A GIS and SWMM Modeling Application for Sewer Separation Design
Partial or complete separation of combined sewers is one of the strategies that can be employed to control or eliminate combined sewer overflow (CSO) discharges. Separation of combined sewers can be expensive and disruptive to communities, since it involves digging up the street. In many urban areas, roof drains from houses are connected to sewers via downspouts (outside piping), or through roof leaders (internal plumbing). Disconnection of roof drainage connected to the sewer via internal plumbing can be very expensive and disruptive. In addition, experience has shown that it is very difficult to remove 100% of rainfall-induced inflow in a sewer. When storm drainage is removed from combined sewers, they become sanitary sewers carrying a small amount of residual inflow. The residual inflow may originate from a variety of sources such as leaky manholes, leaks in pipes, or building sump pumps. Quantification of this residual inflow is important since it affects the CSO activation frequency and thus the degree of roof drainage separation that must be undertaken to achieve CSO elimination goals.
Many hydrologic models employ coefficients to represent the directly connected impervious area (DCIA). These coefficients represent the fraction of the drainage basin that contributes flow to a drainage system. In general, this coefficient is proportional to the total amount of impervious area in the region. If sufficient geographic information and field data is available, it is possible to determine the contribution of runoff from different land-uses, and thus estimate the contribution of runoff from connected buildings to a combined system. This information in turn can be used for hydraulic modeling to estimate the number of buildings to disconnect to prevent the occurrence of CSOs. This chapter presents an application of this approach that was used for a sewer separation project in Boston.
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