Phoenix-Casa Grande Highway Stormwater Interceptor Drop Structures Model Studies

Warren Q. Dahlin, Joseph M. Wetzel

Research output: Book/ReportOther report


The Arizona Department of Transportation is developing the Phoenix-Cas a Grande Highway Stormwater Interceptor Project to handle storm water in the Phoenix area. Rectangular conduits near the ground surface collect the storm water and convey it to a vertical part of the structure which has several components. At the lower elevation is a large excavated chamber called the sump. Extending from the sump to the ground surface is a circular surge shaft containing two vertical walls. A diaphragm wall near the center of the surge shaft forms a dropshaft at the upstream end. The free trajectory inlet and diaphragm wall deflect the incoming water 90 degrees into the vertical dropshaft entraining considerable air. The water-air mixture falls down the dropshaft impinging on the sump invert. The entrained air has the effect of reducing impact pressures on the invert of the sump. A second wall at the downstream end of the surge shaft provides an air vent for returning air to the ground surface. The purpose of the surge shaft is to reduce pressure surges in the system. Downstream of the sump-surge shaft is the deaeration chamber. The sump and deaeration chamber provide for energy dissipation and air removal. Some air rises to the water surface in the surge shaft and escapes; the remaining air is carried into the deaeration chamber where it is removed. The deaeration chamber contains a false crown provided with air slots. The air rises to the false crown and escapes through the air slots to a horizontal chamber above; this chamber conveys the air to the vertical air shaft at the downstream end of the surge shaft. It is desirable to remove most of the entrained air from the water before it enters the tunnels. The entrapped air in the tunnels reduces the capacity for storage and conveyance and introduces the danger of high waterhammer effects upon its sudden release, which could cause damage to the system. The release of high velocity air at ground surface structures could also be hazardous. The effectiveness of the sump and deaeration chamber in dissipating energy and removing the entrained air was one factor considered in the evaluation of the various types tested. At the downstream end of the deaeration chamber a cylindrical surface was provided as an efficient entrance to the transition. In the transition the cross section gradually changed from a square section to the round section of the exit tunnel. Design information was needed for the three drop structures in the project. The decision was made to construct one model which would provide the necessary information. The model was constructed to a scale of 1:21.91 and modelled drop structure No. 3 (Moreland Street) which has a design discharge of 2634 cfs. To provide the necessary information for the other two structures, the model scale could be conveniently changed and the results from the 1:21.91 model extrapolated; or, the existing model could be revised if necessary.
Original languageEnglish (US)
StatePublished - Feb 1985


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