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JD -6 Mine — Drainage Design Plan 5 <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 6 <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 2 of this plan <br />T, = Time of concentration, calculated as discussed above <br />Peak discharge (Qp) for the catchment area is then calculated from: <br />Qv =Q. -A. -q - F, <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 />Am = Sub -basin area (square miles) <br />q = Runoff (inches) <br />Fp = Pond and swamp adjustment factor <br />2.2.3 Computational Methods <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 />Peak discharge was also computed using the NRCS WinTR -55 software (MRCS, 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 />Peak discharge for sub -basins reporting to the Stormwater Retention Pond was also computed using the <br />HydroCAD -10 software, distributed by HydroCAD Software Solutions, LLC. HydroCAD® uses the <br />procedures described in TR -55 and TR -20 with added features for multiple pond routing, variable pond <br />geometry, pond pumping, exfiltration, baseflow and inflow losses (that is, inflow to and exfiltration from <br />reaches), and an increased number of "nodes" (reaches, sub -basin, and ponds) over the limited number <br />available in WinTR -55. <br />2.3 Structure Design <br />2.3.1 Channel Capacity <br />Channel capacity was evaluated with a single section analysis using Manning's Equation. Based on site <br />conditions, channels were modeled as trapezoidal geometry with side slope ratios of 2:1, 4.8:1, 3.4:1, and <br />3.1:1 (Section 4.4, Attachment 3). The Upper Diversion Structure was also modeled using the planned 3:1 <br />side slope geometry (Section 4.4). <br />The value of Manning's n selected for each channel affects channel velocity, conveyance capacity, and <br />peak flows. The most important factors that affect the selection of channel n values are: <br />1. Type and size of the materials that compose the bed and banks of the channel; and <br />2. Shape of the channel. <br />4148A.131028 Whetstone Associates <br />