Coasts: Systems and processes - Energy Flashcards
Souces of energy: winds, waves, currents and tides. Low energy and high energy coasts.
What are the 4 sources of energy in coastal environments?
Wind
Waves
Tides
Currents
What is the primary source of energy for all natural systems?
The sun
Explain why the primary source of energy in a coastal system is the sun
The sun heats the Earth’s surface, causing warm air to rise. This results in low air pressure, causing air to move from areas of high pressure to low pressure, creating wind. Wind can then go on to create waves. The sun also drives the global currents as currents move from cold to warm water.
What is wind?
The movement of air from areas of high pressure to areas of low pressure.
What is prevailing wind direction? What is it in the UK?
The most usual direction the wind reaches the coast. In the UK the prevailing wind direction is from the SW
Why is wind important in a coastal system?
It is a vital input as it provides energy for other processes. It is also an important agent of erosion and transport.
What impact does prevailing wind have in coastal systems?
It is a factor controlling the direction that waves approach the coastline and the direction of transport of material in the coastal zone.
What are the 3 factors affecting wave energy?
Fetch
Wind strength
Wind duration
What is fetch?
The distance of open water over which a wind blows uninterrupted by major land obstacles
What is the most common type of wind erosion?
Abrasion
What causes wind to transfer energy to the sea surface in the formation of waves?
The frictional drag of the wind on the sea surface
How does wind create waves?
When wind blows over the sea surface it transfers its energy due to frictional drag, creating microripples. These ripples offer more surface area for the wind to press against, creating ripples. Ripples increase in size to chop, then fully developed sea, until reaching a maximum when they travel almost as fast as the wind.
Give 3 characteristics of waves
Wave height
Wavelength
Wave frequency
What is wave height?
The height difference between a wave crest and the neighbouring trough
What is wavelength?
The distance between successive wave crests
What is wave frequency?
The time for one wave to travel the distance of one wavelength
How do waves break?
Waves start out with a circular orbit. As waves approach shallow water, friction with the seabed slows the base of the wave. This increases the height and steepness of the wave, making its orbit elliptical, until the crest moves faster than the bottom and it breaks onto shore.
What is swash?
The rush of water up the beach after a wave breaks
What is backwash?
The action of water receding back down the beach towards the sea
How are constructive waves formed?
By distant weather systems in the open ocean.
How are destructive waves formed?
By local storm conditions
What are the characteristics of constructive waves?
Low height
Long wavelength - up to 100m
Low frequency - 6-8 per min
What are the characteristics of destructive waves?
High height
Short wavenlength
High frequency - 10-14 per min
Describe how constructive waves break.
As they approach the beach the wave front steepens only slowly. This gives a strong swash which loses energy quickly as water percolates through the beach material, creating a very weak backwash.
Describe how destructive waves break
As they approach the beach, the rapidly steepen and plunge down. This creates weak swash as there is little forward movement of water but a powerful backwash which can inhibit swash from the next wave.
Describe the net movement of beach material of constructive waves.
Beach gain - the weak backwash has insufficient force to pull sediment off the beach. The strong swash means material is slowly moved up the beach leading to the formation of berms.
Describe the net movement of beach material of destructive waves.
Beach loss - the vertical impact loosens beach material making it more easier to remove. The strong backwash pulls material back down the beach.
Which type of waves have larger longshore bars?
Destructive waves
What type of beach profile are constructive waves associated with?
Gentle beach profiles (however over time they will build up the beach and make it steeper)
What type of beach profile are destructive waves associated with?
Steeper beach profiles (however over time they will flatten the beach)
How can destructive waves form storm beaches?
The force of each wave may project some shingle towards the rear of the beach, forming a large ridge known as the storm beach.
Describe the negative feedback loop between beaches and waves.
Constructive waves build up a beach, making it steeper. This encourages the formation of destructive waves that plunge rather than surge. Redistribution of sediment offshore by destructive waves reduces the beach gradient which in turn encourages the waves to be more constructive.
What is wave refraction?
When waves approaching an indented coastline become increasingly parallel to the coastline
Why do waves refract around headlands?
Waves approaching headlands meet shallower water first
Briefly describe wave refraction around a headland separating two bays.
Waves approaching the coast will drag in the shallower water meeting the headland. The part of the wave in deeper water approaching the bay will move forward faster, causing the wave to bend.
How do wave characteristics change as they approach a headland?
They become more destructive as:
Wave height increases
Wave steepness increases
Wavelength shortens
How does the change in wave energy due to refraction affect headlands?
Wave energy is concentrated on headlands, increasing erosion
How does the change in wave energy due to refraction affect bays?
Low-energy waves spill into the bay resulting in beach deposition
How can the high energy waves at headlands contribute to the build up of beaches in the bays?
The high energy waves can cause a slight local rise in sea level resulting in a longshore current front the headland, moving some of the eroded material towards the bays.
What is a current?
The permanent or seasonal movement of surface water in the seas and oceans
What are the 3 main types of current?
Longshore currents
Rip currents
Upwelling
What are longshore currents?
A flow of water running parallel to the shoreline
Why do longshore currents form?
As most waves approach the coastline at an angle
What do longshore currents do?
Move water along the surf zone and transport sediment parallel to the shoreline
What are rip currents?
Strong, localised, narrow currents of water which move directly away from the shore
How do rip currents form?
A series of plunging waves causes a temporary build up of water at the top of the beach. Met with resistance from the breaking waves, the backwash is forced just below the surface following troughs in the beach profile.
What is upwelling?
The movement of cold water from deep in the ocean towards the surface.
How are upwelling currents formed?
The denser cold water replaces warmer surface water, creating nutrient rich cold ocean currents
Which type of currents form part of the pattern of global ocean circulation?
Upwelling currents
What are tides?
The periodic rise and fall in sea level
What causes tides?
The gravitational pull of the moon and sun
Why does the moon have the greatest influence over the tides compared to the sun?
The moon is closer
How are high tides created?
The moon pulls water towards it, creating a high tide. There is a compensatory bulge on the opposite side of the Earth which is also a high tide.
Describe the regular pattern of tides
Twice in a lunar month (at new moon and full moon), the sun, moon and Earth align. This is when the tide-raising force is strongest, producing spring tides. Also twice a month, the moon and sun are perpendicular to each other in relation to the Earth, giving neap tides.
What factors change the regular patterns of tides?
Morphology of the seabed
Proximity of land masses
Impact of the coriolis force
What is tidal range?
The relative difference in height of sea water at high and low tide
What do tidal ranges determine?
The upper and lower limits of erosion and deposition and the amount of time each day that the littoral zone is exposed to sub-aerial weathering
What landforms can high tidal ranges form?
Wave cut platforms
Are tidal ranges the same everywhere?
No, for example along the coasts of the Mediterranean Sea tidal ranges are low, but in parts of the British Isles tidal ranges are high.
What important sources of energy do tidal ranges generate?
Powerful tidal currents
Why are tidal currents important in coastal systems?
They transfer sediment within and beyond the coastal system
Where can tidal currents be particularly strong?
In estuaries and narrow channels
What are storm surges?
Occasions when meteorological conditions give rise to strong winds which can produce much higher water levels that those at high tide
Give an example of an area affected by storm surges.
The North Sea and east coast of Britain
What are depressions?
Intense low pressure weather systems
How do storm surges form?
Depressions over the sea produce low pressure conditions that cause sea levels to rise. Strong winds drive waves ahead of the storm, pushing the sea water towards the coastline.
What can intensify the effect of storm surges?
Spring tides
What are low energy coastlines?
Coastlines where wave energy is low and the rate of deposition often exceeds the rate of erosion
What are high energy coastlines?
Coastlines where wave energy is consistently high and and the rate of erosion exceeds the rate of deposition
What type of landforms are typical on low energy coastlines?
Depositional landforms such as beaches and spits
What type of landforms are typical on high energy coastlines?
Erosional landforms such as headlands and bays, wave cut platforms
Give an example of low energy coastlines in the UK
Sheltered estuaries and bays in Lincolnshire
Give an example of high energy coastlines in the UK
Stretches of the Atlantic-facing coast such as Cornwall
Do high or low energy coastlines experience a net transfer of material from coastline to sea?
High energy coastlines
Describe the likely wind conditions on high energy coastlines
Strong, steady prevailing winds with a long fetch
Describe the likely wind conditions on low energy coastlines
Weak, onshore winds
