Mechanisms for Stormwater Surges in Vertical Shafts
A number of large scale combined sewer systems have experienced events in which surges have resulted in the return of poor quality stormwater to grade in what are referred to as geysers. In spite of current concerns that these events be avoided by the proper design of new systems, there have been limited investigations to understand the conditions that lead to these surges or how to avoid them. In the course of an ongoing investigation into the dynamics of rapidly filling pipelines, two phenomena have been identified that may result in geyser-like phenomena. Both of these are associated with the movement of a large pocket of entrapped air along the crown of a nearly horizontal pipe. One case occurs when the leading edge of the air cavity arrives at a vertical riser at a location along the pipeline that is already in a surcharged state. A second case occurs as the trailing edge of the same air pocket arrives at the riser and the pipe is not in a surcharged condition.
Experiments have been performed to reproduce both of these situations in a simplified fashion where several of the key variables can be controlled. The features of these experiments are described as well as some of the results obtained to date. These results are interpreted to identify aspects of a storage tunnel system that are under design control and how these affect the magnitude of the observed surges. Finally, since the experiments are performed in a relatively small-scale apparatus, speculation is presented on the scale effects of a reduced scale apparatus, and conclusions regarding the applicability of the observed results to prototype applications are made.
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