Coast

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Rugged coastline of the West Coast Region of New Zealand Knights Point - Tasmanske more - panoramio.jpg
Rugged coastline of the West Coast Region of New Zealand
Southeast coast of Greenland 17-08-islcanus-RalfR-DSC 3282.jpg
Southeast coast of Greenland

The coast, also known as the coastline or seashore, is defined as the area where land meets the sea or ocean, [1] or as a line that forms the boundary between the land and the ocean or a lake. [2] Earth has around 620,000 kilometres (390,000 mi) of coastline. Because coasts are constantly changing, a coastline's exact perimeter cannot be determined; this measurement challenge is called the coastline paradox. The term coastal zone is used to refer to a region where interactions of sea and land processes occur. [3] Both the terms coast and coastal are often used to describe a geographic location or region located on a coastline (e.g., New Zealand's West Coast, or the East, West, and Gulf Coast of the United States.)

Contents

Coasts are important zones in natural ecosystems, often home to a wide range of biodiversity, and important zones such as wetlands which are important for bird populations or mangroves or seagrass which provide nursery habitat for fish or other aquatic species. [4] Some coasts fronts on the open ocean and are called pelagic coast while other coasts are more sheltered coast in a gulf or bay. A shore, on the other hand, may refer to parts of land adjoining any large body of water, including oceans (seashore) and lakes (lake shore).

While there is general agreement in the scientific community regarding the definition of coast, in the political sphere, the delineation of the extents of a coast differ according to jurisdiction. Government authorities in various countries may define coast differently for economic and social policy reasons. According to a United Nations atlas, 44% of all people live within 150 km (93 mi) of the sea. [5] Because of their importance in society and high concentration of population, the coast is important for major parts of the global food and economic system. Important human activities happen in port cities, fisheries, and other spaces beaches and seaside resorts which are used for tourism.

However, the economic importance of coasts makes many of these communities vulnerable to climate change which causes changes in extreme weather and sea level rise and related issues such as coastal erosion, saltwater intrusion and coastal flooding. [6] Other coastal issues, such as marine pollution and debris and marine ecosystem destruction, further complicate the human uses of the coast and threaten coastal ecosystems. [6] International attention to these issues has been captured in Sustainable Development Goal 14 "Life Below Water" which sets goals for international policy focused on preserving coastal ecosystems and supporting more sustainable economic practices for coastal communities. [7]

History

In its beginnings, the Earth's coasts have been an oxygen-producing oasis, but by around 4 billion years ago in the Hadean era, during the Great Oxidation Event, microbes living in the oceans were among the first organisms to appear on land, but then new forms of life, such as eukaryotes, a diverse type of cells, began to evolve into a variety of plants and animals. From prehistoric times onward, humans adapted their way of living through coastal environments.

Today, coasts make up total of 7 percent of the Earth's oceans, with 95 percent of the world's marine productivity of aquaculture. As of 2016, more than 2 percent of the world's coasts have been found in marine protected areas.

Formation

Atlantic rocky coastline, showing a surf area. Porto Covo, west coast of Portugal Porto Covo pano April 2009-4.jpg
Atlantic rocky coastline, showing a surf area. Porto Covo, west coast of Portugal

Tides often determine the range over which sediment is deposited or eroded. Areas with high tidal ranges allow waves to reach farther up the shore, and areas with lower tidal ranges produce deposition at a smaller elevation interval. The tidal range is influenced by the size and shape of the coastline. Tides do not typically cause erosion by themselves; however, tidal bores can erode as the waves surge up the river estuaries from the ocean. [8] :421

Waves erode coastline as they break on shore releasing their energy; the larger the wave the more energy it releases and the more sediment it moves. Coastlines with longer shores have more room for the waves to disperse their energy, while coasts with cliffs and short shore faces give little room for the wave energy to be dispersed. In these areas, the wave energy breaking against the cliffs is higher, and air and water are compressed into cracks in the rock, forcing the rock apart, breaking it down. Sediment deposited by waves comes from eroded cliff faces and is moved along the coastline by the waves. This forms an abrasion or cliffed coast.

Sediment deposited by rivers is the dominant influence on the amount of sediment located in the case of coastlines that have estuaries. [9] Today riverine deposition at the coast is often blocked by dams and other human regulatory devices, which remove the sediment from the stream by causing it to be deposited inland. Coral reefs are a provider of sediment for coastlines of tropical islands.[ citation needed ]

Like the ocean which shapes them, coasts are a dynamic environment with constant change. The Earth's natural processes, particularly sea level rises, waves and various weather phenomena, have resulted in the erosion, accretion and reshaping of coasts as well as flooding and creation of continental shelves and drowned river valleys (rias).

Importance for humans and ecosystems

Somalia has the longest coastline in Africa. Somalia 16.08.2009 08-30-13.jpg
Somalia has the longest coastline in Africa.

Ecosystem services

Coasts make up a total of 7 percent of the Earth's oceans, with 95 percent of the world's marine productivity of aquaculture and the sustainability of marine habitats. Because of their importance to humanity, there are some coasts found in marine protected areas.

Coasts and their adjacent areas on and offshore are an important part of a local ecosystem. The mixture of fresh water and salt water (brackish water) in estuaries provides many nutrients for marine life. Salt marshes and beaches also support a diversity of plants, animals and insects crucial to the food chain.

The high level of biodiversity creates a high level of biological activity, which has attracted human activity for thousands of years.

Coasts also create essential material for organisms to live by, including estuaries, wetland, seagrass, coral reefs, and mangroves. They support 85 percent of U.S. migratory birds. Coasts also provide a habitat for sea turtles, marine mammals, and coral reefs. [11]

Human settlements

The Coastal Hazard Wheel system published by UNEP for global coastal management The CHW system.png
The Coastal Hazard Wheel system published by UNEP for global coastal management

More and more of the world's people live in coastal regions. [12] Many major cities are on or near good harbors and have port facilities. Some landlocked places have achieved port status by building canals.

Nations defend their coasts against military invaders, smugglers and illegal migrants. Fixed coastal defenses have long been erected in many nations, and coastal countries typically have a navy and some form of coast guard.

Tourism

Coasts, especially those with beaches and warm water, attract tourists often leading to the development of seaside resort communities. In many island nations such as those of the Mediterranean, South Pacific Ocean and Caribbean, tourism is central to the economy. Coasts offer recreational activities such as swimming, fishing, surfing, boating, and sunbathing.

Growth management and coastal management can be a challenge for coastal local authorities who often struggle to provide the infrastructure required by new residents, and poor management practices of construction often leave these communities and infrastructure vulnerable to processes like coastal erosion and sea level rise. In many of these communities, management practices such as beach nourishment or when the coastal infrastructure is no longer financially sustainable, managed retreat to remove communities from the coast.

Types

Emergent coastline

According to one principle of classification, an emergent coastline is a coastline that has experienced a fall in sea level, because of either a global sea-level change, or local uplift. Emergent coastlines are identifiable by the coastal landforms, which are above the high tide mark, such as raised beaches. In contrast, a submergent coastline is one where the sea level has risen, due to a global sea-level change, local subsidence, or isostatic rebound. Submergent coastlines are identifiable by their submerged, or "drowned" landforms, such as rias (drowned valleys) and fjords

Concordant coastline

According to the second principle of classification, a concordant coastline is a coastline where bands of different rock types run parallel to the shore. These rock types are usually of varying resistance, so the coastline forms distinctive landforms, such as coves. Discordant coastlines feature distinctive landforms because the rocks are eroded by the ocean waves. The less resistant rocks erode faster, creating inlets or bay; the more resistant rocks erode more slowly, remaining as headlands or outcroppings.

Other coastal categories

Landforms

The following articles describe some coastal landforms:

Coastal landforms. The feature shown here as a bay would, in certain (mainly southern) parts of Britain, be called a cove. That between the cuspate foreland and the tombolo is a British bay. Accreting coast Image6.svg
Coastal landforms. The feature shown here as a bay would, in certain (mainly southern) parts of Britain, be called a cove. That between the cuspate foreland and the tombolo is a British bay.

Cliff erosion

Coastal features formed by sediment

Coastal features formed by another feature

Other features on the coast

Geologic processes

The following articles describe the various geologic processes that affect a coastal zone:

Wildlife

Animals

Animals that live in coastal areas include puffins, sea turtles and rockhopper penguins, among many others. Sea snails and various kinds of barnacles live on the coast and scavenge on food deposited by the sea. Most coastal animals are used to humans in developed areas, such as dolphins and seagulls who eat food thrown for them by tourists. Since the coastal areas are all part of the littoral zone, there is a profusion of marine life found just off-coast, including sessile animals such as corals and sea anemones.

There are many kinds of seabirds on various coasts. These include pelicans and cormorants, who join up with terns and oystercatchers to forage for fish and shellfish. There are sea lions on the coast of Wales and other countries.

Coastal fish

Schooling threadfin, a coastal species Sixfinger threadfin school.jpg
Schooling threadfin, a coastal species

Coastal fish, also called inshore fish or neritic fish, inhabit the sea between the shoreline and the edge of the continental shelf. Since the continental shelf is usually less than 200 metres (660 ft) deep, it follows that pelagic coastal fish are generally epipelagic fish, inhabiting the sunlit epipelagic zone. [13] Coastal fish can be contrasted with oceanic fish or offshore fish, which inhabit the deep seas beyond the continental shelves.

Coastal fish are the most abundant in the world. [14] They can be found in tidal pools, fjords and estuaries, near sandy shores and rocky coastlines, around coral reefs and on or above the continental shelf. Coastal fish include forage fish and the predator fish that feed on them. Forage fish thrive in inshore waters where high productivity results from upwelling and shoreline run off of nutrients. Some are partial residents that spawn in streams, estuaries and bays, but most complete their life cycles in the zone. [14]

Plants

Many coastal areas are famous for their kelp beds. Kelp is a fast-growing seaweed that grows up to a metre a day. Mangroves, seagrasses and salt marsh are important coastal vegetation types in tropical and temperate environments respectively. [4] Restinga is another type of coastal vegetation.

Threats

Coasts also face many human-induced environmental impacts. principal among which are sea-level rise, and associated issues like coastal erosion and saltwater intrusion, and pollution, such as oil spills or marine debris contaminating coasts with plastic and other trash.

Sea level rise due to climate change

Satellite observations of sea level rise from 1993 to 2021. NASA-Satellite-sea-level-rise-observations.jpg
Satellite observations of sea level rise from 1993 to 2021.

Tide gauge measurements show that global sea level rise began at the start of the 20th century. Between 1900 and 2017, the globally averaged sea level rose by 16–21 cm (6.3–8.3 in). [15] More precise data gathered from satellite radar measurements reveal an accelerating rise of 7.5 cm (3.0 in) from 1993 to 2017, [16] :1554 which is a trend of roughly 30 cm (12 in) per century. This acceleration is due mostly to climate change, which is driving thermal expansion of seawater and the melting of land-based ice sheets and glaciers. [17] Between 1993 and 2018, thermal expansion of the oceans contributed 42% to sea level rise; the melting of temperate glaciers, 21%; Greenland, 15%; and Antarctica, 8%. [16] :1576 Climate scientists expect the rate to further accelerate during the 21st century, with the latest measurements saying the sea levels are currently rising by 3.6 mm per year. [18] :62 [19]

Projecting future sea level is challenging, due to the complexity of many aspects of the climate system and to time lags in sea level reactions to Earth temperature changes. As climate research into past and present sea levels leads to improved computer models, projections have consistently increased. In 2007, the Intergovernmental Panel on Climate Change (IPCC) projected a high end estimate of 60 cm (2 ft) through 2099, [20] but their 2014 report raised the high-end estimate to about 90 cm (3 ft). [21] A number of later studies have concluded that a global sea level rise of 200 to 270 cm (6.6 to 8.9 ft) this century is "physically plausible". [22] [16] [23] A conservative estimate of the long-term projections is that each Celsius degree of temperature rise triggers a sea level rise of approximately 2.3 meters (4.2 ft/degree Fahrenheit) over a period of two millennia (2,000 years): an example of climate inertia. [15] In February 2021, a paper published in Ocean Science suggested that past projections for global sea level rise by 2100 reported by the IPCC were likely conservative, and that sea levels will rise more than previously expected. [24]

Pollution

The pollution of coastlines is connected to marine pollution which can occur from a number of sources: Marine debris (garbage and industrial debris); the transportation of petroleum in tankers, increasing the probability of large oil spills; small oil spills created by large and small vessels, which flush bilge water into the ocean.

Marine pollution

Marine pollution occurs when substances used or spread by humans, such as industrial, agricultural and residential waste, particles, noise, excess carbon dioxide or invasive organisms enter the ocean and cause harmful effects there. Since most inputs come from land, either via the rivers, sewage or the atmosphere, it means that continental shelves are more vulnerable to pollution. Air pollution is also a contributing factor by carrying off iron, carbonic acid, nitrogen, silicon, sulfur, pesticides or dust particles into the ocean. [25] The pollution often comes from nonpoint sources such as agricultural runoff, wind-blown debris, and dust. Land and air pollution have proven to be harmful to marine life and its habitats. [26]

A particular concern is the runoff of nutrients (nitrogen and phosphorus) from intensive agriculture, and the disposal of untreated or partially treated sewage to rivers and subsequently oceans. These nitrogen and phosphorus nutrients (which are also contained in fertilizers) stimulate phytoplankton growth, which can provide more food for other marine life, but in excess can lead to harmful algal blooms (eutrophication) which can be harmful to humans as well as marine creatures. Such blooms are naturally occurring but may be increasing as a result of anthropogenic inputs or alternatively may be something that is now more closely monitored and so more frequently reported. [27] A second major concern is that the degradation of algal blooms can lead to depletion of oxygen in coastal waters, a situation that may be exacerbated by climate change as warming reduces vertical mixing of the water column. [28]

Marine debris

Marine debris, also known as marine litter, is human-created waste that has deliberately or accidentally been released in a sea or ocean. Floating oceanic debris tends to accumulate at the center of gyres and on coastlines, [29] frequently washing aground, when it is known as beach litter or tidewrack. Deliberate disposal of wastes at sea is called ocean dumping. Naturally occurring debris, such as driftwood and drift seeds, are also present.

With the increasing use of plastic, human influence has become an issue as many types of (petrochemical) plastics do not biodegrade quickly, as would natural or organic materials. [30] The largest single type of plastic pollution (~10 %) and majority of large plastic in the oceans is discarded and lost nets from the fishing industry. [31] Waterborne plastic poses a serious threat to fish, seabirds, marine reptiles, and marine mammals, as well as to boats and coasts. [32] Dumping, container spillages, litter washed into storm drains and waterways and wind-blown landfill waste all contribute to this problem. This increased has caused serious negative effects such as ghost nets capturing animals, concentration of plastic debris in massive marine garbage patches, and concentration of debris in the food chain.

Microplastics

Great Pacific garbage patch -- Pacific Ocean currents have created three "islands" of debris. Pacific-garbage-patch-map 2010 noaamdp.jpg
Great Pacific garbage patch — Pacific Ocean currents have created three "islands" of debris.

Due to their ubiquity in the environment, microplastics are widespread among the different matrices. In marine environments, microplastics have been evidenced in sandy beaches, [34] surface waters, [35] the water column, and deep sea sediment. Upon reaching marine environments, the fate of microplastics is subject to naturally occurring drivers, such as winds and surface oceanic currents. Numerical models are able to trace small plastic debris (micro- and mesoplastics) drifting in the ocean, [36] thus predicting their fate.

Microplastics enter waterways through many avenues including deterioration of road paint, tire wear and city dust entering the waterways, plastic pellets spilled from shipping containers, ghost nets and other synthetic textiles dumped into the ocean, cosmetics discharged and laundry products entering sewage water and marine coatings on ships degrading. [37]

Ecosystem destruction

Fishing has declined due to habitat degradation, overfishing, trawling, bycatch and climate change. Since the growth of global fishing enterprises after the 1950s, intensive fishing has spread from a few concentrated areas to encompass nearly all fisheries. The scraping of the ocean floor in bottom dragging is devastating to coral, sponges and other long-lived species that do not recover quickly. This destruction alters the functioning of the ecosystem and can permanently alter species' composition and biodiversity. Bycatch, the capture of unintended species in the course of fishing, is typically returned to the ocean only to die from injuries or exposure. Bycatch represents about a quarter of all marine catch. In the case of shrimp capture, the bycatch is five times larger than the shrimp caught.

Statistics

Length of coastline

Britain-fractal-coastline-100km.png
Britain-fractal-coastline-50km.png
An example of the coastline paradox. If the coastline of Great Britain is measured using units 100 km (62 mi) long, then the length of the coastline is approximately 2,800 km (1,700 mi). With 50 km (31 mi) units, the total length is approximately 3,400 km (2,100 mi), approximately 600 km (370 mi) longer.

The coastline paradox is the counterintuitive observation that the coastline of a landmass does not have a well-defined length. This results from the fractal curve-like properties of coastlines, i.e., the fact that a coastline typically has a fractal dimension (which in fact makes the notion of length inapplicable). The first recorded observation of this phenomenon was by Lewis Fry Richardson [38] [39] and it was expanded upon by Benoit Mandelbrot. [40] [41]

The measured length of the coastline depends on the method used to measure it and the degree of cartographic generalization. Since a landmass has features at all scales, from hundreds of kilometers in size to tiny fractions of a millimeter and below, there is no obvious size of the smallest feature that should be taken into consideration when measuring, and hence no single well-defined perimeter to the landmass. Various approximations exist when specific assumptions are made about minimum feature size.

See also

Related Research Articles

Beach Area of loose particles at the edge of the sea or other body of water

A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from rock, such as sand, gravel, shingle, pebbles, etc., or biological sources, such as mollusc shells or coralline algae. Sediments settle in different densities and structures, depending on the local wave action and weather, creating different textures, colors and gradients or layers of material.

Estuary Partially enclosed coastal body of brackish water with river stream flow, and with a free connection to the sea

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.

Coastal erosion Displacement of land along the coastline

Coastal erosion is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of waves, currents, tides, wind-driven water, waterborne ice, or other impacts of storms. The landward retreat of the shoreline can be measured and described over a temporal scale of tides, seasons, and other short-term cyclic processes. Coastal erosion may be caused by hydraulic action, abrasion, impact and corrosion by wind and water, and other forces, natural or unnatural.

Shoal Natural submerged sandbank that rises from a body of water to near the surface

In oceanography, geomorphology, and geoscience, a shoal is a natural submerged ridge, bank, or bar that consists of, or is covered by, sand or other unconsolidated material and rises from the bed of a body of water to near the surface. It often refers to those submerged ridges, banks, or bars that rise near enough to the surface of a body of water as to constitute a danger to navigation. Shoals are also known as sandbanks, sandbars, or gravelbars. Two or more shoals that are either separated by shared troughs or interconnected by past or present sedimentary and hydrographic processes are referred to as a shoal complex.

Landforms are categorized by characteristic physical attributes such as their creating process, shape, elevation, slope, orientation, rock exposure, and soil type.

Zmudowski State Beach is located on Monterey Bay, in Moss Landing, Monterey County, northern California.

Seawall Form of coastal defence

A seawall is a form of coastal defence constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast. The purpose of a sea wall is to protect areas of human habitation, conservation and leisure activities from the action of tides, waves, or tsunamis. As a seawall is a static feature it will conflict with the dynamic nature of the coast and impede the exchange of sediment between land and sea. The shoreline is part of the coastal interface which is exposed to a wide range of erosional processes arising from flowing water sources, wind and terrestrial sources, meaning that a combination of denudational processes will work against a seawall.

Marine debris Human-created solid waste in the sea or ocean

Marine debris, also known as marine litter, is human-created waste that has deliberately or accidentally been released in a sea or ocean. Floating oceanic debris tends to accumulate at the center of gyres and on coastlines, frequently washing aground, when it is known as beach litter or tidewrack. Deliberate disposal of wastes at sea is called ocean dumping. Naturally occurring debris, such as driftwood and drift seeds, are also present.

Marine pollution Pollution of oceans from substances discarded by humans

Marine pollution occurs when substances used or spread by humans, such as industrial, agricultural and residential waste, particles, noise, excess carbon dioxide or invasive organisms enter the ocean and cause harmful effects there. Since most inputs come from land, either via the rivers, sewage or the atmosphere, it means that continental shelves are more vulnerable to pollution. Air pollution is also a contributing factor by carrying off iron, carbonic acid, nitrogen, silicon, sulfur, pesticides or dust particles into the ocean. The pollution often comes from nonpoint sources such as agricultural runoff, wind-blown debris, and dust. Land and air pollution have proven to be harmful to marine life and its habitats.

Coastal geography Study of the region between the ocean and the land

Coastal geography is the study of the constantly changing region between the ocean and the land, incorporating both the physical geography and the human geography 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 coast

Coastal management Preventing flooding and erosion of shorelines

Coastal management is defence against flooding and erosion, and techniques that stop erosion to claim lands. Protection against rising sea levels in the 21st century is crucial, as sea level rise accelerates due to climate change. Changes in sea level damage beaches and coastal systems are expected to rise at an increasing rate, causing coastal sediments to be disturbed by tidal energy.

Abrasion (geology)

Abrasion is a process of erosion which occurs when material being transported wears away at a surface over time. It is the process of friction caused by scuffing, scratching, wearing down, marring, and rubbing away of materials. The intensity of abrasion depends on the hardness, concentration, velocity and mass of the moving particles. Abrasion generally occurs four ways. Glaciation slowly grinds rocks picked up by ice against rock surfaces. Solid objects transported in river channels make abrasive surface contact with the bed and walls. Objects transported in waves breaking on coastlines cause abrasion. And, finally, abrasion can be caused by wind transporting sand or small stones against surface rocks.

Coastal engineering

Coastal engineering is a branch of civil engineering concerned with the specific demands posed by constructing at or near the coast, as well as the development of the coast itself.

Ocean Body of salt water covering the majority of Earth

The ocean is the body of salt water which covers approximately 71% of the surface of the Earth and contains 97% of Earth's water. Another definition is "any of the large bodies of water into which the great ocean is divided". Separate names are used to identify five different areas of the ocean: Pacific Atlantic, Indian, Southern (Antarctic), and Arctic. Seawater covers approximately 361,000,000 km2 (139,000,000 sq mi) of the planet. The ocean is the principal component of Earth's hydrosphere, and therefore integral to life on Earth. Acting as a huge heat reservoir, the ocean influences climate and weather patterns. the carbon cycle and the water cycle.

Marine habitats Habitat that supports marine life

Marine habitats are habitats that support marine life. Marine life depends in some way on the saltwater that is in the sea. A habitat is an ecological or environmental area inhabited by one or more living species. The marine environment supports many kinds of these habitats. Marine habitats can be divided into coastal and open ocean habitats. Coastal habitats are found in the area that extends from as far as the tide comes in on the shoreline out to the edge of the continental shelf. Most marine life is found in coastal habitats, even though the shelf area occupies only seven percent of the total ocean area. Open ocean habitats are found in the deep ocean beyond the edge of the continental shelf.

Cliffed coast Form of coast where the action of marine waves has formed steep cliffs that may or may not be precipitous

A cliffed coast, also called an abrasion coast, is a form of coast where the action of marine waves has formed steep cliffs that may or may not be precipitous. It contrasts with a flat or alluvial coast.

Coastal hazards

Coastal hazards are physical phenomena that expose a coastal area to risk of property damage, loss of life and environmental degradation. Rapid-onset hazards last over periods of minutes to several days and examples include major cyclones accompanied by high winds, waves and surges or tsunamis created by submarine earthquakes and landslides. Slow-onset hazards develop incrementally over longer time periods and examples include erosion and gradual inundation.

Effects of climate change on oceans Effects of climate change on oceans

The effects of climate change on oceans include the rise in sea level from ocean warming and ice sheet melting, and changes in ocean stratification and circulation due to changing temperatures leading to changes in oxygen concentrations. There is clear evidence that the Earth is warming due to anthropogenic emissions of greenhouse gases and leading inevitably to ocean warming. The greenhouse gases taken up by the ocean help to mitigate climate change but lead to ocean acidification.

Human impact on marine life

Human activities affect marine life and marine habitats through overfishing, habitat loss, the introduction of invasive species, ocean pollution, ocean acidification and ocean warming. These impact marine ecosystems and food webs and may result in consequences as yet unrecognised for the biodiversity and continuation of marine life forms.

Hapua

Hapua is the Māori term for river-mouth lagoons on mixed sand and gravel (MSG) beaches which form at the river-coast interface where a typically braided, although sometimes meandering, river interacts with a coastal environment that is significantly affected by longshore drift. The lagoons which form on the MSG coastlines are common on the east coast of the South Island of New Zealand and have long been referred to as hapua by the Māori. This classification differentiates hapua from similar lagoons located on the New Zealand coast termed waituna. Hapua are often located on paraglacial coastal areas where there is a low level of coastal development and minimal population density. Hapua form as the river carves out an elongated coast-parallel area, blocked from the sea by a MSG barrier which constantly alters its shape and volume due to longshore drift. Longshore drift continually extends the barrier behind which the hapua forms by transporting sediment along the coast. Hapua are defined as a narrow shore-parallel extensions of the coastal riverbed. They discharge the majority of stored water to the ocean via an ephemeral and highly mobile drainage channel or outlet. The remainder percolates through the MSG barrier due to its high levels of permeability. Hapua systems are driven by a wide range of dynamic processes that are generally classified as fluvial or marine; changes in the balance between these processes as well as the antecedent barrier conditions can cause shifts in the morphology of the hapua, in particular the barrier. New Zealand examples include the Rakaia, Ashburton and Hurunui river-mouths.

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Further reading