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[[File:Gravel.jpg|thumb|medium sized granular, free from fines]]
[[File:Gravel.jpg|thumb|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 [[Reservoir gravel|coarse aggregate]].
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 [[Reservoir gravel|coarse aggregate]].
These same mid sized granular materials are recommended for use in [[Stormwater planter]] underdrains.
These same mid sized granular materials are recommended for use in [[Stormwater planter]] underdrains and may be useful in the fine grading of foundations courses for [[permeable paving]].
Suitable materials include:
Suitable materials include: