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Line 169: + − ===Sizing and Modelling===+ − Bioretention facilities should be sized to accommodate runoff from approximately 5 to 20 times the footprint area of the facility. i.e. they should have an I/P ratio of 5 to 20. − When the drainage area is too large, silt can accumulate very rapidly, overwhelm the [[pretreatment]] devices, and lead to clogging of the facility. − When the drainage area is relatively small compared to the bioretention facility, it can make the facility unreasonably costly. − *'''[[Bioretention: Sizing| Sizing]]''' − *'''[[Bioretention: TTT| Modelling]]''' − − ===Inlets and pretreatment options=== − Options for [[pretreatment]] include: − *A [[level spreader]], [[gravel diaphragm]] or [[Vegetated filter strip |vegetated filter strip]] for sheet flow − *A [[Forebays|forebay]] for concentrated surface flow − *An [[Oil and grit separators|oil and grit separator]] for concentrated underground flow − − Simple (non-treating) [[inlets]] include: − *Sheet flow from a pavement edge or flush curb − *One or more [[curb cuts]] − *Covered drains − − ===Overflow Configuration=== − {{:Overflow}} − ===Variations of Bioretention Cells=== − − Below, find three alternate Bioretention cell configurations, that differ based off whether developers want the system to infiltrate incoming water fully, partially, or not at all due to sites which possess contaminated soils or shallow bedrock, and/or zero-lot-line developments (i.e. condo developments and dense urban infill). − − Line 320:
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→Design
|Surface covered with turf grass and stone||Surface covered with mulch and dense, deeply rooting vegetation
|Surface covered with turf grass and stone||Surface covered with mulch and dense, deeply rooting vegetation
|}
|}
===Design Variations===
The figures below illustrate three design variations for a parking lot bioretention cell that differ according to whether or not drainage via infiltration into underlying native soils is to be promoted (i.e., lined or unlined) and their outlet configuration (i.e., with or without an underdrain and location of the perforated pipe in the cross-section). Partial infiltration design, that includes an underdrain and internal water storage reservoir is recommended where native soil infiltration rate is less than 15 mm/hr and is the most common. Full infiltration design does not include an underdrain and is suitable on highly permeable soils (>15 mm/hr infiltration rate) and residential rain gardens. No infiltration or "filtration only" designs feature an impermeable liner and underdrain and are suitable on all soil types. All the figures are image map drawings from the following pages to compare side-by-side the differences between varying configurations.
All the images below are image map drawings from the following pages to compare side-by-side the differences between varying configurations.
rect 1392 2865 1439 3193 [[Liner|Impermeable Liner]]
rect 1392 2865 1439 3193 [[Liner|Impermeable Liner]]
</imagemap>
</imagemap>
===Sizing and Modelling===
Bioretention facilities should be sized to accommodate runoff from approximately 5 to 20 times the footprint area of the facility. i.e. they should have an I/P ratio of 5 to 20.
When the drainage area is too large, silt can accumulate very rapidly, overwhelm the [[pretreatment]] devices, and lead to clogging of the facility.
When the drainage area is relatively small compared to the bioretention facility, it can make the facility unreasonably costly.
*'''[[Bioretention: Sizing| Sizing]]'''
*'''[[Bioretention: TTT| Modelling]]'''
===Inlets and pretreatment options===
Options for [[pretreatment]] include:
*A [[level spreader]], [[gravel diaphragm]] or [[Vegetated filter strip |vegetated filter strip]] for sheet flow
*A [[Forebays|forebay]] for concentrated surface flow
*An [[Oil and grit separators|oil and grit separator]] for concentrated underground flow
Simple (non-treating) [[inlets]] include:
*Sheet flow from a pavement edge or flush curb
*One or more [[curb cuts]]
*Covered drains
===Overflow Configuration===
{{:Overflow}}
===Plant Selection===
===Plant Selection===
The nature of bioretention cells is to attenuate stormwater from rainfall events of varying intensities. For this reason, the vegetation used must be suitable for the varying moisture conditions and is often categorized into three zones related to the grading of the feature.
The nature of bioretention cells is to attenuate stormwater from rainfall events of varying intensities. For this reason, the vegetation used must be suitable for the varying moisture conditions and is often categorized into three zones related to the grading of the feature.