From LID SWM Planning and Design Guide
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Stone or gravel can serve as a low maintenance decorative feature, but it may also serve many practical functions in an LID practice. Typical stone functions in LID and direction on selection are provided in the table below:

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 purpose smooth river run stone may be desired.
*The stone sizing is based on flow velocities at inlets and outlets, but typically ranges between 50 mm and 250 mm.
*The thickness of the stone bed is twice that of the largest stone 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."