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ti STATE OF COLORADO qiC VlAgp D. ~. Gov c.~o~ DEPARTMENT OF NATURAL RESOURCES D. Monte Pascoe, Executive Director MINED LAND RECLAMATION 423 Centennial Building, 1313 Sherman Street Denver, Colorado 80203 Tel. (30~) 839-3567 David C. Shelton April 15, 1981 Director T0: Jim Herron FROM: Richard A. Domingue ~, ~,~,n.w.~~~ SUBJECT: GEX Roadside and Cameo Mine - Sediment Control System Our File No. C-041-81 Rule 4.05.3 Diversions and Conveyances of Overland Flow: iii iuiiiiiiiiiiiii 999 1. Peak Flows Estimates: GEX has used the SCS method described in the manual Peak Flows in Colorado to estimate peak flows. None of the estimates appear to have considered basin shape corrections as described in appendix E of the manual. Estimates using the Homographs should also include slope corrections where the slope is not exactly as stated on the graphs. GEX must: a. Show all information necessary to generate the CN's used. I. Hydrologic Soil Group, II. Land Use, or III. Cover type and density. b. Provide an estimate of average watershed slope. c. Provide basin shape and slope corrections where applicable. 2. Design Velocities: In several cases, specifically in cast-over ditch designs the channel geometry is not sufficiently defined to determine area (A) and wetted perimeter (P). These parameters should be shown as functions of depth (d) in order to determine design depth, velocity, and freeboard. (See Barfield and Haan (1978) page 130-131. Several of the ditches had erosive velocities at design flows. GEX must: a. Provide sufficient dimensional detail to completely define the channels, b. Show area (A) and wetted perimeter (P) in terms of depths (d), c. Provide estimates of velocity at design flows, d. Provide information on channel lining necessary to prevent erosion at design velocities (4.05.3(6)(a)), and e. Show that 0.3 ft. freeboard will exist under design flows (4.05.3(6)(b)).