Effect of Raingage Density on Runoff Simulation Modeling

Rainfall and runoff data for a 3.08 square mile urban watershed in Denver, Colorado was used to investigate the effects of raingage density and hyetograph compositing on urban stormwater runoff simulation. This watershed has rainfall data from five raingages and flow data from two gages, all in 5-minute time increments. The data were used to calibrate the EPA Storm Water Management Model (EPA SWMM, version 5.0), and the Urban Drainage and Flood Control District's version of an earlier EPA SWM model (UDSWMM). This calibrated model served two purposes; first, to examine the effects on runoff calculations using a single composite hyetograph for each of the recorded storm rainfall and runoff events modeled and second, to determine the effects of raingage density on volume of runoff and peak flow simulations. The effects of raingage density were investigated by processing rainfall data from each raingage using several different combinations of raingage densities. The data were used to drive the calibrated SWMM model and the results compared against recorded runoff volumes and flow rates for the recorded storms. It was demonstrated that raingage density does affect the accuracy and the scatter of simulated results. The effects of compositing recorded rainfall data at several raingages into a single rainfall hyetograph were also investigated. Two types of compositing were performed. One method simply used area weighted averaging of rainfall recorded at each time increment. This method was defined as straight across compositing. In the second method, each hyetographs at the five gages was examined and all values were time shifted to line up the most intense five-minute rainfall values of the storms. The simulation results were then compared to the recorded flows This paper describes the findings of this study and discusses their implications for urban stormwater runoff modeling. It presents a summary the collaborative work and findings by the authors over the last 17 years.