Geoscience Reference
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activity at the ocean level (steep rise in the greenhouse effect). The
development of the watersheds has greatly reduced the migration of
sediments; on the contrary, the clearing waves, even if temporarily,
have been able to speed up the washing of the finest matter into the
sea. From this point of view, the development of marshes throughout
history is probably responsible as much for these abundant inputs as
for the management deployed by men to develop these lands (Bay of
Mont Saint-Michel but especially the marshes of the center-Atlantic in
France or the establishments on large salt marshes that could not have
happened in a context of a shortage in sediments; the reclamation of
land in the Netherlands, or the coastlines along Korea, China and
especially Japan, etc.). Increasingly, the sustained secular and
complex watercourse management effort has affected the input of
sediments: the placement of dikes has the effect of hardening the
shore and prevents sand and finer matter from being washed into
streams, whereas the implementation of spikes across streams to
facilitate shipping compresses the strength of the current in one part of
the riverbed and traps the sediments. Dams, of course, which are
increasingly being used, form formidable reservoirs of sediments that
are rarely cleaned. On the seaside, the supplying of harbors plays a
similar role to spikes but most importantly, since the start of tourism
development, numerous stations have required the building of
concrete waterfronts and the placing of heavy flood defense structures
(various walls, ripraps and stones) which, there again, reduce the input
of sediments to beaches. The movement can be endless, the poorly
designed spikes (groynes) relay each other and divide up the coastline
in so many discrete cells with low permeability, which in turn
emphasize the sinking of beaches and the discharge toward shallow
sandy stocks. The chronic sediment deficit in many places compels us
to rethink our approaches and methods, as the refilling of beaches in
various forms only maintains sediment stability temporarily.
Finally, the changes in sea level are key elements of the system but
the timescale for these changes is not seasonal or annual. It oscillates
within the duration of transgressions and regressions, with a few
hiccups. These phenomena deserve particular attention in that they are
a more or less quick form of continental submersion.
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