|Subsurface storage layer for stormwater and to surround the underdrain or subdrain
- 50mm diameter clear stone, washed and free of all fines, should be used
- The depth of the gravel subsurface storage layer is a minimum of 300 mm and the underdrain is set at least 100 mm above the bottom to provide a minimum infiltration volume
- A 100 mm pea gravel choking layer and optional drainage geotextile can be used to prevent the bioretention soil from migrating into the gravel storage layer and underdrain
- Geotextile is not recommended around the sides and bottom of the gravel storage layer as it has been found to be unnecessary and a common cause of early clogging
|Dissipate flow and prevent erosion at inlets and outlets
- Angular crushed stone, which will "knit" or lock together and be less likely to shift, is recommended. However, for aesthetic purposes, smooth river-run stone may be desired.
- Determine stone size by flow velocities at inlets and outlets. Typical stone for this purpose ranges between 50 mm and 250 mm. The larger the stone, the more energy dissipation.
- Stone beds should be twice as thick as the largest stone's diameter.
- To prevent erosion of soils beneath the stone and the migration of the stone into the soil, the stone bed should be underlain by a drainage geotextile.
|Direct and spread flow throughout a large LID facility or to protect narrow channel sections where flow will concentrate
- While crushed stone will be less likely to shift, river-run stone may be used to create a dry-stream-bed look.
- The sizing of the gravel will depend on the expected velocities.