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reported by the Bureau of Reclamation (U56R, Design of Sma11 <br />Dams, 1974), the plunge basin needs to be 9 feet deep to be <br />effective for the East tributary flow conditions. These basins <br />will be located on each bench of the pit. They will tend to <br />accumulate sediment and debris under th~~ more Frequent, smaller <br />flows. However, under the 700-}'ear desiyn flows, the accumulated <br />debris will be swept from she basins. .1s shown on the drawing, <br />the minimun bench width in thie vicinity of the channel is 40 <br />feet. <br />The concern raised by the Ct~1LRD is that even with the designed <br />plunge basins, there is a potential t11at the top edge of the <br />first "plunge" could erode upstream and eventually migrate <br />outside of the permit area. An engineered drop structure has <br />been designed to mitigate this potential. The next drawing which <br />is attached shows an elevation view of this drop structure which <br />is to be Located a minimum of 20 to 40 feet inside the permit <br />boundary. The concrete structure will be keyed into the bedrock <br />two feet in all directions. Grouted riprap will be used to <br />protect the downstream face of the drop structure. <br />13. d. Stream Crossing Culverts <br />The purpose of these culverts is to allow equipment and trucks to <br />move across the channels without continually disturbing fhe <br />bottom of the drainage channels and creating additional sediment <br />in the channels. These culverts would be designed as "low water" <br />crossings, i. e. twin 24 inch CMPS would be placed in the channel <br />and covered with a rockfill that functions as the road crossing <br />of the channel. No fine grain material would be used in the <br />rockfill since there is no reason to make the embankment <br />impermeable. The upstream and downstream faces of the road <br />embankment would be protected with large riprap. Overtopping of <br />the road embankment would occur for any flows above the normal <br />spring snowmelt. No traffic would cross the channel during times <br />of overtopping. The channel would be enlarged in the vicinity <br />of the culvert crossing to accommodate the embankment <br />obstruction. <br />There would a primary crossing of Waldo channel in the vicinity <br />of the confluence with the East tributary. During Phase I, there <br />may need to be at least one crossing of the East tributary to <br />allow equipment movement to and from the north and south sides of <br />the channel. <br />4 <br />