1.1 How can coastal landscapes be viewed as systems Flashcards
1.1 How can coastal landscapes be viewed as systems?
Key idea ⮕ Coastal landscapes can be viewed as systems.
Inputs of open systems
-Kinetic energy from wind and waves
-Thermal energy from the heat of the Sun
-Potential energy from the position of material on slopes
-Material from marine deposition
-Weathering and mass movement from cliffs
Outputs of open systems
-Marine and wind erosion from beaches and rock surfaces
-Evaporation
Throughputs of open systems
Stores:
-Beach and nearshore sediment accumulations
-Flows (transfers), such as the movement of sediment along a beach by longshore drift
System feedback in coastal landscapes
When a system’s inputs and outputs are equal, a state of equilibrium exists within it. In a coastal landscape, this could happen when the rate at which sediment is being added to a beach equals the rate at which sediment is being removed from the beach; the beach will therefore remain the same size.
When the equilibrium is disturbed, the system undergoes self-regulation and changes its form in order to restore equilibrium. This is known as dynamic equilibrium, as the system produces its own response to the disturbance. This is an example of negative feedback.
Sediment cells
A stretch of coastline and its associated nearshore area within the movement of coarse sediment, sand and shingle is largely self-contained. A sediment cell is a closed system, which suggests that no sediment is transferred from one cell to another. There are 11 large sediment cells around the coast of England and Wales. There are also many sub-cells of a smaller scale existing within the major cells.
Closed system
A system with no inputs or outputs.
Transfer between sediment cells
It is unlikely that sediment cells are completely closed. With variations in wind direction and the presence of tidal currents, it is inevitable that some sediment is transferred between neighbouring cells.
Wind
The source of energy for coastal erosion and sediment transport is wave action. This wave energy is generated by the frictional drag of winds moving across the ocean surface. The higher the wind speed and the longer the fetch, the larger the waves and the more energy they possess.
Onshore winds, blowing from the sea towards the land, are particularly effective at driving waves towards the coast. If winds blow at an oblique angle towards the coast, the resultant waves will also approach obliquely and generate longshore drift.
Wind is a moving force and as such is able to carry out erosion, transportation and deposition itself. These aeolian processes contribute to the shaping of many coastal landforms.
Waves
A wave possesses potential energy as a result of its position above the wave trough, and kinetic energy caused by the motion of the water within the wave. It is important to realise that moving waves do not move the water forward, but rather the waves impart a circular motion to the individual water molecules.
A ball floating in the sea is an example of this phenomenon. As a moving wave passes beneath the ball, it rises and falls but does not move horizontally across the water surface.
The amount of energy in a wave in deep water formula
P = H²T
P is the power in kilowatts per metre of wave front
H is the wave height in metres
T is the time interval between wave crests in seconds, known as wave period
Swell waves
A wave with a long wavelength, low height and steepness. It has a wave period of up to 20 seconds.
Storm wave
A wave generated locally by high wind energy. It has a short wavelength, greater height and a shorter wave period.
Wave period
The time interval between wave crests in seconds.
Properties of Atlantic Ocean waves
Wave height (m): 5.0
Wave period (sec): 8
Energy (kW per m): 200
Properties of English Channel waves
Wave height (m): 0.6
Wave period (sec): 6
Energy (kW per m): 2.16
Types of breaking waves
-Spilling (Steep waves breaking onto gently sloping beaches; water spills gently forward as the wave breaks)
-Plunging (Moderately steep waves breaking onto steep beaches; water plunges vertically downwards as the crust curls over)
-Surging (Low-angle waves breaking onto steep beaches; the wave slides forward and may not actually break)
Spilling wave
Steep waves breaking onto gently sloping beaches; water spills gently forward as the wave breaks.
Plunging wave
Moderately steep waves breaking onto steep beaches; water plunges vertically downwards as the crust curls over.
Surging wave
Low-angle waves breaking onto steep beaches; the wave slides forward and may not actually break.
Tides
The periodic rise and fall of the sea surface and are produced by the gravitational pull of the Moon and, to a lesser extent, the Sun. The Moon pulls the water towards it, creating a high tide, and there is a compensatory bulge on the opposite side of the Earth. At locations between the two bulges, there will be a low tide. As the Moon orbits the Earth, the high tides follow it. The highest tides will occur when the Moon, Sun and Earth are all aligned and so the gravitational pull is at its strongest. This happens twice each lunar month and results in spring tides with a high tidal range. Also twice a month, the Moon and the Sun are at right angles to each other and the gravitational pull is therefore at its weakest, producing neap tides with a low range.
Geology
The two key aspects of geology that influence coastal landscape systems are lithology and structure.
Lithology
Describes the physical and chemical composition of rocks. Some rock types, such as clay, have a weak lithology, with little resistance to erosion, weathering and mass movements. This is because the bonds between the particles that make up the rock are quite weak. Others, such as basalt, made of dense interlocking crystals, are highly resistant and are more likely to form prominent coastal features such as cliffs and headlands. Others, such as chalk and carboniferous limestone (predominantly composed of calcium carbonate), are soluble in weak acids and thus vulnerable to the chemical weathering process of carbonation.
Structure
The properties of individual rock types such as jointing, bedding and faulting. It also includes the permeability of rocks. In porous rocks, such as chalk, tiny air spaces (pores) separate the mineral particles. These pores can absorb and store water - a property known as primary permeability. The joints are easily enlarged by solution.
Structure is an important influence on the planform of coasts at a regional scale. Rock outcrops that are uniform, or run parallel to the coast, tend to produce straight coastlines. These are known as concordant coasts.
Where rocks lie at right angles to the coast they create a discordant planform: the more resistant rocks form headlands; the weaker rocks form bays.
Structure also includes the angle of dip rocks and can have a strong influence on cliff profiles. Both horizontally bedded and landward-dipping strata support cliffs with steep, vertical profile. Where strata incline seawards cliff profiles tend to follow the angle of the dip of bedding planes.
Primary permeability.
Pores in rock, such as chalk, that can absorb and store water.
Rip currents
An important role in the transport of coastal sediment. They are caused either by tidal motion or by waves breaking at right angles to the shore. A cellular circulation is generated by differing wave heights parallel to the shore. Water from the top of breaking waves with a large height travels further up the shore and then returns through the adjacent area where the lower height waves have broken. Once rip currents form, they modify the shore profile by creating cusps which help perpetuate the rip current, channeling flow through a narrow neck.
Ocean currents
Much larger phenomena, generated by the Earth’s rotation and by convection, and are set in motion by the movement of winds across the water surface. Warm ocean currents transfer heat energy from low attitudes towards the poles. They particularly affect western-facing coastal areas where they are driven by onshore winds. Cold ocean currents do the opposite, moving cold water from polar regions towards the Equator. These are usually driven by offshore winds, and so tend to have less effect on coastal landscapes. The strength of the current itself may have a limited impact on coastal landscape systems in terms of geomorphic processes, but the transfer of heat energy can be significant, as it directly affects air temperature and, therefore, sub-aerial processes.
Terrestrial
Rivers are major sources of sediment input to the coastal sediment budget, and this is particularly true of coasts with a steep gradient, where rivers directly deposit their sediments at the coast. Sediment delivery to the shoreline can be intermittent, mostly occurring during floods. In some locations, as much as 80 per cent of coastal sediment comes from rivers.
The origin of the sediment is the erosion of inland areas by water, wind and ice as well as sub-aerial processes of weathering and mass movement. Wave erosion is also the source of large amounts of sediment and makes a major contribution to coastal sediment budgets.
Cliff erosion can be increased by rising sea levels and is amplified by storm surge events. The erosion of weak cliffs in high-energy wave environments contributes as much as 70 per cent of the overall material supplied to beaches, although typically it contributes much smaller amounts. Some of this sediment may comprise large rocks and boulders, especially if it is derived directly from the collapse of undercut cliffs.
Longshore drift can also supply sediment from one coastal area by moving it along the coast to adjacent areas.
Data from the coastal sediment budget for West Point, Prince Edward Island, Canada
Inputs (average) m³/year:
Marine deposition = 9,532
Cliff erosion = 3,177
Longshore drift = 5,563
Outputs m³/year:
Total = 18,274
