Erosional and depositional behaviour of cohesive sediments across
aquatic environments
Abstract
Muddy sediments are abundant across aquatic ecosystems, consisting of
mineral grains and biological material. Erosional characteristics of
these cohesive sediments are impacted by micro-organisms providing
bio-stabilisation. Deposition may be impacted by chemical and biological
composition, along with turbulence properties, which in turn influence
flocculation of suspended particulate matter. Flocculation processes
affect settling velocity, porosity and density characteristics. Cohesive
sediments absorb contaminants, as well as influencing interactive
processes between sedimentary dynamics and hydrodynamics, through their
bio-physical attributes. It is therefore beneficial to predict muddy
sediment transport processes via numerical modelling. Accurate modelling
relies on quantitative erosional and depositional data for calibration.
Through collation and analysis of field- and laboratory-derived data
sets, this study examined aspects of erodibility and deposition across
several aquatic environments (including estuarine, intertidal and lake
sediments). A range of case studies examined floc properties, sediment
composition, erosion thresholds, turbulent shear stress and suspended
particulate matter concentration. Investigation of floc dynamics in
estuarine sediments revealed larger, faster settling flocs in muddy
sediment (mean settling velocity, Wsmean = 4.1-5.2 mm.s⁻¹; mean floc
effective density, ρe,mean = 317-352 kg.m⁻³). In mixed sediment, flocs
were smaller and settled more slowly (Wsmean = 3.8-4.0 mm.s⁻¹; ρe,mean =
288-508 kg.m⁻³). Comparison of oil-contaminated sediments revealed the
importance of floc size class and mineral type. On the addition of oil,
larger, faster settling flocs were produced in pure bentonite cases,
while smaller, slower settling flocs were observed in kaolinite cases.
In highly organic lake sediments (organic content = 62%), settling
velocity varied over increasing suspended sediment concentration and
between floc size classes (macroflocs faster than microflocs by 0.95
mm.s⁻¹). Such findings may be utilised to increase the understanding of
complex sedimentary and hydrodynamic interactions within aquatic
environments. This study provides quantitative data, applicable to the
improvement of predictive numerical model reliability.