Water Quality Enhancement and Extended Detention

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.

The release structure for an Extended Detention Basin consists of a water quality orifice plate which is designed to slowly release the stormwater holding volume (commonly referrred to as “water quality capture volume”) over a certain period of time.  Typically this period of time is around 40-hours, but can vary with municipalities and different drainage design criteria.

How Does Extended Detention Work? 

Stormwater can hold a number of pollutants:

  • Solids
  • Nutrients
  • Pathogens
  • Suspended Solids
  • Trash 
  • Metals
  • Oils

Extended Detention allows for settling of suspended solids.  Many pollutants bind to suspended solids, and by removing these solids, we can enhance stormwater quality.

Extended Detention Basins act as stormwater settling ponds.  Storm runoff enters the pond, is held for a certain time period, and is released very slowly over a specified holding period.  This holding period allows for solids suspended within the storm runoff to “fall out” of turbid stormwater.

Smaller, more frequent storms are the predominant factor in stormwater pollutant loading.  The Extended Detention Basin’s holding volume is intended to capture runoff from the average storm event.  Typically, Extended Detention Basins are designed to capture runoff from the average storm (approximately a 1-year runoff event) and release over a 40-hour period. Again, this time period can vary with municipalities and drainage criteria.

Photos: A. Cvar