Section 3 - Coastal processes 2

How can Geology affect the rate of erosion?

Differential erosion occurs when rocks wear away at different rates.Limestone has joints which can be exposed by the seaGranite and chalk tend to withstand erosion compared to softer clay based rocks.

If rocks lie parallel to the coast or at right angles they will produce very different coastlines.

In Purbeck the southern coast has rocks lying parallel and this is called concordant. The stronger Limestone has only been broken in a few places e.g. Lulworth Cove where it reaches the weaker clay.

The rocks on the Eastern side run a t right angles known as Discordant. The result is large bays followed by headlands that stick out into the sea.

The Dip of the rock is extremely important is the shape and sturdiness of a cliff.

Longshore Drift (Littoral Drift)

Waves approach the shore at an angle most of the time due to sea bed, refraction etc

Material will be pushed up the beach at the same angle by the Swash.

Backwash is influenced by gravity and so falls back at a right angle to the beach line

Over time there is a lateral shift of material

If the material that is being moved away is greater than the material arriving beach erosion will occur.

In south England the prevailing wind ensures LD that is West to East

http://www.geographyalltheway.com/year8_geography/coasts/imagesetc/longshore_drift.gif

http://www.swgfl.org.uk/jurassic/anim2.html

Groynes are used to control beach material. They are large wooden blocks heading out to sea. Accumulation of material will occur on the windward side.

It is particularly useful if the beach is a large tourist destination

Beaches

Constructed from sand and shingle.

Sand - has lower profile as its small size means it will compact when wet and allow little percolation.

Most of the swash therefore returns as backwash and little energy is lost in friction due to the smooth surface.

Material is often carried down the beach

This will cause the development of ridges and runnels at the lower water mark running parallel to the shore.

Shingle – The larger the material generally causes a steeper gradient. Because more water will percolate through the shingle the backwash is weaker and thus less material is moved down the beach.

During the spring tides the highest waves will throw material to the back of the beach. This creates a storm beach with a steep profile. The ridges that follow show the successively lower high tide marks. These are called berms. The result of constructive waves.

Cusps are semicircular shaped depressions formed when waves break directly onto the beach

They have both a strong swash and backwash. Once developed the swash is channeled into the centre and so a stronger backwash forms in this part increasing the erosion.

Ripples form on the sand by the action of wave movement in the lower part of the beach.

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Longshore drift moves material along the coast. When the coastline changes direction (more North – South) the material is carried into the more sheltered water and begins to build up eastwards. Storms bring more material that rises above the high water mark.

Over time the feature develops and the finer material continues to build eastwards.

Spits

A spit is a long ridge of land that has one end joined to the mainland and projects out into the sea across an estuary.

They composition of a spit depends on the source of the material that has been eroded away.

The end of the spit curves due to wave refraction carrying the material round into more sheltered water. Wind direction may also play a role here.

The river carries material out from the estuary into the sea and prevents the spit connecting to the land

There may be several curves due to periods of dominant weather / winds changing the direction

Sediment stars to infill the water behind the spit and salt marshes may develop.

Dunes can develop if strong winds lift sand and propel it inland

If the spit joins an island to the mainland then a tombolo is formed. If the spit runs across a bay because the flow of water is not strong from an estuary then it is called a bar.

Changes is sea level since the ice age is thought to have contributed to development of these features.

Sand Dunes

Sand is blown inland from the process of saltation. If there is a large tidal range sand dries out and is exposed to wind.

Obstacles such as seaweed and beach debris trap sand at the back of the beach e.g. high berm. Sand has a high PH (carbonate from seashells) and is not hospitable. Pioneers hardy plants are the only ones to colonise the area e.g Lyme Grass, Sea Rocket, Salwort

The first dunes to develop are Embryo dunes. They are colonised by grasses such as couch, lyme and marram. They are able to grow upwards through the wind blown sand and give the ground some stability.

These plants have long roots (marram), leaves that contain water (sea couch). This helps with water retention and adds organic matter to the dunes making it more hospitable for later plants.

The next type of dune is known as a Fore dunes. These are initially yellow but as more organic matter is formed they turn to a grey colour. The smaller ones are around 5mts but can grow to 20 mts on the main ridge. They have a slightly alkaline PH. Much less bare sand will be exposed here.

The Grey dunes further inland become more fixed. With increasing organic matter a humus layer develops. This improves the nutrient supply and more water is retained. Plants that could not survive in previous conditions now flourish. E.g. include lichens, moss, red fescue

Dune Slacks develop in between dunes where conditions are damp. Mosses, willows and reeds tend to favour these conditions. The water table is closer to the surface here.

Behind the grey dunes the sand supply is cut off giving smaller dune features. Often referred to as the dune heath (wasting dunes). Heather is usually a big plant species.

Blowouts may be visible where the wind has been funnelled and removed sand to leave the dune bare.

Beyond the dune heath a woodland may develop and this is the beginning of the climatic climax vegetation for the British isles. This is known as a psammosere. If humans have interfered and this stage is not reached i.e. farm animals or human planted species stop it the development is known as plagioclimax.

Salt Marshes

These are formed in sheltered river estuaries or behind spits. Mudflats are formed from rivers or gentle tides depositing silt and mud. Vegetation develops in a succession known as a Halosere.

Plants able to tolerate slat and the occasional submergence by the sea start to colonise. E.g. glasswort, seablite, spartina. These help secure the mud and bind it together – Spartina in particular.

Vegetation covers up to 15cm high as more organic matter and sediment builds up. 25mm growth upwards a year is common for a developing mudflat. Sea lavender, sea thrift and red fescue develop with the Spartina and early colonisers.

The tidal currents are slowed by the vegetation, helping more material build up.

As mud levels rise the complex creek systems develop and this channels the tides as the marsh grows higher. If the land can build up above high tide colonisation can lead to growth of tress such as oak.