Estimating Unit Hydrograph Peak Rate Factors

Abstract

The Soil Conservation Service (SCS) dimensionless unit hydrograph model TR‐20 is widely used for hydrologic design. The model uses a dimensionless unit hydrograph, with the drainage area, the depth of the runoff volume, the time‐to‐peak, and the SCS peak rate factor required to dimensionalize the unit hydrograph. A standard value of 484 for the peak rate factor is commonly used. While vague guidelines have been provided for modifying the value of the parameter, almost all designs are made using a value of 484. A method is presented that provides a systematic means of obtaining a value of the peak rate factor that is unique to a watershed. The method only requires a runoff curve number map, which should be developed to use the TR‐20 model anyway. Application of the method to six watersheds, including three coastal basins, indicates that the method increases the accuracy of prediction; the peak rate factors computed using the procedure outlined compared very favorably with the values obtained by optimizing on measured rainfall hyetographs and runoff hydrographs. The results indicate that the value of 484 is too large for coastal watersheds on the Delmarva peninsula.