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Coasts Notes

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COASTS Waves Waves are created by a transfer of energy from the wind, blowing across the surface of the sea Constructive waves

Long wavelength - 100m

Low frequency - 6 to 8 per minute

Great swash and weak backwash

Berms and ridges formed

Destructive waves

High frequency - 10 to 14 per minute

Steep and break heavily

Strong backwash

Material projected to the back of the beach to form storm beaches

Wave Refraction Refraction is the change in direction of a wave due to a change in its speed. When waves approach a coastline that is not of a regular shape, they are refracted and become increasingly parallel to the coastline. As waves near the coast, they become higher and steeper with a shorter wavelength, causing the waves to bend. As a result, wave energy becomes concentrated on the headland, causing greater erosion.

Tides Tides are the periodic rise and fall of the level of the sea, caused by the gravitational pull of the sun and moon.

The moon pulls water towards it, creating a high tide. On the opposite side of the moon, there is a compensatory bulge where the tide is also high - between these two bulges is where the tide is lowest

Spring tide - when the moon and sun are in a straight line, the tide is strongest and highest Neap tide - when the moon and sun are at 90 degrees to each other, the tide is lowest

Storm Surge

Meteorological conditions give rise to strong winds which causes higher water levels than those produced at a high tide.

North Sea Storm Surge, 1953 Caused by

Large fetch from the north, together with the strong winds caused waves of 6m high

High spring tides and river discharging into the sea at flood levels

A deep Atlantic depression moved across Scotland, deepening all the time, and by the time it had reached the coast of Denmark, the central pressure had drastically fallen - this rapid fall in pressure caused the surface of the sea to rise by 0.5m

Effects

Sea defences breached in several places

11,000 homeless

Huge livestock losses

250 people drowned

In the Netherlands - 1,800 people dies and 10% of the country's agricultural land was flooded

Sediment sources and cells Sediment comes from a variety of sources, such as the sea bed, beaches and river channels. Sediment movements occur in distinct areas, known as cells, within which input and outputs are balanced. Sediment cells are closed systems in which nothing is gained or lost, furthermore these cells can vary greatly in size.

Coastal Processes Marine Erosion Processes operating upon a coastline that are connected with the sea such as waves, tides and longshore drift. Hydraulic action - a breaking wave traps air as it hits the cliff. The strong force of water compresses the air into any gap in the rock, creating huge pressure in the joint. As the water pulls back, the pressure is suddenly released, causing the rock to break away Abrasion - the material that is picked up from the sea is thrown against the cliff face. Sediment such as sand and shingle also grinds against the rock, wearing it away

Attrition - rocks in the sea which carry out abrasion are broken down into smaller particles of sediment Corrosion - dissolving of calcium based rocks (limestone) by the chemicals in sea water Factors affecting the rate of erosion

Wave steepness - steeper waves have more energy and so more power to erode

Fetch - the further the wave has travelled, the more sediment it has picked up therefore it has more capacity to erode

Coastal configuration - headlands attract more erosion through refraction

Beach presence - beaches absorb wave energy and therefore reduce erosion on the cliff face

Geology Lithology

Very resistant rocks (granite) are less easily eroded than weaker materials (clay)

Rocks that are well jointed (limestone) are easily eroded as the sea can penetrate along their lines of weakness

Structure

Concordant coastlines where the rocks run parallel to the coast - reduce the amount of erosion as the sea can only breakthrough in a few places

Discordant coastlines where the rocks run at right angles to the coast - increased erosion as the sea can penetrate along the weaker rock

Longshore Drift Waves approach the shore at an angle and are pushed up the beach at this same angle. As the water runs back down to the sea, backwash carries material at a right angle to the beach. Sediment is carried in this 'zigzag' fashion down the coast. Groynes are used in an attempt to stop the longshore drift; however this causes material to accumulate behind the groynes leading to entrapment of beach material.

Sub-aerial processes Processes operating on the land but affecting the shape of the coastline. Biological weathering - marine organisms (piddocks) have specially adapted shells that enable them to drill in the rock. Seaweed also attaches itself to rocks, however the force of the water causes it to pull away and as a result, breaks the rock away as well Mass movement - the down slope movement of earth materials under the influence of gravity

Rock falls from cliffs undercut by the sea

Landslides when softer rock or material slips down when wet

Mudflows when heavy rain causes large amounts of mud to fall downhill

Slumping is caused when soft material overlies more resistant material

Runoff - when the soil is infiltrated to full capacity, excess water flows over the land

Headlands and Bays Along the coastline, where there is more resistant rock (granite), there is less erosion and so this rock develops to form a headland. The headland is subject to refraction and therefore receives the highest energy waves. The less resistant rock (limestone) is easily eroded and therefore breaks away easily to form a bay. These bays experience low energy waves that allow sediment to accumulate and form beaches, which act as protection of the coastline. Lulworth Cove, Dorset In Purbeck, the rocks run parallel to the coast and therefore waves can break through the more resistant rock to attack the weaker strata behind. Therefore, in Purbeck, the force of the sea has broken through the more resistant Portland stone to form Lulworth Cove in the clay behind.

Cliffs and wave cut platforms High and steep waves break at the foot of the cliff, concentrating all of their erosive power in one area of the cliff face. This causes the cliff to become undercut, forming a wave cut notch. As waves continue to pound and erode the wave cut notch, the cliff will eventually collapse. The cliff will begin to retreat from the wave cut notch, leaving behind a gently sloping wave cut platform. The platform is extremely vulnerable to abrasion as sediment from the sea grinds and scrapes against the rock, breaking it away. Wave cut platforms can limit the amount of erosion on the cliff as they break the erosive power of the waves. The larger the wave cut platform, the further the water will have to travel and therefore the weaker the force of the wave. Geo - along a joint in the rock, the sea will cut inland, widening the crack to form a narrow, steep-sided inlet Cave - when the cliff becomes undercut and subject to great erosion Blowhole - when the cave extends to the top of the cliff

Arch - when the cave extends backwards to meet another cave, creating a hole all the way through the headland Stack - as the cliff recedes and the wave cut platform develops, the arch will eventually collapse Stump - overtime the sea will exploit the wave cut notch at the base of the stack, causing it to collapse and leave only a small portion of rock known as a stump Example - in Purbeck, the sea has cut the well known arch of 'Durdle Door'. Development of 'Old Harry Rocks' stumps from the Portland Stone.

Landforms produced by coastal deposition Beaches The accumulation of material deposited between low spring tides and the highest point reached by storm waves, constructed from sand and shingle. Sand - produces beaches with a gentle gradient, usually 5 degrees, because the material is very compact when wet, allowing very little percolation. Therefore most of the swash is returned as backwash, so material is carried back down the beach. As a result, ridges and runnels develop in the sand at the low water mark. Shingle - produces steeper beaches as the material rapidly percolates water, reducing the amount of material moved back down the beach by backwash. Storm beach - ridge composed of the largest boulders thrown by the strongest waves. Positioned at the back of the beach when swash is strong at high spring tide Berms - series of ridges positioned below the storm beach, marking the successively lower high tides Cusps - semicircular depressions which form when waves break directly onto the beach and both swash and backwash are strong

Spits and bars Spit - a long, narrow piece of land that is joined to the mainland and projects out into the sea or across an estuary. Composed of sand or shingle, spits are formed when the prevailing wind blows at an angle to the coastline, resulting in longshore drift. Spurn Head spit, found along the Holderness Coast in Humberside. Increasingly, the end of the spit will begin to curve round as wave refraction carries material round into the more sheltered water. Spits are often associated with: Sand dunes - dried out sand is blown to the back of the spit where material accumulates. The sand dune can be stabilised over time with the development of vegetation

Salt marshes - low energy, gentle waves enter the sheltered area behind the spit and deposit fine material, such as clay and silt. This material builds up to form a marsh which then develops vegetation Tombolo - when a spit joins an island to the mainland. For example, Chesil Beach in the south coast of England Bar - when a spit develops across a bay, when there is no strong flow of water from the landward side, the sediment can reach across to the other side

Sand dunes Sand dunes are a dynamic landform, where material is accumulated at the rear of the beach. Sand is mainly moved by the force of the wind, as well as the process of saltation (material is bounced along with the force of water)

1. Sand becomes trapped by obstacles at the back of the beach, possibly on the storm beach - pioneer plants develop (sea rocket)

2. Embryo dune will develop and vegetation (marram grass) will begin to grow

3. Development of embryo dunes causes them to grow upwards to form foredunes that are initially yellow as they contain little organic matter

4. Foredunes develop vegetation and organic matter to become grey dunes that are fixed

5. Dune slacks may develop which are depressions in the sand where the water table is very close to the surface and therefore little vegetation will grow

6. At the very back of the succession is the climax community where large trees (oak, birch) will develop Succession - sand dunes are an example of a succession as they grow and develop over time. The vegetation that develops stabilises the sand dune, and the succession of a sand dune is also known as a psammosere

Salt marshes In sheltered river estuaries or behind spits, gentle low tides deposit smaller sediment such as silt and clay. Over time, vegetation will develop in this area and the succession that develops is known as a halosere

1.

Fine material is deposited to form a mud flat, algae develops

2. Pioneers begin to colonise the area, these are called halophytes (sea blite) - these plants trap even more mud and therefore it becomes the dominant vegetation in the salt marsh

3. Pioneers develop a close vegetation over the mud and this allows colonisation by other plants (marsh grass) and this can build up to 15m high

4. As mud levels rise, complex creek systems develop that channel the tides and these deepen as the marsh becomes higher

Sea level change Eustatic fall - as the climate gets colder, more precipitation falls as snow rather than rain. This snow will freeze in the cold conditions and therefore water is stored as ice, slowing down the hydrological cycle. As a result, less water is returning to the sea and therefore sea levels will fall Isostatic fall - the increased weight of the ice causes the land surface to sink Eustatic rise - as the climate gets warmer, the ice will melt and replenish the store of water, causing sea levels to rise Isostatic readjustment - as the ice melts, the land will begin to rise back to its original position Currently, due to global warming, as temperatures around the world rise, more standing ice is melting, particularly in Greenland and Antarctica. Inevitably, this is causing eustatic rise and sea levels are rising. This is creating huge problems for areas such as The Maldives, and it is estimated that within 15 to 30 years, the Maldivian Islands will no longer be there

Submergent features Rias - created by rising sea levels drowning river valleys. The floodplain of a river will disappear beneath the rising sea levels, but on the edges of uplands only the middle and upper course valleys will be filled with water. Therefore, the higher land will remain dry, producing a ria Example - in Cornwall, sea level rose and drowned the valleys of the rivers flowing off the uplands of Cornwall. The Fowey estuary in Cornwall is 200m deep and 8 km wide Fjords - created by rising sea levels drowning glacial valleys that were cut to a much lower sea level. They have steep valley sides and are fairly straight and narrow. They have a Ushaped cross section with hanging valleys on either side Example - Sogne Fjord in Norway is a straight valley, 1250 km deep and 180 km wide

Emergent features Raised beaches - areas of former wave cut platforms where the beaches are at a higher level than the present sea level. Old cliff lines with wave cut notches, caves, arches and stacks are often found behind the beach - caused by isostatic uplift Example - raised beaches are common in Western Scotland - three levels have been found, 8m, 15m and 30m high

Impacts of sea level rise

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