Waves: energy on the move

Detail

1. Two kinds of ocean movement

Ocean water is always moving. It moves in two directions.

Direction	Forms	Main driver
Horizontal (sideways, across the surface)	Ocean currents, waves	Wind, differences in temperature and density
Vertical (up and down)	Tides, upwelling (cold deep water rising to the surface), sinking (surface water going down)	Pull of the Moon and Sun, density differences

• Waves are a horizontal movement. But inside a wave, each water particle also moves up and down, as explained below.

2. What a wave is

• A wave is energy moving across the sea surface. The water itself does not travel with it.

• The water in a wave hardly moves forward. Waves carry only energy across the sea. Each water particle comes back to where it started after the wave passes [3].

• Two forces make a wave:

• Wind gives the energy. It pushes the water surface.
• Gravity pulls the raised crests back down.

• These two forces take turns, so the surface keeps rising and falling as the energy passes on.

• Wind waves are the most common kind of wave. Two other kinds of wave come from different sources [3]:

• storm surges come from severe storms and low pressure
• tsunamis come from earthquakes or landslides under the sea.

3. Orbital motion of water particles

Orbital motion of water particles means that, under a wave, each bit of water moves in a small circle (an orbit) and does not travel forward with the wave (NCERT Figure 13.1).

• Step by step:
• The crest comes near → the water particle moves up and forward.
• The crest passes → the particle moves down and back.

• The particle ends up almost where it started. One circle is done.

• Result: a floating object such as a bottle or a fishing float only bobs up and down in place. It does not ride along with the wave.

• The motion dies out with depth:
• The circles are largest at the surface and get smaller as you go deeper.
• This motion reaches down only to about half the wavelength [3]. This depth is called the wave base.
• Below the wave base the water is still. So waves seldom stir the deep ocean. A submarine deep under a storm feels almost no waves.

4. How waves grow

• Ripples: a breeze of 2 knots or less (1 knot = 1 nautical mile per hour, about 1.85 km/h) on calm water makes tiny ripples.
• Growth: as the wind gets stronger, the ripples grow into bigger waves.
• White caps: in strong wind, the wave tops break and turn into white foam. These are white caps.
• The wind disturbs the surface where air meets water. The longer it blows, the bigger this disturbance grows [3].

5. Wave height and fetch

Wave height is the vertical distance from the trough (lowest point) to the crest (highest point). The biggest wave height that can form depends on three things:

Factor	Meaning	Effect
Wind strength	How fast the wind blows	Faster wind → higher waves
Duration	How long the wind keeps blowing	Longer time → more energy goes into the water
Fetch	The stretch of open water the wind blows over in one direction	Longer fetch → bigger waves

• Where the biggest waves form: in large open oceans with a long fetch, such as the Southern Ocean. This ocean circles Antarctica, and no land blocks the westerly winds there. These are the "Roaring Forties" and "Furious Fifties", the strong westerly wind belts near 40°S and 50°S.
• Where waves stay small: in small or closed seas, because the fetch is short.

6. Size and shape tell where a wave came from

Feature	Young / local wind waves	Swell
Shape	Steep, short, choppy	Long, low, smooth and regular
Speed and period	Short gap between crests	Slow, long gap between crests
Origin	Raised by wind blowing nearby	Made by distant storms, sometimes in another hemisphere
Link to local weather	Matches the local wind	Can arrive in calm local weather

• Swell means waves that have moved out of the area where they formed. They keep travelling as broad, rounded waves after the wind that made them has stopped.
• Swell can cross a whole ocean basin. This is why swell from the Southern Ocean can reach the Kerala coast (see Kallakkadal below).

7. Characteristics of waves (NCERT terms)

Term	Definition
Crest	Highest point of a wave
Trough	Lowest point of a wave
Wave height	Vertical distance from the bottom of the trough to the top of the crest
Amplitude	Half of the wave height
Wavelength	Horizontal distance between two crests one after another
Wave period	Time between two crests passing the same fixed point
Wave speed	How fast the wave moves through the water; measured in knots
Frequency	Number of waves passing a fixed point in one second

• Trap: amplitude ≠ wave height. Amplitude is half the height.
• Trap: period is a time, and wavelength is a distance.

8. Breaking of waves at the shore

Breaking of waves is when a wave falls over and collapses as it reaches shallow water.

• Cause → process → result:
• The wave enters shallow water near the beach. Now the bottom of the wave touches the sea floor.
• Friction (the rubbing of water against the floor) slows the lower part of the wave. The top keeps moving faster.
• When the water depth becomes less than about half the wavelength, the wave gets steeper. The crest rises higher, leans forward and breaks [3].

• The broken wave turns into foamy surf. It gives up its energy on the coast, wearing away rock and moving sand.

• This energy shapes coastal landforms such as cliffs, wave-cut platforms, beaches, spits and bars (see landforms-evolution).

9. Longshore drift (beyond NCERT)

Longshore drift is the movement of sand and pebbles along the coast. It happens when waves hit the beach at an angle.

• Why it happens: waves usually do not reach the beach parallel to the shore. They arrive at a slight angle, called the angle of wave approach. When they release their energy, they set up a longshore current that flows parallel to the shore [4].
• The zig-zag path:
• Swash (water rushing up the beach) runs up at the same angle as the wave and carries sand with it.
• Backwash (water flowing back to the sea) runs straight down the slope, pulled by gravity.

• Each wave moves the sand a little further along the beach.

• What makes the current faster: a sharper breaking angle, higher waves and a steeper beach slope. A wider angle, a gentler slope and lower waves slow it down [4].

• Effects:
• It builds spits (ridges of sand joined to the land at one end) and bars (ridges of sand lying across a bay or offshore).

• It can erode beaches and damage structures built by people [4].

• Indian examples (east coast, where sand mostly moves south to north on the Tamil Nadu–Andhra coast):

• Chennai (Tamil Nadu): the harbour breakwaters stop the sand moving north.
  • Sand piles up on the south side and has widened Marina beach.
  • Beaches to the north (Royapuram, Ennore side) get no new sand, so they are eroding.

• Visakhapatnam (Andhra Pradesh): the port pumps sand across the harbour entrance (sand bypassing). This keeps sand flowing to beaches on the far side and keeps the channel free of sand.

• Lesson: hard structures such as ports, groynes and breakwaters cut off the sand supply to the coast further along. This is a key point in coastal zone management.

10. Kallakkadal: swell surge on the Kerala coast (beyond NCERT)

Kallakkadal ("sea thief" in Malayalam: kallan = thief, kadal = sea) is sudden coastal flooding on the Kerala and south-west Indian coast. It comes without warning from the local weather.

• Cause → process → result:
• Strong storms blow over the Southern Ocean, far south of India.
• They create long-period swells, which travel thousands of kilometres north across the Indian Ocean.
• The swells reach the Kerala coast (about 8°N–12.8°N) and push seawater onto the land, even when local winds are calm [2].

• Low-lying fishing villages, houses and boats are flooded.

• Why it is called a "thief": the local wind and sky show no sign of danger, so the sea "steals" onto the land.

• Not a tsunami: a tsunami is caused by an earthquake or landslide under the sea. Kallakkadal is caused by wind-made swell from distant storms.
• Forecasting:
• INCOIS (Indian National Centre for Ocean Information Services, Hyderabad, under the Ministry of Earth Sciences) launched the Swell Surge Forecast System in 2020.
• INCOIS also runs SIVAS (Swell-Surge Inundation Vulnerability Advisory System). It is built for the Kerala coast and forecasts how far the sea will flood the land up to 3 days ahead. Its warnings come in English, Hindi and Malayalam [2].

11. Linked topics

• Landforms made by waves (cliffs, sea caves, stacks, beaches, spits, bars) → landforms-evolution.
• Tsunamis (caused by earthquakes or landslides, not wind) → natural-hazards-disasters.

Prelims Hooks

• In a wave, energy moves forward. The water only moves in circles (orbital motion), so a floating object bobs in place.
• Orbital motion reaches down only to about half the wavelength (wave base). Deep water stays still under waves.
• Amplitude = half the wave height. Wave height = trough to crest.
• Wavelength = distance between two crests. Wave period = time between two crests. Frequency = waves per second. Wave speed is measured in knots.
• Wave height depends on wind speed, wind duration and fetch. It does not depend on water salinity or ocean depth in the open sea.
• Biggest wind waves: open Southern Ocean (longest fetch).
• Swell = long, regular waves from distant storms. Steep, short waves = young, local waves.
• A wave breaks when depth < about half the wavelength.
• Longshore drift: swash goes up at an angle, backwash comes straight down. Result: spits and bars.
• Kallakkadal – Kerala – Southern Ocean swell – INCOIS Swell Surge Forecast System (2020). It is not a tsunami.

Mains Points

• Coastal erosion caused by people (GS-I/GS-III):
• Breakwaters at Chennai stop longshore drift. Marina beach grows while the beaches to the north erode.
• Sand bypassing at Visakhapatnam shows one fix.

• Port planning under Sagarmala should include sediment-budget studies and soft measures such as beach nourishment.

• Swell surge as a hidden hazard (GS-III, disaster management):

• Kallakkadal floods the Kerala coast with no local warning, hurting fishers' homes and boats.

• INCOIS's Swell Surge Forecast System (2020) and SIVAS give a lead time of up to 3 days, with warnings in Malayalam [2]. This shows ocean science helping people on the ground.

• Distribution of wave energy (GS-I):

• Long fetch and steady westerlies make the Southern Ocean and the west-facing coasts the most wave-exposed.

• So the west coast of India gets more swell-driven erosion and flooding in the monsoon months than sheltered gulfs do.

• Waves as a resource (GS-III):

• Wave energy comes from wind energy carried across the ocean. Coasts with steady swell are possible sites for wave power, alongside tidal sites at Khambhat, Kachchh and the Sundarbans.