2 Coastal Processes Flashcards
What are the 6 factors that influence coastal processes?
Waves
Tides
Currents
Aeolian
Geology
Human Activity
What are waves?
The transfer of energy through the water, produced by wind exerting a frictional drag on the ocean’s surface - not the transfer of water
What energy do waves input?
Major input of kinetic energy into the coastal system
-Potential energy by virtue of the water’s height above the trough of the wave
Energy allows work (e.g. erosion)
Describe the movement of a molecule of water as a wave passes through
The waves impart a circular motion to the individual water molecules
What is the wave crest/trough?
Highest/lowest point of a wave
Define wavelength
Average distance between successive wave crests
Define wave height
The vertical distance between a wave trough and crest
Define wave velocity
The speed at which the wave travels
Define wave steepness
The ratio of wave height to wave length
Define wave period
The average time between successive waves
Define wave frequency
The average number of waves per minute
Define wave power
The square of wave height multiplied by wave period (P=H^2T)
Describe the characteristics of swell waves
Long wavelength
Gentler gradient
Long wave period (up to 20secs)
Generated by distant winds blowing in the open ocean
Describe the characteristics of storm waves
Short wavelength
Steeper
Short wave period
Generated by local winds
What is the equation for calculating the wave power?
P (wave power) = H^2 (height) T (wave period)
Why do waves break?
-In deep water, not affected by friction (long wavelength and low wave height)
-Shallower water, slowed by friction with the sea bed
-Front is slowed earlier than the back, causing the back of the wave to ‘catchup’ with the front (shortens wave length and increases wave height)
-Base slowed more than the top so top ‘overshoots’ the base, and distorts the circular orbits of water molecules to be more elliptical
-When wave depth is <x1.3 wave height, the wave becomes too top-heavy and unstable, and it breaks
Describe spilling waves
Steep waves breaking onto gently sloping beaches; water spills gently forwards as the wave breaks
Describe plunging waves
Moderately steep waves breaking onto steep beaches; water plunges down vertically as the crest curls over
Describe surging waves
Low angle waves breaking onto steep beaches; the wave slides forward and may not actually break
Describe the characteristics of constructive waves
Low waveheight
Long wavelength
Gentle steepness
Long period
Low frequency (6-8/min)
Long fetch (distant winds)
Broken wave spills up beach
Swash stronger than backwash - long period means backwash returned to sea before next wave arrives, so incoming swash is not disrupted
Material pushed from lower to upper beach, creating steeper upper beach features such as berms
Describe the characteristics of destructive waves
High waveheight
Short wavelength
Steep steepness
Short period
High frequency (12-14/min)
Short fetch (local winds)
Wave breaks vertically down in a plunging motion
Backwash stronger than swash - plunging motion means little energy directed up the beach, and short period means backwash of one wave will disrupt incoming swash
Material combed from upper beach and deposited on lower beach, creating features such as breakpoint bars
Draw a sketch of constructive and destructive waves
What shape of coastline results in wave refraction?
Irregular-shaped
What two coastal landforms in particular encourage refraction?
Headlands and bays
What is an orthogonal
Line drawn at right angles to the wavefront, showing direction of wave energy
Describe orthogonal
- Waves approach the coastline at an angle
- One side of the ave front reaches shallower water before the other
- The side of the wave front in deeper water travels faster, causing the whole wave front to bend
- One side of the wave front is therefore slowed by friction before the other side
- The overall effect is the wave front to become increasingly parallel to the coast
What is wave energy derived from?
Wind blowing over the surface of the water
What is wave energy dependent on?
Wind strength
The length of time for which the wind has blown
The fetch
Define fetch
The distance of open water over which the wind has blown to generate the waves
What is the formula for calculating the maximum wave height?
H = 0.36 square rootF
What are tides?
The periodic rise and fall in the level of the sea
Caused by the gravitational pull of the moon and of the sun
The moon pulls water towards it, creating a high tide, and there is a compensatory bulge on the opposite side of the Earth. In the area between the two bulges, the tide is at its lowest
Does the moon or sun exert a stronger influence?
Moon as it is much closer despite being smaller
What is a spring tide?
Moon, Earth and sun aligned in a straight line
Very high high tides and very low low tides (large tidal range)
Twice a month
What is a neap tide?
Earth, sun and moon are aligned in a 90 degree angle, the gravitational pulls are acting in contrary directions
Not as high or low tides
What is the tidal range?
Vertical distance between high and low tides
What are the numbers for macro-tidal, mesh-tidal and micro-tidal?
macro-tidal (>4m)
mesh-tidal (2-4m)
micro-tidal (<2m)
Why do funnel-shaped coastlines affect tides?
They constrict the high tide bulge, forcing it higher e.g, Bristol Channel
Define currents
Represent the flow of ocean water
Major input of kinetic energy into the coastal system
What do rip currents do?
Role in transport of coastal sediment
Explain how rip currents are formed
(Important in the transport of coastal sediments)
- Water from broken waves moves up through the breaker zone, then parallel to the shoreline (alongshore current)
-Causes water to pool up by the beach where it will flow out to sea through the path of least resistance through a narrow neck
-Once this cellular circulation is set up, erosion of beach material by the offshore rip current creates a beach cusp, which further channels water and intensifying the rip
What is transferred by currents which can be significant?
Heat energy as it directly affects air temperature and therefore sub-aerial processes
Strength of the current itself may have limited impact on coastal landscape systems in terms of geomorphic processes
Explain how ocean currents are formed
Larger scale, Coriolis force and convection
-Movement of winds across ocean surface distribute heat from equatorial oceans to the high-latitude oceans, to maintain global atmospheric equilibrium
-Warm water currents tend to move from the W to E bringing warm, onshore currents to western-facing coastlines
-Cold water currents move from E to W and more offshore –> usually driven by offshore winds so have less effect on coasts
Define gyres
Huge circulations of water in the oceans created by ocean currents
How do winds influence the coastal landscape?
- Wave energy generated by frictional drag of winds moving across the ocean –> waves are a source of energy for coastal erosion and sediment transport
- Higher wind speed and longer fetch = larger waves with more energy
What are onshore winds and what do they do?
Blow from the sea towards the land and drive waves towards the coast
If winds blow at an oblique angle towards the coast, waves approach obliquely and generate longshore drift
What processes can wind carry out by itself?
Erosion, transportation and deposition
How does the tidal range influence coastal landscapes?
Influences where wave action occurs, weathering processes that happen on land exposed between tides and the potential scouring effect of waves along coasts with a high tidal range
In enclosed seas, tidal ranges low so wave action restricted to a narrow area of land
Funnel coast tidal range high as 14m
What is the micro-tidal range?
<2m
What is the meso-tidal range?
2-4m
What is the macro-tidal range?
> 4m
Define differential erosion
The process by which adjacent rock types, of differing levels of resistance, are eroded at different rates
It can also occur with the same rock mass if, for example, sections of it have a higher density of joints, bedding planes and other lines of weakness
Define lithology
The physical and chemical composition of the rocks
What are the 3 characteristics that determine the characteristics of rocks and how they are affected by coastal processes?
Lithology, structure and dip
What is the lithology of clay?
-Weak lithology
-Little resistance to erosion, weathering and mass movements
-Due to unconsolidated nature of the rock –> weak bonds that join the individual particles
What is the lithology of basalt?
Dense, interlocking crystals lead to a highly resistant lithology
What is the lithology of limestone?
-Strong physical lithology tightly bonded particles
-Very resistant rock, but a weaker chemical lithology
-Vunerable to solution in weak acid (corrosion erosion/carbonation weathering)
Why is carboniferous limestone permeable?
Water seeps into limestone because of its many joints - secondary permeability
Joints ae easily enlarged by solution
What is primary permeability?
To be able to absorb and store water
In porous rocks, eg chalk, pores separate the mineral particles
What does structure refer to?
Properties of individual rock types e.g. jointing, bedding and faulting
Relates to the permeability of a rock
Configuration of rock types relative to other geological bands at regional scale
Define porosity
Primary permeability
Water transmitted through pores
How does structure impact cliff profiles?
Structure also includes the angle of dip of rocks
Both horizontally bedded and landward-dipping strata support cliffs with steep, vertical profiles
Where strata incline seawards, cliff profiles follow angle of dip of the bedding planes
How does structure influence the coast?
Influence on the planform (shape) of coasts at a regional scale
Rock outcrops that are uniform or run parallel to the coast, tend to produce concordant coasts
Rocks that lie at right angles to the coast create discordant planform
Define perviosity
Secondary permeability
Cracks/joints allow water through (not through the rock)
How does the shape of the coastline differ on concordant and discordant coastlines?
Discordant - rock bands perpendicular to the coastline, creating irregular coastlines (headlands and bays)
Concordant - rock bands parallel to the coastline, creating straighter coastlines
How does dip impact cliff formation?
Steepest cliffs tend to form in rocks that have horizontal strata or which dip gently inland, whereas rock that dip towards the coast tend to produce much more gently sloping features
What are the 3 types of strata?
Horizontally bedded strata
Seaward dipping strata
Landward-dipping strata
Describe how horizontally bedded strata impacts cliff formations
Undercutting by wave actions leads to rockfall
The cliff retreat inland parallel to the coast
Describe how seaward dipping strata impacts cliff formations
Undercutting by wave action removes basal support, rock layers loosened by weathering slide into the sea along the bedding planes
Describe how landward-dipping strata impacts cliff formations
Rocks loosened by weathering and wave actions are difficult to dislodge, the slope profile is gradually lowered by weathering and mass movement
How does building groynes impact the coastal system?
-Traps sediment being transported by LSD, creates larger store of material (wider, higher beach), which reduces erosion rates
-At adjacent locations, however, the beach will be starved of sediment and the store will be depleted. May lead to increased rates of erosion downdraft
How does extracting sand from a beach and the offshore zone impact the coastal system?
-Depletes store of sediment; as beaches and offshore bars are an effective wave energy buffer; increased rates of erosion of cliffs behind the beach (this is the system trying to replace the lost store by increasing an input)
-Or sediment may be transported from locations along the coast to replenish the lost store
How does beach nourishment impact the coastal system?
-New input
-Larger beach may reduce erosion of the terrestrial environment behind the back
How does stabilising a sand dune ecosystem by planting marram grass impact the coastal system?
Facilitates the process of deposition by disrupting Aeolian transport of sand and fine material
The store of sediment in the sand dune grows
How do aeolian factors affect the coastal system?
Wind is another input of kinetic energy
Capacity to erode, transport and deposit finer material
Waves get energy from wind exerting a frictional drag on the ocean surface
Wind speed will therefore affect coastal processes
Define solution
Dissolved minerals transported in a mass of moving water
Define suspension
Occurs when small particles are carried by currents
Define saltation
Irregular ‘leapfrogging’ motions where material too large to be in suspension is bounced along the sea
After being picked up the material will be deposited after a short distance
This may dislodge other sediment for further saltation
Define traction
Largest particles of load pushed along the sea bed by waves or currents
Partial rotations then periods of rest
When does deposition occur?
-Sheltered places
-When waves slow down due to friction after breaking
-Top of the swash when wave stops momentarily
-Backwash when some water seeps into the beach and weakens it
-Rate of sediment accumulation > rate of removal
Name 5 chemical weathering processes and briefly explain them
Oxidation - react with oxygen and iron oxide decomposes the rock
Solution - Dissolved by water
Hydration - water added to rock create new minerals of a larger volume, causes surface flaking as expand
Hydrolysis - reaction between rock minerals and water
Carbonation - rainwater and CO2 produced carbonic acid which reacts with calcium carbonate in rock to make it soluble
Name 2 biological weathering processes and briefly explain them
Tree roots - grow into cracks or joints
Organic acids - produced during decomposition of plant and animal litter cause soil water to become more acidc and react with some minerals. Blue-green algae can have a weathering effect, producing a shiny film of iron and manganese oxides on rocks. Molluscs secrete acids which produce hollows in rock
Name 4 physical weathering processes and briefly explain them
Thermal expansion - frequent cycles of high and low temps cause layers to flake off
Salt crystallisation - salt solutions seep into porous rocks, forms crystals and creates stress
Freeze-thaw - water enters cracks and expands and causes pressure
Pressure release - overlying rocks removed and underlying rock expand sand fractures parallel to the surface
Define mass movement
Downslope movement of rock, soil and other material, under the influence of gravity
Water is a trigger
What are the 4 types of mass movement
Rockfall
Slides
Slumps (rotational)
Mudflows
Describe fluvial deposition
River velocity decreases at the mouth due to friction - energy reduction
Larger particles deposited first
Describe fluvial transportation
Transport to coast through traction, saltation, suspension, solution
Define flocculation
Fresh and salt water mixes, fine clay particles clump together due to electrostatic attraction in saline conditions.
Define deflation
Removal of fine material by the wind
Describe wind erosion
Wind able to pick up sand particles and move them by deflation
Speeds of 40km/hr, surface creep and saltation as are heavy
Restricts height of abrasion so has limited effect on coastlines and cliffs
Erosive force increases exponentially with wind velocity
Dry sand easier for wind to pick up as moisture increases cohesion
Define surface creep
Surface rolling
Why is attrition effective on land compared to the sea?
As particles carried in the wind lack the protective film of water when they bump into one another
Define wind transportation
-Same mechanisms apart from solution
Once particles have been entrained, can be moved in winds moving at speeds as low as 20km/hr
What are the 2 aspects of geology that influence coastal landscapes?
Lithology and structure
Define abrasion
When the sea’s load is thrown against the rocks of the coast by breaking waves, wearing them away through a ‘sandpapering’ action
Define attrition
Attrition occurs when particles of the load knock against each other or against coastal rocks causing them to become smaller and smoother
Define hydraulic action
Wave breaks against a cliff face, causing air and water in cracks in the cliff to be compressed
As the wave recedes, this pressure is released like a mini-explosion, and the rapidly expanding air can widen the crack
Define pounding
Force of a wave on the rock, even without any load to wear the rock away
Define corrosion
Particular rocks - contain soluble minerals sun has calcium carbonate (eg limestone) or magnesium carbonate - are slowly dissolved by the sea water
Sea water has a pH of 7 or 8, insignificant unless the water is artificially acidic due to pollution
Describe rockfall
- Steep cliff faces (so resistant rocks)
- Rocks detached from the cliff due to weathering processes and then fall to the foot of the cliff due to gravity
- Falling material breaks up as it moves, scree slope at the cliff base
- Promoted by basal undercutting of the cliff by wave action (wave cut notch), line of weakness (joints) and weather conditions that promote weathering processes
Describe slides
- Material moves downslope due to gravity as a block
- Linear slip plane
- Can occur on softer or resistant materials
Describe slumping
- Material moves downslope due to gravity as a block
- Curved slip plane so rotating material as it moves
- Common in weak rocks and during periods of heavy rainfall as clay saturated and very mobile/heavy so increasing downslope force
- A layer of permeable sandstone on top of the clay increases likelihood –> water passes through the sandstone, but if cannot soak into saturated clay, sandstone becomes saturated itself exerting greater pressure on clay
- Slope loading promotes it (eg thick tree growth or building at the cliff top)
- Creates characteristic back-titled blocks in the cliff profile
Describe mudflows
Saturated fine material flows downslope under the influence of gravity
- Slow process
- Usually in combination with slumping
How is sediment delivered to the coastline?
Rivers are major source of sediment input, particularly for coasts with a steep gradient as rivers directly deposit their sediments at the coast
Delivery can be intermittent, most during floods
Where does sediment come from?
Erosion of inland areas by water, wind and ice and sub-aerial processes of weathering and mass movement
Wave erosion
Cliff erosion increases by rising sea levels and storm surges
Longshore drift
How is offshore locations involved in the coast?
Constructive waves, tides and currents bring sediment from offshore and deposit it
Wind blows sediment from exposed sand bars, dunes and beaches - fine sand as wind does not have as much energy
