Systems and Processes

• The primary source of energy for all natural systems is the Sun.
• At the coast the main form of energy is derived from the sea in the form of waves, which are mostly formed by the wind.
• Wind is the movement of air from one place to another.
• Variations in atmospheric pressure primarily reflect differences in surface heating by the Sun. The greater the pressure difference between two places – called the pressure gradient – the faster (stronger) the wind.
• A number of factors affect wave energy:

  ◆ The strength of the wind – determined by the pressure gradient.

  ◆ The duration of the wind – the longer the wind blows, the more powerful waves will become.

  ◆ The fetch – the distance of open water over which the wind blows.
• As air moves across the water, frictional drag disturbs the surface and forms ripples or waves. In the open sea, there is little horizontal movement of water. Instead, there is an orbital motion of the water particles. Close to the coast, horizontal movement of water does occur as waves are driven onshore to break on the beach.
• Wave Formation

  1. The water becomes shallower and the circular orbit of the water particles changes to an elliptical shape.
  2. The wavelength (the distance between the crests of two waves) and the velocity both decrease, and the wave height increases – causing water to back up from behind and rise to a point where it starts to topple over (break).
  3. The water rushes up the beach as swash and flows back as backwash
• Constructive Wave

  • Generated by distant weather systems in the open ocean.
  • Low, surging wave, with a long wavelength.
  • Strong swash, weak backwash.
  • Beach gain.
  • Usually associated with a gentle beach profile - but overtime they will build up the beach and increase gradient.
• Destructive Wave

  • Generated by local storms.
  • High, plunging waves, with a short wavelength.
  • Weak swash, strong backwash.
  • Beach loss.
  • Usually associated with a steeper beach profile, but overtime they flatten the beach.
• Tides are changes in the water level of seas and oceans caused by the gravitational pull of the moon.
• The relative difference in height between high and low tides is called the tidal range
• A high tidal range creates relatively powerful tidal currents, as tides rise and fall, which can be particularly strong in estuaries and narrow channels
• Neap Tide

  • Moon is at right angles to the sun in the first or last quarter.
  • Gravitational pulls act against each other to create lower high tides and higher low tides.
  • Smaller tidal range
• Spring Tide

  • Sun and moon in line at full moon or new moon.
  • Gravitational pulls act together to create higher high tides and lower low tides.
  • Larger tidal range.
• Rip currents are commonly formed when a series of plunging waves cause a temporary build-up of water at the top of the beach. Met with resistance from the breaking waves, water returning down the beach is forced just below the surface following troughs and small undulations in the beach profile.
• Rocky coasts are generally found in high-energy environments.

  • Where the rate of erosion exceeds the rate of deposition.
  • Erosional landforms, such as headlands, cliffs, and wave-cut platforms, tend to be found in these environments.
• In contrast, sandy and estuarine coasts are generally indicative of low-energy environments.

  • Where the rate of deposition exceeds the rate of erosion.
  • Landforms such as beaches, spits and coastal plains tend to be found in these environments.
• Wave refraction causes energy to be concentrated at headlands and dissipated in bays, it occurs as a result of waves dragging in shallow water, causing the wave height to increase. This results in the the part of the wave in deeper water bending, so the energy concentrates on headlands and is dissipated at bays.
• Globally, most beach sediment comes from rivers, streams and coastal erosion.
• A sediment cell is a stretch of coastline, usually bordered by two prominent headlands, where the movement of sediment is more or less contained.
• Sediment Cells

  • Energy comes from the sun, converted into wave energy by the wind
  • Inputs = Primarily derived from the river, coastal erosion and offshore sources, such as bars or banks
  • Transfers = Include littoral drift together with onshore and offshore processes such as rip currents.
  • Stores = Include the beach, sand dunes and offshore deposits.
• The sediment budget is the balance between inputs and outputs of sediment in a sediment cell, which seeks to achieve a state of dynamic equilibrium.
• Weathering is the breakdown or disintegration of rock in situ (its original place) at or close to the ground surface.
• Mechanical (or physical) weathering involves the break-up of rocks without any chemical changes taking place.
• Frost shattering (also known as freeze-thaw) occurs when water enters a crack or joint in the rock when it rains and then freezes in cold weather. When water freezes, it expands in volume by about 10 per cent. This expansion exerts pressure on the rock, which forces the crack to widen. This repeats and the fragments of rock break and become scree which are used by the sea in marine erosion.
• Salt crystallisation. When salt water evaporates, it leaves salt crystals behind. These can grow over time and exert stresses in the rock, just as ice does, causing it to break up.
• Wetting and drying. Frequent cycles of wetting and drying are common on the coast. Rocks rich in clay (such as shale) expand when they get wet and contract as they dry. This can cause them to crack and break up.
• The breakdown of rocks by organic activity is biological weathering.

  • Thin plant roots grow into small cracks in a cliff face. These cracks widen as the roots grow, which breaks up the rock .
  • Birds (e.g. puffins and sand martins) and animals (e.g. rabbits) dig burrows into cliffs.
  • Marine organisms are also capable of burrowing into rocks (e.g. piddocks, which are similar to clams), or of secreting acids (e.g. limpets).
• Carbonation – rainwater absorbs carbon dioxide from the air to form a weak carbonic acid. This reacts with calcium carbonate in rocks, such as limestone and chalk, to form calcium bicarbonate, which is easily dissolved. The cooler the temperature of the rainwater, the more carbon dioxide is absorbed (so carbonation is more effective in winter).
• Oxidation – the reaction of rock minerals with oxygen, for example iron, to form a rusty red powder leaving rocks more vulnerable to weathering.
• Solution – the dissolving of rock minerals, such as halite (rock salt).
• Waves – the rate and type of erosion experienced on a particular stretch of coast is primarily influenced by the size and type of waves that reach that coast.
• Rock type (lithology) – rock lithology (its physical strength and chemistry) is important in determining the rate of erosion. Tough and resistant rocks such as granite erode at very slow rates compared to weaker clays and shales.
• Geological structure – cracks, joints, bedding planes and faults create weaknesses in a cliff that can be exploited by erosive processes.
• Subaerial processes – weathering and mass movement will weaken cliffs and create piles of debris that are easily eroded by the sea, potentially increasing the rate of erosion.
• Presence or absence of a beach – beaches absorb wave energy and reduce the impact of waves on a clif
• Coastal management – the presence of structures such as groynes and sea walls will have an impact on sediment transfer (and the build-up of beaches) and patterns of wave energy along a coastline
• Littoral Drift

  Swash carries material obliquely up a beach at the angle of the prevailing wind. Backwash then carries material perpendicular down the beach under gravity.
• Deposition takes place when the velocity of the water (or wind) falls below a critical value for a particular size of particle and can no longer be transported.
• Hydraulic Action = The impact on rocks of the sheer force of the water, this can exert an enormous pressure upon the rocks