Erosion and Sediment Transport Measurement in Rivers: Technological and Methodological Advances
(Proceedings ol'lhe Oslo W o r k s h o p . June 2002). I A H S Publ. 2 8 3 . 2 0 0 3 .
A new definition of suspended sediment:
implications for the measurement and prediction
of sediment transport
IAN G. D R O P P O
National
Ontario
Water
L7R
Research
4A 6,
Institute,
Environment
Canada,
PO Box 5050,
Burlington,
Canada
[email protected]
Abstract Sediment (floe) structure is discussed in relation to its influence on
the measurement and prediction of sediment transport. Structural charac­
teristics of floe size, porosity, density and compositional matrix are invest­
igated with the aim of creating a new paradigm of suspended sediment. This
new paradigm is intended to influence the development of new techniques and
strategies for the quantification of suspended sediment transport in aquatic
environments.
Key words
deposition;
floe;
flocculation; suspended sediment;
transport
INTRODUCTION
A river's transport characteristics can have a profound effect on the design of sampling
programmes for the measurement and prediction of sediment erosion and transport.
Historically, sampling programmes have relied on infrequent sampling protocols in
conjunction with continuous water quantity monitoring to estimate contami­
nant/sediment loads. While monitoring/measuring errors are generally associated with
the temporal variance of targeted contaminants/sediment and the relationship between
discharge and concentration, little consideration is given to the impact of sediment
structure and how this may impact sediment transport.
Sampling programmes have typically relied on the traditionally measured,
chemically dispersed, mineral fraction to characterize sediment transport character­
istics and for input to numerical models for the prediction of transport. These pro­
grammes implicitly or explicitly subscribe to the traditional definition of suspended
sediment (i.e. the holding-up of small particles of matter, transported by moving water,
by turbulent upward eddies). This traditional definition implies that sediment particles
are individual entities with no other function than to be eroded, transported and settled
within aquatic systems as individual non-cohesive particles. In today's multidisciplinary approach to ecosystem research and assessment, such a physically-based
definition n o longer represents the significance of suspended sediment within the
environment as a physical, chemical and biological moderator of aquatic systems
(Clifford, 2002). Floes have been shown to be the dominant mode of transport within
any aquatic system that carries a significant proportion of cohesive sediment (Petticrew
& Droppo, 2000). Flocculation significantly alters the downward flux of sediments due
to the cumulative effects of a change in the effective particle size, density, porosity and
shape (Li & Ganczarczyk, 1987; Ongley et al, 1992; Phillips & Walling, 1995;
Nicholas & Walling, 1996; Droppo et al., 1997, 1998) over that of the primary
particle.
4
Ian G. Droppo
Settling velocity (measured, derived or assumed) is a key predictor within all
sediment transport models. The knowledge that cohesive particles exist primarily as
flocculated particles, and not as the traditionally viewed primary particle, complicates
the quantification of particle (floe) settling velocity. The settling velocity/transport of
sediment in aquatic systems is controlled to a degree by the structure of the sediment
that in rum is influenced by the conditions prevailing in the fluid/sediment medium
(e.g. shear, organic content, mineralogy of the sediment). The use of traditionally
obtained absolute particle size distributions and Stokes' law derived settling velocities
to characterize sediment in sediment transport models will result in erroneous results.
This paper represents a discussion on how the structural characteristics of floes
(floe size, density, porosity and compositional matrix) can influence the measurement
and prediction of sediment transport. Towards this end, a new definition/paradigm of
suspended sediment is proposed. This new definition provides a new way of thinking
of suspended sediment and is intended to be the motivating factor towards the
development of new techniques for the quantification of suspended sediment transport
within aquatic systems. As this is a discussion paper centred on the general phenom­
enon of flocculation and how it may influence sediment transport in general, no
detailed field descriptions and methods are provided. References to these are, however,
provided where appropriate.
DISCUSSION
A typical floe structure that is composed of what would appear to be sub-flocs made up
of thousands of other individual grains is provided in Fig. 1. It is evident that such a
particle will be transported in suspension very differently than the primary absolute
particles, due to significant differences in effective size, surface area, density, porosity,
shape and settling velocity (Li & Ganczarczyk, 1987; Ongley et al, 1992; Phillips &
Walling, 1995; Nicholas & Walling, 1996; Droppo et al, 1997, 1998). Given the
obvious structural and behaviour differences, and the relative difference in mass
5 0 tim
Fig. 1 An example of a floe illustrating a typical diffuse structure composed of
thousands of individual grains. Visual observations subjectively suggest that this floe
may have developed from 9 sub-flocs (identified on the figure).
A new definition of suspended sediment
5
transport between the two forms, it is evident that to understand and measure sediment
transport correctly, sediment must be viewed and interpreted based on its natural
flocculated state and not as absolute (primary) particles. This is particularly true given
that: (a) flocculation of cohesive sediment is a universal phenomenon, and (b) that the
floe is the dominant mode of sediment transport for rivers with a significant cohesive
sediment load (Petticrew & Droppo, 2000).
The influence of floe size on sediment transport
The individual inorganic grains which make up the floe shown in Fig. 1 are on average
approximately 5 pm in diameter with a corresponding Stokes' settling velocity of
0.02 m m s" (Fig. 2). If such particles were present exclusively within the water
column (i.e. no floes) then they would only settle within a quiescent environment
(Hjulstrom, 1939). Direct measurement of floe settling velocity yields settling rates up
to two orders of magnitude faster than the absolute particles (the majority of natural
floes settle in the range of 1.0 to 2.5 m m s"'; Droppo et al., 1997, 2000) (the method of
settling velocity measurement can be found in Droppo et al., 1997). The elevated
settling velocity of floes over constituent particles for a variety of environments and
the consistent relationship of increased settling velocity with increasing floe size is
illustrated in Fig. 2. The process of flocculation is the only phenomenon that can
explain the presence of the cohesive particles on the bed of flowing environments. The
mechanism of floe deposition and erosion has been described by Droppo et al. (2001)
and Droppo & Amos (2001) to be a combination of the cyclic linkage between floe
recycling and surficial fine grained lamina (SFGL) recycling.
1
As floes become smaller, they approach the particle size of the constituent
particles and as such Stokes' law may represent these particles adequately (Fig. 2).
However, for large floe particles, Stokes overestimates settling velocity by orders of
magnitude (Fig. 2) due primarily to the floe structure (Fig. 1) not supporting the solid
spherical particle assumption assumed by the Stokes equation (Droppo, 2001).
Stokes'
1000
14-Mile Ck.
E
100
- - - Hamilton
Harbour
Lake Biwa
g
10
u
o
- * — C S O
A
Primary
Particles
>
ra
c
5a
0.1
m
0.01
10
100
Particle Size
1000
(pm)
Fig. 2 Relationship between measured and calculated (Stokes) settling velocity and
floes (CSO = combined sewer overflow). Calculated settling velocity is also provided
for a 5 am primary particle. Note that a density of 2.65 g cm'" has been used in the
Stokes equation.
Ian G. Droppo
6
Given the inappropriateness of the Stokes equation for the calculation of floe
settling velocity, this variable must be measured directly (Droppo et al., 1997). Such
measurements are, however, not without their problems as generically the r~ of the
least-squares regression lines are low owing to the wide range of morphologies and
compositions of individual floes. Nonetheless, direct measurement does allow for the
assessment of the factors which influence settling, and has demonstrated a direct
relationship between floe settling velocity and floe diameter. Such a relationship is
counter to that prescribed in the Stokes equation (i.e. that particle settling velocity is
proportional to the particle diameter squared) and is once again a testament to the
structural and behavioural differences between floes and primary particles.
Despite the above knowledge, primary particles are still the most common
sediment parameter measured by sediment monitoring programmes. This arises from a
lack of a standard defining equation to explain the process of flocculation and due to a
lack of simple and cost effective standard techniques for the sampling, measurement
and analysis of flocculated material. Given the primary particle's significantly different
transport properties, it is evident that such measurements and models will yield erron­
eous results and that sediment management decisions based on primary particle
characteristics will be flawed.
The influence of floe density and porosity on sediment transport
The impact of flocculation on the downward flux of sediment is in part related to the
change in the effective density and porosity of the particles. As floe size increases the
number of contacts between particles increases, thus increasing the porosity. Increas­
ing porosity is linked to the concomitant effect of decreasing the density, due to the
propensity for the pores to trap water within the floe. This negative relationship
between floe size and density is consistent for all floe populations, regardless of
environment (Fig. 3).
The significance of water trapping can be seen in Fig. 4 which illustrates the very
small pores developed within the floe due to the secretion of extracellular polymeric
1
0.001
-I
10
-
i
100
-
i -
-
1000
-
,
10000
Particle size (»m)
Fig. 3 Comparative results of excess density (wet density of the floe minus the density
of the water) with floe size for different environments, a = Fennessy et al. (1994)
(marine), b = Gibbs (1985) (estuarine) c = Hawley (1982) (lacustrine), d = Droppo et
al. (2000) (river).
A new definition of suspended sediment
1
Fig. 4 Transmission electron microscope micrograph of a typical floe with inorganic
and organic particle interactions.
fibrils (EPS) by the bacterial population of the floes. The average diameter of an EPS
fibril is 4 - 2 0 nm (Leppard, 1997) and as such, each pore will have a high surface
tension for the retention of water. Large macro pores are possible within large open
matrix floes, with the effect of increased settling velocity due to the movement of free
water through the floe pores reducing mechanical drag on the floe (Masliyah &
Polikar, 1980). However, Liss et al. (1996) suggest that such macro pores may in fact
be filled with EPS that cannot be seen by standard microscopic methods. In addition,
given the likely "tumbling" nature of floes transported in a turbulent environment,
flow through pores as a result of vertical settling is likely to have a limited impact on
floe settling.
While the floe properties of high porosity and low density will have some impact
on the settling velocity of a floe, their significance is limited relative to that of floe
size, due to: (a) the density of the floe being close to the density of the water (due to
the high porosity of floes) and (b) the range in floe densities being relatively small
(Fig. 3). As such, a change in floe density/porosity will have a minimal impact on the
settling velocity of a floe. This also suggests that water temperature effects on water
density will have little mechanical impact on floe settling. However, temperature will
influence floe development and structure (and therefore transport) by influencing the
microbial community and its metabolic function (primarily EPS production).
The influence of overall floe structure/composition on sediment transport
In conjunction with flow characteristics, the transport of a floe is largely related to its
complex composition and structural matrix. The above has discussed the gross scale
influences of floe size, density and porosity on sediment transport. However, to fully
understand the transport behaviour of a floe it is important to also examine floe
structure and composition at the micro-scale. To this end, Droppo (2001) has broken
the floe down into four different structural components: (a) inorganic particles (Fig. 5);
Ian G. Droppo
8
(b) biota and biorganic particles with an EPS sub-class (Figs 6 and 7); (c) pore
structure (Fig. 8); and (d) water (Fig. 9). Each of these components have autonomous
and yet linked physical, chemical and biological processes which will influence the
sediment's transport within the water column. This section borrows heavily from the
Droppo (2001) paper entitled "Rethinking what constitutes suspended sediment" and
as such Figs 5-9 are reproduced with permission from this source (John Wiley and
Sons Limited, ©2001).
Diverse
Structure
(shape/size)
and
Composition
o
>
<
Floe
Hydrodynamic
Change (promotes
Hydrodynamic
settling)
C h a n g e (effect ?)
Fig. 5 The characteristics of inorganic particles that will influence the internal and
external (transport) behaviour of floes.
BIOTA AND
BIOORGAN1C
Negative
Charge
1
Floe H y d r o d y n a m i c
Trapping
Change (promotes
of Water
Nutrient/
Electrochemical
Contaminant
Flocculation
Floe
Adsorption
(Floe Building)
Density
settling)
R e d u c e s Floe
Floe Hydrodynamic
Density
Change (reduces
settling)
Nutrient/Contaminant
Assimilation/
Transformation
Reduces
Promotes
Diffusional
Gradients
Fig. 6 The characteristics of the microbial community/organic particles that will
influence the internal and external (transport) behaviour of floes.
A new definition of suspended sediment
9
Floe Hydrodynamic
Change (reduces
settling)
Floe ITydrodynamic
C h a n g e (promotes settling)
Fig. 7 The characteristics of microbial EPS fibrils and their influence on the internal
and external (transport) behaviour of floes.
Floe Hydrodynamic Change
(promotes settling)
i
H
w
i
Advective Transport
Reduces Floe
of Water and Associated
Density
Contaminants/Nutrients
Floe Hydrodynamic
o
>
<
35
aas
Advective Transport
of Floe Building C o m p o n e n t s
Change (reduces
settling)
Jt
Advection Added Biological
Removal/Transformation
B i o - C h e m i c a l & Diffusion
Gradients
of Contaminants/Nutrients
Fig. 8 The characteristics of water within a floe and its influence on the internal and
external (transport) behaviour of floes.
Figures 5-9 by Droppo (2001) are not intended to include all behaviour or struct­
ural/constituent characteristics for floes. There are just too many to account for them
all. Rather, this series of flow charts is intended to demonstrate the complex link bet­
ween the physical, chemical and biological structures (gross and fine scale) of a floe,
and the outward physical, chemical and biological behaviour. In this regard, informa­
tion is provided in the figures demonstrating how various characteristics will influence
floe building and the subsequent transport of sediment (hydrodynamic effects).
10
Ian G. Droppo
Fig. 9 The characteristics of floe pores and their influence on the internal and external
(transport) behaviour of floes.
Figures 5-9 also provide information on the chemical behaviour effects, such as
the promotion of chemical diffusion, transformation and volatilization and biological
behaviour, including nutrient and contaminant assimilation, transformation, and EPS
production. These flow charts suggest that floes can be viewed as individual microecosystems with autonomous and interactive, physical, chemical and biological
behaviours operating within a floe matrix. The figures also suggest that the floes are
continuously interacting with their environment, as the medium in which they are
transported provides the floes with building materials, energy, nutrients and chemicals
for biological growth, chemical reactions and morphological evolution. While floes
can function as individual microecosystems, they have also been shown to have an
ability to regulate the surrounding water quality (Leppard, 1985; Decho, 1990).
There are too many structural and behavioural linkages within Figs 5-9 to expand
on them all. As such, given that floe biology is believed to be the dominant factor in
promoting, controlling, stabilizing and modifying the physical, chemical and biological
behaviour of flocculated material (Decho, 1990; Liss et al., 1996; Droppo, 2001), the
role that bacteria play in floe development and transport is described here to demons­
trate the complexity of the floe microecosystem. In the floe building process, bacteria
will often preferentially attach to suspended particles, as this is a beneficial association
for their survival, since the sediment represents physical protection and a food source
due to adsorbed dissolved organic mater, nutrients and contaminants (Logan & Hunt,
1987, 1988). The main mechanism of attachment is through the bacteria's production
of EPS that sticks the bacterium to the sediment (some bacteria have stock
attachment). As the bacteria and the EPS are both "sticky" in nature, subsequent
collisions in suspension will result in additional particles (organic and inorganic)
entering into the floe matrix forming larger faster settling particles. Of course, floe
breakage may also occur due to high shear forces. The EPS fibrils themselves are very
fine and form dense porous networks within the floe. These fibrils will facilitate
further nutrient and contaminant adsorption (further bacterial food source) due to their
A new definition of suspended
sediment
large surface area and the small pores will result in the trapping of water. The trapping
of water reduces the floe density, with possible effects on floe settling in addition to
promoting electrochemical and diffusion gradients within the floe (Costerton et al,
1987). The bacteria themselves are of low density and, as such, a highly colonized floe
will have a lower density than a less colonized floe, assuming similar floe sizes and
compaction. As much of the EPS will be concentrated within the core of the floe (Liao
et al., 2002), it may also result in the occurrence of anaerobic processes within the floe
and aerobic process on the outer edges of the floe, where the EPS is more diffuse. The
cumulative effects of the above will also influence floe hydrophobicity (Finlayson et
al., 1998; Liao et al, 2000), surface charge (Liao et al, 2001, 2002), "stickiness"
(Logan & Hunt, 1988; Logan, 1993) and organic coatings (with concomitant electro­
chemical and biological effects) (Gibbs, 1983; van Leussen, 1988; Muschenheim et
al, 1989). Although the above scenario is obviously oversimplified and difficult to
quantify, it demonstrates that the floe does actually function as a microecosystem
within aquatic systems.
CONCLUSION
The above discussion represents a new paradigm of suspended sediment which
encompasses the knowledge that suspended sediment is not made up of discrete sedi­
ment particles, but rather of complex microecosystems (floes) with an active biological
community and microprocesses controlling the structure and transport of sediment.
Droppo (2001) provides a useful definition of floes. He states that a floe is an
"individual microecosystem represented as a composite particle composed of a matrix
of water, inorganic and organic (viable and non-viable) particles, with autonomous and
interactive physical, chemical and biological functions or behaviours operating within
the floe matrix". When suspended sediment is viewed as being composed of such
"microecosystems", then our interpretation of the suspended sediment and the role that
it plays within our environment changes. Not every sediment monitoring programme
will need to assess sediment structure, nor will many programmes at this time have the
ability to do so, however, this paradigm of suspended sediment is suggestive of a new
way of thinking of suspended sediment and is intended to be the motivating factor
towards the development of new techniques for the quantification of suspended
sediment transport. Such developments will greatly improve our ability to effectively
manage our water resources in the future.
Acknowledgements This paper was composed while the author was on a Leverhulme
Trust Visiting Fellowship at the University of Exeter, Exeter, Devon, UK. The author
would like to thank C. Jaskot, G. Leppard and M. West for assistance with image
analysis.
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