Geoscience Reference
In-Depth Information
It is under the constant threat of these supposedly exceptional
marine-meteorological conditions, but which in fact are not that
uncommon [CAR 10, FEU 12], that the shorelines evolve and retreat.
To summarize, all the factors likely to affect the coasts that do not
have the potential to modify things need to be combined: deep-low
pressures, and therefore strong rotating winds that increase in intensity
as the trough advances, so these progressive changes in wind direction
further complicate the progression of swells travelling from afar
combined with the local action of the wind that results in storm
surges. In other words, the surf is much higher than would normally
be expected and is also a function of the tidal coefficients. Many
violent storms have made little damage to the coast because they
developed in neap tides or at a point of the tide where the latter was
not at its maximum. Xynthia made a lot of damage as at dawn, the
intensity of the winds and the violence of the surges coincided with
the maximum day tide. The inflow of water, particularly in confined
zones such as estuaries, which provoke the submersion of the beaches,
is a natural system, and the breaking of the dikes, that had been built
out of necessity long ago, are often poorly maintained (Figure 5.6).
The solution - if any is likely to be implemented in all cases - resides
in the organization of society and partly - only partly - in scientific
research. In the United States, everyone knows how to mobilize
populations quickly in the event of a tropical cyclone, and there are
generally few victims. The same cannot be said of other places. In
France, things only change after there have been victims. A procedure
for the prevention of marine submersion and rapid flooding named
“dike procedure” was drafted in urgency. It involves, among others,
repairing and reinforcing the 1,200 km of dike between 2011 and
2016. This program costs 500 million Euros, of which 40% will be
funded by the risk prevention funds (Barnier funds) and should also
benefit from European funds (FEDER). The strategy aims to control
urbanization in risk areas. On the other hand, another line of the plan
aims to improve the warning, monitoring and response chain. To that
end, a section on “strong waves - marine submersions” has been
added (at the department level) by the weather forecast and has been
implemented by Météo France as a new open and regional warning
service of “rain flood”, whereas each district concerned by risks of
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