2.a. Coastal landforms develop due to a variety of interconnected climatic and geomorphic processes. Flashcards
Formation of spits and tombolos
Spits are long, narrow beaches of sand or shingle which are attached to the land at one end and extend across a bay, estuary or indentation in a coastline.
They are generally formed by longshore drift occurring in one dominant direction which carries beach material to the end of the beach and then beyond into the open water.
The spit becomes more substantial and permanent when storms occur due to the build up of larger material.
-The end of the spit often becomes recurved owing to wave refraction around the end of the spit and perhaps the presence of a secondary wind/wave direction contributes towards this.
Spits may continue to grow overtime and develop a number of recurves or hooked ends. However, if a spit forms across an estuary, it’s length may be limited by the actions of the river current. Behind the spit in the sheltered area, deposition of silt and mud will occur as wave energy is reduced, eventually forming a salt marsh.
example of a spit can be found at Orford Ness in East Anglia. The coastline is eastfacing, which therefore means that it is largely unaffected by Britain’s southwesterly prevailingwinds. Instead northeasterly winds and waves are locally dominant, consequently resulting inlongshore drift from north to south.
-Tombolos are beaches that connect the mainland to an offshore island
- formed from spits that have continued to grow seawards until they reach and join an island.
-An example of a tombolo can be seen at Chesil, which is near Weymouth in Dorset.
It is a 30km long shingle beach and was thought to have formed this way.
However, it is now thought that the onshore movement ofsediments is the more likely cause of this landform at this particular location. At its eastern end atPortland, the ridge of shingle is 13m high and is composed of flinty pebbles, compared to thewestern end, which is near Burton Bradstock, where the ridge is only 7m high and made up ofsmaller, pea-sized shingle.
,Formation of salt marshes
’Salt marshes are features of low energy environments, such as estuaries and on the landward side ofspits and are subjected twice daily to inundation and exposure as tides rise and fall.
The UK has45,000 ha of salt marsh, mainly located in eastern and northwest England.
1.A salt marsh begins life as an accumulation of mud and silt in a sheltered part of the coastline, e.g. in the lee of a spit or a bar.
2.As more deposition takes place, the mud begins to break the surface to form mudflats
3.Salt-tolerant plants such as cordgrass soon start to colonise the mudflats - early colonisers are known as pioneer plants.
4.Cordgrass is tolerant of saltwater and its long roots prevent it from being swept away by waves and tides, as well as helping to trap sediment and stabilise the mud.
5.As the level of mud rises, it is less frequently covered by water.
6.The conditions become less harsh as rainwater begins to wash out some of the salt, and decomposing plant matter improves the fertility of the newly forming soil.
- New plants such as sea asters start to colonise the area and gradually, over hundreds of years a succession of plants develops. (vegetation succession)
8.The development of salt marshes depends on the rate of accumulation of sediment, with rates of10cm per year quite common.
- Deposition of fine sediment occurs as rivers lose energy when theyslow upon entering the sea. This is a key factor, but so too is flocculation.
10.Tiny clay particles carry anelectrical charge and repel each other in fresh water. In salt water, the particles are attracted to eachother, combining together to form flocs, which being larger and heavier are unable to be carried inthe river flow and so settle out of suspension, even at relatively high velocities.
An example of a saltmarsh is Dawlish Warren, which is located in south Devon. It provides the mainroosting site for huge numbers of wading birds and wildfowl that spend the autumn and winter onthe Estuary. The Warren is also designated as a Special Area of Conservation (SAC) for its dunegrassland, humid dune slacks and the tiny, rare ‘petalwort’ that grows there
Onshore Bars
Onshore bars are an example of a depositional coastal landform.
Deposition occurs when there is insufficient energy to move sediments further. In low energy environments, factors such as wave and wind direction, the supply of sediment and the depth of water are often significant.
Large depositional landforms such as onshore bars are only found where the tidal range is less than 3metres.
Onshore bars can be formed as a result of two differing wave approaches.
1.The first type of onshore bar is ‘drift-aligned’ and originates from the development of a spit. When wave crests driven by the prevailing wind arrive at an angle to the coastline.
2.The oblique wave angle creates a near shore current known as longshore drift which is capable of moving large quantities of sediment in a down-drift direction.
- Where there is an abrupt change in the orientation of the coastline, thetransportation and deposition of sediment will continue in a straight trajectory out to open water.
4.If aspit continues to grow across an indentation such as a bay, in the coastline, it will eventually join theland at the other end.
5.This results in the straightening of the coastline and the formation of abrackish lagoon on the landward side.
6.Typical of a drift-aligned onshore bar, the sediment is sorted ascoarser sediment is deposited first and finer sediments are carried across the bay as the bar extends.An example of this can be found at Loe in Cornwall.
Another type of onshore bar is ‘swash-aligned’, this is when waves break parallel to the coast.
Coastlines exposed to constructive waves, those waves with long wavelengths and low periodicity,result in the landward migration of offshore seabed material. The development of these onshorebars is thought to originate from the period of post glacial sea level rise about 6000 years ago. As sealevels rose, waves scoured the glacial seabed deposits and carried them landward. Typical of a swashaligned onshore bar the sediment in unsorted and comprises greater proportions of shingle deposits.An example of an onshore bar that has been created by both swash and drift aligned processes, isSlapton Sands, which is 100m wide and 5km long. It stretches from Torcross to Strete on the southcoast of Devon
‘Explain the formation of beaches’
‘Beaches are the most common landform of deposition and represent the accumulation of material deposited between the lowest tides and the highest storm wavs.
Beach material, which consists of sand, pebbles and cobbles, comes from three main sources:
cliff erosion (around 5%), offshore (around 5%) and rivers (up to 90%).Sand produces beaches with a gentle gradient, usually less than 5˚, because its small particle size means that it becomes compact when wet. This allows little percolation during backwash. As little energy is lost to friction, and little volume is lost to percolation, material is carried back down the beach rather than being left at the top. This result in a gentle gradient and the development of ridge and runnels parallel to the shore.Shingle, a mix of pebbles and small to medium sized cobbles, produces steeper beaches because swash s stronger than backwash so there is a net movement of shingle onshore. Single may make up the upper part of the beach where rapid percolation due to larger air spaces, means that little backwash occurs and so material is left at the top of the beach. There are very few well-developed beaches along the stretch of coastline from Saltburn to Flamborough Head in Yorkshire. However, the best examples are found in the sheltered, low energy environments such as Scarborough and Filey Bay. There is a high tidal range of around 4m
