Wet Detention

Description:

Wet detention ponds are surface ponds that are permanently wet. They are designed to slowly release a portion of the collected stormwater runoff through an outlet structure and to remove runoff pollutants. They receive annual removal credit when installed using acceptable design and maintenance criteria. There is a catchment area associated with each. The normal permanent pool elevation is below the elevation of the surrounding land.

Underground holding ponds are not considered wet ponds. Wet ponds are often effective BMPs for sites with moderate to high water table conditions. They are detention treatment systems that provide removal of both dissolved and suspended pollutants by taking advantage of physical, chemical, and biological processes within the pond for time periods usually greater than 14 days of residence time.

Wet ponds are used frequently and provide predictable recovery of storage volumes. They are easily maintained by the designated maintenance entity.

Average Annual Residence Time (ART)

As shown in the figures below, the treatment effectiveness of a wet pond is directly related to the annual residence time that storm water runoff occupies within the wet pond before discharge. The longer the residence time, the more opportunity the various pollutant load reduction mechanisms within the wet pond system have to function.

Average Annual Residence Time Formula

ART = 365 × V / Q

Where:

  • ART = Average Annual Residence Time (days)
  • V = Permanent pool volume (acre-feet)
  • Q = Average annual flow rate (acre-feet/year)
ART (days)
Removal Efficiency of Total Phosphorus in Wet Detention Ponds
 as a Function of Average Annual Residence Time (ART)
ART (days)
Removal Efficiency of Total Nitrogen in Wet Detention Ponds
as a Function of Average Annual Residence Time (ART)

Input Data

Wet Detention Pond with Littoral Zone and Floating Wetland

  • Permanent pool volume: 0.3 acre-feet
  • Littoral zone included
  • Floating wetland coverage: 5% of pond area
  • Input pond TP value required only if calculating anoxic depth

The floating wetland enhances nutrient uptake and improves overall removal efficiency depending on regulatory credit and maintenance requirements.

Wet Pond with No Littoral Zone and 10% Floating Wetland Coverage

  • No littoral zone
  • Floating wetland coverage: 10% of pond area
  • Enter permanent pool volume
  • Floating wetland or Managed Aquatic Plant (MAP) credit determined by review agency
  • Typically applied to restricted catchment areas (e.g., redevelopment urban sits)

Overall Removal Calculation

The calculation for overall pollutant removal (detention pond + floating wetland) follows the series equation provided in the Applicant’s Handbook, Volume 1 (June 28, 2024), page 9.7.

Example – Total Nitrogen (TN)

For a wet pond with no littoral zone:

  • Pond TN removal = 36%
  • Floating wetland credit = 20% applied to the remaining untreated portion
And as an example, for TN and wet pond TN removal with no littoral zone of 36% and an additional 20% granted for the floating island applied to the remaining amount: 

Series equation:

1 − [(1 − 0.36) × (1 − 0.20)]
= 1 − (0.64 × 0.80)
= 1 − 0.512
= 0.488

Overall removal = 48.8%

This is NOT calculated as 36% + 20% = 58%. Removal efficiencies must be applied in series, not summed.