Using Radar-Rainfall to Improve the Characterization of the Inflow/Infiltration(I/I) Response

The Metropolitan Sewer District of Greater Cincinnati (MSDGC) has developed a computer model of its wastewater collection system, referred to as the System Wide Model (SWM). The study drainage area is approximately 400 square miles, and the model includes 42,208 modeled sanitary and combined pipes. The model will be used to characterize and evaluate nearly 90 sanitary sewer overflows (SSOs) and approximately 230 Combined Sewer Overflows (CSOs), address wet-weather problems related to SSOs and CSOs, and ultimately used for real-time sewer system control evaluations. Correlating rainfall to the sewer system response, for both separate and combined sewer systems, is an integral process of the model development. The proper characterization of rainfall data is critical. This paper describes how radar-rainfall has been used to improve the characterization of the infiltration/inflow (I/I) response throughout the study area. Radar-rainfall data, calibrated with a network of ground rain gages, were used to account for the spatial variability of rainfall across the study area. This approach yielded more reliable estimates of rainfall than ground gages alone, due to the spatial variation phenomenon and systemic errors in collecting rain data from few ground gages. A comparison of the I/I response using radar-rainfall and using rainfall from the closest rain gage was made at several locations in the study area.