The grain size characterisation of coastal sand from the Somme estuary to Belgium: sediment segregation processes and sources consideration.
Résumé
and-rich Holocene to modern clastic deposits in the eastern English Channel and the southern North Sea coasts of France and
Belgium occur extensively as nearshore-sand bank, estuarine-tidal flat, aeolian dune and beach sub-environments. Sand samples
(n = 665) collected from these deposits suggest the presence of three different populations: a largely dominant (83%) medium to
fine quartz sand population (“b”), and finer- (14%) and coarser-grained (4%) populations (respectively “c” and “a”). The
distribution of these populations among the four sub-environments reflects tide- and storm-dominated sorting and transport
processes and a variable degree of mixing. These populations are derived from a mixture of very fine- to very coarse-grained
fluvial, outwash and paraglacial sediments deposited on the beds of the eastern English Channel and southern North Sea during the
late Pleistocene lowstand. The nearshore-sand bank environment, which also corresponds to the main offshore source area of the
coastal deposits, exhibits population heterogeneity reflecting the variability of hydrodynamic conditions and sediment sorting in
this zone. The nearshore topography of tidal ridges, banks and troughs in these tidal seas leads to variable bed and tide- and storm-
induced shear stress conditions. These conditions only allow for the mobilisation and onshore transport of some of the finer
fractions (populations “b” and “c”), leaving an offshore mixture of these finer populations with coarser, less mobilisable sediments
(population “a”). Once in the coastal zone, these two finer populations undergo further hydrodynamic sorting and segregation.
Variably sorted very fine sands to silts (population “c”) are trapped in the low-energy estuarine-tidal flat sub-environment, while the
highly homogeneous population “b” is further sorted in aeolian dune and beach sub-environments. This sorting occurs via a coastal
sand transport pathway linking the Somme estuary mouth to the southern North Sea bight where tidal range and wave energy
decrease relative to the English Channel. Since this sand transport pathway enables longshore transport of hydrodynamically sorted
medium to fine sand derived directly from the immediate nearshore zone, it has further contributed to a net flux of this sand
population from the eastern English Channel sea bed to the southern North Sea.