112
D.M. Paterson and S.E. Hagerthey
The influence of bed particles on the flow becomes critical when they are
sufficiently large to penetrate the viscous sub-layer. If particles protrude into
less than 1/3 of the sub-layer then the flow is unaffected (Brown et al. 1999).
Particles protruding beyond this interfere with flow and the viscous sub-layer
breaks down in the transition into rough turbulent flow. The size of the
viscous sub-layer varies with flow but is usually less than 1 mm. For example,
at a flow of 1 m S-I, the laminar sub-layer is about 60 microns in depth. Thus,
in theory, particles >20 11m can influence the transition between smooth and
rough turbulent flow. This is over-simplified, however, since the shape of the
particles is important. Plate-like or needle-shaped particles may orient in
such as way that no obstruction to flow is caused (cohesive beds); sand grains
are more spherical (equi-dimensional) and have the potential to influence
flow transition. Under conditions where organisms infill the gaps between
larger particles (by mesh work or polymer gels) smooth turbulent flow may
be retained while, alternatively, organisms which create micro-relief features
that penetrate the viscous sub-layer may enhance the transition to rough
turbulent conditions.
5.4 Redefining Intertidal Sediments - The Five Phases
of Depositional Environments
There are many classic texts which describe the physical nature of depositional
systems, both mud and sand (e.g. Allen 1984), from a sedimentological approach. There are fewer sources detailing the nature of the microbiology of
the sediments at more than a basic level, although this is beginning to change
(Jickells and Rae 1997; Riding and Awramik 2000). However, there is still a tendency to separate the biological and physical components of the system, which
may inhibit our understanding of natural processes in depositional environments. The natural deposition system can be consider to have five phases:
1. The mineral phase (sediment particles)
2. The vital phase (the organisms)
3. Non-living organic phase (products of secretion, detritus)
4. Free aqueous phase
5. Gas phase
The separation into five phases is, of course, arguable but serves as a device
to understand the system complexity and to ensure the inclusion of living
organisms in a conceptual framework (Fig. 5.3). Organic material can be considered as part of living biota but there are reasons to consider them sepa-
D.M. Paterson and S.E. Hagerthey
The influence of bed particles on the flow becomes critical when they are
sufficiently large to penetrate the viscous sub-layer. If particles protrude into
less than 1/3 of the sub-layer then the flow is unaffected (Brown et al. 1999).
Particles protruding beyond this interfere with flow and the viscous sub-layer
breaks down in the transition into rough turbulent flow. The size of the
viscous sub-layer varies with flow but is usually less than 1 mm. For example,
at a flow of 1 m S-I, the laminar sub-layer is about 60 microns in depth. Thus,
in theory, particles >20 11m can influence the transition between smooth and
rough turbulent flow. This is over-simplified, however, since the shape of the
particles is important. Plate-like or needle-shaped particles may orient in
such as way that no obstruction to flow is caused (cohesive beds); sand grains
are more spherical (equi-dimensional) and have the potential to influence
flow transition. Under conditions where organisms infill the gaps between
larger particles (by mesh work or polymer gels) smooth turbulent flow may
be retained while, alternatively, organisms which create micro-relief features
that penetrate the viscous sub-layer may enhance the transition to rough
turbulent conditions.
5.4 Redefining Intertidal Sediments - The Five Phases
of Depositional Environments
There are many classic texts which describe the physical nature of depositional
systems, both mud and sand (e.g. Allen 1984), from a sedimentological approach. There are fewer sources detailing the nature of the microbiology of
the sediments at more than a basic level, although this is beginning to change
(Jickells and Rae 1997; Riding and Awramik 2000). However, there is still a tendency to separate the biological and physical components of the system, which
may inhibit our understanding of natural processes in depositional environments. The natural deposition system can be consider to have five phases:
1. The mineral phase (sediment particles)
2. The vital phase (the organisms)
3. Non-living organic phase (products of secretion, detritus)
4. Free aqueous phase
5. Gas phase
The separation into five phases is, of course, arguable but serves as a device
to understand the system complexity and to ensure the inclusion of living
organisms in a conceptual framework (Fig. 5.3). Organic material can be considered as part of living biota but there are reasons to consider them sepa-