the comprehensive plan, as deemed nec- essary by geologist. . Detailed data are essential to define lo- cal slope-stability problems and as a basis for reducing risk. . Regulations should be based on detailed data and adopted comprehensive plan. Framework and guidelines for site-specific studies should be made part of implementing procedures in conjunction with geologist. 1. More detailed geologic hazard data, as determined in conjunction with the geologist, are essential to land-use decisionmaking within local planning area. 2. On the basis of detailed data, the com- prehensive plan provides guidance for the regulation of areas determined unsuitable for urban development. Methods of avoiding or reducing haz- ards are included in plan policy and proposals. 3. Regulations should be developed in conjunction with the geologist indi- cating soils and engineering geologic studies to be required before approv- ing specific projects. 1. Detailed geologic data are essential to determine general potential for devel- opment and to establish the nature of more specific data that will be needed to ensure proper safeguards. 2. On the basis of detailed data, boundary of high-risk area may be modified to reflect local conditions more pre- cisely. 3. High-risk areas are precluded from de- velopment in comprehensive plan and implementing regulations, both of which should be developed in con- junction with the geologist. . In almost every case, some site-specific studies will be necessary. In most cases, only soils studies will be needed. . On the basis of data developed while preparing the comprehensive plan and implementing the regulations, specific engineering geologic studies may be required in local areas. . Only development conforming to rec- ommendations from the approved site-specific investigation is to be per- mitted. Approval of the investigation is based on recommendations of the soils engineer or engineering geolo- gist. Site-specific design and construction 1. Soils and preliminary engineering geo- logic studies will be necessary before approving specific projects unless waiver procedure is established in conjunction with the geologist. 2. Where stability problems are noted in preliminary studies, more detailed analysis will be necessary as a basis for project design and construction. 3. Only development conforming to rec- ommendations from the site-specific study should be permitted. Approval of the study by the jurisdiction based on advice of the soils engineer or engi- neering geologist. 1. High-risk boundaries should be modi- fied in accordance with site-specific studies approved by the local jurisdic- tion and the geologist. 2. Site-specific studies may show that low- density development is appropriate with adequate safeguards. 3. Only development conforming to the recommendations of the study should be permitted. Approval of the study by the jurisdiction is based on recom- mendations of the soils engineer or engineering geologist. 60 RELATIVE SLOPE STABILITY AND LAND—USE PLANNING, SAN FRANCISCO' BAY REGION, CALIF. planning and land-use regulation program. It also helps a local jurisdiction put its slope-stability condi- tions into context with the region as a whole. In summary, while the bay region slope-stability map is a benchmark in regional mapping, the planner and any other map user must be continually aware of the limitations of the data. In addition, it is important for local agencies who think they may have significant slope-stabilty problems to consult with professional geologists to define the magnitude of the problems they need to address in their land-use planning and decisionmaking. The next section will deal specifically with how regional and local agencies can apply slope- stability information in land-use planning, and it will include guidelines on how to acquire more detailed slope-stability information. APPLICATION TO LAND-USE PLANNING Planning for slope stability has already been briefly described, and the planning activities of Federal, State, regional, and local agencies introduced. This earlier discussion indicated very generally the respon- sibilities, data needs, and land-use responses for effec- tive slope-stability planning. The discussion that follows elaborates on these descriptions, detailing the current methods of applying slope-stability informa- tion in land-use planning. Landslides are a local phenomenon; consequently, local agencies (city and county) have the key responsi- bility for reducing risk from landslide hazards. Local planning, however, should be responsive to Federal, State, and regional objectives, standards, and decision criteria relevant to slope stability. The discussion in this section, therefore, describes the Federal, State, and regional requirements and decision criteria that affect local land-use planning and then provides ex- amples of plans, focusing on city and county needs, and details of how agencies have acquired the neces- sary slope-stability data. In considering slope-stability information in land- use planning, some very broad basic guidelines be- came apparent. These guidelines are provided below. The section concludes with a discussion of the roles and responsibilities of the various professionals in— volved in the process of planning for slope stability. For those wishing to acquire only a general under- standing of how slope-stability information can be ap- plied in land-use planning, it is suggested that the basic guidelines and examples of actual planning ef- forts be reviewed, and then go back to the discussion of the objectives, standards, and decisions of Federal, State, and regional agencies if more information is de- sired. ‘ BASIC GUIDELINES Slope instability, in conjunction with other geologic hazards, can be a major factor in determining how land is used. Because of the local nature of the land- slide problem, city and county agencies have the greatest responsibility for ensuring that detailed slope-stability analyses are completed where neces- sary. To assist planners and decisionmakers in dealing with slope-stability concerns, the following basic guidelines are offered: 1. When slope-stabilty hazards have been identi- fied, make such information available to all who might be interested in or potentially affected by the hazard. 2. Pay special attention to landslide hazards when preparing comprehensive land—use policy, plans, and implementation strategies. 3. When potential slope-stability hazards have been identified in an already developing area, make and implement appropriate plans for mitigating the hazards. Where necessary, such measures as reloca- tion of residents where damage is imminent, slope sta- bilization, removal of structures in the highest risk area, and disaster-preparedness plans (particularly in seismically active areas) should be taken. 4. Evaluate potential slope-stability hazards by us- ing maps of adequate scale and detail before any con- templated development or structure reaches the site selection or design stage. 5. Develop and adopt standards of design and con— struction to obtain acceptable levels of safety. 6. See that all proposals for development on slopes of questionable stability are thoroughly reviewed by competent professionals. 7. Require adequate independent inspection dur- ing construction to enforce the safety measures called for in the approved plans. APPLICATION IN PLAN FORMULATION How slope-stability information is used in formu- lating land-use plans varies depending upon such fac- tors as level of government, nature of agency responsibility, professional and financial capabilities, and the physical conditions contributing to slope in- stability. Described below are the key Federal, State, and regional agencies that have shown concern about landslide hazards in the San Francisco Bay region through adopting objectives, standards, and decision criteria that specify local uses of land. Although they are discussed here in connection with plan formula— tion, these higher agencies affect implementation as well. In the development of city and county general plans in the bay region, planning agencies have not only been responsive to the standards and require- USE OF SLOPE-STABILITY INFORMATION IN LAND-USE PLANNING 61 ments of higher levels of government but, as is shown below, have in many cases gone further than man- dated. In recent years, land-capability studies have be- come effective tools for evalutating the physical capa- bility of lands to accommodate a range of uses. A general description of a land capability analysis is pro- vided later in this section to show how slope-stability information might be applied. FEDERAL LEVEL Federal agencies that fund, permit, or review land- disturbing human activities often have very special re- sponsibilities, and some have exhibited only peripher- al concern for slope stability. HUD (Department of Housing and Urban Development) has assumed the most direct responsibility for ensuring that slope-sta- bility problems are addressed in the process of urban development. The following HUD programs and re— quirements are important because they establish guidelines for planning activities at State, regional, and local levels. REQUIRED LAND-USE ELEMENT OF THE COMPREHENSIVE PLANNING ASSISTANCE PROGRAM The Comprehensive Planning Assistance Program administered by HUD provides grants for planning to municipalities, counties, and metropolitan, regional, and State agencies (US. Dept. Housing and Urban Development, 1954). Any planning agency or jurisdic- . tion, including those in the bay region, wishing to re- ceive a grant under this program must have developed or are expected to complete a land-use element and a housing element pursuant to Section 600.72 of the HUD—701 program requirements. The required land- use element must contain integrated policies to guide governmental decisionmaking “on all matters relating to the use of land.” The element must identify land- use needs, land-resource development, and the impact of policies on areas of critical concern. In addition, the element must identify where growth should and should not take place and consider the environmental protection required in determining future growth pat- terns. The land-use element must take potential slope-stability hazards into account in the land—use plans of agencies receiving grants if factors contribut- ing to slope instability are present within their juris- dictions. The importance of such HUD requirements is un- derscored by the fact that under the Housing and Community Development Act of 1974, HUD has been directed to consult with those Federal agencies charged with implementing the nation’s environmen— tal protection policies to insure that comprehensive planning is in accord with national environmental protection policies. Thus an Interagency Agreement between HUD and EPA (Environmental Protection Agency) provides that the results of planning and management efforts completed under EPA 208 pro- gram requirements (areawide waste—treatment man- agement planning assistance) may fulfill HUD—701 requirements in areas where the two programs over- lap. EPA-208 program requirements are permitted to dominate because their planning provisions match up with the HUD—701 program and because their imple- mentation provisions have no counterpart in the HUD program. The agreement helps to rationalize planning assistance and to ensure that land-use planning for water quality is developed within the broader frame— work of comprehensive planning (US. Environmental Protection Agency, 1975). HUD HOUSING PRODUCTION AND MORTGAGE CREDIT/MINIMUM PROPERTY STANDARDS HUD standards define the minimum level of ac- ceptability of design and construction for Federally assisted housing and housing eligible for Federally in- sured mortgages. The Minimum Property Standards consist of four volumes (US. Dept. of Housing and Urban Development, 1973), Volume I, One- and Two- Family Dwellings; Volume II, Multi-family housing; Volume III, Care Type Housing; and Volume IV, a Manual of Acceptable Practices. The fourth volume contains backup and illustrative material for the three volumes of mandatory standards. These standards require that land-development proposals take note of natural hazards such as land- sliding. Project design and review must insure that po— tential hazards from slope instability be addressed. Thus, information on slope stability in both project design and review stages is required. FEDERAL DISASTER ASSISTANCE PROGRAM The Federal Disaster-Assistance Administration (FDAA) directs, manages, and coordinates the Fed- eral disaster-assistance program (US. Dept. Housing and Urban Development, 1975). Under this program, governed by the Federal Disaster-Relief Act of 1974, landsliding may constitute a “major disaster” if, in the opinion of the President, it causes damage of suffi- cient severity and magnitude to warrant major-disas- ter assistance. Such a “major disaster” might result from seismically induced landslides affecting a large area or landslides caused by unusual rains in an urban area. 62 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. The FDAA encourages the development of compre- hensive disaster-preparedness plans by the states and local governments, and, to reduce losses from disas- ters, encourages adoption of hazard-mitigation mea- sures including land-use and construction regulations. In addition, the Disaster Relief Act of 1974 requires, in part, that any state or local government that re- ceives a loan or grant under the Act agrees to study the hazard and act to mitigate it in the disaster area. Where landslides are possible, slope-stability infor- mation is essential for effective disaster-preparedness plans and hazard-mitigation programs. STATE LEVEL OFFICE OF PLANNING AND RESEARCH The OPR (Office of Planning and Research), re- sponsible to the Governor, prepares long-range State goals and policies for land use and environmental quality. The OPR, however, has no direct control over land use. Concern for slope stability was set forth in the office’s report on environmental goals and policy (California Office of Planning and Research, 1973). In part, these goals and policies include the geologic-haz— ard information developed during preparation of the State’s Urban Geology Master Plan (Alfors and oth- ers, 1973). The following goals and policies contained in OPR’s 1973 report regarding “environmental resources and hazards” have particular significance in regard to planning for slope stability: It is the Goal of the State to identify and protect the significant and critical environmental resources and hazards of the State for the benefit and enjoyment of present and future generations. To accomplish this Goal, it is the Policy of the State: 1. to identify through its departments and political subdivisions all potentially significant and critical environmental resources and hazards throughout California, and after thorough evaluation, adopt and define those geographic areas of the State which do con- tain environmental resources and hazards of Statewide importance as being areas of Statewide Interest or of Critical Concern; 2. to evaluate, through its departments and political subdivi- sions, all activities, as they may significantly affect the environ- mental resources and hazards of the State which are areas of Statewide interest or Critical Concern; undertake measures to minimize those activities which will have a detrimental effect on such resources, and encourage the development of programs which will enhance the quality of these resources for future generations. During the interim period before the Critical Concern areas are adopted, this Policy shall apply to all of the areas listed in this Re- port as areas of Statewide Interest or potential areas of Critical Concern; 3. to encourage local units of government to consider the areas listed in this Report as areas of Statewide Interest or potential areas of Critical Concern in the preparation of their individual General Plans, including but not limited to open space, conserva- tion, scenic highways and seismic safety elements; and 4. to consider those areas of Critical Concern as high priority in any Statewide acquisition, lease, or enforcement programs. The report describes those geologic hazards that threaten “life and property” that need to be carefully evaluated before decisions are made to change land use. To deal with these hazards, the report includes an “Environmental Resources Protection Plan” with 11 basic recommendations for land-use legislation, of which the following relate to slope stability: that areas of critical environmental or hazardous concern to the entire state be designated; that guidelines be formulated to encourage orderly development and protection from natural calamities while minimizing adverse impact upon people or resources which have been designated of critical environmental or hazardous concern; that the resolution of conflicts and the performance of regula- tory functions occur at the level of government closest and most responsive to the people affected; that innovative and creative programs affecting land uses or af- fecting these areas of critical concern be encouraged through the efforts of the private sector and government entities. The CIR (Council on Intergovernmental Rela- tions) (now part of OPR) advised and assisted cities, counties, districts, and regional planning agencies. As part of this responsibility, the CIR adopted guidelines for the preparation of local general plans (California Council on Intergovernmental Relations, 1973) that assisted local governments in preparing State-man- dated general-plan elements and discussed the use of slope-stability information in formulating the ele- ments. By action of the State Legislature, the CIR was abolished, its functions transferred to OPR, and a Lo- cal Government Advisory Council created (Assembly Bill No. 551, Sept. 1975). CALIFORNIA RESOURCES AGENCY The California Resources Agency includes a num- ber of departments, boards, and commissions that af- fect the use and development of land through planning and regulation, grants to local government, and their own construction projects and operations. Some functions of key bodies within the agency that are relevant to use of slope stability information are summarized below. The Department of Conservation includes the CDMG (California Division of Mines and Geology). The CDMG prepared the Urban Geology Master Plan for California (Alfors and others, 1973), which evalu- ates the nature, magnitude, and costs of geologic haz- ards in the State and recommends actions for their mitigation. The CDMG also provides technical infor- mation on landsliding and the relative stability of slopes. In several instances, usually under special con- tract with a jurisdiction, the CDMG has developed USE OF SLOPE-STABILITY INFORMATION IN LAND-USE PLANNING 63 technical information and interpreted it for local planning purposes. The Department of Parks and Recreation has the responsibility for acquiring, developing, and main- taining State parks. The department ensures that park use and park development projects consider the slope-stability conditions of the area. It also adminis— ters State grants to local agencies for acquiring and developing parks. STATE LANDS COMMISSION The State Lands Commission has the responsibility for administering the sale and leasing of State-owned public lands. It has power to affect the manner in which lands under its jurisdiction are used, and it can require that slope stability be adequately studied as part of its project-review functions. BUSINESS AND TRANSPORTATION AGENCIES The Department of Transportation, the Depart- ment of Housing and Community Development, and the California Housing Finance Agency seem to have significant concern for slope stability. Caltrans (The Department of Transportation) is re— sponsible for planning and building State transporta- tion facilities. Because knowledge of slope stability is important in the location and design of such facilities, Caltrans makes extensive use of geology in planning and construction. The agency has developed special techniques for measuring slope stability, using for ex- ample subaudible rock noise (Means and Hoover, 1973), and has conducted extensive research into the design of stable cut slopes. The HCD (Department of Housing and Community Development) is broadly responsible for housing and community development activities. The functions of the Department are (Sedway and Cooke, 1975): to ‘assist’ local governments and private enterprise on communi- ty development and housing matters; to establish, administer and enforce minimum housing standards and regulations pursuant to various housing related laws; to maintain a statistics and research service; to ‘make recommendations’ to the Governor for changes in state and federal housing laws; and to ‘encourage’ planning and other activities intended to increase housing supply and quality. It was required to develop the California Statewide Housing Element and is ‘responsible for coordinating federal-state relationships in housing’ and for ‘encouraging full utilization’ of federal programs which assist ‘the residents of this state, the private housing indus- try and local government, in satisfying California’s needs.’ HCD is also empowered to prepare a statewide housing plan (an extension of the State Housing Ele- ment) in cooperation with government and industry. Among other things, the plan is to include an analysis of local building codes. Although the analysis is to fo— cus on flexibility in the uses of new materials and methods of construction and building code enforce- ment, there is also the opportunity to evaluate the adequacy of code requirements with regard to geologic hazards. The overall State goal is to provide enhanced living environments, particularly for people of low or moderate income. Another responsibility of HCD is to develop and propose regulations to guide certain activities of the California Housing Finance Agency. Thus, HCD could recommend hazard-risk criteria for the Finance Agency to use in lending money for new housing. The CHFA (California Housing Finance Agency), in existence since September 1975, is empowered to sell bonds to raise money for lending at below market interest rates to qualified housing sponsors or to ap- proved commercial lenders in order to increase the housing supply for Californians of moderate, low, and very low incomes. In lending money, CHFA is to en— sure that the planning of such developments empha- sizes “superior design.” Thus, CHFA has the opportunity to ensure that developers will provide for identification and mitigation of geologic hazards such as unstable slopes. REGIONAL LEVEL ASSOCIATION OF BAY AREA GOVERNMENTS ABAG (The Association of Bay Area Govern- ments), in carrying out its areawide comprehensive planning responsibilities, has prepared and approved a regional plan that provides a policy framework for considering future growth of the bay region. The re— gional plan is composed of the entire body of goals, objectives, and policies that have been adopted by ABAG (Tranter, 1972) and provides strategies for im- plementing them. Significant attention has been paid to geologic hazards, including potential for slope fail- ure, primarily through goals, objectives, and policies for public safety and open-space preservation. ABAG’s Regional Plan: 1970—90 (Assoc. Bay Area Govts., 1970) outlines broad strategies for guiding ur- ban development in a manner that would preserve ur- ban communities, discourage urban sprawl, protect open land, and minimize disturbance to natural pro- cesses. Open—Space Plan Phase II (Assoc. Bay Area Govts., 1972) provided for identification of the char- acteristics of the region’s remaining open land and es- tablished a framework for preserving open space serving the following functions: managed resource production and preservation; protection of health, 64 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. welfare, and well-being; public safety; outdoor recrea- tion; and guiding urban growth. Areas with landslide hazards (identified in a report by Radbruch and Wentworth, 1971) were included under “open space for public safety” of the Phase II plan. Since completion of the 19372 open-space program, ABAG has instituted the Urbanization and Develop- ment Program to create a conscious strategy for guid- ing urban growth in the bayfregion. This program is designed to carry out the city-centered policies of the regional plan by directing growth into existing devel- oped areas while preserving open land. The concerns for open space, particularly “open space for guiding urban growth” contained in‘ the Phase II plan have been incorporated in the Urbanization and Develop- ment Program. Evolving from ABAG’S earlier planning efforts are policy guidelines for land having regionally significant environmental characteristics. These guidelines are being developed to supplement the “environmental framework” presented earlier in the regional plan and the Phase 11 plan. A report, (Areas of Critical Envi— ronmental Concern, Assoc. Bay Area Govts., 1975, p. 9) includes these recommendations: Review Criteria—Specific regional interests in var- ious land areas throughout the nine counties are iden- tified and criteria presented for use in reviewing the consistency of local plans and projects with regional objectives. This review procedure is a vital function in a regional planning process which endeavors to retain maximum planning flexibility at the local government level. Recommendations to local governments—Recom- mendations are made to local governments and re- gional, State, and Federal agencies on their roles and responsibilities in implementing regional objectives in critical areas. Urbanization policy—The report continues sup- port of an urban development policy that channels fu- ture growth into existing communities and removes development pressure from nonurbanized lands. The following specific recommendations in the critical areas report regarding slope stability (ASSOC. of Bay Area Govts., 1975, p. 45, 46) reflect the local nature of slope problems: Local agency actions. Administering a program of subdivision, hillside, and grading ordinances that require preparation of a geologic and soils engineering report prior to development of areas subject to landsliding, erosion, or hearing material prob- lems. New recommendations. Support state or federal legislation re- quiring local agencies to administer a program requiring the preparation of a geologic and soils engineering report prior to development of an area subject to possible slope stability, ero- sion, or bearing material problems. To complement and reinforce the public safety poli- cies contained in the critical areas report, ABAG has prepared a second policy guideline, Regional Earth- quake Safety Issues and Objectives (ABAG, 1976). This policy guideline was based on two earlier pro- jects: a land capability study and a hazards evaluation study. The land-capability study was a project sponsored by U.S.G.S. as part of the San Francisco Bay Region Study. The project developed a method to define the ability of land to accommodate a particular land use on the basis of geologic and hydrologic costs. The pur- pose of this study was to show how earth-science in- formation could be made more useful to local decisionmakers (Laird and others, 1979). The hazards evaluation study was prepared as part of a project sponsored by the Federal Defense Civil Preparedness Agency. The DCPA program had the following objectives (ABAG, Apr. 1975, memo to members of the Civil Preparedness Tech. Advisory Comm.): To prepare a source file of information on disasters and disaster mitigation. To prepare a risk evaluation methodology. To develop a civil preparedness plan of action for ABAG. The Civil Preparedness Technical Advisory Com- mittee was formed to draw up a plan of action for ABAG. The committee concluded that ABAG should have no operational role in disaster response. ABAG should, however, advocate governmental and citizen support for disaster preparedness and promote re- gional and local efforts in hazard reduction and plan- ning of disaster response and postdisaster recovery. This philosophy later became a part of the policy guideline on earthquake safety. As part of the DCPA- funded project, ABAG prepared a booklet, Hazards Evaluation for Disaster Preparedness Planning as a guide to local governments in establishing priorities for disaster preparedness (Assoc. Bay Area Govts., 1976). The report presents a systematic procedure for describing, analyzing, and evaluating hazards (includ- ing landslides), and suggests ways a local jurisdiction can reach decisions on what hazards are important, what measures can effectively reduce them, and what priorities for action should be set. The evolution of ABAG’s comprehensive planning activities from adoption of the regional plan to prep- aration of the critical environmental areas plan, land capability study, hazards evaluation booklet, and earthquake safety policy guideline reflects the stated position “that the Association must and will develop and adopt the type of guidelines, based on a thorough recognition of the intricacies of the regional setting, which will enable attainment of a desired future by providing guidance without needlessly limiting op- USE OF SLOPE-STABILITY INFORMATION IN LAND-USE PLANNING 65 tions” (Tranter, 1972). ABAG is updating its regional plan in 1977 and 1978 to incorporate this extensive hazards work. METROPOLITAN TRANSPORTATION COM MISSION MTC (The Metropolitan Transportation Commis- sion), in coordinating development of regional trans- portation facilities, has prepared and adopted a regional transportation plan and has developed an en- vironmental impact assessment process for use in plan evaluation and project review. Both the plan and the impact assessment recognize the importance of geo- logic processes, such as landsliding, to location, de- sign, and construction of transportation facilities, and require a geologic evaluation in hazardous areas be- fore a proposal or specific project is approved. The RTP (Regional Transportation Plan) was adopted in 1973 and has been updated and revised annually. The plan contains policies to avoid or minimize adverse impacts on the physical environment. Two policies in the RTP establish the basis for considering geologic hazards (Metr. Trans. Comm., 1974b, p. 14): MTC shall, in conjunction with regulatory agencies such as the State Public Utilities Commission, employ standards of safety and high quality in the regional transportation system. Earthquake and seismic technology shall be used in the plan- ning, location and construction of new transportation facili- ties. The RTP also describes the regional transportation system and its needs and problems and establishes guidelines for the development and revision of the system consistent with adopted policy and as “** *part of a continuing, comprehensive evaluation of trans- portation from a regional perspective” (Metr. Trans. Comm., 1974b, p. 50). “Factors” to be assessed in the ongoing review of proposals include “Health/Safety/ Amenity.” Under this heading, one question that must be addressed is (Metr. Trans. Comm., 1974b, p. 51—52): Will the proposal significantly***change the potential for the occurrence of or damage from natural hazards such as earth- quakes, slides, floods, subsidence, tsunamis? The RTP policies, system description, and review standards 'establish the framework for MTC review of regionally significant transportation projects. MTC cooperates with ABAG in the review process, provid- ing comments related to transportation. MTC ap- proval is required for certain projects including transbay bridges, public multicounty transit systems on exclusive rights of way, all applications from local governments or districts for State or Federal funds re- lated to transportation, and construction of the State Highway System. MTC also reviews required Federal and State environmental documents on projects in the bay region for compatibility with the RTP. To assist MTC in making decisions on regional transportation issues that are consistent with the RTP, a process to assess environmental impact has been developed. This process involves preparing an information base covering all aspects of the regional environment. An important part of the imformation base is a study of the present environment that has been completed for MTC by the consulting firm of Wallace, McHarg, Roberts, and Todd. The study in- cludes an inventory of the environment, technical evaluation of data, environmental assessment proce- dures, and 77 unique maps ranging in scope from re- gional urbanization patterns to habitats of wildlife species (Metr. Trans. Comm., 1975, p. 3). The maps and technical evaluations focus on five “impact areas.” One impact area is the “Natural Process In- ventory,” which includes mapping and technical eval- uation of topography, geology, and hydrology. The data used for this impact area include the landslide information from Brabb, Pampeyan, and Bonilla (1972), Radbruch and Wentworth (1971), and Wright and Nilsen (1974). Data from the Wallace, McHarg study, along with other data contained in the environmental-impact as- sessment, provide a framework for analysis of the en- vironmental impacts of transportation plan elements, specific transportation projects, and changes within a transportation corridor. As the MTC Regional Transportation Plan contin— ues to evolve, it will be coordinated with the regional land-use planning activities of ABAG. MTC has con- cluded an agreement with ABAG by which ABAG’s regional land-use plan will serve as a guide for MTC transportation plans. The RTP, therefore, will be guided by the ABAG plan for critical environmental areas and other ABAG planning standards for use of lands with potential geologic hazards. Under the agreement, ABAG will review the RTP to determine regional impact and will recommend amendments to its own plan as necessary to meet the goals of both agencies. SAN FRANCISCO BAY CONSERVATION AND DEVELOPMENT COMMISSION The BCDC (San Francisco Bay Conservation and Development Commission) was created by the State Legislature to prepare a comprehensive plan for San Francisco Bay and its shores and to control develop- ment within its area of jurisdiction. This area includes San Francisco Bay, a strip 100 feet (30 m) landward along the bay shore, as well as salt ponds, managed wetlands, and certain waterways. In 1969, the State Legislature adopted the bay plan and empowered BCDC to issue or deny permits for projects that would 66 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. fill, extract materials, or substantially change a water, land, or structural use within its jurisdiction. Local governments retain basic land-use controls but, in ef- fect, BCDC holds veto power over any project in con- flict with the San Francisco Bay Plan. The following findings and policies were developed by BCDC, based on extensive studies of stability of bay muds (Goldman, 1967) and the safety of fills (Steinbrugge and others, 1968); they establish the ba- sis for current BCDC regulation of filled lands and other unstable soils within its jurisdiction: Findings a. To reduce risk of life and damage to property, special con- sideration must be given to construction on filled lands in San Francisco Bay. (Similar hazards exist on other poor soils throughout the Bay Area, including soft natural soils, steep slopes, earthquake fault zones, and extensively graded areas.) b. Virtually all fills in San Francisco Bay are placed on top of Bay mud. Under most of the Bay there is a deep, packed layer of old Bay mud. More recent deposits, called younger Bay mud, lie on top of the older muds. The top layer of young mud presents many engineering problems. The construction of a sound fill de- pends in part on the stability of the base upon which it is placed. c. Safety of a fill also depends on the manner in which the filling is done, and the materials used for the fill. Similarly, safety of a structure on fill depends on the manner in which it is built and the materials used in its construction. Construction of a fill or building that will be safe enough for the intended use requires (1) recognition and investigation of all potential haz- ards—including (a) settling of a fill or a building over a long peri- od of time, and (b) ground failure caused by the manner of constructing the fill or by shaking during a major earthquake— and (2) construction of the fill or building in a manner specifical- ly designed to minimize these hazards. While the construction of buildings on fills overlying Bay deposits involves a greater num- ber of potential hazards than construction on rock or on dense hard soil deposits, adequate design measures can be taken to re- duce the hazards to acceptable levels. d. There are no minimum construction codes regulating con- struction of fills on Bay mud because of the absence of sufficient data upon which to base such a code. Hazards vary with different geologic and foundation conditions, use of the fill, and the type of structures to be constructed on new fill areas. Therefore, the highest order of skilled judgment, utilizing the available knowl— edge of all affected disciplines, is required to (1) recognize and investigate all potential hazards of constructing a fill, and (2) de- sign the fill and any construction thereon to minimize these haz- ards. e. In the absence of adequate fill construction standards or codes, the BCDC appointed a Board of Consultants consisting of geologists, civil engineers specializing in soils engineering, struc- tural engineers, and other specialists, to review, on the basis of available knowledge, all new fills that might be permitted in the Bay Plan, so that no fills would be included upon which con- struction might be unsafe. No specific fills are included in the Plan, but the Board of Consultants has completed an initial set of criteria (published separately as “Carrying Out the Bay Plan: The Safety of Fills”) as a guide to future consideration of specific fill proposals. Policies 1. The Bay agency should appoint a Fill Review Board con— sisting of geologists, civil engineers specializing in soils engineer- ing, structural engineers, and architects competent to and adequately empowered to (a) establish and revise safety criteria for Bay fills and structures thereon, (b) review all except minor projects for the adequacy of their specific safety provisions, and make recommendations concerning these provisions, (0) pre- scribe an inspection system to assure placement of fill according to approved designs, and (d) gather, and make available, perfor- mance data developed from specific projects. These activities would complement the functions of local building departments and local planning departments, none of which are presently staffed to provide soils inspections. 2. Even if the Bay plan indicates that a fill may be permissi- ble, no fill or building should be constructed if hazards cannot be overcome adequately for the intended use in accordance with the criteria prescribed by the Fill Review Board. 3. To provide vitally needed information on the effects of earthquakes on all kinds of soils, installation of strong-motion seismographs should be required on all future major land fills. In addition, the Bay agency should encourage installation of strong-motion seismographs in other developments on problem soils, and in other areas recommended by the U. S. Coast and Geodetic Survey, for purposes of data comparison and evalua- tion. The BCDC Engineering Criteria Review Board, es- tablished as recommended in Policy 1, has proved to be effective in implementing safety policies. The Board’s example and influence have extended far be- yond the limits of BCDC’s jurisdiction (San Francisco Bay Conservation and Development Comm., 1974). It should be noted that unstable bay muds within the jurisdiction of BCDC have been included as Cate- gory lA on the relative slope stability map of the bay region contained herein. CALIFORNIA COASTAL ZONE CONSERVATION COMMISSION The CCZCC (California Coastal Zone Conservation Commission) was established by initiative of the State’s voters in 1972. Working with six regional com- missions, the CCZCC was charged with preparing a plan for the future of the California coastal zone. While the California Coastal Plan was being prepared, the commissions controlled all development, through a permit process, to insure consistency with the objec- tives of the legislation and the polices of the emerging plan. Coastal areas of the bay region are represented by two regional commissions: Central (San Mateo County) and North Central (San Francisco, Marin, and Sonoma Counties). The plan (Calif. Coastal Zone Cons. Comm., 1975) was presented to the Governor and State Legislature in December 1975 for adoption and implementation. Under the terms of the initiative, the CCZCC and the six regional commissions were to expire on January 1, 1977, unless legislation was enacted to create suc- cessors to them. In September 1976 the California Coastal Act of 1976 was enacted establishing the California Coastal Commission and six regional coastal commissions as USE OF SLOPE-STABILITY INFORMATION IN LAND-USE PLANNING 67 successors to the commissions created by the 1972 ini- tiative. Under the terms of the Act, the six regional commissions will expire 30 days after the last required local coastal program has been certified, but no later than January 1, 1981. The California Coastal Plan is a policy document with findings and recommendations for conservation and development of the coastal environment. It is sig- nificant to note the importance assigned in the plan to geologic hazards, including slope stability and the po- tential land-use impacts. The Coastal Plan describes landslides and mud- flows as two major geologic hazards in the California coastal zone with substantial risks to life and property (CCZCC, 1975, p. 84). Mapping and regulation to re- duce slope-stability hazards are recommended (CCZCC, 1975, p. 86): Slope Stability Hazards Can be Minimized by Mapping and Regulation. Slope-stability mapping is a primary tool for assess- ing potential landslide hazard, while regulation of land use and site preparation is the chief means of minimizing slope stability hazards. At present, both mapping and regulation are incom- plete Within the coastal counties. Mapping has often been under- taken only when intensive development is contemplated and landslide hazard is suspected; however, the Division of Mines and Geology has or is preparing maps for Sonoma, Marin, Santa Cruz, Ventura, Los Angeles, Orange, and San Diego Counties. Regulation is normally adopted only after damaging landslides occur. Slope-stability maps must be supplemented by specific analysis of individual sites if construction is proposed in areas indicated to be hazardous. Policies based on these findings were approved by the CCZCC recommending that: (1) the State role in geologic programs be strengthened through increased authority for State agencies in identifying and regu- lating land use in hazardous areas; (2) the State adopt legislation requiring more specific response to local geologic hazards, including specific planning guide- lines and land—use regulations that local agencies would have to adopt (for example, Chapter 70 of the Uniform Building Code dealing with grading require- ments and specific geologic studies). Also included are recommendations for development or reconstruction in hazardous areas and preventing public subsidy for hazardous developments. Local land-use decisions within the coastal zone will require detailed evaluation of potential geologic haz- ards. The plan recommendations were based on a bal- ancing of public and private costs and represent the belief that a resource as unique as the coastal zone must be preserved for the enjoyment of future Califor- mans. CITY AND COUNTY GENERAL PLANS California law requires that each city and county prepare and adopt a comprehensive long-term general plan for physical development. The law further stipu- lates that the plan include nine mandatory elements, of which the following directly or indirectly involve slope—stability information (Calif. Council on Inter- governmental Relations, 1973): The land-use element designates the general distri- bution, location, and extent of land used for various purposes and is based on analyses including such fac- tors as topography and geology. Further, the land-use element makes polices regarding natural and man- made hazards, such as slope stability, identified in other mandatory elements, particularly the seismic safety and safety elements. The conservation element is a plan for the preserva- tion, management, and wise utilization of natural re- sources including water, forests, soils, wildlife, minerals, and other natural resources. In part, the conservation element provides the data and policy necessary to evaluate the environmental impact of specific proposals. Conservation policy should address the need for slope—stabilization information. The open-space element is a plan for the preserva- tion and conservation of open space. Open space re- tained for reasons of public health and safety includes areas which require regulation because of hazardous or special conditions such as earthquake fault zones, landslide areas, and other hazards. The seismic-safety element identifies and appraises seismic hazards. Mudslides, landslides, and slope sta- bility must be considered in addition to other hazards. The seismic-safety element provides primary policy inputs to planning for land use, housing, open space, circulation, and safety. The safety element, in part, locates known geologic hazards and provides standards and general criteria for land use relating to such hazards. The element de- fines the general nature of the regulations and pro- grams needed to correct or mitigate the hazards of their effects. These mandatory elements have been added to state law one by one in a piecemeal manner; conse- quently, there is considerable overlap. In addition, the element by element format of the State requirements is not necessarily conducive to the preparation of planning documents that are internally consistent and easily used by decisionmakers. Nevertheless, it is clear the State Legislature intends that geologic hazards such as slope instability be adequately considered in local planning, location of critical facilities, evaluation of environmental impacts of land-use proposals, and conservation and preservation of open space. State general-plan requirements have considerably affected the gathering and use of earth-science infor- mation in many communities, particularly where po— 68 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. tential landsliding, earthquakes, or flooding were either already known or readily identified. Conse— quently, jurisdictions have acquired earth-science in- formation, including slope-stability maps of appropriate scale and level of detail to insure that po- tential hazards will be adequately recognized in gen- eral plans. Many jurisdictions have consulted with en- gineering geologists (or have their own staff geolo- gists) to determine the information necessary to prepare safety elements. PORTOLA VALLEY GENERAL PLAN The experience of the town of Portola Valley in the application of earth-science information, including slope—stability information, is unique and noteworthy. This experience is primarily the result of the town’s physical setting, low-density development at the time of incorporation, a local group of geologists dedicated to bringing extreme geologic hazards to the town’s at- tention, and a political climate receptive to the use of earth-science information in making planning deci- sions. Portola Valley is situated about 30 miles (50 km) south of San Francisco, on the bay side of the Santa Cruz Mountains. Most of the town’s 10 square miles (26 km) lies in a rift valley formed by the active San Andreas fault. West of the fault, the Santa Cruz Mountains are formed of Tertiary marine sedimenta- ry rocks with some interbedded basalt and diabase. Numerous landslide deposits, some of large propor- tions, exist on the steep slopes of this area. East of the San Andreas fault are Tertiary marine sediments to- gether with rocks of the older Franciscan Formation. This area has no serious landslide problems. The town was incorporated in 1964 for the purpose of preserving the natural qualities of the environment. Several geologists who were residents of the commu- nity brought geology to the attention of the Town Council shortly after incorporation. This group per- suaded the town to include geologic considerations in plans and regulations. The town has accomplished much in mitigating geologic hazards and is recognized for its pioneering efforts in seismic-hazard planning. The discussion that follows, however, focuses primar- ily on its activities in planning for slope stability. The general plan for the town was drawn up in 1963—64, before the development of a significant earth-science data base for the community and before State adoption of requirements for open space, con- servation, seismic safety, or safety elements. The plan did, however, propose a basic pattern of development for the western hillside areas that would concentrate development on the ridges and leave the steep and un- stable canyons as open space. Subsequently, with ad- ditonal earth-science information in the vicinity of the San Andreas fault, the general plan was amended to replace previously proposed public institutional uses with open-space proposals. Since adoption of the general plan, Portola Valley has pursued a successful program that makes exten- sive use of geologic information in land-use planning. A key ingredient in this program has been the avail- ability of volunteer professional geologists who have helped formulate the town’s program. The Geologic Hazards Committee, appointed in mid-1967, was com— posed of professional geologists, an attorney exper- ienced in landslide litigation, and a local building inspector. This committee was to “recommend ways in which geologic factors should be taken into account in order to minimize losses by homeowners and develop- ers in the towns of Portola Valley and Woodside.” The committee made three major recommendations: (1) The town should retain a town geologist to consult on ordinance administration and amendments as well as to develop basic geo- logic data. (2) The town should review all ordinances to make certain geologic hazards are taken into consi- deration. (3) The town should compile a “Geologic Hazards Map.” The town has followed these recommendations by retaining a town geologist, reviewing and revising reg- ulations, and mapping geologic hazards. The geologic hazards map has been of great help in developing spe- cific land-use regulations and preparing the recently adopted “Seismic/Safety Element.” In addition, the map is being used in a current study of the adequacy of existing proposals for general-plan land use. The Portola Valley geologic study consists of two detailed maps: a “Geologic Map” and a “Movement Potential of Undisturbed Ground Map” (Geologic Hazards Map), which have been adopted by resolu- tion to guide land—use decisions (Town of Portola Val- ley, 1974). The mapping program included extensive field investigations and took approximately four years to complete. The maps were prepared at a scale of 126,000 by graduate students from a nearby university under the direction of the Town Geologist. The basic geologic information was put on a base map of the town that shows topographic features, property lines, and other cultural features. The geologic map includes landslide features and identifies “active,” “dormant,” “recent,” “old,” and “Quaternary” landslides. More than half of the hill- sides in the western part of the town’s planning area are mapped as subject to landslide activity. USE OF SLOPE—STABILITY INFORMATION IN LAND-USE PLANNING 69 The geologic hazards map separates all land within the town into four categories of relative geologic sta- bility (table 8). [The categories are defined in table 8.] Slope stability was the important consideration in preparing the map. The following policies, based on the two maps con- cerning landslide hazards were included in the Seis- mic Safety Element adopted by the Town Council on August 13, 1975: 1. Review all proposed developments with respect to the Geo- logic Map and Movement Potential of Undisturbed Ground map *** of the Town. Require geologic and soil reports for all significant development of all areas shown as landslides. Reports should be responsive to the infor- mation indicated on these maps. 2. Locate structures for human habitation and most public utilities as not to risk other than minimum disturbances from potential landslides. Give due consideration to miti- gating measures, based on geologic and other reports ac- ceptable to the Town, which can be taken to reduce the risk from seismic and non-seismic hazards to an accept- able level**** TABLE 8.—Description of categories shown on “movement poten- tial of undisturbed ground map,”Portola Valley, Calif. Relatively stable ground Level ground to moderately steep slopes underlain by bedrock within approximately 3 feet (1 meter) of ground surface or less; relatively thin soil mantle may be subject to shallow landsliding, settlement, and soil creep. Unconsolidated granular material (alluvium, slope wash, and thick soil) on level ground and gentle slopes; sub- ject to settlement and soil creep; liquefaction possible at valley floor sites during strong earthquakes. Sls Naturally stabilized ancient landslide debris on gentle to moderate slopes; subject to settlement and soil creep. Sex Generally highly expansive, clay-rich soils and bedrock. Subject to seasonal shrink-swell, rapid soil creep, and settlement. May include areas of nonexpansive mate- rial. Expansive soils may also occur within other map units. Sbr Sun Areas with significant potential for downslope movement of ground me Steep to very steep slopes generally underlain by weath- ered and fractured bedrock; subject to mass wasting by rockfall, slumping, and raveling. Ps Unstable, unconsolidated material, commonly less than 10 feet (3 m) thick, on gentle to moderately steep slopes subject to shallow landsliding, slumping, settlement, and soil creep. Pd Unstable, unconsolidated material, commonly more than 10 feet (3 m) thick, on moderate to steep slopes; subject to deep landsliding. Areas with potential for surface rupturing and related ground dis- placements associated with active faulting Pf Zone of potential permanent ground displacement within 100 feet (30 m) of active fault trace. Unstable ground characterized by seasonally active downslope movement Ms Moving shallow landslides, commonly less than 10 feet (3 In) thick. Md Moving deep landslides, commonly more than 10 feet (3 m) thick. 3. Where roads or utility lines are proposed to cross landslide areas, for reasons of convenience or necessity, they should be permitted only if special design and construction tech- niques can be employed to assure that acceptable risk lev- els will be met. 4. Adopt implementing policies and (or) regulations which are consistent with policies 1—3 above and which will help as- sure that any failures of ground due to landslides will not endanger public or private property beyond levels of ac- ceptable risk defined in this statement. How these policies are being implemented and how slope-stability information is used by Portola Valley in land-use regulation and project review is described in the section on “Application in Plan Implementa- tion.” HAYWARD GENERAL PLAN With limited resources, the city of Hayward is at- tempting to integrate earth-science concerns into all phases of its planning program. The city is located in Alameda County on the east side of San Francisco Bay and south of Oakland. Hayward has grown rapidly in recent years to a current population of approximately 94,000 and continues to be under strong development pressures. Natural features of Hayward include a sizeable stretch of marshlands and shorelines along the bay, a large undeveloped hillside area to the east of the urbanized plain, and the Hayward fault. Major planning studies have been completed or are under- way for all three areas. The discussion that follows fo- cuses on the planning related to the 14,000 acre (5,700 hectare) Hayward hillside. The Hayward Hill Area Study was completed as background for the preparation of a general plan for the Hayward hillside (Hayward Planning Dept., 1971). Basically a land-capability study, the report de- scribes in detail the geology, soils, vegetation, climate, and hydrology of the hill area. Assistance was given by the Cordilleran Section of the Geological Society of America, which held a symposium in 1970 to discuss the area’s environment and the effects of urbanization on the hills. The study defines unstable or potentially unstable lands; lands with soil limitations for develop- ment; and potential ha; ards from fire, flood, and silta- tion. Suggestions included the use of cluster development, preservation of wooded and geologically unstable areas, and a variety of engineering and con- struction practices to minimize potential hazards. In addition, the city staff has suggested that an “environ- mental-hazards” zoning district similar to flood-plain zoning be established for parts of the hill area. For the hill area study, Hayward obtained basic 70 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. geologic information through a special contract with the (CDMG) California State Division of Mines and Geology. Maps of bedrock geology, seismic and non- seismic movement, and slope stability were produced, based on fieldwork and p otogeology. Aerial photo- graphs taken in 1967 at a s ale of 112,000 and in 1950 at a scale of 124,000 were used to determine the inci- dence of sliding during thd 15-year period and to lo- cate debris flows requiring! field checks. In addition to the basic geologic information, other earth-science work was included in the hill study. Soils information was obtained from the Soil Conser- vation Service, and hydrologic data were obtained from a paper presented to the Geological Society of America symposium by Dr. Thomas A. Pagenhart and from Robert E. Ellis of the Alameda County Flood Control and Water Conservation District. The information contained in the hill study was used for the proposed plan for the Hayward hillside, and the geologic data from the study has been incor- porated in the Hayward Earthquake Study (City of Hayward Planning Comm. Subcommittee on Land Use and Development Regulations, 1972). The pro- posed plan for the Hayward hillside takes into consid- eration the slope-stability problems identified by CDMG. Although this plan has not been acted upon by the City Council, the plan findings and recommen- dations, and the basic data these findings and recom- mendations are based upon, have been important to the Council’s evaluation of urban expansion, future city boundaries, and interaction with the Alameda County Local Agency Formation Commission3 (Hayward Planning Dept., 1975). The Hayward Earthquake Study, which has been adopted by the City Council as part of the city’s Seis- mic Safety Element, contains findings and recommen- dations related to potential hazards associated with slopes susceptible to landsliding during an earth- quake. Based on the hill study’s identification of slopes susceptible to landsliding, the earthquake study makes the following findings: The hill area east of the city contains sedimentary rocks which have been broken by faulting and bent into folds. Slope depos~ its are composed of sand, silt, and clays which cover nearly all the bedrock formations. The slope deposits contain expansive clay minerals which cause the entire mass to shrink and swell in periods of dry and wet weather. Debris flows are common on the valley walls when the slope is between 25 and 40 percent. There is a strong possibility of landsliding in this area during an earthquake. “In 1963, Local Agency Formation Commissions (LAFCO’s) were created by the State Legislature (Gov. C., Title 5, Div. 2, Part 1, Chap. 6.5 and 6.6, 1963). There is a LAFCO in each county to oversee the formation and alteration of boundaries of all local government agencies. The District Reorganization Act of 1965 (Gov. C., Title 6, Div. 1) provides uniform procedures for changes in district organization (such as an- nexations, detachments, consolidations, dissolutions, mergers, and complete reorga- nization) and for review by LAFCO’s of proposals for such changes. Ground rupture and cracking are not the only high seismic risks associated with an earthquake. Ground shaking, landslides, and liquefaction can also cause substantial damage to life and property during an earthquake. Therefore it is important that the proper precautions be taken outside of the fault corridor to further protect the safety of the citizens of Hayward. The following measures were recommended to in- sure that future development would consider such problems as slope instability: (1) Detailed soils and geologic reports and grading plans should be submitted with construction plans for any new subdivision within the city of Hayward. (2) A grading permit from the City Engineer should be required before any grading within the city, except for grading meeting very specific criteria. (3) The definition of “Quarry” should be expanded, and the operation of quarries should be more strictly regulated so that grading will be com- patible with natural site conditions. The recommendations for grading and quarrying regulations conclude “***the damage to the environ- ment as a result of these operations can only be effec- tively prevented by the rigid application of these regulations, and this will require adequate personnel to carry out close inspection of all grading operations.” The city planning staff is attempting to bring to- gether the environmental data and recommendations into an innovative response to State general plan re- quirements that will better serve the land use plan- ning needs of the community. This effort is to include four “elements” covering the subject required in the General Plan. The first, a “Conservation Environmental Protec- tion Element”, provides guidelines for current devel- opment. The “Hayward Conservation and Environmental Protection Study” (Hayward Plan- ning Dept., 1975) has been completed as the back— ground document to this element. Included in the background study is a map of the significant environ- mental conditions of the city and a composite “Signifi- cant Factors Map” accompanied by a matrix describing the probable environmental impacts of various development activities (fig. 51). Geologic data from the earlier studies, including the “Hills Area Study,” have been used to define the “Geological Con- ditions” on the map. According to the city planning staff (Martin Storm, written comm., Dec. 1975, and recent telephone con- USE OF SLOPE-STABILITY INFORMATION IN LAND-USE PLANNING 71 versation), proposed development could be reviewed against the Significant Factors Map and environmen- tal impact matrix. If the proposed development is found to affect the environment adversely, either the developer would have to mitigate the adverse effects, or, if public safety is involved and the adverse effects cannot be acceptably mitigated, the development would be denied. Although this innovative effort is still, for the most part, in the conceptual stage, it does indicate a well- organized approach for the use of earth-science infor- mation in planning. SONOMA COUNTY GENERAL PLAN Because of problems from landsliding, Sonoma County has, over the past several years, made increas- ing use of slope-stability data in its land-use planning. The dimensions of the landslide problem in the county are indicated by the fact that during the winter of 1968—69, the countywide public and private cost re- sulting from landsliding was more than $6 million, the highest of any of the nine bay-area counties (Taylor and Brabb, 1972). To deal with these hazards, the county obtained geologic and hydrologic data from the US. Geological Survey and contracted with the California Division of Mines and Geology for geologic- hazard mapping to be applied specifically in land-use planning. Sonoma County is located north of San Francisco and consists of approximately 1,010,500 acres (409,000 hectares) of land (fig. 52). The county is bounded by the Pacific Ocean on the west and north- west and by San Francisco Bay on the southeast. Fairly rugged mountains rise from the coast to an ele- vation of 3,500—4,000 feet (1,000—1,300 m) in the northern half of the county. A large valley, which con- tains the county seat and urban center, the city of Santa Rosa, occupies the south—central area of the county. The southwestern port of the county is gener- ally low, rolling grassy hills ranging in elevation from 500—600 feet (150—180 m). The cities of Petaluma and Sonoma are located in narrow valleys in the south- western and southeastern parts of the county, respec- tively. At the lower ends of Sonoma and Petaluma valleys are tidal flats reclaimed from San Pablo Bay. Because of the physiographic diversity of the county, land-use planners must deal with a variety of earth-science problems, not the least of which is slope stability. As part of the county’s 1973 Open Space Ele- ment Phase II program, a computer-aided environ- mental data system was developed to help collect, store, and evaluate basic earth-science data. The sys- tem utilized 250 acre (100-hectare) grid cells (1,000 m square) for organizing and recording information; computers for storing, manipulating, and displaying information; and a value—setting procedure called the “Delphi system” for identifying and incorporating citizen value judgments. The purpose was to plan for open space in response to California open-space ele- ment requirements. Development of the Phase II Open-Space Element was based on maps showing the environmental char- acteristics of the county. Initially, ten “environmental source” maps were prepared at the scale of 1:62,500 (1 inch equals approximately 1 mile) making use of exist- ing data on a variety of subjects including geologic hazards, slope, soils, hydrology, and climate. The data were recorded by grid cell and stored in the computer. The stored data were manipulated by computer to produce eight more maps showing slope instability, soil erosion, soil shrink/swell, and soil pressure limita- tions. To establish priorities for open-space planning, the environmental source maps were used in a capability analysis to generate three “environmental sensitivity maps” showing “hazardous areas,” “sensitive areas,” and “unique areas.” Weights, or “importance ratios,” were assigned to the various environmental factors through combining planners’ values and citizen com- mittee members’ values and using the Delphi process mentioned earlier (Sonoma County Planning Dept., 1973). Two of the three environmental sensitivity maps were based in part on slope-stability information. The hazardous areas map showed those areas liable to en- vironmental problems including landslides. The sen- sitive areas map indicated areas where man’s activities might have sufficient impact on the environ— ment to lead to deterioration or destruction of the nat- ural equilibrium. Sensitive areas mapped included areas with steep slopes. As described in the Phase II Open Space Element, “In Hazardous Areas, natural forces threaten man; in Sensitive Areas man endan- gers the natural ecobalance.” The studies on slope instability that were used to prepare the “Environmental Sensitivity Maps” were: 1. Preliminary Geologic Map of Western Sonoma County and Northernmost Marin County, Cali- fornia (Blake and others, 1971). This map was prepared at the scale of 1:62,500 and is based on data compiled and modified from a variety of sources. 2. Geology for Planning in the Sonoma Mountain and Mark West Road Areas, Sonoma County, California (Huffman and Armstrong, 1974). 72 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. 52 a: ROUTE 580 W/ r 32F BAY PLAIN l 82 0mm) Java mm u» HILL AREA Ammi‘ ‘uml mm» a»- 11mm) n“ 53 nm&>‘ -unu I) ate-- A say: a: 61/ an an an an FIGURE 51.—-Map of significant factors and probable environmental impacts, Hayward, Calif. E E u 32 53 D B1 ‘52 as 6000 FEET 2000 METE HS USE OF SLOPE-STABILITY INFORMATION IN LAND-USE PLANNING PROBABLE ENVIRONMENTAL IMPACTS 73 ENVIRONMENTAL CONDITIONS DEVELOPMENT ACTIVITIES Utility Landfills Landscaping 5"“? DISPPW 0' Designation Structures Roads pipes and excavations With native nLaerUIrigaevlgtger Irrigation agplitcstgn cables and grading species supplements materiaIs GEOLOGICAL ' HAVWARD FAULT CORRIDOR A-I .- 47 §— ‘7 A? LIOUIFIABLE GRANULAR DEPOSITS A-2 .— 4_ fi- 7‘ ‘. UNSTABLE HILL AREAS A3 7(‘- 7* A— A— 7‘ fi— 7‘ A— Tfi ‘- -¢ UNCONSOLIOATEO SEOIMENTS ['VOUNG MUD'l A-4 7‘}- 7‘ A- g- 7! g _ SOILS DISTURBED AREAS B-l +¢ T¢ SEPTIC TANK LIMITATIONS 8-2 g_ A.. EROSION NAZAROS 3-3 7(‘7 TQ ‘. 7‘ v‘ 7‘ _¢ ‘_ VEGETATION SLOPE STABILIZERS C»I —¢ -¢ —¢ ~¢ —¢ —¢ -¢ FRESHWATER HABITATS C-2 —¢ —¢ —¢ efi -¢ 7¢ —¢ SALT MARSH (EXISTING AND PROPOSEDI 0-: —¢ —¢ —¢ AQ -¢ 7‘ —¢ SALT WATER EVAPORATION PONOs c-A —¢ —¢ 7‘ vfi —¢ -¢ VEGETATIDN WITH SPECIAL ESTNETIC SIGNIFICANCE * _¢ -¢ _¢ 7‘ 7‘ 7‘ 7‘ CLIMATE AND AIR OUALITY HIGH WIND SPEED AND CHILL D r¢‘_ 7¢ §_ ?¢ +¢ ‘. +¢ ._ A- -Q ‘. HYDROLOOY FLOOD HAZARD AREAS E 7(‘. 7‘ ‘. gt- 7‘ +¢ ‘. 7‘ NOISE POLLUTION POTENTIAL NOISE IMPACT F +¢§_ -¢ 11¢ ,¢ +¢ ‘ Probable development impact on environmental conditions + Favorable impacts ‘ Probahle environmental impact on development activities _ Adverse impacts 7 Varying impacts depending on specific sites and activities involved NUMBER OF ENVIRONMENTAL CONDITIONS NOTED 0 "f1: ' 2 3 ‘4 ‘ 5 E: SIGNIFICANT FACTORS MAP FIGURE 51.—Continued This map, completed by M. E. Huffman under CDMG contract to the county, was prepared at the scale of 124,000 and is based on field obser- vation and analysis of aerial photographs. . Geologic Hazards Study—North Sonoma Coast. Two studies covering the coastal area between the Gualala and Estro Americana Rivers were also completed by Huffman, under CDMG con- tract with the county, and included mapping of the study area at the scale of 124,000. The data were gathered by field observation and analysis of aerial photographs. Michael Huffman of CDMG, who prepared several maps in conjunction with the County Advanced Planning Staff, interpreted the data for three critical geologic formations for application in the county en— vironmental data system, particularly in connection with the hazards and slope stability maps. The result of the da.:a gathering and analysis of the Open-Space Element Phase 11 program (Sonoma County Planning Dept., 1973) was the mapping of the hazardous, sensitive, and unique areas. It was recommended that the open-space character of these lands be protected by an interim open-space zoning 74 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. SONOMA COUNTY San Francisco, Bay U0 0 l\ $07 1?va \ Sonoma \ [P t, \ l' Santa Rosa . Petaluma \\f/\/:\ .4 0 10 20 MILES O 15 30 KILOMETERS FIGURE 52.—Sonoma County, Calif. The valley floors and adjacent low hills produce wine grapes, prunes, pears, apples, row crops, and oats for hay. The hills in the southwestern part of the county are used for grazing dairy cattle and sheep. Areas east of the hills are mainly range, pasture, and mixed woodland. Sheep and beef cattle are raised in these areas. Douglas fir and redwood are logged in the northern half of the county. ordinance and permit process until the final open- space element was completed and integrated with the general plan. Although the Phase II Open-Space Element and an interim open-space zoning ordi- nance have been adopted by the County Board of Su- pervisors, the computerized environmental-data system has been set aside because of its complexity. The Sonoma County Planning Department has prepared an ERME (Environmental Resources Man- agement Element), which incorporates the final open-space element and the conservation, seismic safety, and safety elements required by the State (Sonoma County Planning Dept., 1975). Important to the development of geologic-hazard policy and recommendations in the ERME was the report,Ge0- logy for Planning in Sonoma County, prepared for the county by the California Division of Mines and Geology (Huffman and Armstrong, 1974). This report identifies those areas subject to geologic hazards, such as seismically induced ground motion, fault rupture, tsunamis, and slope instability. The ERME recommends that the areas identified as having geologic hazards be regulated by an open- space combining district (Sonoma County Planning Dept., 1975, p. 42). The intent is that before develop- ment can occur in the areas regulated by the combin- ing district, performance standards will have to be met for resource management and public safety. An important recommendation is that a County staff ge- ologist administer an engineering geologic study of all hazardous areas (Sonoma County Planning Dept., 1975, p. 57). Other recommendations include strict enforcement of grading provisions and the planning of future geologic studies to assist the county in imple- menting its general plan. The slope-stability information contained in the Huffman and Armstrong (1974) report identifies four stability categories for the county. The report recom- mends that in three of the four categories engineering- geology reports be required before tentative tract ap- proval for land development. Such a recommendation from the earth scientist has had an important effect on the development of the Sonoma County ERME. The county planning staff believes that the signifi- USE OF SLOPE-STABILITY INFORMATION IN LAND—USE PLANNING 75 cant role assigned to evaluating geologic hazards in the general plan will result in more rational land use and less impact on the natural geologic processes. This goal will, of course, be dependent on the adoption by political decisionmakers of measures to implement the ERME and other general-plan elements. LAND-CAPABILITY STUDIES Land-capability studies have emerged in recent years as effective tools for evaluating the physical ca- pability of lands to accommodate land uses. The basic task of land-capability analysis is to focus on the physical landscape, identify its elements and their characteristics, and determine the capacity of the land to support the land uses under consideration. For example, a capability study might address the problem of determining which lands in a planning area can most easily accommodate residential devel- opment. Land characteristics, or factors, are selected that represent the natural qualities most unsuitable to residential use. Such factors might include hazards such as unstable slopes, flooding, active faults, ero- sion, expansive soils, steep slopes, or thin soils. Posi- tive qualities in the natural landscape such as types of vegetation or sand and gravel reserves are also often included. The capability study is structured to judge the ef- fect of all the selected factors, both individually and collectively, on the land-use option being considered and to express the impact of all factors by a single to- tal score for any designated portion of the planning area. It thus provides a method for comparing the rel- ative capability of land based on an explicit set of as- signed values. Capability studies may vary, but they generally fit the description given above. To illustrate the manner in which earth-science information can be used in such studies, seven distinct steps are listed below: 1. Select the land-use option to be considered. 2. Select the natural physical factors to be included in the evaluation. 3. Define the land units to be used in recording in- formation for each factor. 4. Obtain or prepare maps with information on the factors selected in Step 2. 5. Assign rating values to the factors. 6. Assign weights to each of the factors. 7. Determine weighted capability and total score of each land unit. The use of these steps is illustrated in a simplified hypothetical example of a land capability study con- sidering a residential land use. Our example shows a step-by-step application of the method, using only three earth-science factors to simplify the illustration. The example is intended to be purely illustrative and should not be construed as recommending specific nu- merical values. Obviously, it is impossible to provide a universally applicable description of a capability study. Also, the terms used to describe the study com- ponents vary considerably. Therefore, the following description is purposely general and intended only to serve as an example of the use of earth-science infor— mation in land-capability studies. Step 1. Select the land-use option to be consid- ered. The land use option is selected from various possible land uses for planning area, such as residential, industrial, agri- cultural, or transportation. In our exam- ple we will consider residential use. Step 2. Select the natural physical factors to be in- cluded in the evaluation. These factors might include geologic hazards, soils, steepness of slopes, vegetation, suscepti- bility to flooding, and other natural fac- tors known to be important in relation to the land-use options. In our example, we will consider only slope stability, erosion potential, and vegetation (table 9, first column). Step 3. Define the land units to be used in record- ing information for each factor. The shape, size, and number of land units needed are dependent on the size of the planning area and the nature of the plan— ning problem. Scale, detail, and accuracy of available data, and time, money, and manpower constraints often affect deci- sions regarding the size of land units and the level of detail that can be handled. A grid-cell system of land units is common- ly used and can facilitate computeriza- tion. The land units are delineated on a map of the planning area at a scale appro- priate to the problem. For our example, we will use three grid cells (see fig. 53). TABLE 9.—Weighted capability factors Factor Rating Factor Wei hted Factor conditions values weight capa ilily Slope Stability Stable 10 10 100 Moderately stable 5 10 50 Unstable 0 10 0 Erosion potential Insignificant 10 2 20 Moderate 5 2 10 High 0 2 0 Vegetation Conifer forest 7 3 21 Oak-grassland 10 3 30 Grassland 3 3 9 76 RELATIVE SLOPE STABILITY AND LAND-USE PLANNING, SAN FRANCISCO BAY REGION, CALIF. Stable (0/: q Unstable n a G J u o O 0 0 o SLOPE STABILITY FACTOR lo 0 o a o Overlay grid cells on slope-stability - A /0 0 B Q 0 C map to determine factor conditions 020 O 0 D a O in grid cells o 0 °°o 0 o 0 o 0 ° 2 o “minim ° ‘0 AODO D°O°°¢Z°nooo Weighted capability of factor conditions; determined from capability table (table 100 50 0 9) Cell ‘A Cell B Cell C Insignificant EROSION POTENTIAL FACTOR Overlay grid cells on erosion-potential map to determine factor conditions in grid cells Weighted capability of factor conditions; . .. 10 10 determined from capabilityvtabla (table 10 9) Cell A Cell B Cell C X\ XX \ V x x x x x x x / I77 I ’jfi/jl/ljl/ll Grass and x x x x x x x x x ’l/l/l/l / / // \ x X x x ////// //////// \T ////// ///////// x x x x x x x x x x x VEGETATIONFACTOR /,,,,, ,,,,,,/,,, 9 ‘ ‘ ////// ///// I/l/ x x x x x X X x x x x Overlay grid cells on vegetation Than to ”III/I’ll], A////3 B C determine factor conditions in grid r/x/I/x ////////// x x X X x X x X x X x * ///////;////////// x X X x x x x x x x x cells xx/z/n/x/z/lx/z/l. x l/l/I/ ’1 1‘ x x x x /////////////////// YX X X X X X X X X X X ////////////////// X X . / Oak-grasslandx/zxxz x X ConifeXr 23'9“), x x X /////////////////// X x X Weighted capability of factor conditions; determined from capability table (table 30 9 21 9) Cell A Cell 3 Cell C TOTAL CE LL SCORES Weighted capability, slope stability 100 50 + 10 10 1O Weighted capability, erosion potential 30 9 21 + 140 69 31 Weighted capability, vegetation total cell score Cell A Cell B Cell C USE OF SLOPE-STABILITY INFORMATION IN LAND-USE PLANNING 77 Step 4. Obtain or prepare maps with information on the factors selected in step 2. A set of maps with information describing the fac- tors selected is needed—all with a com- mon scale and level of detail. Typically, each factor is represented by one map in the set. To be of maximum usefulness to the analysis, each map would present in- formation on the selected factor by show- ing conditions (table 11) significant to the land-use options being considered. The number of conditions identified will vary depending on the planning problem being addressed and the level of detail and ac— curacy of the data. For some studies data will be available as interpretive maps, that is, maps which de- fine areas in relation to advantages or limitations for land uses, or relative risk from natural hazards. Most capability studies will depend, at least in part, on data that have not been collected or inter- preted specifically for the analysis. Any interpretation of these data or assign- ment of values for use in the capability analysis should be done by or under the supervision of a qualified professional who understands both the limits of reli- ability of the data and the requirements of the planner. In our example we will use interpretive maps showing the conditions for the three factors (conditions listed in table 9; maps shown in fig. 53). Step 5. Assign rating values to factor conditions. A rating scale is chosen to express the rela- tive conditions identified for each factor in step 4. From the scale a number, or rat- ing value, is assigned to each factor condi— tion. For example, assume that conditions of slope stability range from stable to un-