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03/27/1995 13:11 7192753051 HRM PAGE 05 ~ . .. 11\ <Y GE I Consultants, Inc. March 14, 1995 Project 95050 5660 Greenwood Plaza Blvd, Suite 202 Englewood. CO 80111.2418 .303.779.5565 Ms. Honey R. Moschetti, President' DeWeese.Dye Ditch and Reservoir Company 1320 Grand A venue Canon City, CO 81212 Ie Re: Hydrologic Analyses of DeWeese Dam, near Westcliff, Colorado Dear Ms. Moschetti: We have completed Tasks I and 2 of our hydrologic analyses of DeWeese Dam. The purpose of this letter is to summarize the results of our analyses to date and to provide recommendations for completion of this pha~e of our work. The most significant result of our work thus far is a substantial decrease in the maximum design inflow flood. In our previous work, we used one-half of the probable maximum flood (PMF) reported in the 1980 Phase I Inspection Report prepared by International Engineering Co., Inc. In the Phase I report, the one-half PMF was simply estimated as one-half of the full PMF hydro graph. The peak routed discharge at the dam from this flood was reported to be about 238,000 cubic feet per second (cfs). The peak inflow to the reservoir would he !;omething slightly higher than this amount, due to the attenuation of the flood which results from reservoir storage effects. State dam safety regulations require that Intennediate Class IT dams (such as DeWeese Dam) must be capable of safely passing the inflow flood resulting from 50 percent of the probable maximum precipitation (PMP). Our analysis indicates that the SO-percent PMP flood will have a peak inflow to the reservoir of about 140,000 cfs, with a peak routed outflow at the dam of about 128,000 cfs. This estimate should be considered preliminary, subject to approval by the State Engineer's Office. Our estimate of the peak routed outflow from the 50-percent PMP is about 45 percent less than the one-half PMF estimate from the Phase I report. We believe that this substantial decrease is the result of several factors, including: 1. In general, the peak inflow from the SO-percent PMP flood will be lower than the peak inflow estimated as one-half of the peak inflow from the full PMF.. This decrease is primarily the result of the effects of initial rainfall losses and temporary basin storage. 2. Our analysis of the drainage basin above the reservoir indicates an area of about 351 square miles, which is about 16 percent less than the area reported in the Phase I report (419 square miles). 9~RI4I.TR."ItM Winche5ter, MA Concord. NH Rsleish. NC Carlsbad. CA Chic.al!~). IL