Vulnerability, Impacts and Adaptation of Coastal Zones to Global Change - Vulnerability of Coastal Ecosystems and Adaptation - page 139

of the sea level that can cause temporary flooding or marine
submersion in low elevation coastal areas and erosion of the seashore.
Flooding is particularly serious if during the storm, the sea level
reaches extremely high levels resulting from the coincidence of the
storm surge and the high tide, especially at spring tides. Storm surges
are associated with tropical cyclones and also with strong extratropical
storms, especially those that have a rapid development and
displacement. They trigger a rapid increase in sea level, due to the
accumulation of water in downwind regions and the elevating effect
triggered by the lower atmospheric pressure at the center of the
depression (Figure 4.2). The accumulation of water near the coast can,
again, be accentuated by Ekman pumping, by the shape of the seabed
and by the funneling effects produced by some coastlines.
Figure 4.2. Temporary elevation of sea level during a storm due
to the coincidence of the storm surge and the high tide
What interests us particularly here is the fact that the frequency of
extreme local sea levels is increasing with the increases in local mean
sea level (LMSL). This level is defined as the average sea level at a
specified locality in the seashore, measured in relation to a reference
point on land and over a sufficiently long period of time (a month, a
year), so that the average value will be independent of fluctuations
caused by waves and tides. The variations of the LMSL result from
the combination of vertical movements of the seashore, either of
elevation (of tectonic origin or caused by the postglacial rebound) or
subsidence, with the increase in the global mean sea level (GMSL),