Physical Model Study, CSO Diversion through Bottom Outlet Slot, Site DS-4 Upper Rouge Tunnel System

James W. Lewis, Steven J. Wright, James Sherrill and Mark Pribak


The objective of this study was to determine the discharge characteristics of flow through a bottom discharge slot in a nearly horizontal circular pipe. The study consisted of an experimental model constructed in the University of Michigan Hydraulics Laboratory. The model study was a scaled representation of a proposed bottom outlet slot diversion structure for the Upper Rouge Tunnel (site DS-4) combined sewer overflow project in Southeast Michigan. The purpose of the slot is to allow discharge from the flow conduit into a deeper storage facility until the storage facility becomes filled to capacity after which the flow would continue to overflow in the original conduit. The main goals of the bottom outlet slot design are to control the flow passively and to minimize head losses in the conduit to avoid excessive backwater effects from the diversion structure. Experiments were performed in a scale model of a proposed design for the discharge structure. One important observation in describing the flow was that the flow passed through critical depth near the upstream end of the slot and was supercritical above the slot. Further, the flow patterns above the slot were clearly not two-dimensional because considerable variations were noticed in the lateral direction. The experimental results showed that increasing the pipe slope causes only minor increases to the required slot length. A consistent relationship is observed between the discharge divided by slot width and the required slot length. A simple methodology for developing a relation for the slot length required to intercept a specified discharge is proposed and compared to the experimental data.

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