Evaluating Surge Potential in CSO Tunnels
Tunnels are being proposed to control combined sewer overflow (CSO) in numerous cities in the United States and Canada. The tunnels are intended to capture and store CSO for later dewatering to a wastewater treatment plant (WWTP). These tunnel systems may also provide relief to the interceptor system and transport the captured CSO to a downstream interceptor, pumping station or WWTP.
When a CSO tunnel is filled slowly, the surge waves that are created intuitively are small and there is no established guidance as to the breakpoint filling rate required to create significant surges. In this chapter, some guidance is provided that relates the surging potential to tunnel diameter, tunnel slope and the maximum filling rate expected. These results may be used as a screening tool to decide whether surges and surge control measures need to be evaluated in further detail for proposed CSO tunnel projects. The surge modeling results are not intended to be used to provide absolute answers of potential surge heights. Also, ventilation requirements to prevent geysering should be considered and have been studied by others.
Rapid filling of CSO tunnels, such as from large and intense rain storms, can result in surges, in pockets of air being trapped and pressurized, and in geysers occurring to grade at tunnel manholes and shafts. When open channel conditions exist and the filling creates a pipe filling bore, surges can rise to unacceptably high levels (including grade) which can cause basement flooding, damage structures, and spill sewage to the environment. Surge control measures that are commonly considered for CSO tunnels includeorifices on connections that limit the peak filling rates and inlet gates that close once the tunnel reaches a target part full level. Surge relief can also occur through tunnel outfalls or into surge tanks.
The Transient Analysis Program (TAP) was used to simulate worst-case surge conditions in hypothetical tunnel systems and develop relationships that provide the breakpoint filling rates below which the potential for surges is small. TAP has been shown to be a proper computational hydraulics program for simulating surges in one-dimensional pipelines and TAP model results have compared favorably with laboratory surge data. TAP is useful in analyzing water supply, storm water, drainage, irrigation, combined sewer, sanitary sewer systems, and does an excellent job of calculating detailed steady-state gradually-varied flow profiles through channels/conduits and water/wastewater treatment facilities. TAP can be used to simulate flood routing in open channel/conduit systems as well as evaluate operation and control schemes.
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