Coastal Zone Flashcards
(30 cards)
Destructive Waves
o Operate in storm conditions
o Are about 5 or 6 metres high
o The SWASH is weak and the BACKWASH is strong and there is A LOT OF EROSION
o The FREQUENCY is also high, which means there are more hitting the beach in a minute than constructive waves.
Constructive Waves
o Operate in calm weather
o Are about a metre high
o The SWASH is strong and there is weak BACKWASH as the water percolates through the beach sediment
o Erosion is LIMITED
o They’re involved with the TRANSPORT and DEPOSITION of material creating depositional landforms (beaches, bars etc.).
What will determine how large a wave will be?
The fetch – how far the wave has come from
How strong the wind is
How long the wind has blown for
But how do the waves actually shape the coastline?
o This is due to EROSION, TRANSPORTATION & DEPOSITION.
o Erosion is destructive waves wearing away the coastline
o It happens when the waves are packed with energy
o Erosion can create landforms but eventually wear them away
o The sea erodes the coast in four ways (these are the PROCESSES OF EROSION)
1) HYDRAULIC ACTION/ POWER - water crashes against the cliffs/rocks, and the air and water is trapped and
compressed in cracks. When the sea moves away again the air expands explosively weakening the rocks,
enlarging the cracks and breaking pieces off.
2) ABRASION (sometimes known as CORRASION) - is very effective and is caused by broken rock fragments
battering the land, cliffs etc. and breaking off other pieces of rock. Operates like ‘sandpaper’.
3) ATTRITION - occurs when rock fragments collide into smaller and get worn down into smoother and smaller
pebbles.
4) SOLUTION - involves chemical action of sea on rock. If the rock is limestone, it dissolves in the sea-water some
sea salts can also react with certain rocks and cause them disintegrate.
Once the waves have eroded sediment how is it
transported?
o Transport along the coast is when waves move material
across a beach
o This is called LONGSHORE DRIFT.
o Longshore drift happens when waves break at an OBLIQUE angle to the shore (not right angles) due to the
prevailing wind.
o This means that each wave pushes material along the beach a bit more. (swash carries the material up the beach at an angle and the backwash drags the material down the beach at right angles.)
o Many depositional features such as spits are formed by this process.
What other named processes of
transport are there?
o Suspension - Fine sediment is carried as suspension in water, making it look muddy/murky
o Solution - Dissolved material is carried along in solution, so you can’t see it.
o Traction - Larger pebbles and cobbles are rolled along the sea bed
o Saltation - Small pebbles are moved when one pebble hits another, causing it to bounce. This bouncing can set up a chain reaction
What happens once the sea has finished transporting the sediment?
o DEPOSITION. This is the dumping of eroded material on the land by constructive waves
o It happens when the waves have less energy
o Deposition creates a variety of landforms
What other processes can shape the coastline?
o WEATHERING!
o This is the breakdown of rock in situ (i.e. the stone doesn’t move)
o This can happen due to Mechanical Weathering or Chemical Weathering
o Mechanical Weathering: includes freeze-thaw. Temperatures drop below freezing at night and then rise during the day. As a result, any water in the cracks freezes and expands and then thaws. Over time the expansion when the water turns to ice will break the rocks apart.
o Chemical Weathering: This includes solution. Water reacts with the calcium carbonate in rocks like limestone and chalk. The rocks are dissolved and are washed away
What is ‘mass movement’?
o The waves and the other processes discussed above can result in ‘mass movements’ on cliffs
o There are two main types:
- Sliding – large chunks of rock slide down the slope
quickly without warning.
- Slumping – common where cliffs are made of clay. The clay becomes saturated during heavy rainfall and
oozes down towards the sea as part of a mud or debris
flow (more on this later in the Christchuch Bay case
study)
What are the landforms created by erosion?
o Wave-cut platforms
o Headlands and bays
o Caves, archs and stacks
Wave-cut Platforms :
o Waves erode rocks along the shoreline by hydraulic action, abrasion and corrosion
o A notch is slowly formed at the high water mark which may develop into a cave. Rock above the notch becomes unstable with nothing to support it, and it collapses
o The coastline (cliffs) can retreat over many years and a WAVE-CUT PLATFORM forms as this process continues as only the base of the cliffs are left as the cliffs recedes.
Headlands and Bays :
o If there are alternate bands of hard and softer rock in the
coastline, the harder rocks take longer to erode than the softer rocks- because the sea has less effect
o The hard rock will be left jutting out forming one or more HEADLANDS, usually with cliffs
o The softer rock will be eroded to form BAYS, the
erosion means the bays will usually slope more gently inland, creating room for a BEACH TO FORM.
Caves, Archs and Stacks :
o A crack in a headland can be eroded wave energy is usually strong there because the headland juts out
o This forms one or more wave cut notches/caves
o Further erosion enlarges the cave and it breaks through the headland forming an ARCH
o The roof of the arch is often unstable and eventually collapses leaving a STACK or SERIES OF STACKS
o Areas with limestone or chalk geology are prone to this kind of erosion.
What are the landforms of deposition?
o Beaches
o Spits
o Bars
o Tombolo
Beaches:
o Deposition forms BEACHES
o Depositional landforms are most common on lowland coasts, beaches are the most common depositional landform
o Beaches are accumulations of sand and shingle deposited by waves and currents
o The sediment comes from cliff erosion but most is brought down to the coast by rivers. They can be SWASH
ALIGNED or DRIFT ALIGNED:
- Swash Aligned beaches are where waves approach parallel to the shore and break with swash moving directly up the beach and the backwash returning directly back. These form wide beaches with an even shoreline. Ridges tend to form on beaches such as these called berms.
Example: Chesil Beach, Dorset.
- Drift aligned beaches form when waves approach the beach at an angle resulting in a swash moving diagonally up the beach and the backwash returning through gravity.
These movements create longshore drift (diagram on the previous page). These beaches have an uneven profile- especially when groynes have been used to slow the longshore drift. Example: West Wittering, West Sussex.
Spits :
Deposition also creates SPITS (East Head
Spit!!)
- Spits are beaches joined to the coast at one end.
- At their seaward end shingle ridges form a series of recurves or hooks.
- Spits often develop across river mouths or where the direction of the coastline changes abruptly.
- Spits form by a process called LONGSHORE DRIFT (referred to earlier)
- Tucked away behind spits are quiet backwaters that waves cannot reach, where tidal currents deposit fine silt and build up MUDFLATS and SALT MARSHES.
Bar :
o Deposition also creates BARS
o Bars take spits one stage further and actually extends further and join up to another headland
o This is able to happen due to the lack of a strong river current that would wash the sediment away like in the tidal race of a spit.
o A good example of this is Loe Bar in Cornwall or Slapton Sands in Devon (pictures left).
o If a spit extends and joins up with an island then this is called a TOMBOLO. The best example of this is Chesil Bach joining to the Isle of Portland in Dorset.
Why are sea levels rising?
o The belief is that global warming is causing sea levels to
rise
o The heat is causing the ice caps to melt, which is
releasing water into the oceans
o In addition, the extra heat can cause thermal expansion of the sea too, making it higher
o Estimates suggest that levels could rise between 30 - 140cm by the end of this century
What case study of a location at risk of sea level
rise could be used?
o The Maldives, shown on the map on the right, is a
collection of very low-lying islands.
o The highest point is only 2.4 metres above sea level. o The beautiful landscape and white sandy beaches has resulted in 30% of the country’s GDP being created by tourism
Maldives : Economic
Rising sea levels could end the tourist industry
Maldives : Social
o Traditional way of life lost
o Homes flooded and land lost
o ‘Environmental refugees’
Maldives : Environmental
o Coral reefs will die as the water gets deeper
o The ecosystems associated with reefs will be lost
Maldives : Political
o The whole country could disappear!
o Political tension between the polluters and the Maldives – the Maldives are suffering, but they are not to blame for global warming
What are the differences between ‘hard’ and ‘soft’ coastlines?
o A hard coastline is one made from solid rock - e.g. chalk, limestone or granite.
o These are resistant to erosion and produce steep vertical cliffs like those found around Swanage, Lulworth Cove and Durdle Door (Isle of Purbeck)
o Soft rock coastlines consist of clays, sands and gravels and therefore far less resistant to erosion
o Crucially, these are also affected by weathering processes and slope processes. Sock rock coasts become unstable when wet as their structure is not strong- slumping often results.
o Hard rock cliffs will have a slow rate of recession and will tend to maintain a vertical profile due to their strength. A wave cut notch can form at the base and
eventually, the rock above will fall, but the cliff will stay vertical.
o Soft rock cliffs have a fast rate of recession as they are weak and are attacked by the waves, but also by sub-aerial (weathering) processes.
o At Barton-on-Sea in Hampshire, rates of cliff recession are about 1 metre a year!
o Along the East coast of England, some places are losing 8-10 metres a year.
