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I I I I I I I I I I I I 1 I Based on the seepage calculations, visual observation of the embankment material s, and the nature of the rock in the foundati on and abutments; a best estimate of embankment seepage equal to 10 percent of the total measured seepage appears reasonabl e. It is conceivabl e that if the foundation rock surface was not properly treated, a vast majority of the seepage could be transmitted along the embankment-foundation contact and through the fractured bedrock foundation and abutments. The lush grass cover and marshy conditions downstream of the dam support this hypothesis. 2.5 Summary of Existing Conditions and Recommended Interim Operation Restriction Due to the severe damage to the outlet works conduit and marginal stability of the downstream slope, it is recommended that the zero storage restriction be maintained until rehabilitation is completed. 3.0 HYDROLOGY AND HYDRAULICS 3.1 Probable Maximum Flood The drai nage basi n for Twi n Lakes Reservoi r 1 i es north of Rocky Mountai n National Park in the Roosevelt National Forest, as shown in Figure V.5 in Secti on 5 of thi s Chapter. The basi n di vi de is located at approximately 1 0,700 near the headwaters of the South Fork of the Cache 1 a Poudre Ri ver. The drai nage area is 0.89 square mi 1 es and the basi n has an average slope of 922 feet per mile. The distance from the basin divide to the reservoir is approximately 1.5 miles. From these values, a time of concentration for basin runoff to reach the reservoir of 0.47 hours was computed. Infil trati on losses must be accounted for in order to compute the excess rainfall producing runoff. The Green and Ampt infiltration model, which incorporates several soil characteristics as well as initial and final saturation levels, was used to compute infiltration rates for the storm period. The soil hydraulic conductivity is one of the most critical parameters selected when employing this method. The U.S. Army Corps of -56-