A2GEOGRAPHYREVISIONCOASTALENVIRONMENTS

8.1 WAVESMARINEANDSUBAERIALPROCESSES

KEYTERMSANDDEFINITIONSAbrasion is a type of erosion in which rock fragments carried by waves grind away a surface such as a cliff face.Attrition is the process by which particles of rock being transported by the sea are rounded and become smaller in size due to hitting one another. Particles near the shoreline become smaller and more rounded due to more frequent attrition.Backwash is the movement of water back down the beach dueto the effect of gravity.Constructive wave is a wave with a long wavelength and a lowheight, which helps to build up beaches by deposition.Denudation is the wearing away of the Earth’s surface. It includeserosion, weathering and mass movements.

KEYTERMSANDDEFINITIONSDeposition is the laying-down of material carried by rivers, the wind, glaciers or the sea because of a loss of energy.Destructive wave is a wave with a high height and a short wavelength, which helps erode beach materials and cliffs.Erosion is the wearing away of the Earth’s surface by a moving agent, such as a river, glacier or the sea. In a river, there are several processes of erosion, including hydraulic action, abrasion, attrition and solution. In coastal areas, hydraulic action is the most potent form of erosion.Fetch is the distance of open water over which wind can blow to create waves. The greater the fetch the more potential power waves have when they hit the coast. In the south and west of England the fetch stretches for nearly 6000 km, to South America.

KEYTERMSANDDEFINITIONSHydraulic action is the erosive force exerted by water. It is particularly effective on jointed rocks, especially during storm conditions.Longshore drift is the movement of material along a beach. When a wave breaks obliquely (at an angle to the beach), pebbles are carried up the beach in the direction of the wave (swash). The wave returns to the sea (backwash) at right angles to the beach (direction of steepest slope), carrying material with it. In this way, material moves along a beach. Longshore drift may erode beaches and the formation of spits and bars. Attempts to halt longshore drift include the erection of barriers known as groynes.

KEYTERMSANDDEFINITIONSPrevailing wind is the direction from which the wind most commonly blows in a region. In the British Isles, for example, the prevailing wind is South Westerly, blowing from the Atlantic Ocean and bringing moist and mild conditions.Sea level is an average level of the sea, between high water mark and low water mark.Sediment cell is a length of coastline, which is essentially self contained as far as the movement of sand and other sediment is concerned. The boundaries of the sediment cells generally coincide with large estuaries or prominent headlands.Solution is the process by which the minerals in a rock, notably calcium ions, are dissolved in acid water. It is also referred to as corrosion.

KEYTERMSANDDEFINITIONSSpheroidal weathering is a form of chemical weathering in which concentric or spherical shells of decayed rock are successively separated from a block of rock, which commonly results in the formation of a rounded boulder of decomposition.Swash is the movement of material up the beach in the direction of the prevailing wind.Wave is a circular or elliptical movement of water near the surface of the sea.Wave refraction is the way in which a wave changes shape and loses speed as it comes in contact with the seabed. If refraction is complete, waves break parallel to the coastline. If refraction is not complete, longshore drift occurs.

TOPICSUMMARYCoasts are very varied landscapes and there are a number of factors controlling coastal evolution.● Waves result from friction between wind and the sea surface.● There are three main types of breaker: spilling, plunging and surging.● Constructive waves tend to occur when wave frequency is low, particularly when these waves advance over a gently shelving sea floor.● Destructive waves are the result of locally generated winds,which create waves of high frequency.

TOPICSUMMARYWaves are dominant in some coastal environments, whereas in others it is the tide or winds.● Tides are regular movements in the sea’s surface, caused by the gravitational pull of the Moon and Sun on the oceans.● Coastal areas can be classified into microtidal, which have a very low tidal range of less than 2 m; mesotidal, with a range of between 2 m and 4 m; and macrotidal, with a range of over 4 m.● Storm surges are changes in the sea level caused by intense low-pressure systems and high wind speeds.

TOPICSUMMARYThe change in speed and distortion of the wave fronts is called wave refraction.Waves perform a number of complex and interacting processes of erosion.The main forms of erosion are hydraulic action, abrasion, attrition and solution.In addition, sub-aerial, or cliff-face processes include weathering and mass movements.Sediment transport is generally categorised into two modes: bed load and suspended load.The coastal sediment system, or littoral cell system, is a simplified model in which each cell is self-contained, and where inputs and outputs are balanced.

ADDITIONALWORK1. Outline the main characteristics of constructive and destructive waves.2. Comment on the factors that lead to increased rates of marine erosion.3. Briefly explain the processes of a) longshore drift and b) wave refraction.4. What is a sediment cell? How is an understanding of sediment cells useful in coastal studies?5. With the use of annotated diagrams, distinguish between destructive and constructive waves.6. Explain how human activities can affect sediment cells.

SUGGESTEDWEBSITESwww.seafriends.org.nz/oceano/waves.htm is a very detailed site on wave theory. There are some interesting graphs here.http://webs.cmich.edu/resgi/links.asp?mc=Other+Resource+Links&cad=E&to=265&tod=Coastal+Animations for a number of animations about coasts.http://coastalchange.ucsd.edu/st3_basics/littoralcell.html for littoral cells in California.www.nmm.ac.uk/explore/astronomy-and-time/astronomy-facts/solar-system/tides-and-tidal-forces for tides (this is quite scientific).www.pol.ac.uk/home/insight/tidefaq.html for FAQs on tides.www.eoearth.org/article/Western_Africa_and_coastal_and_marine_environments for a West Africa coastal and marine environments case study.