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n <br /> Transit Mix Concrete Co. <br /> calculate the runoff response to a given precipitation event for specific surface topography, soil, <br /> 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). The County <br /> relies on the City of Colorado Springs for their Drainage Criteria Manual, and the City's Manual <br /> has the same requirements as UDFCD. UDFCD has established spreadsheet for use in <br /> preparing drainage designs, and these spreadsheets, downloaded from UDFCD website, were <br /> used for the rational method calculations. Application of the UDFCD methods involves <br /> subdividing the drainage area into sub-watersheds with relatively uniform surface <br /> 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 Atlas 2 (1973)and are considered out-of-date by the Division. 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 /> runoff peak flows from the major (100-year) storm event. The UDFCD result tables presenting <br /> these calculations are included in Attachment G-1. <br /> Curve Number Method <br /> Design of the channels and calculation of run off quantities were carried out using SEDCAD <br /> Version 4, a commercially available software package that calculates the runoff response to a <br /> given precipitation event for specific surface topography, soil, and vegetative cover conditions <br /> using subroutines from TR-55 (USDA-SCS, 1986). Application of the SEDCAD 4 program <br /> Hitch Rack Ranch Quarry Permit Application <br /> October 3,2017 G-17 <br />