A number of situations of engineering interest are associated with the relatively rapid filling of a nearly horizontal pipeline. The displacement of the air may lead to a number of different transient conditions of concern in engineering design. Our specific application relates to the use of underground storage tunnels to mitigate combined sewage overflows. Historically, the operation of these system have created surges initiated when the tunnel passes to a surcharged condition that have resulted in “geysering” in which an air/water combination is expelled to the ground surface through manholes or other risers. Numerical models that have been created to describe the phenomena generally treat the air as a passive phase that exists at atmospheric pressure and disappears when the water fills the tunnel. In an attempt to understand the conditions leading to extreme surges, a physical model that reproduced the essential elements of the tunnel filling process was created. This model was filled at one end in a fill box in which the maximum water level was controlled with an overflow weir. A surge riser was mounted at the opposite end of the pipeline to observe the magnitudes of surges created under a number of different flows created by varying the inflow rate, the initial water level in the pipe, and the pipe slope. In general, initiation of the filling process generated a hydraulic bore that propagated through the system until the tunnel reached a surcharged state and the maximum surge generated depends on the dynamics of the bore. However, under certain flow conditions, the water level in the surge riser was observed to increase prior to the arrival of the bore, an occurrence that can only be explained by pressurization of the air in the pipe. Subsequent experiments confirmed this explanation and a modified experimental setup was created to elucidate the important effects of the air. Measurements were made of a number of key flow variables and are found to be consistent with the predictions of a numerical model that considers the elementary dynamics of the air.

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