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.
Contents |
Mechanism
Longshore drift is the movement of eroded material, in a shark fin shape way, along the coast line. The process arises when 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. There is also erosion on the beach. 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. 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.
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. 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 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. 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.
See also
References
- Imray, Laurie, Norie and Wilson (publishers). Chart C29, East Coast of England. Orfordness to Blyth (1978)
