Difference between revisions of "Safety correction factors"

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The ppinfiltration rate]] used to design an infiltration BMP (e.g.[[infiltration trenches|infiltration trench]] or [[bioretention]]) must incorporate a safety correction factor that compensates for potential reductions in soil permeability.
#REDIRECT [[Design infiltration rate]]
 
These may be due to:
*compaction or smearing during construction,
*gradual accumulation of fine sediments over the lifespan of the BMP, and
*uncertainty in measured values when less permeable soil horizons exist within 1.5 metres below the proposed bottom elevation of the BMP.
 
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.
<onlyinclude>
{|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>
|-
!Ratio of mean measured infiltration rates
!Safety Correction Factor
|-
|≤ 1
|2.5
|-
|1.1 to 4.0
|3.5
|-
|4.1 to 8.0
|4.5
|-
|8.1 to 16.0
|6.5
|-
|16.1 or greater
|8.5
|}
</onlyinclude>
 
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>
 
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>
 
 
Minnesota say 2 <ref>Design criteria for infiltration. (2018, February 14). Minnesota Stormwater Manual, . Retrieved 00:16, March 14, 2018 from https://stormwater.pca.state.mn.us/index.php?title=Design_criteria_for_infiltration&oldid=35583.</ref>
Pennsylvania say 3 <ref>CRC for Water Sensitive Cities. (2015). Adoption Guidelines for Stormwater Biofiltration Systems: Appendix C - Guidelines for filter media in stormwater biofiltration systems.</ref>
CIRIA say 2-3 for low risk sites <ref>Ballard, B. W., Wilson, S., Udale-Clarke, H., Illman, S., Scott, T., Ashley, R., & Kellagher, R. (2015). The SuDS Manual. London.</ref>

Latest revision as of 18:17, 1 October 2018

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