Waves

Notes

Waves are oscillations of the water surface, mostly caused by wind friction on the sea. The water particles move in circular orbits, so the wave form travels but the water itself does not, until the wave breaks near the shore.

Terms: crest, trough, wave height (trough to crest), amplitude (half the height), wavelength (crest to crest), period (time between two crests), frequency, speed, and fetch (the distance over which the wind blows).

Factors: wind speed, duration and fetch decide the size of the wave.

Types

Sea (wind waves): local, irregular waves.
Swell: regular, long waves that have travelled far from their storm origin.
Surf (breakers): waves that break as they reach shallow water, when the depth is too small for the circular motion.
Constructive waves: low and long; the swash is stronger than the backwash and builds beaches. Destructive waves: high and steep; the backwash is stronger; they erode.
Wave refraction bends the waves to align with the coast and concentrates energy on headlands. Longshore drift moves sediment along the coast.

Tsunami

A Japanese word for "harbour wave"; a seismic sea wave caused by an underwater earthquake (thrust fault), a volcanic eruption or a landslide.
A very long wavelength (over 100 km); it moves at about 700-800 km/h in deep water with an amplitude of less than 1 m, but grows in height in shallow water near the coast.
2004 Indian Ocean tsunami: the Sumatra-Andaman earthquake (magnitude about 9.1, 26 December 2004); over 200,000 deaths. 2011 Tohoku (Japan) tsunami.
India's Indian Tsunami Early Warning Centre (ITEWC) at INCOIS, Hyderabad, has operated since 2007 and works with the Pacific and Indian Ocean warning systems.

Other: storm surges, seiches, rogue waves, and the potential of wave energy.

Key Points

Waves are caused by wind; the form moves, not the water.
Terms: crest, trough, height, wavelength, period, fetch.
Types: sea, swell, surf; constructive vs destructive.
Tsunami: seismic sea wave; fast in deep water; grows near the coast.
ITEWC at INCOIS, Hyderabad.

MCQs

Q1. The distance over which the wind blows across the sea to create waves is called the:

(a) Wavelength
(b) Fetch
(c) Swash
(d) Period
Ans: (b).

Q2. The Indian Tsunami Early Warning Centre is located at:

(a) Pune
(b) Hyderabad
(c) Chennai
(d) Mumbai
Ans: (b). At INCOIS.

Q3. Tsunamis are caused mainly by:

(a) Wind
(b) Undersea earthquakes
(c) Tides
(d) Ocean currents
Ans: (b).