Sedimentation is the physical process by which suspended solid particles, collectively referred to as sediment, settle out of a fluid, most commonly water, under the influence of gravity. In stormwater management and hydrology, sedimentation occurs when the velocity of flowing water is reduced to a point where it no longer has sufficient energy to transport sediment particles, causing those particles to deposit onto the bed of a channel, basin, or other receiving surface.
Sedimentation is governed by several interrelated factors, including particle size, shape, and density, as well as the viscosity and velocity of the water. Larger and denser particles, such as sand and gravel, settle more quickly, while finer particles like silt and clay may remain suspended for extended periods and require very low flow conditions or chemical processes, such as flocculation, to settle.
In the context of stormwater systems, sedimentation plays both beneficial and detrimental roles. It is intentionally utilized in engineered practices such as sediment basins, detention ponds, and forebays, where stormwater runoff is temporarily stored to allow suspended sediments and associated pollutants, including nutrients, heavy metals, and hydrocarbons, to settle out before the water is discharged downstream. This improves water quality and reduces pollutant loads entering natural water bodies.
Conversely, uncontrolled sedimentation can have negative impacts. Excessive deposition in channels, culverts, and storm drains can reduce hydraulic capacity, increase the risk of flooding, and necessitate maintenance dredging. In natural watercourses, sedimentation can smother aquatic habitats, degrade spawning grounds, and alter channel morphology.
Sedimentation is closely linked to erosion, as erosion provides the source of sediment that is later transported and deposited. Together, erosion, transport, and sedimentation form the fundamental processes of sediment dynamics within a watershed. Proper stormwater management seeks to balance these processes by minimizing erosion at the source and promoting controlled sedimentation in designated treatment areas.