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→Detention time
===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 [[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<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: