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Response to DRMS Adequacy Review #2 — Cotter JD -8 Mine Reclamation Plan Amendment <br />excessive. These excessive travel times result in very low peak flow estimates. It should <br />also be noted that travel times less than one minute (0.017 hours) are negligible and <br />should be ignored (reference channel travel times in Table 9). <br />Cotter response: The input variables to the sheet flow travel time equation were re- <br />evaluated in response to this comment by DRMS, and two input variables were <br />modified. Sheet flow travel time is calculated using the simplified form of <br />Manning's kinematic solution, Equation 3.3 of TR55 (NRCS, 1986): <br />0.007 (nL) <br />Tsh — p0.5 <br />With: <br />T = travel time of sheet flow (hr), <br />n = Manning's roughness coefficient, <br />L = length of flow path (ft), <br />P = precipitation, 24 -hour rainfall (in), <br />S = slope (ft /ft). <br />In the preceding equation, slope, flow length and precipitation are essentially fixed <br />for each sub -basin and the value of n is the only subjective variable. The long sheet <br />flow travel time previously presented for Sub -basin 1 -2 -3 resulted from a naturally <br />low slope angle (S = 0.015), a moderately high roughness coefficient (Manning's n = <br />0.2), and an erroneous sheet flow distance (L = 260 ft). <br />The sheet flow distance was corrected to 160 ft and Manning's n was reduced to 0.05 <br />for the relatively flat, stony, irregular ground located near the mine portal. <br />Similarly, Manning's n was reduced from 0.20 to 0.10 for the undisturbed existing <br />pinion - juniper /sagebrush slope although the distance remained unchanged (140 ft) <br />for this portion of the sub - basin, upslope from the mine portal area. Below the <br />mine portal area, travel time also includes concentrated overland flow across the <br />waste rock pile, followed by channel flow down the face of the pile. The short <br />channel flow travel times of concentration flow and channel flow were included for <br />completeness, but are negligible compared to the sheet flow travel times. The <br />recalculated travel times and peak discharge are provided in Table 1 below and are <br />included in the revised Drainage Design Plan. <br />The travel times for Sub -basin 4 -5 -6 were also recalculated using the new values for <br />Manning's n. The resulting travel times and peak discharge are provided in Table 1 <br />below and are included in the revised Drainage Design Plan. Compared to the <br />previous values, estimated travel times are reduced and estimated peak flow rates <br />are increased. <br />Page 7 <br />