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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. Image File history File links Question_book-3. ... Diagram of longshore drift. ... Diagram of longshore drift. ... This article includes a list of works cited but its sources remain unclear because it lacks in-text citations. ... This article or section cites very few or no references or sources. ... For other uses, see Beach (disambiguation). ...

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. 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. 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. 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. 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. Surface waves in water This article is about waves in the most general scientific sense. ... Fetch is a term for the length of water over which a given wind has blown. ... For other uses, see Wind (disambiguation). ... This page is a candidate to be copied to Wiktionary. ... For other uses, see Beach (disambiguation). ... For other uses, see Sand (disambiguation). ... Look up shingle in Wiktionary, the free dictionary. ...

The water molecules are among the particles moved in the process so there is a net movement of water as well. This establishes a longshore current. 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ... A current is a movement or flow of fluids, especially water in a river or ocean. ...

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. 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. In geography, a bay or gulf is a collection of water that is surrounded by land on three sides. ... For other meanings, see Estuary (disambiguation) Río de la Plata estuary 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 sea. ... Sea wave polishing pebbles into rounded corners Pebbles For other uses, see Pebble (disambiguation). ... Gravel (largest fragment in this photo is about 4 cm) Gravel is rock that is of a certain particle size range. ... For other uses, see Sand (disambiguation). ... This article is about a type of online computer game. ...

Constructive process

Coastal accretion

Longshore drift is one of the principal processes in the construction of spits, bars and tombolos, and in the overall sustainability of beach deposits. 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. Image File history File links Accreting_coast_Image6. ... Image File history File links Accreting_coast_Image6. ... A spit is a deposition landform found off coasts. ... Sand bars in the Mississippi River at Arkansas and Mississippi A bar is a linear shoaling landform feature within a body of water. ... Tombolo at Stockton Island, Ashland County (Wisconsin). ...

This link shows incident waves refracting into the entrance to the River Tees and various beach forms produced in a variety of circumstances. 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 Tees is a river in Northern England. ...

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. 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. The problem causes coastal towns in erosional longshore drift areas to employ coastal defences such as groynes, which interrupt the downstream movement of sediment. 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. Map sources for New Milton at grid reference SZ235955 New Milton is a market town in south west Hampshire, England. ... For other uses, see England (disambiguation). ... Dunwich (IPA: ) is a small town in the county of Suffolk in England. ... Suffolk (pronounced ) is a large historic and modern non-metropolitan county in East Anglia, England. ... For morphological image processing operations, see Erosion (morphology). ... A groyne on the East coast of England A groyne (groin in the United States) is a method of coastal defense against erosion. ... Alternate meanings: See Jetty (web server) Alternate meanings: See Jettying in buildings The term jetty, derived from the French jetie, and therefor signifying something thrown out, is applied to a variety of structures employed in river, dock and maritime works which are generally carried out in pairs from river banks...

See also

• Beach evolution
• Beach nourishment

This article or section does not adequately cite its references or sources. ... Before and after photos of beach restoration efforts, Florida coastline, USA. Beach nourishment device Beach nourishment is a complimentary term that describes a process by which sediment (usually sand) lost through longshore drift or erosion is replaced on a beach. ...

References

• Imray, Laurie, Norie and Wilson (publishers). 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