Coasts Flashcards
Swash
The part of the wave crashing onto the shore
Backwash
The part of the wave sweeping back into the sea
Constructive waves
Able to build the beach as the awash is stronger than the backwash
Destructive waves
Destroy the beach as the backwash is greater than the swash, so material is taken back to sea with every wave
Fetch
The distance the wind blows across the water. The longer the fetch, the more powerful the wave
Process of a wave forming
1.Circular orbit in open water
2.Friction with the sea bed distorts the circular orbital motion
3.Increasingly elliptical orbit, top of the wave moves faster
4.Wave begins to break on shelving seabed
Wave frequency
The number of waves passing a point in a given amount of time, expressed in waves per second. The reciprocal of the period.
Wave crest
The highest point of the wave
Wave trough
The lowest point of the wave
Wave period
The time it takes for two successive crests to pass a given point
Wave length
The distances between two identical points on successive waves
Characteristics of constructive waves
Low wave height
Long wave length
Spilling wave
Strong swash
Weak backwash
Gain beach sediment
Low frequency
Characteristics of destructive waves
High wave height
Short wave length
Plunging wave
Weak swash
Strong backwash
Loss beach sediment
High frequency
Definition of weathering
The wearing down or breaking of rocks while they are in place
What are weathering processes ?
Processes that break down the top and the side of the cliff
Chemical weathering
Causes by chemical changes, dissolves into component parts. 4 main types are carbonation, oxidation, hydration, hydrolysis.
Mechanical weathering
Disintegration of rocks into smaller pieces. Piles of rock fragments, called scree, can be found at the base of cliffs.
Process of freeze-thaw
*Water collects in cracks or holes in the rock
*At night this water freezes and expands and makes cracks in the rock bigger
*When the temp rises and the ice thaws, water will seep deeper into the rock
*After repeated freezing and thawing, fragments of rock may break off and fall to the foot of the cliff
Process of salt weathering
*Seawater contains salt. When the water evaporates it leaves behind salt crystals
*In cracks and holes these salt crystals grow and expand
*This puts pressure on the rocks and flakes may eventually break off
Process of carbonation
*Rainwater absorbs CO2 from the air and becomes slightly acidic
*Contact with alkaline rocks such as chalk and limestone produces a chemical reaction causing the rocks to slowly dissolve
What is mass movement ?
The downward movement or sliding of materials under the influence of gravity. Some processes occur slowly such as soil creep, but some are rapid such as rockfall.
Mudflows
Saturated soil and weak rock flow down a slope. Occur when vegetation is sparse so can’t hold the soil in place. They happen after heavy rain. At the base of the mudflow, saturated soil spreads out to make a lobe.
Rockfalls
Rapid but rare, occur on slopes of over 40°. They result from extreme weathering and produce debris slips beneath the cliff because the material from the rockfall breaks down the cliff top. Worsened by marine erosion.
Landslides
Cliffs formed on soft rocks are susceptible to sliding after prolonged rainfall. Raising of the underground water table decreases friction between layers. Slides largely retain structure and move as one.
Rotational slip, slumping
Occurs when movement appears to have a rotational element to it and may produce a curved rupture surface. Cliffs formed on weak or impermeable rock are susceptible to rotational slumping after prolonged rainfall.
Hydraulic action
Sheer power of waves blasts and traps air into cracks and explodes, breaking pieces off.
The force of the water crumbles and removes material from cliffs.
Abrasion
The sea uses materials such as sand and pebbles to scrape and break rock.
Rocks and pebbles are flung at cliffs and break pieces off.
Attrition
Rock and pebbles in the sea collide and break into smaller pieces, rough edges become smooth and round.
Traction
Large pebbles rolled across the seabed
Saltation
A hopping or bouncing motion of particles too heavy to be suspended
Suspension
Particles carried within the water
Longshore drift
Waves approach the beach at an angle similar to the prevailing wind direction.
Swash carries material up the beach following this angle.
Backwash takes material down the beach under gravity.
Material carried along the beach via longshore drift.
Landform definition
A feature of the landscape formed or sculpted by erosion, transportation or deposition.
Discordant coastlines
Where the geology alternates between strata or hard and soft rock perpendicular to the coastline
Concordant coastline
Where strata of soft and hard rock run along parallel to the coastline
How is a wave cut platform formed ?
The sea attacks a weakness in the base of the cliff.
A wave cut notch is created by erosional processes such as hydraulic action and abrasion.
As the notch becomes larger, the cliff becomes unstable and collapses as a result of gravity.
The cliff retreats inland.
The material from the collapsed cliff face is eroded and transported away, leaving a wave cut platform .
How are stacks formed ?
Wave cut notch
Cave
Arch - via fault lines
Stack - sub-aerial processes
Stump
Coastal deposition
Takes place in areas where flow of water slows down. Waves lose energy in sheltered bags and where water is protected by spits and bars. Sediment can no longer be carried or moved and is therefore deposited.
What is a beach ?
An accumulation of eroded material transported and eroded by the sea.
Properties of sandy beaches
In sheltered bays
Strong swash
Low energy constructive waves
Berms ( clear ridges that mark the high tide line )
Gentle slope
Properties of pebble beaches
High energy environment
Strong backwash
High energy constructive waves
Destructive waves
Steel profile
Finer sand is washed away
What is a spit ?
An extended stretch of beach material that projects out to sea and is joined to the mainland at one end.
How are spits formed ?
Longshore drift moves material along the coastline. A spit forms when the material is deposited, over time growing and developing a hook if the wind direction changes further out. Waves cannot pass a spit, therefore creating a sheltered area where silt is deposited and mudflats or salt marshes form.
What is a bar ?
A bar is formed as a spit grows across a bay joining up two headlands. A lagoon is formed behind it in the old bay, getting filled up with deposition.
What is an offshore bar ?
Forms further out to sea. Waves approach a gently sloping coastline and deposit sediment due to friction with the seabed. Causes waves to break at a distance from the shore.
How are sand dunes created ?
Sand is deposited on the beach and is bleib inland by onshore winds to form dunes.
Embryo dunes form around deposited obstacles such as wood or rock.
These develop and become stabilised by vegetation, forming fore dunes and tall yellow dunes. Marram grass is adapted to windy exposed conditions and has long roots to find water, helping to find the sand together and stabilise the dunes.
Rotting vegetation adds matter to the sand making it more fertile. A greater range of plants colonise these back dunes.
Why might a pond form on a beach ?
Wind forms depressions in sand called dune slacks, which have acidic soil and a varying pH, in which ponds form.
Characteristics of embryo and fore dunes
On shore winds
Seaweed, hummus buildup
Sand building up
Transient
Alkaline sand
Plants are scattered, low growing and salt tolerant eg. Sandwort
Characteristics of yellow dunes
Surface continually blown away and replenished by fresh sand
Reduced wind speed
Top of dunes above high tide level
Plants are salt intolerant, thrive in being buried by sand, have inrolled leaves and underground rhizomes to stabilise sand, eg Marram grass
Characteristics of grey dunes
Increased hummus content
Surface lichens
Sand no longer accumulating
Marram grass not able to compete
Many plants now coexist, mainly perennials, increased organic matter content eg. Grey lichen, heather
Characteristics of dune slacks
Damp, low lying hollows
High water table in winter
Soil acidic and pH variable
Moisture loving plants eg. rushes
Characteristics of heath
Acidic soil and increased organic matter content
Nutrient rich
Shelter developed
Acid loving plants coexist, woody perennials plus understory species eg. heather, woodland
What is hard engineering
Whereby the natural environment is protected when artificial structures are put in place to protect the coastline
What is soft engineering ?
Works with the natural environment, is less expensive and often more sustainable
Sea walls
Placed at the back of a beach. Recurved walls rotate the wave backwards so that the energy is reflected back to sea. This impedes the next wave and reduces its erosional power.
Can last for many years and provide security and safety.
Look obtrusive, restrict access to beach, expensive and have high maintenance costs.
Groynes
Stop movement of sediment via longshore drift.
Create a wider beach, sitting areas, not too expensive and last long.
Impede walking on beach, dangerous due to change in water depth on each side, need regular maintenance and restrict supply of sediment down drift.
Rock armour
Piles of large boulders places at the foot of a cliff. The rocks cause the waves to break, absorbing energy and protecting cliffs. The downward slope arrangement deflects the waves energy and the gaps release the pressure so the structure isn’t damaged.
Used for fishing, cheap and easy to maintain, quick to build.More natural.
Makes beach difficult to access, putting people off. Rocks are usually not local, so expensive to ship. Ugly and doesn’t blend in.
Gabions
When the water hits at gabions it loses its energy as it falls through the gaps in the rocks, reducing its erosive power.
Cheap, quick solution, blend in with local environment .
Unattractive, regular maintenance needed, seabirds may damage feet.
Beach recharge ( nourishment )
Material is taken from a bay and shifted further down the coast
Beach recycling ( nourishment )
Sand is moved from one area to another that is down the coastline and not at risk of erosion.
Widened beach, natural.
Access restricted for several weeks, may cause resentment from donor area, destruction of habitats, expensive
Beach reprofiling
Artificial reshaping of a beach using existing beach material. In winter, destructive waves lower a beach, so afterwards, bulldozers move shingle back up.
Residential areas are protected, low cost, natural.
Restrict access to beach temporarily, may look unattractive to tourists.
Sand dune regeneration
Artificial creation of sand dunes, acting as a physical barrier between land and sea, and absorbing wave energy.
Protect land behind them, minimal costs, regeneration helps maintain wildlife habitats.
Deters tourists whilst being established, expensive to maintain due to trampling, no guarantee they will be stable.
What is managed retreat ?
A form of soft engineering, where low lying land is allowed to naturally flood so defences can be improved further inland.
Advantages of managed retreat
Works with natural land
Protects more valuable land
Salt marshes could be a tourist attraction
Inexpensive
Disadvantages of managed retreat
Damage habitats
Damage of property
Relocate people
Compensating is costly
Could lead to mass movement
Medmerry Managed Retreat, Surrey
Used to be protected by sea wall, but costs of repair were too costly, so managed retreat was the preferred option. Costs were £28 million and the controlled breach happened in Nov 2013.
Aimed to create a large saltmarsh to form a natural buffer to the sea, help protect surrounding land, establish valuable wildlife habitats and encourage visitors.
Embankments have been constructed inland to give protection to farmland, roads and settlements. This alteration process is called coastal realignment.
SMPs
Shoreline Management Agency’s incentive the most sustainable approach to managing the flood and coastal erosion risks in the short term and long term.
What is a salt marsh ?
A coastal wetland that is flooded and drained by salt water brought in by tides. Diverse ecosystems that play an important role in protecting shoreline regions from erosion and floods.
How are salt marshes formed ?
Only form in low energy environments where there is shelter from wind and waves, eg behind a spit.
The low energy environment traps material being transported by the tides. This accumulated to form a salt marsh. Over time this is colonised by plants and animals. These salt water tolerant plants also trap material, allowing the salt marsh to expand and form new land.
Coastal management at Lyme Regis
Phase 1 - new seawall + promenade, emergency £1.4 million project needed in 2003 to stabilise cliffs and reprofile beach
Phase 2 - New sea wall + promenade, creation of wide sand and shingle beach, extension of rock armour
Phase 3 - Plan was cancelled
Phase 4 - £20 million focussed on east of town, new sea wall, nailing and drainage to stabilise cliffs
