Hydraulic Transient Modeling to Support Design of a Large Storage Tunnel in Washington, DC

The Challenge

Large storage tunnels are popular for controlling combined sewer overflows (CSOs) across the United States and in Europe. They are often preferred in urban settings because they are less disruptive compared to surface storage facilities. However, large tunnels create unique challenges for design engineers. As tunnels rapidly fill, tremendous forces are unleashed. These forces can create violent hydraulic surges, elevated hydraulic grade lines, geysering caused by trapped air, and even water hammer.

DC Water is designing and implementing a large tunnel storage system for flood control and the capture and storage of combined sewage for treatment. Understanding the dynamics of filling for this large and complex tunnel system is important to prevent accidental discharge of captured sewage, ensure the safety of tunnel operators and the public, and prevent damage to the sewer system and tunnel infrastructure.

The Solution

LimnoTech collaborated with researchers at the University of Michigan and Auburn University to develop and apply a sophisticated and innovative model called Surge and Hydraulic Analysis for Tunnels (SHAFT), which can simulate all stages of the highly dynamic tunnel filling process. For the DC Water system, the SHAFT model was used to predict the formation and movement of both open-channel and pipe-filling bores during rapid filling events, to evaluate the risk of extreme surges. The model also identified locations where pockets of air would potentially be trapped, which can lead to geysers or other potentially hazardous phenomena. A suite of SHAFT models was developed for numerous alternative tunnel geometries, using a variety of tunnel-filling and -dewatering scenarios. Based on these simulations and through interactive collaboration with the design engineering team, we identified potential transient surge problems, evaluated various modifications to the tunnel geometry, recommended a change in tunnel slope, and estimated air release requirements to support the tunnel design.

With the first phase of the tunnel system now under construction, the SHAFT model continues to be used to evaluate surges, transients, and pneumatics of other tunnel phases as their designs move forward. SHAFT model results are being used to design surge protection infrastructure for the proposed Potomac River Tunnel, as well as certain individual tunnel drop shafts still at risk in the tunnel-filling process, and to provide estimates of tunnel venting for filling events of various intensities.