The Shoreline between Nature and Society, a Changing Heritage - Vulnerability of Coastal Ecosystems and Adaptation - page 178

the deflation of sandy shores. The first dunes in line manifest the wind
by their shapes. Surges gather sediments and wash them laterally
(coastal drift) or transversally onto the “profile” of the beaches.
The sediment balance is an essential piece of information whose
effects can be taken into account more effectively. It expresses the
ratio between the amount of existing sediment in a given location and
the ability of the marine agents (the wind, waves and currents) to
mobilize them. The forms of accumulation express this balance at all
times: in mid-latitudes, the beaches lose sand during winter storms (a
simple example) and the long swells at more quiet times bring them
back to the coast. The origin of the sediments is a crucial element. The
catchment area is the primary source: sediments are poured into the
sea through streams and rivers as they are the result of continental
erosion. In estuaries and deltas, an estimate of the input can be
obtained; in tropical countries, streams bring sand and clays in
abundance, whereas at high latitudes, they wash a multitude of
sediments that are often poorly sorted. The sea itself is a source of
inputs, either by its permanent onslaught on the shores with cliffs and
mobile coastlines, or by producing biogenic sediments such as with
coral reefs. The swells that batter the continental shelves dislodge the
sand down to such depths that scientific debates and research are still
struggling to establish a “limit to the effect of swells”, which has
been estimated around isobaths of 20 m, not without discussion, as
the local conditions may vary, and they play a considerable role in the
matter. Research in the last decades has shown a degradation
of the continental shelves battered by large oceanic swells [BIR 85,
PAS 98]: significant examples have been given such as major
barrier islands on the Atlantic coast of the United States where,
after long periods of accumulation of sand, which is a sign of
advancing coastline, the latter has retreated regularly, sometimes
since the 16th Century (Figure 5.2). Although this theory is subject
to debate, it expresses an element which the engineering
behind coastline protection should take into account and