A MULTIPLE-OBJECTIVE RDII QUANTIFICATION APPROACH FOR WET WEATHER CONTROL IN SEPARATE SEWERSHEDS

With ongoing pressures to meet operational and regulatory demands within available budget resources, collection system modeling and evaluation efforts need to meet multiple objectives. The Allegheny County Sanitary Authority (ALCOSAN) and its member municipalities manage wet-weather flow from its 296 square mile (767 km2) service area. To develop and assess a range of alternative sewer rehabilitation measures and storage and conveyance facilities to control wet weather flow, the amount of precipitation that enters the municipal collection systems, the rainfall-dependant infiltration and inflow (RDII), needs to be accurately and reliably quantified. The amount of RDII finding its way into separate sewers depends on complex factors such as the nature and severity of defects along the collection system and the relative elevations of groundwater tables to that of the sewer pipes. This varies from site to site, event to event, and season-to-season. The ALCOSAN monitoring and modeling programs are intended to quantify and characterize site-specific sewershed RDII responses while providing a watershed-based decision-making tool for evaluating rehabilitation, replacement, and control facility alternatives.The continuous simulation modeling approach estimates RDII responses by applying monitored high-resolution, spatially varied precipitation to a series of three unit hydrographs, calculated from corresponding monitored wastewater flow. Precipitation data are obtained from a gaugeadjusted radar rainfall system supported by a network of 32 rain gauges. Wastewater flow was continuously monitored through an extensive network of trunk sewer monitors for approximately 12 months to characterize seasonal variations in separate sewer responses. Using these data sources, unit hydrograph curve-fitting analyses were conducted to match RDII simulations with monitored RDII responses. This process involved the generation of three unit hydrographs that were used to simulate an RDII response hydrograph for each monitored event. From there, a set of monthly-varied R-T-K parameters were developed for each monitored sewershed area to accurately characterize seasonal variations. The resulting monthly parameters were then entered into the hydrologic model to simulate the RDII responses in the separate sewered areas as a function of rainfall.The ALCOSAN monitoring, analysis, and modeling methodologies quantify and characterize site-specific, seasonal variations observed in the RDII responses from the separate sewersheds. The completed analyses and model applications demonstrate the seasonal variability of RDII responses in sewer systems and the utility of models to characterize and quantify these seasonal variations. The monitoring, analysis, and modeling methodologies were completed in approximately 25 separate sewershed areas throughout the ALCOSAN service area. The RDII and model approach provide a valuable watershed-based, decision making tool for evaluating rehabilitation, replacement, and control facility alternatives.