Using CFD to Improve 1D/2D Collection System Hydraulic Model

The Collection System in the City of San Francisco is comprised of approximately 1,000 miles of combined sewers that serve drainage needs of residential, commercial, and industrial properties. The City’s Hydraulic model is used primarily to determine sewer sizes that reduce flooding and maintain adequate drainage conditions for both public and private properties. The model is comprised of a 1D model that represents the pipe sewer network and a 2D component representing the street surface used to determine overland flow depths and extents. Flow-3D is currently used to improve model representation of complex junction structures on a flood control project designed to lower water surface elevations to meet a level of service design storm condition for a 75-acre area that experiences recurring flooding. Previously produced results from physical modelling of a vortex drop structure was used to validate comparable water surface elevations obtained from the Flow-3D model. The geometry of the proposed junction structures at two locations of the project were then modeled to obtain detailed water surface profiles meeting existing design criteria. Mesh size varied for each structure depending on complexity with cell sizes ranging from 0.5-1.0 ft. Simulation results were used to update the City’s 1D/2D Hydraulic model by adjusting head losses to obtain comparable water surface elevations obtain from Flow-3D. The geometry for each junction structure was recommended to proceed to the final design phase of the project.

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