# Safety correction factors

The infiltration coefficient used to design an infiltration BMP must incorporate a safety correction factor that compensates for potential reductions in soil permeability.

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: :${\displaystyle q'={\frac {q}{SCF}}}$ 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.

Safety correction factors for calculating design infiltration rates[1]
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

### Example

For example, 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:

${\displaystyle ratio={\frac {q_{bottom}}{min\ q_{within1.5m}}}}$

the ratio would be 2.5, the safety correction factor would be 3.5, and the design infiltration rate would be 8.6 mm/h. 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.

1. Wisconsin Department of Natural Resources. 2004. Conservation Practice Standards. Site Evaluation for Stormwater Infiltration (1002). Madison, WI.