Changes

===Detention time===

===Detention time===

A minimum detention time of 24 hours should be targeted in all instances, but 48 hours is preferred for water quality benefits. Where this necessitates a very low outflow, a [[Flow control#Vortex valve|vortex valve]] or similar is recommended over an [[Flow through an orifice|orifice]] or pipe restriction. The detention time is approximated by the drawdown time.

A minimum detention time of 24 hours should be targeted in all instances, but 48 hours is preferred for water quality benefits. Where this necessitates a very low outflow, a [[Flow control#Vortex valve|vortex valve]] or similar is recommended over an [[Flow through an orifice|orifice]] or pipe restriction. The detention time is approximated by the drawdown 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 name="MOE"/>. 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 (MOE, 2003)<ref name="MOE"/>. 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>