Wave base

The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. For water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion above.[1]

Wavebase
Wave base diagram.

Process

In seawater, the water particles are moved in a circular orbital motion when a wave passes. The radius of the circle of motion for any given water molecule decreases exponentially with increasing depth. The wave base, which is the depth of influence of a water wave, is about half the wavelength.

At depths greater than half the wavelength, the water motion is less than 4% of its value at the water surface[2] and may be neglected.

For example, in a pool of water 1 metre (3.3 ft) deep, a wave with a 3 metres (9.8 ft) wavelength would be moving the water at the bottom. In the same pool, a wave with a wavelength of 0.5 metres (1.6 ft) would not be able to cause water movement on the bottom.

Distinctions

There are typically two wave bases, the fair weather wave base (FWWB) and the storm wave base (SWB).[1]

The fair weather wave base refers to the depth beneath the waves under normal conditions and the portion of the seafloor that is agitated by this everyday wave action is known as the Upper shoreface.

The storm wave base refers to the depths beneath storm-driven waves and can be much deeper. The portion of the seafloor that is only agitated by storm-driven wave action is known as the Lower shoreface.

Note that another classification exists, which considers that the zone affected by both fair weather and storm waves is to be defined as shoreface, whereas Upper offshore is the name given to the zone only affected by storm waves and Lower offshore a zone not disturbed by any surface wave. (e.g. [3])

Types Of Wave Bases

fair weather base: it is a sea depth of 5 to 15m below sea level where the sea bed is unaffected by the action of waves in the calm wave conditions.

See also

References

  1. ^ a b R. G. Dean & R. A. Dalrymple (1991). Water wave mechanics for engineers and scientists. Advanced Series on Ocean Engineering. 2. World Scientific, Singapore. ISBN 978-981-02-0420-4.
  2. ^ At a depth of half the wave length, the amplitude of the water particle motion by the waves has been reduced to e−π ≈ 0.04 times it value at the water surface.
  3. ^ Bayetgoll et al., 2015, Ichnology and sedimentology of a shallow marine Upper Cretaceous depositional system (Neyzar Formation, Kopet-Dagh, Iran): Palaeoceanographic influence on ichnodiversity, Cretaceous Research, 56, 628-646
Bardahessiagh Formation

The Bardahessiagh Formation is a geologic formation in Northern Ireland. It has been described from a locality lying about 3 km NNE of Pomeroy, south of Craigbardahessiagh. It is now known as comprising the former ‘Bardahessiagh Formation’, or Bardahessiagh Beds and the ‘Junction Beds’ that is underlain by a stratigraphical unit not recognised, until fieldwork by the Ulster Museum staff in 1992. The Bardahessiagh Formation is divided into three units, but the summit of the formation is not known. Field evidence indicates that the local top of the formation is characterised by a thrust contact with the Killey Bridge and Tirnaskea Formation (upper Katian and Hirnantian, respectively), exposed south of the Well Field, which lies 650 metres SSW of Craigbardahessiagh.

Constraints on the age of the formation is based on the brachiopod faunas and an upper Sandbian to lower Katian age has been proposed.The Bardahessiagh Formation preserves a diverse and well-preserved assemblage of fossils. Only the brachiopods, cephalopods, bivalves and kilbuchophyllid corals have been systematically described.

Based on the brachiopod faunas, the Bardahessiagh Formation has been interpreted as deposited in a transgressive regime, below storm-wave base environments, peaking in the second unit, with the presence of typical Sericoidea association (Candela 2001, 2006).

The integrity of the formation is currently endangered by excavation by locals using trenchers to dig irrigation ditches.

Carbonate platform

A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms (usually microbes) which induce carbonate precipitation through their metabolism. Therefore, carbonate platforms can not grow up everywhere: they are not present in places where limiting factors to the life of reef-building organisms exist. Such limiting factors are, among others: light, water temperature, transparency and pH-Value. For example, carbonate sedimentation along the Atlantic South American coasts takes place everywhere but at the mouth of the Amazon River, because of the intense turbidity of the water there. Spectacular examples of present-day carbonate platforms are the Bahama Banks under which the platform is roughly 8 km thick, the Yucatan Peninsula which is up to 2 km thick, the Florida platform, the platform on which the Great Barrier Reef is growing, and the Maldive atolls. All these carbonate platforms and their associated reefs are confined to tropical latitudes. Today's reefs are built mainly by scleractinian corals, but in the distant past other organisms, like archaeocyatha (during the Cambrian) or extinct cnidaria (tabulata and rugosa) were important reef builders.

Contourite

A contourite is a sedimentary deposit commonly formed on continental rise to lower slope settings, although they may occur anywhere that is below storm wave base. Countourites are produced by thermohaline-induced deepwater bottom currents and may be influenced by wind or tidal forces. The geomorphology of contourite deposits is mainly influenced by the deepwater bottom-current velocity, sediment supply, and seafloor topography.

Depositional environment

In geology, depositional environment or sedimentary environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock record. In most cases the environments associated with particular rock types or associations of rock types can be matched to existing analogues. However, the further back in geological time sediments were deposited, the more likely that direct modern analogues are not available (e.g. banded iron formations).

Fezouata Formation

The Upper and Lower Fezouata Formations of Morocco are Burgess shale-type deposits dating to the Early Ordovician, filling an important preservational window between the common Cambrian Lagerstätten and the Late Ordovician Soom Shale. Found fossilized fauna were numerous organisms previously thought to have died out after the mid-Cambrian.

Hobie Wave

The Hobie Wave is an American catamaran that was designed by Morrelli & Melvin and first built in 1994.

Hummocky cross-stratification

Hummocky cross-stratification is a type of sedimentary structure found in sandstones. It is a form of cross-bedding usually formed by the action of large storms, such as hurricanes. It takes the form of a series of "smile"-like shapes, crosscutting each other. It is only formed at a depth of water below fair-weather wave base and above storm-weather wave base. They are not related to "hummocks" except in shape.

Ichnofacies

An ichnofacies is an assemblage of trace fossils that provides an indication of the conditions that their formative organisms inhabited.

Index of wave articles

This is a list of Wave topics.

Kaili Formation

The Kaili Formation is a stratigraphic formation which was deposited during the Lower and Middle Cambrian (~513 to 506 million years ago). The formation is approximately 200 metres (660 ft) thick and was named after the city Kaili in the Guizhou province of southwest China.

The depositional environment of the Kaili formation is not entirely known, and there are two hypotheses for its formation. It may have been a nearshore marine environment with 'normal' levels of oxygenation; or it may have been a deeper water environment further from the shore, on the open continental shelf; in this setting oxygen would not be available below the surface layers of the deposited sediment. The trace fossil assemblages in the formation suggest that it was below the wave base and was reasonably well-oxygenated.

Lower shoreface

Lower Shoreface refers to the portion of the seafloor, and the sedimentary depositional environment, that lies below the everyday wave base. It is also used for the sandstone sedimentary structure rock formations that were produced by this process in an earlier geologic era, such as the Cretaceous Period.

Much Wenlock Limestone Formation

The Much Wenlock Limestone Formation is a series of Silurian limestone beds that date back to the Homerian age, the early part of the Wenlock epoch. The formation comprises nodular to thinly bedded limestones, with variable development of reef bodies that can be found exposed at Wenlock Edge and in a small area near Dudley, England. The formation is made up of three different members which have been classified as the Lower Quarried Limestone Member, the Nodular Beds Member and the Upper Quarried Limestone Member. Many of the members can be subdivided into separate lithofacies that would have been deposited under different environmental conditions. However the overall environment of deposition for the formation is in a shallow, tropical marine setting that is located between the storm wave base and the fair weather wave base.

Oceanic plateau

An oceanic or submarine plateau is a large, relatively flat elevation that is higher than the surrounding relief with one or more relatively steep sides.There are 184 oceanic plateaus covering an area of 18,486,600 km2 (7,137,700 sq mi), or about 5.11% of the oceans. The South Pacific region around Australia and New Zealand contains the greatest number of oceanic plateaus (see map).

Oceanic plateaus produced by large igneous provinces are often associated with hotspots, mantle plumes, and volcanic islands — such as Iceland, Hawaii, Cape Verde, and Kerguelen. The three largest plateaus, the Caribbean, Ontong Java, and Mid-Pacific Mountains, are located on thermal swells. Other oceanic plateaus, however, are made of rifted continental crust, for example Falkland Plateau, Lord Howe Rise, and parts of Kerguelen, Seychelles, and Arctic ridges.

Plateaus formed by large igneous provinces were formed by the equivalent of continental flood basalts such as the Deccan Traps in India and the Snake River Plain in the United States.

In contrast to continental flood basalts, most igneous oceanic plateaus erupt through young and thin (6–7 km (3.7–4.3 mi)) mafic or ultra-mafic crust and are therefore uncontaminated by felsic crust and representative for their mantle sources.

These plateaus often rise 2–3 km (1.2–1.9 mi) above the surrounding ocean floor and are more buoyant than oceanic crust. They therefore tend to withstand subduction, more-so when thick and when reaching subduction zones shortly after their formations. As a consequence, they tend to "dock" to continental margins and be preserved as accreted terranes. Such terranes are often better preserved than the exposed parts of continental flood basalts and are therefore a better record of large-scale volcanic eruptions throughout Earth's history. This "docking" also means that oceanic plateaus are important contributors to the growth of continental crust. Their formations often had a dramatic impact on global climate, such as the most recent plateaus formed, the three, large, Cretaceous oceanic plateaus in the Pacific and Indian Ocean: Ontong Java, Kerguelen, and Caribbean.

Rabbitkettle Formation

The Rabbitkettle Formation is a geologic formation in Yukon, comprising thin bedded silty and occasionally siliciclastic limestones deposited in deep (below storm wave base) waters. It preserves fossils dating back to the Ordovician period.

According to it:

Starts in the Late Cambrian; ends in late Tremadoc

reaches 750m in thickness

transitional slope facies

Alternation of black calcareous mudstones and grey, burrowed wackestones

Swell (ocean)

A swell, in the context of an ocean, sea or lake, is a series of mechanical waves that propagate along the interface between water and air and thus are often referred to as surface gravity waves. These series of surface gravity waves are not wind waves, which are generated by the immediate local wind, but instead are generated by distant weather systems, where wind blows for a duration of time over a fetch of water. More generally, a swell consists of wind-generated waves that are not—or are hardly—affected by the local wind at that time. Swell waves often have a long wavelength, but this varies due to the size, strength and duration of the weather system responsible for the swell and the size of the water body. Swell wavelength also varies from event to event. Occasionally, swells which are longer than 700 m occur as a result of the most severe storms. Swells have a narrower range of frequencies and directions than locally generated wind waves, because swell waves have dispersed from their generation area, have dissipated and therefore lost an amount of randomness, taking on a more defined shape and direction. Swell direction is the direction from which the swell is coming. It is measured in degrees (as on a compass), and often referred to in general directions, such as a NNW or SW swell.

Tempestite

Tempestites are studied throughout sedimentary geology and paleotempestology, they are storm deposits that can be recognized throughout the rock record. The deposit gets its meaning from the word tempest which simply means a violent storm, the tempestite describes the preservation and physical representation of a violent storm event (tempest) within the rock record. Tempestites can be preserved within a multitude of sedimentological environments, including delta systems, estuarian systems, coastal environments, deep sea environments, and even fresh water lacustrine environments. Tempesites most often form in wave dominated delta systems and are preserved within the sedimentological record below fair weather wave base and above storm weather wave base. They are commonly characterized by hummocky cross stratified beds that have an erosive base, and can form under combined flow regimes. This erosive base is often seen in the form of gutter casts.

Thanet Formation

The Thanet Formation is a geological formation found in the London Basin of southeastern England. It is of early to mid-Thanetian (late Paleocene) age and gave its name to that stratigraphic interval. It was previously known as the Thanet Beds, the Thanet Sands and the Thanet Sand Formation. It was named after the Isle of Thanet. The type sections are Herne Bay in Kent for the upper part of the formation and Pegwell Bay for the lower part. It lies unconformably on the Late Cretaceous Chalk Group. It unconformably underlies the Lambeth Group, generally the Upnor Formation but in Essex it is the Reading Formation.

Upper shoreface

Upper Shoreface refers to the portion of the seafloor that is shallow enough to be agitated by everyday wave action, the wave base. Below that is the lower shoreface.

Wind wave

In fluid dynamics, wind waves, or wind-generated waves, are water surface waves that occur on the free surface of the oceans and other bodies (like lakes, rivers, canals, puddles or ponds). They result from the wind blowing over an area of fluid surface. Waves in the oceans can travel thousands of miles before reaching land. Wind waves on Earth range in size from small ripples, to waves over 100 ft (30 m) high.When directly generated and affected by local waters, a wind wave system is called a wind sea. After the wind ceases to blow, wind waves are called swells. More generally, a swell consists of wind-generated waves that are not significantly affected by the local wind at that time. They have been generated elsewhere or some time ago. Wind waves in the ocean are called ocean surface waves.

Wind waves have a certain amount of randomness: subsequent waves differ in height, duration, and shape with limited predictability. They can be described as a stochastic process, in combination with the physics governing their generation, growth, propagation, and decay—as well as governing the interdependence between flow quantities such as: the water surface movements, flow velocities and water pressure. The key statistics of wind waves (both seas and swells) in evolving sea states can be predicted with wind wave models.

Although waves are usually considered in the water seas of Earth, the hydrocarbon seas of Titan may also have wind-driven waves.

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