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The following aze the steps taken to determine the storage volume requirements in the <br />mined areas of the Castle Rock Quarry property: <br />a) Once the post-mining topography was determined, azeas were broken down into <br />sub-basins and their acreages calculated. <br />b) The maximum storage volume available in each sub-basin was computed, in <br />order to compaze with storage volume required to contain runoff. <br />c) Using the design storm recurrence interval discussed in Section III,C,4, and the <br />assumptions discussed above, the required storage volume was calculated. <br />4. Design storm recurrence intervals. The storm recurrence interval used for the <br />calculation of storage capacity required was back-to-back 100-yeaz 24-hour storm events. <br />The 10-yeaz event was not computed, since volumes will be substantially less than the <br />back-to-back 100-yeaz events. <br />D. Hydraulic Criteria <br />' 1. Identification of various capacity references. In apre-application meeting, the Douglas <br />County Public Works Department (DCPWD) determined the sub-basins in the post- <br />mining azea must have the capacity to hold stormwater from consecutive 100-yeaz 24- <br />hour storm events. <br />2. Oher drainage facility design criteria. The minimum required freeboazd for open space <br />detention facilities is 1.0 feet above the computed 100-yeaz 24-hour storm event water <br />surface elevation (as stated in the DCSDDTC manual, Section 14.4.3). The storage <br />' volumes actually provided allow for a minimum of 2 feet of freeboazd above consecutive <br />100-yeaz 24-hour storm events. <br />' Another key drainage element discussed in the pre-application meeting with the DCPWD <br />staff was the Willow Creek mine access road crossing. For design criteria, CMI proposes <br />' to install culverts sized to convey the 100-yeaz flood event without any additional flood <br />damage to adjoining properties beyond what would currently be experienced. <br /> Willow Creek drains approximately 1,840 acres to the south to southwest of the Castle <br />' Rock Quarry site. The 100-year storm event peak flow at the proposed crossing point <br /> was calculated as 959 cubic feet per second (cfs). This peak flood flow was computed <br /> using the Colorado Urban Hydrograph Procedure (CUHP) model, developed by the <br /> Urban Drainage and Flood Control District (UDFCD). This peak flow analysis was <br /> conducted for the previously mentioned 1989 DMG Regulaz Operations (112) permit <br />' application. The model results aze enclosed in Appendix B of this report. <br /> From a Manning's flow analysis it is estimated a 100-yeaz storm event on Willow Creek <br />' would flood a 30- to 50-foot width of the creek channel. This was based upon an <br /> assumed typical creek cross-section from the USGS mapping of the azea. <br /> <br /> <br />