Dry ponds

The following image showcases an extended detention dry pond. For more details click here..A note: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.

Click here to see map of Dry Ponds in Scarborough and surroundings

See also Water squares

Overview

A dry pond is a detention basin designed to temporarily store collected stormwater runoff and release it at a controlled rate through an outlet (MOE, 2003). Also known as infiltration basins or detention basins (according to their features), dry ponds are a grassed alternative to bioretention cells. Dry ponds may have a deep pool of water in the sediment forebay to reduce scour and resuspension of sediment, but do not have a permanent pool of water in the main basin. This permits the dry pond to be integrated into the landscape and used as an amenity space. Dry ponds are recommended as flood control structures to accommodate occasional excess overflow downstream of other structural BMPs. Compared to wet ponds, dry ponds are less expensive to install, require less maintenance, and may involve less liability for the community (Fairfax County, 2025).

Dry ponds are ideal for:

• Managing infrequent extreme flow events,
• erosion management,
• incorporating into parks and other green recreational spaces, and
• distributing across a larger development site.

Click here to read a case study on dry ponds at the The University of Toronto Scarborough campus.

Planning considerations

Multiuse soccer field and intermittent detention pond in Montana during a dry period (top) and after rainfall (bottom) (TD & H Engineering, 2025).

Location

Dry ponds are a useful tool for managing flooding during larger storm events. They are well suited to being placed downstream of other smaller distributed BMPs as an end-of-pipe control for occasional backup flood protection. Where possible, they should be integrated into amenity space, given that users rarely wish to continue outdoor activities during such intense rainstorms.

Infiltration

For information about constraints to infiltration practices, and approaches and tools for identifying and designing within them see Infiltration. For guidance on infiltration testing and selecting a design infiltration rate see Design infiltration rate.

Dry pond public safety sign (City of Toronto, 2015).

Public safety

Where temporary storage of water occurs on the surface, the depth and rate of rise of the water should be sufficiently low that risks are minimized for site users, especially since the temporary nature of the storage facility will mean that the public are not accustomed to its presence. A risk assessment should be undertaken of the frequency and rate of flooding to a range of inundation depths in order that public safety is not jeopardised (Ballard et al., 2015). Signs should be erected around the dry pond at locations highly visible to the public to identify the site as a stormwater management facility and raise awareness of the function and hazards (City of Toronto, 2015).

Water temperature

Dry detention ponds can elevate the temperature of stormwater. This effect can be reduced by shortening detention time (US EPA, 2021), increasing canopy cover (LSRCA, 2022), or installing a cooling trench at the pond outlet (City of Toronto, 2015). Alternative stormwater controls may be better suited for areas that discharge to cold-water streams.

Cold Climates

Dry ponds, especially those without infiltration, are typically the least affected by winter/spring conditions because there is no large permanent pool. Precautions can be taken to guard against freezing of pipes and orifices, and a by-pass (flow splitter) can be employed to limit inflow during spring freshet (MOE, 2003).

Design

Design parameters for extended detention (MOE, 2003)

Element	Design Objective	Criteria
Drainage Area	Minimum orifice size (see flow control)	5 Ha ≥10 Ha preferred
Treatment Volume	Provision of appropriate level of protection	See below
Active Storage	Detention	Suspended solids settling 24 hrs 48 hrs preferred for water quality benefits
Forebay	Pre-treatment	Minimum depth: 1 m (1.5 m preferred) Sized to ensure non-erosive velocities leaving forebay Maximum Area: 20% (preferred) to 33% of total permanent pool
Length-to-Width Ratio	Maximize flow path and minimize short-circuiting potential	Overall: minimum 3:1 4:1 preferred
Depth	Safety	Maximum 3 m Flat bottom to encourage uniform ponding/infiltration (recommended tolerance on base levels 10 mm in 3m)
Side slopes (See also berms)	Safety	4:1 average no steeper than 1:3 to permit vegetation stabilization and maintenance/amenity access (except where pond <0.5 m deep) stepped or benched slopes possible depending on maintenance access (Ballard et al., 2015)
Inlet	Avoid clogging/freezing	Minimum 450 mm diameter inlet pipe; Preferred pipe slope: > 1 %; If submerged, obvert 150 mm below expected maximum ice depth
Outlet (See also flow control)	Avoid clogging/freezing	Minimum: 450 mm diameter outlet pipe; Preferred pipe slope: > 1 %; If orifice control used, 75 mm diameter minimum; Minimum 100 mm orifice preferable
Maintenance access	Access for backhoes or dredging equipment	Provided to approval of Municipality; Provision of maintenance drawdown pipe preferred
Buffer	Safety	Minimum 3 m above maximum water quality/erosion control water level

Volume

Recommended storage volumes per ha by catchment imperviousness for continuous flow dry ponds with basic protection (MOE, 2003)

Imperviousness of Catchment	Storage Volume (m³/ha)
35%	90
55%	150
70%	200
85%	240

The surface storage volume of a dry pond (A_p) is determined: $A_{p}={\frac {RVC_{T}\times A_{c}}{f'\times t}}$

Where:

• RVC_T = Runoff volume control target (mm)
• A_c = Area of the catchment (m²)
• f' = design infiltration rate (mm/hr)
• t = time permitted for ponding to infiltrate (hrs) (typically 48 hours)

Higher catchment imperviousness may require a larger storage volume, as shown in the chart on the right.

Detention time

A minimum detention time of 24 hours should be targeted in all instances, but 48 hours is preferred for water quality benefits. Where this necessitates a very low outflow, a vortex valve or similar is recommended over an orifice or pipe restriction. The detention time is approximated by the drawdown time. The drawdown time in the pond can be estimated using the classic falling head orifice equation which assumes a constant pond surface area. This assumption is generally not valid, and a more accurate estimation can be made if the equation is solved as a differential equation. This is easily done if the relationship between pond surface area and pond depth is approximated using a linear regression:

$A_{o}={\frac {2A_{P}}{t\ C(2g^{0.5})}}\left(h_{1}^{0.5}-h_{2}^{0.5}\right)$

Where

• t = Drawdown time (s)
• A_p = Surface area of the pond(m²)
• C = Discharge coefficient (typically 0.63)
• A_o = Cross-sectional area of the orifice(m²)
• g = Gravitational acceleration constant (9.81 m/s²)
• h₁ = Starting water elevation above the orifice (m)
• h₂ = Ending water elevation above the orifice (m)

C2 slope coefficient from the area-depth linear regression C3 intercept from the area-depth linear regression

Settling Velocity of Particulates

The process of sedimentation is enacted when solids "settle" to the bottom of a sedimentation practice, from suspension in moving or retained water (in this case a dry pond BMP). Due to dry ponds having grassy channels and slopes they are able to over time decrease the velocity of incoming stormwater flow that enters the practice. This combined with the agility of the practice to allow for temporary storage and ponding of water (24 - 48 hr) allows excess sediments (sand, silts, clay and small aggregates) and associated pollutants to settle and be retained within the BMP (Weiss et al. 2010). These sediments should be removed periodically to maintain as designed performance of the feature. The following calculations are for measuring the settling velocities of various solids.

Stoke's Law for settling solids

Stoke's Law measures solids settling in stormwater features and is applicable to fines, clay, silt, and sand in water.

$V=\left({\frac {g\left({\frac {p_{1}}{p}}-1\right)\ d^{2}}{18v}}\right)$

Where

• V = settling velocity of particles (m/s)
• g = gravitational acceleration constant (9.81 m/s²)
• d = diameter of the solid (spherical) (m)
• ρ₁ = mass density of solid (kg/m³)
• ρ: mass density of water (1000 kg/m³
• v: kinematic viscosity of water at (1 mm²/s)

Ferguson & Church (2004)

Ferguson & Church's calculation meanwhile allows for designers to include the relationship between settling velocity and particle diameter size. A relationship for settling velocity that incorporates larger particles, such as sands with Reynolds Number (RE) > 10. The equation becomes Stokes' Law when particles have smaller diameters and allows for a constant drag coefficient to be applied for larger particle diameters.

$V={\frac {gRd^{2}}{18v+(0.75CgRd^{3})^{\frac {1}{2}}}}$

Where

• V = settling velocity of particles (m/s)
• g = gravitational acceleration constant (9.81 m/s²)
• d = diameter of the solid (spherical) (µm)
• v = kinematic viscosity of water (1 mm²/s)
• R = Specific gravity for solid in question (i.e. 1.58kg/cm³ for sand)
• C = Typical constant for spherical solids (0.4) and (1.0) for sand grains

See the table below for average settling velocities of different particle size ranges and particle types based on MOEE (1994) and from Muschalla, 2014

Visual Indicators Framework - Bioretention/Swales

Size Fraction (i)	Particle size range (µm)	Average settling velocity of particles in size fraction i, Vsi (m/s)	Fraction of total mass contained in size fraction i (%) - MOEE	Fraction of total mass contained in size fraction i (%) - measured
1	x ≤ 20	2.54E-06	20	83.4
2	20 ≤ x ≤ 40	1.30E-05	10	9.1
3	40 ≤ x ≤ 60	2.54E-05	10	4.4
4	60 ≤ x ≤ 130	1.27E-04	20	4.1
5	130 ≤ x ≤ 400	5.93E-04	20	-
6	400 ≤ x ≤ 4000	5.50E-03	20	-

Forebay

Dry pond forebay, adapted from MOE (2003)

A sediment forebay facilitates maintenance and improves pollutant removal by trapping larger particles near the inlet of the pond. The forebay should be separated from the main pond by an earthen berm, which should be constructed as a small dam since the downstream portion of the pond remains dry. A weir at the top of the berm will convey flows to the downstream section during storm events. To facilitate maintenance, a pipe with an upstream valve should be installed within the berm to allow the forebay to be drawn down for cleaning (MOE, 2003).

The berm should be planted with emergent vegetation to enhance filtration of water as it passes over. Refer to Wetlands: Plants for a list of Ontario native emergent plants.

Inlet

Dry pond inlet (MOE, 2003)

The number of inlets into the pond should be minimized. To reduce erosion risk, the use of environmental stone or interlocking blocks with large openings that support vegetative growth is recommended at the inlet. A flow deflector or energy dissipation blocks may also be installed to limit scour and prevent the resuspension of previously settled pollutants from the pond bottom (MOE, 2003).

Outlet

Dry pond outlet (MOE, 2003)

The performance of a dry pond can be enhanced via a shallow micropool at the outlet which concentrates finer sediment and reduces re-suspension. The micropool is normally planted with hardy wetland species such as cattail (MOE, 2003).

For excess flow control, see Flow through riser

• https://www.fhwa.dot.gov/engineering/hydraulics/software/hy8/
• https://www.hydrologystudio.com/no-fail-detention-pond-design/

Buffer

A minimum of a 3 m buffer strip from the maximum design water level mark should be installed to meet safety (e.g., restrict access to steep areas or inlet/outlet locations), wind screening, shading, aesthetic, and public amenity objectives. When possible,

• at least least 5 species should be planted in a random pattern to prevent the establishment of monoculture areas,
• a large number of young plant stocks, tree whips and seedlings should be planted along with a small number of large mature shrubs and trees, and
• a naturalized landscape approach should be used which strives for a vegetation community with long-term sustainability and no maintenance requirements (MOE, 2003).

See Salt-Tolerant Turf Grasses, Shrubs: List, and Trees: List for a list of native species which could be planted in the buffer strip.

Modeling

Dry ponds are found in storage element in the LID TTT

The largest area is at the top, level 0 m; each subsequent lower depth has a smaller area

The Low Impact Development Treatment Train Tool can analyze annual and event-based runoff volumes and pollutant load removal for dry ponds.

A dry pond as a storage element (key parameters) in the Treatment Train Tool.

Stage Storage
Name	Important to have a unique name, to connect it with the catchment area
Storage type	Dry detention ponds
Bottom elevation (m)	This is important to correspond with other components, e.g. when the overflow is coupled to another BMP within a treatment train
Maximum depth (m)
Lined/unlined	Unlined (ideally)
Underlying soil	Choose from five; sandy soils drain more quickly.
Evaporation factor	?
Suction head (mm)	?
Saturated conductivity (mm/hr)	?
Initial soil moisture deficit (fraction)	?
Curves
The Curves table is designed to accommodate the side slopes. The top line begins at 0 m, with subsequent depths in the following lines.

Low Impact Development Life Cycle Costing Tool can be used for dry ponds.

Inspection and Maintenance

Dry pond maintenance in Alberta.

Maintenance requirements vary between dry ponds and wet ponds, although many maintenance tasks overlap.

Comparison of dry pond and wet pond maintenance activities (MOE, 2003)

Maintenance Activity	Dry Pond	Wet Pond
Inspection	✓	✓
Grass cutting	✓	✓
Weed control	✓	✓
Removal of accumulated sediments	✓	✓
Upland vegetation replanting	✓	✓
Trash removal	✓	✓
Outlet valve adjustment	✓	✓
Shoreline fringe and flood fringe vegetation replanting		✓
Aquatic vegetation replanting		✓

Maximum allowable sediment accumulation (m³/ha) in dry ponds by catchment imperviousness for basic protection (STEP, 2016)

Catchment Imperviousness	Sediment Accumulation (m³/ha)
35%	40
55%	69
70%	111
85%	133

Inspection points for dry ponds include (MOE, 2003):

• standing water in pond after rain events for longer than designed detention time - may indicate outlet blockage
• pond is always/relatively dry, even throughout designed detention time - may indicate inlet blockage
• vegetation around the pond unhealthy or dying - may indicate poor species selection
• visible sediment accumulation in the bottom of the pond or around the high water line of the pond - may indicate need for sediment removal

A field data sheet for the inspection of constructed wetlands is available below. You can download (downward facing arrow on the top righthand side) and print (Printer emoticon on top right hand side) the Dry Pond or Dry Extended Detention Basin Inspection and Maintenance Checklist developed by the City of Ontario, Ohio.

Gallery

• 

  Wishing well park is a baseball field with flood warning signs, Scarborough ON

• 

  Avondale Park, designed for flood storage, but not optimized for infiltration, North York ON

• 

  A multi-functional dry basin, still due to be fitted with playground equipment. Cambridge, UK. Photo credit: Simon Bunn

• 

  This dry pond features a concrete lined low-flow channel. Photo credit: US EPA

• 

  Stormwater lagoon, Wilmhurst Road, Warwick. UK. Photo credit: Robin Stott

• 

  Dry polder northwest of Vincencov, Prostějov. Czech Republic. Photo credit: Jiří Komárek

External links

• Edmonton Dry Ponds
• County of Fairfax Ponds
• City of Kitchener Dry Ponds

A stormwater design features that provides for the gradual release of a volume of water in order to increase settling of pollutants and protect downstream channels from frequent storm events.

Depressed storage area, designed to capture flood events.

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

A pretreatment basin at the inlet of a practice that allow settling out of sediment and associated contaminants suspended in urban runoff.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

A body of water smaller than a lake, often artificially formed.

A structural best management practice that is located at the end of a flow conveyance route. End-of-Pipe Controls on surface and below ground but are not limited to wet ponds, constructed wetlands and other similar systems.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

Depressed storage area, designed to capture flood events.

Lake Simcoe Region Conservation Authority

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

Annual spring increase in water levels caused by melting snow, ice, and increased rainfall.

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

Includes the protection of soil from dislocation by water, wind or other agents.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

The period between the maximum water level and the minimum level (dry weather or antecedent level).

The period between the maximum water level and the minimum level (dry weather or antecedent level).

Deposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.

Settling-out or deposition of particulate matter suspended in runoff.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

Soil particles with a diameter less than 0.050 mm.

Soil or media particles smaller than sand and larger than clay (3 to 60 m)

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

A pretreatment basin at the inlet of a practice that allow settling out of sediment and associated contaminants suspended in urban runoff.

A compacted earthen wall that diverts runoff or creates shallow ponding of runoff. In some cases, runoff ponds behind the berm and gradually flows through it or is infiltrated.

A vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year.

a gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They function by slowing runoff velocity and filtering out suspended sediment and associated pollutants, and by providing some infiltration into underlying soils.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

The total mass of a pollutant entering a waterbody over a defined time period.

Stormwater management following the hierarchical approach: Source Control measures, Conveyance Control measure and End of Pipe treatment to achieve the water quality and water balance target for lot level development of the preferred strategy.

A combination of lot level, conveyance, and end-of-pipe stormwater management practices.

Abiotic transfer of water vapour to the atmosphere.

A body of water smaller than a lake, often artificially formed.

Sustainable Technologies Evaluation Program

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Water ponded on the ground surface.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.