The demand for large-scale watershed and sewershed planning studies in the United States has been increasing steadily over the past ten years. In large part, the demand is driven by major government programs regulating combined sewer overflows (CSO), sanitary sewer overflows (SSO), and storm water discharges. The implementation of these regulatory programs often results in local or regional public agencies embarking upon large multi-year studies requiring a comprehensive inventory of watershed and sewershed infrastructure, a characterization of the hydrologic and hydraulic function of that infrastructure, and analyses into the mechanisms by which pollutants are discharged into receiving waters. Significant monetary investments are made into comprehensive field investigations and surveys, hydrologic and hydraulic models, and regional facilities planning to develop and implement short-and long-term CSO and SSO control strategies.Accurately determining the quantity of extraneous flow that enters public sewers and private service laterals is a critical component of these comprehensive studies. The amount of rainfall dependent inflow and infiltration (RDII) entering the separate sewer systems varies from site to site and event to event as precipitation over a sewershed may produce different RDII responses within the sewers at different times of the year. The Lower Ohio hydrologic and hydraulic modeling project in Pittsburgh, Pennsylvania provided a unique opportunity to improve upon the accuracy and reliability of model simulations by incorporating monthly variations in sewer system responses to rainfall events. The completed analyses and model implementation were successful in quantifying site specific and seasonal variations observed in RDII responses.
The demand for large-scale watershed and sewershed planning studies in the United States has been increasing steadily over the past ten years. In large part, the demand is driven by major government programs regulating combined sewer overflows, sanitmy sewer overflows, and storm water discharges. The implementation of these regulatory programs often results in local or regional public agencies embarking upon large multi-year studies requiring a comprehensive inventory of watershed and sewershed infrastructure, a characterization ofthe hydrologic and hydraulic function ofthat infrastructure, and analyses of the water-polluting elements and processes. Significant monetary investments are being made into comprehensive field investigations and surveys, hydrologic and hydraulic models, and regional facilities planning to develop and implement short-and long-term combined sewer overflmv (CSO) and sanitary sewer overflow (SSO) control strategies. The amount and rate of rainfall and snowfall are the key driving force in the quantity of extraneous flow that enters public sewers and private service laterals and the frequency and duration of wastewater discharges into receiving water bodies. However, it is not uncommon to observe that disproportionately small investments are being made to improve the precision and accuracy of regional rainfall measurement. It has been demonstrated and documented in cities throughout the United States and around the world that calibrated radar-rainfall systems can provide accurate and precise rainfall measurement for large geographic areas. However, quantitative evaluations are relatively sparse that document whether
The Allegheny County Sanitary Authority (ALCOSAN) is implementing a regional plan to control Combined Sewer Overflow (CSO) and Sanitary Sewer Overflow (SSO) discharges into receiving water bodies within the greater Pittsburgh region. In order to develop and evaluate alternative conveyance and control facilities to regulate the frequency and volume of CSO and SSO discharges, ALCOSAN has developed detailed hydrologic and hydraulic (H&H) computer simulation models. These models are used to quantify and characterize the amount of flow conveyed by tributary service communities and guide the decision making process by evaluating various alternative control facilities to select the most viable and cost effective solutions. To adequately account for seasonal and annual variability on system-wide flow from the regional collection systems, a long-term continuous simulation modeling approach has been selected and utilized. The accurate representation of the diurnal and seasonal variation in dry weather (or base) flows is an important component of an accurate continuous simulation model. Imprecision related to the representation of wastewater base flow conditions can translate into inaccuracies associated with simulation of wet weather peak flows and volumes and can result in potential errors in the selection and design of control facilities.
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 km 2 ) 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 297 and model approach provide a valuable watershed-based, decision making tool for evaluating rehabilitation, replacement, and control facility alternatives. KEYWORDSRain...
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