Minor Loss Coefficients for Storm Drain Modeling with SWMM
In the 1990s, as one of the updates to version 4.0 of SWMM, an EXTRAN routine was included which allowed the modeler to include input parameters on the C1 card for calculating minor losses in flow transitions. The ability to model these types of losses explicitly, and to track them as part of the input file, is an important addition to EXTRAN's capabilities. Particularly in cases where there is no flow data for the storm drain network being modeled, it is important to try to represent all the physical processes that can have an effect on flows and water quality.
The goal of this chapter is to review commonly used loss coefficients to determine their applicability for modeling storm drains in EXTRAN. The literature review found that many of the coefficients in use were derived from experimental data unrelated to the type of flows found in storm drains. There was much more information for pressure flow scenarios than for free surface flow. There was also more information for transitions in pipes than for transitions through junctions such as manholes.
In reviewing the original experimental research used to derive these coefficients, the chapter identifies several methods of estimating them which should be more applicable to storm drain modeling. The chapter also recommends that researchers conduct new experimental studies with the goal of developing loss coefficients that can be used for better estimates of junction losses in pipes with free surface flow.
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