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. _ -~ - ~ iii iii~iiiii~iiiii~ s o°~GOµrRO(~~a 4~ A O „n~ l.3Wtt TuCkB, Execuhve Director S ~ Oi Jib ac ~~baa Nde • II C Ni~+i ~ti r.. ~~ ~ a li D~r,1,1,; ulrve Canm~tlee: Wally Tpey BoWtla County N Carry Frynoltls °o City of Denver Kenneln M. Gorrell ~ Cary OI Arvada ~ c~~~ert~ M MBCIn10sh p Cny of Denver u John J Nionoll w Arapahoe County i W O Ide tubers. w Hal Antlersm 2 Jenwsm County w Boo Briggs Aoams County ~ Ruth A Correll rv Gty or BoulOer 3 Dav~tl A. Day m Engineer N W G Duncan Douglas County m . ~~~am r n+cN¢nms m C,ly of Denver ~ Seely Parsons w Ury or ~mleron 5 Arlen E. Paean Engmcer ' inlnony E R~chler n CiP/ nl Thernlan m M ~ 'Sam" Santlos uv'i Cary of Denver ~ z 0 x a w 1 w r May 11, 1979 Arapahoe County Planning Department 5334 South Prince Street Littleton, CO 80166 Colo. Dcpt. D. :.U,~:r,l ~{esc,i~.~~.: Gentlemen: This letter is in response to your request for our comments concerning rezoning case 79-22, Allen G. Smith. We appreciate the opportunity to review this proposal. We have the following comments: 1. The application has recognized the 100-year flood plain of Sand Creek as defined in "Flood Hazard Area Delineation, Sand Creek" but does not recognize the 100-year flood plain for Murphy Creek as defined in "Flood Hazard Area Delineation, Piney Creek, Cottonwood Creek, Lone Tree Creek and Murphy Creek" (sheet 48). 2. The plan calls for the Sand Creek channel itself to be mined. Over the years we have seen many proposals to mine sand and gravel in flood plains but never one to mine the channel itself. 3. The reclamation plan would leave a large pit into which Sand Creek and Murphy Greek waters would flow. The decrease in the velocity of the water in the pit would cause the water to drop its sediment load. Clear water (essentially sediment free) would leave the pit at the downstream end. This clear water would have a tendency to pick up sediment, thus possibly causing severe erosion downstream of the mined area. This possibility should be thoroughly addressed by an engineer knowledgable in river mechanics. 4. Three drop structures (two on Sand Creek and one on Murphy Creek) have been proposed to prevent erosion of the pit slopes up to a 10-year flood. No calculations have been provided to substantiate the hydraulics of this proposal. In other words, would the entire 10-year flows reach and pass through the drop structures as they are proposed? Also, designs of the drop structures have not been provided for our review.