Choker layer

medium sized granular, free from fines

In bioretention systems a choking layer of ≥ 100 mm is the recommended method to prevent migration of finer filter media into an underlying reservoir of coarse aggregate. Suitable materials include:

High performance bedding (HPB)

Clean, angular aggregate screened to between 6 - 10 mm. Widely available and designed specifically for drainage applications. Free from fines by definition.

HL 6

Is a clean, angular aggregate screened between 10 - 20 mm. Free from fines by definition.

Pea Gravel

Rounded natural aggregate, screened between 5 - 15 mm, and washed free from fines.

In most scenarios, a geotextile layer is unnecessary and have been associated with rapid decline and clogging in some circumstances.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

Soil particles with a diameter less than 0.050 mm.

A 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.