Simply put a coastal flood is when the coast is flooded by the sea. The cause of such a surge is a severe storm. The storm wind pushes the water up and creates high waves.
A storm is formed in al low pressure area, as you may know. An interesting fact is that beneath a low pressure area the sea level is higher. Think of a balloon. Press it hard and it flattens. Release pressure and it bounces back. The rule of thumb is that with each millibar of pressure less, the sea level rises a centimetre. This contributes to the high sea level, but the wind can have a larger effect.
A flood starts when waves move inland on an undefended coast or overtop or breach the coastal defence works like dunes and dikes. The waves attack the shore time and again. When it is a sandy coast, each wave in a storm will take sand away. Eventually a dune may collapse that way.
Very characteristic of a coastal flood is that the water level drops and rises with the tide. At high tide the water may flow in and at low tide it may recede again. When a sea defence is breached, low tide is the time to repair the breach. In the animation you see the build-up of force by the sea and how the sea floods the coast. Once it overtops and breaches the defences, the sea enters fast, but slows down when it spreads over a larger area.
Wave at Hartlepool, UK.
Source: HR Wallingford
Wave overtopping a coastal embankment.
Source: Leichtweiss-Institute for Hydraulics (LWI)
Back to flood types