Evaluating Surge Potential in CSO Tunnels

Karen E. Ridgway


The Transient Analysis Program (TAP) was used to develop criteria for estimating the worst-case potential for surges in CSO tunnels. Sets of TAP model runs were made for multiple tunnel diameters, typical design slopes and a range of filling rates. The surge heights were calculated, tabulated, and plotted so relationships between the tunnel diameter, tunnel slope, filling rate and maximum surge height were observed. These results can be used as a screening tool for deciding whether surge control measures need to be evaluated in detail for proposed CSO tunnel projects.

Tunnels are being proposed in numerous cities with combined sewers as a Combined Sewer Overflow (CSO) control measure. These proposed tunnels would capture and store CSO for later dewatering to a wastewater treatment plant (WWTP). The tunnel also may provide relief to the interceptor system and transport the captured CSO to a downstream interceptor, pumping station or WWTP.

Significant surges can develop in CSO tunnel systems under rapid filling conditions. Surges can rise to unacceptably high levels (including grade), cause basement flooding, cause damage to structures and spill CSO to the environment.

Surge control measures that are commonly considered include orifices that limit the peak filling rate and inlet gates that close once the tunnel reaches a target percent full level. Also, surge relief from the tunnel may occur through existing outfalls or new tunnel outfalls or into surge tanks.

There is no established guidance as to the filling rate that is required to create significant surges in a CSO tunnel. When a CSO tunnel is filled slowly, the surge waves created are not significant. The worst-case surges occur when the tunnel is partly full and rapid filling occurs and creates a pipe filling bore that moves in an upslope direction. When this kind of pipe filing bore reaches the upstream end shaft, a rapid rise in sewer level occurs in the shaft and a mixture of air and sewage will be expelled. The surge is reflected back downstream as a closed conduit transient and these reflected waves also may be problematic.

Worst-case surge potential is related to 1) tunnel diameter; 2) tunnel slope and 3) the maximum filling rate expected. If the maximum filling rate expected is small relative to the tunnel diameter, the potential for creating strong surges is shown to be not significant and detailed surge analyses may not be needed.

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