Sedimentation

Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration, or electromagnetism. In geology, sedimentation is often used as the opposite of erosion, i.e., the terminal end of sediment transport. In that sense, it includes the termination of transport by saltation or true bedload transport. Settling is the falling of suspended particles through the liquid, whereas sedimentation is the termination of the settling process. In estuarine environments, settling can be influenced by the presence or absence of vegetation. Trees such as mangroves are crucial to the attenuation of waves or currents, promoting the settlement of suspended particles.In a sedimentation experiment, the applied force accelerates the particles to a terminal velocity v t e r m {displaystyle v_{mathrm {term} }}   at which the applied force is exactly canceled by an opposing drag force. For small enough particles (low Reynolds number), the drag force varies linearly with the terminal velocity, i.e., F d r a g = f v t e r m {displaystyle F_{mathrm {drag} }=fv_{mathrm {term} }}   (Stokes flow) where f depends only on the properties of the particle and the surrounding fluid. Similarly, the applied force generally varies linearly with some coupling constant (denoted here as q) that depends only on the properties of the particle, F a p p = q E a p p {displaystyle F_{mathrm {app} }=qE_{mathrm {app} }}  . Hence, it is generally possible to define a sedimentation coefficient s   = d e f   q / f {displaystyle s {stackrel {mathrm {def} }{=}} q/f}   that depends only on the properties of the particle and the surrounding fluid. Thus, measuring s can reveal underlying properties of the particle.In geology, sedimentation is the deposition of particles carried by a fluid flow. For suspended load, this can be expressed mathematically by the Exner equation, and results in the formation of depositional landforms and the rocks that constitute sedimentary record. An undesired increased transport and sedimentation of suspended material is called siltation, and it is a major source of pollution in waterways in some parts of the world. High sedimentation rates can be a result of poor land management and a high frequency of flooding events. If not managed properly, it can be detrimental to fragile ecosystems on the receiving end, such as coral reefs. Climate change also affects siltation rates.In chemistry, sedimentation has been used to measure the size of large molecules (macromolecule), where the force of gravity is augmented with centrifugal force in an ultracentrifuge.