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. I I I I I I I I I I I I I I I I I I I magnitude. If pervious seams are prominent in the embankment, the overall permeabi 1i ty governi ng flow cou1 d easily be one or two orders of magni tude hi gher than the average used above. Therefore, the seepage estimate cou1 d approach 30 gpm, with the reservoir at normal pool elevation 9510 (G.H. 32); Although the seepage being measured is relatively small, its apparent location is critical to the dam's safety. On~ of the leading causes of embankment dam failure is piping along conduits and foundation or abutment contacts; as a resu1 t of uncontrolled seepage. EVlin though thi s seepage has occurred for numerous years, it should not be taken for granted or assumed to be an acceptable condition. 2.5 Summary of Existing Conditions and Recommended Interim Operating Restriction Based on the generally satisfactory conditions of the outlet works it is considered safe to store water in the reservoir. However, due to the danger posed by uncontrolled seepage, it it recommended that storage be limited to elevation 9493 (G.H. 151. 3.0 HYDROLOGY AND HYDRAULICS 3.1 Probable Maximum Flood The drai nage basi n for Peterson lake Reservoi r 1 i es north of Rocky Mountai n National Park in the Roosevelt National Forest as shown on Figure V.6 in Secti on 5 of thi s chapter. The basi n di vi de is located at approximately el evati on 10,500 near the headwaters of the Cache 1 a Poudre Ri ver; The drai nage area is 2.65 square miles and the basi n has an average slope of 550 feet per mi 1 e; The dl stance from the basi n di vi de to the reservoi r is approximately 1.S miles. From these values, a time of concentration for basin runoff to reach the reservoir of 0.63 hours was computed. Infil tration losses must be rainfall producing runoff. accounted for in order to compute the excess The Green and Ampt infiltration model which -57-