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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 fractionwhere:tdrawdown time in secondsApsurface area of the pond(m²)Cdischarge coefficient (typically 0.63)A0cross-sectional area of the orifice(m²)ggravitational acceleration constant (9.81,/s2)h1starting water elevation above the orifice (m)h2ending water elevation above the orifice (m)<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>
→Outlet
Detention time
Detention time
A detention time of 24 hours should be targeted in all instances. Where this necessaitates a very low outflow, a [[Flow control#Vortex valve|vortex valve]] or similar is recommended over an orifice or pipe restiction. The detention time is approximated by the drawdown time.
A detention time of 24 hours should be targeted in all instances. Where this necessaitates a very low outflow, a [[Flow control#Vortex valve|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.
The drawdown time in the pond can be estimated using the classic falling head orifice equation which assumes a constant pond surface area<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>. 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.
<math>A_O=\frac{2A_{P}}{t\ C(2g^{0.5})}\left ( h_{1}^{0.5}-h_{2}^{0.5} \right )</math>
<math>A_O=\frac{2A_{P}}{t\ C(2g^{0.5})}\left ( h_{1}^{0.5}-h_{2}^{0.5} \right )</math>
{{planlist|1=Where
* t = Drawdown time (s)
* A<sub>p</sub> = Surface area of the pond(m<sup>2</sup>)
* C = Discharge coefficient (typically 0.63)
* A<sub>O</sub> = Cross-sectional area of the orifice(m<sup>2</sup>)
* g = Gravitational acceleration constant (9.81 m/s<sup>2</sup>)
* h<sub>1</sub> = Starting water elevation above the orifice (m)
* h<sub>2</sub> = Snding water elevation above the orifice (m)}}
C2
C2
slope coefficient from the area-depth linear regression
slope coefficient from the area-depth linear regression
C3
C3
intercept from the area-depth linear regression
intercept from the area-depth linear regression
===Excess flow control===
===Excess flow control===