What changes the frequency of coastal flooding over time?
As I explained in the previous post, coastal floods are driven by extreme sea levels, which arise as combinations of four main factors: waves, astronomical tides, storm surges and relative mean sea level. Longer-term changes in any, or all of the four components can lead to variations in extreme sea levels.
Changes in relative mean sea level (due to changes in sea level associated with climate change and vertical land movement), affects extreme sea levels both directly and indirectly without any change in the occurrence of extreme events. The direct effect of a rise in mean sea level results in a lower storm surge elevation at high tide being necessary to produce a sea level high enough to cause flooding. The indirect effect occurs as changes in relative mean sea level alter water depths and hence modify the propagation and dissipation of the wave, tide and storm surge components of sea level. In addition, extreme sea levels may change as a result of variations in the strength and tracks of weather systems, related to climate change, which alter the intensity, and/or duration, of waves and storm surges (and river discharge), although this is less certain than rises in mean sea level.
Another important factor that is a major driver of coastal flood risk is socio-economic changes that generally increase or reduce flood consequences. Population growth, changes in land use and increasing asset values in floodplain areas have lead to enhanced exposure to flooding. Historically this was the main driver of increasing flood exposure and risk, but in the future we expect sea-level rise to become a much more important driver. However, to date improvements in flood defenses, together with advancements in flood forecasting and warning and emergency planning, have typically reduced flood consequences since the 1950s.
Changes in coastal morphology can also influence flood pathways and thus flood risk. Coastal erosion is expected to increase with accelerating sea level rise, so flooding linked to erosion is of increasing concern.