Longshore drift (sometimes known as shore drift, LSD or littoral drift) is a geological process by which sediments such as sand or other materials, move along a beach shore. Geology (from Greek γη gê, "earth" and λόγος Logos, "speech" lit Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of
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Mechanism
Longshore drift is the net movement of sand and other fine particles like shell along the coast line. The process occurs naturally and constantly on any shoreline where waves approach the shore obliquely. A wave is a disturbance that propagates through Space and Time, usually with transference of Energy. That is to say, at an angle other than 90° (because the backwash leaves the shore at 90°). The effect of this is determined by factors such as the direction and fetch of the present wind and, in the long term, of the prevailing wind. Fetch, often called the fetch length, is a term for the length of water over which a given Wind has blown Wind is the flow of Air or other Gases that compose an Atmosphere (including but not limited to the Earth's) Waves striking the shore at an angle as opposed to straight on will cause the wave swash to move up the beach at an angle. Swash ( uprush and backwash) in Geography, is the water that washes up on Shore after an incoming wave has broken The swash moves the sediment particles (typically sand or shingle) up the beach at this angle, while the [backwash] brings them, solely under the influence of gravity, directly down the beach. Sand is a naturally occurring Granular material composed of finely divided rock and Mineral particles This has the net effect of gradual movement of the particles along the shore by the use of swash and backwash. Erosion on the beach works concurrently with longshore drift to straighten the overall shape of the beach; by making it conform to the action of the waves so that any particles of sand that are not deposited parallel to the wave action are areas that receive the most pressure from incoming waves and wind.
The water molecules are among the particles moved in the process so there is a net movement of water as well. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by This establishes a longshore current. A current, in a River or Stream, is the Flow of Water influenced by Gravity as the water moves Downhill to reduce its
Where the wave front falls exactly squarely onto a major segment of coast, it will still meet parts of the shore in bays and estuaries obliquely, so that longshore drift will still occur on a more detailed scale. Headlands and bays are two related features of the coastal environment An estuary is a semi-enclosed Coastal body of Water with one or more Rivers or Streams flowing into it and with a free connection to the open The waves carry sediment from headlands into coves, as any seaside visitor will have noticed. Waves and currents can sort the pebbles, gravels, sands, and the muds to settle according to the energy remaining in the waves in the different locations. A pebble is a clast of rock with a Particle size of 4 to 64 Millimeters based on the Krumbein phi scale of Sedimentology Gravel is rock that is of a specific Particle size range In Geology, gravel is any loose rock that is larger than two millimeters (2mm Sand is a naturally occurring Granular material composed of finely divided rock and Mineral particles In computer gaming, a MUD ( Multi-User Dungeon, Domain or Dimension) is a multi-player computer game that combines elements of
Constructive process
Longshore drift is one of the principal processes in the construction of spits, bars and tombolos, and in the overall sustainability of beach deposits. A spit is a deposition Landform found off Coasts. At one end spits connect to land while at the far end they exist in open water A shoal or sandbar (also called sandbank) is a somewhat Linear Landform within or extending into a body of Water, A tombolo is a deposition Landform such as a spit or bar which is attached to the mainland by a narrow piece of land Longshore currents and longshore drift are generally considered to be constructive processes. Unlike storm waves, they are not normally regarded as significant in coastal erosion, though the sedimentary particles have to come from somewhere. They are the continuing processes that nourish the beach and carry sand along the shore or a barrier spit to deposit it at the end of the spit so that the spit grows in length and size.
This link shows incident waves refracting into the entrance to the River Tees and various beach forms produced in a variety of circumstances. The Tees is a river in Northern England. It rises on the eastern slope of Cross Fell in the Pennines, and flows eastwards for about 85 miles The longshore drift from the east has brought material to the eastern side of the central groyne while the nearby reef has prevented deposition in quite the classical manner for such a position. On the leeward side of the groyne, inside the harbour entrance, small tombolos have formed. At whichever angle the wave front meets the general coast from time to time, it is diffracted into a constant direction as it enters the harbour so that on the wave-absorption beaches just inside, there are arcs of sediment ridges which the wave front always meets squarely. The sediment washes straight up the ridge and back.
The ebb tide is here, scouring the entrance and the silt carried by the harbour water is carried northwards. Close inshore there is an element of longshore current but most of this transport is being done by the tidal ebb stream which follows the coast northwards from three hours after high water to five hours before the next high water. (chart)
Destructive process
In storm conditions particularly, the process can cause significant erosion to beaches (an example being Barton on Sea on the English coast) and therefore is a threat to coastal communities that rely on their beaches for protection from more serious erosion. Barton on Sea is a coastal Village situated in Hampshire, England. The need is most obvious where there are permanent structures on the shore. The loss of first the beach, then of old, glacially deposited, sediment to longshore drift resulted in the loss of the English town of Dunwich in Suffolk to the sea. England is a Country which is part of the United Kingdom. Its inhabitants account for more than 83% of the total UK population whilst its mainland Dunwich (ˈdʌnɨtʃ is a small town in Suffolk, England, within the Suffolk Coast and Heaths AONB. Suffolk (ˈsʌfək is a non-metropolitan county of historic origin in East Anglia, England. The problem causes coastal towns in erosional longshore drift areas to employ coastal defences such as groynes, which interrupt the downstream movement of sediment. Erosion is the carrying away or displacement of solids ( Sediment, Soil, rock and other particles usually by the agents of currents such as wind A groyne ( groin in the United States is a rigid Hydraulic structure built out from the shore (in Coastal engineering) or from the bank (in rivers and Jetties and groynes generally do not stop longshore drift, but interfere with it to the extent of retaining some of the beach material which might otherwise be removed. When a groyne is constructed, once equilibrium has been regained, the longshore drift resumes but, if the groyne has been well-sited, some of the beach material is retained on its upstream side. This is useful in dissipating the energy in incoming waves as the retained beach breaks some of the force. The part of the shore in the “shadow” of the groyne, downstream from it, is sheltered by having the waves broken by the structure.
See also
- Beach evolution
- Beach nourishment
- thermoluminescence
References
- Imray, Laurie, Norie and Wilson (publishers). The Shoreline is where the land meets the sea and it is continually changing Some mineral substances such as Fluorite store energy when exposed to Ultraviolet or other Ionising radiation. Chart C29, East Coast of England. Orfordness to Blyth (1978)
External links
- Photos, animation and explanation for schools
- Intranet.lissjunior.hants.sch.uk has a brief animation on longshore drift.
- USGS — Coastal Erosion on Cape Cod
- This link calls the process “shore drift”
- This link talks about long shore drift in South Carolina
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