What causes coastal flooding

Coastal floods are caused by extreme sea levels, which arise as combinations of four main factors: waves, astronomical tides, storm surges and relative mean sea level. The additional influence of river discharge may also be important in some estuaries.

A storm surge is a short-lived large-scale rise in water level, driven by low atmospheric pressure and strong winds associated with a storm, and enhanced locally by coastal topography. The worst coastal flooding occurs when the peak storm surge coincides with high spring tide. Local or remote storms produce large wind or swell waves, which can overtop coastal defences/beaches and cause flooding and erosion.

Each of these four components of sea level exhibits considerable natural variability, which influences the frequency of flooding on inter-annual and multi-decadal time scales, and makes isolating changes due to climate change difficult. Natural variability in the wave, storm surge and mean sea level components is stochastic and linked to regional climate, such as the North Atlantic Oscillation. In contrast the tidal component is deterministic, manifesting regular and predictable modulations at 4.4-year and 18.6-year time scales, relating to the perigean and nodal cycles of our Moon’s orbit (see a paper I wrote on this here). Flood rich (poor) periods occur when the variability is in (out) of phase.