Deep Tunnel filling transient model in Washington, D.C.
The District of Columbia Water and Sewer Authority is in the process of designing a deep tunnel system to capture and store combined stormwater and sewage for later treatment. Until recently, engineers have had only a limited understanding of the tremendous forces involved when large volumes of water enter the tunnels during rain events. Predicting the filling dynamics of these large tunnel systems is important to prevent accidental venting of captured stormwater and sewage, to ensure the safety of tunnel operators and the public, and to prevent damage to the sewer system and tunnel infrastructure.
In collaboration with the University of Michigan, LimnoTech developed a software program called SHAFT (Surge and Hydraulic Analysis for Tunnels). The model can predict all stages of the tunnel filling process, including the creation of open channel or pipe filling bores as the tunnel fills, and the locations where air can potentially be trapped. The SHAFT model was used to evaluate proposed tunnel geometry to determine whether peak inflows during extreme rain events would overwhelm the storage tunnel as designed, capturing large pockets of air that could push stormwater and sewage to the surface as they left the tunnel system, or causing flooding at the surface due to tunnel sloshing. These simulations will be used to support design modifications to prevent adverse impacts of tunnel-filling surges.
SHAFT Model Demonstration
This simulation shows how the proposed tunnel system would fill during a large storm event. At the beginning of the rain event, the tunnel captures stormwater and sewage that would otherwise be discharged into the river. The mix of stormwater and sewage flows down to the lowest point in the tunnel system at the Blue Plains Wastewater Treatment Plant. As stormwater and sewage continue to fill the system, the lowest portion of the tunnel pressurizes. The large amounts of stormwater and sewage still traveling toward the plant has a lot of energy and this quickly moving stormwater and sewage piles up as it hits the ceiling of the tunnel. This creates a “surge” that travels up the tunnel. If the hydraulic grade line, shown by the blue line, goes above critical elevations, stormwater and sewage can rush out of the tunnel onto streets or into basements.
Within three hours the tunnel has captured the required amount of stormwater and sewage. In real-world implementation, the stormwater and sewage within the tunnel will then be pumped out and treated to provide storage for the next storm.