New method for the offline solution of pressurized and supercritical flows

An offline approximation of the Saint Venant equations is proposed for combined sewer overflow (CSO) prediction. Precalculated tabulations of the mass and momentum equations allow online interpolation, accelerating run-time hydraulic computations above those yielded by standard industry software, while preserving accuracy. Previous methods of backwater profile compilation account onlyfor subcritical, open-channel flows. This work further extends the backwater profiles to include pressurized flows, which are key for CSO conditions. Incorporation of the Darcy-Weisbach equation eliminates potential lookup table discontinuities between open-channel and pressurized conditions.Graphical depiction of supercritical flows causes iteration errors as nonunique flow rates appear for equivalent water surface elevations. Iteration errors can be eliminated by allowing the solution to proceed separately upstream and downstream from the governing critical water surface. This precalculated curve approach addresses discrepancies in supercritical flow and submerged weir calculations in the EPA SWMM model. The implicit computational approach surpasses SWMM run times by as much as 57% and proves an accurate tool for evaluation of sewer water levels for real-time optimization model incorporation. copy; 2013 American Society of Civil Engineers.