Sediment Transport

What happens to weathered material?Some will remain where it is, the rest will be transported away. Transport can be due to: gravity - moves loose weathered sediment down slopes wind - lifts and transports finer grained sediment rivers - lift and transport sediment of all sizes, including heavy and large grains the sea - moves sediment of all sizes ice - depending on the climatic conditions, can carry large amounts of rock debris The grains of sediment are involved in the process of erosion. They come into contact with rocks over which they are travelling, perhaps in a desert, in the bed of a stream or along the coast. These rocks are subject to abrasion. They may be sandblasted by wind blown sediments in deserts, ground down by boulders rolling along a riverbed or chipped away by shingle carried by the sea when waves crash into cliffs.Transported grains are not only in contact with the Earth's surface but are also in contact with each other. Attrition is the wearing down of the transported sediment by rolling, rubbing and crushing together of sedimentary grains during transport. The more contact there is between grains and the longer they have been in transport, the smaller and more rounded they become. Harder minerals like quartz survive better than less hard minerals, like mica, which may be completely crushed.Methods of TransportIce carries sediment embedded within it. Larger grain sizes can be transported when the velocity of the current is greater. Bed loads consist of larger grains that are moved by traction and smaller grains that bounce along the bed by saltation. Finer grains do not touch the bed and so are the suspended load. Dissolved material transported in solution is invisible and so cannot be shown. Grains become smaller the longer they have been transported.Clay sized particles require high velocities for erosion because they are flat and platy in shape and are cohesive so tend to stick together. The clay particles remain in suspension even at very low current velocities because they are small, have low mass and are buoyant.Grain Shape and RoundessGrains get rounder the longer they have been transported and the degree of roundness can be described from very angular to well rounded. The shape of grains depends on the type of rock or mineral from which they are made, rather than on transport. One way to compare shape is to a sphere.Grain SizeMeasurement and classification of the grain size of sediments is made using the Wentworth-Udden scale. As the range of grain sizes found in nature is large, a logarithmic scale is used.The grain size is obtained from the average diameter of the grains of sediment being studied. The diameter in mm is not always easy to remember, so the phi scale is also used. >2mm: phi -2 to -8 - gravel, pebbles, cobbles, boulders 2mm: phi -1 - sand: very coarse 1mm:  phi 0 - sand: coarse 0.5mm: phi 1 - sand: medium 0.25mm: phi 2 - sand: fine 0.125mm: phi 3 - sand: very fine 0.0625mm: phi 4 - silt 0.039mm: phi 8 - clay

New Page