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

Also known as infiltration basins or detention basins (according to their features). Dry ponds are a grassed alternative to bioretention cells. This permits the landscape to be accessed and used as an amenity space.

Overview

Dry ponds are recommended as flood control structures to accommodate occasional excess overflow downstream of other structural BMPs. They should be integrated into the landscape as useful, accessible public space.

Dry ponds are ideal for:

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

Planning considerations

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 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.

Compared to wet ponds “Dry ponds… …are less expensive to install, require less maintenance and may involve less liability for the communities around them.” https://www.fairfaxcounty.gov/soil-water-conservation/understanding-stormwater-ponds

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.

Risk

Where temporary storage of water occurs on the surface the depth and rate of rise of the water should be sufficiently low that risks posed by the water body are minimized for site users (taking into account the temporary nature of the storage facility which 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.

Design

Design parameters for extended detention

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)
Forebay	Pre-treatment	Minimum depth: 1 m; Sized to ensure non-erosive velocities leaving forebay;
Length-to-Width Ratio	Maximize flow path and minimize short-circuiting potential	Overall: minimum 3:1; 4:1 preferred
Depth	Safety	Maximum 3 m
Side slopes (See also berms)	Safety	4:1 average
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

The bottom of a dry pond should be flat to encourage uniform ponding and infiltration across the entire surface. Recommended tolerance on base levels 10 mm in 3m.

The side slopes should be no steeper than 1:3 to permit vegetation stabilization and access for maintenance and amenity. This may be relaxed where the pond area is very shallow (0.5 m). stepped or benched slopes are also a possibility, but consideration should be made of maintenance access.

Volume

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)

Outlet

http://www.iowastormwater.org/documents/filelibrary/files/infiltration_bmps/Outlet_Structures_48B13497A39A1.pdf

Detention time

A detention time of 24 hours should be targeted in all instances. Where this necessitates a very low outflow, a vortex valve or similar is recommended over an orifice or pipe restiction. 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	-

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/

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

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.

Materials

Resilient turf grasses are particularly useful in the design of vegetated filter strips, dry ponds and enhanced grass swales. The Ministry of Transportation have standardized a number of grass mixes. The 'Salt Tolerant Mix' is of particular value for low impact development applications alongside asphalt roadways and paved walkways.

Canada #1 Ground Cover (salt tolerant mix)

Common name	Scientific name	Proportion
Tall Fescue	Festuca arundinacea	25 %
Fults Alkali Grass	Puccinellia distans	20 %
Creeping Red Fescue	Festuca rubra	25 %
Perennial ryegrass	Lolium perrenne	20 %
Hard Fescue	Festuca trachyphylla	10 %

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
• Calgary Braeside Dry Pond
• County of Fairfax Dry Ponds
  • County of Fairfax 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.

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

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.

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

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

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

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

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

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

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

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.

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.

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 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.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

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

Penetration of water through 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.