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Hydraulics <br />Hydrology <br />Water Resources <br />To: Mr. Garrett Varra, Varra Companies, Inc. <br />Mr. Brad Jones, Varra Companies, Inc. <br />From: Mr. Douglas Trieste, Flow Technologies <br />P.O. Box 6100 <br />Breckenridge, CO 80424 <br />970 - 547 -3823 <br />www.flowtechnologies.biz <br />MEMORANDUM <br />Date: January 14, 2014 <br />Subject: Response to Mr. Tim Cazier's Colorado DRMS, Memorandum (Varna- Coulson <br />Resource Project, Permit No. M- 2010 -049; Exhibit G — Water Information, Preliminary <br />Adequacy Review, December 13, 2013 (Re: Coulson Gravel Pit Mine Berm Failure Analysis and <br />Flood Control Mitigation Plan prepared by Flow Technologies and dated September 2, 2013) <br />Comment 3 <br />"Section 3.2.1, P. 13: The last paragraph of Section 3.2.1 discusses the need to characterize the <br />geotechnical composition for a complete erosion estimate. Please provide a discussion on <br />summarizing what values were used and how they were obtained for the following parameters: <br />a. Representative particle size, <br />b. Percent clay fraction, <br />c. Plasticity index, <br />d. Total unit weight, <br />e. Undrained shear strength, and <br />f. Critical shear stress. <br />Please also provide some discussion as to how these parameters are related to the erodibility <br />factor, Kd. " <br />Representative Particle Size. This value is the representative particle diameter in inches of the <br />soil material on the surface of the downstream face gravel pit berm. The representative particle <br />diameter is used in determining the soil grain roughness, and thereby erosional effective stress, <br />on the soil of the downstream face. This value is used in computations only when the <br />representative particle diameter is greater than 0.05 inches and value is required only when a <br />vegetative surface is being modeled. Because the headcutting /erosion modeling of Coulson Pit <br />did not consider a vegetative slope — rather bare soil for conservativeness — input of <br />representative particle size was not necessary. <br />Percent Clay Fraction. A geotechnical engineering report performed by Terracon Consultants, <br />Inc. ( Geotechnical Engineering Report, Coulson and BearsonSM Gravel Pits, Various <br />Locations, Weld County, Colorado, June 17, 2013, Terracon Project No. 21135012) indicates <br />that percent clay fraction is zero. <br />RECEIVED <br />JAN 2.2 20 1 4 <br />Division of Reclamation, Page 1 <br />Mining & Safety g <br />