Conveyance analysis of Chicago's 'deep tunnel' system

The Tunnel and Reservoir Plan (TARP) is a system of 175 kilometers of deep underground tunnels and large reservoirs designed to capture and store combined-sewer overflows of greater Chicago. Conveyance studies in sewer systems often discretize the sewer into reaches delimited by manholes or junctions, effectively ignoring the water-surface profile within a pipe. Another approach is to performing backwater calculations, on small spatial steps, for every flow condition in the system. This is not unreasonable for problems such as flood-plain studies, where only a few flows are considered. However, a much larger range of flows is possible when considering operational decisions for a sewer system. Previous research demonstrated the effective use of the hydraulic performance graph (HPG) for open-channel capacity determination. The HPG stores the results of backwater calculations for a reach so that these calculations do not need to be repeated. The HPG has been extended from that for open-channels to also describe pressurized flow in sewers. This paper describes a procedure that uses the HPGS for each reach of a sewer system, along with a description of the junction losses, to describe the conveyance of any bottlenecks in a system of sewers or tunnels for free-fsurface, pressurized, or mixed flow conditions. This method has been integrated into a geographic information system (GIS). The GIS stores the topology and geometric description of a pipe network, the HPGs for all reaches in the system, and coefficients needed to estimate junction losses. This allows for visualization of bottleneck locations and flow conditions throughout the system for any flow configurations and operation plan.