Coastal Hazards

Flooding Hazards

Coastal flooding is a hazard not only because of the damage that saltwater can inflict on property but also because the associated deep water, strong currents, and turbulence can cause drowning. Moreover, floating objects torn free by turbulent waters can injure and exacerbate inundation damage.

The water level at the coastlines at a single instant in time is made up of (1) the mean sea level, (2) storm surge, (3) the tide, and (4) run-up. The first influence on mean sea level to consider is that the ocean is intimately connected to the atmosphere. Currents and water levels respond to changing wind, pressure, and air temperature patterns. These properties change dramatically from season to season, from year to year because of climatic effects such as El Niño/La Niña, and over much longer timescales because of influences such as the arrival and passing of ice ages. The sea level can also experience relative changes, as the landmasses can sink or rise slowly due to effects such as isostatic rebound (the lift in land masses that follows retreat of continental glaciers). It is therefore difficult to measure mean sea level at a site, let alone determine if it is changing in any permanent sense. Thus, the definition of mean sea level depends on the time interval over which the mean is calculated. For the most part, this is a yearly average and so varies from year to year and depending on location.

Storm surge is the rise in water level caused by the atmospheric changes that accompany storm events. The reduced air pressure lifts the surface of the water up in an effect called the inverse barometer effect. Strong winds that circulate around low-pressure events can cause seawater to pile up against the coastline. The relative importance of the pressure and wind effects depends on the location and intensity of the storm, with the latter more important in tropical cyclones and hurricanes. The lifting of the water surface (by wind or pressure) can initiate a large-scale wave or a long wave, which propagates away from the storm at a speed that is controlled by the depth of the coastal waters. If the storm travels at the same speed and direction as the wave, there is ample time for energy transfer, and the wave can get large. Some semi-enclosed areas such as bays and basins can resonate with the same period as the long wave, resulting in an extra large storm surge. This is the case of storm surges in the North Sea and the reason why the Netherlands has needed to invest so much money in storm surge defense structures.

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