OCEANIC WAVE Introduction A wave is a disturbance

OCEANIC WAVE

Introduction: A wave is a disturbance in a medium that carries energy without a net movement of particles. Ocean waves are formed as wind blows across the surface of the ocean, creating small ripples, which eventually become waves with increasing time and distance. When waves reach shallow water, they become unstable and begin to break and can impose large hydrodynamic forces on organisms living in these regions.

Formation of Wave: The ocean is in constant motion. Waves form as a result of the water’s motion, gravitational forces, and winds. The most common waves we see are created by wind. However other waves include those created by gravitational forces (e. g. tidal waves) and those created by underwater disturbances, such as earthquakes (e. g. tsunamis).

There are three main factors that affect wave formation: wind velocity, fetch, and duration. Wind velocity is the speed of the wind. Fetch is the area of ocean surface where wind blows in an essentially constant direction, thus generating waves. It is the distance over the water that the wind can blow uninterrupted (which can be huge distances out at sea) Duration is the amount of time the wind blows over the fetch. The greater the wind velocity, the longer the fetch, and the greater duration the wind blows, then the more energy is converted to waves and the bigger the waves. However, if wind speed is slow, the resulting waves will be small, regardless of the fetch or duration. It takes all three factors acting together to create big waves.

Terminology related to Wave Amplitude (A): The amplitude of a wave is the maximum displacement of any particle of the medium from its equilibrium position. Period (T): Period (T) of a wave is the time taken by any particle of the medium to complete one vibration during a period (T). Wavelength (λ): Wavelength (λ) is equal to the distance between two consecutive particles of the medium which are in the same state of vibration & it is equal to the distance travelled by the wave by its time period (T). Crest & Trough: A crest point on a wave with the maximum value of upward displacement within a cycle. A trough is the opposite of a crest, so the minimum or lowest point in a cycle.

Frequency (f): It is the number of vibrations made per second by any particles of the medium (f = 1/T). Since the frequency of a wave is a characteristic property of the source which is producing the wave motion, hence, the frequency of a wave does not change when a wave travels from one medium to another medium. The Height (H) of a wave is the distance between the top, or crest, of the wave, and the bottom, or trough, of the wave. The Steepness is simply the ratio of height and length (H/L)

Phase or Phase Angle (Φ): It represents the state of vibration of the particle of a medium with respect to its mean position. Phase Difference Δ(Φ): It represents the different state of vibration of a particle at two different instants (or) any pair of particles at the same instant. ΔΦ = Φ 2 – Φ 1. Wave Velocity (v): It is the distance travelled by the wave in one second (v = λ/T). It is determined by the mechanical properties of the medium through which the wave propagates. The velocity of wave motion is measured with respect to the medium, the wave velocity changes when the medium is in motion.

Types of Wave

Longitudinal and Transverse Longitudinal Waves In a longitudinal wave the particle displacement is parallel to the direction of wave propagation. The particles do not move down the tube with the wave; they simply oscillate back and forth about their individual equilibrium positions. The P waves (Primary waves) in an earthquake are examples of Longitudinal waves. The P waves travel with the fastest velocity and are the first to arrive.

Transverse Waves In a transverse wave the particle displacement is perpendicular to the direction of wave propagation. The particles do not move along with the wave; they simply oscillate up and down about their individual equilibrium positions as the wave passes by. The S waves (Secondary waves) in an earthquake are examples of Transverse waves. S waves propagate with a velocity slower than P waves, arriving several seconds later.

Standing Wave and Progressive Wave Progressive waves or Travelling Wave: The disturbance produced in the medium travels onward, it being handed over from one particle to the next. Each particle executes the same type of vibration as the preceding one, though not at the same time. The amplitude of each particle is the same but the phase changes continuously, No particle is permanently at rest. Different particles attain the state of momentary rest at different instants, All the particles attain the same maximum velocity when they pass through their mean positions.

Progressive waves

Stationary waves or standing Wave There is no onward motion of the disturbance as no particle transfers its motion to the next. Each particle has its own characteristic vibration. The amplitudes of the different particles are different, ranging from zero at the nodes to maximum at the antinodes. All the particles in a given segment vibrate in phase but in opposite phase relative to the particles in the adjacent segment. The particles at the nodes are permanently at rest but other particles attain their position of momentary rest simultaneously. Energy is not transported across any plane.

Classification Period band Generating forces Restoring forces Capillary waves <0. 1 s Wind Surface tension Ultragravity waves 0. 1– 1 s Wind Surface tension and gravity Gravity waves 1– 20 s Wind Gravity Infragravity waves 20 s to 5 min Wind atmospheric pressure gradients Gravity Long‐period waves 5 min to 12 h Atmospheric pressure gradients and earthquake Gravity Ordinary tidal waves 12– 24 h Gravitational attraction Gravity and Coriolis force Transtidal waves >24 h Storms and Gravity and Coriolis gravitational attraction force

Capillary Waves The shortest‐period waves, and the first to be noticed on the ocean surface when wind starts blowing, are the capillary waves , which resemble cat 's paws ripping the otherwise smooth surface (Kinsman, 1965). This peculiar wavy structure is generally forced by a light breeze of speeds of about 3 m/s (taken at a reference height of 10 m from the water level) and assumes a fine structure of small ripples with a wavelength of less than 1. 5 cm and period less than 0. 1 s

Capillary Wave

Gravity Waves: Wind Sea and Swell A consistent blowing of wind over a substantial fetch (i. e. , the distance over which the wind blows) forces waves to become much longer than the threshold wavelength of 1. 7 cm. As the wavelength grows longer than 1. 5 m (i. e. , wave period becomes larger than 1 s), surface tension becomes negligible and gravity remains the sole restoring mechanism. Under these circumstances, waves are classified as gravity waves. Generally speaking, gravity waves assume periods ranging from a minimum of about 1 s up to maximum of approximately 25 s (i. e. , wavelength varies roughly between 1. 5 and 900 m).

Swell 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 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

Infra-gravity Waves Nonlinear interactions between wave components convert part of the energy associated to wind‐generated gravity waves into subharmonics with periods ranging from about 20 to 30 s up to a maximum of approximately 5 min (Herbers, Elgar, and Guza, 1995 a). These long oscillations, which are driven primarily by swell (Tucker, 1950), are bound to the generating wave trains and are normally known as infra-gravity waves. Infra-gravity waves are indirectly formed by the wind. Their creation is linked to the presence of short-wave groups, which are formed due to the superposition of two different short-wave trains, with wave lengths and frequencies that are very similar. When the waves are in phase their amplitudes are added, and when they are out of phase their amplitudes damp each other out. This results in a wave group structure, which is irregular in shape due to the various frequencies present in a natural wave field.

Long‐Period Waves (Tsunamis, Seiches, and Storm Surges) Well‐defined waves with periods longer than 5 min are routinely recorded in the ocean (Nielsen, 2009). Although different originating mechanisms can be responsible for such waves, meteorological conditions and earthquakes remain the primary cause. Normally, long oscillations generated by atmospheric conditions are known as seiches and storm surges , while tsunamis identify waves originated from earthquakes. Despite the long wavelength, the restoring mechanism is still dominated by gravity.

Tsunamis: Tsunamis are ocean waves triggered by: § Large earthquakes that occur near or under the ocean § Volcanic eruptions § Submarine landslides § Onshore landslides in which large volumes of debris fall into the water Tsunamis are long waves with period varying between 1 and 20 min (wavelength from a few kilometers up to a few hundreds of kilometers) that are generated by sudden tectonic changes to the sea bed or landslides that are usually attributed to earthquakes and submarine volcanic activity

Tsunamis typically consist of multiple waves that rush ashore like a fast-rising tide with powerful currents. When tsunamis approach shore, they behave like a very fast moving tide that extends much farther inland than normal water. If a tsunami-causing disturbance occurs close to the coastline, a resulting tsunami can reach coastal communities within minutes. A rule of thumb is that if you see the tsunami, it is too late to outrun it. Even small tsunamis (6 feet in height, for example) are associated with extremely strong currents, capable of knocking someone off their feet. As a result of complex interactions with the coast, tsunami waves can persist for many hours.

Seiches: Seismic Seiches are standing waves set up on rivers, reservoirs, ponds, and lakes when seismic waves from an earthquake pass through the area. They are in direct contrast to tsunamis which are giant sea waves created by the sudden uplift of the sea floor. A Seiches is a standing wave in an enclosed or partially enclosed body of water. Seiches and Seiches-related phenomena have been observed on lakes, reservoirs, swimming pools, bays, harbours and seas. The key requirement formation of a Seiches is that the body of water be at least partially bounded, allowing the formation of the standing wave. Seiches are often imperceptible to the naked eye, and observers in boats on the surface may notice that a seiche is occurring due to the extremely long wavelengths.

Storm Surges Storm surge is the abnormal rise in seawater level during a storm, measured as the height of the water above the normal predicted astronomical tide. The surge is caused primarily by a storm’s winds pushing water onshore. The amplitude of the storm surge at any given location depends on the orientation of the coast line with the storm track; the intensity, size, and speed of the storm; and the local bathymetry. Storm tide is the total observed seawater level during a storm, resulting from the combination of storm surge and the astronomical tide.

Surface Wave: surface wave is a mechanical wave that propagates along the interface between Water and atmospheric air. Waves affect a large number of marine activities and biochemical processes that take place on and below the ocean surface. Surface waves can be divided into two types. The first type is called the Love or L waves only move along the earth's surface and travel slightly slower than S waves. The second type of surface wave is known as a Rayleigh or R wave. The pieces of material disturbed by a Rayleigh wave move both vertically and horizontally.

Internal Wave: Internal waves are waves of the interior ocean. They can exist when the water body is stratified, i. e. , when it consists of water layers of different densities. This difference is usually caused by a difference in water temperature, but it can also be caused by a difference in salinity as in the Strait of Gibraltar. In order to generate internal waves, the interface must be disturbed. There are many ways to disturb the interface, e. g. , by: water being pushed by the action of the tide over irregularities in shallow bathymetry, the transit of a surface ship or a submarine, river outflow.

Breaking of Wave: As waves approach the shore, the bottom of the wave meets the ocean floor. As they drag across the bottom, the front waves slow down, and wavelength is reduced. The following waves start to build up behind the slow ones, and as the wavelengths get shorter, the wave energy gets transferred upwards, increasing wave height. The friction along the bottom slows the base of the wave down while the water at the surface continues forward. When the wave steepness (the ratio between wave height and wavelength) exceeds a ratio of 1: 7, it becomes unstable and breaks. The slope of the sea floor greatly influences how quickly the sea floor affects the waves as the waves get closer to shore, and therefore how the waves break.

Swash & Backwash Swash is when the waves comes towards the beach/land. Backwash is the opposite - it moves away from the beach/land. If Backwash is stronger that their swash so they remove material from the beach. They are frequent in number – between 10 -15 a minute. They are tall so have more power when they break scouring away at beach material. They create a steep narrow beach. If swash is stronger than their backwash causing them to deposit material on the beach. They are less frequent – 6 -9 a minute Long but not high waves so they roll onto the beach so do not erode. Create a wide gently sloping beach.

Beaufort scale The Beaufort scale is an empirical measure that relates wind speed to observed conditions at sea or on land. Its full name is the Beaufort wind force scale. Developed in 1805 by Sir Francis Beaufort, U. K. Royal Navy The Beaufort scale is not an exact nor an objective scale; it was based on visual and subjective observation of a ship and of the sea. The corresponding integral wind speeds were determined later, but the values in different units were never made equivalent.

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