Waves and tides

The sea is never still. Three kinds of movement keep it busy: waves, tides and currents. They shape the coast, feed sea life, help fishing and ships, and even affect the climate. This lesson covers waves and tides.

Waves. When the sea surface rises and falls again and again, that is a wave. Most waves are made by wind blowing over the water, and they carry energy across the ocean for long distances. The high point of a wave is the crest; the low point is the trough (Fig. 1.3). The distance from one crest to the next is the wavelength. The time a crest takes to move from one point to the next is the wave period, and the number of crests passing a point each second is the wave frequency. (You meet waves in your science book too.)

Waves as coastal architects. Waves are called architects because they build and break the coastline. Picture a headland: hard rock sticking out into the sea, left standing after the softer rock around it wore away. Waves smash into it day after day (Fig. 1.4):

• first a crack opens, forced wider by the water's push (hydraulic action);
• the crack grows into a sea cave;
• the waves cut through the back of the cave, making an arch;
• the top of the arch falls in, leaving a lone pillar, a sea stack;
• the stack wears down into a short stump.

So if you see a stack, you can tell its past: it was once an arch, before that a cave, before that a crack. Waves also drop sand, building beaches.

Storm winds can whip up huge waves. Giant waves caused mostly by undersea earthquakes are tsunamis. On reaching land, a tsunami can climb up to 30 metres above sea level or even more: this height is its run-up height.

Tides (jwar-bhata). Tides are the slow, regular rise and fall of the sea, twice a day in most places. At its highest, the water covers much of the beach: that is high tide (flood tide). At its lowest, it pulls far back from the beach: that is low tide (ebb tide).

Why does this happen? The Moon and the Sun pull on the Earth's water, so the water bulges out on two opposite sides of the Earth (Fig. 1.5).

• Spring tides come when the Sun, Earth and Moon are in a straight line: on full moon (purnima) and new moon (amavasya) days. The pulls add up, so high tides are extra high and low tides extra low. (Spring has nothing to do with the season.)
• Neap tides come at the Moon's first and third quarters. The Sun and Moon pull in different directions, so the gap between high and low tide becomes smaller.

How tides help people.

• Ships: high tide raises the water near the shore, so ships can enter harbours. In some ports the water level changes by 4-5 metres between tides. One of the biggest changes anywhere is in the Gulf of Khambhat: about 12 metres. Before a ship docks (berths), it checks the depth and the tide, especially at low tide. Very large oil tankers (VLCCs) wait hours to cross the shallow Singapore Strait only at high tide, so they do not run aground. Tidal currents can be strong too: 5-6 knots, that is roughly 9-11 km/h.
• Fishing: at high tide many fish come closer to shore for food and shelter, so fishing communities get a better catch.
• Energy: the rising and falling water can turn turbines to make electricity. India has a coastline of 11,098.81 km. Tidal energy is not yet used on a large scale, but good sites have been found in the Sundarbans and in the Gulfs of Kutch and Khambhat.

A tide you can see at work: the Bhadkeshwar Mahadev Temple in Dwarka stands on a small hill in the Arabian Sea. You can walk to it only at low tide, because the stone path goes under water at high tide. High and low tide come twice every day, so visitors simply wait for the low-tide hours.