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Whetypne <br />Associates Technical Memorandum <br />b. Please clarify the statement that "... channel protection will be placed to the full channel depth <br />listed for each structure. " Will channel protection be placed in existing channels? If so <br />allowances need to be made for the reduction in conveyance capacity. Please elaborate. <br />c. The fourth bullet states "Riprap shall extend 1.0 x flow depth above flow level. " This may <br />conflict with the previous statement " "... channel protection will be placed to the full channel <br />depth listed for each structure. " Channel protection will be required to extend to the full <br />design depth, including freeboard. (Note: DRMS requires channel freeboard be evaluated for <br />all engineered channels: channels shall be designed with a minimum of 0.5 feet of freeboard <br />unless the velocity head (v212g) is significant, then the minimum required freeboard is half <br />the velocity head, or v214g). Please clam the design /redesign intent. <br />d. There is no reference to design drawings (as built or proposed). Please provide channel <br />design/redesign drawings, stamped and signed by a professional engineer registered in <br />Colorado. <br />e. The last paragraph discusses using "gravel mulch ". Please describe where gravel mulch will <br />be used and why. If flow velocities are expected to exceed five feet per second for gravel <br />mulch applications, please demonstrate the specified material will be stable under design <br />flow conditions. <br />Cotter response: Channel stability and erosion potential has been evaluated in the <br />revised DDP using the public domain software WinXSPro, distributed by the U.S. <br />Forest Service (Hardy, et al., 2005). Specifically, the mean velocity and Fronde number <br />calculated using the Thorne and Zevenbergen (1985) resistance equations were <br />compared to the Maximum Permissible Velocity or the Fronde number established for <br />channel stability. <br />(a) The Urban Storm Drainage Criteria Manual (UDFCD 2001, 2008), recommends <br />that structures be designed with a permissible velocity of 5 ft /sec or Fronde number <br />of 0.6 for soil types A and B and of 7 ft /sec or Froude number of 0.8 for soil types C <br />and D. Engineered channels at the JD -6 Mine are constructed in soil types C and D <br />(rock) and are thus designed to meet the v = 7 ft /sec or Fr = 0.8. <br />(b) The stability analysis indicates that channel protection is not required for existing <br />MDS and LDS structures. Channel protection is required in six segments of the <br />redesigned UDS, where drop structures will be installed to dissipate energy. The <br />remaining portion of the UDS will be constructed with a slope of 0.035 ft /ft and no <br />channel protection will be required, as described in the revised DDP. Channel <br />protection will be installed on the planned spillway crest and exit section as <br />described in the revised DDP. <br />(c) The rock protection will be installed to a height of 1 ft above the planned spillway. <br />The modeling indicates the maximum height of water above the spillway crest for <br />the 100 -year 24 -hr storm event is less than 1.0 ft. At the UDS drop structures, rock <br />protection will be installed to 1 ft above the design flow depth for the 100 -yr 24 -hr <br />storm event. <br />4148A.131030 <br />