2008-03-05_REVISION - M1997054 (31)

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1 DRAINAGE DESIGN CRITERIA d. Drainage Criteria The elements of this narrative and the corresponding Drainage Plan have been assembled in adherence to the applicable portions of Section VI, Sub-Section G and Appendix 1, Sub-Section III of the Fremont County Subdivision Regulations, as available on the Fremont County Planning and Zoning website. Additionally, this Drainage Report has been prepared in adherence to applicable stormwater control standards set forth by the Colorado Department of Public Health and Environment. e. Development Criteria The methods of site-specific stormwater management employed within this plan have been developed to mitigate potential disturbances to naturally occurring hydrogeologic and ecological systems within, or ' immediately adjacent to the Parkdale Quarry property. By retaining run-on within the sand and gravel pit and granite quarry during the mining processes, historic flow regimes shall be relatively maintained through the use of dewatering systems in the case of the quarry, analogous to those used to dewater ' generated process-waters, and through the large infiltration capacity of the gravel deposit within sand and gravel pit. ' Runoff intercepted by the granite quarry is not expected to be met with nearly the same capacity for infiltration due to the nearly impermeable nature of the granite deposit. Thus, alternative stormwater management techniques have become necessary to develop for use within the quarry to ensure proper compliance with stormwater management regulations. Following reclamation of both the sand and gravel pit and granite quarry, it is the intention that no increases in flood elevations shall be induced by the remaining reservoir facilities. The expectation is that flood flows from contributing basins will be retained within the pit or reservoir, until the water surface reaches the elevation of the spillway, at which time all remaining inflows will be routed through the system. Once final reclamation is complete and final reservoir operating criteria are developed, an analysis will be performed to fine-tune the site-specific stormwater management techniques. r f. Hydrological Criteria For this drainage design, on-site basins were analyzed in HEC-HMS using the NRCS SCS Type II rainfall distribution. Please refer to Appendix C for results and documentation. All of the basins were assumed to be five percent impervious, which is a very conservative value for greenbelts or agricultural areas. An NRCS soil type of D was used for these basins based on the predominant soil types on the site. A curve number of 84 was used for each basin, which corresponds to ' a land use condition of "Pasture Grassland Range (FAIR)" with a soil type of D. Due to the proposed 50-year mining duration for the granite quarry, the design rainfall event selected for this project was the 100-year, 24 hour event. In the interest of conservancy, the 100-year, 24 hour storm served as the design storm for all stormwater quantification for both the granite quarry and sand and gravel pit. A rainfall depth of 3.4 inches was used for the respective storm event, as defined within the Precipitation- Frequency Atlas of the Western United States Volume III- Colorado for this region of Colorado. ' Parkdale Quarry Drainage Report Fremont Count File No. CUP 07-003