and the diversion would have to be limited to about 2,000 cubic feet per second. 187. A number of minor items that would tend to change the above relationship of costs and benefits could be included, but, because of their uncertainty, would not be conclusive one way or the other. On the one hand, there is the value of increased secondary power at possible hydro plants in the river section and the value of additional secondary power which could be generated at Pickwick and Gilbertsville from the excess installations at those two plants caused by the diversion of a portion of the primary flow; on the other hand, there is the possibility of greater cost of additional transmission lines for the canal section plants, since many of the Government-owned transmission lines are already constructed in the Tennessee and Tombigbee Valleys. There is also the possibility of using higher lifts for the proposed locks in the canal section, thus decreasing the cost of power facilities. However, this does not appear to be justified as the increased costs of foundations and canal excavation and increased flowage damages would, it is believed, offset any possible savings. 188. Although the diversion of about 2,000 cubic feet per second appears to be practical and would possibly give some additional benefits, the margin of justification is so small that the installation of power facilities has not been included in the proposed plan of improvement. 189. Development of power for operation of locks. For estimating purposes, it was assumed that electrical energy supplied from commercial sources would be used for operating the lock machinery. Alternate sources of power which would be suitable are gasoline or Diesel engines and hydraulic turbines. To determine which source of power would be the most economical requires a very careful and detailed study of local conditions. Before actual construction such studies should be made, but they were not considered warranted for this report. Should further studies indicate that the installation of hydraulic turbines in connection with the lock structures for the operation of the locks would be the most economical, it would be necessary to divert about 10 percent more water from the Tennessee River than given in table No. 26, page 72. FLOOD CONTROL 190. The flood-control problem.-Local interests urging the improvement have stressed the benefits that might accrue from the control of floods. It was suggested that among the purposes to be accomplished by this improvement was the control of floods on the Tombigbee River, and that by the diversion of the flood waters of the Tennessee River into the Tombigbee Valley flood peaks on the Tennessee and Mississippi Rivers would be reduced. In view of the emphasis placed upon the control of floods, careful consideration was given to the suggested plan and to any other plan that might logically be considered for obtaining the desired results. In analyzing the problem, the alleviation of flood damages in the Tombigbee River Valley_was considered separately from the proposed diversion scheme. Each subject will be discussed separately. 191. Flood protection, Tombigbee Valley.-Above Demopolis, flood damages are confined to a flood plain varying from about 6 miles wide near Demopolis to about 2 miles wide near the source of the river. For practically its entire length the Tombigbee River is subject to overflows resulting from excessive rains. The flood characteristics vary with the territory through which the stream flows. For the purpose of this discussion, the valley is divided into two areas: (a) above Aberdeen, Miss.; and (b) between Aberdeen, Miss., and Demopolis, Ala. (a) Above Aberdeen, Miss.-The numerous headwater tributaries of the Tombigbee River drain a fan-shaped area in Prentiss and Itawamba Counties. The East Fork of the Tombigbee River is formed by the confluence of two of these tributaries, Browns and Mackeys Creeks. From the confluence of these streams to Aberdeen, a distance of 82 river-miles, the channel is narrow and tortuous, meandering through a flood plain 4 to 6 miles in width. The flood plain in this section is flanked by low hills. The banks of the stream are relatively low and the alluvial flood plain is especially susceptible to overflow following excessive rains. Along many of the tributaries, particularly in Itawamba and Monroe Counties, Miss., extensive drainage districts have been established for the improvement of bottom lands and large drainage ditches have been constructed. As a result, a very large percentage of the tillable bottom lands have been placed in cultivation in the areas drained by ditches. These ditches during the floods have transported large amounts of materials and deposited them in the bed of the Tombigbee River. The accumulation of these deposits in the main channel has attained such proportions that the discharge capacity of the stream has been greatly reduced. The combination of the reduced discharge capacity of the main stream and quick run-off resulting from the drainage ditches has caused frequent floods with comparatively small discharges. The Flood Control Act of June 22, 1936, approved a project for bank clearing and channel rectification on the upper Tombigbee River to relieve this flood situation. The work originally authorized was subsequently supplemented by allotments from relief funds and regular funds and the work has been extended to include tributary streams in Itawamba County and Prentiss County.. (b) Between Aberdeen, Miss., and Demopolis, Ala.-This section of the valley, which is about 163 river-miles in length, lies in the heart of the so-called Black Belt, a region containing some of the best soils in Alabama and Mississippi. About 50 percent of the bottom land is in cultivation and a large percentage of the remaining portion is covered with timber. The towns in this vicinity are all located well above the flood plain. Only a few dwellings of small value are located in the flood plain itself. High stages are frequent in this section during winter months, but little flood damage results except from infrequent floods which occur during the crop growing season from April to November. The problem of flood control was considered in the 308 Report (H. Doc. 56, 73d Cong., 1st sess.), and a very comprehensive study of its various phases was made in connection therewith. 192. Since the submission of the 308 report, additional maps and other data have become available, but no factors concerning the flood-control problem have materially changed. A review of the proposed flood-control plans contained in House Document 56 substantiates the conclusions of this report that the cost of flood control would greatly exceed the benefits to be drived. The proposed plan of improvement would have little effect on the floods in the Tombigbee Valley. 193. It is believed that the only work in the Tombigbee Valley which can be justified is that similar to the bank clearing and channel rectification now in progress in Itawamba County. 194. Diversion of flood waters from the Tennessee River.-Any plan of crossing the divide by means of a canal at the level of the Pickwick pool suggests a possible means of diverting flood water from the Tennessee River into the Tombigbee River. For navigation purposes it would not be necessary to have a canal more than about 115 feet wide through the divide. It is proposed to provide a depth of 12 feet in the divide section with the low-water pool of 408. This cut would provide a depth of 22 feet with the surcharged Pickwick pool at elevation 418. To avoid excessive slopes, scouring of the channel, and impeding navigation, the velocities would have to be limited to about 5 feet per second. With this velocity the flow through the divide cut would be about 12,650 cubic feet per second. If the canal through the divide were constructed 150 feet wide, a diversion of about 16,500 cubic feet per second would be possible. The natural channel of the Tombigbee River is not large enough to accommodate this flow within its banks above Aberdeen and would, therefore, have to be enlarged to prevent excessive flood damages in the Tombigbee Valley. A flow of 16,500 cubic feet per second is only 10 percent of the bank-full stage of the Tennessee River at Pickwick Dam and less than 4 percent of the flood of 1897. When it is considered that the Tennessee watershed above Pickwick Dam is only 3.6 percent of the drainage basin of the Mississippi above Cairo, it is apparent that the diversion of 16,500 cubic feet per second into the Tombigbee River would have an inappreciable effect on floods on the Mississippi River. The flood benefits on the Ohio and Tennessee Rivers derived from the diversion of such a small flow would be negligible, certainly not sufficient to justify the cost of channel enlargements, increased spillway capacities required, and the increased flood damages in the Tombigbee Valley. 195. To be of any appreciable benefit, provision would have to be made for diverting more than the flow which can be carried by the proposed divide cut for navigation. Even the flow of 16,500 cubic feet per second diverted down the Tombigbee River would increase flood hazards in the Tombigbee Valley, as floods are very apt to occur at the same time on the two watersheds. Increased diversion would therefore, have to be stored in impounding reservoirs. 196. To study the effect of a greater diversion than would be possible through the proposed Yellow Creek cut for navigation, a larger cut, capable of accommodating the diversion of 100,000 cubic feet per second of floodwater from the Tennessee River, was investigated. The diversion of such a quantity of water would cause great damage in the Tombigbee Valley, unless confined. In the broad, flat flood plain of the Tombigbee River, there are no sites at which high impounding dams might be constructed economically. However, from the data available two impounding reservoir sites were selected in the valley above Amory with a combined storage capacity estimated at 3,386,000 acre-feet. The two reservoirs would be filled in 17 days by the diversion of 100,000 cubic feet per second continuously for this period. While this diversion would probably have an effect on floods on the lower Tennessee River and the Mississippi River below Cairo if properly synchronized with flood peaks on the Ohio and Mississippi Rivers, the estimated cost of this storage is about $84 per acre-foot. In a comprehensive reservoir report, the Mississippi River Commission estimates that it would be possible to obtain about 95,000,000 acre-feet of storage. on tributary streams of the Mississippi River at a cost of about $12 per acre-foot. On the Tennessee River alone, there would be available about 10,589,000 acre-feet of flood storage at an estimated cost of about $16 per acre-foot. It is obvious, therefore, that storage in the headwaters of the Tombigbee River for control of floods on the Tennessee and Mississippi Rivers is not as economical as other possible plans. ECONOMIC COST For 197. Division of cost.-The total estimated first cost of the proposed improvement, as shown in paragraph 175, is $67,472,000. This total includes items of cost that would be borne by both Federal and nonFederal agencies. In considering the division of cost between Federal and non-Federal agencies, the Board took cognizance of the fact that the Tennessee-Tombigbee Waterway would be an entirely new route which would connect two growing and industralized sections of the country and afford an interchange between two large water transportation systems, viz., the Ohio-upper Mississippi and the Intracoastal Waterway. The benefits, therefore, while local to some degree, would be more general in their effects than is usually the case. this reason, the Board felt that Federal contributions should be greater in scope than for improvements which afford major benefits to restricted areas. In the Board's opinion, the Federal Government should bear the cost of construction of all works of navigation, all railroad bridges (but not maintenance and operation), all railroad relocation, the cost of rights-of-way and flowage damages in the summit section, and the cost of alteration or relocation of all utilities except necessary alterations to sewers, drainage, and water-supply works. The non-Federal agencies, in the Board's opinion, should bear the construction cost of all highway bridges and highway relocations, the cost of all rights-of-way and flowage damages except in the summit section, new transfer facilities, reconstruction or alteration of sewers, drainage, and water-supply works, all claims for damages due to the construction of the proposed waterway, and maintain and operate all bridges and utility crossings. 198. Although under the existing law, bridges over navigable sections of the waterway would have to be altered or replaced at their owners' expense, in the opinion of the Board the cost of alterations to railroad bridges and relocation of railroad lines should be borne by the United States, as the proposed bridges would be constructed with clearances sufficient to accommodate a through traffic, the requirements of which would be much greater than that which might reasonably be expected to use the Tombigbee River. Alterations to highway bridges and highway relocations are considered to be chargeable to local interests as the States and counties concerned would be directly benefited. 199. The first cost of the improvement to be borne by the Federal Government would be $65,339,300 and the non-Federal cost is estimated to be $2,132,700. 200. Annual charges.-The estimated annual carrying charges are shown in the following table: 1 201. Interest during construction.-As outlined in paragraph No. 170 and shown graphically on chart No. 9,1 the construction period proposed for the improvement is 8 years. Interest during construction would be 3 percent of the Federal first cost and 41⁄2 percent of the non-Federal first cost, computed on expenditures year by year in accordance with the construction program. 202. Annual interest on investment.-The interest at 31⁄2 percent on the Federal investment and 41⁄2 percent on the non-Federal investment amounts to a total annual charge of $2,668,900. This item is 75 percent of the net annual carrying charge. 203. Amortization of depreciation. As the proposed waterway would be a through route serving a wide and varied territory and not dependent for the majority of its commerce on a limited natural resource, it may be reasonably expected that the useful life of the improvement would extend indefinitely. No item, therefore, has been included for obsolescence of the waterway. The economic life of the fixed structures has been taken at 50 years and that of the movable parts at 25 years. The amortization charge, compounded at 31⁄2 percent for Federal works and 4% percent for non-Federal, would repay the cost of the structures, less any amounts recoverable from tangible property or structures, at the end of the amortization period. It is estimated that 25 percent of the fixed structures and 10 percent of the movable parts would be recoverable at the end of their economic life. Amortization was computed only on the cost of the locks and dams, bridges and terminals, as such items as earthwork, rights-ofway, flowage damages, highway, railway, and utility relocations would not have to be replaced at the end of 50 years. The annual charges 1 Not printed. 86805-46- -8 |