Sedimentation

Sedimentation is the process by which particulate matter suspended in a fluid settles out under gravity and forms a deposit. In natural waters and engineered systems, settling is governed by particle size, density, and shape, as well as fluid viscosity and flow regime. For small, spherical particles in laminar conditions, settling velocity can be estimated by Stokes' law; however, most natural systems involve turbulence, flocculation, and hindered settling, which alter both the rate and pattern of deposition.

Natural sedimentation occurs in rivers, lakes, deltas, and oceans when flow energy declines, causing suspended particles to deposit. Over time, clastic sediments form beds with graded or cross-bedded structures; fine silts and clays settle more slowly to form mudstones and shales. Chemical and biogenic sediments arise from precipitation of minerals or from biological processes, and may accumulate in quiet waters or settings such as reefs and evaporite basins. Deposition is followed by burial, compaction, and lithification to form sedimentary rocks.

Factors that control sedimentation include the transport energy of the system, particle size and density distributions, settling velocity, aggregation by flocculation, and water chemistry. Turbulence can keep particles suspended, while calm conditions promote deposition. Human activities such as dam construction, land use changes, and dredging can alter sedimentation patterns by changing flows and sediment supply.

Measurement and significance: Sedimentation rates help quantify sediment budgets, inform interpretations of the rock record, and guide the design of settling basins in water and wastewater treatment. In geology and sedimentology, the study of deposition, sorting, and layering supports reconstruction of past environments and the development of sedimentary rocks.