Detention
basins are located in or beside larger washes within the City of
Sierra Vista (see Detention Basin Map). Detention basins work by capturing large amounts
of fast moving water and releasing it slowly. This is accomplished
by providing a large storage space for the water within the basin.
Incoming water begins to fill the storage space within the basin during
the first part of a storm. The basins are equipped with an outlet
structure that releases the water more slowly than it comes in.
As the storm progresses, the outlet structure causes the water to back
up within the basin. When engineers design the basins, they are
really performing a balancing act. They want to release the water
as slowly as possible, but the more slowly it is released, the larger
(and more expensive) the basin has to be. The designer has to
balance these two components of the design to get the most efficient
facility possible.
Detention basins help the community
in two significant ways. As land is developed, the amount of impervious
surface increases and the amount of vegetation decreases. Impervious
surfaces are surfaces such as asphalt, concrete and rooftops that do
not allow water to soak into the ground, which increases both the volume
of water in the washes and the peak flow. By releasing water more
slowly than it comes in, the detention basins reduce the peak flows
and therefore reduce the danger of flooding downstream. Slowing
the flow provides a second critical benefit to desert communities by
increasing the contact time the water has in the bottom of the washes
and the bottom of the basin, thereby giving runoff a greater chance
to infiltrate back into the ground and recharge the aquifer. Increasing
the chance for infiltration provides additional recharge to groundwater
supplies. This reduces some of the “extra” runoff
produced by development and stores it in the aquifer.
