Section 2: Coastal systems, waves, storms Flashcards
Define coasts
Where the land meets the sea
Define estuary
partly enclosed coastal body of brackish water with on or more rivers flowing into it, and a free connection to the open ocean
Length of UK coastline (not including islands)
~11,000 miles
Human activities at coasts
Habitat and source of food
Means of transport
Accessible, fertile land
natural resources
Water extraction and discharge of waste
Recreational activities
Bruntland report 1987
Must meet the needs of the present generations with comprising the ability of the future generation to meet their own needs
Number of people currently living in Low Elevation Coastal Zone (LECZ)
680 million (1 Billion by 2050)
Number of people living within 60 miles of the coast
2.4 billion (40% of global population)
Define Mangroves
Various types of salt-tolerant plant species (trees or shrubs) that occur in intertidal zone of tropical and subtropical coastlines
Define Seagrass
A group of flowering plants found in marine or estuarine waters, that tend to develop extensive underwater meadows
Define saltmarshes and mudflats
Ecosystems of brackish, shallow water with salt-tolerant plants such as herbs, grasses or shrubs. Usually found in the intertidal zone of sheltered marine and estuarine coastlines, in temperature and high latitudes
Define coral reefs
Carbonate structures which gradually built by stony corals, calcareous algae and other reef building organisms/ Warm-water corals reefs occur in coastal areas of tropical and subtropical regions
Define kelp forests
Kelp- a large algae seaweed which tends to occur in high density forests worldwide through temperature and polar coastal regions and some tropical waters
Ecosystem services
Provisioning (food, biomass, fuel and water)
Regulating (climate control, natural hazards, disease)
Supporting (nutrient cycles, soil formation, photosynthesis, PP, carbon sequestration)
Cultural (spiritual, aesthetic, recreational, ethical values)
Hazards to coastal areas
Flooding, erosion, big storms, tsunamis, sea level rise, harmful algal blooms, hypoxia, warming sea temperature, habitat degradation and loss
Indian Ocean Tsunami facts
Year: 2004 (boxing day)
Deaths: 228,000
UK flooding 2019/20 damages
£333 million
Drivers of flooding
Terrestrial: rivers, run-off, ground water
Atmosphere: wind, rain
Marine: waves, tides, tsunamis, storm surges, mean sea level
Morphology: geology (hard/soft), sediment characteristics supply
Space and time scale variation:
Turbulence
1mm, 1sec
Space and time scale variation:
Seiches waves
1m/1km, 1min
Space and time scale variation:
Tsunamis
100kms, mins to hours
Space and time scale variation:
Tropical surges
10/100kms, 12 hours
Space and time scale variation:
Extra-tropical surges
1000/10000kms, 10 days +
Up ? sea level rise in 2100
1.15m
What is a wicked problem
Complex, challenging, have multiple feedbacks, high uncertain and have ambiguous solutions
Coastal Risk management options
Do nothing
Protect (hold-the-line): seawalls, offshore breakwaters, beaches
Adapt (accommodate): flood proofing, hazard mapping, awareness, land use zoning
Retreat (re-align): land use zoning, coastal set back, managed retreat
What is resilience
The ability and time taken to build back better
Minimise loss, minimise recovery time, minimise cost
Causes of sea level change
Add/subtract: glacial melt, (rainfall- non existent)
Move: wind, currents, tides (biggest mover)
Change properties: thermal expansion
Define non-tidal residual
The meteorological component of sea level that remains once the tidal component has been removed and primarily contains the meteorological contribution to sea level
Most dramatic meteorological sea level change occur during storms, when low atmospheric pressure allows sea level to rise and strong winds force water towards the coastline and generate large waves
Why are waves and storm surges important (6)
1.Strong influence on sea level- coastal flooding, cause damage
2. Influence on currents and sediment transport -coastal erosion
3. Design of coastal and offshore structures eg light houses
4. Renewable energy generation eg tidal energy
5. Recreation eg surfing
6. Navigation, safety at sea
Global circulation patterns
The air in the atmosphere and the water of the ocean are continually moving. Powerful air currents, jet stream, Gulf stream, small swirls and eddies
Driven by the energy from the sun and the rotation of the Earth
Highest angle of insolation
At the equator 2% of radiation reflected if sun overhead
High/low latitudes solar radiation spread over a larger surface area (40% radiation reflected)
Northern hemisphere summer, angle of earth
North pole tilted towards the sun, sun’s rays strike N.Hem more directly
Northern Hemisphere winter, angle of Earth
North pole tilted away, light from the sun is more spread out over a larger area
How is the global heat balanced
Excess heat from the equatorial zone is transferred towards the poles, in global circulation patterns (convection cells)
**Global convection cells from equator to poles
Hadley cell, Ferrel cell, Polar cell
Why do differences occur in circulation patterns over land and sea
Mountains, large solid mass, absorb heat better, land masses change heat more rapidly
What direction do these current flows in the Northern hemisphere:
Doldrums
Trade winds
Westerlies
Polar easterlies
Doldrums: no circulation
Trade winds equator: east to west
Westerlies 30o: west to east
Polar easterlies 60o: north/east to west
Idealised pressure belts and resulting wind systems are significantly modified by:
- Earth’s tilt- produces seasons
- The air of continents get colder in the winter and warmer in the summer than the air over the adjacent oceans (because of the heat capacity of water compared to rock)
In winter, continents usually develop high pressure cells, during summer develop low pressure cells
Extratropical: N.Hem Cyclone
Easterly wind (cold front) passes westerly wind (warm front)
Easterlies deflect to the left, westerlies to the left, create an detached cyclone
High and low pressure front interact
Direction of rotation in the N Hemisphere
Low pressure system: anti-clockwise depression, cyclone
High pressure system: clockwise, anticyclone
Direction of rotation in the S Hemisphere
Low pressure system: clockwise
High pressure system: anti-clockwise
Characteristics of a wave
Transfers a disturbance from one part of a material to another
The disturbance is propagated through he material without any substantial overall movement of the material itself
The disturbance appears to be propagated with constant speed
Regular waves
Monochromatic
Sinusoidal
One single frequency
Irregular waves
Random
Confused
Changes in elevation, angle, direction, size frequency
Features of a sinusoidal wave
Crest: maximum vertical disturbance above mean sea level
Trough: Maximum disturbance above mean sea level
Wave length: distance between two points of the same height (eg crest)
Wave height: maximum vertical distance between the highest and low points of the wave
Wave period: time it takes to observe a whole wave
Amplitude (sinusoidal wave): maximum displacement under the crest or above the trough to the still sea level
Slope: steepness of the wave, the change in vertical elevation over horizontal distance
How to calculate wave steepness
Hight divided by length
How to calculate wave number
Number of wavelgnths per unit distance (1/L)
How to calculate wave frequency
Number of peaks (or troughs) which pass a fixed point per second (1?T)
What is the wave period
Time interval between two successive peaks (or troughs) passing a fixed point - in seconds
Irregular wave quantitative description
Significant wave height (Hs): average wave height of one third of the highest waves in the record. Approximately corresponds to visual estimates of wave periods. Used in prediction of flooding
Mean wave period (Tz): mean period of all waves in the wave record (zero-crossing wave period)
How does the wind generate capillary waves
Frictional stress between air and ocean surface creates a transfer of energy. This deforms the surface into small rounded waves, short wavelengths (cm). Surface tension is dominant restoring force trying to move back to smooth ocean surface
How do capillary waves become gravitation waves
An increase in capillary waves means the sea gets rougher in character. This catches more wind- transferring more energy. Gravity waves reacher greater heights, gravity replaces surface tension as the dominant restoring force
How does a wave system become a storm
Storms characterised by choppy waves, short wavelengths moving in many directions
1. Wind speed increase
2. Increase of duration wind blows in one direction
3. Increase in fetch (distance)
How does a wave system become a storm
Storms characterised by choppy waves, short wavelengths moving in many directions
1. Wind speed increase
2. Increase of duration wind blows in one direction
3. Increase in fetch (distance)
What is the critical value for wave steepness when whitecaps form
1/7
What is a fully developed sea
For a given wind speed, a maximum fetch and duration beyond which waves cannot grow
What waves are fastest within a region of blowing wind
Longest waves are the fastest, so can escape from the region of generation
What are the characteristics of waves that escape a region of generation
Have propagated into a region without wind generation, become swell waves, regularly spaces, long period/length, can indicate an oncoming storm
Deep water waves
If the water depth (D) is greater than one half of the wavelength (L) the waves are not affected by the ocean floor
D > L/2
No bottom friction
No net movement, local movement is cancelled within the circular motion
Shallow water waves
If the depth (D) is less than one-twentieth of the wavelength
D < L/20
Interaction with the seabed, wave slows down, wave height increases. Waves shoal and break
What is shoaling
Wave experiences fraction, deforms wave and eventually breaks
Water entering shallow water change in wave height
Refraction as transformative in shallow waves
Wave approaches coast at angle to bottom contours
Deeper part of the wave travels faster than the rest, friction
Results in a rotation of the wave crest with respect to the bottom contours
Nearshore currents, sediment transport and coastal morphology impact this
Headland vs bay (refraction)
Headland: wave convergence, high energy zone
Bay: waves spread, deposition, low energy zone
Diffraction as transformative in shallow water
Energy is transferred along the wave crest rather than the direction of wave propagation
Occurs when a wave encounters a feature: island breakwater or offshore reef
A wave shadow zone is created behind the obstacle, but diffraction causes the wave energy to spread into this zone
Reflection as transformative in shallow water
In certain conditions (eg presence of wall/breakwater) waves do not break in shallow water - reflect on shore
Interaction reflected and incoming waves result in the formation of standing waves
Water particles move vertically and horizontally
Properties of the water do not change, only the direction
When will a wave break
The wave reaches shallow water, experiences friction, slows down, potential energy rises (wave height increases) to balance total energy, deforms, crest becomes steeper and more angular, shape cannot be sustained, breaks
What is a spilling wave
Large wave height relative to the wave length- shallow beach slope
White line as propagates
What is a plunging wave
(most desirable for surfers) shallow to intermediate beach slope
Crest curls as it deforms, injects into the trough, classic barrel shape
What is a collapsing wave
Front face collapses- intermediate to steep slope
Undefined shape
What is a surging wave
Wave slides up the beach without breaking- steep beach slope
Significant sliding of the water up and down the beach face, dangerous, break over a long distance
