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Line 128:
Line 128: − + − *n' = effective porosity of the active storage fill material(s), (effective porosity of surface ponding depth and filter media depth combined for practices with no underdrain; porosity of storage reservoir aggregate or effective porosity of void-producing structure system for practices with an underdrain) − +
→Determine the required surface area of the practice
<math>A_{p}=i\times D\times A_{i}/[d_{a} + (f' \times D)]</math>
<math>A_{p}=i\times D\times A_{i}/[d_{a} + (f' \times D)]</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*d<sub>a</sub> = Active water storage depth (m)
*d<sub>a</sub> = Active water storage depth (m), see above equations
*f' = design infiltration rate of the underlying native soil (m/h)
*f' = design infiltration rate of the underlying native soil (m/h)
*D = Design storm duration (h)}}<br>
*D = Design storm duration (h)}}<br>
* Step 11: Compare required surface area of the practice to available space.<br>
* Step 11: Compare required surface area of the practice to available space.<br>
To decrease A<sub>p</sub>, consider increasing ponding depth if feasible and would not create a safety hazard, or decreasing catchment impervious area.
To decrease A<sub>p</sub>, consider increasing ponding depth if feasible and would not create a safety hazard, or decreasing catchment impervious area.
* Step 12: Calculate catchment impervious area to practice permeable (footprint) area ratio:
* Step 12: Calculate catchment impervious area to practice permeable (footprint) area ratio, R, also referred to as I/P ratio:
<math>R=A_{i}/A_{p}</math><br>
<math>R=A_{i}/A_{p}</math><br>
Adjust either A<sub>i</sub> or A<sub>p</sub> to keep R between 5 and 20.
Adjust either A<sub>i</sub> or A<sub>p</sub> to keep R between 5 and 20.