coastal systems and landscapes

what are the inputs to coastal systems?

e.g. sediment can be brought into the system in various ways. energy inputs come from wind, waves, tides, and currents.

what are the outputs from coastal systems?

e.g. sediment can be washed out to sea, or deposited further along the coast

what are flows/transfers within coastal systems?

e.g. processes such as erosion, weathering, transportation, and deposition can move sediment within the systems (e.g. from beach to dune)

what are stores/components within coastal systems?

landforms such as beaches, dunes, and spits are stores of sediment

are coastal systems in dynamic equilibrium?

yes, they are generally in dynamic equilibrium - inputs and outputs are balanced

give an example of negative feedback within coastal systems

as a beach is eroded, the cliffs behind it are exposed to wave attack. sediment eroded from the cliffs is deposited on the beach, causing it to grow in size again

give an example of positive feedback within coastal systems

as a beach starts to form, it slows down waves, which can cause more sediment to be deposited, increasing the size of the beach. the new equilibrium is reached when long-term growth of the beach stops.

how is energy transferred in the coastal system?

by air (as wind) and by water (as waves, tides and currents)

how are winds created?

by air moving from areas of high pressure to areas of low pressure

what creates strong winds?

during events such as storms, the pressure gradient is high and winds can be very strong

what can strong winds generate?

powerful waves

• in some areas, wind consistently blows from the same direction (prevailing wind), causing higher-energy waves than winds that change direction frequently

what creates waves?

the wind blowing over the surface of the sea

• the friction between the wind and the surface of the sea gives the water a circular motion

what does the effect of the wave on the shore depend on?

its height

what is wave height affected by?

the wind speed and the fetch of the wave

• a high wind speed and a long fetch create higher and more powerful waves

what is the fetch?

the maximum distance of sea the wind has blown over in creating the waves

what happens as waves approach the shore?

they break

• friction with the sea bed slows the bottom of the waves and makes their motion more elliptical

• the crest of the wave rises up and then collapses

what is the swash?

water washing up the beach

what is backwash?

water washing back towards the sea

what are the two types of wave?

constructive and destructive

• the waves in an area are usually mainly constructive or mainly destructive

what are the features of constructive waves?

• low frequency (6-8 per minute)

• low and long, giving them a more elliptical cross-profile

• powerful swash carries material up the beach and deposits it

• weak backwash

what are the features of destructive waves?

• higher frequency (10-14 per minute)

• high and steep, giving them a more circular cross-profile

• weak swash

• strong backwash removes material from the beach

what is wave frequency?

how many waves pass a point in a particular time

what are tides?

the periodic rise and fall of the ocean surface

what are tides caused by?

the gravitational pull of the moon and the sun

what do tides affect?

the position at which waves break on the beach

• at high tide they break higher up the the beach

where are most landforms created and destroyed?

the area of land between maximum high tide and minimum low tide

what is a current?

the general flow of water in one direction

what are currents caused by?

wind or variations in water temperature and salinity

what do currents do?

move material along the coast

what are the two types of coast?

high energy and low energy

how do high energy coasts receive energy?

receive high inputs of energy in the form of large, powerful waves caused by strong winds, long fetches, and steeply shelving offshore zones

what are the features of high energy coasts?

• often have sandy coves and rocky landforms (e.g. cliffs, caves, stacks, and arches)

• rate of erosion is often higher than the rate of deposition

how do low energy coasts receive energy?

receive low inputs of energy in the form of small, gentle waves caused by gentle winds (e.g. in sheltered areas), short fetches, and gently sloping offshore zones

• some coastlines are low energy because there is a reef or island offshore that protects the coast from the full power of the waves

what are the features of low energy coasts?

• often have saltmarshes and tidal mudflats

• rate of deposition is often higher than the rate of erosion

what are the inputs of sediment into the coastal system? (5)

• rivers carry eroded sediment into the coastal system from inland

• sea level rise can flood river valleys, forming estuaries (sediment in the estuary becomes part of the coastal system)

• sediment is eroded from cliffs by waves, weathering, and landslides

• sediment can be formed from the crushed shells of marine organisms

• waves, tides, and currents can transport sediment into the coastal zone from offshore deposits (e.g. sandbanks)

what is the sediment budget?

the difference between the amount of sediment that enters the system and the amount that leaves

• enters > leaves = positive sediment budget

• leaves > enters = negative sediment budget

what is the coast divided into?

sediment cells (littoral cells)

what are sediment cells?

lengths of coastline (often between two headlands) that are pretty much entirely self-contained for the movement of sediment

why are sediment cells closed coastal systems?

sediment doesn't move between cells, so processes going on in one cell don't affect the movement of sediment in another cell

what are the six erosional processes?

1. corrasion (abrasion)

2. hydraulic action

3. cavitation

4. wave quarrying

5. solution (corrosion)

6. attrition

what is corrasion?

bits of rock and sediment transported by the waves smash and grind against rocks and cliffs, breaking bits off and smoothing surfaces

what is hydraulic action?

air in cracks in cliffs is compressed when waves crash in. the pressure exerted by the compressed air breaks off rock pieces.

what is cavitation?

as waves recede, the compressed air expands violently, again exerting pressure on the rock and causing pieces to break off

what is wave quarrying?

the energy of a wave as it breaks against a cliff is enough to detach bits of rock

what is solution?

soluble rocks (e.g. limestone, chalk) get gradually dissolved by the seawater

what is attrition?

bits of rock in the water smash against each other and break into smaller bits

what is transportation?

the process of eroded material being moved

what are the four main transportation processes?

1. solution

2. suspension

3. saltation

4. traction

what is solution?

substances that can dissolve are carried along in the water

e.g. limestone is dissolved into water that's slightly acidic

what is suspension?

very fine material, such as silt and clay particles, is whipped up by turbulence and carries along in the water

• most eroded material is transported this way

what is saltation?

larger particles, such as pebbles or gravel, are too heavy to be carried in suspension. instead, the force of the water causes them to bounce along the sea bed.

what is traction?

very large particles, e.g. boulders, are pushed along the sea bed by the force of the water

what is longshore (littoral) drift?

• swash carries sediment (e.g. shingle, pebbles) up the beach, parallel to the prevailing wind. backwash carries sediment back down the beach, at right angles to the shoreline

• when there's an angle between the prevailing wind and the shoreline, a few round of swash and backwash move the sediment along the shoreline

what is deposition?

when material is being dropped on the caost

what are the two types of deposition?

marine and aeolian

what is marine deposition?

when sediment carried by seawater is deposited

what is aeolian deposition?

when sediment carried by the wind is deposited

when does deposition occur?

when the sediment load exceeds the ability of the water or wind to carry it

• this can be because sediment load increases (e.g. due to a landslide) or because wind/water flow slows down (so it has less energy)

why may wind/water slow down?

• friction increases: if waves enter shallow water or wind reaches land, friction between the water/wind and ground surfaces increases, which slows down the water/wind

• flow becomes turbulent: if water/wind encounters an obstacle (e.g. a current moving in the opposite direction, or an area of vegetation), flow becomes rougher and overall speed decreases

what happens when wind drops?

wave height, speed, and energy will also decrease

what is sub-aerial weathering?

the gradual break down of rock by agents such as ice, salt, plant roots and acids

what does weathering do?

weakens cliffs and makes them more vulnerable to erosion

what types of weathering affect coasts? (5)

• salt weathering

• freeze-thaw weathering

• wetting and drying