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Prince Albert Mine — General Stormwater Comments <br />Page 5 <br />October 7, 2011 <br />e. Channel Data — Design Flow: Exhibit U -1 does not list a peak flow result for <br />either subbasin PA -N1 or PA -N2. Please provide appropriate calculations, <br />including: <br />i. CN rationale for the curve numbers (CN) selected other than the "Arid <br />Rangeland, Pinyon - Juniper, Poor Condition ". <br />ii. Time of concentration estimates. <br />iii. Contributing areas. <br />f. Channel Data — Slope: The maximum slope is listed as 0.063 ft/ft. The Plan <br />View shows a reach with a 6.63% slope. Please revise the calculations to reflect <br />the steeper design slope. <br />Channel Data — Manning's n: The value of 0.028 appears too high for bare soil <br />and too low for riprap. Please provide rationale for the selection of 0.028. Note <br />flow velocities exceeding 5.0 fps will require a discussion on armoring/erosion <br />protection. <br />h. Section Thru Spillway: The specified geotextile/filter fabric indicated is Mirafi <br />140N. DRMS believes the unit weight of this material is approximately 5 oz/yd <br />Given the size and thickness of the riprap layer, we strongly recommend using a <br />minimum 12 oz/yd fabric. <br />i. Section Thru Spillway: The designed riprap layer is 1.5 thick on the embankment <br />slope and for some distance downstream before the thickness is reduced to 0.76 <br />feet. Please provide a distance dimension for each of the two riprap blanket <br />thicknesses. <br />g. <br />Section Thru Spillway: No cross - sections perpendicular to the flow direction are <br />provided. Please provide lateral cross - sections through the embankment and <br />through the run out downstream to enable an evaluation of flow containment. <br />11. Exhibit U -4 — Designs, calculations and methodology. <br />a. Detention Pond Volumes: The columns in the three "Volume Provided" tables <br />are not labeled. DRMS assumes these columns are (from left to right) <br />"Elevation ", "Area (sq. ft.) ", "Cumulative Volume (cu. ft) ", and "Cumulative <br />Volume (cu. ft) ", respectively. Please label the columns. Also, DRMS assumes <br />one cumulative volume is obtained using average end area the other is obtained <br />using the frustum of a cone or pyramid. Please select only one volume to display <br />to avoid confusion <br />J• <br />If either you or the applicants have any questions regarding the comments above, please call me <br />at (303) 866 -3567, extension 8169. <br />c: \documents and settings \grm \desktop\current projects \prince albeit m -11- <br />040\.stormwatercomments mem07oct l 1 .docx <br />