From LID SWM Planning and Design Guide
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This rain garden in a school yard uses stone as both decorative edging and for erosion control.
This bioswale in a parking lot uses stone at the inlets and along the bottom of the swale to prevent erosion, as the sides are sloped.

Stone or gravel can serve as a low maintenance decorative feature, but it may also serve many practical functions in an LID practice. For advice on aggregates used in underdrains, see gravel.

Stone functions and specifications
Function Recommended Specification
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.