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Flooding & Coastal Communities

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What is a flood?

Why does it flood?Where does the water come from? The water cycleWhere is water stored?

Examples of Pluvial vs. Coastal flooding

Impacts of flooding

Flood defences

What else influences flooding:

Human impactsUrbanisationDeforestationSea level rise

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What is a flood?

Flooding is a natural event

Flooding can be caused by:1. Heavy rainfall2. Storm surges3. Very high tides

The coast is particularly vulnerable to flooding as it can be effected by all three of these simultaneously.

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Coastal flooding

Waves

Rain

Storm surge High

tides

Run off

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Storm surges

Low pressure systems such as large storms and hurricanes push down on the ocean. This creates a wave like mass of water in the ocean. When a storm surge combines with a high tide a storm tide occurs which can easily flow over the top of any coastal sea defences, flooding the coastal zone.

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Pluvial flooding

Pluvial flooding is rain generated flooding.

Heavy rainfall increases the volume of water on the land and in the river systems. If too much rain falls too quickly, it cannot drain away and forms a flash flood.

Heavy rainfall on Mweel Rea in Mayo poured off the land and into the surrounding rivers. This caused a flash flood in minutes!

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Pluvial flooding Rivers have a natural tendency to flood during certain times of the year.

When there is too much water in the river system, the river ‘bursts it’s banks’ and floods the floodplain.

Flood plains are flat areas of land adjacent to the river which are particularly prone to flooding. They usually have very nutrient rich soil and are popular agricultural areas.

The Mississippi, is on of the most famous rivers in the world. The river floods regularly during heavy rainfall submerging the flood plains in water. This satellite image shows how far the river expands after heavy rain.

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Impacts of flooding

What is the impact of this flood for the people living and working here?

Look at the amount of water compared to measurable objects: the man, the buildings, the car.

What happens to the water? Where does it go? Why is does this happen?

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Flood Defences

Reflects the energy of the waves

Sea walls and coastal defences protect the coast from storm surges and waves.

Large rock armour disperses the energy ‘breaking up’ the waves power so it is not as strong when it reaches land.

Sea walls are built high to stop waves crashing over and are curved to reflect the wave’s energy back out to sea.

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Flood Defences

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What else influences flooding?

Unfortunately in some areas, the human impact has increased the risk of flooding. This is due to:

• Clearing of land and trees. Trees have immense root systems which soak up water and root the soil together.

• Urbanisation. When we build roads, buildings and car parks we cover the natural land . Water soaks into the land through the soil and permeable rock . Concreted areas do not provide areas for water to soak away. Without correct drainage and during heavy rainfall events these areas are prone to flooding.

• Damage to natural defence structures such as dunes and beaches. Dunes act as a ‘shock absorber’ reducing the impact of large storms and waves. By removing sand or trekking through dune systems, their natural ability to protect the land behind is reduced.

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For further exploration on this topic search the following:

• NOAA National Weather Service (Keywords: Storm surge, Storm tide, Coastal inundation)

• The Climate Institute• Harnessing the Ocean’s energy through wave and tidal energy• Sea level rise and the impact on our coast• The Thames Barrier

Some images portraying the power of the sea:La Jument Lighthouse, Photographer: Jean GuichardA storm in Lahinch, Photographer: George KarbusThe 100 foot wave, Nazare ,Portugal.