Dry ponds

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.

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

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. </ref name=Ballard>

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

Detention time A detention time of 24 hours should be targeted in all instances. Where this necessaitates 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 Equation 4.10. Equation 4.10 is 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 Equation 4.10 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.

Equation 4.10: Drawdown Time

t = start fraction 2 A subscript p over C A subscript o left-parenthesis 2 g right-parenthesis superscript 0 point 5 end fraction left-parenthesis h subscript 1 superscript 0 point 5 minus h subscript 2 superscript 0 point 5 right-parenthesis

or if a relationship between Ap and h is known (i.e., A = C2h + C3)

t = start fraction 0 point 66 C subscript 2 h superscript 1 point 5 + 2 C subscript 3 h superscript 0 point 5 over 2 point 75 A subscript o end fraction

where:

t drawdown time in seconds Ap surface area of the pond(m²) C discharge coefficient (typically 0.63) A0 cross-sectional area of the orifice(m²) g gravitational acceleration constant (9.81,/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

Excess flow control

A two stage riser with low flow controlled with an orifice and high flow controlled with a weir.

There are many configurations for multistage risers according to the flow control requirements. It is easiest to begin downstream and design with the the highest return period required in mind. e.g. start by specifying the pipe/culvert component to control the 100 year flow. Then to work backwards to size the lip or weir at the top of the structure to meet the next return period flow.

The riser is then designed with flow through one or more orifices.

$Q=\left\{{\begin{matrix}0,&Z\leq Z_{0}\\C_{w}L_{w}\left(Z-Z_{0}\right)^{1.5},&Z_{0}\leq Z\leq Z_{0}+H_{0}\\C_{d}A_{0}{\sqrt {2g\left(Z-Z_{0}-0.5H_{0}\right)}},&Z_{0}+H_{0}\leq Z\leq Z_{1}\\C_{w}L_{w}\left(Z-Z_{1}\right)^{1.5}+C_{d}A_{o}{\sqrt {2g\left(Z-Z_{1}\right)}},&Z_{1}<Z\end{matrix}}\right.$

Where:

• Z = the depth of ponded water in m,
• Z₀ = the elevation of the bottom of the orifice in m,
• H₀ = the height of the orifice in m,
• Z₁ = the elevation of the weir in m,
• C_w = the weir coefficient,
• L_w = the length of the weir in m,
• C_d = the discharge coefficient of the orifice (often defaulted to 0.6 for a sharp edged orifice),
• A_o = the cross-sectional area of the orifice in m²,

See also

• Detention basins
• Flow control

For review

https://www.hydrologystudio.com/no-fail-detention-pond-design/ 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

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

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

Depressed storage area, designed to capture flood events.

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 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.

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

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.

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.

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.