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b. Peak flow results are attributed to TR -55 methodology. DRMS was able to replicate some, but not <br />all the peak flow estimates. Note the structures need to be designed for the worst case runoff <br />scenario, which would be expected to correspond to the maximum disturbed area during <br />operations and before reclamation. <br />i. Please provide CN rationale for the curve numbers (CN) selected other than the "Arid <br />Rangeland, Pinyon- Juniper, Poor Condition ". <br />Response - The sandy nature of the waste material will have a significantly lower runoff <br />potential than the existing soil and vegetative cover. Potentially the runoff will increase as <br />the dumps are topsoiled during the ongoing reclamation, but this will still have a lower CN <br />than the base site (topsoil will removed from dump areas just prior to expansion of the <br />dumps again leading to a situation where the combined runoff potential is less than the base <br />condition). <br />ii. Please provide time of concentration estimates. <br />Response - Time of concentration estimates are being provided. <br />8. Exhibit U -2 - Design, calculations and methodology. <br />a. Spillway Riprap: The methodology and assumptions for the riprap calculations are adequate. There is <br />an error in the interstitial flow velocity calculations after the D50 is changed to 9 inches to account <br />for low density riprap (i.e., Cu = 1.95, instead of 2.0 and q ^ 5.86 cfs, instead of 5.01 cfs). This <br />does not affect the thickness result. However, because of the low density and the specification to <br />use well cemented sandstone, DRMS is concerned about the durability of the proposed riprap <br />material. Please provide a durability specification for the riprap. <br />Response - Due to obtaining the sizing formula, riprap calculations have been redone. Additional <br />riprap specifications have been added <br />b. Riprap Specification: Please provide a shape specification for the riprap. The maximum dimension <br />for a particular stone should be no more than 3 times the minimum dimension. <br />Response - A dimension ratio specification for the individual riprap stones has been added <br />c. Section Thru Spillway: The specified geo textile/filter fabric indicated is Mirafi 140N. DRMS <br />believes the unit weight of this material is approximately 5 oz/yd Given the size and thickness of <br />the riprap layer, we strongly recommend using a minimum 12 oz/yd" fabric. <br />Response - The geotextile specification has been amended to Mirafi 1120N or better. An alternate <br />design for a granular filter has been added. <br />