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Show 10. Extent to Which Engineering and Design of Major Works Should Take Account of All Probable Future Water Needs The Problem The extent to which dams and other major en- gineering works should be designed to meet future water needs. The Situation Estimates of future water requirements for irri- gation, domestic and industrial supply, power, and other purposes recently have been revised upward. Moreover, additional flood protection has been found warranted in many suburban areas as a result of population increases. The correct procedure for considering these future needs is first to estimate them on the most realistic basis possible, and then to provide for them in the planning, design, and construction stages in accordance with best engineering and economic practices. Some of the particular problems which have been encountered in the Central Valley are: Two-step versus one-step construction of dams.- The question here is: Where it is known that a reservoir site ultimately should be built to its prac- ticable limit, but need for its full capability does not presently exist, should a low dam be constructed initially with provisions for future raising, or should the dam be built to ultimate height? The question arose, for example, in connection with the Folsom Project on American River, where it was concluded that provision of the maximum practicable capac- ity of 1 million acre-feet in one step would be the most economical procedure. Two-step construc- tion also was considered for the Table Mountain and Monticello Projects. The Hetch Hetchy Dam on the Tuolumne River was constructed by San Francisco in two steps, the initial reservoir capacity being 206,000 acre-feet, and the ultimate capacity 360,000 acre-feet. Installation of penstocks and other features for probable future power generation in dams presently under construction.-Such action is logical en- gineering practice. Penstocks are being provided, for example, at the Pine Flat Dam for future power installation. Provision for future irrigation needs.-Should special irrigation outlets be provided at reservoirs under construction for future needs, or should these facilities await the need, at which time their in- stallation would be much more costly? Irrigation outlets are being provided at all multiple-purpose projects designed for future irrigation demands. However, a special outlet for a future high level canal from Folsom Reservoir was considered, but for economic reasons was not incorporated. Design of flood control works.-To what extent should the design of levees take into account future upstream reservoir storage? Such coordinated design is good engineering practice. An example occurs in the Cache Creek flood control project, where levees will be designed to provide needed local flood protection in conjunction with a future upstream reservoir at the Indian Valley site, or some alternative. Use of adjacent reservoir sites.-What consider- ation should be given in the design of dams and reservoirs to prevent foreclosure of future use of adjacent sites? This problem was encountered in the Iron Canyon Project where the dam site was moved downstream after project authorization, so as not to preclude use of the original site (Table Mountain) for a much larger storage capacity than presently is authorized. Conclusions The design of major engineering works for the development of the water resources of the Central Valley, as in all river basins, should give full con- sideration to all probable future water needs and provide the most economical and practicable methods for meeting those needs. The problems involved are primarily in the fields of engineering and economics. In the valley such consideration is especially important because all available fore- casts suggest an almost certain need for full develop- ment of water resources in the basin for all purposes. Water may eventually even have to be imported from other basins. Plans of the several agencies indicate an awareness of these needs. However, more complete data should be made available, par- ticularly on economic trends and strategic indus- trial needs, so as to permit planning with the desired accuracy. 11. Interbasin Diversions of Water into and out of the Central Valley The Problem The extent to which interbasin diversions of water into and out of the Central Valley may be required as a part of the ultimate plan. 911610-51- -li 139 |