Coastal erosion

COASTAL EROSION Natural forces such as wind, waves and currents are constantly shaping the coastal regions. The combined energy of these forces moves land materials. The landward displacement of the shoreline caused by the forces of waves and currents is termed as coastal erosion. It is the loss of sub-aerial landmass into a sea or lake due to natural processes such as waves, winds and tides, or even due to human interference

Major Causes of Coastal Erosion Natural Causes Action of Waves: As waves move towards the shore, waves break and the turbulent energy released stirs up and moves the sediments deposited on the seabed. The wave energy is a function of the wave heights and the wave periods. Winds: Winds acts not just as a generator of waves but also as a factor of the landwards move of dunes Tides:. During high tides, the energy of the breaking waves is released higher on the foreshore or the cliff base (cliff undercutting) Catastrophic events events like tsunamis which result in major coastal changes over very short time periods.

Near-shore currents: Currents are generated by the action of tides (flood currents), waves breaking at an oblique angle with the shore (long-shore currents), and the backwash of waves on the foreshore (rip currents). All these currents contribute to coastal erosion processes. Storms. Storms result in raised water levels (known as storm surge) and highly energetic waves induced by extreme winds (Cyclones). Combined with high tides, storms may result in catastrophic damages Slope processes. The term “slope processes” encompasses a wide range of land-sea interactions which eventually result in the collapse, slippage, or topple of coastal cliff blocks. These processes involve on the one hand terrestrial processes such as rainfall and water Vertical land movements (compaction). Vertical land movement – including tectonic movement, or sediment settlement

Rising sea level: Size of polar ice caps and valley glaciers which change the amount of water available for oceans and seas Expansion and contraction of ocean volume due to changes resulting from plate tectonics Rising and sinking of coasts due to plate tectonic changes Warming of ocean water which leads to increased volume for the same amount of water Increase in total water available in the ocean due to human activities which release groundwater and make it part of the surface water system

Rising sea level: Sea level has risen about 40 cm in the past century and is projected to rise another 60 cm in the next century. Sea level has risen nearly 110 meters since the last ice age. Due to global warming, average rise of sea level is of the order of 1.5 to 10 mm per year. It has been observed that sea level rise of 1 mm per year could cause a recession of shoreline in the order of about 0.5 m per year.

Anthropogenic causes Human influence, particularly urbanisation and economic activities, in the coastal zone cause coastal erosion construction of artificial structures, mining of beach sand, offshore dredging,or building of dams or rivers triggers beach erosion

Corrosion Hydraulic Action Processes of Erosion Abrasion Attrition

Abrasion/Corrosion This is the process by which the coast is worn down by material carried by the waves. Waves throw these particles against the rock, sometimes at high velocity. Hydraulic Action This process involves the force of water against the coast. The waves enter cracks (faults) in the coastline and compress the air within the crack. When the wave retreats, the air in the crack expands quickly causing a minor explosion. This process is repeated continuously. Corrosion This is the chemical action of sea water. The acids in the salt water slowly dissolve rocks on the coast. Limestone and chalk are particularly prone to this process. Attrition Material carried by the waves bump into each other and so are smoothed and broken down into smaller particles.

Landforms of Coastal Erosion 1) Headlands and Bays 2) Cliffs and Wave Cut Platforms 3) Caves, Arches, Stacks and Stumps

Takes an initially irregular coast and straightens it

Headlands and Bays Formation wave attack This is the bay which is made from less resistant rock, and therefore erodes more quickly. This is the headland which is made from more resistant rock than the bay.

An example of headlands and bays on the Dorset coastline……. chalk clay Swanage Bay limestone

Waves travel more slowly in the shallow water near the headlands Waves travel more slowly in the shallow water near the headlands. This causes the waves to bend and concentrate their energy on the headlands. This, in turn, protects the bay from the full force of the waves. Wave Refraction Wave fronts

Cliff Recession Wave cut platform Old wave cut notch High tide Low tide Wave cut platform The waves attack the base of the cliff through the processes of abrasion, corrosion, hydraulic action and attrition. Over time the cliff will be undercut and a wave cut notch is formed. Eventually the cliff becomes unstable and collapses. Further cliff retreat will form a wave cut platform.

Caves, Arches, Stacks and Stumps The stack erodes to become a STUMP. The waves undercut the arch base and it becomes widened. After a time the arch becomes unsupported and collapses to form a STACK. The waves erode faults (cracks) in a headland through the processes of erosion. The faults eventually widen into a CAVE. The waves eventually erode through the headland to form an arch.

This is a cave (‘Stair Hole’) 1 2 This is a cave (‘Stair Hole’) This is an arch (‘Durdle Door’) 3 4 This is a stack (Old Harry’s Rocks) These are stumps

Features of coastal erosion

cliff

crack

cave

arch

stack

stump

coastal retreat

TYPE OF WAVE NATURE OF COASTAL ROCKS FACTORS influencing THE RATE OF EROSION IMPACT OF HUMAN ACTIVITIES THE GRADIENT OF THE COAST

TYPE OF WAVE Destructive waves cause MORE EROSION than constructive waves – have a more powerful backwash than swash

In terms of lines of weakness: NATURE OF COASTAL ROCKS Rocks that make up the coast may have different resistance to erosion In terms of lines of weakness: Coastal rocks that have more lines of weakness will be MORE EASILY ERODED than, Those with fewer lines of weakness In terms of mineral composition: Coasts that are made up of granite, basalt, and dolomite – MORE RESISTANT TO EROSION than, Coasts that are made up of limestone, shale, and clay

THE GRADIENT OF THE COAST Coasts with a STEEPER GRADIENT tend to COLLAPSE FASTER

IMPACT OF HUMAN ACTIVITIES Human through their activities can speed up or slow down the process of coastal erosion Construction works – speed up Dredging – speed up Constructing groynes to trap sand and rock debris and others - slow down

Coastal landforms resulting from sea level change - THE CYCLE: Temperatures decrease Glaciers and ice sheets grow Sea level falls. Ice thickness increases and land lowers isostatically Temperatures rise Ice melts and sea levels rise eustatically Melting releases pressure and land rises isostatically

SUBMERGED COASTLINES Fjords These are coastlines that have been formed due to rising sea levels (eustatic). Fjords Valleys that have been created by the slow progression of glaciers & then subsequently filled with rising sea water. Norway

Rias Rias are similar to Fjords. However rather than being a drowned glacial valley, they are drowned river valleys. The shape of the Ria depends on the structure of the coastline. Examples can be seen along the south-west of Ireland as well as the Dalmatian coast of Croatia

These are coastal landforms created as a result of isostatic changes. EMERGENT COASTLINE These are coastal landforms created as a result of isostatic changes. Raised Beaches Glacial ice melted here 15,000 years ago, returning the water which had been locked up in the ice to the sea. Consequently sea levels rose rapidly and prominent beaches were cut all round Cowal. With the removal of the ice load, however, Central Scotland has lifted up since the Ice Age, thus raising the earlier levels into the well known 'raised beaches'

Coastal erosion impacts danger to life in the case of sudden onset landslide events structural damage or destruction of buildings and infrastructure damage or destruction of lifeline infrastructure such as water, sewage and gas pipes and roads loss of land, resulting in coastal cliffs or shorelines retreating closer to other buildings land instability at neighbouring slopes and properties loss of beach amenity due to cliff collapse or sea wall construction.

Mitigation methods Groynes are designed to slow down longshore drift and build up the beach. They are wooden barriers built at right angles to the shore prevent the movement of beach material along the coast by longshore drift. They allow the build up of a beach which are a natural defence against erosion and an attraction for tourists. The disadvantages are that they can be seen as unattractive and costly to build and maintain.

Breakwater

A breakwater is often used to protect a harbour but may be used to protect a stretch of coastline, usually made out of concrete or blocks if stone. They have to be strong enough to take the full force of the waves. Since they have to be built in deep water they are, like sea walls, expensive to build.

gabions

The gabion is a metal cage filled with rocks stacked to form a simple wall. They are used to protect a cliff or area in the short term only, since they are easily damaged by powerful storm waves and the cages tend to rust quite quickly. Gabions have the advantage of ease of use and are relatively cheap but their life span is short.

revetments

A cheaper alternative to sea walls is the revetment. This is a sloping feature which breaks up and absorbs the energy of the waves but may let water and sediment pass through. The disadvantages are that it only protects against erosion and that have a negative impact on the sediment along the shoreline and it decreases the release of sediments.

Sea wall

The sea wall is the more traditional 'hard' defence used to protect coasts. Sea walls have a slope and curved top which breaks up the energy of the wave and prevents water going over the top of the wall during heavy storms. Sea walls are very expensive but should last 20-30 years. Another disadvantage is that they increase the backwash therefore the beach can be eroded away very quickly.