Defining impact zones by grain size
T. G. Milligan (Canada)
ABSTRACT
Grain size is the most basic of classification criteria for sediments.
Sediments can be characterised by the way in which the material they
are made up of was deposited on the bottom. The size differentiation
that produces a given sediment size distribution is a function of
physical transport processes and a record of those processes will be
preserved. The erosion of terrestrial material tends to create
straight size distributions with the volume of material in each size
class varying regularly. After being delivered to a bay or estuary by
run off or shore erosion, these straight size distributions are
modified to reflect the transport energy of the water. The fine
particulate material, which also makes up the suspended load, is unable
to settle through the water column unless it flocculates with other
particles to form large, fast settling aggregates or marine snow.
Studies have shown that flocculation rate is controlled by
concentration, size and particle adhesion forces and that the
particles in suspension deposit in proportions equal to that in which
they are found in the suspension. By using precise Coulter grain size
analysis and the model developed by the late Kate Kranck, it is
possible to break down a sediment into the three major components from
which it was formed: material deposited as flocs, material deposited as
single grains from suspension and material carried under higher energy
conditions. Using this method, both the amount of material deposited
in a flocculated state and the maximum size, or floc limit, of
the particles composing the floc can be determined. The increased fine
particulate load, and the possible increase in the amount of biological
"glue" in the form of polysaccharides, which results from mariculture
can increase the rate of flocculation in an inlet and change the
deposition rate and distribution of fine sediments. By examining the
particle size distribution of sediments in regions of aquaculture it is
possible to determine zones of fine particle deposition and to assess
changes in the particle dynamics of an area.