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*Consideration of desired BMP drawdown period.
*Consideration of desired BMP drawdown period.
===Step 1: Background Evaluation===
====Background Evaluation====
Prior to performing testing and developing a detailed site plan, existing site conditions should be inventoried and mapped including, but not limited to:
Prior to performing testing and developing a detailed site plan, existing site conditions should be inventoried and mapped including, but not limited to:
*Proposed location of development features (buildings, roads, utilities, etc.).
*Proposed location of development features (buildings, roads, utilities, etc.).
===Step 2: Test Pit or Soil Boring Observations===
====Test Pit or Soil Boring Observations====
Test pits or soil borings provide information regarding the soil horizons and overall soil conditions both horizontally and vertically in that portion of the site. Multiple observations can be made across a site at a relatively low cost and in a short time period.
Test pits or soil borings provide information regarding the soil horizons and overall soil conditions both horizontally and vertically in that portion of the site. Multiple observations can be made across a site at a relatively low cost and in a short time period.
*For sites with multiple infiltration BMPs, each with footprint surface areas less than 50 square metres, a minimum of one test pit is required and one soil boring per infiltration BMP location is recommended.
*For sites with multiple infiltration BMPs, each with footprint surface areas less than 50 square metres, a minimum of one test pit is required and one soil boring per infiltration BMP location is recommended.
===Step 3: Infiltration Testing===
====Infiltration Testing====
The preferred testing period is during April and May. If testing is conducted between June 1 and December 1, the soil should be soaked for 24 hours prior, to simulate field saturated conditions.<ref>http://www.extension.umn.edu/environment/housing-technology/moisture-management/how-to-run-a-percolation-test/index.html</ref>
The preferred testing period is during April and May. If testing is conducted between June 1 and December 1, the soil should be soaked for 24 hours prior, to simulate field saturated conditions.<ref>http://www.extension.umn.edu/environment/housing-technology/moisture-management/how-to-run-a-percolation-test/index.html</ref>
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=== Step 4: Design Considerations===
====Design Considerations====
The infiltration rate used to design an infiltration BMP must incorporate a safety correction factor that compensates for potential reductions in soil permeability 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 infiltration rate used to design an infiltration BMP must incorporate a safety correction factor that compensates for potential reductions in soil permeability 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 (in millimetres per hour) at the proposed bottom elevation of the BMP must be divided by a safety correction factor to calculate the design infiltration rate. 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.
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, 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.
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