Dry Extended Detention Pond
A dry extended detention pond is a stormwater basin designed to temporarily capture and store runoff, releasing it slowly over a period of about 24 hours. This extended detention time allows fine sediment and associated pollutants to settle out of the water column before it is discharged downstream. Unlike a wet pond, a dry extended detention pond does not maintain a large permanent pool of water between storm events, though it incorporates a small micropool of water at the outlet. The floor of the basin is typically vegetated with grasses that can tolerate periodic inundation.
The primary function of this practice is water quality improvement through gravitational settling. The slow release of captured runoff also helps reduce peak discharge rates, which can mitigate downstream channel erosion and flooding. These facilities are a common and widely applied stormwater management practice, often used to treat runoff from larger commercial, residential, and institutional developments.
Applicability
Dry extended detention ponds are one of the most broadly applicable stormwater management practices, suitable for many development scenarios, though they are less common in highly dense urban settings. Designers can use a BMP selector tool to evaluate if a dry extended detention pond is appropriate for specific site conditions and watershed goals.
Regional Suitability
This practice can be implemented in nearly all regions of the United States. In cold climates, the detention volume may need to be increased to handle spring snowmelt, and salt-tolerant vegetation should be selected if the pond receives road runoff. In arid or semi-arid climates, landscaping should feature drought-tolerant species, and forebays may be designed without a permanent pool of water. In areas with karst (limestone) topography or highly permeable soils, an impermeable liner may be required to prevent sinkhole formation and protect groundwater.
Ultra-Urban and Space-Constrained Areas
Dry extended detention ponds require a significant land area, typically 2% to 3% of their contributing drainage area, making them difficult to implement in dense, ultra-urban environments where land is scarce. They are more feasible in these settings when a suitable open area is available downstream of the developed site.
Stormwater Hotspots
These ponds can receive runoff from stormwater hotspots—land uses that generate high concentrations of pollutants. However, when used for this purpose, they require significant vertical separation from the groundwater table to prevent contamination. An impermeable liner may also be necessary as an additional safeguard.
Retrofit Applications
Dry extended detention ponds are well-suited for stormwater retrofitting. Many older flood control basins, originally built with no water quality features, can be modified to provide extended detention. This typically involves retrofitting the outlet structure to slow the release of smaller, more frequent storms. New ponds can also be constructed in existing open spaces within a developed watershed to capture and treat runoff from previously uncontrolled areas.
Cold Water Stream Considerations
Studies have shown that extended detention in shallow, unshaded basins can increase the temperature of discharged stormwater (Galli, 1990). In watersheds that drain to cold water streams, detention times may be limited to less than 12 hours to minimize thermal impacts. Providing shade through landscaping around the outlet and pilot channel can also help mitigate stream warming.
Design Criteria
A successful dry extended detention pond design incorporates features for feasibility, conveyance, pretreatment, treatment, and landscaping. These elements ensure the practice functions effectively and minimizes long-term maintenance needs. A comprehensive stormwater pond sizing calculator can assist with determining the required storage volumes for water quality and channel protection.
Feasibility
- Drainage Area: A minimum drainage area of 10 acres is recommended. For smaller sites, the required low-flow orifice can become extremely small and prone to clogging.
- Soils and Groundwater: The practice is adaptable to most soil types. The bottom of the pond should be located above the seasonal high groundwater table to ensure the basin drains properly and does not create mosquito breeding habitat.
- Head: Sufficient elevation drop, or head, is needed between the pond’s inlet and outlet to allow for positive drainage and gravity flow through the outlet structure.
- Slopes: Ponds can be sited in areas with slopes up to 15%, but the basin floor and embankments require relatively flat grades for stability and safety.
Conveyance
- Inlets and Outlets: All inlets and outfalls must be stabilized with riprap or other measures to prevent erosion as water enters and exits the pond.
- Pilot Channel: A concrete or armored low-flow channel should be constructed across the basin floor to convey minor flows and prevent soggy conditions.
- Emergency Spillway: A stabilized emergency spillway must be included in the embankment to safely pass large flood events that exceed the capacity of the primary outlet structure.
Pretreatment
- Sediment Forebay: A sediment forebay is a small, separate basin located at each major inlet to the pond. It is designed to capture coarse sediment before it reaches the main treatment area, which simplifies maintenance and prolongs the life of the facility.
- Forebay Sizing: The forebay should be sized to contain a volume of 0.1 inches per impervious acre of contributing drainage and should be four to six feet deep. This volume counts toward the total required water quality volume.
- Access: Direct, stabilized access must be provided for heavy equipment to periodically remove accumulated sediment from the forebay.
Treatment
- Detention Time: The pond must be sized to capture the water quality volume and release it over approximately 24 hours.
- Flow Path: The geometry of the pond should provide a long flow path from inlet to outlet, with a length-to-width ratio of at least 1.5:1. This prevents short-circuiting and maximizes detention time.
- Micropool: A small permanent pool of water, or micropool, should be established at the outlet structure. The micropool prevents sediment resuspension and helps keep the low-flow orifice clear of debris.
- Non-Clogging Outlet: The low-flow orifice must be designed to resist clogging. The preferred configuration is a reverse-slope pipe that draws water from at least one foot below the micropool surface. The orifice should have a minimum diameter of 3 inches.
Landscaping
- Side Slopes: Interior side slopes should not exceed a 3:1 (horizontal:vertical) ratio for safety and ease of maintenance.
- Vegetation: The basin floor and side slopes should be planted with water-tolerant grasses or meadow vegetation. A buffer of at least 25 feet should be maintained around the pond’s perimeter. Woody vegetation should not be planted on or near the embankment.
- Access: A 12-foot-wide maintenance right-of-way with a maximum slope of 15% must extend from a road to the forebay and outlet structure.
Pollutant Removal
Dry extended detention ponds provide moderate pollutant removal, primarily by settling suspended solids. They are less effective at removing soluble pollutants because they lack the biological treatment mechanisms found in practices with a large permanent pool, such as a stormwater wetland. The performance is highly dependent on providing adequate detention time and incorporating key design features like a sediment forebay and micropool. The table below presents typical pollutant removal rates based on monitoring data. For more detailed information, consult the national pollutant removal database.
| Pollutant | Median Removal Rate (%) |
|---|---|
| Total Suspended Solids (TSS) | 61 ± 321 |
| Total Phosphorus (TP) | 20 ± 13 |
| Total Nitrogen (TN) | 31 ± 16 |
| Nitrate-Nitrogen (NOx) | -2 ± 23 |
| Metals (Cu, Pb, Zn) | 29 – 54 |
| Bacteria | 782 |
| Source: Winer (2000) 1 ± values represent one standard deviation. 2 Data based on fewer than five data points. |
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Construction and Cost Considerations
On a cost-per-area-treated basis, dry extended detention ponds are one of the least expensive structural stormwater practices. Construction costs vary based on site conditions, storage volume, and local markets. The cost for design, permitting, and construction can be estimated using an equation developed from a regional study (Brown and Schueler, 1997): C = 12.4V0.760, where C is the cost in dollars and V is the 10-year storm storage volume in cubic feet. Based on this relationship, a 1 acre-foot pond might cost approximately $42,000, while a 10 acre-foot pond could cost around $240,000.
While construction costs are relatively low, these facilities have a significant land footprint, typically requiring 2% to 3% of the contributing drainage area. Unlike practices such as a grass channel that can fit into linear spaces, ponds require a large, consolidated parcel of land. Research has also suggested that, unlike wet ponds which can be an amenity, dry ponds may slightly detract from the value of adjacent residential properties (Emmerling-Dinovo, 1995).
Maintenance
Regular maintenance is critical for the effective long-term performance of dry extended detention ponds. A legally binding maintenance agreement should be established to assign responsibility for these activities. The following table outlines a typical maintenance schedule.
| Activity | Frequency |
|---|---|
| Inspect for erosion, embankment damage, and standing water. | Semi-Annually |
| Inspect inlet/outlet structures for debris, clogging, and damage. | Annually |
| Mow side slopes and maintenance access areas. | As needed (Standard) |
| Remove litter and debris. | As needed (Standard) |
| Repair eroded areas and reseed bare ground. | Annually (As needed) |
| Remove sediment from the forebay. | Every 5-7 Years (or when 50% full) |
| Remove sediment from the main basin. | Every 25-50 Years (or when volume is reduced by 25%) |
Limitations
Despite their wide applicability, dry extended detention ponds have several limitations. Their pollutant removal capability is only moderate and is often poor for dissolved nutrients like nitrate. If not designed or maintained properly to ensure complete drainage after storms, they can create boggy conditions and become breeding grounds for mosquitoes. The large land requirement makes them unsuitable for many dense urban sites. There is also a risk of thermal pollution, as the shallow, sun-exposed water can warm significantly before being discharged to downstream channels, potentially harming sensitive aquatic life.
Frequently Asked Questions
What is a dry extended detention pond?
A dry extended detention pond is a stormwater management basin that is normally dry between rain events. It is designed to capture runoff and release it slowly over about 24 hours. This temporary storage, or extended detention, allows suspended solids and other pollutants to settle out of the water. The basin floor is typically covered in grass. While its primary goal is water quality improvement, it also helps reduce peak flow rates downstream, providing flood and channel erosion control.
How does a dry pond differ from a wet pond?
The primary difference is the presence of a permanent pool of water. A wet pond maintains a large, deep pool year-round, which provides treatment through biological processes in addition to settling. A dry extended detention pond is designed to drain completely within 24 to 48 hours after a storm, leaving the basin floor dry. Modern dry ponds include a very small “micropool” at the outlet to prevent clogging and resuspension of sediment, but this is minor compared to the permanent pool of a wet pond.
What is the purpose of the micropool at the outlet?
The micropool is a small, permanent pool of water located directly at the low-flow outlet structure. It serves two main functions. First, it prevents the outlet orifice from becoming clogged with floating debris, as the intake for the orifice is positioned below the water surface. Second, it helps prevent settled pollutants from being washed out during subsequent storms, a process known as resuspension. This small feature significantly improves the reliability and pollutant removal performance of the pond.
Why is a sediment forebay important in a dry pond design?
A sediment forebay is a small basin at the pond’s inlet that slows down incoming runoff, causing heavier sediments and debris to settle out before they enter the main part of the pond. This pretreatment step is crucial for long-term performance. It concentrates the coarsest sediment in one small, easily accessible area, making maintenance much simpler and more cost-effective. By capturing this material, the forebay protects the main basin from premature filling and preserves its treatment capacity.
What is the typical detention time for a dry ED pond?
The target detention time for water quality treatment in a dry extended detention pond is approximately 24 hours. The outlet structure is specifically engineered to release the captured water quality volume over this period. This duration provides a good balance, allowing sufficient time for fine particles to settle out of the water column without holding the water so long that it becomes a nuisance (e.g., mosquito breeding) or becomes excessively warmed by the sun, which could harm downstream aquatic ecosystems.
Are dry ED ponds effective at removing all pollutants?
No, they are not. Their primary removal mechanism is gravity settling, which is effective for suspended solids, heavy metals, and particle-bound nutrients. However, they are generally ineffective at removing dissolved pollutants like nitrates, and can sometimes even be a source of them. Their performance for phosphorus is modest. For sites where removing soluble pollutants is a high priority, other practices like stormwater wetlands or bioretention may be more appropriate choices.
Can these ponds be used for flood control?
Yes, providing flood control is a common secondary function of dry extended detention ponds. In a typical design, the outlet structure has multiple stages. A small, low-flow orifice provides the 24-hour extended detention for water quality storms. Larger openings or weirs at higher elevations are designed to control peak flows from larger, less frequent events like the 10-year or 100-year storm. This dual-purpose design makes them a versatile tool in stormwater management.
What are the most important maintenance tasks for a dry pond?
The most critical maintenance tasks are ensuring the outlet does not clog and removing accumulated sediment. Regular inspections are needed to clear debris from the low-flow orifice and trash rack. Periodic sediment removal from the forebay, typically every 5 to 7 years, is essential to maintain pretreatment capacity. Less frequently, the main basin will need to be dredged. Routine tasks like mowing, managing vegetation, and repairing eroded areas are also necessary to keep the facility safe and functional.
Can a dry extended detention pond be used for a very small site?
It is generally not recommended to use dry extended detention ponds for sites with drainage areas smaller than 10 acres. On smaller sites, the orifice required to achieve a 24-hour detention time becomes extremely small (less than 3 inches in diameter). These tiny openings are highly susceptible to clogging from leaves, trash, and other debris, which leads to frequent maintenance problems and potential failure of the system. Other small-scale practices are better suited for smaller drainage areas.
What are key design considerations for cold climates?
In cold climates, designs must account for snowmelt and freezing conditions. The total detention volume may need to be increased to manage the large, rapid runoff from spring snowmelt events. The low-flow orifice should be at least 3 inches in diameter and designed to prevent freezing and ice blockage. Inlet pipes should not be submerged to avoid ice formation. If the pond receives runoff from sanded roads, more frequent sediment removal from the forebay will be necessary.