Abstract

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.

There are over 400 points of connection to the ALCOSAN interceptor system. Many have physical conditions that make it infeasible to conduct accurate and reliable wastewater flow monitoring and it would be cost prohibitive and unnecessary to monitor for a multi-year analysis period. To perform multi-year continuous simulations, analytical results from monitored dry weather flow periods were extrapolated and applied to locations and simulation periods for which there is no directly monitored wastewater flow data available. The dry weather flows used as input into the model represent an increased level of refinement and accuracy as they were derived directly from time series of monitored (observed) flow data.

This chapter documents the approach, assumptions, and results associated with compiling the dry weather base flows for the multi-year ALCOSAN system-wide models.

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