Abstract

The Allegheny County Sanitary Authority (ALCOSAN) and its 83 member municipalities in the greater Pittsburgh, Pennsylvania region are in the process of implementing a comprehensive wet weather sewer overflow management program to improve and preserve the water environment in the service area. Simulation models are required to develop, evaluate and select technically viable, cost-effective regional facilities and operational changes needed to control wastewater flow. High resolution spatially- and temporally-varied precipitation data is needed for the region so that models can accurately quantify and characterize complex interrelationships between rainfall, runoff, wastewater flow, and overflow volumes. The costs to develop and implement control facilities can be high and ALCOSAN aggressively is pursuing all opportunities to significantly reduce this cost for its customers. Therefore, the investment was made to implement a regional radar-rainfall system with the belief that improved precipitation estimates would significantly reduce the ultimate cost for wastewater management and regulatory compliance.

It has been demonstrated and documented in cities throughout the United States and around the world that calibrated radar-rainfall systems significantly can improve the accuracy and precision of rainfall measurement for large geographic areas. However, quantitative evaluations are relatively sparse documenting whether enhanced rainfall measurement actually translates to improved wastewater flow prediction performance from hydraulic/hydrologic models. The ALCOSAN Lower Ohio River basin project provided a unique opportunity to characterize and quantify the impacts of improved precipitation estimates on the accuracy and reliability of model-predicted wastewater flow and CSO/SSO frequencies and volumes. Analyses were conducted for 46 monitored storm events to determine if simulated/monitored storm peak and volume relationships improved significantly as a result of improved rainfall measurement.

The Lower Ohio basin study was successful in quantifying and characterizing the levels of improvement to modeling accuracy, resulting from improvements to the accuracy and resolution of precipitation data. In general, significant improvements consistently were observed between simulated and observed storm peaks as refinements to model input precipitation data were made. In contrast, within many of the analyzed sewersheds relatively little improvement was observed between simulated and observed total storm volumes as refinements were made to the precipitation data. Results were highly mixed within the analyzed sewersheds regarding the continued level of improvement to simulation model results when the source of precipitation data was upgraded from the gauge network to the radar rainfall system. The study demonstrated that the geographic distribution of storm depths, intensities and durations is significant in the greater Pittsburgh region. If a traditional gauge network were to be employed, many gauge sites would be needed to monitor the orographic distribution of rainfall, and the level of effort and cost to maintain the gauge network would be high. It was concluded that developing and implementing a regional gauge adjusted radar-rainfall system was a worthwhile investment that will result in multiple benefits to the region and reduce the ultimate cost for wastewater management and regulatory compliance.

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