Coastal Flooding (Edexcel A Level Geography)

Factors Increasing Flood Risk

• Local factors increase flood risk on some low-lying and estuarine coasts due to:

  • Height of land

  • Degree of subsidence

  • Vegetation removal

• Global sea level rise further increases risk 

• Densely populated coastal areas also increase the number of people at risk

• Low-lying coastal areas are often densely populated due to:

  • Attractive scenery and beaches

  • Deltas are fertile and ideal for agriculture

  • Suitable for ports and trade

• Globally, almost 300 million people live in coastal areas below 2m

• Of the 10 world's largest cities, 8 are in coastal areas

• Countries and regions which are particularly low-lying include:

  • Bangladesh - over 10% of the land is 1m or less above sea level

  • Maldives - has an average height of 2m above sea level with highest point being 2.4m

  • Netherlands - 27% of the country is below sea level and protected by sea defences

Height of land

• Low lying areas are more vulnerable to rising sea levels and storm surges

• Storm surges and spring tides leads to water flooding areas of the back-shore and beyond

Degree of subsidence

• Coastal areas are vulnerable to subsidence particularly deltas and land reclaimed from the sea

  • Clearing land for agriculture and ground water abstraction, increases the risk of subsidence

  • Building of settlements, along with the weight of the buildings, increases the risk of subsidence

• When subsidence occurs, the land is then lower and more vulnerable to coastal flooding

Vegetation removal

• The removal of vegetation, including salt marshes and mangroves, increases coastal flood risk

• It is estimated that 50% of salt marshes and 35% of mangroves have been lost since the 1950s due to:

  • Reclamation for development - housing and transport

  • Coastal management such as sea walls

  • Tourism, to create beaches with open views to the sea

  • Shrimp farming

• Where vegetation exists, it significantly impacts wave height, which reduces the risk of flooding: 

  • In the UK, salt marshes are estimated to reduce wave height by up to 80% 

  • 100m of mangroves is estimated to reduce wave height by between 13-66%

• The vegetation:

  • Traps sediment, which helps to increase the land height 

  • Reduces the impact of waves and erosion because, it absorbs the wave energy

Storm Surges

• Storm surge events can lead to severe coastal flooding with dramatic short-term impacts

• Storm surges are caused by:

  • Depressions (low pressure) leading to storm events

  • Tropical cyclones 

• Warmer ocean temperatures combined with higher sea levels are predicted to make storms and tropical cyclones more frequent and more severe

Depressions

• A depression is an area of low pressure 

  • The low-pressure area forms due to air rising from the surface

  • As it rises, it cools, condenses and forms clouds, bringing rain

  • In the northern hemisphere, depressions move:

    • West to east 

    • In an anticlockwise direction

  • Due to the low-pressure, sea levels are higher - for every 10mb decrease in pressure, the sea level is 10cm higher

  • This is called the inverse barometer effect

  • Storms are deep, active low-pressure areas, which bring strong winds and heavy rain

    • In the UK, when the storm is likely to have a significant impact on people and the environment, it becomes a named storm

Tropical cyclones

• A tropical cyclone is an area of very low air pressure

• As in a depression, it is formed by warm rising from the ocean surface, forming towering cumulonimbus clouds, which rotate around a central eye 

• Tropical cyclones form in the zone between 5-30^° north and south of the equator, in areas where sea temperatures are 27^°C or above

• A tropical cyclone brings heavy rain and strong winds

• When wind speeds reach 39mph it is categorised as a tropical storm, when they reach 74mph it is a tropical cyclone

Storm surges

• A storm surge is a rise in the sea level as a result of a storm or tropical cyclone and affect approximately 1.5 million people a year with a range of short-term impacts including:

  • Deaths and injuries

  • Homes destroyed leading to homelessness

  • Increase in water borne diseases

  • Destruction of infrastructure

  • Businesses destroyed leading to loss of employment and decrease in GDP

  • Loss of power

  • Loss of agricultural land

  • Destruction of crops and livestock

Climate Change & Flood Risk

Climate change & flood risk

• Climate change may increase coastal flood risk 

• The two main factors which increase the flood risk are:

  • Global sea level rise due to warming oceans and melting ice sheets and glaciers

  • Increased frequency and magnitude of storms

• The pace and magnitude of this increased risk are uncertain

Global sea level rise

• According to National Oceanic and Atmospheric Administration (NOAA) global sea levels have risen about 21-24cm since 1880 

• The rate of sea level rise is accelerating: 

  • Throughout the 1900s, the rate averaged 1.4mm a year

  • Since 2006, the rate has averaged 3.6mm a year

• In some areas, when combined with subsidence, this has led to a rise as high as 15-20cm since the start of satellite records

• The IPCC predicts a rise of between 26-77cm by 2100 

  • A 50cm sea level rise would: 

    • Impact 800 million people around the world

    • In the UK, 200km of coastal flood defences would be vulnerable; leaving £120 billion worth of infrastructure and resources at risk of flooding

    • Coastal flooding events would become more frequent

Increased storm magnitude and frequency

• The IPCC report (2013) states that:

  • The frequency and intensity of the strongest tropical cyclones have increased since the 1970s

  • It is likely that the frequency of tropical cyclones will decrease or remain unchanged

  • It is likely that the intensity of tropical cyclones will increase with higher maximum wind speeds and precipitation

• At the current time, there are no studies which have linked an increase in storm frequency and intensity in the UK, to climate change

• The UK is affected by extra-tropical storms - a type of storm system forming in the mid-latitudes, and some studies suggest that climate change may affect the paths of these storms, making parts of the UK more vulnerable to high winds and rain

Impact on flood risk

• The increase in sea levels, intensity of tropical storms and changing weather patterns caused by climate change, all have the potential to increase the risk of coastal flooding

• The magnitude and timing of these changes is uncertain, as the extent of climate change and the impacts it will have, are, at present, uncertain