Definition

Flooding of coastal lands is primarily due to inundation by the sea during storms and other natural events (e.g., tsunamis) that increase coastal population risk. Flooding also occurs from inland waters when storm water levels hinder evacuation.

Description

Coastal risks have very different origins and etiologies, but current vulnerability assessments focus mainly on erosion and climate-induced floods. Generalized fluid-dynamic erosion is the reason for most shore and coastal protection measures. Tectonic plate movements may generate tsunami waves, and the resulting coastal risks can reach great levels for vulnerable settlements. Hurricanes and regional monsoons can also cause vast erosion and damage to coastal zones.

Eustatic sea level is increasing due to climate change (Figure 1), but crustal isostasy, tectonics, and coastal plain subsidence cause variability of sea level at the local level. Accordingly, this trend of increasing global sea level can be accentuated in some places or attenuated and even reversed in others. The intensity and frequency of coastal flooding depend not only on eustasy but also on other climate-related factors such as low-pressure systems and strong winds that can raise the average sea level above the current tide and generate temporary increases in basin water levels that cause inland coastal floods (Diez et al., 2011, 2012). A temporary rise in sea level can act as a dam at the mouth of a river causing blockage of river drainage and a rise in water levels on the river, on its floodplain, and on surrounding areas (Audiencia Territorial de Valencia, 1991).

Coastal Risks: Floods, Figure 1
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Global Climate Evolution since last Glaciation expressed on mean sea level and based on geo-historic approaches. Scales vary decreasing to present time and level.

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Unlike marine floods that are caused by seawater in limited areas, coastal floods may be due to a variety of different causes, and they may affect much larger coastal zones. When such floods are directly sourced to pluvial or fluvial waters, persistent high sea level is the main cause of the flood duration.

Historically, structural coasts were selected for settlements mainly because of security, health, safety, and economic reasons. However, sedimentary coasts offered greater productivity, and these areas soon attracted large populations to their plain littoral/deltaic hillocks. Mediterranean seaside (“maritime”) cities were always located beside a castled rock-hill (e.g., Athens, Haifa, Genoa, Malaga, and Monaco), whereas coastal plain cities were established on the landward side of relatively dry and high grounds (e.g., Rome on its seven hills, Valencia or Alexandria on delta hillocks, Venice on a relict barrier island of the Po Delta, etc.).

Coastal cities and towns often spread into lowland areas. Enormous conurbations are at a huge risk of flooding today as a result. The case of New Orleans is paradigmatic: its older settlements barely suffered from floods caused by Hurricane Katrina, meanwhile most of its later developments and lower quarters were catastrophically flooded. The great European delta formed at the outlet of the Elm, Rhine, and Meuse rivers has required drastic transformations to protect large cities from flood risk.

Flood risk can never be totally eradicated. Therefore, each vulnerable coastal development now requires a risk management plan to deal with the hazard. Flood risk has become a datum for analysis and resiliency management.

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