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<br />014 ; <br /> <br />In review, however, it is our judgment that the basic assumptions adopted in the <br />analyses are much too severe to be accepted as a realistic evaluation. Our comments <br />are as follows: <br />a. The very brief (normally less than 10 days) and transitory seepage <br />induced by possible reservoir incursions above about Elev. 4410 could not conceivably <br />saturate a significant portion of the alluvium above that level under the dam to the ex- <br />tent of establishing steady seepage conditions through and under the dam. In other <br />words, the assumption used of steady seepage under the extreme, Probable Maximum <br />Flood reservoir level of Elev, 4448.5 is excessively pessimistic. A supportable as- <br />sumption, in our judgment, would be Elev. 4420, and certainly no more than Elev. <br />4425. <br /> <br />b. The possible concept that increasing pressures in the deep aquifer <br />would act to pressurize the overlying fine sandy alluvium to and above the ground sur- <br />face at the dam, during flood routing through the reservoir, is not supported by the <br />referenced USGS study. <br />c. Any possible future studies of seepage under the non-cutoff reach of <br />the dam should concentrate on defining the chronologie downstream progress of the sat- <br />uration surface under the dam as a funotion of the estimated hydro graphs of reservoir <br />water surface elevation for various sized flood inflows. Incidentlllly, such hydrographs <br />should not contain excessive conservatism as regards antecedent reservoir levels and <br />rainfall, or undue restriction on reservoir releases. <br />5. Construction Materials. We have only briefly reviewed the background <br />data on embankment construction materillls. It is clear from such review, however, <br />that while there is an ab=dance of fine grained soils ranging from medium sands to <br />silty clays on and near the damsite, no massive deposits of coarse grained materials <br />such as gravel, cobbles and rockfill have been located. This is unfortunate, for a de- <br />sign which must cope with foundation settlement and seepage problems will normally <br />employ large amounts of coarse, free draining material in locations where seepage <br />and piping tendencies might be anticipated. <br />As regards the proposed silty clay core material, it is our impression that the <br /> <br />14. <br />