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DESIGN CRITERIA <br />Desiqn Criteria and Constraints <br />This report complies with technical criteria set forth in the Weld County Storm Design Criteria and <br />the Urban Storm Drainage Criteria Manual, Volumes 1, 2, and 3, by the Urban Drainage and Flood <br />Control District. As previously discussed, the Parsons Mine site lies within the Bracewell Basin <br />and previous drainage studies and/or master plans have not been developed for this basin. <br />The constraints related to mining and reclamation of the Parsons Mine site deal primarily with <br />mining setbacks from property boundaries, wetlands, floodway boundaries, critical wildlife habitats, <br />and existing gas/oil wells. During mining phases, temporary topsoil and overburden stockpiles will <br />be constructed adjacent to the mined areas. These temporary stockpiles will be located outside of <br />the floodway boundaries and oriented parallel to the direction of overbank flow. Upon completion <br />of the reclamation phase of the project, all temporary stockpiles will be removed and final grading <br />within the limits of the 100-year floodplain will be to an elevation at or below pre-mining grades. <br />Hydrologic Criteria <br />The rational method has been used to estimate peak stormwater runoff for the 100-year storm return <br />period for subbasins with drainage areas of 160 acres or less. Runoff from subbasins with drainage <br />areas greater than 160 acres was estimated using the Colorado Urban Hydrograph Procedure <br />(CUHP). Supporting calculations are provided in Appendix B. Rainfall intensity-duration- <br />frequency data is based on information provided in the City of Greeley Storm Drainage Design <br />Criteria and Construction Specification Document. The 100-year 1-hour storm has a magnitude of <br />2.78 inches. <br />Hydraulic Criteria <br />During the mining phases of this project, temporary minor conveyance channels will be constructed <br />to keep runoff from frequent storm events from entering the mining areas. In many places, these <br />minor channels will be constructed alongside temporary topsoil and overburden stockpiles. The <br />design of these temporary conveyance channels is presented in the Erosion Control Plan. Direct <br />precipitation on the mined areas and runoff from major storm events will be captured in the mining <br />areas. A dewatering trench and dewatering pump will be utilized to remove this captured runoff <br />from the mining areas. If necessary, discharge permits will be obtained for each of these <br />dewatering discharge points to allow discharge to the natural drainageways. <br />The runoff from the site is dramatically reduced after reclamation is completed as compared with <br />the existing conditions. The unlined reclaimed ponds will capture and attenuate surface runoff that <br />historically flowed across the parcels. Existing natural drainageways will remain undisturbed. For <br />reclamation, there are no proposed hydraulic components such as conveyance channels, rundown <br />channels, culverts, grade control structures, or detention outlets. Each of the unlined ponds will be <br />constructed with an outlet pipe installed to maintain the normal pool elevation. <br />-5- Revised Preliminary Dminage Report.docx