Surge Modeling in Sewers using Alternative Hydraulic Software Programs
Rapid filling, such as from large and intense rain storms, can result in the formation of open channel surge waves (hydraulic bores) with steep wave fronts in sewers/tunnels. Hydraulic bores travel in the sewers/tunnels until they reach an upstream, downstream or internal boundary (e.g. at shafts or manholes). As the sewer/tunnel system fills, the bores wholly or partially reflect off the boundaries and discontinuities of the sewer/tunnel system causing surges in hydraulic grade line (HGL) elevations that rise above ground surface elevations. Surges are important to evaluate and control in deep relief sewers and tunnels since they can cause high internal pressures that can stress the conduit walls, damage manhole/shaft structures, and cause spilling of combined sewage to the environment. Stress reversals can loosen structures and their anchors.
It is important that the hydraulic modeling software used for the design of the sewer/tunnel facility accurately compute surge conditions since the cost for surge control measures can be significant, and failures due to improperly controlled surges can be catastrophic. The required surge control measures depend on the proposed operational scheme, and the control measures may be over- or under-sized depending on the hydraulic software program used for the design.
Hydraulic modeling software programs currently in common use for sewer system hydraulic modeling were not developed to simulate rapidly varying open channel transient conditions. These programs, as commonly applied to sewer systems, will under predict and over damp the surge waves.
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