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1 <br />I:Jlf!j rj 0 to] j GEOTECH II <br />Civil, Structural & Geotechnical Engineers <br />222 truth Park Ave. ? monfr p(e",.?) mmlf)I <br />F'h.: (9 M) 249-6828 FP,X: f9?',' 1 *a? <br />September 22, 2008 <br />Ms. Karmen King <br />Gault Group <br />36 West Main Street <br />Cortez, CO 81321 <br /> <br />REVIEW OF INFORMATION FOR CHANNELIZATION FO DEADMAN GULCH <br />Deor Ms. King: <br />Following our review and recommendations regarding the proposed design of the <br />Deadman Gulch channelization, prepared by Mr. Steve Belz of Black Creek Hydrology, <br />revised plans have been issued and we have completed a more detailed analysis of the <br />hydrology of the contributing watershed. Design criteria are as follows: <br />Watershed Runoff: Buckhorn Geotech completed a HEG-1 analysis of the potential <br />runoff from a 100 year 24-hour storm. The SCS Type IIA storm distribution was used as <br />it simulates a cloudburst type storm which is common in Western Colorado during late <br />summer and often causes the heaviest runoff. The watershed size above the Golden <br />Wonder Mine was estimated at 80 acres with a lag time to the point of design of 1.2 <br />hours. Based on Mr. Belz's description of the watershed a runoff curve number of 60 <br />was selected for "mountain brush" cover and soils with a hydrologic soil group <br />classification of C. A total rainfall of 3.14 inches was utilized based on Mr. Belz's <br />information. Based on these parameters the 100-year 24-hour storm is estimated to <br />yield a peak runoff of 7-cfs. <br />Pipe Entrance Design: Entrance losses can limit the amount of flow that can enter the <br />piped system. Estimates of the entrance capacity of the proposed 21" T.D. pipe <br />indicates the pipe can handle in excess of 7-cfs. The sloped pipe face and the flared <br />wing walls shown on the headwall design at the pipe entrance will improve entrance <br />capacities up to 10-percent. A bar screen is shown at the entrance to prevent debris <br />entering the pipe. A notched headwall will allow any excess flow, due to excessive <br />storm event or pipe blockage, to overflow to the surface drain to be constructed above <br />the pipe. <br />Ike Profile: A profile of the pipe was provided along with a cross-section showing a <br />riprap overflow channel above the pipe. The pipe is capable of conveying in excess of <br />the estimated peak flow from the 100-year 24-hour storm event with the overflow <br />channel providing conveyance of excess flows.