Wet Pond
A wet pond is a constructed stormwater basin that maintains a permanent pool of water, functioning as a cost-effective and widely used treatment practice. Runoff entering the pond is treated through physical settling of suspended solids and biological uptake of nutrients by algae and aquatic vegetation. The extended time that runoff resides in the permanent pool is the primary mechanism for pollutant removal. A common design modification is the wet extended detention pond, which provides temporary storage above the permanent pool to slow the release of stormwater, offering enhanced settling and downstream channel protection.
Wet ponds are broadly applicable across many regions and development types, though some site constraints and climatic factors must be considered. They are a common choice for new development and are also frequently used in retrofit scenarios. For assistance in selecting the most appropriate practice for a specific site, a BMP selector tool can help evaluate various options.
Applicability
Regional Suitability
Wet ponds are effective in most temperate and humid regions of the United States. In arid or semi-arid climates, maintaining the permanent pool can be challenging and may require supplemental water, which is often not a sustainable practice. For example, one study in Austin, Texas, found that 2.6 acre-feet of supplemental water were needed annually to maintain a 0.29 acre-foot pool (Saunders and Gilroy, 1997). In cold climates, design modifications are necessary to address issues like freezing of the permanent pool and outlet structures, as well as increased sediment and salt loads from road treatment. In karst (limestone) topography, an impermeable liner is typically required to prevent groundwater contamination and maintain the pool.
Ultra-Urban Areas
The relatively large footprint of wet ponds (typically 2% to 3% of their drainage area) can make them difficult to site in dense, ultra-urban settings where land is scarce. However, they can be a viable option if a sufficiently large open space is available. In many ultra-urban contexts, other practices like bioretention may be more feasible.
Stormwater Hotspots
Wet ponds are suitable for treating runoff from stormwater hotspots—land uses that generate higher pollutant loads, such as gas stations or industrial sites. When used for this purpose, they should be designed with an impermeable liner and sufficient separation from the groundwater table to prevent contamination.
Retrofits
Wet ponds are a popular choice for stormwater retrofits. Many communities have existing dry detention basins built solely for flood control. These can often be excavated and modified with a new outlet structure to create a permanent pool, adding significant water quality treatment benefits. New wet ponds can also be constructed in available open spaces or even online within stream corridors to address water quality impairments in previously developed watersheds.
Cold Water Streams
A significant limitation of wet ponds is their tendency to cause thermal pollution. The large, open surface of the permanent pool absorbs solar radiation, heating the water. This warmer water, when discharged, can degrade downstream habitat for cold-water fish species like trout. A study in Maryland found that wet ponds increased outlet temperatures by an average of 9°F (Galli, 1990), making them generally unsuitable for watersheds that drain to cold water streams.
Design Criteria
Proper design is essential for the effective and sustainable performance of a wet pond. Key criteria focus on site feasibility, conveyance of flows, and the specific elements that provide pretreatment and treatment. These criteria ensure the pond functions as intended, removes pollutants effectively, and minimizes long-term maintenance needs. A comprehensive stormwater pond sizing calculator can assist with determining the required volumes for treatment and detention.
Feasibility
A wet pond typically requires a minimum drainage area of 10 acres, with 25 acres being preferable in many climates, to ensure sufficient runoff and baseflow to maintain the permanent pool. A water balance analysis may be performed to confirm pool viability for smaller drainage areas. The site must have sufficient head to allow for gravity flow through the system. Ponds should not be located within jurisdictional wetlands or other protected waters, and larger ponds may be subject to dam safety regulations.
Conveyance
Stormwater must be conveyed into and out of the pond safely and non-erosively. Inlets should be stabilized, and the principal spillway outlet must be designed with energy dissipation, such as a riprap stilling basin, to prevent downstream channel erosion. An emergency spillway is required to safely pass large flood events that exceed the capacity of the principal spillway.
Pretreatment
A sediment forebay is a required pretreatment element for a wet pond. The forebay is a separate, smaller basin located at the pond inlet, designed to capture coarse sediment before it enters the main pool. It should be sized to hold approximately 10% of the total water quality volume (WQv), or 0.1 inches of runoff per impervious acre of drainage area. The forebay should be 4 to 6 feet deep and have direct maintenance access for periodic sediment removal.
Treatment Geometry and Features
The main pool provides the primary water quality treatment. To prevent short-circuiting and maximize residence time, the pond should have a length-to-width ratio of at least 1.5:1. Internal berms or baffles can be used to create a longer, more sinuous flow path. The perimeter of any deep pool areas (4 feet or deeper) must include two benches:
- Aquatic Bench: A shallow shelf extending up to 15 feet inward from the normal shoreline, with a maximum depth of 18 inches. This bench supports emergent wetland vegetation, which enhances nutrient uptake and shoreline stability.
- Safety Bench: A gently sloped area extending 15 feet outward from the water’s edge to the toe of the pond embankment. This bench provides a safety buffer and maintenance access.
Outlet Works
The outlet structure, or riser, controls water level elevations and discharge rates. It must include a non-clogging low-flow orifice, typically a reverse-slope pipe with a minimum diameter of 3 inches, to draw water from below the surface and prevent clogging by floating debris. The riser should be located within the embankment for easy access and equipped with a trash rack. A pond drain with an adjustable valve is also required to allow the pond to be dewatered for maintenance.
Landscaping
A detailed landscaping plan is a critical component of pond design. A vegetated buffer at least 25 feet wide should be established around the pond’s maximum water surface elevation to stabilize soils and discourage nuisance geese. The aquatic and safety benches should be planted with appropriate native, non-invasive species. Woody vegetation should not be planted on or near the embankment to protect its structural integrity.
Design Variations
Several design variations adapt the basic wet pond concept to meet specific site constraints or treatment objectives.
Wet Extended Detention Pond
This common hybrid design combines a permanent pool with extended detention (ED) storage above it. At least 50% of the required water quality volume is provided in the permanent pool, while the remaining volume is temporarily stored and released over 12 to 48 hours. This approach provides excellent pollutant removal and robust channel protection, often in a smaller footprint than a standard wet pond. It shares some functional characteristics with a dry extended detention pond but offers superior water quality performance due to the permanent pool.
Micropool Extended Detention Pond
This variation is essentially a dry ED pond with a very small permanent pool (micropool) at the outlet. The micropool is designed to prevent the outlet from clogging and to treat low flows during dry weather. While an improvement over a standard dry pond, its overall pollutant removal is lower than that of a full wet pond.
Multiple Pond System
In this configuration, two or more ponds are constructed in series. This “treatment train” approach provides redundant treatment, increases the overall flow path and residence time, and can significantly enhance pollutant removal, particularly for nutrients.
Pocket Pond
A pocket pond is a smaller wet pond used for drainage areas typically less than 10 acres. To maintain a permanent pool with such a small contributing area, these ponds are sited where they can intercept the local groundwater table. Their pollutant removal performance is generally lower than that of larger wet ponds but can be an effective solution for smaller sites with suitable hydrogeology.
Pollutant Removal
Wet ponds are one of the most reliable and effective structural stormwater practices for pollutant removal. The combination of a long residence time, a large permanent pool, and biological activity allows for significant reductions in sediment, nutrients, and other common stormwater pollutants. The table below presents typical removal efficiencies based on a comprehensive review of monitored performance data.
| Pollutant | Removal Efficiency (%) |
|---|---|
| Total Suspended Solids (TSS) | 80 |
| Total Phosphorus (TP) | 51 |
| Total Nitrogen (TN) | 33 |
| Nitrate-Nitrogen (NOx) | 43 |
| Metals (Cd, Cu, Pb, Zn) | 29 – 73 |
| Bacteria | 70 |
Source: Adapted from Winer, 2000.
Performance can be variable and is highly dependent on factors like the size of the permanent pool relative to the watershed, the pond’s geometry, and the quality of maintenance. Properly designed and maintained ponds consistently achieve high removal rates. For more detailed performance information across a wider range of practices, consult the national pollutant removal database. The performance of wet ponds can sometimes be enhanced by pairing them with other practices, such as a stormwater wetland, to create a more complex treatment system.
Construction and Cost Considerations
Wet ponds are a relatively low-cost stormwater management practice compared to their treatment effectiveness. Construction costs vary based on site conditions, storage volume, and local labor rates. An analysis by Brown and Schueler (1997) produced a cost equation for estimating construction, design, and permitting costs: C = 24.5V0.705, where C is the cost and V is the storage volume in cubic feet for the 10-year storm. Based on this formula (adjusted for inflation), a 1 acre-foot facility would cost approximately $45,700, while a 10 acre-foot facility would be around $232,000.
Land acquisition can be a significant cost, though ponds typically occupy only 2% to 3% of their contributing drainage area. Annual maintenance costs are generally estimated at 3% to 5% of the initial construction cost. Despite these costs, well-maintained wet ponds can be a community amenity, with studies showing that properties adjacent to ponds can have increased property values of 10% or more (US EPA, 1995).
Maintenance
Routine maintenance is crucial for ensuring a wet pond functions correctly and remains an aesthetic asset. A legally binding maintenance agreement should be established to assign responsibility for these tasks.
| Activity | Schedule |
|---|---|
| Inspect for erosion, damage, and trash accumulation. Check for signs of hydrocarbon buildup. Ensure inlets and outlets are clear of debris. | Annual |
| Clean and remove debris from inlet and outlet structures. Mow side slopes and maintenance access areas. | Monthly (during growing season) |
| Repair any eroded areas on side slopes or embankment. | As Needed |
| Remove accumulated sediment from the forebay. | Every 5 to 7 Years (or when 50% full) |
| Remove accumulated sediment from the main pool. | Every 20 to 50 Years (or when volume is significantly reduced) |
Source: Adapted from WMI, 1997.
Limitations
While widely applicable, wet ponds have several important limitations:
- Thermal Impacts: They can significantly warm stormwater discharges, making them unsuitable for watersheds draining to sensitive cold-water streams.
- Space Requirements: Their footprint of 2-3% of the drainage area can make them infeasible in dense urban environments.
- Climate Restrictions: They are generally not feasible in arid climates without a supplemental water source due to high evaporation rates.
- Habitat Alteration: Improper siting can lead to the destruction of natural wetlands or high-quality upland forest.
- Safety Concerns: The permanent pool of water can pose a safety risk, requiring design features like safety benches and managed side slopes to mitigate hazards.
Frequently Asked Questions
What is a wet pond?
A wet pond, also known as a retention pond, is a constructed basin designed to hold a permanent pool of water. It treats incoming stormwater runoff primarily through the gravitational settling of pollutants and sediment. Additional treatment occurs through biological processes, such as nutrient uptake by algae and aquatic plants. Wet ponds are a common and cost-effective stormwater best management practice (BMP) used to improve water quality from developed areas. They can also be designed to provide flood control by including storage volume above the permanent pool.
How is a wet pond different from a dry pond?
The key difference is the presence of a permanent pool of water. A wet pond is designed to always contain water, which provides continuous water quality treatment through settling and biological activity. In contrast, a dry extended detention pond is designed to be dry between storm events. It detains runoff temporarily and releases it slowly, primarily providing flood control and some limited pollutant removal. Wet ponds offer significantly higher and more reliable pollutant removal, especially for fine sediment and nutrients, compared to dry ponds.
How much land area does a wet pond require?
A wet pond and its associated buffer typically occupy about 2% to 3% of the total land area that drains to it. For example, a 100-acre development would require a 2- to 3-acre area for the pond. This relatively large footprint can be a constraint in dense urban settings where land is expensive or unavailable. The exact size depends on local rainfall, the amount of impervious surface in the watershed, and the specific water quality and quantity control objectives the pond is designed to meet.
What is the purpose of a sediment forebay?
A sediment forebay is a small, deep pool located at the inlet of a wet pond. Its primary purpose is to act as a pretreatment device by capturing coarse sediment, trash, and debris before they enter the main pond. This protects the main pool from premature sedimentation, which preserves its treatment volume and extends its functional lifespan. By concentrating sediment in a small, easily accessible area, the forebay simplifies maintenance and significantly reduces the frequency and cost of dredging the entire pond.
Can wet ponds be used in cold climates?
Yes, wet ponds can be adapted for use in cold climates, but several design modifications are necessary. The outlet structure must be designed to resist freezing, often by using larger diameter pipes and weirs instead of small orifices. Road salt can impact vegetation, so salt-tolerant species should be selected for landscaping. Additionally, the large volume of spring snowmelt, which carries a high pollutant load, must be managed. Some designs use a seasonally operated outlet that lowers the permanent pool in winter to create extra storage capacity for the spring melt event.
How often do wet ponds need to be dredged?
The maintenance schedule for sediment removal is divided into two parts. The sediment forebay, which captures the majority of coarse sediment, typically needs to be cleaned out every 5 to 7 years, or whenever it is 50% full. The main pool of the pond accumulates sediment much more slowly. It generally requires dredging only once every 20 to 50 years, or when sediment accumulation has significantly reduced the permanent pool volume and impaired its treatment function. Regular forebay maintenance is the most effective way to prolong the life of the main pool.
Do wet ponds pose a safety risk?
The permanent pool of water can be a safety concern, particularly for children. Modern design standards address this risk through specific safety features. Ponds are designed with gentle side slopes (no steeper than 3:1) that terminate on a flat safety bench extending 15 feet from the water’s edge. This design provides a stable area for egress and prevents a sudden drop-off into deep water. Fencing is generally discouraged in favor of these landscape-based safety measures, which can also include dense vegetation to limit access to the permanent pool.
Why are wet ponds not recommended for cold water streams?
Wet ponds can harm cold water streams through thermal pollution. The large, relatively shallow surface of the permanent pool absorbs significant solar radiation, causing the water temperature to rise. When this heated water is discharged during and after a storm, it can raise the temperature of the receiving stream. Cold-water fish species like trout and sculpin are highly sensitive to temperature increases, which can reduce dissolved oxygen and disrupt their life cycles. Therefore, wet ponds are generally considered unsuitable for watersheds that drain directly to sensitive cold-water fisheries.
What is an aquatic bench?
An aquatic bench is a shallow, flat shelf constructed along the inside perimeter of a wet pond’s permanent pool. It typically extends up to 15 feet into the pond from the normal shoreline and is designed to have a water depth of no more than 18 inches. This shallow zone creates ideal conditions for establishing emergent wetland vegetation. These plants help stabilize the shoreline against erosion, provide habitat for wildlife, and contribute to pollutant removal through nutrient uptake. The aquatic bench also improves safety by creating a shallow area around the deeper central pool.
Can a single pond manage both water quality and flood control?
Yes, wet ponds are frequently designed to manage both water quality and flood control. This is achieved by providing storage volume for different purposes at different elevations. The permanent pool at the bottom of the pond provides the water quality treatment volume (WQv). Above the permanent pool, additional “extended detention” or “flood control” storage is provided. During a storm, this upper volume temporarily stores runoff and releases it slowly through the outlet structure, reducing peak flows downstream and preventing flooding. This dual-purpose design is a very efficient use of land and infrastructure.