Wave base

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Wave base diagram. Wavebase.jpg
Wave base diagram.

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]

Physical oceanography The study of physical conditions and physical processes within the ocean

Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.

Contents

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.

Seawater Water from a sea or ocean

Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5%. This means that every kilogram of seawater has approximately 35 grams (1.2 oz) of dissolved salts. Average density at the surface is 1.025 kg/L. Seawater is denser than both fresh water and pure water because the dissolved salts increase the mass by a larger proportion than the volume. The freezing point of seawater decreases as salt concentration increases. At typical salinity, it freezes at about −2 °C (28 °F). The coldest seawater ever recorded was in 2010, in a stream under an Antarctic glacier, and measured −2.6 °C (27.3 °F). Seawater pH is typically limited to a range between 7.5 and 8.4. However, there is no universally accepted reference pH-scale for seawater and the difference between measurements based on different reference scales may be up to 0.14 units.

Radius segment in a circle or sphere (from its center to its perimeter or surface) and its length

In classical geometry, a radius of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the Latin radius, meaning ray but also the spoke of a chariot wheel. The plural of radius can be either radii or the conventional English plural radiuses. The typical abbreviation and mathematical variable name for radius is r. By extension, the diameter d is defined as twice the radius:

Exponential decay probability density

A quantity is subject to exponential decay if it decreases at a rate proportional to its current value. Symbolically, this process can be expressed by the following differential equation, where N is the quantity and λ (lambda) is a positive rate called the exponential decay constant:

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.

Upper shoreface The portion of the seafloor that is shallow enough to be agitated by everyday wave action

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

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.

Lower shoreface The portion of the seafloor, and the sedimentary depositional environment, that lies below the everyday wave base

Lower Shoreface refers to the portion of the seafloor, and the sedimentary depositional environment, that lies below the everyday wave base.

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

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References

  1. 1 2 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