Stone

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.

A lot level bioretention cell designed to receive and detain, infiltrate and filter runoff, typically used for discharge from downspouts.

Includes the protection of soil from dislocation by water, wind or other agents.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

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.

A perforated pipe used to assist the draining of soils.

Soil particles with a diameter less than 0.050 mm.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.