THE LIBRARY OF THE UNIVERSITY OF NORTH CAROLINA THE COLLECTION OF NORTH CAROLINIANA PRESENTED BY Tennessee Valley Authority C387.2 S97t UNIVERSITY OF N.C. AT CHAPEL HILL 00044665308 This book must not be taken from the Library building. Form No. 471 Digitized by the Internet Archive in 2013 http://archive.org/details/floodsonswannanoOOtenn_0 FLOODS ON SWAN NAN OA RIVER AND FLAT CREEK — — — — Vicinity of BLACK MOUNTAIN & MONTREAT NORTH CAROLINA TENNESSEE VALLEY AUTHORITY DIVISION OF WATER CONTROL PLANNING Tennessee Valley Authority KNOXVILLE. TENNESSEE May 2k, 1962 Colonel Harry E. Brown, Director State of North Carolina Department of Water Resources Post Office Box 9392 Raleigh, North Carolina Dear Colonel Brown: In response to the request of the Black Mountain Town Council and the Vice President of the Mountain Retreat Association, the governing body for Montreat, through the North Carolina Department of Water Resources, TVA has prepared the report, Floods on Swanna- noa River and Flat Creek, Vicinity of Black Mountain and Montreat, North Carolina . The purpose of this report is to provide "basic information on floods that have occurred or may occur which would be helpful in the state and local programs of city planning and development at Black Mountain and Montreat. We are furnishing you copies of the report for distribution to the appropriate state and city agencies and individuals. Also, copies of the report are being furnished interested Federal agencies for their information and use. Very truly yours, Division of Navigation and Local Flood Relations - ■ ■ ■ . . ■ ■ : TENNESSEE VALLEY AUTHORITY DIVISION OF WATER CONTROL PLANNING FLOODS ON SWANNANOA RIVER AND FLAT CREEK IN VICINITY OF BLACK MOUNTAIN AND MONTREAT NORTH CAROLINA REPORT NO. 0-6172 KNOXVILLE, TENNESSEE APRIL 1962 CONTENTS Page Foreword i I. Resume of Flood Situation 1 II. Past Floods 7 Swannanoa River Valley . 8 Settlement 8 The Stream and Its Valley 9 Developments in the Flood Plain 11 Bridges across the Stream 14 Obstructions to Flood Flow 19 Flood Situation 19 Flood Records 19 Flood Stages and Discharges 20 Flood Occurrences 21 Rate of Rise and Velocities during Floods . 22 Flooded Area, Flood Profiles, and Cross Sections ..... 22 Flood Descriptions 24 April 1791 24 1796 and 1810 25 May 1845 25 1850-1915 25 July 16, 1916 26 August 16, 1928 31 August 13, 1940 . 32 June 16, 1949 32 August 28, 1949 . 34 Flat Creek Valley 34 The Stream and Its Valley 34 Developments on the Flood Plain . 35 Bridges across the Stream 37 Obstructions to Flood Flow 42 Montr eat Dam . 44 f Flood Situation 44 CONTENTS — (Continued) Page Flood Records 44 Rate of Rise and Velocities during Floods 44 Flooded Area, Flood Profiles, and Cross Sections 45 Flood Descriptions 45 III. Regional Floods . 47 Maximum Known Floods in the Region 47 Upper Swannanoa River and Flat Creek vs. Regional Flood Discharges 50 IV. Maximum Probable Floods 57 Determination of Flood Discharges 57 Observed Storms 58 Observed Floods . 59 Maximum Probable Flood Discharges 59 Frequency 60 Possible Larger Floods 60 Hazards of Great Floods 60 Areas Flooded and Heights of Flooding 60 Velocities and Rates of Rise 61 TABLES Table Page 1 Relative Flood Heights in Vicinity of Black Mountain and Montreat 5 2 Drainage Areas of Streams in Swannanoa River Watershed ..... 11 3 Bridges across Swannanoa River in Vicinity of Black Mountain and Montreat, North Carolina 17 4 Swannanoa River at Biltmore, North Carolina — Flood Crest Elevations and Discharges above Bankfull Stage, 1791-1962 ... 20 5 Highest Known Floods in Order of Magnitude — Swannanoa River at Biltmore 21 6 Monthly Flood Distribution, Swannanoa River at Biltmore, North Carolina— 1791-1962 21 7 Bridges across Flat Creek in Vicinity of Black Mountain and Montreat, North Carolina 38 TABLES - - (Continued) Table Page 8 Maximum Known Flood Discharges on Streams in Region of Black Mountain and Montreat, North Carolina 49 9 Regional Flood Peak Discharges 51 10 Selected Maximum Observed Storms Transposable to the Region of Black Mountain and Montreat, North Carolina 58 11 Selected Maximum Observed Floods in Vicinity of Black Mountain and Montreat, North Carolina. . 59 PLATES Follows Plate Page 1 Upper Swannanoa River Watershed 10 2 Floods above Bankfull Stage — Swannanoa River at Biltmore, North Carolina 20 3 Stage Hydrographs— Floods of August 13, 1940, & June 16, 1949— Swannanoa River at Biltmore, North Carolina . 20 4 Upper French Broad Area Showing July 1916 Estimated Storm Rainfall 26 5 Upper French Broad Area Showing Mid- August 1940 Storm Rain- fall 32 6 Storm of June 13-16, 1949— Upper French Broad Watershed ... 32 7 Maximum Known Flood Discharges — Region of Black Mtn. and Montreat, N. C. ..................... 50 8 Flooded Areas — Swannanoa River and Flat Cr. — Vicinity of Black Mtn. and Montreat, N. C. ................ . 62 9 Flooded Areas — Swannanoa River, Mile 17. 45 to 20. 55 ..... 62 10 Flooded Areas— Swannanoa River, Mile 20. 55 to 22. 30 ..... 62 11 High Water Profiles --Swannanoa River, Vicinity of Black Mountain, N. C. 62 12 Flooded Areas — Flat Creek, Vicinity of Black Mountain and Montreat, North Carolina ................. 62 13 High Water Profiles — Flat Creek, Vicinity of Black Mountain and Montreat, N. C. . 62 14 Cross Sections — Swannanoa River and Flat Creek, Vicinity of Black Mountain and Montreat, N. C 62 FIGURES Figure Page 1 Swannanoa River Bridges near Grovestone 15 2 Swannanoa River Bridges at Black Mountain 16 3 Typical Highway Damage in July 1916 23 4 Overflow at Black Mountain, July 16, 1916 28 5 Barn on Black Mountain Avenue in July 1916 28 6 Railway Damage on Swannanoa River, July 1916 29 7 Flood Effects on Flat Creek, July 1916 30 8 Flat Creek Bridges in Black Mountain and Vicinity 39 9 Flat Creek Bridges in Vicinity of Montreat 40 10 Lake Susan Development at Montreat 43 11 General Precision, Inc., Plant at Grovestone 52 12 Radio Station WFGW at Old Nichols Airport 52 13 Flood Heights along Black Mountain Avenue 53 14 Flood Heights in Vicinity of Lakey Street Bridge 54 15 Flood Heights near Mouth of Flat Creek, Black Mountain 55 16 Flood Heights at Padgett Town Road 56 17 Flood Heights at Pine Lodge Cottages 56 X-2717 COVER PHOTO Flood of July 1916 destroys Southern Railway bridge at Mile 21. 33 on Swannanoa River in Black Mountain, North Carolina. Photograph furnished by Gragg Studio, Black Mountain, North Carolina. FOREWORD Tennessee Valley Authority Division of Water Control Planning FOREWORD This report relates to the flood situation along the Swannanoa River and Flat Creek in the vicinity of Black Mountain and Montreal North Carolina. It has been prepared at the request of the Black Mountain Town Council and the Vice President of the Mountain Retreat Association, the governing body for Montreat, through the North Carolina Department of Water Resources to aid (1) in the solution of local flood problems and (2) in the best utilization of lands sub- ject to overflow. The report is based upon work TVA has been carrying on since its beginning in connection with its water resource operations throughout the Tennessee Valley. TVA has assembled information on rainfall, runoff, historical and current flood heights, and other technical data bearing upon the occurrence and magnitude of floods in localities throughout the region which provide the basis for preparation of this report. The report does not include plans for the solution of flood problems. Rather, it is intended to provide the basis for further study and planning on the part of the town of Black Mountain and the Mountain Retreat Association in arriving at solutions to minimize vulnerability to flood damages. This might, involve (1) the construction of flood protection works, (2) local planning programs to guide developments by controlling the type of use made of the flood plain through zoning and subdivision regulations, or (3) a combination of the two approaches. The report covers three significant phases of the Black Mountain and Montreat flood problem. The first brings together a record of the largest known floods of the past on the Swannanoa River and Flat Creek. The second treats of Regional Floods. These are derived from consideration of the largest floods known to have occurred on streams of similar physical characteristics in the same general geographical region as that of the Swannanoa River and Flat Creek and generally within 50 miles of Black Mountain and Montreat. The third develops the Maximum Probable Floods for the Swannanoa River and Flat Creek. Floods of this magnitude on most streams are considerably larger than any that have occurred in the past. They are the floods of infrequent occurrence that are considered in ii planning protective works, the failure of which might be disastrous,, Such floods are used by TVA in the design of physical features of reservoirs, dams, power- houses, and local flood protection works. The report contains maps, profiles, and valley cross sections which indicate the extent of flooding that has been experienced and that might occur in the future in the Black Mountain and Montreat vicinity. In problems concerned with the control of developments in the flood plains of the Swannanoa River and Flat Creek, and in reaching decisions on the magnitude of floods to consider for this purpose, appropriate consideration should be given to the possible future occurrence of floods of the magnitude of (1) those that have occurred in the past, (2) the Regional Floods derived from a study of those that have occurred on streams of similar physical characteristics in the region of Black Mountain and Montreat, and (3) the Maximum Probable Floods that it is estimated might occur. This report should be useful in planning new developments in the flood plain. From the maps, profiles, and cross sections, the depth of probable flooding by either recurrence of the largest known flood or by occurrence of the Regional or Maximum Probable Floods at any location may be ascertained. By having this information, floor levels for buildings may be planned either high enough to avoid flood damage or at lower elevations with recognition of the chance and hazards of flooding that are being taken. I. RESUME OF FLOOD SITUATION I. RESUME OF FLOOD SITUATION Black Mountain, North Carolina, is located on the Swannanoa River 21 miles above the confluence of Swannanoa River and the French Broad River at Asheville. Flat Creek, a tributary with a drainage area of 6. 05 square miles, joins Swannanoa River at Black Mountain,, Montreat is a development owned by the Mountain Retreat Association, under the control of the Presbyterian Church in the United States, and having the powers and responsibilites of a municipality under the laws of the State of North Carolina. The headwaters and all but the lower 1. 5 miles of the channel of Flat Creek are within the property of the Mountain Retreat Association. The corporate limits of Black Mountain extend for 0. 3 mile along Flat Creek and for 1. mile along the Swannanoa River, This investigation covers the Swannanoa River from Mile 17. 36 to Mile 22. 3 and Flat Creek from the mouth to Mile 2. 6. The total area of the Swannanoa River watershed above the lower limit of the reach studied in this report is 22. 2 square miles. In recent years there has been increased development of the flood plains of the upper Swannanoa River and Flat Creek. Many commercial establish- ments and residences are subject to flooding. Black Mountain has experienced over- flows a number of times since it was incorporated 69 years ago, the largest of these being in July 1916. The most recent serious flooding occurred in June 1949. There are no records of river stage or discharge on the Swannanoa River at Black Mountain or on Flat Creek at Montreat. Records of river stage and discharge were maintained from 1920 to 1926 and from 1934 to date on the Swannanoa River just above the mouth at Biltmore. In compiling a record of early floods on the two streams, it has been necessary to interview residents along the streams who have knowledge of past floods and to conduct research in newspaper files and historical documents. From these investigations and from studies of possible future floods on the Swannanoa River and Flat Creek in the vicinity of Black Mountain and Montreat, the flood situation, both past and future, has been developed. The following paragraphs summarize the significant findings with regard to the flood situation which are discussed in more detail in succeeding sections of this report. 2 THE GREATEST KNOWN FLOOD on the upper Swannanoa River and Flat Creek of which there is definite knowledge occurred on July 16, 1916. Two high water marks are known for this flood in the Black Mountain area, but there is not sufficient data to develop high water profiles. Overflow was "hill to hill" on Flat Creek below the Montreat property and on the Swannanoa River through Black Mountain, but there was little development along the streams at that time. * * * A GREAT FLOOD occurred on the Swannanoa River in April 1791, soon after settlement of the region began. The available evidence indicates that this flood exceeded the flood in 1916 by five feet or more on the lower Swannanoa River. It is probable that the flood was greater than the 1916 flood on the upper Swannanoa River and on Flat Creek also. * * * OTHER BIG FLOODS on the Swannanoa River, for which data were obtained, occurred on August 16, 1928; August 13 and 30, 1940; and on June 16, 1949. The flood of August 13, 1940, was the highest of these, reaching within about lto 3 feet of the flood of July 1916 in the Black Mountain vicinity. The August 13, 1940, and June 1949 floods were also important on Flat Creek. The 1940 flood was slightly higher than the 1949 flood and had about the same relation to the 1916 flood as on Swannanoa River. * * * REGIONAL FLOODS on the Swannanoa River and Flat Creek in the vicinity of Black Mountain and Montreat are based upon floods experienced on streams within 50 miles of the two communities, a number of which are larger than any known floods on the two streams. This indicates that greater floods than those of which there is definite knowledge so far may reasonably be expected in the future. Based upon the magnitude of floods that have occurred on the neighboring streams, a Regional Flood may occur on the Swannanoa River at Black Mountain that would be generally 1 to almost 14 feet higher than the August 13, 1940, flood crest. This flood would not be much greater than the 1791 flood under present channel and flood plain con- ditions and would be only a few feet higher than the 1916 flood. A Regional Flood on Flat Creek in the vicinity of Black Mountain and Montreat would reach stages generally 2 to 13 feet higher than the June 1949 flood crest. * * * 3 MAXIMUM PROBABLE FLOOD determinations indicate that floods couid occur on the Swannanoa River in the Black Mountain vicinity up to about 14 feet higher than the 1940 flood crest. Here again, this flood would not be appreciably greater than the 1791 flood under present conditions. A Maximum Probable Flood on Flat Creek would be 3 to 15 feet higher than the 1949 flood. FLOOD DAMAGES that would result from a recurrence of floods as large as those of July 1916 on the Swannanoa River and Flat Creek would be considerably greater than in the actual flood because of the development now in the flood plains and the higher stages caused by the obstructions on the flood plains of the two streams. The Regional and Maximum Probable Floods, with their greater depths and veloci- ties, would cause even heavier damages. * * * MOST FREQUENT FLOOD OCCURRENCES on the Swannanoa River and Flat Creek have been in the late spring and summer and especially in August. Most of the higher floods have been summer floods resulting from very heavy hurricane rain- fall or intense thunderstorms. However, floods occur also in the fall and winter months, and on small drainage areas such as that above Black Mountain and Mon- treat large floods may come at any time. * * * VELOCITIES OF WATER during the July 19 16 flood ranged up to 13 feet per second in the channel of the Swannanoa River in the Black Mountain vicinity and up to 8 feet per second on the flood plain. Along the lower reaches of Flat Creek, veloci- ties in the channel and flood plain were probably comparable with those on the Swannanoa River. Farther upstream near Montreat the velocities were as much as 19 feet per second in the channel and up to 6 feet per second in the overbank areas. During a Maximum Probable Flood, velocities on the Swannanoa River and on lower Flat Creek would range up to 15 feet per second in the channel and up to 9 feet per second on the flood plain. For Flat Creek at Montreat the cor- responding figures would be 19 and 6 feet per second. * * * 4 DURATION OF FLOODS is short on both the Swannanoa River and Flat Creek. In the flood of June 1949 both streams were above bankfull stage in the Black Mountain vicinity for about 12 hours. On the lower river at Biltmore, where this flood was less severe, the river was above bankfull stage 6 hours. In the flood of August 13, 1940, the period above bankfull stage at Biltmore was 14 hours; the average rate of rise was 0. 6 foot per hour and the maximum rate was about 2. 4 feet per hour. It is probable that at Black Mountain and Montreat the rates of rise exceeded these figures. During the Maximum Probable Flood the rise on Swannanoa River at Black Mountain would be about 10 feet in 3 hours, with a maximum rate of about 4. 5 feet per hour. On Flat Creek at Montreat, the rate of rise would be 14 feet in 1^ hours and the maximum rate about 10 feet in one-half hour. * * * HAZARDOUS CONDITIONS would occur during very large future floods as a result of the rapidly rising streams, the high velocities, and the deep flows. Many build- ings and bridges would be swept away and the lives of those occupying homes and other structures would be endangered. * * * FUTURE FLOOD HEIGHTS that would be reached if floods of the magnitude of the Regional and Maximum Probable occurred on the Swannanoa River and Flat Creek are shown in Table 1. The table compares these future flood crests under present conditions with the crest of the August 13, 1940, flood on Swannanoa River and June 16, 1949, flood on Flat Creek. 5 TABLE 1 RELATIVE FLOOD HEIGHTS IN VICINITY OF BLACK MOUNTAIN AND MONTREAT Flood Aug. 13, 1940 Regional Maximum Probable Aug, 13, 1940 Regional Maximum Probable Location Mile Swannanoa River Southern Railway 17. 58 at Lakey Street 20. 96 Estimated Peak Discharge cfs 10,000 24, 500 28,000 7,000 18, 500 21, 200 Above 1940 Flood feet 8. 6 8. 9 5. 7 6.0 Above 1949 Flood feet Flat Creek June 16, 1949 Regional Maximum Probable at Montreat Dam 2. 49 1, 600 7,000 10,000 4.0 4. 6 n. PAST FLOODS PAST FLOODS* 1 * This section of the report is a history of floods which have occurred on the Swannanoa River and its tributary, Flat Creek, in the vicinity of Black Mountain and Montreat, Buncombe County, North Carolina. The portion of Swannanoa River considered extends from Mile 17. 36, at the mouth of North Fork Swannanoa River and 3. 6 miles downstream from Black Mountain, to Mile 22. 3, 0.8 mile above the upper corporate limit of Black Mountain. The investi- gations on Flat Creek cover the 2. 6 miles of the stream from the mouth to the head of Lake Susan in Montreat. Flat Creek joins the Swannanoa River at Mile 21. 45, in Black Mountain. One mile of the Swannanoa River and 0. 3 mile of Flat Creek are within the corporate limits of Black Mountain. Flat Creek above Mile 1. 5 is within the property owned by the Mountain Retreat Association, a corpora- tion under the direction of the Presbyterian Church in the United States. The Swannanoa River flows generally westward in the reach investi- gated. Flat Creek flows generally southward through Montreat to its confluence with Swannanoa River. In the center of the Presbyterian Church assembly devel- opment is Montreat Dam, 23 feet high, forming Lake Susan with a surface area of about 1^ acres. The region is heavily forested and steeply mountainous except for the relatively wide bottom land along the lower portion of Flat Creek and along Swannanoa River. The larger part of the developed area at Black Mountain, includ- ing the main business district, lies on high ground but some commercial and indus- trial development and a number of residences are on the flood plain. Below the Montreat boundary the land along Flat Creek has been largely developed to com- mercial and residential use. Within the assembly grounds there is little or no bottom land along the stream and developments are generally above flood height. No records of river stages or discharges have been maintained on the Swannanoa River at Black Mountain or Flat Creek at Montreat. Streamflow records have been kept intermittently since 1920 on the Swannanoa River at Bilt- more, 20 miles downstream from Black Mountain. (1) Prepared by Hydraulic Data Branch. 8 Flood history investigations made by TVA Hydraulic Data Branch engineers in 1941 developed information on floods on the Swannanoa River down- stream from the mouth of North Fork Swannanoa River. In 1950 investigations were made to extend the information on floods on the Swannanoa River upstream through Black Mountain and on Flat Creek from the mouth through Montreat. Local residents have been interviewed and research has been carried on in news- paper files and historical documents. From these sources it has been possible to develop a history of known floods on Swannanoa River and Flat Creek at Black Mountain and Montreat covering the past 60 years or more. 1. SWANNANOA RIVER VALLEY Settlement Buncombe County was formed in 1792 from a part of Burke and Ruther- ford Counties and at first contained all the land in North Carolina lying west of the Blue Ridge. This had been Cherokee Indian land until the North Carolina General Assembly declared it open to the white man in 1783. The route crossing the Blue Ridge at Swannanoa Gap and following the Swannanoa River was an important artery of travel from the earliest days. Most of the settlers of the Swannanoa and French Broad River valleys came by this route. The first railroad to be built into this part of North Carolina, the Western North Carolina Railroad, crossed the Blue Ridge at Swannanoa Gap and was completed to Biltmore, near the mouth of Swannanoa River, on October 3, 1880. This line is now a part of the Southern Railway system. The name Swannanoa comes from "Shawano, " one of the common early spellings of "Shawnee, " the tribe of Indians who were once residents of the valley. The settlement which developed at the site of Black Mountain was first known as Gray Eagle. It was renamed Black Mountain for the nearby range of mountains of that name in 1870. Black Mountain was incorporated on March 4, 1893. The 1960 Census Report gives the population of Black Mountain as 1313 persons. However, a large portion of the community lies outside the present corporation limits. Property to the north and west of the original limits has been annexed in recent years and other additions are being contemplated. 9 The town of Black Mountain has for some time been one of the more important resort centers in the Asheville area. A number of private summer camps surround it and five religious assembly grounds are located nearby. The largest assembly area is owned by the Presbyterian Church at Montreat, in the Flat Creek valley upstream from Black Mountain. The tract contains some 3000 acres of land, mostly in forest, and includes essentially all of the watershed of Flat Creek above Mile 1. 5. The name Montreat comes from "Mountain Retreat Association, " the corporation which was chartered by the State of North Carolina to operate the facility March 2 S 1897, The charter created the town of Montreat at the same time and gave to the corporation the powers and functions of a municipality. First operated as an interdenominational religious assembly area, Montreat was purchased by the Presbyterian Church in the United States in 1906, to be used for summer conferences and as a site for summer homes for church people. Montreat College was established on the church ground in 1916. Operated for some time as a four-year girls' school, it now has a two-year coedu- cational program and operates under the name of Montreat-Anderson College. The present enrollment is about 250. Some 400 homes are on the Montreat grounds. Only about 50 of these are occupied the year around so that only a few hundred persons are there during the winter months. In the summer the homes are fully occupied and with the attendance at the various camps and conferences there may be 2500 persons in residence at any one time. Planning studies are currently being made which envision doubling the capacity of the college, making extensive improvements to the buildings now in use, and adding new facilities. The Stream and Its Valley The Swannanoa River drains a rugged mountain area of 133 square miles, lying wholly within Buncombe County, North Carolina. The river basin is about 17 miles long and 7 to 10 miles wide. The stream flows generally west- ward and joins the French Broad River at Asheville, 21 miles downstream from Black Mountain. In the vicinity of the towns of Black Mountain and Montreat the prin- cipal streams are the upper reach of the Swannanoa River and Flat Creek. This 10 part of the watershed, shown on Plate 1, is about 8 miles long and l\ to 4^ miles wide. The total drainage area at the lower limit of the reach studied in this report is 22. 2 square miles. Flat Creek flows through Montreat and joins the Swannanoa River within the corporate limits of Black Mountain. Both Flat Creek and Swannanoa River have their origin on the western slopes of the Blue Ridge, which separates the Tennessee Valley drainage from the Atlantic Coast drainage to the east. The watershed above Black Mountain is mountainous and heavily forested except for some reaches along the streams. Elevations range from 2340 feet at the town of Black Mountain to as high as 5360 feet at Graybeard Mountain, 4. 5 miles north. The Swannanoa River is formed at Ridgecrest, where a number of small streams converge from the steep coves and ridges. The stream flows west- ward to Black Mountain, makes a semicircle to the south of the main business dis- trict, then oontinues westward to the confluence with North Fork Swannanoa River at Grovestone. The principal tributary to upper Swannanoa River is Flat Creek, which will be discussed later. Tomahawk Branch, which heads up on the slopes of Middle Mountain, flows through the western part of Black Mountain and enters Swannanoa River at Mile 19. 39. Two streams drain the steep slopes of the Blue Ridge and the Swannanoa Mountains which lie south and southwest of Black Mountain. Camp Branch collects the flow from Brittin Cove and Lynch Cove and flows into Swannanoa River at Mile 20. 27. Wolfpit Branch enters the stream at Mile 18. 97. Pertinent drainage areas in the upper Swannanoa River basin are shown in Table 2. Channel slopes are steep on the upper Swannanoa River, averaging 36 feet per mile for the reach covered by this investigation. The rate of fall changes gradually from about 60 feet per mile above Black Mountain to about 25 feet per mile near Grovestone. The bottom lands are relatively broad and consist of a thin topsoil underlain by heavy gravel and boulders, evidently deposited by floods of the past. These bottoms are cut up by numerous flood channels and show progres- sive meandering of the stream channel in the erodible material. The agricultural lands are mostly in pasture. Flood-plain widths vary from between 300 and 600 feet above the mouth of Flat Creek up to as much as 1600 feet downstream from Black Mountain, Within the corporate limits of the town the flood plain is generally 800 to 1000 feet in width. The widest section is about Mile 18. 6 near radio station WFGW. Elsewhere PLATE I WATERSHED Scale { I Mile APRIL 1962 11 TABLE 2 DRAINAGE AREAS OF STREAMS IN SWANNANOA RIVER WATERSHED Mile above Drainage Stream Location Mouth Area sq, mi. Swannanoa River Upper end of reach 22. 30 3. 35 Below Flat Creek 21.40 10. 9 Lakey Street 20. 96 11. 2 Southern Railway- 17.58 22. 1 Lower end of reach 17. 36 22. 2 Biltmore (stream gage) 1.56 130 Mouth 133 Flat Creek Upper end of reach 2.59 2. 76 Montreat Dam 2.49 2. 80 Montreat boundary 1.51 4. 65 U. So Highway 70 0. 10 6.02 Mouth 6.05 Camp Branch Mouth 2. 93 Tomahawk Branch Mouth 1. 94 Wolfpit Branch Mouth 1.75 downstream from Black Mountain, the flood-plain width varies between 1000 and 1500 feet except near Mile 19. where it narrows to about 600 feet. The corporate limits of Black Mountain include a one-mile length of the Swannanoa River between Miles 20. 56 and 21. 55. Developments in the Flood Plain Plates 8, 9, and 10 show the flood plain of the Swannanoa River. Except for the immediate area at Black Mountain and along the lower end of the study reach, near Grovestone, the use of the flood-plain lands is limited generally to agriculture. The Salisbury Line of the Southern Railway crosses the Swannanoa River at Mile 17. 58, near Grovestone, and follows the right bank of the river to Black Mountain. It crosses to the left bank at Mile 21. 33, in Black Mountain, then L2 follows that bank to Ridgecrest, where it leaves the basin through a tunnel at Swan- nanoa Gap. A spur track at Grovestone serves the industries which are in that vicinity- Except at the bridge crossings the track is generally above the elevation of the highest known floods on Swannanoa River, Uo S. Highway 70 carries heavy east-west traffic between Asheville and the Piedmont and Coastal sections of North Carolina. West of Black Mountain, U. S. Highway 70 is a three-lane concrete highway,, From Black Mountain east it is a four-lane divided highway which also carries the designation of Temporary Interstate Highway 40, The highway crosses Swannanoa River at Mile 17, 56 s just downstream from the Southern Railway bridge at Grovestone . Its location lies north of and closely parallel to that of the railway through the basin. Numerous develop- ments of a service nature are along the highway but these are generally above any flood danger. Planning studies are currently being made to determine the location for Interstate Highway 40 in the vicinity of Black Mountain. It is anticipated that the highway will pass through the Swannanoa River basin between Ridgecrest and Ashe- ville. However s planning studies have not progressed to a point to foretell what effect the highway might have upon developments in the flood plain and upon future floods. The principal business development of Black Mountain is north of the river along State Street, the main east-west artery of the town, and on Broadway and Cherry Street between State Street and the railway. All the developments in this section are at elevations well above that of the highest known floods on the Swannanoa River. Some of the developments south of the railway are subject to flooding. These include a bulk oil plant on Lakey Street, and a grocery and a rid- ing stable on Black Mountain Avenue. A trailer court is on the left bank, on Sycamore Street between Lakey Street and Black Mountain Avenue. A building supply warehouse and a hosiery mill near the intersection of Lakey and Vance Streets are above the highest floods of the past but are within range of the Maxi- mum Probable Flood. Two churches and about 25 houses are within the area which would be overflowed in a recurrence of the July 1916 flood on the Swannanoa River. A Maximum Probable Flood would affect about 75 houses and a number of the businesses in the vicinity of Sutton Avenue and West Midland Avenue. 13 Begley's Lake is a private fishing lake development on the left-bank flood plain below the Southern Railway Mile 21. 1 to 21. 2. Included is a lake of about three acres, an eating place, and a number of trailer parking sites. Nearly all this area is within the range of any large flood. The land on the right-bank flood plain Mile 18. 1 to 18. 7, was used for a number of years for a private flying field. In more recent years the site has been used by a firm manufacturing a special low-altitude parachute. Former hangar buildings were converted for use in the assembly of the parachutes and the testing was done from two steel towers erected near the riverbank. This firm is presently in the process of moving its operations to a site outside the Swannanoa River basin and a radio broadcasting station is being built on the land. Three transmitting towers on the right-bank flood plain have bases which range in eleva- tion from 2259.4 to 2265, 8 feet. Offices, studios and transmitting equipment will be in a building near the railway, with its principal floor at elevation 2279. 1 feet. A recurrence of the flood of August 13, 1940, would put water over one or more of the tower bases. A Maximum Probable Flood would cover all tower bases and reach an elevation of 2275 feet near the main building, and the Regional Flood would reach an elevation of 2274 feet. A plant of the Kearfott Division of General Precision, Incorporated, is located on the right bank of Swannanoa River just below U. S. Highway 70, The plant manufactures small electrical components. The buildings are on graded land raised some 18 feet above the original flood -plain elevation. The principal floors are at elevation 2254. 4 feet, about 14 feet above the elevation reached by the flood of August 13 1940, at the location, and about 7 or 6 feet above the height expected at the site in a Regional or Maximum Probable Flood, respectively. A number of other industries are in the Grovestone vicinity. Two wood products firms are northeast of the Kearfott plant, A manufacturer of pre- cision tools and components is located south of U. S. Highway 70 at Stepp Branch and 2000 feet southwest of the Kearfott plant. All three of these plants are outside of the Swannanoa Ri ver flood plain. Black Mountain obtains its principal water supply from an intake and reservoir located in Dunsmore Cove \\ miles east of the town. An intake on a branch north of Burgin Cove, the original supply for the town, is not used at pres- ent. A reservoir located on Miami Mountain, at the east corporate limit, supplies 14 pressure to the town distribution systenio An interconnection at the Montreat property line ties the system to that of the Mountain Retreat Association. Mon- treat supplies water to Black Mountain at this point as it is available in exchange for fire fighting services. Interconnection to the Asheville water supply is made at a 6-inch line in the northwest section of Black Mountain, Water is available to the system at this point, but the other sources are normally ample for the town's needs. Black Mountain and vicinity are served by the sewerage system of the Swannanoa Sanitary District, An outfall sewer follows the Swannanoa River from Ridgecrest through Black Mountain to Asheville, where it discharges into the French Broad River. The sewer line, constructed in 1929, is now in poor con<- dition. Floods have damaged the line, and settling and shifting have caused num- erous leaks and blockages. The system is to be incorporated into a newly organized Metropolitan Sanitary District, This district will include Asheville and the larger part of Buncombe County. A treatment plant to be built near Ashe- ville will treat the sanitary waste from the whole area. It is anticipated that the project will include extensive renovations of the existing collecting system in the Black Mountain vicinity. Bridges across the Stream Two railway bridges, six highway bridges, and two private bridges cross the Swannanoa River in the reach included in this investigation. Table 3 lists pertinent elevations for these bridges and shows the relation to the crest of the flood of August 13, 1940, on the river. Plate 11 shows the relation of the floor and underclearance at the bridges to the flood profiles for the reach. Figures 1 and 2 are photographs of all but the two private bridges. Only one of the bridges, the Southern Railway bridge at Black Mountain, has an underclearance which is above the crest height determined for the flood of July 1916. It is probable, however, that the bridge would be overtopped if a flood of this magnitude were to occur under the present conditions at the site. All the other bridges have floors or approaches which would be affected by floods con- siderably less severe than the 1916. Heavy east-west truck and auto traffic is carried by the three-lane U. S. Highway 70 bridge which is at Mile 17. 56. This is a three-span steel and U. S. Highway 70 bridge at Mile 17. 56, upstream side, from railway embankment near bridge shown below. Blue Ridge Road bridge at Mile 20. 01, downstream side. Figure 1. — SWANNANOA RIVER BRIDGES NEAR GROVESTONE State Highway 9 bridge at Mile 20. 89, upstream side. Lakey Street bridge at Mile 20. 96, upstream side. Southern Railway bridge at Mile 21. 33, upstream side. Bridge at Mile 22. 30, downstream side. Note sewer line crossing the stream. Figure 2. — SWANNANOA RIVER BRIDGES AT BLACK MOUNTAIN 17 TABLE 3 BRIDGES ACROSS SWANNANOA RIVER IN VICINITY OF BLACK MOUNTAIN AND MONTREAT, NORTH CAROLINA Underclearance Mile above Mouth Identification Stream Bed iii lev. Aug. 13, 1940, Floor Flood Crest Elev. Elevation TP 1 SWT iiiiev. Above Flood Crest Below Flood Crest feet feet feet feet feet feet U. bo xiwy. (U 2246.0 2241.0 9 9/1 Q Q 9 Q 17.58 Southern Rwy. 2231. 3 2244. 3 (a ^ 2241. 8 2240. 8 1.0 19.07 Blue Ridge Rd. 2267. 1 2276. 9 2277. 7 2274. 3 3.4 20.01 Blue Ridge Rd. 2293. 2 2302.4 2303.4 2299.4 4.0 20. 89 N. C. Hwy. 9 2330.0 2341. 5 2337. 9 2338. 9 1.0 20. 96 Lakey Street 2332.4 2341. 6 2340. 1 2338. 6 1. 5 21. 33 Southern Rwy. 2343. 2365. 2^ 2353. 6 2358. 1 4. 5 21. 61 Private road 2357. 8 2367. 1 2366.0 2364. 6 1.4 22. 11 Private road 2391. 2 2399. 1 2396. 7 2397.4 0. 7 22. 30 Road 2404. 1 2415. 2 2410. 2413.5 3.5 (a) Top of rail. concrete bridge built in 1947. Approach fills block the full 1500 -foot width of the flood plain to a height of 5 to 9 feet. The underclearance at the bridge is at eleva- tion 2243. 3 feet, 2. 3 feet above the crest height for the flood of August 13, 1940. A Maximum Probable Flood would overtop the bridge and road by about five feet. Blue Ridge Road is a loop highway which serves local traffic south of the river as well as the Blue Ridge Assembly area in Johnson Cove. The road crosses Swannanoa River at two points. Miles 19.07 and 20.01. The lower bridge is a two-span steel girder, timber deck bridge with an approach fill on the left bank which averages about 5 feet in height. The upper bridge is of similar con- struction with one span. Approach fills there average about 5 feet high on the right bank and 3 feet high on the left bank. Bridges at both of these sites were washed out in 1940. They were replaced and were overtopped again during the flood of June 1949. 18 Two highway bridges cross Swannanoa River within the corporate limits of Black Mountain. State Highway 9, which carries traffic from Montreat and Black Mountain south to the Lakey Gap area and Bat Cave, crosses at Black Mountain Avenue, Mile 20. 89, This three-span steel and concrete bridge was built in 1950 to replace a smaller bridge which had been overtopped by the floods of 1916, 1940, and 1949. The underclearance at the present bridge is above the crest height reached by all except the 1916 flood, but raising of the approach fills for the new bridge at least partially overcame the advantage of the increased opening under the bridge. The floor of the bridge is at elevation 2341. 5 feet, but overflow of the left approach will start at 2339 feet, when water will overtop Sycamore Street which parallels the river on that bank. A Maximum Probable Flood would overtop the bridge by about Z\ feet. Just upstream from Highway 9, at Mile 20. 96, is the Lakey Street bridge. This is a one-span steel girder, wood floor bridge which replaces a lighter steel girder bridge that was at the site in 1949. The underclearance at the present bridge is about equal to the 1949 flood crest height but lower than the 1940 flood crest. The floor of the bridge is lower than the 1916 flood crest height. A low approach fill crosses the right-bank flood plain. Access to the bridge is affected when overflow of this approach begins at elevation 2339. 8 feet. The flood of July 1916 reached an elevation of 2343. 1 feet at this point. A Maximum Probable Flood would reach an elevation of 2346. 1 feet. Three small bridges upstream from the mouth of Flat Creek serve residences south of the railway. All are low structures within range of any large flood. At Mile 21. 61 is a single-span steel girder bridge built in 1958 by a private real estate developer to provide access to the Burgin Cove section. A private bridge at Mile 22. 11 is of light log and plank construction. The highway bridge at Mile 22. 30 is a steel girder wood floor bridge. The Southern Railway bridge at Mile 17. 58, 100 feet upstream from the U. S. Highway 70 bridge, consists of a heavy steel girder span with an under- clearance elevation of 2240. 8 feet. This is 2. 5 feet lower than the underclearance elevation at the highway bridge. The net opening under the bridge is also less than at the highway bridge. The approach fills parallel those of the highway and are slightly lower. The top of the tracks at the bridge is at elevation 2244. 3 feet and overflow begins at this elevation. A Maximum Probable Flood at this point is expected to have a crest elevation of 2250. 8 feet. 19 The railway bridge which crosses Swannanoa River in Black Mountain at Mile 21. 33, just below the mouth of Flat Creek, has a fill which blocks the flood plain to a height of 13 to 18 feet- The bridge was installed in 1917 to replace one washed out in the 1916 flood. The present opening is wider than at the old bridge but the girders are heavier and deeper, so that the net increase in opening is small. A recurrence of the 191.6 flood under present conditions would probably overtop the bridge and fill. Obstructions to Flood Flow The numerous bridge approaches on the flood plain of Swannanoa River present substantial obstruction to flood flows. These have been discussed pre- viously. Grading for the Kearfott plant at Grovestone has blocked the right-bank flood plain at that point. Other developments which are on the flood plain do not present any significant obstruction to flood flows. FLOOD SITUATION Flood Records There are no records of river stages or discharges available for the Swannanoa River in the immediate Black Mountain vicinity. Records have been maintained for 33 of the last 41 years at a gaging station on the Swannanoa River at Biltmore, North Carolina, near the mouth of the stream and 20 miles downstream f rom Black Mountain. The U. S. Geological Survey made observations on a staff gage at the Biltmore Avenue bridge, at River Mile 1. 58, from December 1, 1920, to September 30, 1926. Since May 8 r 1934, continuous records of streamflow are available from a recording stream gage 100 feet downstream from the bridge. The Geological Survey made observations of streamflow on the Swannanoa River at Swannanoa, 5 miles downstream from Black Mountain, from May 1907 to June 1909 and from January 1926 to December 1931, and at Azalea, 13 miles below Black Mountain, from August 1946 through September 1947. To supplement the record obtained at these gaging stations, local resi- dents were interviewed for information on dates and heights of floods. Files of the Asheville newspapers were searched, as were historical records and docu- ments. Following the flood of June 16, 1949, field investigations were made in 20 TABLE 4 SWANNANOA RIVER AT BILTMORE, NORTH CAROLINA FLOOD CREST ELEVATIONS AND DISCHARGES ABOVE BANKFULL STAGE 1791-1962 This table includes all known floods above bankfull stage of 12 feet at the U. S. Geo- logical Survey gaging station, River Mile 1. 56, just downstream from the Biltmore Avenue bridge. Drainage area is 130 square miles. Gage Heights Peak Discharge Date of Crest Stage Elevation Amount Per Sq. Mile feet feet cfs cfs April 1791 26 2003 40,000 308 August 1796 15 1992 10, 600 82 1810 15 1992 10, 600 82 May 1845 18 1995 16, 400 126 August 1850 13 1990 7, 300 56 August 1852 15 1992 10, 600 82 February 1875 17 1994 14, 400 111 June 17, 1876 15 1992 10, 600 82 March 19, 1899 14 1991 8, 800 68 May 21, 1901 16 1993 12,400 96 December 30, 1901 15 1992 10, 600 82 February 28, 1902 14 1991 8, 800 68 July 10, 1916 14 1991 8, 800 68 July 16, 1916 20. 7 1997. 3 23,000 177 August 16, 1928 18. 7 1995.3 17, 800 137 August 13, 1940 19.00 1995. 58 18, 400 142 August 30, 1940 15. 34 1991. 88 11, 200 86 June 16, 1949 14.65 1991. 23 9, 930 76 August 28, 1949 12.56 1989. 14 6, 760 52 the Black Mountain-Montreat area and the reports on these have supplied valuable information. Flood Stages and Discharges Table 4 lists peak stages and discharges for known floods exceeding bankfull stage of 12 feet at the Geological Survey gage on the Swannanoa River at Biltmore. Table 5 lists the highest 8 floods in order of magnitude. For floods in the period 1920-1926 and from 1934 to date the flood crest stages are those observed at the gage. Stages for floods occurring prior to these records are from high water marks or are estimated from newspaper and historical accounts or from interviews with local residents. PLATE 2 SBONsaanDDO jo «3?wnN 0- ; a *■ 00 >- z O I I- D < > u _) _1 111 UJ =- 10 L. 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Pi o PC rH CD > •rH P£ cd o d Ed c0 r2 CO Ph CD' CD rH o CD Pi o rH 03 > •r— Ph r— ( r^H .5 CD Pj cd r^ rH O PtJ rH r=3 cd W PQ rH CD > ■ rH Ph H rH 5 CD o > CD .h bfl Pi Ph cO * § rH S fa § CD CD rH U rC Q U o Ph > CD P ^; CD CD rH O +j cd i — i Ph rH CD •rH Ph cO cd cd CO Ph Pi CD CD u U cd 03 03 HI u rH cd HP o rH o rH fa o CM CO LO co 00 OS CM CO LO CO OO 50 was experienced on many streams in the region as a result of this rainfall, which totaled up to 15 inches in some areas in the upper Watauga and Swannanoa River basins. The June 14-16. 1949, rainfall was part of a widespread disturbance that produced floods of considerable magnitude throughout much of the southeastern part of the Tennessee Valley, The highest flood of record was experienced on the North Fork Swannanoa River near Black Mountain, North Carolina, where 8, 50 inches of rainfall was recorded in 21 hours. Approximately 2, 5 inches of rain had fallen during the two days prior to this intense burst of rainfall. Storms similar to those that caused these large floods in the region could occur over the Swannanoa River watershed. In that event, floods comparable in magnitude with those experienced on the neighboring streams whose watersheds are similar to that of Swannanoa River could also occur on the Swannanoa River. Based upon the magnitude of storms that have occurred on neighboring streams, a Regional Flood may occur in the vicinity of Black Mountain and Mon- treat about 1 to almost 14 feet higher than the mid-August 1940 flood on the upper Swannanoa River and 2 to 13 feet higher than the June 1949 flood on Flat Creek. Upper Swannanoa River and Flat Creek vs. Regional Flood Discharges Plate 7 is a diagram of the discharges tabulated in Table 8, together with a map showing the locations of the discharge measurements. The upper Swannanoa River watershed, with its headwaters on the Tennessee Valley Divide, lies in the narrow band or spillover area that is subject to heavy, widespread, orographic precipitation. This watershed, therefore, is as susceptible to heavy rainfall as any of the watersheds in this region. The largest five floods observed in this region have occurred on water- sheds which lie in this spillover area. All of these floods resulted from either the tropical hurricanes of July 1916 and August 1940. or the storm of May 1901 which was general over the eastern Tennessee Valley. The fact that these three floods occurred within a 40 -year period emphasizes the fact that floods of these magnitudes cannot be considered as rare events in this region and, therefore, it must be antici- pated that such floods will occur again in the future. PLATE 7 32 Numbers on chart and bars refer to those in Table 8. O Estimated Maximum Probable Flood determined by Flood Control Branch. -4- 28 7 24 =3"' / SW A^N ANOA RIVER, SOUTHERN RWr. BRIDGE AT RIVE R MILE 1 7.58 20 < ~9' z -10- I 2 J2. 73' 4 / 8 Y4 -15- -16- 45 50 55 60 TENNESSEE VALLEY AUTHORITY DIVISION OF WATER CONTROL PLANNING HYDRAULIC DATA BRANCH ^ f LAT C REEK , MONTREAT DAM AT MONTREAT, N.C. 5 10 15 20 25 30 35 DISCHARGE IN THOUSAND CUBIC FEET PER SECOND 40 MAXIMUM KNOWN FLOOD DISCHARGES REGION OF BLACK MTN.AND MONTREAT, N.C. APRIL 1962 51 The experience line on Plate 7 that is used to determine the Regional Flood includes the largest known flood on the Swannanoa River at Black Mountain, that of July 16, 1916. Plate 7 shows that two flood discharges, that of May 1901 on Cane Creek above Bakersfield, North Carolina, and that of August 1940 on the Watauga River near Valle Crucis, North Carolina, exceed this experience line. Particularly heavy rainfall over these two watersheds resulted in discharges that are in the order of a Maximum Probable Flood. For this reason, these two dis- charges, even though they have been experienced in the region, have not been considered in estimating the Regional Flood. Table 9 lists the estimated Regional Flood discharges for the Swan- nanoa River and Flat Creek. TABLE 9 REGIONAL FLOOD PEAK DISCHARGES River Drainage Stream Location Mile Area Discharge sq. mi. cfs Swannanoa River Southern Railway 17.58 22.1 24,500 at Lakey Street 20. 96 11. 2 18, 500 Flat Creek at Montreat Dam 2.49 2.80 7,000 The profiles of the Regional Floods on the Swannanoa River and Flat Creek are shown on Plates 11 and 13. The heights of the Regional Flood, and of the Maximum Probable Flood discussed in Section IV, at several locations along the Swannanoa River and Flat Creek are shown in Figures 11 through 17, which present the pictures in upstream order. 52 Figure 11. — GENERAL PRECISION, INC. , PLANT AT GROVESTONE The Kearfott Division of this corporation is located just below the U. S. Highway 70 bridge over the Swannanoa River. The factory is above all floods considered in this report, but the parking lot would be inundated by the Regional and Maximum Probable Floods to the extent shown. The June 1949 flood elevation was 3. 9 feet lower than the base of rod. Figure 12. —RADIO STATION WFGW AT OLD NICHOLS AHIPORT The building at right, under construction for the studio and transmitter, is 4 feet above the Maximum Probable Flood, but the tower in the foreground is in the flood plain as shown by the arrows on the rod. The view is northeast at Mile 18. 36. 5G Figure 13. —FLOOD HEIGHTS ALONG BLACK MOUNTAIN AVENUE Upper view is south along the avenue, which is State Highway 9 and crosses the Swan- nanoa River at Mile 20. 9. The barn at left is at the site of a barn damaged in the 1916 flood and shown in Figure 5. The bridge and fill were built in 1950. Lower view shows Huggins Chapel, which lies in the left-bank flood plain on the east side of the avenue. The floor was flooded 1 foot deep August 13, 1940, and 0. 4 foot in June 1949. Arrows show the Maximum Probable and Regional Flood heights in both views. The drop through the bridge in these floods would be 3. 9 feet; the arrows refer to the height on the upstream side. 54 Figure 14. —FLOOD HEIGHTS IN VICINITY OF LAKEY STREET BRIDGE Upper view is the Church of God on Lakey Street, which was surrounded by flood water in June 1949. In 1916 the flood height was about 2 feet above the floor level (base of rod). Lower view shows the plant area of the Gulf oil distributor between the church and the river, at Mile 21.0. The Regional and Maximum Probable Flood heights are shown by arrows; note that the difference between them is only 0. 4 foot. 55- MAX. PROB. FLOOD REGIONAL FLOOD J Figure 15. — FLOOD HEIGHTS NEAR MOUTH OF FLAT CREEK, BLACK MOUNTAIN Top view is west along U. S. Highway 70, State Street. The road is over the Flat Creek culvert. Middle view shows the Black Mountain Elementary School, formerly the high school, on Flat Creek Road on left bank, near Mile 0. 2. Bottom view shows the studio and transmitter building of Station WBMT on left bank at Mile 0. 24. Ar- rows show heights of the Maximum Probable and Regional Floods on Flat Creek, which differ from each other in each case by 0. 7 foot. 56 Figure 16. —FLOOD HEIGHTS AT PADGETT TOWN ROAD The Fred Miller Grocery is at the southwest corner of Padgett Town and Flat Creek Roads, at Mile 0. 98 on left bank. The June 1949 flood was 0. 8 foot lower than the floor level. Arrows show heights of the Regional and Maximum Probable Floods. Figure 17. —FLOOD HEIGHTS AT PINE LODGE COTTAGES This development is on the right bank of Flat Creek at Mile 1. 18. The June 1949 flood height was 0. 9 foot lower than the base of the rod that stands at the porch corner. Arrows show heights of the Regional and Maximum Probable Floods. IV. MAXIMUM PROBABLE FLOODS IV. MAXIMUM PROBABLE FLOODS This section discusses the Maximum Probable Floods on the upper Swannanoa River and Flat Creek and some of the hazards of great floods. Floods of the magnitude of the Maximum Probable are the kind considered in planning, construction, and operation of protective works, the failure of which might be disastrous. They represent reasonable upper limits of expected flooding. The two streams involved in this study— upper Swannanoa River and Flat Creek--have drainage areas of 22. 2 and 6. 05 square miles respectively at the downstream limits of the study. At the upstream limits the corresponding drainage areas are 3. 3 and 2 7 square miles. Extreme floods on these streams are most likely to result from either of two types of storms --intense periods of rainfall during winter storms of fairly long duration, or short-duration storms of the cloudburst or hurricane type usually occurring during summer. Infiltration and other losses are generally low in winter and generally high in summer. DETERMINATION OF FLOOD DISCHARGES To determine the Maximum Probable Floods on the upper Swannanoa River and Flat Creek in the vicinity of Black Mountain and Montreat, consideration was given to great storms and floods that have already occurred on this watershed and to those which have occurred elsewhere but could have occurred on these areas. This procedure provides information about possible floods and storms additional to that which can be gained from the short-term local hydrologic records alone. The maximum known flood on the Swannanoa River in the vicinity of Black Mountain occurred in July 1916. The peak discharge at the Lakey Street bridge, Mile 20. 96, is estimated to have been 17,000 cubic feet per second. On Flat Creek the maximum known flood also occurred in July 1916. The peak dis- (1) Prepared by Flood Control Branch. "3 charge at Montreat Dam is estimated to have been 4, 000 cubic feet per second. It is reasonable to expect that greater floods will occur on these streams. Observed Storms Observed storms are meteorologically transposable to the Black Mountain and Montreat area from within a broad region extending generally from the Atlantic Ocean to the Appalachian Divide and from Florida through Pennsylvania. The moisture source for storms in this region is the warm, moist air flowing north- ward from the tropical Atlantic Ocean. In general, the moisture potential for a given region decreases with its increased distance from the moisture source. When transferring storms within the broad region to the Black Mountain and Montreat area, appropriate adjustments were made for differences in this moisture potential. Table 10 lists known rainfall depths for several large storms trans- posable to these watersheds. TABLE 10 SELECTED MAXIMUM OBSERVED STORMS TRANSPOSABLE TO THE REGION OF BLACK MOUNTAIN AND MONTREAT, NORTH CAROLINA Date Location Duration hours Area sq. mi. Rainfall Depth inches June 1937 Ivy, No C. 1.5 4.0 6. 25 July 1938 Hayesville, N. C. 1 4.0 6.0 July 1916 North Carolina 6 54 7. 8 September 1940 New Jersey 6 54 18.4 October 1941 Florida 6 54 11. 2 September 1957 Barnardsville, N. C. 2 3.56 5.5 July 1960 Grizzle Creek, Ga. 3 Point 12.5 June 1961 Mitchell County, N. C. 2.5 3.49 8.5 Upon the basis of these and other data, as adjusted for conditions in the watersheds, a rainstorm of 16. 4 inches in 4 hours was adopted for computing the Swannanoa River Maximum Probable Flood at the lower limit of the study and 16. 6 inches in 3 hours for the Flat Creek Maximum Probable Flood at the lower limit of the study . Storms greater than these can occur., From a meteorological stand- point, storms as much as 40 percent greater than those adopted to compute the Maximum Probable Floods are possible. Observed Floods Factors such as the meteorology of the region and flood-producing characteristics of the watershed were given consideration in determining whether peak discharges on other streams are applicable. Tables 8 and 11 list peak dis- charges for observed floods on several streams of approximately the size of the Swannanoa River and Flat Creek. For comparison, the discharges of the maxi- mum known floods on these streams are listed. TABLE 11 SELECTED MAXIMUM OBSERVED FLOODS IN VICIN ITY OF BLACK MOUNTA IN AND MONTREAT, NORTH CAROLINA Peak Discharge Drainage Per Stream Location Area Date Amount Sq. M sq. mi. cfs cfs Big Creek Sunburst., N. C. 1. 32 1940 12, 900 9, 800 Big Creek Sunburst, N. C. 1. 69 1940 12, 400 7, 340 Dutch Creek Valle Crucis, N. C. 2.42 1940 9, 200 3, 800 N. Fk. Catawba R. Asheford, N. C. 5. 20 1940 15,000 2, 900 Pigeon River Spruce, N. C. 8.4 1940 16,400 1, 950 N. Fk. Catawba R. Wood lawn, N. C, 41. 8 1940 55,000 1, 320 Elk Creek Elkville, N. C. 50.0 1940 70,000 1,400 Watauga River Sugar Grove, N. C. 55. 1 1940 41,000 745 Linville River Branch, N. C. 65.0 1940 39,500 608 Wilson Creek Adako, N. C. 66.0 1940 99,000 1, 500 Swannanoa River Black Mountain 11. 2 1916 17,000 1, 520 Flat Creek Montreat Dam 2. 80 1916 4,000 1, 430 Maximum Probable Flood Discharges From consideration of the flood discharges in Table 11 and of the trans- position to the Black Mountain and Montreat area of outstanding storms, the peak 60 discharge of the Swannanoa River Maximum Probable Flood at the Southern Rail- way bridge near the lower limit of the study was determined to be 28 , 000 cubic feet per second and at Lakey Street, 21, 200 cubic feet per second. The Flat Creek Maximum Probable Flood was determined to be 13, 000 cubic feet per second at the mouth and 10, 000 cubic feet per second at Montreat Dam. Frequency It is not possible to assign a probability of occurrence or frequency to the Maximum Probable Flood. The occurrence of such a flood would be a rare event; however, it could occur in any year. Possible Larger Floods Floods larger than the Maximum Probable are hydrologically possible; however, the combination of factors that would be necessary to produce such floods would seldom occur. The consideration of floods of this magnitude is of greater importance in some problems than in others but should not be overlooked in the study of any problem. HAZARDS OF GREAT FLOODS The amount and extent of damage caused by any flood depend in general upon how much area is flooded, the height of flooding, the velocity of flow, and the rate of rise. Areas Flooded and Heights of Flooding The areas flooded by the Maximum Probable Floods and maximum known floods are shown on Plates 8, 9, 10, and 12. Depths of flow can be esti- mated from the crest profiles which are shown on Plates 11 and 13. The Maximum Probable Flood profile on the Swannanoa River is from 2 to 14 feet higher than elevations experienced in the August 13, 1940, flood. The maximum difference occurs just upstream from the Southern Railway bridge at Mile 21. 33 and is the result of heading-up due to the small opening through the large railroad fill. On Flat Creek the Maximum Probable Flood profile is 3 to 61 15 feet higher than elevations experienced in the June 16, 1949, flood. The maxi- mum difference occurs at the road bridge at Mile 2, 10 „ The elevations shown on Plates 11 and 13 and the overflow areas shown on Plates 8, 9, 10, and 12 have been determined with an accuracy consistent with the purposes of this study and the accuracy of the basic data. More precision would require costly surveys not warranted by this study. Figures 11 to 17 on pages 52 to 56 show the height that would be reached by the Maximum Probable Flood at several locations in the vicinity of Black Mountain and Montreat. Velocities and Rates of Rise Water velocities during the Maximum Probable Flood depend largely upon the size and shape of the cross section, the condition of the stream, and the bed slope, all of which vary on the different streams and at different locations on the same stream. During the Maximum Probable Flood, velocities in the main channel of the Swannanoa River in the vicinity of Black Mountain and Montreat would range from more than 4 to 15 feet per second. In the overflow area, velocities would be about 1 to 9 feet per second. The Maximum Probable Flood on the Swannanoa River at Mile 21. 14, between Lakey Street and the Southern Railway bridge, would rise about 10 feet above low water to its crest stage in about 3 hours with a maximum rate of rise of about 4. 5 feet in 1 hour. On Flat Creek during the Maximum Probable Flood, velocities in the main channel would range from about 3 to 19 feet per second. In the overflow area, velocities would range from about 1 to 6 feet per second. The Maximum Probable Flood on Flat Creek near the mouth below U, S. Highway 70 bridge would rise about 15 feet above low water to its crest stage in about 3^ hours with a maximum rate of rise of 5 feet in one-half hour. Immediately above the bridge at Mile 2,40, Flat Creek would rise about 14 feet above low water to its crest stage in about If hours with a maximum rate of rise of 10 feet in one-half hour. These rapid rates of rise and high stream velocities in combination with deep flooding would create a hazardous situation in developed areas. PLATE TENNESSEE VALLEY AUTHORITY DIVISION OF WATER CONTROL PLANNING HYDRAULIC DATA BRANCH HIGH WATER PROFILES SWANNANOA RIVER VICINITY OF BLACK MOUNTAIN, N.C. APRIL 1962 19 20 MILES ABOVE MOUTH 2525 PLATE 13 MILES ABOVE MOUTH PLATE 14 2400 2390 2380 2370 * / «• f IDs / j — 2360 2350 2340 400 800 SECTION 17-MILE 21.80 2330 to / % 2 1- 1 1 if 21 y- j / i— -ance 'ound LEGEND: Maximum Probable Flood Regional Flood "August 13,1940 flood on Swannanoa River. June 16,1949 flood on Flat Creek. Sections taken looking downstream. 15 sections on Swannanoa River not shown. 17 sections on Flat Creek not shown. Elevations of Maximum Probable and Regional Floods computed by Flood Control Branch. 800 400 400 800 SECTION 13- MILE 20.89 2290 2280 2270 2260 r 1 * < s, * — *- 2260 2250 2240 2230 — Top c f Rail wy 70 . c leer an c atural tresm f tuthern r round t up Si rom 8 00 400 400 800 SECTION 7 -MILE 18.62 1200 400 400 800 1200 SECTION 3&3A-MILES 17.56 8.17.58 SWANNANOA RIVER 2660 2650 2640 2630 .8 1 6 «, 1 F/c Cleans or • J nee ■ 2550 2540 2530 2520 2450 2440 2430 2420 -A * ' Creei or A - - ^ VH j~ — -Clearance 400 200 SECTION 17 - MILE 2.44 200 400 SECTION II- MILE I. 78 200 200 400 SECTION 6- MILE 0.98 600 2370 2360 2350 — > TENNESSEE VALLEY AUTHORITY DIVISION OF WATER CONTROL PLANNING HYDRAULIC DATA BRANCH 600 400 200 200 400 600 SECTION I - MILE 0.02 HORIZONTAL DISTANCE IN FEET FLAT CREEK CROSS SECTIONS SWANNANOA RIVER AND FLAT CREEK VICINITY OF BLACK MOUNTAIN AND MONTREAT, N. C. APRIL 1962