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Coastal Processes - WavesThe most common force for creating waves is the wind. As the wind blows across a smooth sea surface, the friction between the air and water results in small wrinkles or ripples. As the surface becomes rougher it is easier for the wind to grip the roughened water surface and add more energy, increasing the size of the waves. The energy of the wave is related to the height of the wave, the larger the wave the more energy it has.
Only when all three of these factors occur together are waves of large sizes produced. For example; a small breeze blowing over a large fetch will only produce small waves, likewise a strong wind blowing for a short time over a large fetch will only produce small waves and if strong wind blow over a long time with a small fetch only small waves form.
The highest part of a wave is called the crest and the lowest part is called the trough. Waves are generally divided into two categories; deep-water waves and shallow water waves. Deep-water waves are independent of the seabed where as shallow water waves are influenced by the seabed topography. In shallow waters the waves behaviour can be affected by the topography of the seabed. When a wave feels the seabed it slows down and reduces the speed, distance and time between waves. This results in increased height and steepness of waves. At a certain point the seabed is shallow enough to form high steep waves that topple forward. The structure and shape of the shallow seabed can affect waves in other ways. Waves can have their course altered, be focused onto a certain point or be spread out over a wider area. As a wave crashes on to the shore it releases it's energy. On a sandy or sedimentary shore this can result in the sediment being lifted into suspension. This process can dissipate a large amount of energy from the waves. Storm waves can often be more destructive than initially expected if they coincide with a storm surge. A storm surge occurs when low pressure associated with the storm raises the sea level, allowing the waves to travel further inshore without interference with the shore. High-energy events such as earthquakes, volcanic eruptions, landslides and meteor impacts can cause huge waves, known as tsunami, which can have devastating effects on coastal areas and easily reach heights of 10m. These waves can travel at very high speeds with large wavelengths. If you were in the middle of the ocean you would hardly notice these waves, they only become a threat in shallow coastal waters.
Different beach gradient can have an effect on how waves break onto the shore. Steep gradients see the waves breaking higher up the shore than on shallow beaches. The other features on the coast are more natural and are shaped by the waves and tides. The dunes differ along the coast from accreting to eroding. In the areas where the dunes are accreting the foreshore is generally wider and thus the waves have their energy greatly reduced before they reach the dunes. Only on the highest tides and stormy conditions do waves impact directly onto the dunes. On eroding sections of the dune system the foreshore is generally narrower and the waves often impact directly onto the foot of the dunes. During storms this can result in cliffs being formed which gradually slump into a more gentle slope. The sand eroded from the dunes by these storm and other waves will be redistributed across the shore. Other features that have an influence on the energy of the waves reaching the dunes or seawalls are the areas of salt marsh at Marshside and embryo dunes/incipient slack/marsh at Birkdale that are covered by the tide on occasion and greatly reduce the waves energy. Wave conditions along the Sefton shoreline vary due to the complex nearshore bank and channel arrangements that apply on the approaches to the Mersey and Ribble. The wave climates for Formby Point suggest that waves heights are increased off the point compared to offshore, possibly due to interaction with the Crosby Channel. This concurs with the view that changes in wave climate conditions at the point was the primary cause of erosion of Formby Point.
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Three important factors control wave height over open water;
On the Sefton Coast the impact waves have on different shore types can be seen. At Crosby the narrow foreshore takes some of the energy out of the waves but at high tide waves can crash into the sea wall resulting in huge plumes of spray. In contrast the wide foreshore at Southport removes the majority of the energy from the waves before they reach the seawall and little if any spray is produced. The amount of spray is also influenced by the type of defence. This can best be observed just north of Southport Pier where the lake sluice gates have vertical walls and result in spray being produced in the area first.
