Chapter 3 How Do Coastal Erosion And Sea Level Change Alter The Physical Characteristics Of Coastlines And Increase Risks? Flashcards
Eustatic seas level change
Fall- glacial periods, ice sheets form on land in high latitudes
Water evaporated from sea is locked up on land as ice= global fall in sea level
Rise-end of glacial period, melting ice sheets return water to sea and rise
Global temp increases cause volume of ocean water to increase (thermal expansion) = sea level rise
Isostatic sea level change
Fall- build up of land based ice sheets, colossal weight of ice causes earths crust to sag
Ice melts, land surface slowly rebounds
upward over thousands of years
Rise- land ‘sink’ at coast due to deposition of sediment
Especially in large river deltas where it leads to ‘crustal sag’ and delta subsidence
Uk sea level change
End of last ice age (12,000 years ago)
Scotland still rebounding upward (up to 1.5mm per year) POST GLACIAL ADJUSTMENT
England and Wales subsiding (up to 1mm per year)
Uk is ‘pivoting’ south sinking and north rising
Sea level rise caused by global warming (eustatic) compounds the effect in the south and reduces it in the north
Post glacial adjustment
Uplift experienced by land following removal of weight of ice sheets
Emergent coastlines
Extent of isostatic/ eustatic changes during and after ice age were large:
Global sea levels fall 120m
Equal sea level rise happened when ice melted
N America and Europe post glacial isostatic adjustment was up 300m
Two linked changes happen a different rates:
Post glacial sea level rise very rapid, submerging coastlines
Isostatic adjustment v slow with land gradually rising out of sea
Raised beaches
Fossil stump/ cliff
Submerging coastlines
Post glacial sea level rise
Southern England
Common landform= ria (drowned river valleys in unglaciated areas, caused by sea level rise flooding up the river valley making it much wider than would be expected based on the river flowing into it)
Fjords (Norway)
Drowned valley is U shaped glacial valley
Deeper than adjacent sea (up to 1000m deep)
Seaward end = submerged ‘lip’
USA east coast Barrier islands ( offshore sediment bars, usually sand dune covered but not attached to coast 500m-30km offshore)
Contemporary sea level change
Today 2mm per year
Stable between 1800-1870
Rose slowly 1870-1940 but accelerated after that
Between 1870 and today sea level measurements = more accurate
Tide gauges
Satellite measurements = more precise
Future IPCC some expect increase over 100cm 2100
Sea level hard to predict
Thermal expansion (main driver of sea level rise, occurs because volume of ocean water increases as global temps rise)
Melting of mountain glaciers (Himalaya)
Melting of major ice sheets (Antarctica)
Locally change due to tectonic forces (earthquakes force land up/down as much as 2m)
Rapidly eroding coastlines have physical features in common
Long wave fetch
Large destructive ocean waves
Soft geology
Cliffs with structural weaknesses (seaward Rock dip)
Cliffs vulnerable to mass movement and weathering
Strong LSD (sediment removed and cliff base exposed)
Human actions interfere in sediment cell and influence coastal retreat
Aswan high dam on river Nile 1964
Reduced sediment volume from 130m tonnes to 15m tonnes
Erosion rates jumped from 20-25m per year to 200m+ as delta starved of sediment
Dredging (scooping or sucking sediment up from sea/river bed)
Removes sediment source
Knock on effects further along coast by increasing erosion
Variations in erosion rate
Holderness coast East Yorkshire
Average erosion rate 1.25m year
Coastal defences (Hornsea/mappleton/withernsea) stopped erosion but starved further south as grounds interrupt LSD
Erosion rate generally increase from N to S
Boulder clay areas = more vulnerable
Some cliffs susceptible to mass movement
Erosion of holderness varies over time
winter: 2-6m of erosion common (storms combined with spring tides increase erosion)
Summer erosion (constructive waves) much lower erosion
Northeasterly storms cause most erosion( long wave fetch 1500km from north Norwegian coast)
Shape of holderness beach can change and promote erosion
Ords- deep beach hollows parallel to cliff
Concentrate erosion in particular location by allowing waves to directly attack cliff with little energy dissipation
slowly migrate downdrift (500m per yr) location of most erosion changes
Ord locations erode four times faster
Dissipation
Energy of waves is decreased by friction with beach material during wave swash up the beach
Wide beach slows waves down and saps their energy so when break most energy has gone
Coastal flooding risk
Low lying coasts densely populated for several reason:
Popular with tourists (Access to beach)
Deltas and estuaries ideal locations for trade between up river and places along coast
Deltas and coastal plains fertile and ideal for farming
Worlds major deltas (some few m above sea level)
Some have largest cities
Ganges/brahmaputra- Dhaka (14.3million)
Causes of coastal flood risk in river deltas
River straightening for navigation means faster riverflow and sediment propelled to far offshore
Ground water extraction causes subsidence and accelerated sinking
Destruction of mangrove forests for wood and charcoal exposed the coast to erosion
Delta subsidence caused by weight of sediment deposition which is balanced by new deposition on delta surface
Maldives
Indian Ocean
Population of 340,000 across 1200 islands
Highest pint is 2.3m above sea level
Sea level rise of 50cm by 2100 would see 77% of Maldives disappear into sea
Storm surge
Localised, short term rise in sea level caused by air pressure change (1millibar fall in air pressure = 1cm rise in sea level
Causes of storm surge
Depression- (low pressure weather system) in mid-latitudes (uk)
Tropical cyclone- (hurricane) in areas just north and south of equator
Strong winds from weather systems push waves onshore (increase height of sea)
High tide and storm surge at same time
North Sea storm surge
Coastal topography had an effect
Coastline narrows into funnel shape for storm from north
Storm funnelled into increasingly narrow space between coasts
As sea shallow towards coast the effect is severe coastal flooding
Impacts of North Sea storm surge
2013
80+mph winds
Storm surge 6m
Coastal flooding hull and Skegness
Scotland’s Rail network shut and 100,000 homes lose power
2500 coastal homes and businesses in uk flooded
Across country= 15 deaths
Bangladesh vulnerable to impacts of tropical cyclone storm surges
Low lying river delta (1-3m)
Incoming storm surge meets out flowing discharge from Ganges and Brahmaputra (river and coastal flooding combine)
Intense rainfall from tropical cyclones contributes
Much of coastline is unconsolidated delta sediment
Deforestation of coastal mangroves removes veg that stabilised coastal swamps and dissipated wave energy
Bangladesh cyclones
1970 bhola cyclone
Storm surge height 10m
Lowest air pressure 966mb
Deaths and economic loss 300,000-500,000
US$90 million
