A wet detention pond is a constructed storm water retention pool wherein physical, biological, and chemical processes remove pollutants from storm water runoff. Pollutants removed include suspended solids, organic matter, dissolved metals, and nutrients. In addition, wet detention ponds control storm water flow which prevents downstream flooding. As storm water enters the pond, treated water is displaced and discharged into a receiving body of water. Enhanced treatment of storm water runoff can be achieved through extended detention and the use of aquatic plants on the perimeter of the pond. Sediment removal can also be increased through the use of a sediment forebay.

Before construction of a pond begins, local, state, and federal permits should be in place for all aspects of construction including wetlands, water quality, dam safety, grading, erosion control, and land use. Wet detention ponds rely on the maintenance of a permanent pool of water within the pond and therefore, should be placed in areas with adequate baseflow from groundwater or from the drainage area to maintain the permanent pool. Soils under the pond should have a low permeability (10-5 to 10-6 cm/sec) to maintain a permanent wet pool. Pond placement should optimize reuse of a topographic area that allows for maximum detention while requiring minimal earth removal, thus lowering construction costs. Pond construction should not be undertaken near utilities or underlying bedrock.

Pollutant removal in the pond is achieved through one of two methods: solids settling or eutrophication. The solids settling method relies on pollutant removal through sedimentation. The eutrophication method removes nutrients using natural biological processes. According to the National Urban Runoff Program (NURP), up to 2/3 of the suspended sediments, trace  metals, and nutrients settle out within 24 hours. Other studies, examining biological removal, suggest hydraulic residence times (HRTs) of close to 2 weeks are required for phosphorus removal.

¹hydraulic residence time varies
²hydraulic residence time of 2 weeks

Two ratios are useful for predicting pollutant removal efficiencies: volume ratio and area ratio. Volume ratio (VB/VR) is the ratio of permanent pool storage (VB) to the mean storm runoff (VR). Area ratio (A/As) is the ratio of the contributing drainage area (A) to the permanent pool surface area (As). Both of these ratios are correlated with treatment efficiencies. Large volume ratios result in increased retention and treatment between storms, while low pollutant efficiencies are achieved with low volume ratios.

Pool depth can play a critical role in pollutant removal and storage, but caution should be taken when increasing the depth of the pool. A pond with an HRT of 2 weeks would function optimally at depth ranges from 3 to 8 feet; shallower depths with the same pond surface area have shorter HRTs.

Water within the pond is discharged through a wet pond outlet. A wet pond outlet consists of a vertical riser, either concrete or corrugated metal, attached to a horizontal barrel that conveys storm water flow under the embankment to a receiving stream. The outlet is designed to pass excess water while maintaining a permanent pool. Risers are typically placed in or on, the edge of the embankment and are capped with a trash rack to prevent clogging.

As with any storm water best management practice (BMP), proper maintenance will ensure continued proper functioning of the wet detention pond. Proper maintenance may include any or all of the following:

• Clearing trash and debris.
• Conducting routine inspections of the embankment and spillway to check structural integrity and look for signs of erosion or animal habitation.
• Conducting periodic repairs on the embankment, emergency spillway, inlet, and outlet.
• Removing sediment and algae.
• Removing woody vegetation or trees from the embankment that could potentially weaken the embankment.
• Maintaining the outfall area (i.e., replacing rip-rap, removing sediments, etc.)

Sediments collected by the wet detention pond typically meet toxicity limits and can be landfilled safely. Testing of the sediments may be required if the upstream drainage area is industrial and/or results in highly contaminated runoff. Non-toxic sediments can also be disposed on site, but away from the shoreline to prevent their re-entry into the pond. The removal of sediments in a pond may be necessary every 20 years. This may be decreased to every 50 years if a sediment forebay is used prior to the wet pond. The sediment forebay would require maintenance every 5 to 7 years or when 50% of forebay capacity is silted.

The compliance benefits listed here are only meant to be used as a general guideline and are not meant to be strictly interpreted. Actual compliance benefits will vary depending on the factors involved, e.g. the amount of workload involved.

• Wet detention ponds can decrease the potential for downstream flooding and streambank erosion, and provide improved downstream water quality.
• Water quality is improved through removal of suspended solids, metals, and dissolved nutrients using natural biological and physical processes.
• Properly designed and maintained wet detention ponds can also enhance landscape aesthetics as well as provide wildlife habitat.

• Pond construction should not be conducted near utilities or underlying bedrock.
• Sediments from upstream industrial or highly contaminated runoff areas may constitute a hazardous waste requiring special disposal/treatment.
• In Maryland, use of wet ponds is strongly discouraged in cold water fisheries (streams) due to potential
  thermal impacts.

Assumptions:

• Labor rate $45/hour.
• Labor costs for operating a wet pond include mowing and debris removal requiring an estimated three hours per month.
• Labor costs for operating a sand filter includes filter changing, gravel and sand replacement, and debris removal, requiring an estimated 3 hours/year.
• Material costs for the gravel layer, filter fabric, and top portion of sand for sand filter are approximately $1,700 annually.
• The figures in the table are based on one impervious acre treated.
• Annual disposal of sediments is estimated at 350 lbs. at $0.02/lb.
  

Annual Operating Cost Comparison for Wet Ponds and Sand Filter

Economic Analysis Summary

Annual Savings for Wet Detention Pond: -$215
Capital Cost for Diversion Equipment/Process: $18,000
Payback Period for Investment in Equipment/Process: > 30 years

Based on the above analysis it appears that the operational cost of a sand filter system is similar to that of a wet pond system. However, a wet pond system has a high land requirement. Where land is available, wet pond systems may be more appropriate.

Click Here to view an Active Spreadsheet for this Economic Analysis and Enter Your Own Values. To return from the Active Spreadsheet, click the reverse arrow in the Tool Bar.

*There are multiple MSDSs for most NSNs.
The MSDS (if shown above) is only meant to serve as an example.

Civilian:
Mr. Ken Pensyl
Maryland Department of the Environment
Non-Point Source Program
2500 Broening Highway
Baltimore, MD 21224
Phone: (410) 631-3543
FAX: (410) 631-3553

Mr. Glenn Moglen
University of Maryland, Department of Civil and Environmental Engineering
College Park, MD 20742
Phone: (301) 405-1964
FAX:  (301) 405-2585
Email:  moglen@eng.umd.edu