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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 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)
<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>
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===