Sustainable practices such as extended detention, flat grading of pond bottom, micropools, and trickle channels can often be incorporated into the lower stage of detention ponds.
Extended Detention is probably one of the most effective green stormwater practices, and is also one of the most practical means of achieving stormwater sustainability goals.
Extended Detention has been shown to have a very high total suspended solid (TSS) removal rate. Extended detention can often be incorporated into the lower stage of detention ponds, and is therefore very practical to implement. Typically, Extended Detention volume is roughly 5% to 10% of required detention volume. Adding an Extended Detention component to a detention pond is typically not a significant encumbrance.
One of the main challenges with integration of extended detention with detention is pond maintenance and outlet clogging.
An important question is, how will the pond be landscaped? Do you expect a high volume of vegetation that can clog outlets?
In the picture to the left, a standard extended detention water quality pond was designed in an area with very dense landscaping. The maintenance crew complained of the outlet constantly clogging, and eventually removed the water quality plate.
Many of the trash screens used for the water quality component will clog even with the smallest of debris. A standard trash screen is shown in the picture to the right. The trash screen is positioned in between the trash rack and the water quality plate, which has a series of circular holes intended to control the release of water from the pond.
Clogged outlets lead to standing water, which leads to issues with mosquitos as well as other health concerns.
Photos: A. Cvar