- Shingle beaches typically have a steep gradient (over 10 degrees) because the waves easily flow through the coarse, porous surface of the beach, decreasing the effect of backwash erosion and increasing the formation of sediment into a steep sloping back.
- Sandy beaches are typically flatter (>5 degrees) and wider as the smaller particles are evenly distributed and water takes longer to percolate down into the sand so more sand is removed with the backwash.
Beach Profile Features
Ridges and runnels: parallel "hills and valleys"of sand found at the low water mark. These are formed due to interaction of tides, currents and shallow beach topography and so are often formed as breakpoint bars.
Storm Beaches: A ridge of boulders and shingle found at the back of the beach which have been thrown up to the back of the beach by the largest waves at high tides.
Cusps: Semicircular depressions formed by waves breaking directly on the beach with a strong swash and backwash.
Ripples: Develop on sandy beaches as a result of wave and tidal movements.
Hydraulic Action: When a wave impacts a cliff face, air is forced into cracks under high pressure, widening them. Over long periods of time, the growing cracks destabalise the cliff and fragments of rock brak off of it.
Abraison: The repeated action of waves breaking on a cliff is enough to remove material from it over time. If sand and shingle are present in the water, it will act like sandpaper and erosion will take place faster.
Attrition: Beach material is knocked together in water reducing their ize and increasing their roundness and smootness.
Corrosion: Carbon dioxide in the atmosphere is dissolved into water turning it into a weak carbonic acid. Several rocks are vunerable to this acidic water and will dissolve into it. The rate of dissolution is affected by concerntration of carbonates and other minerals in the water. As it increases, dissolution becomes slower.
Factors effecting rate of erosion
- Strength of the waves breaking along the coastline (strength is controlled by its fetch and the wind speed. Longer fetches & stronger winds create bigger, more powerful waves that have more erosive power)
- Landforms (Beaches increase the distance travelled by the wave and so reducing its energy. Headlands refract waves around them, reducing their erosive power at one location and increasing it at another)
- Weathering (Creates weakness in the rocks that are explioted by processes of erosion. Freeze-thaw weathering creates cracks in rocks, increasing the rocks susceptibility to hydraulic action)
- Humans (Activities have a variety of complex effects on coastal erosion but most commonly the activities increase the strength of waves. Dredging, is commonly carried out to improve shipping capacities but it reduces the amount of energy dissipated from incoming waves and so increases erosion)
Concordant coasts have alternating layers of hard and soft rock that run parallel to the coast. The hard rock acts as a protective barrier to the softer rock behind it preventing erosion. If the hard rock is breached though, the softer rock is exposed and a cove can form (e.g., Lulworth Cove).
On a discordant coastline, alternating layers of hard and soft rock are perpendicular to the coast. Because the soft rock is exposed, it is eroded faster than the hard rock. This differential erosion creates headlands and bays along discordant coastlines.
Sub-aerial Processes (Weathering)
Freeze Thaw involes water entering cracks in rocks and freezing. When the water freezes it expands, fracturing the rock.
Exfoliation the repeated action of heating and cooling rocks casuing them the 'shed' off layers.
Biological Weathering plant seeds get into cracks in the rocks and begin growing. As they grow, they exert pressure o the rocks, casuing them to fracture.
Chemical Weathering hydrolysis and oxidation can weather away rocks. Hydrolysis involves splitting if minerals due to their reactivity with water. Oxidation, is basically, rusting. Elements such as iron are susceptible to oxidation and an be found within minerals on coastlines.
Sub-aerial Processes (Mass Movement)
Mass Movement can be defined as the large scale movement of weathered material in the response to gravity. There are 5 types: rockfall, soil creep, landslides, mudflow and slumping.
Rock fall Freeze thaw weathering on a cliff breaks the rocks up into smaller pieces which can then free fall. This occurs commonly on cliffs with lots of joints as the joints make it easier to break up the rock. If the cliff is undercut by the sea, it can loose some of its stability, increasing the likelihood that a rockfall will occur.
Soil Creep Soil creep is an incredibly slow process. It occurs on very gentle slopes and produces an undulated (wavy) surface. Damp soil moves very slowly down hill due to the increase in its mass (since it’s wet).
Landslides After being soaked by water, cliffs made from soft rock will begin to slip due to the rock being lubricated. Landslides are very similar to slumps.
Rotational Slumping Slumping happens for similar reasons to landslides. Heavy rainfall makes the rock heavier due to it absorbing the water and the water also acts as a lubricant. The difference with slumping is that it happens on a concave surface, which causes the cliff to form a crescent shape.
Mudflow Mudflow is a very dangerous form of mass movement which occurs on steep slopes with saturated soil and little vegetation. The lack of vegetation means that there is nothing to bind the soil together, promoting mass wasting. The saturated soil becomes heavier and is lubricated, leading to the rapid movement of a lot of mud downhill.
Features of erosion
Headlands and Bays form on discordant coastlines. Alternating layers of hard and soft rock allow the sea to erode the soft rock faster, forming a bay but leaving hard rock sticking out, known as a headland.
Wave Cut Notches & Platforms A wave cut notch is simply a small indent at the base of a cliff formed when a cliff is undercut by the sea. When a wave breaks on a cliff, all of the wave’s energy is concentrated on one specific point and this section of the cliff experiences more rapid erosion. This eventually leads to the formation of a wave cut notch, when the cliff has been undercut. As the cliff has been undercut, the section of the cliff above the notch (the overhanging rock) no longer has any support and will, eventually, collapse. The repeated process of the collapse of the cliff and then the undercutting of the cliff is often referred to as the “retreat” of the cliff. As the cliff retreats, a gentle platform (with a gradient less than 5˚), referred to as a wave cut platform, is left behind.
Features of erosion...
Caves The first way is simply eroding the crack and causing it to collapse, forming a geo, a steep sided inlet into the side of the coast. Alternatively, the area below the crack or joint is undercut and a small cave will form.
Arches As the two caves are eroded and cut into the headland, they will eventually meet. The resulting landform is then referred to as an arch.
Stacks The roof of the arch has no support however and is highly susceptible to weathering via exfoliation, salt crystallisation and biological weathering. As the weathering continues, the roof of the arch will collapse leaving a stack, a tall, lone piece of land sticking out in the sea.
Stumps The base of the stack receives a lot of erosion from hydraulic action and corrosion and, eventually, the stack will collapse into the sea leaving behind a small piece of land called a stump.
Features of erosion....
Blowhole A blowhole forms in a cave. As a cave moves inland, the roof above it is weakened and as waves crash into the cave, they can be reflected upwards, eroding the roof of the cave. At the same time, weathering on the roof of the cave will help weaken it further and eventually water will be able to break through it, leaving a blowhole.
Barton-on-Sea and its cliffs are located in the south of England, just to the west of the Isle of Wight where it forms a coast with the English Channel. The section of the English Channel which it borders experiences strong winds and has a long fetch, resulting in a fairly fast rate of erosion on the cliffs. The rate of erosion is quickened by the geology of the cliffs, which are mainly composed of salt clay (also known as Barton Clay) and Barton sand in two layers. The combination of the long fetch and weak rocks is resulting in the rapid erosion and mass movement of sections of the cliff which is posing a risk to people and buildings along the stretch of coastline at Barton-on-Sea.In order to prevent or reduce the risk of a cliff collapse, the local council has carried out several protective measures over the past few decades to try and slow cliff collapse.
- Loss of gomes
- Home values go down - people can't move
- House insurance very expensive
- Dangerous to walk on the cliff tops
- Roads under threat
- Tourists may not visit
- Cliif collapse can look unattractive
- Cliff collapse exposes different rocks and fossils
Sea Wall - Concrete structures which absorb the energy of the waves and provide a promenade for tourists - Costs £2000 per metre, a permentant structure that m ay last for many years.
Groynes - Long wooden fences, or piles of large rocks, built out into the sea to stop longshore drift and help build up sand on one side - Costs £2000 per metre, they stop transport of sediment and increase the risk of erosion along the coastline/
Rock Armour - Large boulders piled up at the foot of cliffs to absorb the energy of the waves and stop them eroding the cliff. Costs £300 per metre, an effective and simple method that looks more natural
Gabions - Rocks or boulders held in wire mesh cages and used to protect vunerable areas form destructive waves - Costs £100 per meter, the cheapest option but its not very strong.
- Local people with homes in danger
- Local tourist businesses with caravans and hotels at risk
Not in favor:
- Local taxpayers who don't live next to the coast
- Environmentalists - fear that habitats and natural beuaty will be effected
- People living downdrift could be starved of sediment
Soft Engineering Methods
Beach Nourishment - building up beaches by adding more sand in front of cliffs - its natural protection, because the beach absorbs wave energy, but the sand has to come from somewhere else, costs around £3000 per km and is a cheap method.
Sand Dune regeneration - Allowing sand dunes to build up arond wooden structures - sand dunes absorb wave energy and create new habitats, costs £2,000 per 100m.
Managed retreat - Abondoning the existing sea defences and building new ones further inlad, creating a salt marsh which also floods in storm conditions - some people will lose land, housesor buisnesses, but new habittas are created and flooding is reduced in other areas.
Spits stretch of sand or shingle extending from the mainland out to sea. They develop where there is a sudden change in the shape of the coastline such as at a headland. Normally, longshore drift transports beach sediment along a coastline.Refraction around the end of a spit curves it into a “hook” forming a recurved spit. As the area behind a spit is sheltered from waves and the wind, it provides the perfect environment for salt marshes to develop.
Bars If the bay between two headlands is blocked off by a spit then that spit is known as a bar. The body of water behind the bar is known as a lagoon.
Tombolos Sometimes a spit extends far enough out to sea to join a coastal island to the mainland. When it does so, the spit is known as a tombolo
Salt Marshes Salt marshes only form in low energy environments where there is shelter from the wind and waves.
Depostional Landforms (Sand Dunes)
Dunes require a large supply of sediment, and area with a large tidal range, strong and continuous wind and an obstacle.
- Pioneer species of plants - traps further sediment
- Halophytic plants
- Marram grass
- Depressions in the snd dune succession
- Depressions in the snd dune succession
Sea level change
Eustatic - sea level changes due to an alteration in the volume of water in the oceans or, alternatively, a change in the shape of an ocean basin and hence a change in the amount of water the sea can hold. Eustatic change is always a global effect.
Isostatic - Isostatic sea level change is the result of an increase or decrease in the height of the land. When the height of the land increases, the sea level falls and when the height of the land decreases the sea level rises. Isostatic change is a local sea level change whereas eustatic change is a global sea level change.
Features of sea level change
- Raised beaches (Wave cut platform and beaches that are above current sea level)
- Relic Cliffs (Behing raised beaches)
These features no longer experiencre coastal erosion, but are still weathered
- Ria (flooded river valley thats been flooded by eustatic rise in sea level, found on the lower course of the river, floodplain is also slooded)
- Fjord (They have a U-shaped cros profileand are found in particulary icy areas of the world. Really deep, but have a shallow mouth, where the glacier deposits it's load)
- Dalmation Coastline (Where valleys lie parallel to each other. When the valleys are flooded by the rise in sea level, the tops of the vallet remain above the surface of the sea and appear to be a series of islands that run parrallel to the coastline)
Storm Surges: sudden rises in sea level caused by very strong winds, normally those found in hurricanes and cyclones. The strong winds essentially push the water on an ocean’s surface on top of more water, increasing the sea level and flooding coastlines. The conditions needed to create these strong winds are generally associated with low pressures, further increasing the sea level. The strong winds can create large and powerful waves that can overtop coastal defences so even if the rise in sea level doesn’t flood the coastline, the resulting waves likely will.
Rising Sea Levels: due to climate change or isostatic rebound low lying coastal areas are permanently flooded by the sea. The likelihood and severity of storm surges also rises since weaker winds will also be able to increase the sea level enough to flood coastal areas.
Tsunamis: Tsunamis are giant waves resulting from earthquakes, volcanic eruptions, meteor impacts, any sort of major displacement of water in the ocean.
Hurricane Katrina began as a very low pressure weather system, which strengthened to become a tropical storm and eventually a hurricane as it moved west and neared the Florida coast on the evening of 25 August 2005.
At this point, Katrina's sustained wind speed was approximately 200 km/h. The storm passed directly through New Orleans, destroying many lighter buildings and causing extensive damage to others.Hurricane force winds were recorded along a 200km stretch of coastline, with scenes of similar destruction and flooding in Louisiana, Mississippi and Alabama. Storm surges from the sea caused flooding several kilometres inland in some places.
Flood walls, were only ever designed to cope with a category 3 storm, not enough to cope with the strength of the storm surge of Katrina, a category 5 storm.
Hurricane Katrina - Economic
The Bush Administration sought $105 billion for repairs and reconstruction in the region, this didn’t include potential interruption of the oil supply, destruction of the Gulf Coast's highway infrastructure, and exports of commodities such as grain.
Katrina damaged or destroyed 30 oil platforms and caused the closure of nine refineries
1.3 million acres (5,300 km2) of forest lands were destroyed costing about $5 billion
Before the hurricane, the region supported approximately one million non-farm jobs, with 600,000 of them in New Orleans.
It is estimated that the total economic impactin Louisiana and Mississippi may exceed $150 billion
Additionally, some insurance companies have stopped insuring homeowners in the area because of the high costs from Hurricanes Katrina and Rita, or have raised homeowners' insurance premiums to cover their risk
Hurricane Katrina - Environmental
The storm surge caused substantial beach erosion, in some cases completely devastating coastal areas. InDauphin Island, approximately 90 miles (150 km) to the east of the point where the hurricane made landfall, the sand that comprised the barrier island was transported across the island into the Mississippi Sound, pushing the island towards land
The US Geological Survey has estimated 217 square miles (560 km2) of land was transformed to water by the hurricanes Katrina and Rita
The lands that were lost were breeding grounds for marine mammals, brown pelicans, turtles, and fish
The damage from Katrina forced the closure of 16 National Wildlife Refuges.
The storm caused oil spills from 44 facilities throughout southeastern Louisiana, which resulted in over 7 million U.S.gallons (26 million L) of oil being leaked.
Finally, as part of the cleanup effort, the flood waters that covered New Orleans were pumped into Lake Pontchartrain, a process that took 43 days to complete. These residual waters contained a mix of raw sewage, bacteria, heavy metals, pesticides, toxic chemicals, and oil, which sparked fears in the scientific community of massive numbers of fish dying
Hurricane Katrina - Social
Katrina redistributed over one million people from the central Gulf coast elsewhere across theUnited States. For example,Houston, Texas, had an increase of 35,000 people
By late January 2006, about 200,000 people were once again living in New Orleans, less than half of the pre-storm population.
By July 1, 2006, when new population estimates were calculated by the U.S. Census Bureau, the state of Louisianashowed a population decline of 219,563, or 4.87%
Many people were totally traumatised
Racial tensions were exposed and intensified, as many of the victims were black African Americans
When engaging in coastal management, there’s four key approaches that can betaken:
- Hold the line - Where existing coastal defences are maintained but no new defences are set up.
- Advance the line - New defences are built further out in the sea in an attempt to reduce the stress on current defences and possibly extend the coastline slightly.
- Retreat the line (surrender) - Move people out of danger zones and let mother nature unleash take control.
- Do nothing - The easy option, deal with the effects of flooding and erosion as they come or just ignore them. This is generally what happens in areas where there’s no people, and so nothing of “value” (to the government) to protect.
Breakwaters -Breakwaters are offshore concrete walls that break incoming waves out at sea so that their erosive power is reduced to next to none when they reach the coast. Breakwaters are effective but they can be easily destroyed during a storm and they don’t look particularly nice.
Revetments - Revetments are concrete (or in some cases wooden) structures that are built along the base of a cliff. They’re slanted and act as a barrier against waves not too dissimilar to a sea wall. The revetments absorb the energy of the waves, preventing the cliffs from being eroded.