(e) Evacuation of the overflow area. Of these five methods, diversion is clearly inapplicable because of topographic characteristics of the basin. Evacuation would reduce flood losses materially only at Galax, but is impracticable there since the flood plain is the only area suited for the local industries, which are of major economic importance to the town and basin. 43. Protection of both the town of Galax and the farm lands along Chestnut Creek could be afforded by construction of a flood-control dam and reservoir, but the cost would be prohibitive. Studies of United States Geological Survey topographic maps and a thorough field reconnaissance determined that the most favorable flood-control reservoir project in the Chestnut Creek Basin would consist of one with a dam located on Chestnut Creek 3.5 miles above Galax. Pertinent data on the dam and reservoir which preliminary studies indicated would be best suited to this site are summarized below: Location of dam: On Chestnut Creek, 16.5 miles above the mouth. Reservoir: Capacity: 13,700 acre-feet (8 inches of run-off). Maximum pool elevation: 2,436 feet mean sea level. Dam: Type: Concrete gravity, with uncontrolled overflow spillway. Construction cost. Total Annual costs.. $1,450, 000 25, 000 1, 700, 000 70,000 The construction of this dam and reservoir would afford complete protection from Chestnut Creek floods to the town of Galax and would protect the farm lands along Chestnut Creek below the dam site, which is just upstream from the mouth of Wards Mill Branch. However, the estimated average annual benefits which would accrue from this protection amount to only $17,000 and the cost-benefit ratio is unfavorable in the relation of 1 to 0.2. Additional preliminary studies of the possibilities of combining storage for power development or beneficial increase in low flow with the needed storage for flood control indicated that no more favorable project, from the economic viewpoint, could be developed than that considered for flood control alone. 44. Protection of most of the flood-plain area along Chestnut Creek by means of levees or channel improvement, or a combination of the two methods, would be practicable from an engineering standpoint, but the costs would be so high in comparison with the benefits from such protection, except at Galax, that there clearly could be no economic justification for such protection to any area outside of Galax by these methods at the present time. In Galax the situation is more avorable, and detailed investigations of protection of that town have been made. PLAN OF IMPROVEMENT flow l the le exava tre 45. General. As a result of preliminary investigations, the finding only and can during the rails of schedul protection 9-foot th The locati of tow 46. Galax, Va., local protection project. The local protection project area behind 1 Not printed. 48. C the cons adequa downstre highway of Highw and the Nos. 58 al with adeq Stucted br a small tw end ard 670 ac these areas watercourse Wastes are The largest ettering the mended) s 47. An earth levee on each bank with grade lines 2 feet above the flow line of the "design flood," corresponding to a discharge of 15,000 cubic feet per second in the new channel, is proposed from the upstream end of the project down to the United States highway bridge, routes 58 and 221. The total length of the levees is 6,200 feet along the left bank and 4,460 feet along the right bank. The levees are to be set back 10 feet from the top of the slope of the improved channel. Levee heights generally average about 6 feet except in the downstream 1,500 feet where the heights will increase to a maximum of 9 feet. The levees will have side slopes of 3 horizontal to 1 vertical and a minimum crown width of 5 feet. The earth levees will be constructed of dumped fill, with materials being obtained from the new channel excavation. Levee widths will be increased as necessary for economical disposal of materials excavated from the channel. A rock-filled toe trench is to be provided back of the levee in order to intercept seepage and to prevent damage due to piping and sand boils. One opening will be needed through the levee, where the single-tracked Norfolk & Western Railway passes through the left-bank levee at a point directly below the United States highway bridge. Here a closure will only be needed during floods greater than the maximum of record, and can be effected with sandbags. The maximum height required during the "design flood" would be but 2 feet above the bottom of the rails. 48. Construction of the proposed channel and levees will necessitate the construction of a new railroad bridge to replace the present inadequate plate-girder railroad bridge (No. 1557) located 600 feet downstream from the lower corporation line; the relocation of a small highway bridge near the upper end of the proposed project; the abandonment of a low water bridge at Webster Street; minor sewer and drainage alterations; and the construction of two small sump pump installations. New bridges of adequate vertical and horizontal dimensions are now being constructed by the Virginia Department of Highways to replace the present highway bridge at Oldtown Street and the one at the lower end of town which carries U. S. Highways Nos. 58 and 221 across Chestnut Creek. Completion of these bridges is scheduled for July 1941. These two structures, as reconstructed with adequate waterway areas, will be in harmony with the proposed protection project. It is planned to replace the present railroad bridge below the town by a structure having three 50-foot and one 90-foot through plate-girder spans, with low steel at elevation 2340. The location of this bridge and an elevation of the proposed reconstructed bridge are shown on exhibit No. 10. It is planned to relocate a small two-span light steel stringer highway bridge at the upper end of town a short distance upstream from its present location to beyond the upper project limits. 49. A total of 1,430 acres now drains to Chestnut Creek from the area behind the levees, 760 acres being back of the left-bank levee and 670 acres back of the right bank levee. Storm drainage from these areas is carried to Chestnut Creek by a number of small natural watercourses and drainage ditches, while sanitary sewage and industrial wastes are discharged into the creek by a number of small sewers. The largest of the sanitary sewers, a 12-inch vitrified clay trunk sewer entering the creek from the left near the foot of Grayson Street (extended) serves the sewered portion of Galax upstream from Center slo this r Tot Street. The total population served by this sewer is approximate arb 1,000. The next most important sewers are a 6-inch vitrified clature sanitary sewer which empties into the creek just downstream from are fa Webster Street and an 8-inch vitrified clay sanitary sewer which ater discharges a short distance below the foot of Virginia Street. The two sewers, which drain into the creek from the left, serve the remainder e requ of the sewered area in Galax, an area also having a population stimat of approximately 1,000. In addition, a number of small industrie esti drains, mostly of 6-inch diameter, enter the creek from both sides, #ensing shown on exhibit No. 10. It is proposed to install gated drainage amage culverts under the levee at the point where each important storm $42,00 drainage ditch enters the proposed channel, each culvert having 52.7 sufficient discharge capacity to handle the peak storm run-off to be pro expected from its tributary area. The locations of these storm water relimin outlets through the levees are shown on exhibit No. 10. The instal lation of storm-water pumps is not considered warranted, since studis struct indicate very little likelihood of rainfall in Galax coincidental with fights-ofdamaging stages in the new channel. Closing of the gates on the storm drains would only be necessary when channel discharges exced approximately 8,000 cubic feet per second, or only about once in The esti every 15 or 20 years, and then only for 4 to 8 hours; and studies of yea available data indicate that on these rare occasions insufficient prend main cipitation will occur in Galax coincidentally with the highest stags on on to cause any damages due to a lack of storm-water pumps. Inve tigations of the effects of the closing of gates on the sanitary sewer and industrial drains during floods determined that pumping insta lations would be necessary only in the cases of the three princip. Th sanitary sewers. Small pumping installations are necessary to hand at all f the flow from these three sewers to prevent undesirable overflow of ence of a sanitary sewage in the lower areas of the town during floods. It is flood proposed to gate or close off the smaller private drains; to provide gates and a sump pump installation of 300 gallons per minute capacityars. In (based on estimated ultimate peak sanitary flow) at the 12-ind sanitary sewer near the foot of Oldtown Street; to construct a short extension joining the 6-inch sewer at the foot of Webster Street to the 8-inch sewer at the foot of Virginia Street; and to provide another sump pump installation of 300 gallons per minute capacity (estimated ultimate peak sanitary flow) near the foot of Virginia Street to handle the discharge of the two sewers during floods. econd, is the stag amage cu uency dia hich wou. ESTIMATES OF FIRST COSTS AND ANNUAL CHARGES pancy hich the p 50. Construction costs.-Estimates of construction costs of thesed on t Galax local protection project, based on the preliminary design features discussed in the foregoing paragraphs and indicated by exhibits Nos. 10 and 11,' are shown on exhibit No. 12. The total estimated cost. No in 18 of the project as herein proposed, exclusive of rights-of-way costs, $280.000. ood-dama redited w flood com dicated E T route considered by local interests in connection with their planning for application for a Work Projects Administration project (see par. 39 and 48). The only improvements which will be affected, other 1 Not printed. reased land Na printed. industrial, residential, and agricultural lands, along the same generaler, the pr roject. ect. ods as C amage pre pments in 51. Rights-of-way and damages. The proposed channel and leveest provide follow generally the course of the present channel through undeveloped her dev than bridges and sewer outlets, which are included as construction bat features of the project, will be four inexpensive dwellings, one furniture factory storage building, a small pump house at the town's water plant, and a water intake at the milk-condensing plant. A total of 35 acres of land in addition to the present channel area will be required. The total estimated value of this land is $17,000. The estimated cost of relocation of the affected buildings is $8,000 and berds the estimated cost of adjustments at the water plant and milk-condensing plant is $2,000. The total estimated costs of rights-of-way and wag damages, including an estimated $15,000 cost of property acquisition, is $42,000. 52. Total costs and annual charges.-The estimated total costs of the proposed Galax project, based on surveys of the affected area, preliminary field appraisals, and preliminary design features presented in this report, are summarized below: Construction costs.-- $280,000 42, 000 Total project costs---- 322, 000 The estimated annual cost of the project, based on an economic life of 50 years and including an estimated $500 annual cost of operation and maintenance, is $13,060. Computation of the annual cost is shown on exhibit No. 13.1 ESTIMATE OF AVERAGE ANNUAL BENEFITS 53. The proposed local protection project would completely eliminate all flood damages in Galax which would result from the occurrence of any Chestnut Creek flood equal to or smaller than the "design flood," which, with a peak discharge of 15,000 cubic feet per second, is 36 percent greater than the maximum flood in the past 60 years. In estimating the value of this protection, use has been made of the stage-frequency relationship derived in paragraph 32, and the food-damage curve shown on exhibit No. 8. The project has been credited with an assurance factor by assuming a single occurrence of a flood comparable to the "design flood" in place of the maximum flood indicated by the frequency studies in the 50-year economic life of the project. Using the "design flood" as the greatest flood, and other floods as derived in the frequency studies, together with the flooddamage curves, it has been possible to prepare a flood-damage-frequency diagram (see exhibit No. 8) and estimate the total damages which would be prevented during the economic life of the project. Based on the assumption of continued present status of flood-plain Occupancy and reduced to an annual basis, the total annual damages which the project would prevent are estimated to be $14,400. 1 54. No increase in the average annual value of the project by flooddamage prevention has been made to allow for possible future developments in the flooded area which might occur if flood protection is not provided, since the available data indicates little likelihood of further developments unless the flood situation is remedied. However, the project would have some additional value as a result of increased land values if protection is provided. Analyses of present 1 Not printed. |