Water Environment Federation (WEF)

Development of a Passive Operating Scheme for a CSO Capture Tunnel

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Courtesy of Courtesy of Water Environment Federation (WEF)

The Upper Rouge Tunnel (URT) is a proposed seven mile long CSO capture tunnel within the Detroit Water and Sewerage Department (DWSD) combined sewer system that will be filled at drop shafts along its length. During filling, surge waves will result that, if left uncontrolled, could damage structures. Surge control facilities will depend on the operational scheme selected and will be required along the entire length of tunnel.

The design team’s philosophy was that tunnel filling and surge control should occur as passively as possible. That is, the DWSD should operate as few pieces of equipment (such as control gates) as possible during filling. There is risk of human error, mechanical failure, electrical failure, communication failure, and computer control system failure with equipment operation. Mechanical equipment also requires significant maintenance.

Detailed analyses of the tunnel system were performed to determine the surge control facilities required for alternative operational control schemes. The main objective of the surge modeling was to determine the most economical control scheme that will effectively and passively handle surge, limit tunnel construction risk, and reduce the amount of preventative and long term maintenance for DWSD. All the goals needed to be met while also providing the required tunnel capture volume.

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 (i.e. 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) elevation. Surges can cause high internal pressures that can stress the conduit walls, damage manhole/shaft structures, and cause spilling of combined sewage to the environment.

The DWSD strongly desires to operate a proposed CSO capture tunnel as passively as possible. Passive operation requires less mechanical/electrical equipment that needs to be installed, operated, and maintained. Also, passive operation minimizes the risk of many types of failures that occur from time-to-time such as: human; mechanical; electrical; communication; and computer. Another benefit of passive operation is that the entire tunnel system volume is
available to capture CSO for every storm. If a gated operating scheme is used and the control gates are closed too early, then CSO could occur unnecessarily before the tunnel capture volume is filled.

Operational schemes that rely on closing gates on each connection to control surges during rapid filling are in-use in other cities. One example is the Chicago TARP system. With TARP, the gates on each drop connection are closed when the tunnel system reaches the 40% full level. The surges that develop in the tunnel during rapid filling are damped in the remaining open volume of the tunnel. Consequently, significant tunnel volume is not used for CSO capture. If this approach were to be used in Detroit, the tunnel volume would need to be about twice the required CSO capture volume and the project would be significantly more costly.

Some CSO tunnels, such as the one in Rochester, New York, are allowed to overflow as they become full. These overflows (sometimes used in combination with gate closures) dampen surges in the tunnel system. Having tunnel overflows was not considered an option that would be approved by the State of Michigan DEQ (MDEQ) for the URT. The tunnel system needed to be analyzed for transient conditions in order to gain basis of design approval by the MDEQ.

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