2020-06-16_PERMIT FILE - C1981035 (28)

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Stoner Engineering & ,5urve[J. ing Engineering, Testing &Surveying Date: .tune 30, 2015 To: Marcia L.Talvitie, P.E. Environmental Protection Specialist 11 DRMS-Durango Field Office 691 Co. Rd. 233, Suite A-2 Durango, CO 81301 From: Ryan Griglak, P.E. Stoner Engineering& Surveying Re: King Coal II — Preliminary "As-Built" Hydrology Analysis Permit C-1981-035 Enclosed is the preliminary "as-built" hydrological analysis for GCC Energy, LLC's King it Mine located at 6473 County Road 120, Hesperus, Colorado 81326. The analysis includes all the drainage control structures located on the site. The computer modeling was done with SEDCAD version 4.0. The information for the analysis was obtained from aerial topography supplemented with a detailed field survey. The aerial topography has a much greater degree of accuracy than what was available for the original study. The basin size was slightly reduced and the soil properties modified to more accurately reflect the actual site conditions included the original report. Please see the enclosed information for the actual values. These changes resulted in an increase in the amount of run-off the East and West Clear Water Ditches experience during the design event. The increased flows do not exceed the capacity of either of the clear water ditches. The minimum culvert installed on-site is a 16-inch CMP due to maintenance considerations. This results in the internal culverts being generally oversized from the results of the SEDCAD modeling. The detention pond has the necessary volume available to store the 25-year, 24- hour storm event. Please let me know if you have any questions or comments in regards to the information provided, Sincerely, Ryan M. Griglak, P.E. P.O. Box 1163 28 South Washington Street Cortex, Colorado 81321 970.565.7483 stanerengmeer #rontier.net 970.565.7704