Improved Outfall Representation in EXTRAN Using HEC-2
This chapter describes a methodology used to represent the complex hydraulics of diversion dams, backwater gates, and outfalls in EXTRAN. This methodology was utilized in the development of EXTRAN model of 126 outfall systems in the Greater Detroit Regional Sewer System (GDRSS). Backwater profiles for the outfalls were calculated using the U.S. Army Corps of Engineers computer program HEC-2. The EXTRAN representation utilizes a pipe system to represent the diversion dam and outfall structure. The multiple pipe system utilizes equivalencies based on pipe orifice and weir equations. An iterative process was used to obtain the best fit between the head versus flow relationships developed from EXTRAN and the HEC-2 model for the outfall systems. The iterative process utilizes a variation of the roughness coefficient in combination with pipe size in the EXTRAN model to obtain the best fit. The range of flow conditions to be expected at the outfall should be determined first to set the limits for hydraulic representation as well as define the most important flow regime of interest.
Comparisons between EXTRAN improved outfall representations and HEC-2 representations were found to vary by less than 5% throughout the flow range of interest. The EXTRAN outfall representation presented in this chapter has widespread application to design and planning efforts for combined sewer overflow (CSO) facilities generally. The proper representation of outfalls in hydraulic models may have significant impacts on the sizing of conveyance and end of pipe treatment facilities and associated costs.
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