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JD-8 Mine—Drainage Design Plan 6 <br /> 4110 2.3 Run-on/Run-off Peak Discharge Estimates <br /> Peak discharge from each sub-basin area was estimated using the Natural Resources Conservation Service <br /> (NRCS) graphical peak discharge method (TR55) developed by the U. S. Department of Agriculture for <br /> estimating surface runoff(NRCS, 1986). <br /> This method graphically generates a unit peak discharge rate (Q„) based upon the sub-basin time of <br /> concentration (Ta), curve number (CN) and associated initial abstraction (Ia), precipitation (P) and rainfall <br /> distribution type(type II for Colorado). Peak discharge(Qp)for the sub-basin area is then calculated from: <br /> Qp=QuAmq <br /> where: Qp= Peak discharge <br /> Q„= Peak unit discharge for the appropriate time of concentration(Ta) <br /> Note that'Fe includes Travel time for sheet flow(Tsh)+shallow concentrated flow(TOE)+ <br /> channel flow(Tch) <br /> Am= Sub-basin area in square miles <br /> q= Runoff in inches <br /> 2.4 Channel Design <br /> Channel capacity was evaluated with a single section analysis using Manning's Equation applied to a <br /> trapezoidal shape per the Colorado Department of Transportation Drainage Design Manual (CDOT 2004). <br /> Total depth has been specified at 2.0 x flow depth to maintain sufficient freeboard. <br /> 2.5 Culvert Design <br /> Values for hydraulic headwater, tailwater, barrel flow rate and barrel velocity were determined using <br /> methods and parameters provided in the Federal Highway Administration (FWHA) document HEC 22 <br /> (FWHA 2009) and modeled in the FHWA software program HY-8, Version 7.2 January 17, 2012. <br /> Although these design calculations indicate that smaller culverts could provide the carrying capacity <br /> required in some sub-basins, all culverts installed by Cotter will be 18-inch diameter minimum to reduce <br /> the potential of plugging with wind-blown debris and animal denning activities. <br /> 2.6 Spillway Design <br /> A broad crested weir, as described in HEC 22, chapter 8 (FWHA 2009), was used to establish spillway <br /> design parameters with additional weir coefficient data obtained from HDS #5, pg 38 (FWHA 2001) and <br /> the Urban Storm Drainage Criteria Manual (UDFCD 2011). All retention ponds have been designed to <br /> contain the 10-year 24-hour runoff volume with 1.0 ft of freeboard. This allows for 0.5 ft of spillway <br /> headwater with 0.5 ft of remaining freeboard whilst passing the 100 yr 24-hour event. <br /> 2.7 Riprap /Granular Bedding/Gravel Mulch <br /> Channel protection requirements and riprap sizing were established from HEC 15 (FWHA 2005), as were <br /> rip rap grading specifications. <br /> Riprap will meet the criteria of HEC-15 (FWHA 2005), including but not limited to: <br /> • Gradations will fall in the range of D100/D50 and D50/D20 between 3.0 to 1.5. <br /> • Placed thickness should equal the diameter of the largest rock size in the gradation. <br /> • Material shall be durable and angular but not slabby(length:thickness<3). <br /> • Riprap shall extend 1.0 x flow depth above flow level. <br /> 4122A.120702 Whetstone Associates <br />