Dry ponds

From LID SWM Planning and Design Guide
Revision as of 20:25, 15 March 2018 by Jenny Hill (talk | contribs)
Jump to navigation Jump to search

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[edit]

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

Design[edit]

Volume[edit]

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

Where:

Outlet[edit]

Detention time A detention time of 24 hours should be targeted in all instances. Where this necessaitates a very low outflow, a [[Flow control#Vo|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

[1]


Excess flow control[edit]

Modeling[edit]

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

TTT.png

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[edit]

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[2]. 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[edit]


  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
  2. Ontario Provincial Standard Specification. (2014). Construction Specification and for Seed and Cover OPSS.PROV 804. Retrieved from http://www.raqsb.mto.gov.on.ca/techpubs/ops.nsf/0/3a785d2f480f9349852580820062910a/$FILE/OPSS.PROV 804 Nov2014.pdf