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 surge waves (hydraulic bores) with steep wave fronts in sewers/tunnels. The hydraulic bores will 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 will reflect off the boundaries 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.

It is important that the hydraulic modeling software used for the design of the sewer/tunnel facility be able to accurately predict 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 measures may be over or undersized depending on the hydraulic software program used for the design.

Physical model data obtained from the University of Michigan hydraulics laboratory were used to evaluate the three commercially available software programs against a program developed by Applied Science, Inc. (ASI), the Transient Analysis Program (TAP). Two physical model setups were used in the evaluations. Model “A” is a single 50 foot long, 4 inch diameter pipe with a fill box at one end and riser shaft at the other end. A high filling rate is suddenly put into the fill box, and the resulting surge elevations were measured at the riser. This model simulates the conditions of a tunnel filling from one end.

Model “B” is also a 50 foot long, 4 inch diameter pipe split into two segments. Model “B” is filled from the upstream end and at an intermediate junction. The downstream end has a free discharge condition. A reservoir is connected to the upstream end and a constant, small flow rate is added to the reservoir. Steady-state open channel conditions are reached in the pipe and are used as initial conditions for the surge conditions. A large flow rate is suddenly put into the intermediate junction along the pipe. The resulting hydraulic pressure is measured at two separate locations in the pipe. This model simulates the conditions of a tunnel being filled from an intermediate station, causing surge waves to travel in two directions in the pipe.

The one-dimensional hydraulic software programs used in this comparison include SWMM5 by the US EPA, MOUSE by DHI, InfoWorks by Wallingford Software, and TAP by ASI. The results of each model run are compared to each other and the physical model results, and possible explanations for discrepancies are discussed. The models were setup and run with each program with a range of user specified setup conditions.

The results show the following.

• TAP most properly simulates surge conditions.
• Surges are generally underestimated using the typical sewer system model setup procedures for the other programs.
• Setup conditions required to achieve proper surge simulations is extremely complex and prone to errors in the other programs.