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 benefits.

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.

Why retrofit basins:
Recent studies in stormwater management have shown that the smaller, more frequent storm events typical to this region degrade water quality and increase streambank 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.

Benefits:
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.

Cost:
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 retroffiting device.

Maintenance:
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.