Longshore drift from longshore current is a geological process that consists of the transportation of sediments (clay, silt, pebbles, sand and shingle) along a coast parallel to the shoreline, which is dependent on oblique incoming wind 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.
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 zig zag 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 waves, the motion of sediment due to the impact of breaking waves and bed shear from long-shore current. 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.
There are numerous calculations that take into consideration the factors that produce longshore drift. These formulations are:
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:
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:
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 a 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.
This section consists of features of longshore drift that occur on a coast where long-shore drift occurs uninterrupted by man-made structures.
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. 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.
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 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 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 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.
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 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, sediment rate and by passing mechanism. Channel 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.
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 are shore protection structures, placed at equal intervals along the coastline in order to stop coastal erosion and generally cross the intertidal zone. 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 surges and further down the coast.
There are numerous variations to groyne designs with the three most common designs consisting of:
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 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.
Artificial headlands can occur due to natural accumulation or also through artificial nourishment.
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 breakwater 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.
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 in the late 19th century 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, which currently lies to the north of the Timaru port, is undergoing erosion and may eventually breach, causing loss of another lagoon environment.
The Afon Dwyfor is a river in Gwynedd, north-west Wales, in total the river is 12 1⁄2 miles (20.1 km) in length. It rises in Cwm Dwyfor at the head of Cwm Pennant, gathers to itself numerous streams which drain the surrounding mountains from Mynydd Graig Goch in the west to Moel Hebog in the east, then flows southwest towards Dolbenmaen and out of the Snowdonia National Park.After a brief diversion west, it turns south, then southwest again, heading for the village of Llanystumdwy. Beyond Llanystumdwy it heads for the coast and Tremadog Bay. Its mouth has been diverted eastwards by almost one mile by the Pen-y-chain shingle spit resulting from longshore drift.Its principal tributaries are the Afon Henwy which enters on its left bank above Dolbenmaen, and the Afon Dwyfach which joins it as a right-bank tributary to the west of Llanystumdwy. The Dwyfach itself rises in an area of flat ground to the west of the A487 road between Bryncir and Llanllyfni and flows in a generally southerly direction.'Afon Dwyfor' signifies the 'big holy river' in Welsh whilst the 'Afon Dwyfach' is the 'little holy river'.The river is bridged by numerous minor roads and paths but also by the A487, B4411 and A497 roads as well as the railway line between Criccieth and Pwllheli. At Dolbenmaen it is believed the Roman road to Segontium forded the river. A motte-and-bailey castle, once the residence of Llywelyn the Great, guarded the ford during the Middle Ages.Baymouth bar
A baymouth bar is a depositional feature as a result of longshore drift. It is a sandbank that partially or completely closes access to a bay.These bars usually consist of accumulated gravel and sand carried by the current of longshore drift and deposited at a less turbulent part of the current. Thus, they most commonly occur across artificial bay and river entrances due to the loss of kinetic energy in the current after wave refraction.Breakwater (structure)
Breakwaters are structures constructed near the coasts as part of coastal management or to protect an anchorage from the effects of both weather and longshore drift.Coastal geography
Coastal geography is the study of the constantly changing region between the ocean and the land, incorporating both the physical geography (i.e. coastal geomorphology, geology and oceanography) and the human geography (sociology and history) of the coast. It includes understanding coastal weathering processes, particularly wave action, sediment movement and weather, and the ways in which humans interact with the coastCuspate foreland
Cuspate forelands, also known as cuspate barriers or nesses in Britain, are geographical features found on coastlines and lakeshores that are created primarily by longshore drift. Formed by accretion and progradation of sand and shingle, they extend outwards from the shoreline in a triangular shape. Some cuspate forelands may be stabilised by vegetation, while others may migrate down the shoreline. Because some cuspate forelands provide an important habitat for many flora and fauna, effective management is required to reduce the impacts from both human activities and physical factors such as climate change and sea level rise.Fetch (geography)
The fetch, also called the fetch length, is the length of water over which a given wind has blown. Fetch is used in geography and meteorology and its effects are usually associated with sea state and when it reaches shore it is the main factor that creates storm surge which leads to coastal erosion and flooding. It also plays a large part in longshore drift as well.
Fetch length, along with the wind speed (wind strength), determines the size (sea state) of waves produced. The wind direction is considered constant. The longer the fetch and the faster the wind speed, the more wind energy is imparted to the water surface and the larger the resulting sea state will be.Fire Island Inlet
Fire Island Inlet is an inlet on the south shore of Long Island, New York, USA.
It connects the Great South Bay with the Atlantic Ocean, passing between Robert Moses State Park (the western end of Fire Island) on the south and Oak Beach and Captree State Park (the eastern end of Jones Beach Island) on the north. The inlet is directly south of West Islip, the nearest town on the main part of Long Island.
Fire Island Inlet has evolved over the years due to natural processes, especially longshore drift. Jones Island and Fire Island at one time were connected.The Fire Island Light was at the mouth of the inlet when built in 1825, but is now five miles (8.0 km) east of the inlet.The northwest side of the mouth of the inlet is known as the Sore Thumb, and is a man-made barrier that was created to try to combat the extensive erosion of the beaches inside the inlet. The southeast side of the mouth of the inlet is known as Democrat Point and is known as a popular surfing spot for beach goers, as well as a popular 4x4 fishing area. It is also the site of extensive preserves for the area's birds.Groyne
A groyne (in the U.S. groin) is a rigid hydraulic structure built from an ocean shore (in coastal engineering) or from a bank (in rivers) that interrupts water flow and limits the movement of sediment. It is usually made out of wood, concrete or stone. In the ocean, groynes create beaches or prevent them being washed away by longshore drift. In a river, groynes slow down the process of erosion and prevent ice-jamming, which in turn aids navigation. Ocean groynes run generally perpendicular to the shore, extending from the upper foreshore or beach into the water. All of a groyne may be under water, in which case it is a submerged groyne. The areas between groups of groynes are groyne fields. Groynes are generally placed in groups. They are often used in tandem with seawalls. Groynes, however, may cause a shoreline to be perceived as unnatural.
The term is derived from the Old French groign, from Late Latin grunium, "snout".Hard engineering
Hard engineering involves the construction of physical structures to protect coasts against the erosive power of waves.Lagoon
A lagoon is a shallow body of water separated from a larger body of water by barrier islands or reefs. Lagoons are commonly divided into coastal lagoons and atoll lagoons. They have also been identified as occurring on mixed-sand and gravel coastlines. There is an overlap between bodies of water classified as coastal lagoons and bodies of water classified as estuaries. Lagoons are common coastal features around many parts of the world.Morfa Harlech National Nature Reserve
Morfa Harlech National Nature Reserve (grid reference SH571337) is a nature reserve in Wales, located north of Harlech.
The reserve reaches across expanses of open sand and sea towards Snowdonia and contains one of the two extensive sand dune systems which make up much of the sandy Meirionnydd coastline, it carries particular importance as the only growing dune system in Wales.
Morfa Harlech sand dunes is an extensive dune system stretching northwards from the town of Harlech. The dunes cover 6 km², of which an area of about 1.5 km² in the middle has been afforested with Corsican Pine. Nearby is Harlech Castle, which due to the expanding dune system has been taken back 1,000 metres (3,300 ft) from its original position on the coastline 600 years ago. Morfa Harlech is one of Britain's actively growing sand dune systems due to the longshore drift which is currently eroding the dunes at Morfa Dyffryn.Napatree Point
Napatree Point in Rhode Island, often referred to simply as Napatree, is a long sandy spit created by a geologic process called longshore drift. Up until the Hurricane of 1938, Napatree was sickle-shaped and included a 1.5-mile (2.4 km) long northern extension called Sandy Point. Napatree now extends 1.5 miles (2.4 km) westward from the business district of Watch Hill, a village in Westerly, Rhode Island forming a protected harbor. It is the southernmost and westernmost point of mainland Rhode Island.River Annas
The River Annas is a minor river in Cumbria in northwest England. It is formed as the Kinmont Beck and Crookley Beck which drain the southwestern fells of the Lake District, meet on the eastern edge of the village of Bootle. Their combined waters flow southwest towards Annaside on the Irish Sea coast. However longshore drift has diverted the river northwestwards parallel to the shore for a further 1.2 miles (2 km) so that it enters the sea at Selker. This section of river is followed by the Cumbria Coastal Way. The river is bridged by the A595 road and the Cumbrian coast railway line.Sedimentary 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 coastal environment the rate of change of sediment is dependent on the amount of sediment brought into the system versus the amount of sediment that leaves the system. These inputs and outputs of sediment then equate to the total balance of the system and more than often reflect the amounts of erosion or accretion affecting the morphology of the coast.To assess the sedimentary budget the coast has to be divided into two separate morphologies, commonly known as littoral cells and compartments. Sediment compartments can usually be defined as two rocky barriers which mark the ends of a beach and have a fixed sediment budget, although usually leaky to some extent. Littoral cells can either be free or fixed and can occupy a hierarchy of scales, from individual rip cells to entire beaches.There are various types of natural sources and sinks within a coastal system. Sediment sources can include river transport, sea cliff erosion and longshore drift into an area. Sediment sinks can include longshore drift of sediment away from an area and sediment deposition into an estuary.
Anthropogenic activities can also influence sedimentary budgets; in particular damming of a river and in stream gravel mining of a river bed can reduce the sediment source to the coast. In contrast beach nourishment can increase sediment source.
In 1966, Bowen and Inman defined a littoral cell and separated sediment inputs, accretion by longshore drift and outputs.Sedimentary budgets are used to assist in the management of beach erosion by trying to show the present sediment movement and forecast future sediment movement.Spit (landform)
A spit or sandspit is a deposition bar or beach landform off coasts or lake shores. It develops in places where re-entrance occurs, such as at a cove's headlands, by the process of longshore drift by longshore currents. The drift occurs due to waves meeting the beach at an oblique angle, moving sediment down the beach in a zigzag pattern. This is complemented by longshore currents, which further transport sediment through the water alongside the beach. These currents are caused by the same waves that cause the drift.Swash
Swash, or forewash 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 materials up and down the beach, which results in the cross-shore sediment exchange. The time-scale of swash motion varies from seconds to minutes depending on the type of beach (see Figure 1 for beach types). Greater swash generally occurs on flatter beaches. The swash motion plays the primary role in the formation of morphological features and their changes in the swash zone. The swash action also plays an important role as one of the instantaneous processes in wider coastal morphodynamics.
There are two approaches that describe swash motions: (1) swash resulting from the collapse of high-frequency bores (f>0.05 Hz) on the beachface; and (2) swash characterised by standing, low-frequency (f<0.05 Hz) motions. Which type of swash motion prevails is dependent on the wave conditions and the beach morphology and this can be predicted by calculating the surf similarity parameter εb (Guza & Inman 1975):
Where Hb is the breaker height, g is gravity, T is the incident-wave period and tan β is the beach gradient. Values εb>20 indicate dissipative conditions where swash is characterised by standing long-wave motion. Values εb<2.5 indicate reflective conditions where swash is dominated by wave bores.Tetrapod (structure)
Tetrapods are a type of structure in coastal engineering used to prevent erosion caused by weather and longshore drift, primarily to enforce coastal structures such as seawalls and breakwaters. Tetrapods are made of concrete, and use a tetrahedral shape to dissipate the force of incoming waves by allowing water to flow around rather than against them, and to reduce displacement by interlocking.Tombolo
A tombolo, from the Italian tombolo, derived from the Latin tumulus, meaning 'mound', and sometimes translated as ayre, is a deposition landform in which an island is attached to the mainland by a narrow piece of land such as a spit or bar. Once attached, the island is then known as a tied island. A tombolo is a sandy isthmus.
Several islands tied together by bars which rise above the water level are called a tombolo cluster. Two or more tombolos may form an enclosure (called a lagoon) that can eventually fill with sediment.Training (civil)
Training or entrance training refers to coastal structures built to constrain a river discharging across a littoral coast so that it discharges only where desired. Untrained entrances on sandy coasts tend to move widely and violently to discharge into the ocean, often upsetting those enjoying land nearby. With many cities (and buildings) constructed close to rivers, such management has historically been considered a necessary course of action, even though ecologically, non-intervention would be better and more sustainable.A trained entrance often consists of rock walls that force the water into a deeper more stable channel. Trained entrances can provide better navigation, water quality and flood mitigation services, but can also cause beach erosion due to their interruption of longshore drift. One solution is the installation of a sand bypass system across the trained entrance.
Training is also used on mountainous rivers and streams, and ensures that a fast-flowing river is reduced in violence (and hence erosive capability), usually by the use of weirs and other structures like gabions. In many countries, gabion stepped weirs are commonly used for river training and flood control; the stepped design enhances the rate of energy dissipation in the channel, and it is particularly well-suited to the construction of gabion stepped weirs.