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JD -7 Mine — Drainage Design Plan 9 <br />where: <br />r = hydraulic radius = cross sectional area / wetted perimeter (A/Pw) (ft) <br />S = slope of the hydraulic grade line (channel bottom) (ft/ft) <br />n = Manning's n (roughness coefficient) for open channel flow (as discussed <br />in Section 2.3.1 and Table 6) <br />The Tc values calculated using WinTR -55 are provided in Attachment 1. <br />Runoff peak discharge is calculated from the TR -55 graphical peak discharge method. This approach <br />graphically generates a unit peak discharge rate (Q„) based upon the general catchment parameters of curve <br />number (CN), initial abstraction (Ia), precipitation (P) and rainfall distribution type (type 11 for Colorado) <br />and the individual catchment time of concentration (TJ. The input variables used in determining Q„ using <br />the graphical method were determined as follows: <br />CN = curve number based soil type, as discussed in Section 3 and Table 9 <br />Ia = initial abstraction, lookup value in Table 4 -1 of TR -55 based on CN <br />P = Precipitation, based on design storms listed in Section 2.1 and Table 3 of this plan <br />Tc = Time of concentration, calculated as discussed above <br />Peak discharge (Qp) for the catchment area is then calculated from: <br />Q, = Qu A. - q - FP <br />where: Qp = Sub -basin peak discharge (cubic feet per second [cfs]) <br />Q„ = Sub -basin unit peak discharge (cfs /mil /in [csm /in]) <br />A,,, = Sub -basin area (square miles) <br />q = Runoff (inches) <br />Fp = Pond and swamp adjustment factor <br />2.2.3 Computational Methods <br />2.2.3.1 Spreadsheet Simulation <br />Runoff volumes and peak discharge were calculated in Excel spreadsheets using the equations provided <br />above. These manual /spreadsheet calculations generally require that the unit peak discharge (Q„) be <br />determined graphically by interpolating from Figure 4 -II of TR -55. The graphical method was used for the <br />initial basin analysis and to check the accuracy of input values to the TR -55 basin model. <br />2.2.3.2 NRCS WinTR -55 <br />Peak discharge was also computed using the NRCS WinTR -55 software (NRCS, 2009). WinTR -55 is a <br />single -event rainfall - runoff, small watershed hydrologic model. The model generates hydrographs from <br />urban, agricultural, and rural areas and at selected points along the stream system. Runoff hydrographs <br />were generated by the model and routed downstream through channels. Multiple sub -areas were modeled <br />within the watershed and routed to the applicable diversion structures to outfalls or to the retention pond. <br />Although the computational methods employed in WinTR -55 are similar to the worksheet calculations in <br />TR -55 (NRCS, 1986), WinTR -55 uses the TR -20 software engine for more accurate analysis of the <br />hydrology of small watershed systems. <br />4149A.140207 Whetstone Associates • <br />