Dry ponds

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The following image showcases an extended detention dry pond. For more details click here.[1]. 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.


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[edit]

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


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.


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. [2]


Design parameters for extended detention[3]
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. [2]


The surface storage volume of a dry pond (Ap) is determined:




Detention time[edit]

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[3]. 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:


  • t = Drawdown time (s)
  • Ap = Surface area of the pond(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

Settling Velocity of Particulates[edit]

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)[4]. 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[edit]

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


  • 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)[5][edit]

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 = 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)[6] and from Muschalla, 2014[7]

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[edit]

See Flow through riser


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) ?
The Curves table is designed to accommodate the side slopes. The top line begins at 0 m, with subsequent depths in the following lines.


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[8]. 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 %


External links[edit]


  1. Ministry of the Environment. Stormwater Management Planning and Design Manual. https://dr6j45jk9xcmk.cloudfront.net/documents/1757/195-stormwater-planning-and-design-en.pdf. 2003. Accessed 3 September, 2021
  2. 2.0 2.1 Ballard, B. W., Wilson, S., Udale-Clarke, H., Illman, S., Scott, T., Ashley, R., & Kellagher, R. (2015). The SuDS Manual. London.
  3. 3.0 3.1 Ontario Ministry of Environment. (2003). Stormwater Management Planning and Design Manual. Retrieved January 15, 2017, from https://www.ontario.ca/document/stormwater-management-planning-and-design-manual/stormwater-management-plan-and-swmp-design
  4. Gulliver, J.S., A.J. Erickson, and P.T. Weiss (editors). 2010. "Stormwater Treatment: Assessment and Maintenance. University of Minnesota, St. Anthony Falls Laboratory. Minneapolis, MN. https://stormwaterbook.safl.umn.edu/
  5. Ferguson, R.I. and Church, M. 2004. A simple universal equation for grain settling velocity. Journal of sedimentary Research, 74(6), pp.933-937. for settling solids. http://geoweb.uwyo.edu/geol5330/FergusonChurch_GrainSettling_JSR04.pdf
  6. MOEE (1994). Stormwater management practices planning and design manual. Ministry of Environment and Energy, Ontario, Canada.
  7. Muschalla, D., Vallet, B., Anctil, F., Lessard, P., Pelletier, G. and Vanrolleghem, P.A., 2014. Ecohydraulic-driven real-time control of stormwater basins. Journal of hydrology, 511, pp.82-91.
  8. Ontario Provincial Standard Specification. (2023). Construction Specification and for Vegetative Cover OPSS.PROV 803. Retrieved from https://tcp.mto.gov.on.ca/notice/000-0140