Surge predictions in a large stormwater tunnel system using SWMM

Stormwater tunnels often have massive geometries, with conduit lengths of several kilometers and a wide range of diameter sizes. Modeling rapid filling of these systems is a complex task and needs adequate methodology. One model used in hydraulic analysis of stormwater tunnels is the EPA’s Storm Water Management Model (SWMM). However, model setup conditions related to a pressurization algorithm can significantly affect SWMM’s accuracy in surge prediction. This work evaluates SWMM 5.1 accuracy in simulating rapid filling of tunnels, particularly surging conditions. This evaluation is done using a real-world tunnel geometry, the Upper Des Plaines Tunnel, which is part of Chicago’s TARP tunnel system. Variables considered in the SWMM model setup include discretization strategy and pressurization algorithm, and its results are compared with HAST predictions, a model specifically designed to represent surges in tunnels. This work shows that, with adequate setup, SWMM can represent surging in stormwater tunnels much more precisely.