2011-07-11_PERMIT FILE - M2011001 (12)

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information. For flood plain information refer to the Flood Hazard Development application dated February, 2011, prepared by Tetra Tech. 4.0 ANALYSIS CRITERIA Design Criteria and Constraints This report was prepared in compliance with the Urban Storm Drainage Criteria Manual (USDCM). The Criteria was followed in accordance with the Weld County Code and the Weld County Storm Drainage Criteria Addendum to the Urban Storm Drainage Criteria Manuals Volumes 1, 2 and 3. Based on these criteria, a 100 -year storm was used as the major storm in the drainage analysis. The constraints related to mining and reclamation of the Iverson Mine site deal primarily with mining setbacks from property boundaries, wetlands, river banks, and existing gas /oil wells and appurtenances. During mining phases, temporary topsoil and overburden stockpiles will be constructed adjacent to the mined areas. These temporary stockpiles will be located outside of the floodway boundaries and oriented parallel to the direction of overbank flow. Upon completion of the reclamation phase of the project, all temporary stockpiles will be removed and final grading within the limits of the 100 -year floodplain will be to an elevation at or below pre - mining grades. Hydrologic Criteria Since all basins were less than 160 acres in area, the Rational Method was used in stormwater runoff calculations. Runoff coefficients (C), applied in the Rational Method, were weighted based on the historic and existing land use, and on the types of soils on the site. The site rainfall depth information was obtained using the information provided in the City of Greeley Storm Drainage Design Criteria and Construction Specification Document. The 5 -year and 100 -year 1 -hour storms have magnitudes of 1.49 inches and 2.78 inches, respectively. The percent imperviousness was determined for the anticipated land use from Table RO -3 of the Urban Drainage and Flood Control District Drainage Criteria Manual (UDFCDM). The runoff coefficients were then taken from Table RO -5 of the UDFCDMbased on the soil map average. Historic runoff coefficients are C5 = 0.128, and C100= 0.442 for basin H -1 and C5 = 0.112, and C100 = 0.409 for basin H -2. Historic flow calculations are presented in Appendix D. Please refer to Historic Drainage Plan presented in Appendix D. Hydraulic Criteria During the mining phases of this project, direct precipitation on the mined areas and runoff from minor on -site basins will be captured in the mining areas. A dewatering trench and dewatering pump will be utilized to remove this captured runoff from the mining areas. If necessary, discharge permits will be obtained for each of these dewatering discharge points to allow discharge to the natural drainage ways. The runoff from the site is significantly reduced after reclamation is completed as compared with the existing conditions. The reclaimed pond will capture and attenuate surface runoff that historically flowed across the parcels. Existing natural drainage ways will remain -2 Final Drainage Report Iverson Mine February 2011 P:\23511\133-23511-10002 \Deliverables \Drainage Report \Drainage Report.doc