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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Longshore drift | 2/3 | https://en.wikipedia.org/wiki/Longshore_drift | reference | science, encyclopedia | 2026-05-05T07:35:20.233864+00:00 | kb-cron |
=== The sediment budget === The sediment budget takes into consideration sediment sources and sinks within a system. This sediment can come from any source with examples of sources and sinks consisting of:
Rivers Lagoons Eroding land sources Artificial sources e.g. nourishment Artificial sinks e.g. mining/extraction Offshore transport Deposition of sediment on shore Gullies through the land This sediment then enters the coastal system and is transported by longshore drift. A good example of the sediment budget and longshore drift working together in the coastal system is inlet ebb-tidal shoals, which store sand that has been transported by long-shore transport. As well as storing sand these systems may also transfer or by pass sand into other beach systems, therefore inlet ebb-tidal (shoal) systems provide good sources and sinks for the sediment budget. Sediment deposition throughout a shoreline profile conforms to the null point hypothesis; where gravitational and hydraulic forces determine the settling velocity of grains in a seaward fining sediment distribution. Long shore occurs in a 90 to 80 degree backwash so it would be presented as a right angle with the wave line.
== Natural features == This section consists of features of longshore drift that occur on a coast where long-shore drift occurs uninterrupted by man-made structures.
=== Spits ===
Spits are formed when longshore drift travels past a point (e.g. river mouth or re-entrant) where the dominant drift direction and shoreline do not veer in the same direction. As well as dominant drift direction, spits are affected by the strength of wave-driven current, wave angle and the height of incoming waves. Spits are landforms that have two important features, with the first feature being the region at the up-drift end or proximal end (Hart et al., 2008). The proximal end is constantly attached to land (unless breached) and may form a slight “barrier” between the sea and an estuary or lagoon (called peresyp in the Russian tradition of geomorphology). The second important spit feature is the down-drift end or distal end, which is detached from land and in some cases, may take a complex hook-shape or curve, due to the influence of varying wave directions. As an example, the New Brighton spit in Canterbury, New Zealand, was created by longshore drift of sediment from the Waimakariri River to the north. This spit system is currently in equilibrium but undergoes alternate phases of deposition and erosion.
=== Barriers ===
Barrier systems are attached to the land at both the proximal and distal ends and are generally widest at the down-drift end. These barrier systems may enclose an estuary or lagoon system, like that of Lake Ellesmere / Te Waihora enclosed by the Kaitorete Spit or hapua which form at river-coast interface such as at the mouth of the Rakaia River. The Kaitorete Spit in Canterbury, New Zealand, is a barrier/spit system (which generally falls under the definition of barrier, as both ends of the landform are attached to land, but has been named a spit) that has existed below Banks Peninsula for the last 8,000 years. This system has undergone numerous changes and fluctuations due to avulsion of the Waimakariri River (which now flows to the north of Banks Peninsula), erosion and phases of open marine conditions. The system underwent further changes c. 500 years Before Present, when longshore drift from the eastern end of the “spit” system created the barrier, which has been retained due to ongoing longshore transport.
=== Tidal inlets ===
The majority of tidal inlets on longshore drift shores accumulate sediment in flood and ebb shoals. Ebb-deltas may become stunted on highly exposed shores and in smaller spaces, whereas flood deltas are likely to increase in size when space is available in a bay or lagoon system. Tidal inlets can act as sinks and sources for large amounts of material, which therefore impacts on adjacent parts of the coastline. The structuring of tidal inlets is also important for longshore drift: if an inlet is unstructured, sediment may by-pass the inlet and form bars at the down-drift part of the coast. This may also depend on the inlet size, delta morphology, sediment rate and by-passing mechanism. Channel location variance and amount may also influence the impact of longshore drift on a tidal inlet. Arcachon lagoon in southwest France is an example of a tidal inlet system, which provides large sources and sinks for longshore drift sediments. The impact of longshore drift sediments on this inlet system is highly influenced by the variation in the number of lagoon entrances and the location of these entrances. Any change in these factors can cause severe down-drift erosion or down-drift accretion of large swash bars.
=== Sand Islands ===
Where longshore drift is interrupted by other natural features, sufficient sediment deposition can occur to form long-term land structures extending off the coast. The formation process is similar to that of a Barrier island. K'gari is the largest sand island in the world, located on Australia's east coast, and was formed from interrupted northerly longshore drift. Over extensive periods, drifting sediment can 'leak' into deeper water, where the wind and waves driving longshore drift are weaker. This allows extensive sediment deposits to be built up off-shore, which is gradually transferred back to the coast as the sea level falls in long-term glacial cycles.
== Human influences == This section consists of long-shore drift features that occur unnaturally and in some cases (e.g. groynes, detached breakwaters) have been constructed to enhance the effects of longshore drift on the coastline but in other cases have a negative impact on long-shore drift (ports and harbours).
=== Groynes ===