The purpose of this study is to examine the discharge coefficient as it pertains to
flow through a circular orifice cut into a thin-walled vertical riser pipe. Perforated riser
pipes are a popular outlet control structure for stormwater detention basins. These
basins are used to store and release stormwater runoff from impervious areas of
developed sites. Federal, state, and local regulatory agencies provide requirements for
the quality of stormwater runoff, as well as the maximum peak discharges from a
developed site. Accurately determining the flow rate from a perforated vertical riser
pipe is crucial to meeting these requirements and protecting the environment from
pollutants found in stormwater runoff.
This study therefore investigated several factors that may affect the value of the
discharge coefficient. A physical model was built and various size riser pipes were
installed in the tank to simulate a detention basin. The discharge through the orifice
was determined by measuring the rate of change of the water level in the tank versus
time. A water level versus volume drained calibration was used to find the rate of
change of volume over time, and hence the discharge coefficient.
The study determined that the discharge coefficient increased with decreasing
head values. The study also found that the discharge coefficient decreased as the height
above the floor was increased, up to a certain point. Another factor found to affect the
discharge coefficient was the orifice diameter to riser diameter, or d/D ratio. The discharge coefficient decreases as the d/D ratio is increased. It is postulated that most
of the changes to the discharge coefficient are a result of changes to the contraction of
the jet exiting the orifice.
For orifices away from the influence of the bed, the discharge coefficient vlaues
were normalized and compiled to fit a single curve that could be used to determine the
discharge coefficient for any orifice size in any riser pipe diameter, for a particular head
to orifice diameter ratio.
Multiple orifices in the same vertical plane were investigated as a secondary part
of the study, but no effect on the discharge was found for the orifice spacings tested.