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~ * III IIIIIIIIIIIIIIII ~~ ~~ != TUTTLE APPLEGATE RINDAHL, INC. Consultants for Land and Resource Development June 16~ 1988 Mr. El11L De6root~ P.E. Urban Drainage and Flood Control District 2460 W. 26th Rvenue - E3uite 136-E9 Denveer~ Colorado 80211 RED Arapahoe County Case No. BA BB-6-3q Coal Creek Resoue•Ce Mining Plan Dear Elilli Enclosed is a copy of tfie drainage study we submitted to Arapahoe County for the Coal Creek Project. I apologize for the inconvenience you have had !n the review. It wee my understanding that a copy of this was given to you when it was referred to Urban Drainage by~the County. In addition, I have included supptementat information on some oT the issues we discussed on June 7, 1988 at your office. The issues that we need to adtlress based on our conversation are l1/ Calculate the amount of Tloodplain storage lost by the mining operation to determine !f it is sipnificanL~ i21 6how calculations for the amount of sediment load generated by thin basin to see 1f,lt is significant to the 6outh Platte Eiasin~ and <31 Show design calculations justifying the type of rundown structures proposed to prevent headcuttlnp or revise them to match design criteri^ used by the District. I can provide you the following information Tor each of these issuesf 1. The amount of storage lost in the fioodpiain is based on using five cross sections taken along the valley and comparing the cross sectional area of tfie 100 year flood for pre and post mining conditions. The mayor concern stated is that there is lees overbank storage along this reach of stream after mining !s completed. The stream channel is approximately 4.3 miles long. Open channel calculations were done Tor five cross sections using pre and post mining conditions. There are eight channel restrictions designed Into the project as shown on the mining plan that will create a downstream control and result in higher backwater and thus more storage under developed conditions. The project will actually result in a net average increase in cross section flow area of 1200 square feet. Extrapolating this Tigure Tor the entire reach of channel gives • total increase in flood storage volume of 660 acre-feet during a 100 year flood. This would provide a benefit to downstream property owners by reducing the flood peaks along Coal Creek.