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<br />0447 <br /> <br />new dike to about once in 100 years. A very low head, gated or flash bonded, inflow- <br />outflow structure would be provided near the outer end of the new dike to flood and <br />later dewater the approximately 2 sq. mi. of farm lands in the area between the Dam <br />and the Dike. Superfloods would quietly overtop the Dike. <br />This plan is believed to have two attractive selling points. It would move the <br />end of the slurry trench cutoff nearly two miles upstream from the Dam to a position <br />which, even to a layman, would appear to be safer; and it would preserve for continuing <br />farming in all but a superflood year, on the order of 1000 acres of farmland. <br />CONCLUSIONS <br /> <br />The following conclusions are in essence the result of a reconnaissance level <br />review of the large amoWlt of information presented to us by the USBR. They are not <br />guaranteed, but are presented as our best judgment as it could be developed by approx- <br />imately a week spent in reviewing the information and a day spent inspecting the site. <br />1. The foundation conditions for proposed Narrows Dam are complex and par- <br />ticularly challenging because the dam would be so long and the conditions vary exten- <br />sively along that length. <br />2. The varying foundation conditions require, as has already been shown on the <br />USBR feasibility design, that the dam design be varied from location to location to adapt <br />to the foundation variations. Further local changes in dam design are likely to be <br />needed as final design investigations and actual construction progress. <br />3. The USBR hydrologic studies indicate to us that it is highly improbable that <br />the reservoir. level need ever exceed Elev. 4428.5, the top of the flood control pool. <br />Appropriate positioning of the spillway lip and operation of spillway gates could vir- <br />tually assure this. <br />4. The USBR analysis of seepage effects near the end of the slurry trench cut- <br />off in our judgment employed severely pessimistic assumptions regarding effective res- <br />ervoir level and speed of foundation saturation. Hence, we consider the results, while <br />of some value in illustrating a useful analytic procedure, do not constitute a supportable <br />judgment as to probable seepage exit gradients or need for relief wells. We believe <br />that e.ut gradients would not even develop, even during the few days that Probable <br /> <br />17. <br />