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*''f''' = design infiltration rate (mm/hr)
*''f''' = design infiltration rate (mm/hr)
*''t'' = [[drainage time|time permitted for ponding to infiltrate]] (hrs) (typically 48 hours)}}
*''t'' = [[drainage time|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 [[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
<ref>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</ref>
===Excess flow control===
===Excess flow control===