Enquiry question 2: How do characteristic coastal landforms contribute to coastal landscapes?
2B.4 Marine erosion creates distinctive coastal landforms and contributes to coastal landscapes.
2b.4a. Different wave types (constructive/destructive) influence beach morphology and beach sediment profiles, which vary at a variety of temporal scales from short term (daily) through to longer periods (4)
Constructive Waves
Small oceans with small fetch develop constructive waves. Constructive waves have low wave height and long wave length with low frequency, between 6 and 8 waves per minute. Constructive waves are associated with weak backwash and strong swash, which builds up wide flat beaches and so more associated with coasts of deposition. Constructive waves also tend to form sandy beaches. |
Destructive Waves
Large oceans with large fetch produce large waves, called destructive waves. These waves have large wave height and short wave length and are characterised by tall breakers that have high downward force and a strong backwash. They have high frequency, between 13 and 15 waves per minute. This downward energy helps erode cliifs. In addition, due to a dominant backwash they erode the beach making for narrow steep beach profiles. Localised storms with high wind speed also form destructive waves as well as steep depth gradients around headlands. Destructive waves also tend to form pebbly beaches. |
Beach sediment profiles, vary at a variety of temporal scales from short term (diurnal or daily) through to longer periods (seasonal),
2b.4b. The importance of erosion processes (hydraulic action, corrosion, abrasion, attrition) and how they are influenced by wave type, size and lithology.
Corrasion is when waves pick up beach material (e.g. pebbles) and hurl them at the base of a cliff.
Abrasion occurs as breaking waves which contain sand and larger fragments erode the shoreline or headland. It is commonly known as the sand paper effect. When waves hit the base of a cliff air is compressed into cracks. When the wave retreats the air rushes out of the gap. Often this causes cliff material to break away. This process is known as hydraulic action. Attrition is when waves cause rocks and pebbles to bump into each other and break up. Corrosion/solution is when certain types of cliff erode as a result of weak acids in the sea. |
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2b.4c. Erosion creates distinctive coastal landforms (wave cut notches, wave cut platforms, cliffs, the cave-arch-stackstump sequence).
The Cave-arch-stack sequence
2B.5 Sediment transport and deposition create distinctive landforms and contribute to coastal landscapes.
2B.5a. Sediment transportation is influenced by the angle of wave attack, the process of longshore drift, tides and currents. (5)
Tides
Tides exist thanks to the gravitational pull of the moon and the sun, but vary depending on where the moon and sun are in relation to the ocean as the earth rotates on its axis. The moon and sun’s pull cause two bulges or high tides in the ocean on opposite sides of the earth. The moon, being so much closer, has more power to pull the tides than the sun and therefore is the primary force creating the tides. The sun and moon can reinforce each other’s gravitational pulls, creating larger-than-normal tides called spring tides. The two forces of the sun and moon cancel each other out and create a neap tide. |
The Bay of Fundy in New Brunswick, Canada has the highest tidal range. The tides range from 3.5m (11ft) to 16m (53ft) and cause erosion to the landscape, creating massive cliffs
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Process of sediment transportation
Long Shore Drift
Longshore drift is a process responsible for moving significant amounts of sediment along the coast. This usually occurs in one direction as dictated by the prevailing wind. For example, the prevailing wind along the Holderness Coast is north-easterly. As the result waves break on to the beach obliquely at an angle of around 45 degrees. The swash moves beach material along the beach and the backwash, under gravity, pulls the material back down the beach at right angles to the coastline. Over time this creates a net shift of material along the coast.
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2B.5b. Transportation and deposition processes produce distinctive coastal landforms (beaches, recurved and double spits, offshore bars, barrier beaches and bars, tombolos and cuspate forelands), which can be stabilised by plant succession.
2B.5c. The Sediment Cell concept (sources, transfers and sinks) is important in understanding the coast as a system of dynamic equilibrium, with both negative and positive feedback (ü Portland Bill to Selsey Bill)
Research suggests that coastlines can be divided up into a series of sediment cells (littoral cells), with smaller sub-cells within them. Sediment movement is relatively self-contained within each sediment cell. The UK has 11 such cells recognised by scientists (see map). Within each sediment cell, there are inputs of sediment (e.g. from rivers, cliff erosion and from the sea bed), movement of sediment (e.g. by constructive waves, destructive waves and longshore drift) and outputs of sediment in the form of depositional landforms or to deeper water). Most sediment is moved by longshore drift, longshore currents and tidal currents. Because of this, sediment cell boundaries usually occur at headlands and estuaries (see map).
Sediment cannot cross the cell boundaries so it can only be lost out to sea. Tidal currents are especially important here. They surge up an estuary with the rising tide (flood tide) and surge out with the falling tide (ebb tide). The seaward ebb currents are usually the strongest, since they are enhanced by river flow. The result is the removal of large amounts of mud, silt and sand by suspension into deeper water where sediment is lost from the cell.Sediment cells and sub-cells form ‘coastal systems’ and are the natural units on which shoreline management plans (SMPs) can be based. SMPs are an essential step towards integrated coastal management by which coastal management strategies can be devised to protect one stretch of coastline without creating and adverse effect on another stretch of coastline further downdrift.
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2B.6 Subaerial processes of mass movement and weathering influence coastal landforms and contribute to coastal landscapes.
2B.6a. Weathering (mechanical, chemical, biological) is important in sediment production and influences rates of recession.
Weathering is the decay and disintegration of rock in situ. There are 3 main types of weathering that affect the coast. These are mechanical, chemical and biological weathering.
Several factors control the type of weathering and the rate at which rock weathers.
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The graph above shows the relationship between climate and weathering
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2B.6b. Mass movement (blockfall, rotational slumping, landslides) is important on some coasts with weak and/or complex geology.
Sub-aerial processes – mass movement
Mass movement is the movement of material downslope as the result of gravity. This can be a slow process in the case of soil creep or fast in the case of rockfalls. Water commonly acts as a lubricant in mass movement. Examples include: Falls, Topples, Translational slides, Rotational slide/slumping and Flows Falls - Hunstanton Cliffs, Norfolk
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Land Flow - Hell's Mouth, Cornwall
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Translational slide - Nefyn Bay, Wales
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2B.6c. Mass movement creates distinctive landforms (rotational scars, talus scree slopes, terraced cliff profiles).