Spit (landform)

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Diagram showing a spit Spit diagram.svg
Diagram showing a spit
A spit contrasted with other coastal landforms. Accreting coast Image6.svg
A spit contrasted with other coastal landforms.
Curonian Spit, divided between Russia and Lithuania Curonian Lagoon.jpg
Curonian Spit, divided between Russia and Lithuania

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. [1] [2] [3]

Contents

Hydrology and geology

Where the direction of the shore inland re-enters, or changes direction, for example at a headland, the longshore current spreads out or dissipates. No longer able to carry the full load, much of the sediment is dropped. This is called deposition. This submerged bar of sediment allows longshore drift or littoral drift to continue to transport sediment in the direction the waves are breaking, forming an above-water spit. Without the complementary process of littoral drift, the bar would not build above the surface of the waves becoming a spit and would instead be leveled off underwater.

Spits occur when longshore drift reaches a section of headland where the turn is greater than 30 degrees. The spit will continue out into the sea until water pressure (e.g. from a river) becomes too great to allow the sand to deposit. Vegetation may then start to grow on the spit, and the spit may become stable and often fertile. A spit may be considered a special form of a shoal. As spits grow, the water behind them is sheltered from wind and waves, and a salt marsh is likely to develop.

Dungeness Spit in the Strait of Juan de Fuca, on the U.S. Pacific coast. Dungeness National Wildlife Refuge aerial.jpg
Dungeness Spit in the Strait of Juan de Fuca, on the U.S. Pacific coast.

Wave refraction can occur at the end of a spit, carrying sediment around the end to form a hook or recurved spit. [4] Refraction in multiple directions may create a complex spit. Waves that arrive in a direction other than obliquely along the spit will halt the growth of the spit, shorten it, or eventually destroy it entirely. [4]

The sediments that make up spits come from a variety of sources including rivers and eroding bluffs, and changes there can have a major effect on spits and other coastal landforms. Activities such as logging and farming upstream can increase the sediment load of rivers, which may hurt the intertidal environments around spits by smothering delicate habitats. Roads or bulkheads built along bluffs can drastically reduce the volume of sediment eroded, so that not enough material is being pushed along to maintain the spit.

If the supply of sediment is interrupted the sand at the neck (landward end) of the spit may be moved towards the head, eventually creating an island. If the supply is not interrupted, and the spit is not breached by the sea (or, if across an estuary, the river), the spit may become a bar, with both ends joined to land, and form a lagoon behind the bar. If an island lies offshore near where the coast changes direction, and the spit continues to grow until it connects the island to the mainland, it is called a tombolo.

The end of a spit attached to land is called the proximal end, and the end jutting out into water is called the distal end.

Notable spits around the world

Farewell Spit, on New Zealand's South Island Farewell Spit.jpg
Farewell Spit, on New Zealand's South Island

The longest spit in the world is the Arabat Spit in the Sea of Azov. It is approximately 110 kilometres (68 mi) long.

The longest spit in a freshwater body of water is Long Point, Ontario, which extends approximately 32 km (20 mi) into Lake Erie.

Farewell Spit in New Zealand, at 32 km (20 mi), in the north-west area of South Island, is believed to be caused by the strong prevailing winds and currents, bringing sand eroded from the Southern Alps of the South Island and depositing these into Golden Bay.

A well-known spit in the UK is Spurn Point at the Humber; it is approximately 4.8 km (3.0 mi) long. Another is Chesil Beach in the UK, which connects the Isle of Portland to the mainland.

The Curonian Spit, off the coast of Lithuania and Kaliningrad Oblast of Russia, separates the Curonian Lagoon from the Baltic Sea; it is 98 km long (61 mi). In a similar fashion, the Vistula Spit separates the Vistula Lagoon from the Gdańsk Bay off the coast of Poland.

Zlatni Rat, a popular pebble beach jutting southward from the harbor town of Bol, on the Croatian island of Brač, is formed by Adriatic currents flowing east and west through the Hvar Channel, along the southern side of the island. The spit bends slightly west or east, changing its direction gradually, depending on the conditions of the tides and weather.

Human settlement patterns

Since prehistory humans have chosen certain spit formations as sites for human habitation. In some cases, these sites have been chosen for proximity to marine resource exploitation; the Chumash Native American prehistorical settlement on the Morro Bay is one such location. [5]

See also

Related Research Articles

Coast Area where land meets the sea or ocean

The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the ocean or a lake. The Earth has around 620,000 kilometres (390,000 mi) of coastline. Coasts are important zones in natural ecosystems, often home to a wide range of biodiversity. On land, they harbor important ecosystems such as freshwater or estuarine wetlands, which are important for bird populations and other terrestrial animals. In wave-protected areas they harbor saltmarshes, mangroves or seagrasses, all of which can provide nursery habitat for finfish, shellfish, and other aquatic species. Rocky shores are usually found along exposed coasts and provide habitat for a wide range of sessile animals and various kinds of seaweeds. Along tropical coasts with clear, nutrient-poor water, coral reefs can often be found between depths of 1–50 m.

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.

Tombolo Deposition landform in which an island is connected to the mainland by a sandy isthmus

A tombolo is a sandy isthmus. A tombolo, from the Italian tombolo, meaning 'pillow' or 'cushion', and sometimes translated as ayre, is a deposition landform by which an island becomes 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.

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.

Longshore drift Sediment moved by the longshore current

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

Barrier island Coastal dune landform that forms by wave and tidal action parallel to the mainland coast

Barrier islands are coastal landforms and a type of dune system that are exceptionally flat or lumpy areas of sand that form by wave and tidal action parallel to the mainland coast. They usually occur in chains, consisting of anything from a few islands to more than a dozen. They are subject to change during storms and other action, but absorb energy and protect the coastlines and create areas of protected waters where wetlands may flourish. A barrier chain may extend uninterrupted for over a hundred kilometers, excepting the tidal inlets that separate the islands, the longest and widest being Padre Island of Texas. Sometimes an important inlet may close permanently, transforming an island into a peninsula, thus creating a barrier peninsula. The length and width of barriers and overall morphology of barrier coasts are related to parameters including tidal range, wave energy, sediment supply, sea-level trends, and basement controls. The amount of vegetation on the barrier has a large impact on the height and evolution of the island.

Groyne Structure extending into a body of water

A groyne is a rigid hydraulic structure built perpendicularly from an ocean shore or a river bank, interrupting water flow and limiting the movement of sediment. It is usually made out of wood, concrete, or stone. In the ocean, groynes create beaches, prevent beach erosion caused by longshore drift where this is the dominant process and facilitate beach nourishment. There is also often cross-shore movement which if longer than the groyne will limit its effectiveness. In a river, groynes slow down the process of erosion and prevent ice-jamming, which in turn aids navigation.

Vistula Spit Peninsular stretch of land in the Baltic Sea on the border between Russia and Poland

The Vistula Spit is an aeolian sand spit, or peninsular stretch of land that separates Vistula Lagoon from Gdańsk Bay, in the Baltic Sea, with its tip separated from the mainland by the Strait of Baltiysk. The border between Poland and Kaliningrad Oblast, a semi-exclave of Russia, bisects it, politically dividing the spit in half between the two countries. The westernmost geographical point of Russia is located on the Vistula Spit. The Polish part contains a number of tourist resorts, incorporated administratively as the town of Krynica Morska.

Beach nourishment

Beach nourishment describes a process by which sediment, usually sand, lost through longshore drift or erosion is replaced from other sources. A wider beach can reduce storm damage to coastal structures by dissipating energy across the surf zone, protecting upland structures and infrastructure from storm surges, tsunamis and unusually high tides. Beach nourishment is typically part of a larger integrated coastal zone management aimed at coastal defense. Nourishment is typically a repetitive process since it does not remove the physical forces that cause erosion but simply mitigates their effects.

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

Beach ridge Wave-swept or wave-deposited ridge running parallel to a shoreline

A beach ridge is a wave-swept or wave-deposited ridge running parallel to a shoreline. It is commonly composed of sand as well as sediment worked from underlying beach material. The movement of sediment by wave action is called littoral transport. Movement of material parallel to the shoreline is called longshore transport. Movement perpendicular to the shore is called on-offshore transport. A beach ridge may be capped by, or associated with, sand dunes. The height of a beach ridge is affected by wave size and energy.

Kaitorete Spit Spit in New Zealand

Kaitorete Spit is a long finger of land which extends along the coast of Canterbury in the South Island of New Zealand. It runs west from Banks Peninsula for 25 kilometres, and separates the shallow Lake Ellesmere / Te Waihora from the Pacific Ocean. It is technically a continuous barrier beach, though at its western end it tapers to a point less than 100 metres in width which is occasionally breached at high tide. The spit is noted for its isolation and for its pebbly beaches. At its eastern end is the small settlement of Birdlings Flat, and west of its narrowest point is the settlement of Taumutu.

Cuspate foreland Geographical features found on coastlines and lakeshores

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.

Beach evolution occurs at the shoreline where sea, lake or river water is eroding the land. Beaches exist where sand accumulated from centuries-old, recurrent processes that erode rocky and sedimentary material into sand deposits. River deltas deposit silt from upriver, accreting at the river's outlet to extend lake or ocean shorelines. Catastrophic events such as tsunamis, hurricanes, and storm surges accelerate beach erosion.

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.

Washdyke Lagoon is a brackish shallow coastal lagoon approximately 1 kilometre (0.62 mi) 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 (0.48 km2) in area. It is enclosed by a barrier beach that is 3 kilometres (1.9 mi) long and 3 metres (9.8 ft) above high tide at its largest point. The reduced lagoon size is due to the construction of the Timaru Port breakwater which is preventing coarse sediments from reaching and replenishing Washdyke Barrier. This is important as the lagoon and the surrounding 250 hectares are classified as a wildlife refuge and it demonstrates the role human structures have on coastline evolution.

Canterbury Bight Body of water

The Canterbury Bight is a large bight on the eastern side of New Zealand's South Island. The bight runs for approximately 135 kilometres (84 mi) from the southern end of Banks Peninsula to the settlement of Timaru and faces southeast, exposing it to high-energy storm waves originating in the Pacific Ocean. The bight is known for rough conditions as a result, with wave heights of over 2 metres (6.6 ft) common. Much of the bight's geography is shaped by this high-energy environment interacting with multiple large rivers which enter the Pacific in the bight, such as the Rakaia, Ashburton / Hakatere, and Rangitata Rivers. Sediment from these rivers, predominantly Greywacke, is deposited along the coast and extends up to 50 kilometres (31 mi) out to sea from the current shoreline. Multiple hapua, or river-mouth lagoons, can be found along the length of the bight where waves have deposited sufficient sediment to form a barrier across a river mouth, including most notably Lake Ellesmere / Te Waihora and Washdyke Lagoon

Flat coast Shoreline where the land descends gradually into the sea

At a flat coast or flat shoreline, the land descends gradually into the sea. Flat coasts can be formed either as a result of the sea advancing into gently-sloping terrain or through the abrasion of loose rock. They may be basically divided into two parallel strips: the shoreface and the beach.

Hapua

A hapua is a river-mouth lagoon on a mixed sand and gravel (MSG) beach, formed 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.

References

  1. Duane, D.B. and James, W.R., 1980, "Littoral transport in the surf zone elucidated by an Eulerian sediment tracer experiment:" Journal of Sedimentary Petrology. vol. 50, p. 929-942
  2. https://link.springer.com/referenceworkentry/10.1007/0-387-30843-1_432
  3. https://link.springer.com/referenceworkentry/10.1007/978-3-319-93806-6_297
  4. 1 2 Evans, O.F. 1942, "The origin of spits, bars and related structures:" Journal of Geology, v. 50, p. 846-863
  5. Hogan, C. Michael (December 2012). Burnham, A. (ed.). "Morro Creek". The Megalithic Portal.