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.
PRC Engineering; Arizona Department of Transportation; Howard Needles Tammen and Bergendoff
Dahlin, Warren Q.; Wetzel, Joseph M..
Phoenix-Casa Grande Highway Stormwater Interceptor Drop Structures Model Studies.
St. Anthony Falls Laboratory.
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