2. Interactions between oceans and coastal places

Coasts

The highest surface part of a wave is called the crest, and the lowest part is the trough. The vertical distance between the crest and the trough is the wave height. The horizontal distance between two adjacent crests or troughs is known as the wavelength.

Wave height is affected by wind speed, wind duration, or how long the wind blows, and fetch, which is the distance over water that the wind blows in a single direction. If wind speed is slow, only small waves result. If the wind speed is great but it only blows for a few minutes, no large waves will occur. Also, if strong winds blow for a long period of time but over a short fetch, no large waves form. Large waves occur only when all three factors combine (Duxbury, et al, 2002.)

A coastal zone is the interface between the land and water. These zones are important because a majority of the world's population inhabit such zones. Coastal zones are continually changing because of the dynamic interaction between the oceans and the land.

The littoral zone is the near shore area where sunlight penetrates all the way to the sediment and allows aquatic plants (macrophytes) to grow. Light levels of about 1% or less of surface values usually define this depth.

Over 70 percent of the planet's surface is covered by water, the majority of which is in the world's seas and oceans. Half the world's population lives within 60 km of the sea, and three-quarters of all large cities are located on the coast.

Constructive WAves

Small oceans with small fetch develop constructive waves. Constructive waves have low wave height and long wave length with low frequency, between 6 and 8 waves per minute. Constructive waves are associated with weak backwash and strong swash, which builds up wide flat beaches and so more associated with coasts of deposition. Constructive waves also tend to form sandy beaches.

Destructive waves

Large oceans with large fetch produce large waves, called destructive waves. These waves have large wave height and short wave length and are characterised by tall breakers that have high downward force and a strong backwash. They have high frequency, over 10 waves per minute. This downward energy helps erode clifs. In addition, due to a dominant backwash they erode the beach making for narrow steep beach profiles. Localised storms with high wind speed also form destructive waves as well as steep depth gradients around headlands. Destructive waves also tend to form pebbly beaches.

Tides

The ocean’s tides are controlled by the gravitational pull of the Moon, and to a lesser extent the Sun. The Moon pulls the water in the ocean towards it, creating a bulge of water; a high tide. The Moon not only pulls the water but also pulls the Earth towards it, this creates a second bulge of water and the second high tide on the other side of the Earth. Twice a month the Earth, Moon and Sun are aligned: this puts an extra gravitational pull on the tidal bulge, to produce an extra high tide called a spring tide.

When the Sun and Moon are at right angles to each other, neap tides occur, when the tidal range is lowest. Figure 4 shows the influence of the Moon and Sun on the Earth’s tides. When a spring tide coincides with an onshore gale, a storm surge can occur, which can lead to exceptionally high seas and flooding, as in the East coast floods of 1953 and the ‘near miss’ of November 2007.

The tidal range is the vertical distance between high tide and low tide, and this coincides with the sweep zone for the September 2008. The slope of the shoreline and the tidal range determine the amount of shore exposed to wave action A low tidal range tends to produce a narrower beach, which is prone to higher erosion; such beaches are found on the shores of seas such as the Mediterranean, rather than oceans. Higher tidal ranges are found on ocean coasts, such as the Atlantic coasts of Britain and Canada

Types of Erosion

Sediment transportation

Traction

Saltation

Suspension

Solution

Solution - minerals are dissolved in the water and carried along in solution.
Suspension - fine light material is carried along in the water.
Saltation - small pebbles and stones are bounced along the sea bed.
Traction - large boulders and rocks are rolled along the sea bed.

Longshore Drift (littoral drift)
Longshore drift is a process responsible for moving significant amounts of sediment along the coast. This usually occurs in one direction as dictated by the prevailing wind. For example, the prevailing wind along the Holderness Coast is north-easterly. As the result waves break on to the beach obliquely at an angle of around 45 degrees. The swash moves beach material along the beach and the backwash, under gravity, pulls the material back down the beach at right angles to the coastline. Over time this creates a net shift of material along the coast.