DEVELOPMENT AND EXPANSION OF A CALIBRATED RADAR RAINFALL MONITORING SYSTEM

The 3 Rivers Wet Weather Demonstration Program (3RWWDP) was established in 1997 to provide municipalities located in Allegheny County, Pennsylvania, the technical means, institutional structures and financial mechanisms needed to control existing sanitary sewer overloading and overflow problems. The 3RWWDP is an 8–10 year, 120 million program, funded by Federal, State and local resources, and in-kind services from the partnership of the Allegheny County Sanitary Authority (ALCOSAN) and the Allegheny County Health Department. Over 850,000 people are served by this system, which drains approximately 300 square miles of the greater Pittsburgh “Three Rivers” area of southwest Pennsylvania.The Allegheny County Sanitary Authority (ALCOSAN) and the communities within Allegheny County are in the process of refining and implementing a regional long-term wet weather control plan (LTCP) so the sewage systems of the region will be in compliance with the National Combined Sewer Overflow (CSO) Control Policy and the evolving Sanitary Sewer Overflow (SSO) regulations. Allegheny County communities and ALCOSAN continuously monitor and quantify wastewater flows during dry and wet weather periods. Of the 317 outfall structures in the ALCOSAN system; EPA has identified over 50 of these outfalls as SSOs. The remaining outfalls are classified as CSOs. The distribution and number of SSOs and CSOs within the satellite sewer systems owned by the municipalities increases this number. Even during dry weather some tributaries to the main rivers do not meet water quality standards for human contact. Elimination or reduction of wet weather flow in the Three Rivers Area will require considerable capital improvements. Design of these improvements requires accurate rainfall data in conjunction with wastewater flow monitoring data, to perform the analyses. Characterizing the hydrologic/hydraulic performance of the sewer using distant rain gauges can cause oversizing and wasted expenditures. Advanced technology has improved our ability to accurately measure rainfall over large areas. Weather radar when combined with rain gauge measurements provides detailed information concerning rainfall intensities over specific watersheds. There are many demonstrated benefits of using a combination of radar and gauges in a GIS context for comprehensive watershed management in an urban environment. A network of 21 gauges located within the ALCOSAN service area has been increased to a network of 34 gauges located throughout Allegheny County. The corresponding rain gauge density is approximately one gauge per 25 sq-mi (65 sq-km). In addition, 3RWWDP has improved the communication system associated with the gauge network to make measured precipitation data (radar and gauge) available on a real-time basis over the Internet.3RWWDP recognized the need for spatially distributed rainfall estimates early in the development of the Long Term Wet Weather Control Plan (LTWWCP). Installation of rain gauge networks with sufficient density over the service area is not feasible. Leveraging a new technology application of weather radar, 3RWWDP embarked on a quest for better rainfall estimates over the area. A geo-referenced grid system of 1.0 by 1.0 km pixels was superimposed over the project area for analysis and reporting purposes. Current efforts have expanded this grid to report radar rainfall estimates for more that 2000 sq. kilometers. The original 864 reporting cells have been extended to a reporting grid of 2,276 cells. Predictive rainfall and improved Internet access to calibrated radar rainfall have been added to the original gauge network system first developed in 1993 and the initial calibrated radar-rainfall system developed in 2000.This paper presents system objectives and results, as well as the organizational, technical, and economic achievements of this project.