1.a. Coastal Landscapes Flashcards
1
Q
Inputs to the coastal system
A
- kinetic energy from wind and waves
- thermal energy from the heat of the sun
- gravitational potential energy from the position of material on slopes and cliffs
- material from marine deposition, weathering and mass movement from cliffs
2
Q
Throughputs in the coastal system
A
- Movement of sediment along a beach by longshore drift
3
Q
Stores in the coastal system
A
- Beaches
- Nearshore sediment accumulations
4
Q
Outputs from the coastal system
A
- Marine and wind erosion from beaches and rock surfaces
- Erosion
5
Q
What is a sediment cell?
A
- A stretch of coastline and its associated nearshore area within which the movement of coarse sediment, sand and shingle is largely self-contained
- Generally regarded as a closed system, however it is unlikely that sediment cells are completely closed due to variations in wind direction and the presence of tidal currents
6
Q
How are the boundaries of sediment cells determined?
A
- By the topography and shape of the coastline
- Large physical features such as Land’s End act as huge natural barriers that prevent the transfer of sediment to adjacent cells
7
Q
How does wind impact the coastal landscape?
A
- 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
- Higher the wind speed and longer they fetch, the larger the waves and the more energy they possess
- 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 landscapes
8
Q
How do waves impact the coastal landscape?
A
- A wave possess 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
- Important to realise that moving waves do not move water forward but rather the waves impart a circular motion to the individual water molecules
- Energy in a wave in deep water is approximated by the formula P = H x T, P = power in kilowatts per metre of wave front, H + wave height in metres and T = time interval between wave crests in seconds known as wave period
- Relationship between wave height and wave energy is non-linear
9
Q
What is the highest surface part of a wave called?
A
The crest
10
Q
What is the lowest part of a wave called?
A
The trough
11
Q
What is the vertical difference between the crest and trough called?
A
The wave height
12
Q
What is the horizontal distance between two adjacent crests or troughs called?
A
The wavelength
13
Q
How can shallow water be defined?
A
- Depth of half of the wavelength
14
Q
How do waves behave in shallow water?
A
- The deepest circling water molecules come in contact with the seafloor, friction between the seafloor and water changes the speed, direction and shape of the waves
- Waves slow down as they drag across the bottom, the wavelength decreases and successive waves start to bunch up
- Deepest part of the wave slows down more than the top of the wave
- Wave begins to steepen as the crest advances ahead of the base
- Eventually, when water depth is less than 1.3x wave height, the wave topples over and breaks against the shore, it is only at this point that there is significant forward movement of water as well as energy
15
Q
What are the three types of breaking waves?
A
- Spilling
- Plunging
- Surging
16
Q
What is spilling?
A
Steep waves breaking onto gently sloping beaches, water spills gently forward as the wave breaks
17
Q
What is plunging?
A
Moderately steep waves breaking onto steep beaches, water plunges vertically downwards as the crest curls over
18
Q
What is surging?
A
Low angle waves breaking onto steep beaches, the wave slides forward and may not actually break
19
Q
What happens after a wave breaks?
A
Water moves up beach as smash, driven by transfer of energy that occurs when the eave breaks and moves in the same direction
Speed of the water movement decreases the further it travels due to friction and the uphill gradient of the beach
When it has no more available energy to move forward, water is drawn back down the beach as backwash
- Energy for this movement comes from gravity and always occurs perpendicular to the coastline, down the steepest slope angle
20
Q
Constructive waves
A
- Low in height
- Long wavelength
- Low frequency (around 6-8 per minute)
- Usually break as spilling waves and the strong slash travels a long way up the beach
- Due to the long wavelength, backwash returns to the sea before the next wave breaks, so the next swash movement is uninterrupted and retains its energy
- Essentially, swash energy exceeds backwash energy
21
Q
Destructive waves
A
- Greater height
- Shorter wavelength
- Higher frequency (12-14 per minute)
- Tend to break as purging waves and so there is little forward transfer of energy to move water up the steeply sloping beach as swash
- Friction from the steep beach slows the swash and so it does not travel far before returning back down the beach as backwash
- With a short wavelength, the swash of the next wave is often slowed by the frictional effects of meeting the returning backwash of the previous wave
- Swash energy less than backwash energy
22
Q
Relationship between beach gradient and wave type
A
- High energy waves often occurring during winter months tend to remove material from the top of a beach and transport it to offshore zones, reducing beach gradient
- In contrast, low energy waves, typical of summer months, build up the beach face, steepening the profile
- Wave steepness is thought to be a critical factor in this relationship, but the angle of wave approach and sediment particle size are also important
23
Q
How do tides impact the coastal landscape?
A
Tides are periodic rise and fall of the sea surface, produced by the gravitational pull of the moon, and to a lesser extent, the sun
- Moon pulls water towards it, creating a high tide and there is a compensatory bulge on the opposite side of the Earth, low tide occurs at locations between the two
- As the moon orbits the Earth, the high tides follow it, the highest tides 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 result in spring tides with a high tidal range
- Also twice a month, the Moon and Sun are at right angles to each other and the gravitational pull is therefore at its weakest, producing neap tides with a low range
24
Q
How can tidal range contribute to the development of coastal landscapes?
A
- In enclosed seas, such as the Mediterranean, tidal ranges are low and so wave action is restricted to a narrow area of land
- In places where the coast is funnelled, such as the Severn Estuary, tidal range can be as high as 14m
- The tidal range therefore influences where wave action occurs, the weathering processes that happen on land exposed between tides and the potential scouring effect of waves along coasts with a high tidal range
25
How does geology impact the coastal landscape?
The two key aspects of geology that influence coastal landscape systems are lithology and structure
26
How does lithology impact the coastal landscape?
- 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, eg. basalt, are highly resistant and are more likely to form prominent coastal features such as cliffs and headlands due to being made up of dense interlocking crystals
- Rocks 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
27
How does structure impact the coastal landscape?
- Concerns the properties of individual rock types such as jointing, bedding and faulting, also includes permeability of rocks
- In porous rocks, eg. chalk, tiny air spaces (pores) separate the mineral particles,these pores can absorb and store water (primary permeability)
- Carboniferous limestone is also permeable, but this is because water seems in due to its many joints, which is known as secondary permeability, the joints are easily enlarged by solution
- Structure is an important influence on the platform of coasts at a regional scale, rock outcrops that are uniform or run parallel to the coast tend to produce straight coastlines, known as concordant coasts
- Where rocks lie at right angles to the coast they create a discordant planform, more resistant rocks form headlands, weaker rocks form bays
- Structure also includes the angle of dip of rocks and can have a strong influence on cliff profiles, horizontally bedded and landward-dipping strata support cliffs with steep, vertical profiles, where strata incline seawards, cliff profiles tend to follow angle of dip of the bedding planes
28
How do global ocean currents impact the coastal landscape?
- Rip currents play important role in the transport of coastal sediment, they are caused either by tidal motion or waves breaking at right angles to the shore
- 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 currents, channelling flow through a narrow neck
- Ocean currents are much larger scale phenomena, generated by the Earth’s rotation and by convection, they are set in motion by the movement of winds across the water surface
- Warm ocean currents transfer heat energy from low latitudes towards the poles, they particularly affect western-facing coastal areas where they are driven by on-shore winds
- Cold ocean currents do the opposite, moving cold water from polar regions towards the Equator, they are usually driven by off-shore winds and so tend to have less effect on coastal landscapes
- Strength of the current itself may have a limited impact on coastal landscape systems in terms of geomorphological processes, but transfer of heat energy can be significant, as it directly affects air temperature and therefore, sub-aerial processes
29
Terrestrial sources of coastal sediment
- Rivers are major sources of sediment input to the coastal sediment budget, particularly true of coastal with a steep gradient, where rivers directly deposit their sediments at the coast
- Coasts with a low gradient may lose river sediment to estuaries
- Sediment delivery to the shoreline can be intermittent, mostly occurring during floods, in some locations as much as 80% of coastal sediment comes from rivers
- 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% 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 derived directly from the collapse of undercut cliffs
- Longshore drift can also supply sediment to one coastal area by moving it along the coast to adjacent areas
30
Offshore sources of coastal sediment
- Constructive waves bring sediment to the shore from offshore locations and deposit it, adding to the sediment budget, tides and currents do the same
- Wind also blows sediment from other locations, including exposed sand bars, dunes and beaches elsewhere along the coast
- This aeolian material is generally fine sand, as wind has less energy than water and so cannot transport very large particles
31
Human sources of coastal sediment
- When a coastal sediment budget is in deficit, beach nourishment is one way in which a sediment equilibrium can be maintained, this type of management has been adopted worldwide to preserve and protect coastal environment
- Sediment can be brought in by lorry and dumped on the beach before being spread out by bulldozers
- Alternatively, sand and water can be pumped on-shore by pipeline from offshore sources, low bunds hold the mixture in place while the after drains away and leaves the sediment behind
- Wind, waves and longshore drift movements can also remove sediment from the coastal sediment budget, by subtracting the amount of sediment lost from the amount of sediment gained, it can be determined if the sediment budget is in surplus, deficit or equilibrium
