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Response to DRMS General Stormwater Comments — Cotter JD -9 Mine Drainage Design Plan <br />depths can be seen on the top of the "Master Design Storm Summary" included in <br />Attachment 1. <br />3. Page ESWMP -6, last paragraph, DDP Drawing 3 of 7, and FlowMaster output pages. It <br />is stated the channels are "capable of transporting the 100 year flows ". A Manning's n <br />value varying from 0.030 to 0.045 is used for different channel design analyses. <br />However, no rationale is provided for the selected roughness coefficients. Furthermore, <br />channel roughness is seldom uniform, the DBMS requires channels be evaluated for both <br />stability and capacity, i. e., minimum and maximum expected roughness. For example, an <br />excavated earth channel, after weathering would be expected to have a minimum <br />n = 0.018 (use to evaluate stability or maximum expected velocity); and a maximum <br />n = 0.025 (use to evaluate capacity). In addition, the DRMS requires channel freeboard <br />be evaluated for all engineered channels: channels shall be designed with a minimum of <br />0.5 feet of freeboard unless the velocity head (v212g) is significant, then the minimum <br />required freeboard is half the velocity head, or v2 14g. <br />a. Please provide a rationale for the selected roughness coefficients, and evaluate <br />each designated channel /ditch design slope for both capacity and stability. <br />Attachment A identifies missing design analyses. <br />Our selection of Manning's "N" coefficients involves looking at a variety and range <br />of possible values found in several reliable and respected resources such as: tables <br />from the SCS TR -55 Manual (Urban Hydrology for Small Watersheds), the <br />National Engineering Handbook (Section 4, Hydrology), V.T. Chow (Open <br />Channel Hydraulics), and the Mesa County Stormwater Management Manual <br />which includes tables from many of these sources. Copies of the tables referred to <br />here are included with this response in Attachment 2. The values used in the <br />calculations are determined by comparisons of similar materials from a variety of <br />sources and, based'on experience, selecting those from the tables which most <br />closely match the'particular surfaces being evaluated. A range of values was used, <br />as suggested, to evaluate channels for capacity and freeboard using the roughest <br />numbers and to evaluate them for velocity and armoring requirements using the <br />smoothest numbers. For onsite channels, existing or proposed, these values <br />typically ranged from n =0.035 to n= 0.045. For offsite channels with existing trees, <br />brush, and large rocky material, the values ranged from 0.045 to 0.060. <br />b. Please design all engineered ditches with the appropriate freeboard and provide <br />channel design depths for construction. <br />All engineered channels were designed with appropriate freeboard. The channel <br />geometries and flow depths are shown on the details included in the drawings, <br />(Attachment 5) and on the calculation worksheets included in Attachment 3. <br />2 <br />