2B.9B - Storm Surges
Storm surge events can cause severe coastal flooding with dramatic short term impacts (depressions, tropical cyclones)
Depressions are areas of low air pressure generating surface winds that spiral into the centre of low pressure in an anti-clockwise direction. They occur in mid-latitutes, like the UK.
Storms are depression, areas of low surface pressure that generate strong winds (UK 90 kph, tropical regions 65 kph) They occur in areas just north and south of the equator.
Tropical cyclones are areas of very low surface air pressure (deeper depressions) generating very strong winds (118 kph +)
High air pressure depresses the ocean surface, lowering local sea level.
Low air pressure allows the ocean surface to dome upwards, raising local sea level.
A 1 millibar reduction in air pressure leads to a 1 cm rise in sea level.
A storm surge is a temporary rise in local sea level produced when a depression, storm or tropical cyclone, reaches the coast.
The rise in sea level during the storm surge is accentuated:
Storm surges can produce severe coastal flooding on low-lying coastlines.
Force of onshore current of storm surge water can cause rapid coastal erosion.
Impact of storm surge increased by large destructive waves whipped up by strong storm winds on top of the already higher sea level - rapid coastal erosion.
Short term impacts:
Bangladesh, Tropical Cyclone Sidr 2007
United Kingdom, Storm Xavier, December 2013
Storms are depression, areas of low surface pressure that generate strong winds (UK 90 kph, tropical regions 65 kph) They occur in areas just north and south of the equator.
Tropical cyclones are areas of very low surface air pressure (deeper depressions) generating very strong winds (118 kph +)
- They're classified on the Saffir Simpson scale into 5 categories, where the fifth has winds of over 250 km.
High air pressure depresses the ocean surface, lowering local sea level.
Low air pressure allows the ocean surface to dome upwards, raising local sea level.
A 1 millibar reduction in air pressure leads to a 1 cm rise in sea level.
A storm surge is a temporary rise in local sea level produced when a depression, storm or tropical cyclone, reaches the coast.
The rise in sea level during the storm surge is accentuated:
- At high tide, particularly spring tide
- Shape of coastline funnels into increasingly narrow space
- Sea bed shallows towards coast
Storm surges can produce severe coastal flooding on low-lying coastlines.
Force of onshore current of storm surge water can cause rapid coastal erosion.
Impact of storm surge increased by large destructive waves whipped up by strong storm winds on top of the already higher sea level - rapid coastal erosion.
Short term impacts:
- Deaths and injuries to people immediately through drowning or collapsing buildings
- Subsequent deaths from hypothermia (homes destroyed), water borne diseases (sewer systems and freshwater pipe destroyed), natural causes (transport routes to medical care cut)
- Destruction of infrastructure - roads, railways, ports, and airports flooded or destroyed
- Damaged water pipes, electricity transmission lines and sewage systems - no power or water.
- Homes destoyed - older houses worse standards, cheap in poor areas - homes on marginal low lying land (slums and shanty towns) most vulnerable - reconstruction may take several years, richer (insurance) likely to be rehoused first
- Businesses destroyed - factories, offices - loss of power, interruption of raw material delivery, workers killed/injured/can't get there - agricultural land contaminated - crop harvest lost
Bangladesh, Tropical Cyclone Sidr 2007
- Category 4 cyclone, air pressure 944 mb, 240 kmph and 6 m storm surges
- Impact worsened by:
- funnel shape of the Bay of Bengal focussing water on Bangladesh at the bay apex.
- Out flowing discharge from the Ganges and Brahmaputra rivers combine with coastal flooding.
- Intense rainfall from cyclone increases flooding.
- Coastline from unconsolidated delta sediment - easily eroded.
- Deforestation of mangrove swamps.
- funnel shape of the Bay of Bengal focussing water on Bangladesh at the bay apex.
- 60% of Bangladesh low lying, less than 3 m above sea level
- 15,000 people killed and 55,000 injured. 1.6 million homes destroyed.
- 8,000 km of roads, 700 km of electricity transmission lines and 900 fresh water tube wells destoyed
- Crops destroyed on 600,000 ha of agricultural land
- Total damage estimated at $1.7 billion
- However, the impacts of deaths were much lower than in the 1970 Bhola Cyclone where 300,000 were killed. ($90 million economic loss) Improved warnings, embankments and cyclone shelter network saves many lives.
United Kingdom, Storm Xavier, December 2013
- 80+ mph winds
- Storm coincided with spring tide
- In the North Sea the coastline narrows into a funnel shape for a storm approaching from the north - storm surge funnelled - sea shallows towards coast - severe coastal flooding.
- Average 3 m storm surge in East Anglia, but 6 m at Blakeney in North Norfolk
- 2 people killed. 18,000 evacuated. Coastal defences breached in Yorkshire and Kent and 1,400 homes flooded.
- At Hemsby in Norfolk sand dunes were eroded, seven houses and a lifeboat station destroyed
- East coast rail services suspended for one day
- Total economic loss estimated at $100 million
- Impacts much lower than the 1953 storm surge when 307 people were killed, 65,000 ha of farmland flooded and there were an economic loss equivalent to $1.2 billion today
- This was prevented by: improved flood defences in 2013, including the Thames Barrier (raised during the storm), which protected 800,000 homes according to Environment Agency estimates
- Improved forecasting and efficient evacuation also saved lives and mitigated in areas where flooding or erosion still occurred.
