Planters: Sizing

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A flow-through planter comprises a ponding zone, mulcha top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales. layer, filter mediaThe 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. for planting, and a supporting gravel drainage layer

This article is specific to Flow-through stormwater planters, vegetated systems that do not infiltrate water to the native soilThe natural ground material characteristic of or existing by virtue of geographic origin..
If you are designing a planted system which does infiltrate water, see advice on Bioretention: Sizing.

The dimensions of a stormwater planterA vegetated practice that collects and treats stormwater through sedimentation and filtration. Contributions to water cycle/water balance are through evapotranspiration only; no infiltration. are largely predetermined according to the function of the component. As they do not contain a storage reservoir the planters rely more upon careful selection of materials. Both the filter mediaThe 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. and the perforations of the pipe play critical roles for flow control.

Component Recommended depth (with underdrainA perforated pipe used to assist the draining of soils. pipe) Typical void ratio (VR)
Ponding (dp) ≥ 300 mm 1
Mulch 75 ± 25 mm
  • 0.7 for wood based
  • 0.4 for aggregateA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.
filter media (dm)
  • 300 mm to support turf grass (and accept only rainwater/roof runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface.)
  • 600 mm to support flowering perennials and decorative grasses
  • 1000 mm to support trees
0.3
Pipe diameter reservoir Is equal to underdrainA perforated pipe used to assist the draining of soils. pipe diameter 0.4
Pipe bedding (db) 50 mm (although commonly omitted altogether). 0.4

Calculate the maximum overall depth