Longshore drift from longshore current is a geological process that consists of the transportation of

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...

s (clay, silt, pebbles, sand, shingle) along a coast parallel to the shoreline, which is dependent on the angle incoming wave direction. Oblique incoming wind squeezes water along the coast, and so generates a water current which moves parallel to the coast. Longshore drift is simply the sediment moved by the longshore current. This current and sediment movement occur within the surf zone. The process is also known as littoral drift. 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 beach (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 A shingle beach (also referred to as rocky beach or pebble beach) is a beach which is armoured with pebbles or small- to medium-sized cobbles (as opposed to fine sand). Typically, the stone composition may grade from characteristic sizes ranging ...

). Sand is largely affected by the oscillatory force of breaking

waves Waves most often refers to: *Waves, oscillations accompanied by a transfer of energy that travel through space or mass. * Wind waves, surface waves that occur on the free surface of bodies of water. Waves may also refer to: Music * Waves (ban ...

, the motion of sediment due to the impact of breaking waves and bed shear from long-shore current.Reeve et al., 2004 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 an extended

surf zone As ocean surface waves approach shore, they get taller and break, forming the foamy, bubbly surface called ''surf''. The region of breaking waves defines the surf zone, or breaker zone. After breaking in the surf zone, the waves (now reduced i ...

Overview

Longshore drift formulas

There are numerous calculations that 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) 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 and

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

transport *Waves e.g. breaking and non-breaking *The

shear Shear may refer to: Textile production *Animal shearing, the collection of wool from various species **Sheep shearing *The removal of nap during wool cloth production Science and technology Engineering *Shear strength (soil), the shear strength ...

exerted by waves or the flow associated with waves.

Features of shoreline change

Longshore drift plays a large role in the evolution of a

shoreline A shore or a 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 a ...

, as if there is a slight change of sediment supply,

wind direction Wind direction is generally reported by the direction from which it originates. For example, a ''north'' or ''northerly'' wind blows from the north to the south. The exceptions are onshore winds (blowing onto the shore from the water) and offsho ...

, 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.

The sediment budget

The

sediment budget Sedimentary budgets are a coastal management tool used to analyze and describe the different sediment inputs (sources) and outputs (sinks) on the coasts, which is used to predict morphological change in any particular coastline over time. Within a ...

takes into consideration sediment sources and sinks within a

system A system is a group of Interaction, interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment (systems), environment, is described by its boundaries, ...

.Brunn, 2005 This sediment can come from any source with examples of sources and sinks consisting of: *

River A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...

s *

Lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') a ...

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 An inlet is a (usually long and narrow) indentation of a shoreline, such as a small arm, bay, sound, fjord, lagoon or marsh, that leads to an enclosed larger body of water such as a lake, estuary, gulf or marginal sea. Overview In marine geogra ...

ebb-tidal shoals, which store sand that has been transported by long-shore transport.Brunn, 2005, Michel and Howa, 1997 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.Hart et al., 2008 As well as dominant drift direction, spits are affected by the strength of wave-driven

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

, wave

angle In Euclidean geometry, an angle is the figure formed by two Ray (geometry), rays, called the ''Side (plane geometry), sides'' of the angle, sharing a common endpoint, called the ''vertex (geometry), vertex'' of the angle. Angles formed by two ...

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 An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environment ...

or lagoon.Hart et al., 2008, Petersen et al., 2008 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 The Waimakariri River is one of the largest rivers in Canterbury, on the eastern coast of New Zealand's South Island. It flows for 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 alternate 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.Kirk and Lauder, 2000 These barrier systems may enclose an estuary or lagoon system, like that of Lake Ellesmere enclosed by the Kaitorete Spit or hapua which form at river-coast interface such as at the mouth of the

Rakaia River The Rakaia River is in the Canterbury Plains in New Zealand's South Island. The Rakaia River is one of the largest braided rivers in New Zealand. The Rakaia River has a mean flow of and a mean annual seven-day low flow of . In the 1850s, Euro ...

. The Kaitorete Spit 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 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. The South Island's largest cit ...

for the last 8,000 years.Soons et al., 1997 This system has undergone numerous changes and fluctuations due to avulsion 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 A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...

and ebb shoals. Ebb-deltas may become stunted on highly exposed shores and in smaller spaces, whereas flood

delta Delta commonly refers to: * Delta (letter) (Δ or δ), a letter of the Greek alphabet * River delta, at a river mouth * D ( NATO phonetic alphabet: "Delta") * Delta Air Lines, US * Delta variant of SARS-CoV-2 that causes COVID-19 Delta may also ...

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.Michel and Howa, 1997 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 Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...

, sediment rate and by passing mechanism.

Channel Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body of water. Australia * Channel Country, region of outback Austral ...

location variance and amount may also influence the impact of long shore drift on a tidal inlet as well. For example, the Arcachon lagoon 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 A groyne (in the U.S. groin) is a rigid hydraulic structure built perpendicularly from an ocean shore (in coastal engineering) or a river bank, interrupting water flow and limiting the movement of sediment. It is usually made out of wood, concre ...

s, detached

breakwater Breakwater may refer to: * Breakwater (structure), a structure for protecting a beach or harbour Places * Breakwater, Victoria, a suburb of Geelong, Victoria, Australia * Breakwater Island Breakwater Island () is a small island in the Palme ...

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 A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Ham ...

s and

harbour A harbor (American English), harbour (British English; see spelling differences), or haven is a sheltered body of water where ships, boats, and barges can be docked. The term ''harbor'' is often used interchangeably with ''port'', which is a ...

s).

Groynes

Groynes A groyne (in the U.S. groin) is a rigid hydraulic structure built perpendicularly from an ocean shore (in coastal engineering) or a river bank, interrupting water flow and limiting the movement of sediment. It is usually made out of wood, concre ...

are shore protection structures, placed at equal intervals along the coastline in order to stop coastal erosion and generally cross the

intertidal zone The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species o ...

. 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 A storm surge, storm flood, tidal surge, or storm tide is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low-pressure weather systems, such as cyclones. It is measured as the rise in water level above the n ...

s and further down the coast. 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.

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 Accretion may refer to: Science * Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity * Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucl ...

of sediments on the up-drift side of the

headland A headland, also known as a head, is a coastal landform, a point of land usually high and often with a sheer drop, that extends into a body of water. It is a type of promontory. A headland of considerable size often is called a cape.Whittow, John ...

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 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

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

lee side Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...

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, 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. As an example, the creation of a port in

Timaru, New Zealand Timaru (; mi, Te Tihi-o-Maru) is a port city in the southern Canterbury Region of New Zealand, located southwest of Christchurch and about northeast of Dunedin on the eastern Pacific coast of the South Island. The Timaru urban area is home to ...

in the late 19th century led to a significant change in the longshore drift along the

South Canterbury South Canterbury is the area of the Canterbury Region of the South Island of New Zealand bounded by the Rangitata River in the north and the Waitaki River (the border with the Otago Region) to the south. The Pacific Ocean and ridge of the Southe ...

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 is a brackish shallow coastal lagoon approximately north of Timaru, South Canterbury, New Zealand. The lagoon has drastically reduced in size since 1881 when it was approximately , now it is less than in area. It is enclosed by a ...

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

References

Citations

Books

* * *

Journal articles

* * * *

External links

Photos, animation and explanation for schools
geography-site.co.uk
Intranet.lissjunior.hants.sch.uk
has a brief animation on longshore drift.

woodshole.er.usgs.gov

ecy.wa.gov

cofc.edu {{DEFAULTSORT:Longshore Drift Coastal geography Physical oceanography Coastal erosion Oceanographical terminology