Laserfiche WebLink
Transit Mix Concrete Co. <br /> management plan will be periodically reviewed and adjustment plans will be prepared and <br /> submitted to the CDRMS for review and approval prior to implementation. It is likely that storms <br /> larger than the 100-year, 24-hour event will be controlled to some extent, but sediment loads <br /> and volumes from larger storms will probably exceed the capability of the system to provide <br /> full control. <br /> 3. Hydrologic Model <br /> Design of the channels and calculation of run off quantities were carried out using a combination <br /> of the Rational Method for watersheds less than 100 acres in drainage area and the curve <br /> number method using SEDCAD Version 4, a commercially available software package, for the <br /> remaining watersheds. Both methods calculate the runoff response to a given precipitation event <br /> for specific surface topography, soil, and vegetative cover conditions. <br /> Rational Method <br /> Determination of the run off peak flows were carried out using spreadsheets based on and <br /> verified from the Urban Drainage and Flood Control District (UDFCD) Storm Drainage Criteria <br /> Manual equations which calculate the impervious percentage, the Rational `C' Coefficient, the <br /> travel time, and the runoff response to a given precipitation event (UDFCD, 2016). Application <br /> of the UDFCD methods involves subdividing the drainage area into sub-watersheds with <br /> relatively uniform surface characteristics. <br /> The only modification for the Rational method from the UDFCD was that the precipitation data <br /> were determined from National Oceanographic and Atmospheric Administration (NOAA) annual <br /> maximum point frequency estimates for the site area rather than from the City of Colorado <br /> Springs Drainage Criteria Manual (DCM) Volume 1. The DCM values were based on the NOAA <br /> Atlas 2 (1973) and are considered out-of-date by the DRMS. The NOAA values were generated <br /> for a point near the center of the proposed operation. The intensity-duration-frequency values <br /> for the 100 year storms were calculated using equation that consider the travel time/time of <br /> concentration duration. This equation was used in the peak flow determination spreadsheet to <br /> determine the precipitation intensities for each watershed based on the time of concentration <br /> values. The intensity-duration-frequency data and equation are presented in Attachment G-1. <br /> Based on the UDFCD methods, the peak flow determinations consisted of four steps. First, the <br /> impervious percentage for each watershed was determined based on the different land uses <br /> within the watershed. Second, each watershed was assigned a Rational `C' value descriptive of <br /> the watershed land surface characteristics. Separate C values were selected for the minor and <br /> major storm events. Third, the travel time or Time of Concentration was determined for each <br /> watershed providing an estimate of the duration for the maximum flow to occur. This time <br /> determination addresses both the overland flow and channel flow segments of the runoff <br /> duration. Fourth, these values were used, along with the precipitation data, to determine the <br /> Hitch Rack Ranch Quarry Permit Application M-2016-010 <br /> September 15, 2016 G-11 <br />