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Nature Gallery (Earth - Ocean)

Tides and Waves

Tides

Anyone who has visited a beach can understand the importance of tides. At low tide, beach-goers can walk along the exposed shoreline or construct sandcastles; high tide brings water further up the beach, washing away the castles and forcing people to higher ground.

In the world of commerce, tides are vitally important to the shipping and fishing industries. Navigators of heavy freighters heading for shallow ports must schedule their arrival for high tide, when the water is deepest. Fishing boats are also subject to the ebb and flow of tides. Predicting these changes is an essential aspect of maritime travel, and highly detailed tide tables are available for almost all coastal areas.

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Lunar Tides
The gravitational pull of the moon and the sun on the Earth’s oceans is the major force involved in the creation of tides. Because the moon is much closer to the Earth than the sun, it has more gravitational attraction. When the moon is directly over a given point on the Earth’s surface, it exerts a powerful pull on the water there, which consequently rises above its normal level.

At the same time, the water covering the part of the Earth that is most distant from the moon bulges outwards as a result of the centrifugal force of the revolving Earth-moon system. This is the same force that pulls a child away from the centre of a revolving merry-go-round. The Earth-moon system revolves around a common centre, which is similar to the centre of a merry-go-round. A child close to the outer edge of a merry-go-round is pulled outwards more than one near the centre. Likewise, as the Earth revolves around the common centre of the Earth-moon system, the area furthest from the moon is subject to the greatest centrifugal force. As a result, the waters on that side swell outwards.

This means there are always two high-water areas on the Earth at any given time: the area under the moon and the area opposite the moon. Low tides exist in the areas between these high-tide bulges.

Low and high waters alternate in a continuous cycle. High tides move with the moon as it revolves around the Earth. At most shores throughout the world, two high tides and two low tides occur every lunar day, which is the time it takes for the moon to return to a point on the Earth—about 24 hours and 50 minutes. Thus, a typical seaport will alternate between high and low tides about every 6 hours. Some coasts, such as the shores along most of Antarctica, experience only one high tide and one low tide each lunar day. These variations in the tidal cycles occur because of several factors, including topography and latitude.

The difference in height between high and low tides is called the range of tide. The greatest tides in the world occcur in the Bay of Fundy, a large, funnel-shaped inlet of the North Atlantic Ocean that separates the Canadian provinces of New Brunswick and Nova Scotia. The Bay of Fundy divides into two arms: Chignecto Bay to the north, and Minas Basin to the south. The funnel effect on the water in these narrowing arms increases the tidal range of the bay: the water may rise by as much as 15 metres (50 feet). At Fundy National Park in New Brunswick, Canada spectacular inundations occur over the course of a few hours.

Solar Tides

Like the moon, the sun exerts a gravitational pull on the Earth but, because it is so far away, has a tide-raising force of only about half that of the moon. The sum of the forces exerted by the moon and the sun creates a tide with two crests. Where these extremes occur depends on the relative positions of the sun and moon at the time. During the periods of new and full moon—when the sun, moon, and Earth are directly in line—the solar and lunar tides coincide. This alignment results in a spring tide, when the high tide is higher than usual, and the low tide is lower than usual. When the moon is in its first or third quarter, it is at right angles to the sun relative to the Earth, and water is subject to the opposing forces of the sun and moon. This produces a neap tide, with a lower than normal high tide and higher than normal low tide.

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Tidal Currents and Bores
Accompanying the vertical rise and fall of water are various horizontal or lateral movements commonly known as tidal currents or tidal streams. When the high tide is moving in, a tidal current known as the flood current flows towards the shore for about six hours. At high tide, the current reverses and flows away from shore. This is the ebb current.

At low tide about six hours later, the current reverses again. The exact times may vary depending on local conditions. During the period of reversal, the water is in a state of rest or calm, known as slack water. Like the cyclical tides, these reversing currents are important to seagoing vessels. Ships entering a river’s mouth, heading for an upstream port, receive a boost from a flood current.

When a large volume of water from a flood current accumulates in the mouth of a funnel-shaped river or estuary more rapidly than it can flow upstream, a crested wave, or bore, gradually forms and rushes up the channel. The highest bores in the world reach heights of more than 4.5 metres (15 feet). Notable bores occur at the mouths of the Qiantang Jiang estuary on the Hangzhou Wan in China, of the Petitcodiac River in the Bay of Fundy, of the River Severn of the United Kingdom, and of the Amazon in Brazil. During a spring tide, the Amazon’s bore may travel 650 kilometres (400 miles) upstream at a speed of over 65 kilometres per hour (40 miles per hour).

Tidal Energy

The energy from tides has been harnessed to produce electricity. In 1966 a tidal power plant with a capacity of 240,000 kilowatts went into operation in northwestern France on the Rance River, an estuary of the English Channel. The incoming tide of the river flows through a dam near the city of Saint-Malo, driving turbines, and then is trapped behind the dam. When the tide ebbs, the trapped water is released and flows back through the dam, again driving the turbines. Such tidal powerplants are most efficient if the difference between high and low tides is great, as in the Rance estuary, where the tidal range is 8.5 metres (28 feet).

Waves

Waves are generated by winds. Strong winds create rough waters, and light winds cause the water to be more calm. During storms, violent winds create storm surges that can temporarily raise the sea level in low-lying areas and cause serious flood damage to coastal villages.

Waves are the most significant erosive force along coasts. Wave action constantly batters shorelines, causing the erosion of beach sands and the undercutting of cliffs. Over time, waves sculpt caves in the sides of outcroppings, and these deepen until the outcroppings become distanced from the shore. The resulting rocky promontories, such as those in the Algarve region of southern Portugal, are called sea stacks.

Tides and waves also affect river deltas by transporting sediment along the shore or out to sea. Wave action, under certain conditions, creates sandbars and spits along coastlines.

Tsunamis
Tsunami is the Japanese word for a seismic sea wave, which is a large wave generated by an underwater earthquake. Tsunamis have erroneously been called tidal waves, but are not in fact connected with tides. Tsunamis are triggered when the ocean floor is tilted or offset during a quake. 

Other possible causes are an underwater landslide or volcanic eruption. Most tsunamis originate along the so-called Ring of Fire, a zone of volcanoes and seismic activity that encircles the Pacific Ocean. Since 1819, about 40 tsunamis have struck the Hawaiian Islands.

Reaching speeds of about 725 to 800 kilometres per hour (450 to 500 miles per hour), a tsunami can cross an entire ocean in a few hours. In the deep ocean, the wave is virtually imperceptible, usually less than 1 metre (3 feet) high. 

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Upon entering shallow coastal waters, however, a tsunami is forced to slow down and suddenly grows in height. By the time it reaches the shore, it may be a towering wall of water 15 metres (50 feet) high or more, capable of destroying entire coastal settlements.

On 17 February 1996 an earthquake measuring 7.0 on the Richter scale occured off the coast of Indonesia’s Biak Island, north of New Guinea, causing 6-metre (21-foot) waves to wash over the coast. The death toll reached 102, and 50 people were reported missing. More than 3,000 homes on Biak and the surrounding islands were washed away by the tsunami.

Scientists can predict when and where tsunamis will occur by determining the focus and strength of the precipitating earthquake. Residents in low-lying coastal areas may have time to move to higher ground after warning of a distant underwater earthquake, but they may be caught off guard by a nearby offshore quake.