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<br />HYDROLOGIC ANALYSIS <br /> <br />1. Methodoloqv <br />Discharge frequency relationships for Ralston Creek were <br />based on a rainfall-runoff analysis using the US Army Corps of <br />Engineers HEC-1 Flood Hydrograph Model. Rainfall' runoff <br />modeling was selected for developing the discharge frequency <br />relationships since no streamflow records exist for Ralston <br />Creek. Also, the rainfall runoff modeling allows for <br />evaluating the effects of existing and future reservoirs and <br />future urbanization. <br /> <br />2. HEC-1 Model Confiquration <br />The Ralston Creek basin was divided into 125 <br />subcatchments, which averaged 0.73 square miles (470 acres) <br />per subcatchment. Because of the substantial variability in <br />conditions across the basin, the basin was separated into two <br />distinct regions for modeling flood routing and to impart the <br />effects of precipitation primarily over the urban areas. The <br />high elevations of Ralston Creek, with its relatively large <br />subbasins, used the Snyder unit Hydrograph Method for <br />rainfall-runoff transformation and both the Muskingum-Cunge <br />and Kinematic Wave Methods of channel ro.uting. This upper <br />reach was divided into 5 subcatchments with an average of 9.1 <br />square miles (5,800 aqres) each. For the lower elevations and <br />plains/urban region the Kinematic Wave Method was used for <br />rainfall runoff transformation. The Kinematic Wave Method of <br />rainfall runoff transformation is preferable for relatively <br />small subbasins, particularly in urban watersheds, which exist <br />in the downstream portion of Ralston basin. In this lower <br />region, each subcatchment averaged 0.35 square miles (220 <br />acres). Channel routing in the lower region, similar to the <br />upper region, was accomplished using both the Muskingum-Cunge <br />and Kinematic Wave Methods. Several irrigation and <br />water-supply canals transverse this lower region. since it is <br />assumed that, during a storm, these canals would be running <br />full due to runoff transported from outside the basin, no <br />losses are calculated for them and they are not modeled in the <br />analysis. <br /> <br />a. Precipitation <br />Two different hypothetical storm systems were evaluated <br />for use in the rainfall-runoff calculation. In both cases, <br />precipitation data from the NOAA Atlas 2, Volume III <br />Colorado (USDOC, 1973) was used in the analysis. The first <br />storm was centered over the entire basin and used the median <br />rainfall rates found in the basin. The second storm was <br />centered over the portion of the basin below Ralston Reservoir <br />and used the rainfall rates found at the center of the storm <br />for the lower basin with residual precipitation over the upper <br />basin. This second storm was chosen for use in the model <br />since its concentration in the lower basin results in higher <br />peak flows in the urbanized area of the study. <br /> <br />4 <br />