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<br />0021~6 <br /> <br />I. Foundation and Embankment <br /> <br />The inner. core of the dam embankment will be constructed from the most <br /> <br /> <br />impervious materials selected from the borrow areas, and the outer zones will be <br /> <br /> <br />constructed from the sand and gravel pervious deposits found in the river bottom <br /> <br /> <br />and on the terraces upstream from the dam. Rockfill for providing weight and <br /> <br /> <br />stability on the downstream tow of the dam and to protect the toe from wave <br /> <br /> <br />action of releases from the power plant will be quarried from rock in the <br /> <br /> <br />spillway and access road excavation. The upstream slope of the dam will be <br /> <br /> <br />protected against wave action with a blanket of rock riprap quarried from the <br /> <br /> <br />more resistant sandstone near the right abutment of the dam. <br /> <br />The cutoff trench for the dam will be excavated to bedrock throughout <br /> <br /> <br />its length and grouting of the bedrock will be done to minimize seepage and <br /> <br /> <br />reduce pore pressures under the dam. <br /> <br />2. Spillway <br /> <br />The service spillway will be a reinforced concrete structure on the left <br /> <br /> <br />abutment with an uncontrolled overflow crest elevation of 5,Ol5 feet, a crest <br /> <br /> <br />length of 185 feet and a capacity of 6,800 cfs sufficient to pass the 25-year flood. <br /> <br /> <br />The service spillway will discharge into a ski-jump type stilling basin energy <br /> <br /> <br />dissipator near stream level below the dam to quiet the swift flow down the chute <br /> <br /> <br />prior to returning safely to the downstream channel. Normally the reservoir will <br /> <br /> <br />spill in May and June each year up to 2,300 cfs of flow. <br /> <br /> <br />The auxiliary spillway to pass the maximum probable flood over and above <br /> <br /> <br />what the service spillway will carry amounting to IO,700 cfs will be located at the <br /> <br /> <br />extreme end of the embankment on the left abutment. The crest of the auxiliary <br /> <br /> <br />spillway will be at elevation 5,Ol9 feet, width 400 feet, and the road will be <br /> <br /> <br />located on the crest. In periods of spillover the auxiliary spillway vehicle <br /> <br /> <br />traffic across the spillway will probably cease depending on the depth of water <br /> <br /> <br />overflowing. The existing earth and rock draw into which flood flows will discharge <br /> <br /> <br />after overflowing the crest will be enlarged and graded and a training dike <br /> <br /> <br />constructed to direct the flood flow into the river below the stilling basin of <br /> <br /> <br />the service spillway. (See figure 7.) <br /> <br />- 13 - <br /> <br />