Wave height (H) is the vertical distance between a crest and its adjoining through (nachbar Talsohle).
Wave length (L) is the horizontal distance between two successive (aufeinanderfolgenden) crests.
Wave period (T) is the time taken for two successive crest to pass a fixed point.
Wave velocity (V) is the speed of wave crests.
Wave steepness (H/L) is the ratio of wave height and wave length.
Wave steepness cannot exceed a ratio of 1:7, or 0.14, because at that point the wave breaks. It is possible to calculate the wave length if the wave period is known. This can be done using the formula L=1.56T².
The size of a wave determines (bestimmt) how much energy it exerts (benötigt). Larger waves posses more energy, therefore have a greater potential to cause damage. Wave energy (E) is proportional to the square of the wave height (H), and directly proportional to the wave period (T), using the formula E α LH². In other words, a 4 metre high wave has 16 times more energy than a 1 metre wave, assuming the same wave length. Similarly, a wave with an 8 second period has 4 times more energy than one of the same height with a period of 2 seconds, assuming the same wave height. This shows us that long period, high waves can do as much work in a few days as it takes smaller waves several weeks or months to do.
Wind velocities and therefore wave heights, are greatest in the mid-latitudes (mittlere Breitengrade) where strong winds blow for long periods across large stretches of ocean. Wave velocities and wave heights decrease towards both the equator and the poles. Because the prevailing wind over the oceans blow from the west, more moderate waves tend to be found along the east coasts of continents in the mid and low-latitudes, as these coasts are protected from the west coast swell. In the tropics, where very gentle winds blow or calms prevail, low waves are most common.
Wave approaching (annähern) a coastline do not distribute their energy evenly along the shoreline. We can regard (beachten, feststellen) each wave as a bundle of energy moving towards the shoreline. Each wave (or bundle of energy) can be divided into equal ‘parcels’ (Päckchen) of energy, divided by lines drawn at right-angles to the wave crest, known as wave orthogonal (rechtwinklig). As each wave approaches the shoreline, it meets shallow water in front of the headlands, while still allowing the wave to travel more quickly into the bays. The effect of this is that waves refract (brechen), or bend, around the headlands and meet the shoreline approximately at right angles.
The pattern of the wave orthogonals shows that energy is concentrated on to the headlands and is dispersed (zerstreut) in the bays. This uneven distribution of energy focuses erosion in most bays. For this reason, cliffs are common on headlands while beaches will usually form inside bays.
Wave refraction (Brechung) 9s also responsible for forming tombolos (verbundene Inseln), or tied islands. When an island is located close to the coastline, waves refract round it as if it were a headland, and meet behind it. When the waves collide they lose competence to carry sediment (Ablagerungen). Deposition therefore occurs behind the island, first in the form of a cuspate foreland that eventually grows to join the island to the mainland to form a tombolo.
Waves can be constructive or destructive. Waves are generally constructive during periods of low energy, when deposition occurs. On the other hand, waves are destructive during periods of high energy, when erosion is likely to occur. Beaches tend to go through cycles of erosion or deposition depending on the wave energy available. In the depositional sequence, decreasing wave energy changes the shape of the beach from stage 6 to stage 1, unless it is interrupted by an increasing wave energy. In the erosional sequence, increasing waver energy changes the shape of the beach from stage A (or whatever the starting point is) through to stage F, unless interrupted by lower energy conditions.
Landforms are changed when there is a change in the levels of water, sediment or energy. Deposition will occur if there is an extra input of sediment. The extra sediments available will be formed into depositional landforms such as deltas, beaches, dunes and so on. On the other hand, if there is a los of sediment, erosion will occur. Whether a landform is erosional or depositional depends on the supply of sediment and energy available. By comparing the appearance of a beach with the model in figure 8.8 on the following pages, we can get good indication as to whether a beach is undergoing a depositional or an erosional sequence at the time.
Coastal sediment comes from two main sources. The first is the land, which supplies gravels, quartz sand, silts and muds. These sediments may be brought to the coast by rivers, or they may result directly from the erosion of cliffs. The second source of sediments is the sea. When marine organism die, their shells and skeletons supply carbonate sediments to the coast. We can therefore say that coastal sediments are supplied by river, by marine erosion, or they can be produced on site.
A rare (though extreme) type of wave is the tsunami. These form as a result of disturbances in the earth’s crust, such as an earthquake or volcanic explosion. These disturbances create long wavelengths at the ocean surface, radiating out from the earthquake centre at speeds of up to 900 kilometres per hour. The height of a tsunami in the open sea might be only a few centimetres, but like tides, their height increases in shallower coastal waters. The combination of their great speed and height can result in immense devastation and loss of life.
The twice-daily rises and falls in local sea level caused by the gravitational attraction of the sun and moon are known as tides. Although the moon is smaller than the sun, it is much closes to the earth and so it exerts a 2.16 times stronger gravitational force. Consequently, lunar tides are more significant than solar tides. Every two weeks, at the new and full moon, the moon is aligned (ausgerichtet) with the sun, and this alignment produces the highest tides called spring tides. At the moon’s first and last quarter, it is perpendicular (senkrecht) to the sun, and lower neap tides result. When the spring tides coincide with periods of high winds or storm activity, a strong force capable of severe erosion may result.
The difference between high and low tide on a particular day is known as the tidal range. The tidal range can vary greatly from place to place around the world. In the deep oceans, the tides average only 18 centimetres in height. On the other hand, the height of the tides usually increases at shallow continental shelves and in coastal inlets (Buchten). Depending on the shape of the sea bed and coastline, some coasts receive tides with a small range (less than 2 metres), while others receive tides over 10 metres in range. South Wales is one example of an area with a large tidal range.
Other Coastal Processes
Atmospheric processes
We have already seen that winds generate ocean waves, which provide most of the energy that leads to change the coastal environments. The role of the atmosphere in coastlines is much greater than this, however. The atmosphere produces precipitation (Niederschlag), which in turn leads to weathering of nearby rocks. Together with the force of gravity, this force produces and delivers sediments that are then available to be rebuilt into beaches, deltas and continental shelves in the coastal zone.
Atmospheric processes shape the land’s surface by reworking marine sediments and chemicals in areal well above the reach of the waves and tides. The wind provides energy to blow beach sand inland to form dunes. In tropical areas, temperature influences the growth of coral and algal organisms that produce the coral reefs on many tropical coasts. Temperature also affects plants (such as mangroves and salt plants) and animals of the intertidal zone in the mid-latitudes. Atmospheric forces also play a role in the polar latitudes; when the sea temperature falls below -4°C the water freezes, forming ice that stops most activity of the coast.
Biological processes
The most important biological contribution to coastal landforms is the supply of calcium carbonate sediments from the skeleton and shells of marine organisms. This occurs especially in tropical and temperate regions of the world. In the tropics, reefs of coral and algae are important in influencing the development of the coast. In other areas, the plants and animals of the coastal dunes, rocky coasts and intertidal regions all contribute sediments. As a general rule, biological influences on the coast decrease towards the poles.
Chemical processes
Chemical processes tend to be related closely to weathering caused by atmospheric processes. Nonetheless, there are some additional and unique effects that are found in the coastal zone. Salt spray carried by the wind speeds up weathering and the breakdown of the coastal land surface. As the salt water evaporates the dissolved salt forms crystals in small cracks and pores in the rock. As the crystals grow, they can cause the fracturing of the rock. Salt crystallisation often leads to a distinctive pattern of honeycomb weathering in coastal areas where evaporation rates are high.
In warm tropical waters in the inter-tidal zone of many beaches calcium carbonate is precipitated, cementing the sand grains together to form beachrock. In warm to hot semi-arid regions, calcium carbonate cements the grains of coastal sand dunes together to form dunerock.
Sea level changes
The earth’s temperature has risen and fallen on many occasion over the past few million years. In general, these changes in temperature are just a few degrees Celsius. During the cooler periods, called glacial periods, much of the world’s water is stored in glaciers and continental ice sheets, resulting in lowering of world sea levels. During warmer periods, called interglacials, this trapped water is released once again into the world’s oceans, causing a rise in sea levels.
The changes in sea level resulting from these changes in temperature have been between 100 and 150 metres during the last two million years. At present the earth is relatively warm compared with last times, so the world’s ice sheets ate smaller than their average sizes, and sea level is also higher than average. Indeed, at the end of the last great ice age only 18,000 years ago, the world sea level was about 125 metres below the present level. At that time, most of the world’s shorelines lay near the edge of the continental shelves. The previous occasion when sea levels were as high as now was about 120,000 years ago.
The last ice age, called Pleistocene, ended about 18,000 years ago. At that time the large ice sheets began to melt, causing oceans to rise to their present levels. This process took some time to complete, and the oceans continued rising until about 6,000 years ago. This postglacial rise in sea level was called the Holocene, and because it was very recent in geologic time, almost of todays coastal landforms are very young, being 6,000 years old or less. All the older coasts were submerged out on the continental shelf, except the old coastal features formed 120,000 years ago.
At the sea rose across the continental shelf between 6,000 and 18,000 years ago, it swept along with it many of the sand size sediments lying on the shelf, such as soils, river beds, old beaches and dunes. These sediments were reworked and moved inland with the rising waters, and eventually they were deposited along the present shoreline. As a result of the recent rise in sea level, the continental shelf has been a major source of coastal sediment. This is particularly so in areas with moderate to high waves, such as in the southern half of Australia.
With the rise in sea level, many coastal valleys were drowned. This formed deep estuaries such as the fjords in the formerly glaciated valley of Norway and New Zealand, and drowned river valleys (or rias) in temperate countries such as Australia. Because the fjords and rias are very young in geological terms, the rivers which flow into them are still depositing their coarser materials (sand and gravel) in the upper reaches of the estuaries rather than at the coastline. At the same time as this is happening, waves and tides are pushing sediments up into the estuaries, and the sediments are lost to the coastal system. In this way, many coastal estuaries are acting as sediment sinks, trapping sediment that would otherwise be available at the coast. This will continue until sediments fill the estuaries and the rivers can flow directly to the coast.
Not all sea level changes result from climatic change. Sometimes the land will rise or fall because of local tectonic pressure. Unless an area is compared with other areas, it is difficult to tell in the field whether it is the land or the sea that has changed height. If the land has rise relative to the seal, then raised beaches may result. These are relict features found at a level well above the zone where marine processes currently operate. Changes in sea level are known as eustatism. Therefore, relative changes in the levels of the land and the sea caused by climatic change are often referred to as glacioeustatic changes.