Application of Radar-Rainfall Technology in Combined Sewer Model Development and Calibration

In May 2000, the Metropolitan Sewer District of Greater Cincinnati (MSDGC) initiated the development of a computer model of its collections system, referred to as the System Wide Model (SWM). Upon completion, the calibrated model will be used to characterize and evaluate nearly 90 sanitary sewer overflows (SSOs) and approximately 230 combined sewer overflows (CSOs), address additional wet-weather problems that impact pump station and treatment plant operations and ultimately evaluate the use of real-time controls (RTC) within the system. This paper describes the use of radar-rainfall technology in the development and calibration of combined sewer models.Precipitation is the most important model input and the accuracy of the model results directly depends on this input. Historically, precipitation data for sewer modeling have been provided by individual rain gauges distributed throughout the area to be modeled. The accuracy of the data obtained from these gauges is a function of the equipment, its location and maintenance. The biggest limitation on developing and calibrating computer models, however, is the resolution of the rain gauge network and resulting accuracy of the spatial variability. To alleviate the impact of spatial variation on a rain gauge network, radar can be used to provide a more complete precipitation cover.The radar measurements were collected using the National Weather Service's (NWS) NEXRAD (NEXt generation RADar). The radar images provide reliable aerial templates of rainfall at the desired resolution; they may not, however, provide data that represent actual ground capture rates. The individual precipitation gauges, if installed and maintained properly, provide reliable point estimates of the rainfall reaching the ground surface. Combining these two types of rainfall measurement techniques provides reliable aerial distribution of the rainfall, accomplished by calibrating (or ground truthing) radar data using rain gauge measurements.