Safety correction factors

The infiltration rate used to design an infiltration BMP (e.g.infiltration trench or bioretention) 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. 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.

Safety correction factors for calculating design infiltration rates

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

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:

$ratio={\frac {q_{bottom}}{min\ q_{within1.5m}}}\ =\ {\frac {30}{12}}\ =2.5$

From the table the SCF is 3.5, and the design infiltration rate can be calculated:

$q'={\frac {q}{SCF}}\ =\ {\frac {30}{3.5}}=\ 8.6\ mm/hr$

Minnesota say 2 Pennsylvania say 3 CIRIA say 2-3 for low risk sites

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.