TABLE II.-United States Weather Bureau precipitation stations in and adjacent to 96894-44- 3 Basin (miles) 2232 12 Period of 1940-41 1905-40 1894-1913 33 1872-87 21 1873-1940 44 1927-40 34 1903-40 47 1896-1940 29 1913-40 39 1933-40 13 1903-40 21 1934-40 37 1923-30 29 1921-40 1 Stations outside Kanawha River drainage basin. 22. The records for the stations in this general area indicate that the annual precipitation over the Chestnut Creek Basin generally amounts to between 35 and 47 inches with an average of about 41 inches. The minimum annual precipitation over the Chestnut Creek Basin is probably about the weighted mean minimum for all stations, or 26.9 inches. The maximum annual precipitation in the basin is believed to be at least equal to that of the weighted mean maximum for all Kanawha Basin stations listed in table II, that is, 59.82 inches. Practically all of the precipitation is in the form of rain, the average snowfall being only about 17 inches (unmelted). The maximum, so far as available records of nearby stations show, does not exceed 36 inches although this depth may be exceeded in the highest elevations. 23. The maximum, minimum, and mean monthly precipitation for the six stations listed in table II which are located in the Kanawha Basin, together with the weighted means for the group, are shown in table III. Data for these stations indicate that the monthly rainfall over the Chestnut Creek area generally varies between 2 and 6 inches, with extremes of 23.51 inches at Jefferson in August 1940 and 0.00 inch at Ivanhoe in March 1916. The weighted means for the six stations indicate average monthly rainfall of 3.03 inches in winter, 3.45 inches in spring, 4.35 inches in summer, and 2.86 inches in the fall. TABLE III.-Monthly precipitation, U. S. Weather Bureau Stations in Kanawha Basin, surrounding and nearest Chestnut Creek Basin Jefferson continuous Fince the rict in Feb Mean Min. Max. Mean 885 3.91 4.39 4. 42 5. 43 3.76 1.01 84 .78 1. 46 1. 12 1. 62 .92 recorder on Basin prior 26. Ther he summer Wytheville 62-year record Oss rates of 9.03 8.47 5. 51 7.65 may equal dicate the ummer or recipitatio ribute to eneral in te winter are occurr 3. 48 4. 24 4. 27 4. 45 Max. Mean Min. Max. Mean Min. Max. Mean Min. Max. Mean Min. Though he se swiftly ainfall fro 0 ands, are Buring the Streams. Hoods on to cause rea, the rossing 1.20 ΟΙ 09 1.20 10 mountain ose the n mean an 2.44 3. 17 3.60 4.81 the New storms a 2. 16 59 ropical Intense Fastward Mean Min. 3.21 2.94 3. 54 3. 18 3.63 57 1.27 67 24. Types of storms.-The Chestnut Creek Basin lies in the paths of storms which, originating in two widely separated regions, generally travel in fairly well defined directions at approximately right angles to each other. The major portion of the precipitation in the watershed occurs as the result of storms that regularly move north eastward from the central and western Gulf region, but the most intense rainfall in the area results from storms accompanying major tropical disturbances which sweep inland over the south Atlantic coast and move northwesterly toward the Blue Ridge Mountains. The chief centers of precipitation of these latter hurricane-type storms are in the mountains of western North Carolina, just outside the New River watershed, in a region that has one of the greatest mean annual rainfalls in the United States. Although these storms lose the major portion of their moisture along the eastern and southern mountain slopes, and the intensity of the rainfall abates rapidly in crossing the mountains into the New River and Chestnut Creek area, the accompanying rainfall over these watersheds, is still sufficient to cause extreme floods, and has resulted in practically all the major floods on Chestnut Creek, as well as the other New River headwater streams. These hurricane-type storms usually occur in July, August, and September, which accounts for the high average precipitation during these months. Winter and spring storms, usually most severe in the Ohio Valley, rarely cause floods of major proportions on Chestnut Creek or other New River headwater streams. The most important storms of record affecting Chestnut Creek and the "design storm" are discussed in paragraphs 29 to 31, inclusive. 25. Run-off. The rolling to mountainous nature of the topography, together with the large areas of cut-over timber and pasture. lands, are conducive to high rates of run-off. As a result excess rainfall from intense storms collects rapidly in the streams, which rise swiftly and, because of their steep slopes, as swiftly subside. Though heavy snowfall in any considerable portion of the basin is of rare occurrence, melting snow does contribute occasionally to moderate winter and early spring floods. Freezing of the ground, quite general in winter, and a lack of vegetation in these months also contribute to an increase in the run-off rate. However, due to higher precipitation rates, the greatest peak discharges are experienced in the summer or fall. Rainfall and stream flow data for nearby basins indicate that the maximum ratio of volume of run-off to total rainfall may equal 100 percent during the winter months, and that infiltration loss rates of as low as 0.05 inches per hour may be experienced during the summer and fall months. 26. There had been no stream-gaging station in the Chestnut Creek Basin prior to the temporary installation of a portable water-stage recorder on Chestnut Creek at Galax by the Huntington Engineer District in February 1941 in conjunction with the present investigation. Since the establishment of this temporary station there has been a continuous period of unusually low flows and no significant rises in the stream have occurred. It has therefore been necessary to approximate the stream discharges from the records of other gaging stations in this same general area and to compute flood hydrographs and estimate peak discharges by use of snythetic unit-graphs and field peak flow determinations. From records for adjacent basins, the mean run-off per square mile in the basin is estimated to be approximately 1.25 cubic feet per second and the mean discharge of the stream about 75 cubic feet per second. Immediately after the flood of August The ma Chestnu higher t this poi thị 1916, thi mered in erly dire North C rapidly maintain 27. General flood situation.-The Chestnut Creek Basin is subjet to extreme floods, caused principally by storms of the West Indies hurricane type, and to more frequent lesser floods which result from both the northerly moving tropical disturbances and the geneal Ohio and Mississippi Valley storms. While no gage records are available, data obtained from the residents of Galax indicate that Chestnut Creek exceeds bankfull stage in that vicinity on an average of once a year. About 10 destructive floods are stated to have curred since 1878, and there are records of extreme floods in September 1878, May 1901, July 1916, and August 1940, all coincidental with extreme floods on upper New River, which caused widespread destru tions to crops and improvements existing at the time of the floods. Valley occupancy, while it has probably contributed slightly to accelerated run-off through clearing of the land, has not otherwise materially added to the flood problem except in Galax, where bridge constrictions have increased flood heights. There are no lakes or was 18. watersh estimat The cre discharg amounts rainfall f " N.C.. 1 N.C.. al New Riv Creek, m Chestnut graph wa besin by drainage storage reservoirs in the basin to reduce flood flows and the step for this m gradients of the headwater streams minimize the effects of valley to 28. Flood records.-From statements of residents of Galax, the it of discharge 31. Des area it is 1916, or 1 result of storms th that as he mountain on the nor that consi Chestnut heaviest p barrier in t Occurred in Occurred in eastward s precipitatio over the ri generally s able to test Conditions at Transon Not printed The maximum rainfall recorded on the western slope of the mountains was 18.05 inches in 3 days at Transon, N. C., located in the New River watershed about 30 miles southwest of Chestnut Creek drainage basin. The average total rainfall over the Chestnut Creek Basin is estimated to have been 7 inches for the period July 14 to 18, inclusive. The crest stage at Galax was about 12.8 feet and the approximate peak discharge at that point 7,800 cubic feet per second. An isohyetal of the storm is included as exhibit No. 3.1 map 30. Flood of August 14, 1940.-The flood of August 14, 1940, was one of the two highest, if not the highest, of definite record in the Chestnut Creek Basin. The stage at Galax, Va., was about 2 feet higher than the July 1916 flood with an estimated peak discharge at this point of 11,000 cubic feet per second. Like the flood of July 1916, this flood was also caused by a major tropical disturbance which moved inland over the South Atlantic Coast and progressed in a northerly direction to center over extreme southwestern Virginia and central North Carolina. Normally, storms of this type dissipate energy rapidly when moving inland. However, this storm apparently maintained its intensity at higher levels, since it also deposited large amounts of rain on the western slopes of the mountains. Excessive rainfall fell during August 12 to 15 with 11.52 inches at Laurel Springs, N. C., 12.47 inches at Jefferson, N. C., and 10.68 inches at Helton, N. C., all of which are on the western slope of the mountains in the New River watershed. The isohyetal map for the vicinity of Chestnut Creek, mass precipitation curves, and the computed hydrograph of Chestnut Creek at Galax are shown on exhibit No. 4.1 The hydrograph was computed from the interpolated average rainfall over the basin by use of the synthetic distribution graph, developed for the drainage basin. The peak discharge of the hydrograph computed by this metod has been modified to agree with that estimated by peak discharge determinations using field data (see par. 26). 31. Design flood.-From studies of storms and floods in this general area it is apparent that somewhat greater floods than those of 1878, 1916, or 1940 can reasonably be expected on Chestnut Creek as the result of the inland movement of the same sort of hurricane-type storms that resulted in these floods. Although it appears unlikely that as heavy precipitation as that experienced on the southeastern mountain slopes just outside the New River watershed would occur on the northwestern slopes, records for the 1916 and 1940 storms show that considerably heavier rainfall than that experienced over the Chestnut Creek Basin may be expected from similar storms. The heaviest precipitation recorded to the north and west of the mountain barrier in the vicinity of the Chestnut Creek Basin during these storms occurred in July 1916 at Transon, N. C., where 18.05 inches of rain occurred in a 3-day period. There is no apparent reason why a slight eastward shift in the storm path would not have caused the 18-inch precipitation center at Transon to move 30 miles northeast to a point over the ridge at the head of Chestnut Creek, since the topography is generally similar in both areas. For this reason, and since it is desirable to test the design of flood-control works for the worst flood which conditions indicate may reasonably be expected, the 1916 storm center at Transon has been shifted to the headwaters of Chestnut Creek, as 1 Not printed. |