Editing Longshore drift

{{short description|Sediment moved by the longshore current}}
{{Use dmy dates|date=October 2024}}
[[File:Longshore i18n.png|thumb|Diagram demonstrating longshore drift:{{Ordered list|beach|sea|longshore current direction|incoming waves|swash|backwash}}]]
 
'''Longshore drift''' from '''longshore [[Current (fluid)|current]]''' is a geological process that consists of the transportation of [[sediment]]s (clay, silt, pebbles, sand, shingle, shells) along a coast parallel to the [[Coast|shoreline]], which is dependent on the angle of incoming wave direction.  Oblique incoming wind squeezes water along the coast, generating a water current that moves parallel to the coast.  Longshore drift is simply the sediment moved by the longshore current. This current and sediment movement occurs within the [[surf zone]]. The process is also known as '''littoral drift'''.<ref>Gomez-Pina G (2002) "Sand dune management problems and techniques, Spain", ''Journal of Coastal Research'', Iss '''36''': 325–332.</ref>

[[Beach sand]] is also moved on such oblique wind days, due to the swash and backwash of water on the beach. Breaking surf sends water up the coast (swash) at an oblique angle and gravity then drains the water straight downslope (backwash) perpendicular to the shoreline.  Thus beach sand can move downbeach in a sawtooth fashion many tens of meters (yards) per day.  This process is called "beach drift", but some workers regard it as simply part of "longshore drift" because of the overall movement of sand parallel to the coast.

Longshore drift affects numerous sediment sizes as it works in slightly different ways depending on the sediment (e.g. the difference in long-shore drift of sediments from a sandy beach to that of sediments from a [[shingle beach]]). Sand is largely affected by the oscillatory force of breaking [[water waves|waves]], the motion of sediment due to the impact of breaking waves and bed shear from long-shore current.<ref name="Reeve et al., 2004">Reeve et al., 2004</ref> Because shingle beaches are much steeper than sandy ones, plunging breakers are more likely to form, causing the majority of longshore transport to occur in the [[swash zone]], due to a lack of an extended surf zone.<ref name="Reeve et al., 2004"/>

==Development of longshore drift theories==
{{Unreferenced section|date=October 2024}}
The concept of longshore drift or transportation of sediment parallel to the shore by wave action has evolved considerably with time. Early observations related to sediment displacement can be traced back to coastal communities, but the formal scientific understanding of this started crystallizing in the 19th and early 20th centuries. While such early perceptions were imprecise, this evolution has encouraged a gradually more sophisticated understanding of the processes occurring at coastlines. Understanding of the coastline processes has continued to evolve through a succession of developments that began many years ago.

===Early observations===
Erosion of coasts and sediment transport was known in ancient times, mostly in those parts of the world where dramatic changes of shores take place. However, these early observations were largely anecdotal. Fishermen, sailors and locals would note that sand and gravel seemingly "moved" down the beaches; they didn't fully understand the mechanics, however. Because of the general scientific knowledge, this was an interesting but somewhat misunderstood phenomenon.

===19th century: first scientific studies===
The systematic investigation into the coast processes, including those responsible for longshore drift, began in the mid-1800s when scientists tried to explain the processes of sediment movement along coasts. Among the first of such theories were those proposed by a French engineer, Jean-Baptiste Fourier, and a British geologist, Robert Mallet. They studied wave action and sediment transport; however, at that time, the term "longshore drift" was not yet coined. Instead, the principal focus was to understand the processes of waves and their impact on the resuspension and movement of sand and pebbles. The subject was of primary importance because it helped to explain the morphological features of any coast. However, while much is covered, the complete significance of such mechanisms was yet to be fully realised.

===20th century: longshore drift defined===
In the early years of the 20th century, longshore drift became much more refined in its explanation through oceanographers and coastal engineers. They realized that the angle of wave approach to the coast is of paramount importance to sediment transport. This then led to the development in the concept of "longshore currents," which in turn transport sediment along the coast. These currents then became recognized as the main agent of longshore drift. An important concept which emerged during this generation was that of the "drift-aligned" beach. It explained how beaches get to form as a result of prevailing wind and wave directions and that on one side of the beach deposition takes place, while on the other side, erosion does. While the mechanics were becoming more apparent, the interrelationship of the forces in play still proved quite problematic for those trying to manage coasts.

==Overview==

===Longshore drift formulas===
Numerous calculations take into consideration the factors that produce longshore drift.
These formulations are:
#Bijker formula (1967, 1971)
#The Engelund and Hansen formula (1967)
#The Ackers and White formula (1973)
#The Bailard and Inman formula (1981)
#The Van Rijn formula (1984)
#The Watanabe formula (1992)<ref name="autogenerated1">Bijker, E.W., 1971. Longshore transport computation. J. Waterways Harbors Division 97, WW4, 687—701.</ref>

These formulas provide a different view of the processes that generate longshore drift. The most common factors taken into consideration in these formulas are:
*[[Suspension (chemistry)|Suspended]] and [[bed load]] transport
*Waves, e.g., breaking and non-breaking
*The [[Shear (fluid)|shear]] exerted by waves or the [[Fluid dynamics|flow]] associated with waves.<ref name="autogenerated1"/>

===Features of shoreline change===
Longshore drift plays a large role in the evolution of a [[shoreline]], as if there is a slight change of sediment supply, [[wind direction]], or any other coastal influence longshore drift can change dramatically, affecting the formation and evolution of a beach system or profile. These changes do not occur due to one factor within the coastal system, in fact there are numerous alterations that can occur within the coastal system that may affect the distribution and impact of longshore drift. 
Some of these are:

#	Geological changes, e.g. erosion, backshore changes and emergence of headlands.
#	Change in hydrodynamic forces, e.g. change in wave diffraction in headland and offshore bank environments.
#	Change to hydrodynamic influences, e.g. the influence of new tidal inlets and deltas on drift.
#	Alterations of the sediment budget, e.g. switch of shorelines from drift to swash alignment, exhaustion of sediment sources.
#	The intervention of humans, e.g. cliff protection, groynes, detached breakwaters.<ref name="Reeve et al., 2004"/>

===The sediment budget===
The [[sediment budget]] takes into consideration sediment sources and [[Endorheic basin|sinks]] within a [[system]].<ref name="Brunn, 2005">Brunn, 2005</ref> This sediment can come from any source with examples of sources and sinks consisting of: 
*	[[River]]s
*	[[Lagoon]]s
*	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.<ref name="Brunn, 2005, Michel and Howa, 1997">Brunn, 2005, Michel and Howa, 1997</ref> 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.<ref name="Brunn, 2005, Michel and Howa, 1997"/>

[[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===
[[File:Provincetown Spit Cape Cod.jpg|thumb|left|[[Provincetown]] Spit, at the northern end of [[Cape Cod]], was formed by longshore drift after the end of the last [[Ice age]].]]
[[Spit (landform)|Spit]]s 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.<ref name="Hart et al., 2008">Hart et al., 2008</ref> As well as dominant drift direction, spits are affected by the strength of wave-driven [[Ocean current|current]], wave [[angle]] and the height of incoming waves.<ref>IPetersen et al., 2008</ref>

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<ref name="Hart 2008">Hart et al., 2008, Petersen et al., 2008</ref> (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.<ref name="Hart 2008"/>

As an example, the [[New Brighton, New Zealand|New Brighton]] spit in Canterbury, New Zealand, was created by longshore drift of sediment from the [[Waimakariri River]] to the north.<ref name="Hart et al., 2008"/> This spit system is currently in equilibrium but undergoes alternate phases of deposition and erosion.

===Barriers===
[[File:Aerial view Kaitorete Spit.jpg|left|thumb|[[Kaitorete Spit]] in the [[Canterbury Region]] of [[New Zealand|New Zealand's]] southern island.]]
Barrier systems are attached to the land at both the proximal and distal ends and are generally widest at the down-drift end.<ref>Kirk and Lauder, 2000</ref> 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.<ref name="Soons et al., 1997">Soons et al., 1997</ref> This system has undergone numerous changes and fluctuations due to [[avulsion (river)|avulsion]] of the Waimakariri River (which now flows to the north of Banks Peninsula), erosion and phases of open marine conditions.<ref name="Soons et al., 1997"/> The system underwent further changes {{circa}}&nbsp;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.<ref name="Soons et al., 1997"/>

===Tidal inlets===
[[File:PassesBassin.JPG|left|thumb|[[Arcachon Bay]] in Southwest France.]]
The majority of tidal inlets on longshore drift shores accumulate sediment in [[flood tide|flood]] and ebb shoals.<ref name="Brunn, 2005"/> Ebb-deltas may become stunted on highly exposed shores and in smaller spaces, whereas flood [[River delta|delta]]s are likely to increase in size when space is available in a bay or lagoon system.<ref name="Brunn, 2005"/> Tidal inlets can act as sinks and sources for large amounts of material, which therefore impacts on adjacent parts of the coastline.<ref name="Michel and Howa, 1997">Michel and Howa, 1997</ref>

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.<ref name="Michel and Howa, 1997"/> This may also depend on the inlet size, delta [[river morphology|morphology]], sediment rate and by-passing mechanism.<ref name="Brunn, 2005"/> [[channel (geography)|Channel]] location variance and amount may also influence the impact of longshore drift on a tidal inlet.

[[Arcachon Bay|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.<ref name="Michel and Howa, 1997"/> Any change in these factors can cause severe down-drift erosion or down-drift accretion of large swash bars.<ref name="Michel and Howa, 1997"/>

=== Sand Islands ===
[[File:Fraser Island.png|thumb|[[K'gari]] on Australia's east coast is an example of sand islands formed from longshore drift.]]
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|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.<ref name=":0">{{Cite web |title=New research reveals how the world's largest sand island was formed |url=https://www.uwa.edu.au/news/article/2022/november/new-research-reveals-how-the-worlds-largest-sand-island-was-formed |access-date=2025-03-18 |website=www.uwa.edu.au |language=en}}</ref> 

Over extensive periods, drifting sediment can 'leak' into deeper water, where the wind and waves driving longshore drift are weaker.<ref name=":1">{{Cite web |last=Ellerton |first=Daniel |last2=Shulmeister |first2=James |date=2022-11-14 |title=At least 700,000 years ago, the world’s largest sand island emerged as the barrier that helped the Great Barrier Reef form |url=https://theconversation.com/at-least-700-000-years-ago-the-worlds-largest-sand-island-emerged-as-the-barrier-that-helped-the-great-barrier-reef-form-192014 |access-date=2025-03-18 |website=The Conversation |language=en-US}}</ref> 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 cycle|glacial cycles.]]<ref name=":0" /><ref name=":1" />

==Human influences==
This section consists of long-shore drift features that occur unnaturally and in some cases (e.g. [[groyne]]s, detached [[Breakwater (structure)|breakwater]]s) 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 ([[port]]s and [[harbour]]s).

===Groynes===
[[Image:Groynes, Swanage Bay - geograph.org.uk - 49755.jpg|thumb|Timber [[groyne]] from [[Swanage|Swanage Bay]], UK]]
[[Groynes]] are shore protection structures, placed at equal intervals along the coastline in order to stop coastal erosion and generally cross the [[intertidal zone]].<ref name="Reeve et al., 2004"/> Due to this, groyne structures are usually used on shores with low net and high annual longshore drift in order to retain the sediments lost in [[storm surge]]s and further down the coast.<ref name="Reeve et al., 2004"/>

There are numerous variations to groyne designs with the three most common designs consisting of:
# zig-zag groynes, which dissipate the destructive flows that form in wave-induced currents or in breaking waves.
# T-head groynes, which reduce wave height through wave diffraction.
# ‘Y’ head, a fish-tail groyne system.<ref name="Reeve et al., 2004"/>

===Artificial headlands===
Artificial headlands are also shore protection structures, which are created in order to provide a certain amount of protection to beaches or bays.<ref name="Reeve et al., 2004"/> Although the creation of headlands involves [[coastal management|accretion]] of sediments on the up-drift side of the [[headland]] and moderate erosion of the down-drift end of the headland, this is undertaken in order to design a stabilised system that allows material to accumulate in beaches further along the shore.<ref name="Reeve et al., 2004"/>

Artificial headlands can occur due to natural accumulation or also through artificial nourishment.

[[Image:Maumee Bay State Park aerial view.jpg |thumb|left|Picture showing the use of artificial headlands and detached [[Breakwater (structure)|breakwaters]] in a coastal system]]

===Detached breakwaters===
Detached breakwaters are shore protection structures, created to build up sandy material in order to accommodate [[Drawdown (hydrology)|drawdown]] in storm conditions.<ref name="Reeve et al., 2004"/> In order to accommodate drawdown in storm conditions detached breakwaters have no connection to the shoreline, which lets currents and sediment pass between the [[Breakwater (structure)|breakwater]] and the shore.<ref name="Reeve et al., 2004"/> This then forms a region of reduced wave energy, which encourages the deposition of sand on the [[lee side]] of the structure.<ref name="Reeve et al., 2004"/>

Detached breakwaters are generally used in the same way as groynes, to build up the volume of material between the coast and the breakwater structure in order to accommodate storm surges.<ref name="Reeve et al., 2004"/>

===Ports and harbours===
The creation of ports and harbours throughout the world can seriously impact on the natural course of longshore drift. Not only do ports and harbours pose a threat to longshore drift in the short term, they also pose a threat to shoreline evolution.<ref name="Reeve et al., 2004"/> The major influence, which the creation of a port or harbour can have on longshore drift, is the alteration of sedimentation patterns, which in turn may lead to accretion and/or erosion of a beach or coastal system.<ref name="Reeve et al., 2004"/>

As an example, the creation of a port in [[Timaru, New Zealand]] in the late 19th century led to a significant change in the longshore drift along the [[South Canterbury]] coastline.<ref name="Hart et al., 2008"/> Instead of longshore drift transporting sediment north up the coast towards the Waimataitai lagoon, the creation of the port blocked the drift of these (coarse) sediments and instead caused them to accrete to the south of the port at South beach in Timaru.<ref name="Hart et al., 2008"/> The accretion of this sediment to the south, therefore meant a lack of sediment being deposited on the coast near the Waimataitai lagoon (to the north of the port), which led to the loss of the barrier enclosing the lagoon in the 1930s and then shortly after, the loss of the lagoon itself.<ref name="Hart et al., 2008"/> As with the Waimataitai lagoon, the [[Washdyke Lagoon]], which currently lies to the north of the Timaru port, is undergoing erosion and may eventually breach, causing loss of another lagoon environment.

== See also ==
* [[Beach evolution]]
* [[Beach#Erosion and accretion|Beach erosion and accretion]]
* [[Coastal management]], to prevent coastal erosion and creation of beach
* [[Coastal erosion]]
* [[Coastal geography]]
* [[Sand dune stabilization]]

== References ==

=== Citations ===
{{Reflist|2}}

=== Books ===
* {{cite book |editor-last=Bruun |editor-first=Per  |title= Port and coastal engineering developments in Science and technology |year= 2005 |publisher=P. Bruun |location=South Carolina}}
* {{cite book |last1=Hart |first1=D.E |last2= Marsden |first2= I |last3=Francis |first3=M |year=2008 |contribution=Chapter 20: Coastal systems |editor-last=Winterbourne |editor-first=M |editor2-last=Knox |editor2-first=G.A. |editor3-last=Marsden |editor3-first=I.D. |editor4-last=Burrows |editor-first4=C |title= Natural history of Canterbury |edition=3rd |publisher= Canterbury University Press |pages=653–684 }}
* {{cite book |last1= Reeve |first1=D |last2= Chadwick |first2=A |last3=Fleming |first3=C |title= Coastal engineering-processes, theory and design practice |year=2004 |publisher=Spon Press |location= New York}}

=== Journal articles ===
* {{cite journal |last1=Kirk |first1=R.M |last2=Lauder |first2=G.A |year=2000 |title= Significant coastal lagoon systems in the South Island, New Zealand |journal= Science for Conservation |publisher=DOC 46p |pages=13–24}}
* {{cite journal |last1=Michel |first1=D |last2=Howa |first2=H.L |year=1997 |title= Morphodynamic behaviour of a tidal inlet system in a mixed-energy environment |journal= Physics and Chemistry of the Earth |volume=22 |issue=3–4 |pages=339–343 |doi=10.1016/s0079-1946(97)00155-9|bibcode=1997PCE....22..339M }}
* {{cite journal |last1=Peterson |first1=D |last2=Deigaard |first2=R |last3=Fredsoe |first3=J |date=July 2008 |title= Modelling the morphology of sandy spits |journal=Coastal Engineering |volume=55 |issue=7–8 |pages=671–684 |doi= 10.1016/j.coastaleng.2007.11.009|bibcode=2008CoasE..55..671P |url=http://orbit.dtu.dk/en/publications/modelling-the-morphology-of-sandy-spits(1064c739-9d6a-4576-82a2-2ba3fb90221a).html |url-access=subscription }}
* {{cite journal |last1= Soons |first1= J.M |last2= Schulmeister |first2= J |last3= Holt |first3= S |date=April 1997 |title= The Holocene evolution of a well nourished gravelly barrier and lagoon complex, Kaitorete "Spit", Canterbury, New Zealand |journal= Marine Geology |volume= 26 |pages= 69–90 |doi= 10.1016/S0025-3227(97)00003-0 |issue=1–2|bibcode= 1997MGeol.138...69S }}

==External links==
* [http://www.geography-site.co.uk/pages/physical/coastal/longshore.html Photos, animation and explanation for schools], geography-site.co.uk
* [https://web.archive.org/web/20060425183719/http://intranet.lissjunior.hants.sch.uk/water/picsweb_ks2geography/flash/g2reswa0015.swf Intranet.lissjunior.hants.sch.uk] has a brief animation on longshore drift.
* [https://woodshole.er.usgs.gov/staffpages/boldale/capecod/quest.html USGS — Coastal Erosion on Cape Cod], woodshole.er.usgs.gov
* [https://web.archive.org/web/20060222011618/http://www.ecy.wa.gov/programs/sea/pugetsound/bluffs/drift.html Shore drift], ecy.wa.gov
* [https://web.archive.org/web/20081205004337/http://www.cofc.edu/CGOInquiry/longshoredrift.htm Longshore drift in South Carolina], cofc.edu
* [https://earthwise.bgs.ac.uk/index.php/Category:Portable_streamer_traps_for_longshore_sediment_transport_measurement British Geological Survey: portable streamer traps for longshore sediment transport measurement]

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