Analysis of Transient Surge in the Proposed District of Columbia Water and Sewer Authority Deep Tunnel System

Tunnels are an increasingly popular method of capture for combined sewer overflow control. As tunnels fill, it is important to consider the potential effects of transient surges, including high hydraulic grade lines, geysering caused by trapped air, and the force of rapidly moving bores on tunnel infrastructure. The District of Columbia Water and Sewer Authority (DCWASA) is currently designing a large tunnel system to meet the requirements of their Long Term Control Plan and reduce flooding in sections of the city. Plans currently call for the completed tunnel system to be placed in operation in two phases; one phase by 2018 and the other by 2025. An innovative model, based on research by Vasconcelos and Wright, was used to simulate transient surges as part of the evaluation of the proposed tunnel geometry. The model, called SHAFT, simulates both open-channel and pipe-filling bores, and predicts locations of air entrapment. The model utilizes a shock capturing technique that decouples hydrostatic pressure from surcharge pressures occurring only in pressurized conditions, and takes advantage of the identity between unsteady incompressible flow equations in elastic pipes and unsteady open channel flow equations in the model governing equations. These two concepts allow SHAFT to simulate both flow regimes using the same generalized set of equations and enables SHAFT to readily model flow regime transitions (i.e. from open channel flow to closed conduit flow). Locations where air entrapment during the filling process is common can also be located and provisions for adequate ventilation can be provided. SHAFT simulations were performed for the proposed DCWASA tunnel system for a matrix of hydrographs, initial fill levels, and tunnel profiles. The simulations enabled designers to determine critical conditions relative to the tunnel filling process, and to adapt the design as needed to identify and refine passive controls to avoid destructive geysers and air pockets in the proposed tunnel system.