108
D.M. Paterson and S.E. Hagerthey
are described as comprising grains greater than 62.5 flm in diameter and are
generally spherical, showing no significant inter-particle attraction (cohesive
behaviour). Smaller sediments (below 62.5 flm) become increasingly more
elongated or flat and have a greater surface-to-volume ratio. The consequence
of this is that the charge distribution over the surface of the particle can cause
significant attraction (cohesive behaviour) through Van der Waals forces.
Thus cohesive particles do not act independently while non-cohesive sandy
sediments are made up of particles behaving as individual units. Thus, the
erosional behaviour of sandy sediment has been empirically determined and
can now be mathematically described (Soulsby 1997).
ecr= 0.30/ (1 + 1.2D.) + 0.055 [1-exp( -0.02D.)]
where ecr is the dimensionless threshold Shields parameter and D. is the
dimensionless grain size.
The understanding of cohesive sediment erosion is more complex and even
the description and classification of the cohesive erosion process is a matter
of debate (Amos et al.1997; Paterson and Black 2000; Tolhurst et al. 2000). It is
clear, however, that no single mathematical formulation exists to describe
cohesive sediment erosion and that the modelling of cohesive sediment behaviour is much more complex (Parker 1997; Teisson 1997). However, much of
the sedimentological discussion over equations and processes still neglects
the biology of natural systems although this is changing rapidly. The cohesion, and therefore erosion threshold, of clay minerals is affected by the
organic content of the sediment (Blanchard et al. 2000) and much of this organic material is produced by the activity of microphytobenthos. The organic
material secreted by microbes is given the generic term "Extracellular Polymeric Substances" (EPS; Underwood et al. 1995; De Winder et al. 1999) and
there is a strong relationship between microphytobenthos biomass and operational fractions of the organic material (Underwood and Smith 1998). This
influence is well known for cohesive sediment but less so for sand. However,
under field conditions, various forms of microbial development prevent sand
grains behaving as individual particles and instead they clearly act in a cohesive manner (Yallop et al. 1994). The binding of sand particles by cyanobacteria is well known, while the influence of EPS has been shown in laboratory (Dade et al.1990) and field experiments (Yallop et al.1994). Thus, the idea
of defining cohesion on the basis of particle size is limited to the sediment010gical behaviour of the particles devoid of organic materials. In nature,
cohesive or non-cohesive behaviour is dependent on the extent of microbial
development and varies in response to biological patchiness, zonation and
seasonality.
D.M. Paterson and S.E. Hagerthey
are described as comprising grains greater than 62.5 flm in diameter and are
generally spherical, showing no significant inter-particle attraction (cohesive
behaviour). Smaller sediments (below 62.5 flm) become increasingly more
elongated or flat and have a greater surface-to-volume ratio. The consequence
of this is that the charge distribution over the surface of the particle can cause
significant attraction (cohesive behaviour) through Van der Waals forces.
Thus cohesive particles do not act independently while non-cohesive sandy
sediments are made up of particles behaving as individual units. Thus, the
erosional behaviour of sandy sediment has been empirically determined and
can now be mathematically described (Soulsby 1997).
ecr= 0.30/ (1 + 1.2D.) + 0.055 [1-exp( -0.02D.)]
where ecr is the dimensionless threshold Shields parameter and D. is the
dimensionless grain size.
The understanding of cohesive sediment erosion is more complex and even
the description and classification of the cohesive erosion process is a matter
of debate (Amos et al.1997; Paterson and Black 2000; Tolhurst et al. 2000). It is
clear, however, that no single mathematical formulation exists to describe
cohesive sediment erosion and that the modelling of cohesive sediment behaviour is much more complex (Parker 1997; Teisson 1997). However, much of
the sedimentological discussion over equations and processes still neglects
the biology of natural systems although this is changing rapidly. The cohesion, and therefore erosion threshold, of clay minerals is affected by the
organic content of the sediment (Blanchard et al. 2000) and much of this organic material is produced by the activity of microphytobenthos. The organic
material secreted by microbes is given the generic term "Extracellular Polymeric Substances" (EPS; Underwood et al. 1995; De Winder et al. 1999) and
there is a strong relationship between microphytobenthos biomass and operational fractions of the organic material (Underwood and Smith 1998). This
influence is well known for cohesive sediment but less so for sand. However,
under field conditions, various forms of microbial development prevent sand
grains behaving as individual particles and instead they clearly act in a cohesive manner (Yallop et al. 1994). The binding of sand particles by cyanobacteria is well known, while the influence of EPS has been shown in laboratory (Dade et al.1990) and field experiments (Yallop et al.1994). Thus, the idea
of defining cohesion on the basis of particle size is limited to the sediment010gical behaviour of the particles devoid of organic materials. In nature,
cohesive or non-cohesive behaviour is dependent on the extent of microbial
development and varies in response to biological patchiness, zonation and
seasonality.