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5 Z <br />SMITH WILLIAMS CONSULTANTS, INC. <br />detention structures were not incorporated into the surface water models for sizing the diversion <br />ditches because the addition of the detention ponds will tend to reduce both the peak flow and <br />runoff volumes. Therefore, the diversion channels are designed to be conservative. <br />The data listed in the following table are a summary of the general hydraulic input parameters <br />used in the TR55 model: <br />Input Parameters for Derivation of Design Storm Runoff Flows <br />Storm Frequent -Duration 100- ear/24-hour Storm - <br />Storm Depth 3.5 inches <br />SCS Storm Type II <br />SCS Weighted Curve No. See Section 5.4 <br />Time of Concentration Basin De endent: -0.1 hours minimum <br />Travel Time Basin Dependent: 0.1 hours minimum <br />Sub-Watershed Area Drawings A500 and A610 <br />All composite curve numbers were developed based on curve numbers approved by the DRMS <br />(CC&V 1997a). A copy of this letter is presented in Appendix F. The detailed peak flow <br />calculations for Phase 5 VLF surface water models are included in Appendix F. Results from the <br />• hydraulic analyses indicate that the operational peak flow will be approximately 37.5 cfs entering <br />the sedimentation pond located north of the Phase 5 VLF. The peak flow from the southeastern <br />diversion channel is estimated to be 17 cfs, this flow will be routed to the base of Squaw <br />Mountain. <br />5.4 Runoff Curve Numbers <br />The composite curve numbers used in the hydrological modeling were calculated from curve <br />numbers for three general-land uses inside the sub-watershed areas that contribute to the Phase 5 <br />VLF. surface-water diversions. These uses are classified as "original ground,"_"roads and <br />stockpiles," and "fill slopes." As noted above, the values of the Soil Conservation Service (SCS) <br />[now Natural Resource Conservation Service (MRCS)] curve numbers are identical to those <br />submitted by CC&V to the DRMS, as documented in a March 4, 1997, letter from CC&V <br />(1997a) titled, "Cresson Project: Permit M-1980-244: Surface Water-Drainage Modifications <br />Arequa Gulch Overburden Storage Area, "for the Cresson Project. A copy of this letter is <br />presented in Appendix F. <br />The three land use categories are described as follows: <br />^ Original Ground: SCS Soil Type B (sandy soils and loess) was assumed in these <br />areas and consists primarily of 4 to 12 inches of growth media covered with grass, <br />S:\PROJECTS\1125 CCBV PHASE 5 VLF\H2 - DESIGNWREQUA VLF\VLF PHASE 5 FINAL REPORT V2.DOC 3 p <br />