Longshore drift
Overview
Longshore drift consists of the transportation of sediments (generally sand but may also consist of coarser sediments such as gravels) along a coast at an angle to the shoreline, which is dependent on prevailing wind direction, swash
Swash
Swash, in geography, is a turbulent layer of water that washes up on the beach after an incoming wave has broken. The swash action can move beach material up and down on the beach, which results in the cross-shore sediment exchange. The time-scale of swash motion varies from seconds to minutes...

 and backwash. This process occurs in the littoral zone, and in or within close proximity to the surf zone
Surf zone
As ocean surface waves come closer to shore they break, forming the foamy, bubbly surface we call surf. The region of breaking waves defines the surf zone. After breaking in the surf zone, the waves continue to move in, and they run up onto the sloping front of the beach, forming an uprush of...

. The process is also known as longshore transport or littoral drift.

Longshore drift is influenced by numerous aspects of the coastal system, with processes that occur within the surf zone largely influencing the deposition
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

 and erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 of sediments.
Encyclopedia
Longshore drift consists of the transportation of sediments (generally sand but may also consist of coarser sediments such as gravels) along a coast at an angle to the shoreline, which is dependent on prevailing wind direction, swash
Swash
Swash, in geography, is a turbulent layer of water that washes up on the beach after an incoming wave has broken. The swash action can move beach material up and down on the beach, which results in the cross-shore sediment exchange. The time-scale of swash motion varies from seconds to minutes...

 and backwash. This process occurs in the littoral zone, and in or within close proximity to the surf zone
Surf zone
As ocean surface waves come closer to shore they break, forming the foamy, bubbly surface we call surf. The region of breaking waves defines the surf zone. After breaking in the surf zone, the waves continue to move in, and they run up onto the sloping front of the beach, forming an uprush of...

. The process is also known as longshore transport or littoral drift.

Longshore drift is influenced by numerous aspects of the coastal system, with processes that occur within the surf zone largely influencing the deposition
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

 and erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 of sediments. Longshore currents can generate oblique breaking waves which result in longshore transport.

Longshore drift can generally be defined in terms of the systems within the surf zone as seen in figure 1. This figure shows that sediment transport along the shore
Shore
A shore or shoreline is the fringe of land at the edge of a large body of water, such as an ocean, sea, or lake. In Physical Oceanography a shore is the wider fringe that is geologically modified by the action of the body of water past and present, while the beach is at the edge of the shore,...

 and surf zone is influenced by the swash (occurs in the direction of prevailing wind), which moves the pebble up the beach at the angle of wind direction and also backwash, which moves the pebble back down the beach due to the influence of gravity.

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
Shingle beach
A shingle beach is a beach which is armoured with pebbles or small- to medium-sized cobbles. Typically, the stone composition may grade from characteristic sizes ranging from two to 200 mm diameter....

). Sand is largely affected by the oscillatory force of breaking waves
WAVES
The WAVES were a World War II-era division of the U.S. Navy that consisted entirely of women. The name of this group is an acronym for "Women Accepted for Volunteer Emergency Service" ; the word "emergency" implied that the acceptance of women was due to the unusual circumstances of the war and...

, the motion of sediment due to the impact of breaking waves and bed shear from long shore current. Whereas because shingle beaches are much steeper than sandy ones, plunging breakers are more likely to form, causing the majority of long shore transport to occur in the swash zone, due to a lack of surf zone.

Longshore drift formulas

There are numerous calculations that take into consideration the factors that produce longshore drift.
These formulations are:
  1. Bijker formula (1967,1971)
  2. The Engelund and Hansen formula (1967)
  3. The Ackers and White formula (1973)
  4. The Bailard and Inman formula(1981)
  5. The Van Rijn formula (1984)
  6. The Watanabe formula (1992)


These formulas all provide a different view into the processes that generate longshore drift. The most common factors taken into consideration in these formulas are:
  • Suspended
    Suspended
    Suspended: A Cryogenic Nightmare is an interactive fiction computer game written by Michael Berlyn and published by Infocom in 1983. Like most Infocom titles, it was available on most popular personal computers of the day, such as the Apple II, PC, Atari ST and Commodore 64...

     and bed load
    Bed load
    The term bed load or bedload describes particles in a flowing fluid that are transported along the bed. Bed load is complementary to suspended load and wash load.Bed load moves by rolling, sliding, and/or saltating ....

     transport
  • Waves e.g. breaking and non-breaking
  • The shear exerted by waves or the flow
    Fluid dynamics
    In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...

     associated with waves.

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
Wind direction
Wind direction is reported by the direction from which it originates. For example, a northerly wind blows from the north to the south. Wind direction is usually reported in cardinal directions or in azimuth degrees...

, or any other coastal influence longshore drift can change dramatically, impacting on the formation and evolution of a beach system or profile
Profile
- Computing and technology :* Profile , a concept in Unified Modeling Language* Apple ProFile, a hard drive* User profile refers to the computer representation of user information...

. 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:
  1. Geological changes, e.g. erosion, backshore changes and emergence of headlands.
  2. Change in hydrodynamic forces, e.g. change in wave diffraction in headland and offshore bank environments.
  3. Change to hydrodynamic influences, e.g. the influence of new tidal inlets and deltas on drift.
  4. Alterations of the sediment budget, e.g. switch of shorelines from drift to swash alignment, exhaustion of sediment sources.
  5. The intervention of humans, e.g. cliff protection, groynes, detached breakwaters.

The sediment budget

The sediment budget takes into consideration sediment sources and sinks within a system
System
System is a set of interacting or interdependent components forming an integrated whole....

. This sediment can come from any source with examples of sources and sinks consisting of:
  • River
    River
    A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

    s
  • Lagoon
    Lagoon
    A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...

    s
  • Eroding land sources
  • Artificial sources e.g. nourishment
  • Artificial sinks e.g. mining/extraction
  • Offshore transport
  • Deposition of sediment on shore


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
Inlet
An inlet is a narrow body of water between islands or leading inland from a larger body of water, often leading to an enclosed body of water, such as a sound, bay, lagoon or marsh. In sea coasts an inlet usually refers to the actual connection between a bay and the ocean and is often called an...

 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
Shoal
Shoal, shoals or shoaling may mean:* Shoal, a sandbank or reef creating shallow water, especially where it forms a hazard to shipping* Shoal draught , of a boat with shallow draught which can pass over some shoals: see Draft...

 systems provide a good sources and sinks for the sediment budget.

Natural features

This section consists of features of long shore drift that occur on a coast where long shore drift occurs uninterrupted by man-made structures.

Spits

Spit
Spit (landform)
A spit or sandspit is a deposition landform found off coasts. At one end, spits connect to land, and extend into the sea. A spit is a type of bar or beach that develops where a re-entrant occurs, such as at cove's headlands, by the process of longshore drift...

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. As well as dominant drift direction, spits are affected by the strength of wave driven current
Ocean current
An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...

, wave angle
Angle
In geometry, an angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle.Angles are usually presumed to be in a Euclidean plane with the circle taken for standard with regard to direction. In fact, an angle is frequently viewed as a measure of an circular arc...

 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
Barrier
A barrier or barricade is a physical structure which blocks or impedes something.Barrier may also refer to:-Physical barriers:* Automatic full barriers, which serve to block roads at railway crossings...

” between the sea and an estuary
Estuary
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....

 or lagoon. 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
New Brighton, New Zealand
New Brighton is a coastal suburb of Christchurch, New Zealand, about to the east of the city centre.-Naming:The naming of New Brighton was apparently done on a 'spur of moment' decision by William Fee, an early settler of the area...

 spit in Canterbury, New Zealand, was created by longshore drift of sediment from the Waimakariri River
Waimakariri River
The Waimakariri River is the largest of the North Canterbury rivers, in the South Island of New Zealand. It flows for 151 kilometres in a generally southeastward direction from the Southern Alps across the Canterbury Plains to the Pacific Ocean....

 to the north. This spit system is currently in equilibrium but undergoes phases of deposition and erosion.

Barriers

Barrier systems are attached to the land at both the proximal and distal end and are generally widest at the down-drift end. These barrier systems may enclose an estuary or lagoon system, like that of Lake Ellesmere
Lake Ellesmere
Lake Ellesmere / Te Waihora is located in the Canterbury Region of the South Island of New Zealand. It is actually a broad, shallow lagoon located directly to the west of Banks Peninsula, separated from the Pacific Ocean by a long narrow sandy spit called Kaitorete Spit, or more correctly Kaitorete...

 enclosed by the Kaitorete Spit
Kaitorete Spit
Kaitorete Spit is a long finger of land which extends along the coast of Canterbury in the South Island of New Zealand. it runs southwest from Banks Peninsula for 25 kilometres, and separates the shallow Lake Ellesmere from the Pacific Ocean...

.

The Kaitorete Spit
Kaitorete Spit
Kaitorete Spit is a long finger of land which extends along the coast of Canterbury in the South Island of New Zealand. it runs southwest from Banks Peninsula for 25 kilometres, and separates the shallow Lake Ellesmere from the Pacific Ocean...

 in Canterbury, New Zealand, is a barrier/spit system (which generally falls under the definition barrier, as both ends of the landform are attached to land, but has been named a spit) that has existed below Banks Peninsula
Banks Peninsula
Banks Peninsula is a peninsula of volcanic origin on the east coast of the South Island of New Zealand. It has an area of approximately and encompasses two large harbours and many smaller bays and coves...

 for the last 8000 years. This system has undergone numerous changes and fluctuations due to avulsion
Avulsion (river)
In sedimentary geology and fluvial geomorphology, avulsion is the rapid abandonment of a river channel and the formation of a new river channel. Avulsions occur as a result of channel slopes that are much lower than the slope that the river could travel if it took a new course.-Deltaic and...

 of the Waimakariri River (which now flows to the north or Banks Peninsula), erosion and phases of open marine conditions. The system underwent further changes c.500 year BP, 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
Flood
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...

 and ebb shoals. Ebb-deltas may become stunted on highly exposed shores and in smaller spaces, whereas flood delta
River delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...

s 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 as if an inlet is unstructured sediment may by pass the inlet and form bars at the down-drift part of the coast. Although this may also depend on the inlet size, delta morphology
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

, sediment rate and by passing mechanism. Channel
Geography
Geography is the science that studies the lands, features, inhabitants, and phenomena of Earth. A literal translation would be "to describe or write about the Earth". The first person to use the word "geography" was Eratosthenes...

 location variance and amount may also influence the impact of long shore drift on a tidal inlet as well.

For example, the Arcachon lagoon
Arcachon Bay
Arcachon Bay is a bay of the Atlantic Ocean on the southwest coast of France, situated between the Côte d'Argent and the Côte des Landes, in the region of Aquitaine. The bay covers an area of 150 km² at high tide and 40 km² at low tide...

 is a tidal inlet system in South west France, 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.

Human influences

This section consists of long shore drift features that occur unnaturally and in some cases (e.g. groyne
Groyne
A groyne is a rigid hydraulic structure built from an ocean shore or from a bank that interrupts water flow and limits the movement of sediment. In the ocean, groynes create beaches, or avoid having them washed away by longshore drift. In a river, groynes prevent erosion and ice-jamming, which...

s, detached breakwater
Structure
Structure is a fundamental, tangible or intangible notion referring to the recognition, observation, nature, and permanence of patterns and relationships of entities. This notion may itself be an object, such as a built structure, or an attribute, such as the structure of society...

s) have be constructed to enhance the effects of longshore drift on the coastline, but in other cases have a negative impact on long shore drift (port
Port
A port is a location on a coast or shore containing one or more harbors where ships can dock and transfer people or cargo to or from land....

s and harbours).

Groynes

Groynes are shore protection structures, placed at equal intervals along the coastline in order to stop coastal erosion and generally cross the intertidal zone
Intertidal zone
The intertidal zone is the area that is above water at low tide and under water at high tide . This area can include many different types of habitats, with many types of animals like starfish, sea urchins, and some species of coral...

. 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
Storm surge
A storm surge is an offshore rise of water associated with a low pressure weather system, typically tropical cyclones and strong extratropical cyclones. Storm surges are caused primarily by high winds pushing on the ocean's surface. The wind causes the water to pile up higher than the ordinary sea...

s and further down the coast.

There are numerous variations to groyne designs with the three most common designs consisting of:
  1. zig-zag groynes, which dissipate the destructive flows that form in wave induced currents or in breaking waves.
  2. T-head groynes, which reduce wave height through wave diffraction.
  3. ‘Y’ head, a fish tail groyne system.

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. Although the creation of headlands involves accretion
Coastal management
In some jurisdictions the terms sea defense and coastal protection are used to mean, respectively, defense against flooding and erosion...

 of sediments on the up-drift side of the headland
Headland
A headland is a point of land, usually high and often with a sheer drop, that extends out into a body of water.Headland can also refer to:*Headlands and bays*headLand, an Australian television series...

 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.

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

Detached breakwaters

Detached breakwaters are shore protection structures, created to build up sandy material in order to accommodate drawdown
Drawdown (hydrology)
In water-related science and engineering there are two similar but distinct definitions in use for drawdown.*In subsurface hydrogeology, drawdown is the change in hydraulic head observed at a well in an aquifer, typically due to pumping a well as part of an aquifer test or well test.*In surface...

 in storm conditions. 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
Structure is a fundamental, tangible or intangible notion referring to the recognition, observation, nature, and permanence of patterns and relationships of entities. This notion may itself be an object, such as a built structure, or an attribute, such as the structure of society...

 and the shore. This then forms a region of reduced wave energy, which encourages the deposition of sand on the lee side of the structure.

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.

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. The major influence 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.

As an example, the creation of a port in Timaru, New Zealand in the late 1800s led to a significant change in the longshore drift along the South Canterbury coastline. 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. 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. As with the Waimataitai lagoon the Washdyke Lagoon
Washdyke Lagoon
Washdyke Lagoon is a brackish shallow coastal lagoon approximately 1 kilometre north of Timaru, South Canterbury, New Zealand. The lagoon has drastically reduced in size since 1881 when it was approximately 253 hectares, now it is less than 48 hectares in area...

, which currently lies to the north of the Timaru port is undergoing erosion and may eventually breach causing loss of another lagoon environment.

External links

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