What is a retrofitted basin:
A stormwater basin retrofit usually involves the modification of an existing
basin's outlet structure. Stormwater basins typically contain large outlet
pipes. These basins are designed to temporarily store and re-route runoff
from large storms. Their primary purpose is to help control floods. Retrofitted
basins still provide flood control protection, but through structural
modifications that can also provide water quality and erosion control
Instead of just one
outlet hole, retrofitted basins usually have two or more, of varying sizes.
The additional holes can be added to an existing outlet structure or through
the construction of a low wall inside the basin. The use of weirs
with v-notches, and special attachments to outlet structures can also
be used to retrofit a stormwater basin.
Recent studies in stormwater management have shown that the smaller, more
frequent storm events typical to this region degrade water quality and
erosion. Most stormwater basins built in the past did little or
nothing to reduce velocity or filter out pollutants from these smaller
storms. In fact, many basins, especially those with concrete low-flow
channels, move stormwater quickly into and out of the basins,
causing problems for local streams.
Retrofitting can be
accomplished on both existing dry and wet stormwater basins.
By small modifications to the basin's outlet structure, runoff
can be slowed and water quality can be greatly enhanced.
Retroffited basins are designed to hold back and slow the velocity of
smaller storms, those that typically result in water quality problems.
Slower velocities mean reduced erosion in the streams where most basins
ultimately discharge. Reduced localized flooding can result from a retrofitted
basin's ability to detain small storms for longer periods of time. And
this ability to hold back storm water allows time for pollutants such
as sediments, oils, grease, nutrients, pesticides to settle out and be
filtered through longer contact with basin vegetation.
Material costs to retrofit dry basins should be minimal, as long as the
entire outlet structure does not have to be replaced. Retrofitting a wet
pond may be more costly due to the need to first drain the basin. There
will also be varying costs associated with the design of the appropriate
All stormwater basins require regular maintenance, particularly after
large storms. The maintenance requirements for a retrofitted basin are
similar to the normal maintenance requirements of a non-retrofitted basin
such as clearing debris from outlet structures and checking the stability
of the outlet structure.