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→Detention time
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:
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>
{{plainlist|1=Where
{{plainlist|1=Where
* t = Drawdown time (s)
* ''t'' = Drawdown time (s)
* A<sub>p</sub> = Surface area of the pond(m<sup>2</sup>)
* ''A<sub>p</sub>'' = Surface area of the pond(m<sup>2</sup>)
* C = Discharge coefficient (typically 0.63)
* ''C'' = Discharge coefficient (typically 0.63)
* A<sub>O</sub> = Cross-sectional area of the orifice(m<sup>2</sup>)
* ''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>)
* ''g'' = Gravitational acceleration constant (9.81 m/s<sup>2</sup>)
* h<sub>1</sub> = Starting water elevation above the orifice (m)
* ''h<sub>1</sub>'' = Starting water elevation above the orifice (m)
* h<sub>2</sub> = Snding water elevation above the orifice (m)}}
* ''h<sub>2</sub>'' = Snding water elevation above the orifice (m)}}
C2
C2