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Show now under way, especially on streams carrying heavy sedi- ment loads, as in the Missouri River Basin, the average storage loss may be expected to reach 600,000 acre-feet annually within a decade or two. In recognition of the potentially serious effects of reser- voir storage losses, several Federal, State, and local agen- cies have made sedimentation surveys of approximately 600 reservoirs and have established or authorized range systems for making such surveys on an additional 84 reser- voirs. Approximately 417 detailed surveys have been or will be made in such a way that they can be repeated from time to time to maintain a periodic check on storage loss. Sedimentation survey ranges are being established on prac- tically all reservoirs constructed by the Federal Govern- ment, and resurveys are made at frequent intervals. The Soil Conservation Service, in cooperation with various State and local agencies, carries on a program of surveys on non-Federally owned reservoirs. Basic data obtained from reservoir surveys are neces- sary for several purposes, namely: 1. To permit periodic correction of capacity curves which is required for efficient reservoir operation. 2. To maintain a periodic check on storage loss and determine the timing of storage replacement. 3. To provide data on rates of sediment production from watershed areas as a basis for developing design criteria for new reservoirs, and for determining the need for sediment control measures. The present reservoir survey programs of all Federal agencies engaged in the construction and operation of reservoirs should be continued and, where necessary, expanded to provide complete coverage by survey ranges and periodic resurveys at appropriate intervals, depending on the rate of storage loss. Federally-owned and operated reservoirs at present constitute less than 3 percent of the total number of reservoirs in the United States, but con- tain about three-quarters of the estimated total developed storage capacity of 200,000,000 acre-feet. Of the re- mainder of State, local, and privately owned reservoirs, a representative selection of 10 percent of the total number should be resurveyed with approximately the same frequency. Improved survey techniques promise to expedite this kind of operation and to reduce its cost so that it would appear feasible to carry out such a program. Altogether, approximately 1,400 reservoirs should be in- cluded in the program of detailed surveys, or 14 percent of the estimated total number constructed or expected to be constructed during the next 10 years. Of these, sur- veys have already been made on approximately 400 reservoirs. After the initial surveys are completed on the remainder, resurveys on an average of once every 10 years would require approximately 140 surveys annually. Figure 31 shows the adequacy of reservoir sedimentation surveys to date. It is based on the above estimates of need with relation to those already made, as well as the number of existing or authorized reservoirs available for survey. Furthermore, it assumes that the general practice is to survey the reservoirs once every 5 to 10 years, except where local sediment conditions may dictate other in- tervals. This figure does not show, however, the priority of need for additional surveys in different areas. For example, there is need for a much larger number of surveys in some sections of the Missouri River Basin than in Maine or Florida, although the areas show the same percentage adequacy. Moreover, sedimentation surveys of a large number of farm ponds or small reservoirs may be needed to develop rates of sediment production from different physiographic and soil problem areas. In addition to the need for more reservoir surveys, a large amount of existing basic data as well as that to be obtained should be analyzed and interpreted to develop more adequate information on such factors as: (1) Sedi- ment distribution with respect to operating levels, (2) specific weight of sediment, (3) trap efficiency of dif- ferent kinds of reservoirs, and (4) sediment production rates indicated for various conditions of inflow and water- shed characteristics, including land use and management. Channel Stability Surveys A design and maintenance problem which is receiving attention in several agencies of the Government is that of anticipating the rate and extent of degradation (cutting and lowering of stream beds) of stream channels down- stream of dams and aggradation (filing and raising of stream bed) above the reservoirs. In recent years, several attempts have been made, and are continuing, to evolve methods of analysis; and these have brought out the necessity for, and have stimulated basic laboratory re- search. In parallel with the analytical approach, it is necessary to make field observations of degradation and aggradation of stream channels in selected areas. Several agencies currently require that surveys be made above and below authorized or existing dams on rivers where moderate or severe channel changes will occur. The Colorado River, where surveys have continued since construction of Hoover Dam, the Rio Grande, the Ar- kansas River, and the Missouri River are streams where extensive survey programs are in progress at the most critical locations. Data which have been obtained to date are not sufficient to foretell adequately future channel conditions. Lack of adequate basic relationships expressing the laws of sediment transportation is felt also in other design and maintenance problems for projects such as some canal and irrigation systems, control of meandering streams and aggrading streams which because of rising ground water tables decrease agricultural production. Surveys of the existing channels provide information which aids in solu- tion of design and maintenance problems. Such surveys generally are made by the agency which operates the project giving rise to the sedimentation problem. Occa- sionally a survey is a joint undertaking of several agencies coordinated by the Subcommittee on Sedimentation of the Federal Inter-Agency River Basin Committee. These methods of conducting surveys are satisfactory and should be continued. In addition to the channel surveys made in connection with project design and operation there are a number of critical areas not involved in active projects where channel cross sections should be established and resurveys made from time to time. It is not feasible to estimate the percent adequacy of the existing channel stability investi- gations, but provision should be made for collection of basic data in areas where project investigations are not providing the minimum essential information on the condi- tion of unstable channels. 349 |