Laserfiche WebLink
Prince Albert Mine — General Stormwater Comments <br />Page 2 <br />October 7, 2011 <br />provide details for this channel and design calculations demonstrating that it can pass the <br />peak flow resulting from the 100 -year, 24 -hour storm. <br />a. No stage- storage or storage volume capacity information is provided for the <br />Orepad Pond (as is for the South Pond on Exhibit U -2). Please provide this <br />information for the Orepad Pond. <br />5. Page 23 (EPP), and Figure E.1 — Primary Waste Pile. There is a discrepancy between the <br />Primary Waste Pile area shown on Figure E.1 (1.130 acres) and that on page 23, section <br />U(6.b.1) as 1.146 acres. Please correct the discrepancy. <br />6. Page 29 (EPP), and Figures Exhibit U -3 and Exhibit U -4 — North and Central Ponds. The <br />second paragraph indicates a storage capacity of 4,624 and 5,837 cubic feet for the North <br />Detention Pond and the Central Detention Pond, respectively. However, no stage- storage <br />or information is provided for either pond on Exhibits U -3 and Exhibit U-4, respectively <br />(as is for the South Pond on Exhibit U -2). <br />a. Please provide this information for the North Detention Pond on Exhibit U-4. <br />b. Please provide this information for the Central Detention Pond on Exhibit U -3 <br />7. Exhibit U-1 — Overall Plan, calculations and methodology. <br />a. The 10 -year runoff volume appears to utilize a method involving a 10 -year <br />coefficient, C10. Please provide a source for the methodology and rationale for <br />the selected coefficients. Alternatively, use the widely accepted TR -55 <br />methodology used for the peak flows and provide rationale for the curve numbers <br />(CN) selected other than the "Arid Rangeland, Pinyon - Juniper, Poor Condition ". <br />Note: other CNs will be needed to represent the roads, industrial areas, waste <br />dump tops and waste dump out slopes. An estimate of the acreages for each and <br />the selected CN. <br />b. Peak flow results are attributed to TR -55 methodology. DRMS was able to <br />replicate some, but not all the peak flow estimates. Note the structures need to be <br />designed for the worst case runoff scenario, which would be expected to <br />correspond to the maximum disturbed area during operations and before <br />reclamation. <br />i. Please provide CN rationale for the curve numbers (CN) selected other <br />than the "Arid Rangeland, Pinyon - Juniper, Poor Condition ". <br />ii. Please provide time of concentration estimates. <br />8. Exhibit U -2 — Design, calculations and methodology. <br />a. Spillway Riprap: The methodology and assumptions for the riprap calculations <br />are adequate. There is an error in the interstitial flow velocity calculations after <br />the D50 is changed to 9 inches to account for low density riprap (i.e., Cu = 1.95, <br />instead of 2.0 and q = 5.86 cfs, instead of 5.01 cfs). This does not affect the <br />thickness result. However, because of the low density and the specification to use <br />well cemented sandstone, DRMS is concerned about the durability of the <br />proposed riprap material. Please provide a durability specification for the riprap. <br />c: \documents and settings \grm \desktop\current projects \prince albert m -11- <br />040\stormwatercomments mem07octl 1 .docx <br />