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SURFACE WATER RESPONSES TO COl4~NTS BY CMI.RD on Rocky Mountain Asphalt's Summit Pit Application These responses are to comments made by C. M. Farrell, Colorado Mined Land Reclamation Division (CMRD), Reclamation Specialist, to Rob Mangone, General Manager of Rocky Mountain Asphalt, in a memorandum dated October 27, 1988. Desi n Storm Hydrology Several of the comments relate to conveyance of the design storm in the existing and constructed channels. The design storm standard for channels is specified by CMLRD to be the 100-year, 24-hour storm event, No available stream gaging records were located for Waldo Canyon or adjacent small watersheds from which to perform a statistical analysis of the flood characteristics. Therefore, a synthetic method of deriving the 100-year flood was required to develop the design discharges. Several methods were evaluated to calculate the 100-year .- discharges. The most straigt~Arrward method is referred ~to as the Colorado Tehnical Manual 'f--rhethod, prepared by the Colorado Water Conservation Board (1976). This procedure is a regression type method based upon watershed and precipitation characteristics for natural-flow streams in Colorado. Waldo Canyon and the East tributary are certainly natural-flow streams; however, two limitations of the procedure prevented its application to the area: 1) Waldo Canyon is eight times smaller than the smallest basin used to develop the regression relationship; 2) Waldo Canyon is at an elevation that makes both the Plains region and the Mountain region procedures appropriate. Both relationships were applied but the difference between the two procedures was a factor of 100. This large variation prevented any meaningful averaging of the procedures. The procedure that ~~~ ~~.~..~~ to eotoite the design discharged was the HEC1 computerised flood hpdrograph package developed by the US Army Corps olr:.$giaeers (1985). This procedure has the advantages that: 1) it is a widely used and accepted hydrologic tool; 2) it incorporates multiple watershed charateristics that allow the simulation of all types and sizes of watersheds; and 3) HEC1 produces complete hydrographs, not just peak flow rates as does Technical Manual 1. Basin characteristics were measured for the hydrologic simulation. The watershed boundaries for Waldo Canyon and the East tributary are shown in figure Exhibit B (figures are labeled according to the appropriate CMLRD permit application exhibit). The measured watershed characteristics are listed in Table 1. The topographic relief map of Exhibit B shows the steep slopes of Surface Water Responses: Summit Pit Application paq• 1