- Created by: Procrastination101
- Created on: 22-02-19 17:27
Wave types and their characteristics
How waves are formed - Wind blows over the sea and friction with the surface of the water causes ripples that develop into waves.
Fetch = the distance that wave-generating winds blow across the water.
When waves reach the coast:
- Circular orbit in open water until friction with the seabed distorts the circular motion.
- Crest of wave moves faster - increasingly elliptical orbit as water becomes shallower and waves move forward.
- Waves break and collapse onto beach
Constructive waves - Formed by storms more than 100km away, common in summer, low waves with wave crest far apart, strong swash, weak backwash, waves push sand and pebbles up the beach 'constructing it.'
Destructive waves - Formed by local storms close to the coast, common in winter, waves close together, waves become high with steep wave front, waves plunge down onto beach with little swash, strong backwash erodes sand and pebbles - can destroy beach.
Weathering and mass movement
Weathering (weakening and breakdown of rock) - Mechanical (physical): Freeze thaw - water collects in cracks in rock, at night the water freezes and expands making cracks larger, as temperature rises the ice thaws and water seeps deeper into the rock. Repeated freezing and thawing makes rock fragments break off and collect as scree at the cliff foot.
Chemical: Carbonation - Rainwater absorbs CO2 from the air and becomes slightly acidic. Contact with alkaline rocks e.g. limestone, produces a chemical reaction causing rocks to slowly dissolve.
Biological: Plant roots grow in rock cracks and animals (e.g. rabbits) burrow into weak rocks.
Mass movement - the downward movement of material -
Rockfall: Rock breaks away, often due to freeze-thaw.
Landslide: Blocks of rock slide downhill.
Mudflow: Saturated soil and weak rock flows downhill.
Rotational slip: Slump of saturated soil and weak rock along a curved surface.
Coastal erosion and deposition
Solution - Dissolving of soluble chemicals in rock (e.g. limestone).
Corrasion - Rock fragments picked up by the sea are thrown at the cliff - they scrape and wear away the rock.
Abrasion - The 'sandpapering' effect of pebbles grinding over a rocky platform.
Attrition - Rock fragments carried by the sea knock against each other, becoming smaller and more rounded.
Hydraulic power - The power of the waves as they hit a cliff - trapped air is forced into cracks in the rock, eventually causing it to break up.
Deposition happens when water slows down and waves lose their energy:
- Beaches are formed of sediment deposited in bays.
- Mudflats and saltmarshes are often found in sheltered estuaries behind spits.
Transportation and Longshore drift
Solution - dissolved chemicals often derived from limestone or chalk.
Suspension - particles carried (suspended) within the water.
Traction - large pebbles rolled along the seabed.
Saltation - a 'hopping' or ' bouncing' motion of particles too heavy to be suspended.
The movement of sediment depends on the direction that waves approach the coast, as a result of the prevailing wind direction. Where waves approach head on, sediment moves up and down the beach.
Longshore drift - Where waves approach at an angle due to prevailing wind and sediment is moved along the beach in a zigzag pattern.
Coastal erosion landforms
Headlands and bays:
- Tougher, resistant bands of rock are eroded slowly to form headlands.
- Weaker rock erodes more easily to form bays. Bays are sheltered, deposition occurs, and a beach is formed.
Caves, arches and stacks:
- Lines of weakness (joints or faults) in resistant rock are vulnerable to erosion.
- Abrasion and hydraulic action widen the weakness, forming a cave.
- Erosion may lead to two back-to-back caves, breaking through a headland to form an arch.
- The arch is enlarged by erosion and the roof is attacked by weathering processes and collapes, leaving an isolated stack.
- The stack is eroded and collapes, leaving a stump.
Wave-cut platforms: When waves break against a cliff, erosion close to the high tide line will form a wave-cut notch. Over time, the notch deepens, undercutting the cliff, causing it to collapse. This repeats, the cliff retreats and leaves behind a wave-cut platform.
Coastal deposition landforms
Beaches - Sandy beaches are mainly found in sheltered bays and are created by constructive waves. Along high-energy coasts, sand is washed away leaving behind a pebble beach.
- Embryo dunes form around obstacles such as rocks.
- Dunes develop and are stabilised by vegetation (e.g marram grass) to form fore dunes and tall yellow dunes.
- Decomposing vegetation makes sand more fertile and a wider range of plants colonise the back dunes. Ponds (dune slacks) can form in depressions.
Spits and bars:
Spit = a long finger of sand or shingle, jutting out into the sea.
Bars: formed when longshore drift causes spits to grow across a bay.
Offshore bars form further out to sea where waves approaching a gently sloping coast deposit sediment due to friction with the sea bed.
Hard engineering strategies
Sea walls - concrete or rock barrier at the foot of cliffs or top of beach. Curved to reflect waves out to sea. Effective at stopping the sea and often creates a walkway. Can look obtrusive and unnatural. Very expensive; high maintenance costs.
Groynes - rock or timber structures built at right angles to beach. They trap sediment moved by longshore drift and enlarge the beach - wider beach reduces wave damage. Create a wider beach - good for tourism, and not too expensive. Interrupting longshore drift can lead to increased erosion elsewhere. Unnatural and are unattractive.
Rock armour - piles of large boulders at foot of cliff. Rocks absorb wave energy to protect the cliff. Relatively cheap; easy to maintain. Can add interest to the coast. Rocks are often from elsewhere and don't fit in with local geology. Expensive to transport rock and can be obtrusive.
Gabions - rock-filled wire cages that support a cliff and provide a buffer against the sea. Cheap to produce, can improve cliff drainage, and eventually become vegetated and merge into landscape. Unattractive initially. Cages rust within 5-10 years.
Soft engineering strategies
Beach nourishment - sand or shingle is dredged offshore and transported to the coast by barge. It is dumped on the beach and shaped by bulldozers, creating a wider, higher beach (known as re-profiling). The beach protects land and property. Relatively cheap and easy to maintain and blends in with existing beach. Bigger beach increases tourist potential. Needs constant maintenance.
Dune regeneration - marram grass is planted to stabilise dunes and help them develop, which makes them effective buffers to the sea. Fences keep people off newly planted areas. Maintains a natural environment - good for wildlife. Relatively cheap. Time-consuming to plant grass and construct fencing. Can be damaged by storms.
Dune fencing - fences are constructed along the seaward side of existing dunes to encourage new dune formation. New dunes help to protect existing dunes. Little impact on natural systems. Controlling access protects other ecosystems. Can be unsightly and needs regular maintenance.
River erosion and transportation
Hydraulic action- The force of water hitting the river bed and banks
Abrasion- The load carried by the river hits the bed or banks, dislodging particles
Attrition- Stones carried by the river knock against each other, becoming smaller/more rounded
Solution- Alkaline rocks, e.g. limestone, are dissolved by slightly acidic river water
Solution- (dissolved load)
Suspension- small sediment held in the river
Traction- large particles rolled on the riverbed
Saltation- 'bouncing' of particles too heavy to be suspended.
When does deposition happen?
When a river's velocity decreases and it no longer has the energy to transport its load, it deposits it.
- Larger rocks transported mainly by traction are only carried short distances during periods of high flow. They are deposited in a river's upper course.
- Smaller sediment is carried further downstream - mostly in suspension. It is deposited on a river's bed and banks where velocity slows due to friction.
- Lots of depostion occurs at a river's mouth where its velocity reduces because of the gentle gradient and also by interaction with tides.
Upper course - mostly erosion landforms e.g. waterfalls.
Middle course - mostly erosion and deposition landforms, e.g. meanders and transportation.
Lower course - mostly depostion landforms e.g. levees.
River erosion landforms
Interlocking spurs - A mountain stream erodes vertically creating a v-shaped valley. It winds around areas of resistant rock to create interlocking spurs which jut out into the valley.
Waterfalls - As a river flows downstream it crosses different rock types. More resistant rocks are less easily eroded than less resistant rocks, forming steps in a river's long profile. It is formed by the undercutting of softer rock by hydraulic action and abrasion which causes the hard rock overhang to eventually collapse and the waterfall retreats. They can also form when sea level drops causing a river to cut down into its bed creating a step (a knick point) or in glacial hanging valleys.
Gorges - A gorge is a narrow, steep-sided valley found downstream of a retreating waterfall.
Gorges also be formed:
- at the end of the last glacial period masses of water from melting glaciers poured off upland areas forming gorges,
- on limestone, when large underground caverns can accommodate an entire river.
River erosion & deposition landforms
Meanders - They are formed when moving water in a stream erodes the outer banks and widens its valley, and the inner part of the river, with less energy, deposits silt. The thalweg is the line of fastest current that swings from side to side causing erosion on the outside bend and deposition on the inside bend. This causes meanders to migrate across the floor.
Oxbow lakes - The neck of the meander is gradually eroded and the water begin to take the shortest route. Depostion seals off the old meander until the meander neck is cut through completely - leaving an oxbow lake.
Floodplains - Are wide, flat areas on either side of a river in its middle and lower courses. They are created by migrating meanders and floods depositing layers of silt to form alluvium.
Levees - Formed when, in low flow, deposition raises the river bed so the channel can't carry as much water. During flooding, velocity decreases, and coarser sediment is deposited on the banks, followed by sand and mud, rasing the height of the levees.
Estuary - This is where the river meets the sea. They are affected by tides, wave action and river processes. As the tide rises, rivers can't flow into the sea so velocity falls and sediment is deposited forming mudflats, which develop into salt marshes.
Factors increasing flood risk
Physical causes of flooding:
- Precipitation - torrential rainstorms and/or prolonged periods of rain.
- Geology - impermeable rocks don't allow water to pass through, so it flows into rivers.
- Relief - steep slopes mean water flows quickly into river channels.
Human causes of flooding:
- Urbanisation - impermeable surfaces e.g. tarmac roads, mean water flows quickly into drains, sewers and river channels.
- Deforestation - when trees are removed, much of the water usually evaporated from leaves or stored on leaves, flows rapidly into river channels.
- Agriculture - exposed soil can lead to increased surface runoff
Hydrograph - shows how a river reacts to a rainfall event. It shows rainfall and discharge (volume of water flowing in a river measured in cumecs).
Factors affecting its shape - basin size, drainage density, rock type and permeability, land use, relief, soil moisture, rainfall intensity, antecedent rainfall.
Managing floods - hard engineering
Dams & reservoirs - Widely used to regulate river flow and reduce risk of flooding. Often multi-purpose e.g. flood prevention, HEP generation, and water supply. Can be effective in regulating water flow and can store water in reservoir. Expensive and reservoirs often flood large areas of land.
Channel straightening - Cutting through meanders creating a straight channel which speeds up water flow. This can increase flood risk downstream and they may be lined with concrete which can be unattractive and can damage wildlife habitats.
Embankments - Raise the level of river banks, allowing the channel to hold more water to help prevent flooding. Concrete or stone walls are often used in towns though mud dredged from the river can be used. This is cheaper, more sustainable and looks more natural.
Flood relief channels - Can be built to by-pass urban areas. At times of high flow, sluice gates allow excess water to flow into the flood relief channel, reducing the threat of flooding.
Managing floods- soft engineering
Afforestation - Trees obstruct the flow of water, and slow down its transfer to river channels. Water is taken up by trees and evaporated from leaves and branches. It is relatively cheap with environmental benefits.
Wetlands and flood storage areas - Wetlands are deliberately allowed to flood, forming storage areas. Reduces the risk of flooding downstream.
Floodplain zoning - Restricts different land uses to certain zones on the floodplain. Areas at risk from flooding can be used for grazing, parks, and playing fields. Can reduce losses caused by flood damage. Can be difficult to implement on already developed land.
River restoration - When a river's course has been changed artificially, it can be restored to its original course. It uses the natural processes and features of a river e.g. meanders and wetlands to slow down and reduce the likelihood of flooding downstream.