Laserfiche WebLink
At Design Point DP8 which includes Subbasins H8, P1-1, P1-3, P1-4, P1-5, P1-6, RB5, and R136, <br />the peak flow rate reduces from the historical condition rate of 55.4 cfs to a reclaimed condition of <br />22.8 cfs and a Phase 1 mining condition rate of 11.5 cfs. The drainage area contributing to flow <br />leaving the site reduces from the historical condition of 85 acres to a reclaimed condition of 28 <br />acres and a Phase 1 mining condition of 15 acres. <br />At Design Point DP9 which includes Subbasins H9, P1-2, P1-7, P1-8, P1-9, P1-10, P1-11, P1-12, <br />P1-13, and R137, the peak flow rate reduces from the historical condition rate of 32.5 cfs to a Phase <br />1 mining condition rate of 30.1cfs. The peak flow rate for a reclaimed condition remains unchaged <br />at 32.5 cfs. The drainage area contributing to flow leaving the site increases from the historical <br />condition of 25 acres to a Phase 1 mining condition of 30 acres and returns to a reclaimed condition <br />of 25 acres. <br />RECLAIMED CONDITIONS <br />Upon completion of the gravel mining, the site will be reclaimed as open water ponds, wetlands, <br />and upland areas as shown in Exhibit 3. The designed spill elevation for each of the open water <br />ponds is based on and groundwater model of the site. Each unlined pond has a minimum of <br />approximately 2 feet of freeboard before overtopping could occur. The total effective detention <br />volume of the reclaimed project is approximately 200 acre-feet. <br />The surface hydrology associated with the reclaimed condition will not adversely impact the <br />adjacent properties. Final grading will result in contours that either match or are below pre-mining <br />contours. Surface runoff from the majority of the site will flow directly into the unlined ponds, <br />wetlands, and upland areas. <br />CONCLUSIONS <br />Compliance with Standards <br />All drainage analyses have been performed according to the Weld County Storm Drainage Criteria <br />and the Urban Storm Drainage Criteria Manuals, Volumes 1, 2, and 3 by the Urban Drainage and <br />Flood Control District. <br />Drainage Concept <br />The proposed Parsons Mine development will dramatically reduce the runoff from the site due to <br />the capture of runoff within the mining area. Although only Phase 1 of the mining plan is presented <br />in this Preliminary Drainage Report, each of the subsequent mining phases will significantly reduce <br />surface runoff compared to existing conditions. Upon completion of each phase of the mining <br />project, the screening/overburden stockpile berms will be removed and used for reclamation of the <br />unlined open water ponds, wetlands, and upland areas. <br />-14- Revised Preliminary Dminage Repor[.doca