Coasts
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課程筆記
What is Coastal Erosion?
- Coastal erosion is the loss or displacement of land along the coastline due to waves, currents, tides, wind-driven water, and storms.
- It involves the long-term removal of sediment and rocks, causing the shoreline to retreat landwards.
- Erosion can be measured over different timescales, from individual tides to seasons and longer cycles.
- On coasts with rocks of varying resistance, softer areas erode faster, creating landforms like tunnels, bridges, columns, and pillars.
- Over time, the coast tends to even out as softer areas fill with sediment from harder rocks, and rock formations are worn away.
- Erosion is common where there are strong winds, loose sand, and soft rocks; blowing sand grains create a sandblasting effect that smooths and polishes rocks.
- According to the IPCC, sea level rise caused by climate change will increase coastal erosion worldwide, significantly changing coasts and low-lying areas.
Processes of Coastal Erosion
- Hydraulic action: waves strike a cliff face, compressing air in cracks; this exerts pressure, splintering and removing rock pieces, and over time can form caves.
- Attrition: loose rock debris (scree) collides and grinds against itself, becoming smaller, smoother, and rounder; it also chips the cliff base.
- Solution: acids in seawater dissolve certain rocks, such as chalk and limestone.
- Abrasion (also called corrasion): waves break on cliff faces and use scree to batter and break off rock pieces from higher up the cliff.
- Corrosion: seawater with a pH below 7.0 corrodes rocks, particularly limestone; wave action removes reacted material, increasing the reaction rate.
Factors Influencing Erosion Rates
- Primary factors include the hardness of sea-facing rocks (controlled by rock strength and presence of fissures, fractures, and non-cohesive materials like silt and fine sand).
- The rate at which cliff fall debris is removed depends on wave power; debris lobes can persist for many years if wave energy is insufficient.
- Beaches dissipate wave energy and protect the land behind them; a stable, wide foreshore reduces the number and power of waves reaching the cliff.
- The adjacent bathymetry (seafloor configuration) controls wave energy arriving at the coast; shoals and bars cause storm waves to break and dissipate energy before reaching shore.
- Changes in the location of shoals and bars can shift the focus of erosion along the shore.
- Secondary factors include weathering and transport slope processes, slope hydrology, vegetation, cliff foot erosion, cliff foot sediment accumulation, and resistance of cliff foot sediment to attrition and transport.
- Tertiary factors include resource extraction and coastal management.
Landforms of Erosion
- Headlands and bays form where rocks of different resistance meet the sea; softer rocks erode faster to form bays, while harder rocks remain as headlands.
- Cliffs are steep rock faces formed by wave erosion at the base, causing the cliff to collapse over time.
- Wave-cut platforms are flat, rocky areas left at the base of cliffs after the cliff has retreated.
- Caves form when hydraulic action and abrasion exploit weaknesses in a headland, hollowing out the rock.
- Arches form when a cave is eroded through a headland to the other side.
- Stacks are isolated pillars of rock left standing in the sea after an arch collapses.
- Stumps are the remains of stacks that have been eroded down to just above sea level.
Longshore Drift and Landforms of Deposition
- Longshore drift is the movement of sediment along the coast by waves approaching at an angle, driven by the prevailing wind.
- Sediment is carried up the beach at an angle by swash and returns straight back down by backwash, creating a zigzag movement.
- Beaches are accumulations of sand and shingle deposited by constructive waves; they dissipate wave energy and protect the coast.
- Spits are ridges of sand or shingle that extend into the sea, formed by longshore drift where the coastline changes direction.
- Bars form when a spit grows across a bay, cutting off the water behind it to form a lagoon.
Coastal Management
- Hard engineering involves building structures to prevent erosion, such as sea walls, groynes, and rock armour.
- Soft engineering works with natural processes, for example beach nourishment and dune stabilisation.
- Relocation (managed retreat) allows the coastline to erode naturally in some areas, often with compensation for landowners.
- Hard engineering can be expensive and may cause increased erosion elsewhere by disrupting longshore drift.
- Soft engineering is often cheaper and more environmentally friendly but may need ongoing maintenance.
UK Coastline Case Study
- The UK has a diverse coastline with many examples of erosion and deposition landforms.
- The Holderness Coast in Yorkshire is one of Europe's fastest eroding coastlines, with soft boulder clay cliffs.
- Erosion at Holderness leads to the loss of farmland and properties, and contributes to the growth of Spurn Head spit.
- Coastal management at Holderness includes hard engineering (sea walls, groynes) and soft engineering (beach nourishment).
- The Jurassic Coast in Dorset is famous for its erosional landforms, including Lulworth Cove, Durdle Door arch, and Old Harry Rocks stacks.
投影片
練習題
免費預覽——59 題中的 8 題。註冊即可查看全部。
1.Which process describes waves compressing air in cracks in a cliff face, causing pieces of rock to splinter and break off?
Easy- AHydraulic action
- BAttrition
- CSolution
- DLongshore drift
2.Which erosion process involves loose rock fragments colliding with each other and becoming smaller, smoother and rounder?
Easy- AAbrasion
- BAttrition
- CSolution
- DHydraulic action
3.Which coastal erosion process is also known as corrasion and involves waves using sediment to batter and break off pieces of rock from a cliff face?
Easy- AAttrition
- BSolution
- CAbrasion
- DHydraulic action
4.Which type of rock is most affected by solution because acids in sea water dissolve it?
Medium- AGranite
- BChalk or limestone
- CBasalt
- DSlate
5.Which statement best explains why softer rock layers erode faster than harder rock layers along a coast?
Medium- AHarder rocks are more resistant to erosion, so softer areas are worn away first, creating features such as bays and headlands.
- BSofter rocks are more resistant to erosion, so they stand out as headlands.
- CAll rocks erode at the same rate regardless of hardness.
- DHarder rocks dissolve more easily in sea water.
6.Which human activity is listed as a tertiary factor influencing coastal erosion?
Medium- AVegetation growth
- BCoastal management
- CSlope hydrology
- DCliff foot sediment accumulation
7.Which of the following are processes of coastal erosion? (select all that apply)
Medium- AHydraulic action
- BAttrition
- CLongshore drift
- DSolution
- EDeposition
8.Which of the following are hard engineering methods used to control coastal erosion? (select all that apply)
Medium- ASea walls
- BGroynes
- CBeach nourishment
- DRock armour
- EManaged retreat
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