Waves

1. Nature and terminology of ocean waves

A wave is a propagating disturbance that transfers energy through water. In an ideal surface gravity wave, water particles mainly oscillate around their mean positions rather than travelling with the crest. The familiar statement that waves transfer energy, not water, is therefore a useful first approximation. Real waves nevertheless produce some net particle drift, called Stokes drift, and breaking waves drive currents and sediment movement.

The crest is the highest point and the trough the lowest point. Wave height is the vertical crest-to-trough distance, while wavelength is the horizontal distance between successive crests. Wave period is the time between successive crests passing a fixed point; frequency is the number of cycles per unit time. Phase speed, or celerity, equals wavelength divided by period. Wave steepness is the ratio of height to wavelength and helps determine wave instability.

Wave energy increases approximately with the square of wave height. Thus, a doubling of height represents roughly four times the energy per unit surface area under comparable conditions. Forecasts commonly report significant wave height: traditionally, the average height of the highest one-third of waves in a record. It is not the maximum possible wave height.

• Surface tension restores very small capillary waves; gravity is the main restoring force for larger surface waves.
• Internal waves occur along density interfaces within the ocean and may have large amplitudes without prominent surface crests.

2. Generation, wind seas and swell

Wind transfers energy to the sea through friction and pressure variations over an initially disturbed surface. Small ripples develop, allowing further energy transfer and growth into gravity waves. Three controls are especially important: wind speed, the duration for which wind blows and fetch, the uninterrupted distance over water across which it blows in a broadly consistent direction.

A short fetch limits wave growth even under strong winds. Conversely, sustained winds over a large ocean basin can produce high waves with long periods. Growth is not unlimited: breaking, friction and other dissipative processes balance energy input. Waves generated directly by local winds form a wind sea, typically irregular and containing several wavelengths and directions.

After waves leave their generating area, they become swell. Swell is generally more regular and longer-period than local wind seas and can travel thousands of kilometres. In deep water, longer-period waves travel faster, separating from shorter-period waves through dispersion. For deep-water gravity waves, phase speed increases with the square root of wavelength, while the wave group carrying energy travels at half the phase speed.

India's west coast commonly experiences energetic seas during the southwest monsoon. It can also receive long-period swell from distant Southern Ocean storms despite relatively calm local weather. Such swell can cause unexpected overtopping and coastal flooding, demonstrating why local wind conditions alone do not reliably indicate marine safety.

3. Behaviour in deep and shallow water

Deep-water waves occur where water depth exceeds approximately half the wavelength. Their motion is largely unaffected by the seabed. Water particles follow nearly circular paths whose size decreases rapidly with depth. The conventional wave base, approximately half a wavelength, is a practical limit below which orbital motion is small rather than an abrupt boundary.

Shallow-water waves occur where depth is less than approximately one-twentieth of the wavelength; intermediate depths form the transitional zone. Particle paths become flattened ellipses because the seabed restricts vertical motion. Shallow-water wave speed is approximately the square root of gravitational acceleration multiplied by water depth. Consequently, speed depends primarily on depth rather than wavelength.

As a wave approaches a gently sloping shore, it usually slows and its wavelength shortens, while its period remains nearly constant. Shoaling can increase wave height as energy is concentrated into slower-moving waves. Increasing steepness eventually makes the wave unstable, producing breakers. Spilling breakers usually occur on gentle slopes, plunging breakers on steeper slopes, and surging breakers can occur on very steep shores. Beach slope and incident wave steepness jointly influence breaker type.

• Refraction: bending caused by unequal wave speeds along a crest; wave fronts tend to become more parallel to depth contours.
• Diffraction: lateral spreading of wave energy around obstacles or through gaps, including harbour entrances.
• Reflection: return of wave energy from a steep coast or structure; interaction with incoming waves may produce standing patterns.

4. Waves as coastal geomorphic agents

Breaking waves reshape coasts through hydraulic action, abrasion, sediment transport and deposition. Repeated attack at the base of a cliff may form a wave-cut notch. Collapse of the unsupported rock causes cliff retreat and leaves a wave-cut platform. Erosion along weaknesses in headlands can produce caves, arches, stacks and eventually stumps, although actual sequences depend on rock structure and local conditions.

Wave refraction commonly concentrates energy on projecting headlands and spreads it within bays. Headlands therefore tend to undergo stronger erosion, while sheltered bays often favour sediment accumulation. This is a general tendency, not a universal rule: geology, sediment supply, currents and coastal structures can modify the outcome.

When waves approach a beach obliquely, swash moves sediment up the beach at an angle, while backwash usually returns downslope more directly. Combined with wave-generated longshore currents, this produces alongshore sediment transport. Beaches, spits and some bars reflect this redistribution. Gentle, low-steepness waves often favour beach accretion, whereas energetic storm waves commonly remove sediment offshore.

Rip currents are concentrated seaward flows that return water accumulated near the shore by breaking waves. They are currents, not waves or tides. Groynes and breakwaters can protect particular sites or trap sediment but may reduce sediment delivery to downdrift beaches. Coastal management should therefore consider the wider sediment-sharing coastal unit rather than isolated stretches.

5. Tsunamis, extreme waves and risk reduction

Tsunamis arise from sudden water displacement, commonly caused by vertical seabed movement during major submarine earthquakes. Landslides and volcanic activity can also generate them. Because their wavelengths can extend over hundreds of kilometres, tsunamis behave as shallow-water waves even in the deep ocean. They may have modest heights offshore but extremely high speeds; in 4,000 metres of water, the shallow-water speed formula gives approximately 710 kilometres per hour.

Near land, tsunami speed decreases, wavelength shortens and water levels can rise destructively through shoaling and local amplification. Run-up is the maximum vertical elevation reached above a reference sea level, whereas inundation describes horizontal penetration inland. A tsunami arrives as a sequence of waves, and the first need not be the largest. A receding sea can be a natural warning, but it does not precede every tsunami.

A storm surge is an abnormal rise in sea level generated mainly by strong cyclone winds, with low atmospheric pressure also contributing. It differs from individual wind waves, although both can combine with astronomical tide to intensify coastal flooding. Rogue waves are exceptionally large individual waves relative to the surrounding sea state; they are neither synonymous with tsunamis nor restricted to shallow water.

India's Indian Tsunami Early Warning Centre, established at INCOIS in 2007, combines seismic observations, sea-level measurements, modelling and decision-support systems. Effective risk reduction also requires evacuation maps, drills, accessible warnings and protection of natural coastal buffers where appropriate. Strong or prolonged coastal shaking should prompt immediate movement to higher ground without waiting for an official message.

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