Coasts (LJ) Flashcards
How are waves created?
winds
friction
shallower
elliptical
wave height/wave velocity
collapse
Winds move across the surface of the water, causing frictional drag (resistance to the wind by the water) which creates small ripples and waves. This leads to a circular orbital motion of water particles in the ocean.
As the seabed become shallower towards the coastlines, the orbit of the water particles becomes more elliptical, leading to more horizontal movement of the the waves.
Wave height increases, but the wavelength (distance between to waves) and wave velocity both decrease.
This causes to back up from behind the wave until the wave collapses and surges of the beach.
How does the strength of the wind influence the size and energy of waves?
Wind is essentially air that moves from an area of high pressure to an area of low pressure. The differences in pressure are caused by variations in surface heating by the sun. The large the difference in pressure between to areas (pressure gradient) the stronger the winds. As waves are caused by winds, stronger wind means stronger waves.
How does the duration of the wind influence the size and energy of waves?
If wind is active for longer periods of time, then energy of the wave will build up and increase.
How does the size of the fetch influence the size and energy of a wave?
Fetch is the distance over which the wind blow. The larger the fetch, the more powerful the waves will be.
Define swash
The movement of the wave onto the beach after a wave breaks. Material being carried by the waves is deposited onto the beach.
What is backwash?
The movement of the wave back down the beach. Backwash drags any material of the beach.
Explain the process of constructive waves.
Constructive wave tend occur when wave frequency is low (6 to 8 arriving onshore per minute), especially when these waves advance over a gently shelving sea sea floor formed by fine material such as sand.
These waves are generated far offshore. The gentle offshore slope creates a gradual increase in friction, which will cause a gradual steepening of the wave front. Thus a spilling breaker is formed, where water movement is elliptical.
As this breaker collapses, the powerful constructive waves surges up the gentle gradient. Because of the low frequency, the backwash of each wave has time to return to the sea before the next wave breaks. This means that swash of the wave is not impeded and retains maximum energy
State the characteristics of constructive waves.
Constructive waves are formed by open weather systems that operate in open ocean.
They have a long wavelength.
They have a frequency of 6-8 per minute.
Low waves which surge up the beach.
Strong swash, weak backwash.
Occurs on gently sloping beaches.
Explain the process of destructive waves.
Destructive waves are the result of locally generated winds, which create wave of high frequency (12 to 14 per minute). This rapid approach of waves - particularly if they are moving onshore up a steeply shelving coastline formed by coarse material such as grave of shingle - creates a rapid increase in friction and thus a very steep, plunging breaker where water movement is circular.
Due to the curving and rapid steepening of the wave breaker, the energy of the wave is transmitted down the beach (on breaker collapse), accelerated by the steeper gradient, and so the wave becomes destructive, breaking down beach material.
State the characteristics of a destructive wave.
Destructive waves are formed by localised storm events with stronger winds operating closer to the coast.
Short wavelength
They have a frequency of 12-14 per minute.
High waves, which plunge into the beach.
Weak swash, strong backwash.
Occurs on steeply sloping beaches.
Explain the characteristics of high-energy coastlines.
High-energy coastlines are associated with more powerful waves, so occur in areas where there is a large fetch. They typically have rocky headlands and landforms and fairly frequent destructive waves.
As a result these coastlines are often eroding as the rate of erosion exceeds the rate of deposition.
Explain the characteristics of low-energy coastlines.
Low-energy coastlines have less powerful and occur in sheltered area where constructive waves prevail and as a result these often fairly sandy areas. There are landforms of deposition as the rate of deposition exceed the rates of erosion.
Explain the process of wave refraction.
Wave refraction is the process by which waves turn and lose energy around a headland on coastlines varying geomorphology. The wave energy is focussed on the headlands, creating erosive features in these areas. The energy is dissipated in bays leafing to the formation of features associated with lower energy environment such as beaches.
How to wave fronts break if wave refraction is complete?
What happen if refraction is not complete?
The wave fronts break parallel to the shore. However, primarily due the varying geomorphology of the coastlines, refraction is not always totally achieved - this leads to a gradual move of sediment along the shore, as the swash moves in the direction of the prevailing winds, whereas the backwash moves straight down the beach following the steepest gradient. This process is known as longshore drift.
How does wave refraction distribute energy along the stretch of a coast?
Along a complex transverse coastline with alternating headlands and bays, wave refraction concentrates wave energy and therefore erosional activity on the headlands, while wave energy is dispersed in bays, hence deposition tends to occur in bays.
What are tides?
Tides regular movements in the seas surface caused by the gravitational pull of the moon and the sun of the oceans. The moon accounts for the majority of the gravitational pull.
What influences tides?
Tides are influenced by the size and shape of ocean basins, the characteristic of the shoreline, the Coriolis force and meteorological conditions
Low spring tides occur…
just after a new moon. Spring tides have a greater tidal range than neap tides.
High spring tides occurr…
after a full moon.
Neap tides occur…
when the sun and moon are at right angles to the earth.
Describe the tidal cycle.
High tides occur 12 hours and 25 minutes apart. It take 6 hours and 12.5 minutes for the water at the shore to go from high to low or from low to high. A lunar day last 24 hours and 50 minutes.
What is the tidal range?
Tidal range is the difference between high and low tide. The times and lengths of these varies between locations. There is almost no tide for example in the Mediterranean as it is enclosed. Tidal range varies with distance from the amphidromic point, the shape of the coast and the strength of the tidal currents. Tides range is the greatest in bays and along funnel-shaped coastlines.
Describe the formation of a tidal bore.
If the coast is funnelled, as the tide advances it is concentrated in an every narrowing space. Therefore, its height rises rapidly producing a tidal bore.
For example, the Severn Bore occurs in the Severn Estuary between Wales and England as a wave of up to 1m in height travelling at a speed of 30km/h.
How are tides classified in coastal areas?
Coastal areas can be classified into microtidal, which have a very low tidal range (less than 2m), mesotidal (2-4m) and macrotidal (over 4m).
How does tidal range have an important influence on coastal processes?
It controls the vertical range of erosion and deposition.
Weathering and biological activity is affected by the time between tides.
Velocity of tidal flow is influenced by tidal range and has an important scouring effect.
What are rip currents caused by and how are they important for transporting sediment?
Rip currents are caused by tidal motion or by waves breaking along the shore. A cellular circulation is caused by differences in wave height parallel to the shore. Water from the higher sections of the breaker travels further up the shore and returns back through the points where lower sections have broken.
Once rip currents are formed, they modify the beach by creating cusps. which perpetuate currents.
Rip currents are important for transporting sediment.
What are storm surges?
Storm surges are changes in the sea level caused by intense low-pressure systems and high winds speeds. High air pressure depresses the ocean surface, lowering local sea level, low air pressure allows the ocean surface to dome upwards, raising the local sea level.
What are the impacts of storm surges?
Storm surges can produce severe coast flooding on low lying. Forces of onshore current of storm surge water can cause rapid coastal erosion which is where the local sea level rise strong wave action and coastal flooding wear down or carry away rock, soils and sand along the costs.
For every drop in air pressure of 10 millibars…
sea water is raised by 10 centimetres.
Where are storm surges common?
Surges are common in the Bay of Bengal, on the south-east coast of the USA and Japan.
In which areas is sea level rise intensified?
(storm surges)
In areas where coastline if funnel shaped.
Describe storm surges in the Ganges delta.
The Ganges delta experiences many storm surges. These may exceed 4m and the accompanying storm waves can add a further 4m to wave height. The funnel shape of the Bay of Bengal forces water to build up, thereby intensifying the rise in sea level.
Explain the process of hydraulic erosion and how it plays an important role in coastal erosion.
As a wave crashes onto a rock or cliff face, air is forced into cracks, joints and faults within the rock. As the wave retreats, high pressure causes the cracks to widen as air expands. Over time this causes the rock to fracture. Bubbles found within the water may implode under high pressure creating tiny jets of water that over time erode the rock. This erosive process is called cavitation.
Explain the process of corrasion
(marine erosion)
Sands and pebbles are picked up by the sea from an offshore sediment sink or temporal store are hurled against the cliffs at high tide, thereby wearing the cliff face.
The shape, size, weight and quantity of sediment picked up, as well as the wave speed, affects the erosive power of this process. For example, it could be argued that heavier rock will have more erosive power the lighter rock. However, the lighter rock may have a more erosive power in coastal environments that have faster wave speed.
Explain the process of solution
(marine erosion)
The process of water dissolving rocks and material into solutions. In areas of calcareous rock (lime-rich) rock, the mildly acidic seawater can cause alkaline rock such as limestone to be eroded.
Eval: Igneous rocks, such as granite and basalt, are primarily composed of silicate minerals that are less soluble in water. Therefore, solution may not be significant erosive process in coastal environments composed of igneous rocks.
Explain the process of attrition
(marine erosion)
Wave action cause rocks and pebbles to hit against each other, wearing each other and so becoming round and eventually smaller.
Eval: Although attrition is an erosive process within the coastal environment, it has little to no effect on erosion of the coastline itself.
Weathering
The break down of rocks in situ. It differs from erosion in that no movement or transportation of the broken-down material takes place.
Freeze-thaw
On cool temperature coastlines, freeze-thaw weathering can be very effective. Water seeps into cracks in the cliff face and freezes overnight. When liquid water freezes to form ice, it expands by 10%. This widens and weakens the cracks. Repeated freezing and thawing can make the rock crumble and split. This process takes place on the exposed cliff face.
Heating and cooling
Repeated heating and cooling of rocks can also cause rocks to be broken down and weathered away. Rocks can EXPAND RAPIDLY when hot and CONTRACT RAPIDLT when cold, causing the breaking apart of layers of rock.
For example, on desert coastlines when days are extremely hot and nights are extremely cold (high diurnal range), the changes in temperature causes shear stress on the outer layers of the rock. This causes the outer layer of the rock to peel off in a process called exfoliation. The increased availability of water (which leads to chemical weathering) at the coastline (e.g. morning fog) can accelerate this process.
Where does disintegration occur?
Occurs in areas with large diurnal differences eg deserts. Excessive heating and cooling results in exfoliation occurring.
Salt Crystallisation (salt weathering)
In the spray zone above high water, salt crystallisation will be an effective weathering process as salt water soaks into the pores of the rock.
Salt weathering occurs due to SALT CRYSTAL GROWTH inside the cracks and pores in rocks.
When SALINE solutions (i.e. salt water) gets into cracks and evaporates, it leaves behind salt crystals that were in solution. As these salt crystals accumulate over time, pressure builds up and expands the gaps in the rocks. Eventually this causes the rocks to DISINTEGRATE.
