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Table 1. Hydrolog,y Summary <br />Peak Discharges Runoff Volume <br />Storm Event (cfs) (acre-feet) <br />24-hour General PMF <br />24-hour 1/2 General PMF <br />6-hour Local PMF <br />6-hour 1 /2 Local PMF <br />24-hour 100-year <br />15,927 <br />7,964 <br />8,172 <br />4,086 <br />1,477 <br />7,272 <br />3,636 <br />2,480 <br />1,240 <br />680 <br />Under the State's current rules and regulations for dam safety and construction, any proposed <br />rehabilitation to the dam should be designed to accommodate 1/2 of the Probable Maximum <br />Flood (PMF). According the hydraulic analysis completed in the 1987 Feasibility Study, the <br />existing spillway has the capacity to pass the 100-year storm with 1.8 feet. of freeboard. <br />However, 1/2 of the PMF would overtop the dam by about 2.7 feet. The existing spillway will <br />convey only about 20% of the PMF without overtopping the dam. <br />V. DESCRIPTION OF ALTERNATIVES <br />An attempt was made to estimate the potential loss of storage capacity of the reservoir caused by <br />sedimentation. The storage loss was to be estimated by comparing the calculated reservoir <br />volume determined using two different topographic maps prepared at different times. However, <br />after lengthy research it was determined that the only mapping available is the mapping prepared <br />on November 5, 1984. A site visit was made to look at the inlet location where the East Fork <br />Creek flows into the reservoir. The area is fairly clean and there aze no sandbars or other <br />indications of sediment build up in the reservoir. Without a second topographical map, an <br />expensive survey of the reservoir would be required to estimate the current reservoir storage <br />capacity. This information was discussed with the City and it was decided not to proceed any <br />further with this particular task. However, it was agreed to complete the study of alternatives for <br />raising the spillway as described above. Both alternatives were analyzed for 1/2 of the PMF <br />using the U.S. Army Corps of Engineers HEC-1 Flood Hydrograph Package in keeping with the <br />analysis completed in the 1987 Feasibility Study. <br />Alternative No. 1 evaluates the potential of raising the dam embankment about 7.5 feet to an <br />elevation of 10,406 and enlarging the existing spillway to convey 1/2 of the PMF. The <br />embankment would be extended along the downstream slope at 2.25H:1 V to provide additional <br />stability. Figure 3 presents the embankment modifications for both alternatives. The proposed <br />E <br />