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The measured infiltration rate (q, mm/hr) at the proposed bottom elevation of the BMP must be divided by a safety correction factor (SCF) to calculate the infiltration coefficient. To select a safety correction factor, calculate the ratio of the mean (geometric) measured infiltration rate at the proposed bottom elevation of the BMP to the rate in the least permeable soil horizon within 1.5 metres below the bottom of the BMP. Based on this ratio, a safety correction factor is selected from the Table below. Where the soil horizon is continuous within 1.5 metres below the proposed bottom of the BMP, the mean infiltration rate measured at the bottom elevation of the BMP should be divided by a safety correction factor of 2.5 to calculate the design infiltration rate.
The measured infiltration rate (q, mm/hr) at the proposed bottom elevation of the BMP must be divided by a safety correction factor (SCF) to calculate the infiltration coefficient. To select a safety correction factor, calculate the ratio of the mean (geometric) measured infiltration rate at the proposed bottom elevation of the BMP to the rate in the least permeable soil horizon within 1.5 metres below the bottom of the BMP. Based on this ratio, a safety correction factor is selected from the Table below. Where the soil horizon is continuous within 1.5 metres below the proposed bottom of the BMP, the mean infiltration rate measured at the bottom elevation of the BMP should be divided by a safety correction factor of 2.5 to calculate the design infiltration rate.
<includeonly>
{|class="wikitable"
{|class="wikitable"
|+ Safety correction factors for calculating design infiltration rates<ref>Wisconsin Department of Natural Resources. 2004. Conservation Practice Standards. Site Evaluation for Stormwater Infiltration (1002). Madison, WI.</ref>
|+ Safety correction factors for calculating design infiltration rates<ref>Wisconsin Department of Natural Resources. 2004. Conservation Practice Standards. Site Evaluation for Stormwater Infiltration (1002). Madison, WI.</ref>
|8.5
|8.5
|}
|}
</includeonly>
Where the mean infiltration rate measured at the proposed bottom elevation of the BMP is 30 mm/h, and the mean infiltration rate measured in an underlying soil horizon within 1.5 metres of the bottom is 12 mm/h:
Where the mean infiltration rate measured at the proposed bottom elevation of the BMP is 30 mm/h, and the mean infiltration rate measured in an underlying soil horizon within 1.5 metres of the bottom is 12 mm/h:
:<math>ratio=\frac{q_{bottom}}{min\ q_{within 1.5m}}\ =\ \frac{30}{12}\ =2.5</math>
:<math>ratio=\frac{q_{bottom}}{min\ q_{within 1.5m}}\ =\ \frac{30}{12}\ =2.5</math>
From the table the SCF is 3.5, and the design infiltration rate can be calculated:
From the table the SCF is 3.5, and the design infiltration rate can be calculated:
:<math>q'=\frac{q}{SCF}\ =\ \frac{30}{3.5}=\ 8.6\ mm/hr</math>
:<math>q'=\frac{q}{SCF}\ =\ \frac{30}{3.5}=\ 8.6\ mm/hr</math>