Dry ponds

(Redirected from Detention basins)
Click here to see map of Dry Ponds in Scarborough and surroundings

Also known as infiltration basinsGround depression acting as a flow control and water treatment structure, that is normally dry. 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. [1]

Design

Design parameters for extended detentionA 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.[2]
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 DetentionThe temporary storage of stormwater to control discharge rates, and allow for sedimentation. Suspended solids settling 24 hrs (48 hrs preferred)
ForebayA pretreatment basin at the inlet of a practice that allow settling out of sediment and associated contaminants suspended in urban runoff. Pre-treatment
• Minimum depth: 1 m;
• Sized to ensure non-erosive velocities leaving forebayA pretreatment basin at the inlet of a practice that allow settling out of sediment and associated contaminants suspended in urban runoff.;
Length-to-Width Ratio Maximize flow path and minimize short-circuiting potential
• Overall: minimum 3:1;
• 4:1 preferred
Depth Safety Maximum 3 m
• 4:1 average
Inlet Avoid clogging/freezing
• Minimum 450 mm;
• 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 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 controlIncludes the protection of soil from dislocation by water, wind or other agents. water level

The bottom of a dry pondDepressed storage area, designed to capture flood events. 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 pondA body of water smaller than a lake, often artificially formed. area is very shallow (0.5 m). stepped or benched slopes are also a possibility, but consideration should be made of maintenance access. [1]

Volume

The surface storage volume of a dry pondDepressed storage area, designed to capture flood events. (Ap) is determined$A_{p}=\frac{RVC_T\times A_{c}}{f'\times t}$

Where:

• RVCT = Runoff volume control target (mm)
• Ac = Area of the catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. (m2)
• f' = design infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. (mm/hr)
• t = time permitted for ponding to infiltrate (hrs) (typically 48 hours)

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

A detentionThe temporary storage of stormwater to control discharge rates, and allow for sedimentation. 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 detentionThe temporary storage of stormwater to control discharge rates, and allow for sedimentation. time is approximated by the drawdown timeThe period between the maximum water level and the minimum level (dry weather or antecedent level).. The drawdown timeThe period between the maximum water level and the minimum level (dry weather or antecedent level). in the pondA body of water smaller than a lake, often artificially formed. can be estimated using the classic falling head orifice equation which assumes a constant pondA body of water smaller than a lake, often artificially formed. surface area[2]. 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 pondA body of water smaller than a lake, often artificially formed. surface area and pondA body of water smaller than a lake, often artificially formed. 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 timeThe period between the maximum water level and the minimum level (dry weather or antecedent level). (s)
• Ap = Surface area of the pondA body of water smaller than a lake, often artificially formed.(m2)
• C = Discharge coefficient (typically 0.63)
• Ao = 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

Modeling

Dry ponds are found in storage element in the LIDLow 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. 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 catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. area
Storage type Dry detentionThe temporary storage of stormwater to control discharge rates, and allow for sedimentation. ponds
Bottom elevation (m) This is important to correspond with other components,
e.g. when the overflow is coupled to another BMPBest 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. within a treatment trainStormwater 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.
Maximum depth (m)
Lined/unlined Unlined (ideally)
Underlying soil Choose from five; sandy soils drain more quickly.
EvaporationAbiotic transfer of water vapour to the atmosphere. 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[3]. The 'Salt Tolerant Mix' is of particular value for low impact developmentA 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. applications alongside asphaltA mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces. 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 %