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| *''f''' = design infiltration rate (mm/hr) | | *''f''' = design infiltration rate (mm/hr) |
| *''t'' = [[drainage time|time permitted for ponding to infiltrate]] (hrs) (typically 48 hours)}} | | *''t'' = [[drainage time|time permitted for ponding to infiltrate]] (hrs) (typically 48 hours)}} |
| + | |
| + | ===Outlet=== |
| + | Detention time |
| + | A detention time of 24 hours should be targeted in all instances. Where this necessaitates a very low outflow, a [[Flow control#Vo|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. |
| + | |
| + | Equation 4.10: Drawdown Time |
| + | |
| + | 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) |
| + | C2 |
| + | slope coefficient from the area-depth linear regression |
| + | C3 |
| + | intercept from the area-depth linear regression |
| + | |
| + | <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> |
| + | |
| | | |
| ===Excess flow control=== | | ===Excess flow control=== |