Employing Hydraulic Modeling to Ensure Surges and Geysers are not Transient Problems in Deep Tunnel Systems

Over the course of the next 15 years, the District of Columbia Water and Sewer Authority (DC Water) will design and construct its DC Clean Rivers Project (DCCRP) to reduce Combined Sewer Overflows (CSOs) to the District's receiving waters. The Project consists of a series of large underground storage and conveyance tunnels, drop shafts, diversion sewers and chambers, overflow structures, and a tunnel dewatering pump station. The backbone of the Project is a 7.0-meter (23 ft) diameter tunnel system that spans the length of Washington, DC, from the southwest to northeast quadrants of the city. Owing to its length and the rapid filling associated with storm events, the system is at risk of transient phenomena such as hydraulic surges and geysers. A computational model has been applied throughout the design of the system to identify potential issues and to evaluate design alternatives that serve to minimize risk. This paper summarizes the basis for the modeling framework, and provides examples of modeling results were used to evaluate design alternatives.