Section 2: Storm surges, coastal flooding, sea level Flashcards
define storm surge
A rise in the normal water level along a shore resulting from reduced atmospheric pressure and strong onshore winds
What is atmosphere pressure
Pressure exerted by the weight of air in the atmosphere of Earth. The force per unit area exerted on a surface by the weight of air above that surface in the atmosphere
What is atmosphere pressure
Pressure exerted by the weight of air in the atmosphere of Earth. The force per unit area exerted on a surface by the weight of air above that surface in the atmosphere
What is the atmospheric pressure at mean sea level
1 atmosphere
Factors influencing how much winds piles up water levels
Wind strength
Wind direction
Fetch
Topography
Water depth
Coriolis force
What happens when two layers of moving fluid are in contact
energy and momentum are transferred from the more rapidly moving layer to the slower layer.
In equilibrium, what must wind stress balance
wind stress must balance the pressure gradient causes by the slope in the sea surface
What is set-up
The rise in water level in coastal areas.
Can be caused by tides, storm surges, wave action
Influenced by shape and characteristics of the coastline and direction of the wind
Important for understanding coastal management and coastal engineering as it can help predict and mitigate the impacts of coastal flooding and erosion
Upwelling
What is set-down
The fall in water level in coastal areas.
Can be caused by tides, storm surges, wave action
Influenced by shape and characteristics of the coastline and direction of the wind
Important for understanding coastal management and coastal engineering as it can help predict and mitigate the impacts of coastal flooding and erosion
Downwelling
What are tropical cyclones
500-200km
Serval hours to up to half a day
Usually <200km
Compact and nearly symmetrical
November-April
Short distances and time scales
What are extra-tropical cyclones
1000-500km
2-5 days
several hundred 100km
Sprawling geometry
April-August
Driven by weather front, large, longer
Where are tropical cyclones found and not found
Not on the equator, tropics and deflect with Coriolis to the North or South
How do tropical cyclones form
Form over large relatively warm bodies of water
Derive their energy from the evaporation of water, condenses and forms clouds etc
How do extra-tropical cyclones form
‘depressions’ or ‘lows’
Fuelled by horizontal pressure contrasts
usually between 30-60o latitides
Contrasting extra-tropical and tropical cyclone storm surges
tropical: very large but last less than a day
Extra-tropical: smaller but last much longer (3 days)
Where is the largest storm surge located in the N.Hem and S.Hem
N.Hem: Southern side of the storm
S.Hem Northern side of the storm
Due to deflection of the wind and Coriolis
What is a negative storm surge
Wind direction blows the water away from the coast, causing sea level to drop
Less chance of flooding but can strand boats
What is a storm surge
An abnormal rise of water generated by storm, over and above predicted astronomical tides
What is a storm tide
The water level rise due to the combination of storm surge and astronomical tide
When would coastal flooding occur
Normally dry, low-lying land is flooded by sea water. Extent of flooding is reliant on elevation of inland controlled by the topography
What is the S-P-R-C model
Source: The origin of the hazard (eg storm, rainfall, storm surge etc)
Pathway: Route that a hazard takes to reach receptors (eg overtopping, overflow)
Receptor: the entity that may be harmed (eg people, property)
Consequence: an impact such as economic, social, environmental damage (eg loss of life, material, environmental degradation)
What are the different pathways in the SPRC model
Direct inundation: sea height exceeds the elevation of the land
Overtopping of a barrier: swell conditions, height of the wave exceeds the barrier, high velocities can cause erosion
Breaching of the barrier: broken by waves and water enters inland
Define mean sea level
Average height of the sea over longer periods of time (usually month of a year)
Short term variations are filtered out
Why are changes in mean sea level important
- Inundation, flood, sea damage
- Wetland change or loss (coastal squeeze), wildlife will attempted to retreat up shore if possible
3.Erosion (of ‘soft coastlines’) - Saltwater intrusion
- Rising water table and impeded drainage, aquifers etc
Indirect measures of mean sea level
Salt marshes
Archaeology
Speleothems
Corals
Geology
Sediments (sediment cores)
Direct measurements of mean sea level
Tide pole
Tide gauge (acoustic and radar) - discrete locations, 1 min-1hour
Satellites - global coverage, every 10 days
How can saltmarshes be used to reconstruct historical sea levels
Zonation in saltmarshes
Dependent on degree of submersion
Testate amoebae, very hard and can survive change
What can cause vertical land movement
Subsidence: groundwater withdrawal, reduced sedimentation
Uplift/rising land: sedimentation, tectonics
Subsidence and up life: Sedimentation, tectonic, groundwater with drawal
What is Glacial Isostatic Adjustment
On-going movement of land once burdened by ice-age glaciers
Why does land subside with ground water extraction
Total volume of sediment is reduced, sediments compact resulting in the lowering of the surface
What is steric expansion
ocean warming, expansion, thermal
What are Milankovitch cycles
Changes in:
Eccentricity: The shape of Earths orbit
Obliquity: Angle of the earth’s axis, tilt
Precession: direction that Earth’s spin is pointed
result in long term changes in the earth’s climate
The climate change impacts on global mean sea level
Thermal expansion
Melting glaciers
Loss of ice sheets (Greenland, Antarctica)
Why are glaciers shrinking
Greater-than-average- summer melting, diminished snowfall, later winters, earlier springs
Imbalance results in significant net grain in runoff versus evaporation for the ocean, sea levels rise
What causes the loss of land bases ice
Meltwater from above and seawater below seeping beneath the ice sheets, effectively lubricating ice streams and causes them to quickly slide into the sea
If all the world’s ice melted, what would be the sea level rise
~80m
what is do No Active Intervention in the shoreline management plan
No planned investment in defence against flooding
Cost benefit analysis
what is Hold the Line in the shoreline management plan
defences (hard/soft) maintained, upgraded so the shoreline position remains where it is
what is Advance the Line in the shoreline management plan
New defences built seaward of existing defences
Significant reclamation of land
What is Managed realignment in the shoreline management plan
Realignment of the shore (forwards or back) to control or limit movement
flood risk managed
what is Adaptation/accommodation in the shoreline management plan
Flood proofing
Hazard mapping, awareness, warning and evacuation systems, land use zoning, habitat restoration
