2B.9A - Local Factors that Increase Coastal Flood Risk
Local factors increase flood risk on some low lying and estuarine coastline (height, degree of subsidence, vegetation removal); global sea level rise further increases risk.
Sea level rise affects a disproportionate number of people because:
Local Factors
These also increase the flood risk of some coastlines
Global Sea Level Rise
Sea level rise affects a disproportionate number of people because:
- Many low lying coastlines are densely populated as beaches and the sea attract a large number of tourists
- Low lying deltas are extremely fertile and ideal for agriculture
- Estuaries and deltas are ideal for trade with good navigable access inland up rivers
- Many river deltas support megacities,
- e.g. Shanghai, Yangtze Delta China - 24 million people
- Dhaka, Bangladesh, Ganges-Brahmaputra delta - 14 million people
- Karachi, Pakistan Indus delta - 23.5 million people
Local Factors
These also increase the flood risk of some coastlines
- Height
- Low lying coastlines are only 1-2 m high above (high tide) sea level so at risk from flooding
- Temporary flood risk from storm surges, permanent flooding from global sea level rise
- The Maldives archipelago in the Indian Ocean has a population of 340,000 spread across 1,200 islands
- The highest point in the Maldives is only 2.3 m above sea level
- Malé, the main island and capital, is protected by a 3 m sea wall
- Bangladesh occupies the Ganges-Brahmaputra delta, 60% of the country is less than 3 m high above sea level
- The Kiribati archipelago in the Pacific Ocean is composed of 33 coral atolls.
- Most of the population lives on the island of Tarawa where the maximum height above sea level is 3 m
- Subsidence
- Low lying coastlines in estauries, deltas or outbuilding zones are subject to natural subsidence through the settling and compaction of recently deposited sediment
- However, subsidence is usually outpaced by fresh deposition and the bioaccretion of organic matter
- Deltas experience periodic isostatic subsidence when the weight of the delta sediment reaches the threshold sufficient to cause the crust to depress - leading to marine transgression and flooding
- Human activity can also cause local subsidence:
- Drainage of saturated sediment/soil for agriculture e.g. Fens of East Anglia, or ground water abstraction to supply cities, e.g. Venice, reduces sediment volume and causes subsidence
- Weight of cities and built environment can also compress sediment, leading to subsidence (also happening in Venice)
- Land reclaimed from the sea, e.g. Ijsselmeer polders in the Netherlands, subject to subsidence due to water abstraction via evapo-transpiration by agricultural crops.
- Volcanic islands and coral atolls - seafloor spreading away from hotspot or mid-ocean ridge
- Isostatic readjustment after ice sheet retreat (southern England)
- Bangladesh:
- 50 large islands in the Ganges-Brahmaputra delta subsided by 1.5 m since 1960
- Partly due to isostatic crustal depression and partly due to water abstraction by occupying populations, partly due to natural settling of sediment while the earth bund flood protection prevents compensation fresh sediment deposition
- Vegetation Removal
- Vegetation, like salt marshes and mangrove forest, reduces flood risk
- Vegetation stabilises existing sediment and traps new sediment, raising the height of the land above sea level
- Vegetation absorbs wave energy, reducing wave impact and erosion, and reduces the distance waves travel onshore before the energy is exhausted
- An 100 m belt of mangrove forest is estimated to reduce wave height by 40%
- A 1 km belt of mangrove forest reduces the height of a storm surge by 0.5 m
- An estimated 50% world's mangrove forest removed since 1950 - 1/4 of the loss for the creation of shrimp farms, and lots removed for tourist beaches
- Bangladesh contains the 180 km Sundarbans, the largest mangrove forest in the world. However, 71% is experiencing some vegetation removal. Some parts are eroding at 200 m p.a
Global Sea Level Rise
- Global sea level rise increases the risk of flooding in low lying coastlines (duh)
- Mean global sea level rose by 20 cm in the 1900. 50% of the Netherland and large areas of the East Anglian Fens are now below sea level, but protected by coastal defences
- IPCC predicts a further 18-59 cm rise in sea level by 2100
- Bangladesh - a 40 cm sea level rise would permanently submerge 11% of Bangladesh, creating 7-10 million environmental refugees
- Maldives - 50 cm sea level rise would permanently flood 77% of the Maldive Islands' land area.