Coasts Flashcards
The components of a landscape system
A system is a set of interrelated objects comprised of components and processes that are connected to form a working unit.
Energy available to a coastal system is kinetic, potential and thermal – allowing geomorphic processes to happen.
Input - energy and matter transferred from neighbouring systems.
Output – transferred to neighbouring systems.
Open and closed systems (inc. coasts)
Open systems – sediment is transferred between systems. There are inputs and outputs.
Coasts: inputs - kinetic from wind/waves, thermal from sun, potential from position of deposition material on slopes.
Outputs – marine and wind erosion, evaporation.
Throughputs – sediment accumulation, sediment movement (longshore drift).
Closed systems – no transfer of sediment.
e.g., sediment cells
Dynamic equilibrium
A systems inputs and outputs are equal.
Sediment being added is equal to sediment being removed.
If this equilibrium is disturbed, the system self-regulates to restore the equilibrium (negative feedback).
what are the features of a sediment cell?
how many?
boundaries?
Large stretch of coastline
Movement of coarse sediment is self-contained
Closed system
Due to wind, sediment is transferred between some cells
There are 11 on the coast of England and Wales
Sub-cells within major cells
Boundaries are determined by topography e.g.; Land’s End is a natural barrier to sediment being transferred to adjacent cell.
Coasts are influenced by physical factors - wind
Wave energy is generated by the frictional drag of winds moving across the surface.
The higher the wind speed and the longer the fetch, the larger the waves and the more energy they have.
P=H^2T
If winds approach at an oblique angle, waves will approach coast obliquely and cause longshore drift.
Wind also carries out erosion, deposition and transportation.
anatomy of waves
swell and storm waves
Moving waves do not move water forward but the waves impart a circular motion to the individual molecules.
Anatomy: crest – highest, trough – lowest and the distance between is wave height. Distance between two crests is wavelength.
Swell waves – formed in open oceans, long wavelength, 20s wave period.
Storm waves - short wavelength, greater height and shorter wave period.
Types of waves
As waves approach the shoreline, friction with the seafloor changes the speed, direction and shape of waves.
As waves drag across the bottom they slow down, wavelength decreases and waves bunch up. The deepest part of wave slows down more than top and so it begins to steepen, eventually toppling over.
Spilling – steep waves break onto gently sloping beaches.
Plunging - moderately steep waves breaking onto steep beaches.
Surging – low angle waves break onto steep beaches.
Constructive and destructive
Constructive: low height, long wavelength, 6 min frequency. Break as spilling waves. Due to long wavelength, backwash returns to sea before next wave breaks, so next uninterrupted swash movement retains energy. Swash energy exceeds backwash energy.
Destructive: greater height, shorter wavelength, 12/minute frequency. Break as plunging waves. There is little energy to move water as swash, friction from steep beach slows the swash so it does not travel far before returning as backwash. Swash energy is less than backwash energy.
what are the features of tides?
high tide/neap tide
tidal range
Tides are the periodic rise and fall of sea surface, caused by gravitational pull of moon and sun.
The moon pulls the water towards it, creating a high tide. Between two bulges is a low tide.
High tide follows the moon as it orbits the earth, and the spring tide will be when moon, sun are earth are aligned (twice each lunar month).
Twice a month there’s weak pull as sun and moon are at right angles, so a neap tide.
tidal range is vertical distance between high and low tide.
low tidal range produces a narrow beach which is prone to higher wave erosion. (Mediterranean).
Range influences where wave action occurs and weathering processes.
Currents
Rip currents – caused by waves breaking at a right angle. Water from top of large waves travels further up shore and when travels as backwash it meets where lower height waves broke. The rip currents create cusps.
Ocean currents – generated by Earth’s rotation and convection. Warm ocean currents transfer heat energy to poles and cold currents move water to equator. Driven by off-shore wind so has little impact on geomorphic processes, but transfer of heat energy affects sub-aerial processes.
Lithology and structure
Rocks with weak lithology have limited resistance to erosion, weathering and mass movement e.g., clay.
Basalt and stronger rocks are more resistant and likely to form cliffs and headlands.
Concerns jointing, bedding, faulting and permeability. Porous rocks (chalk) can absorb and store water.
Primary permeability – pores absorb water.
Secondary permeability - water seeps into joints.
Rock outcrops parallel to coast produces concordant coastline.
Rocks at right angle to coast produce discordant coastline - headlands and bays.
Sediment sources
Terrestrial: coastlines with steep gradient have rivers directly deposit sediment or during floods. 80% is from rivers. Sediment originates from the erosion of inland areas by water, wind, ice.
Waves: cliff erosion amplified by rising sea level and storm surges – can supply up to 70%. Longshore drift moves sediment.
Offshore: constructive waves deposit and add to sediment budget. Wind transports fine sediment from sand bars, dunes and other beaches.
Human: beach nourishment maintains sediment equilibrium; dumped by lorry or through pipeline.
Physical Weathering
Physical: surface area of the rock is increased, allowing further weathering to take place.
Freeze thaw - water in cracks expands 10% when it freezes.
Pressure release - when overlying rocks are removed, the underlying rock expands and fractures parallel to the surface.
Thermal expansion - rocks expand when heated and contract when cooled. If there is a repeated cycle of temperature change, layers of rock flake off.
Salt crystallization - solutions of salt seep into pore spaces and form crystals, this causes stress, and the rock disintegrates. Sodium sulphate expands 300%.
Chemical Weathering
Chemical – reactions between moisture and minerals in the rocks. Reduces the rock to its chemical parts or alters its composition. Produces weak residues which are removed by erosion/transportation.
Oxidation - reacts with oxygen in air or water . Iron becomes soluble under acidic conditions and its structure is destroyed.
Carbonation - rainwater combines with dissolved CO2 producing a weak carbonic acid, it reacts with calcium carbonate in rocks to produce calcium bicarbonate which is soluble.
Solution - some salts are soluble in water, any process by which a mineral dissolves in water.
Hydrolysis - silicates combine with water producing secondary materials such as clay.
Hydration - water molecules added to rock minerals creates new minerals of a larger volumes. Hydration causes surface flaking.
Biological Weathering
Biological: consist of physical actions such as the growth of plant roots or chemical processes by organic acids.
Tree roots – they grow into cracks/joints in rocks and exert pressure. When trees fall, their roots exert leverage and pull rocks to the surface, exposing them to further weathering processes. Burrowing animals have a similar effect.
Organic acids - cause soil water to become more acidic and react with some minerals (chelation). Molluscs on shore platforms secrete acids which produce small surface hollows in rocks.
Mass Movement - rock fall/slides/slumps
When gravity exceeds the forces trying to keep the material on the slope, friction.
Mass movement on cliffs adds material to the sediment budget by transferring rocks and regolith onto the shore below.
Rock fall - 40-degree cliffs, rocks become detached due to physical weathering and fall to the foot of cliff under gravity. Wave processes remove this, or it may accumulate as a scree slope.
Slides - linear (along a straight-line slip plane, such as a fault or a bedding plane) or rotational (along a curved slip plane, also known as slumps).
Occur due to undercutting which removes support for materials above.
Slumps common in weak rocks such as clay which becomes heavier when wet, adding to downslope force.
Wave Processes – Erosion
Abrasion – rock rubbing against rocks.
Attrition - rock particles collide with each other and get worn away, becoming smoother, smaller and rounder.
Hydraulic action – air and water get trapped in crevices and become compressed, expands and the crack is widened.
Pounding - mass of a breaking wave exerts pressure on a rock and weakens it.
Corrosion - dissolving minerals in coastal rocks. Only significant if water is locally polluted or acidic and if there’s significant amounts of soluble minerals.
Wave Processes - Transportation
Solution – minerals dissolved in water.
Suspension - small particles of sand carried by currents.
Saltation – irregular movements of heavy materials, too heavy for suspension. Carried for a short distance and dropped again.
Traction - largest particles are pushed along sea floor by the force of flow. Largs take rest after partial rotations.
Wave Processes – Deposition
Material is deposited when there is a loss of energy, caused by a decrease in velocity or volume of water.
Takes place when: accumulation exceeds removal, waves slow down immediately after breaking, at the top of swash when water briefly stops moving, during backwash when water percolates into beach material, low energy environments sheltered from winds and waves.
Fluvial Erosion
Erosion in the upper catchment is the main source of a river’s sediment load.
Most channel erosion occurs during high-flow, high-energy events.
Similar erosional processes to waves.
Sediment is also derived from weathering and mass movement from valley sides.
Fluvial Transportation
Traction
Suspension
Saltation
Solution
