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<br />MINED LAND RECLAMATION DIVISION <br />Department of Natural Resources <br />1313 Sherman St., Room 215 <br />Denver, CO 80203 <br />303 8663567 <br />FAX 303 832 8106 <br />DATE: October 10, 1991 <br />T0: Carl Mount V~ ~Q~ <br />FROM: Margot Anderson ~ 1~~`-"' ' "~ <br />RE: Sediment Pond 008 Eme'rge`1'ic'\y Spillway Design <br />and Diversion 1 Design Changes, Seneca II Mine, <br />C-80-005, TR-22 <br />pF COO <br />r.~ `~90 <br />NP %~ (((\~~~ Q <br />f ~ <br />~ fe'!6 3 <br />Roy Romer, <br />Governor <br />FreO R. Banta, <br />Dinsion Director <br />I have revielred the designs proposed in the technical revision from Peabody <br />Coal Company dated August 23, 1991. I have the following comments: <br />The proposed emergency spillway for Pond 008 will function to <br />retain a 10-year, 24-hour event and safely discharge a 25-year, <br />24-hour event. <br />2. Diversion 1 proposed as a vegetated channel is not recommended for <br />approval for the following reasons: <br />a. limiting velocities of loam-type Soils from 3.00 to 3.50 fps <br />would be exceeded by the minimum computed velocities of <br />approximately 3.7 to 7.7 fps, generated using SedCAd+ and <br />Oklahoma Technical Press hydrology and sedimentology computer <br />programs; <br />b. while sufficient retardance of flow may occur in vegetated <br />stretches of the ditch, with sufficient establishment of <br />vegetation, the higher velocities attained by flow over <br />non-vegetated (bedrock) stretches of the ditch would be <br />sufficiently erosive to entrain soil and vegetation at the <br />transition between bedrock and vegetated stretches of the <br />ditch; <br />c. velocity of flows over bedrock segments would be sufficient <br />to remove soil and vegetation from the sides of the ditch. <br />3. I am proposing Diversion 1 be provided with appropriate non-erosive <br />material, such as rip rap over appropriately selected geotextile, <br />along its sides and bottom where soil is encountered. An alternate <br />suggestion is the installation of drop structures adequately sized <br />and spaced to eliminate erosion from the channel. <br />STA~ OF COLORADO <br />8694E/gaw <br />