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Prince Albert Mine — General Stormwater Comments <br />Page 3 <br />October 7, 2011 <br />b. Riprap Specification: Please provide a shape specification for the riprap. The <br />maximum dimension for a particular stone should be no more than 3 times the <br />minimum dimension. <br />c. 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 />d. 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 0.75 <br />feet. Please provide a distance dimension for each of the two riprap blanket <br />thicknesses. <br />e. 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 />f. Diversion Dike, Section B -B: The bottom channel width is required to evaluate <br />hydraulic stability. Please provide a minimum bottom with dimension and a <br />maximum side slope (e.g., 3H:1 V) on the inside of the diversion dike. <br />Channel Data — Slope: The maximum slope is listed as 0.120 ft/ft. The Plan <br />View shows a reach with a 12.71% slope. This also appears to be the narrowest <br />reach (refer to Comment 5.0. Please revise the calculations to reflect the steeper <br />design slope. <br />h. 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 />i. Detention Pond Volume: The columns are not labeled. DRMS assumes these <br />columns are (from left to right) "Elevation ", "Area (sq. ft.) ", "Cumulative <br />Volume (cu. ft) ", and "Cumulative Volume (cu. ft) ", respectively. Please label <br />the columns. Also, DRMS assumes one cumulative volume is obtained using <br />average end area the other is obtained using the frustum of a cone or pyramid. <br />Please select only one volume to display to avoid confusion. <br />9. Exhibit U -3 — Designs, calculations and methodology. <br />a. Orepad Detention: The topography between the Orepad Pond and the decline is <br />not clear. Please provide some designs (e.g., minimum channel depth, width, side <br />slopes, etc.) for the conveyance structure to demonstrate this channel has the <br />capacity to pass the 100 -year, 24 -hour peak flow to the decline. <br />b. Central Detention Overflow, Section E -E /Channel Data: The maximum slope is <br />listed as 0.125 ft/ft. The Plan View shows a reach with an 18.08% slope. Please <br />revise the calculations to reflect the steeper design slope. Also, please confirm in <br />the data that the channel is intended to be a "V" ditch by listing the bottom with <br />as 0.0 feet. <br />g. <br />c. Central Detention Overflow, Channel Data — Manning's n: The value of 0.028 <br />appears too high for bare soil and too low for riprap. Please provide rationale for <br />c: \documents and settings \grm\desktop\aurent projects \prince albert m -11- <br />040\stormwatercomments mem07oct 1 1 .docx <br />