Large tunnels are popular for combined sewer overflow control across the United States and in Europe. They are preferred in many urban settings as less disruptive than surface facilities, but large tunnels pose 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. Predicting filling dynamics is crucial to avoiding accidental venting of captured sewage, ensuring safe operation, and preventing damage to tunnel infrastructure connecting sewers and lift stations.
LimnoTech collaborated with scholars at the University of Michigan to tackle the challenge of accurately predicting tunnel-filling dynamics. The result was a sophisticated, new-generation tunnel model titled SHAFT (Surge and Hydraulic Analysis for Tunnels) that can predict all stages of the tunnel-filling process. SHAFT can simulate the emergence of open-channel or pipe-filling bores as a tunnel fills, and the locations where air can potentially be trapped and become a problem. Applied to various tunnel-filling and dewatering scenarios, SHAFT simulations can identify design adjustments and enhancements that avoid adverse air release and tunnel surges while maximizing capture performance. This can contain construction and operating costs and dramatically reduce the need for sewage releases, thereby improving receiving water quality.
LimnoTech has employed the SHAFT model for tunnel designers in two world capitals, and has several other projects pending. In Washington, DC, LimnoTech used the SHAFT model to evaluate and refine CSO tunnel designs. Model simulations identified design modifications and hydraulic controls to prevent the adverse effects of tunnel-filling surges. SHAFT forecasts and tunnel design enhancements have undergone extensive and successful peer review by the international tunnel experts on the Program Review Board.
In London, England, the Thames Water Authority is designing its deepest-ever tunnel system, up to 260 feet deep and extending nearly 20 miles long beneath the River Thames, to capture and store diluted sewage for later treatment. LimnoTech coordinated with designers to simulate multiple “what if” scenarios to assess transient hydraulic grade lines and air pocket formation using a wide variety of critical tunnel-filling and -dewatering scenarios. These simulations were used to identify design adjustments and enhancements throughout the tunnel system.