1
Manuscript submitted to “The Methods of Coastal Models”
Modelling of cohesive sediment dynamics (chapter 6)
Ulrik Lumborg and Hans Jacob Vested
2
Modelling of cohesive sediment dynamics
Abstract
Cohesive sediment is defined as sediment with a grain size less than 63µm. Estuarine processes
as flocculation, settling/scour lag and others makes it difficult to predict the behavior of cohesive
sediment without extensive knowledge of the study area and field data. The uses of numerical
modelling tools have however increased the understanding of fine-grained sediment dynamics.
The chapter gives a short introduction to the estuarine processes and describes the required
considerations in setting up a numerical model. The chapter is concluded with examples of
modelling studies from the Danish Wadden Sea.
Introduction
Throughout the world intertidal deposits of fine-grained sediment is found in abundance. The
fine-grained sediment tends to accumulate in tidal environments where the estuarine processes
are responsible for the large accumulations. Estuaries are the compulsion zone between the open
marine environment and the river environment. A combination of hydrodynamic and
sedimentological conditions creates the interesting phenomena of a natural accumulation of
sediment that has formed land and where man has settled since the earliest civilizations.
Estuaries are reported to present sediment concentrations orders of magnitude higher than those
seen in both the open marine and in the river environments. Many major cities, ports and
industrial activities are located in estuarine environments where sediments can cause problems
with water quality and accumulation in navigation channels, harbor basins and water intakes.
Fine-grained sediment is difficult to quantify as it interferes with other aspects in estuaries, those
being biology, hydrodynamics, chemistry etc. Numerical modelling has during the recent
decades become an increasingly useful tool in cohesive sediment management. The dynamics
3
behind the cohesive is mainly empirical since its behavior is affected by numerous parameters
that cannot be determined theoretically. Thus field data collection is still an important aspect of
cohesive sediment studies.
Fine-grained sediment with grain size less than approximately 63µm is different from sand in
being cohesive. This means that the sediment is usually not present as individual grains but
rather as flocs or aggregates composed of thousands of small particles. The individual particles
consist mainly of clay minerals with diameters in the order of 0.5–5 µm1,2. These particles will
practically never settle in a natural environment. The flocs on the other hand have particle sizes
orders of magnitude higher but at the same time the effective density is lower as the flocs consist
of a mixture of sediment and water. Flocs have typical settling velocities on the order of 0.1–1
mm sec-1. The formation of flocs depends mainly on the amount of sediment in suspension, the
level of turbulence, salinity, and the organic content. Flocs can be transported over long
distances before settling at the sea bed. After deposition the sediment can be resuspended by
increasing currents and/or waves. In a tidally dominated environment the sediment will finally
deposit in the upper parts of the tidal flats. After settling, the strength of the sediment towards
erosion will gradually increase due to consolidation. The importance of biological activity can
significantly alter the sediment properties in the water column as well as in the sediment bed.
The combination of hydrodynamic, sediment and biological processes make it difficult to predict
cohesive sediment dynamics.
The present chapter gives a short introduction to cohesive sediment dynamics. The first part
describes how to set-up, calibrate and apply sediment transport models from the first
development of a conceptual understanding to an advanced numerical model. The second half of
the chapter presents examples of site-specific projects where cohesive sediments are involved.
4
Concepts of cohesive sediment dynamics
As stated above the nature of cohesive sediment dynamics is complex and the description is
therefore restricted to a summary of the most important processes to take into account when
modelling cohesives.
Tidal asymmetry
Tidal asymmetry is normally observed in shallow estuaries with large tides3. The wave crest
moves faster than the through and the crest of the tide may partially overtake the through,
resulting in a shorter flood and a longer ebb. In addition is the effect of bottom friction that
depends on the square of the current. Its effect is to produce greater friction in shallow water.
This also slows down the water movement at around low water relative to high water. The
combined effect of these two processes produces a short duration flood phase of the tide, also
known as flood dominance. Ebb dominance can also be produced essentially by interactions
between the deep channels and the shallow areas, and the varying friction during the tide3. The
existence of flood and ebb dominance is important for understanding the behavior of cohesive
sediments. Flood dominance favors landwards while ebb dominance favors seawards sediment
transport, respectively.
Settling/scour lag
The settling and scour lag are two uncoupled processes that both leads to a landwards transport
of sediment independent of the tidal asymmetry. Both processes were described by Postma 4 and
van Straaten and Kuenen 5. The settling lag is occurring when a particle is settling from a
slackening flood current. Here the particle will not deposit directly below the position where the
settling begins but at a position further landward. When the water parcel that deposited the
5
particle is passing again (now as an ebb current) it will not have a speed fast enough to resuspend
the particle which will therefore be resuspended by a water parcel located further landwards.
Therefore the particle will not be transported as far offshore as it once was. As a consequence the
particle will each tidal period be transported further and further landwards until it reaches a
position where it can no longer be resuspended.
It has been found that the force required to erode a particle from the sediment bed is higher than
the force required keeping a particle in suspension. Therefore when a particle is deposited by a
flood current it will require an ebb current with a higher velocity before it is resuspended. This
process will also lead to a stepwise landward transport of the sediment.
Flocculation
The process of individual sediment grains aggregating into flocs can be affected by numerous
parameters. Suspended clay minerals are usually negatively charged and therefore repulse each
other 6. In order to form flocs this force has to be neutralized which can happen in saline water.
This process is known as salt flocculation. Salt flocculation in itself can however not explain the
degree of flocculation that has been observed in especially estuarine waters. First of all the
probability that the individual particles actually collide is important. The most important
processes in this context are Brownian motions (random collisions due to molecule movements),
suspended sediment concentration (the higher the concentration the higher the probability),
turbulence in the water column, and differential settling (collision of flocs with different settling
velocities). Another very important aspect in flocculation is the pelagic biology. Organic matter
is sometimes sticky and can literally glue the mineral grains together 7. The settling velocity is
6
not constant neither in time nor space and it is considered one of the most important parameters
in cohesive sediment transport models8.
Fluid mud / hyper concentrated suspensions
The concentration of suspended sediment in the water column increases towards the bed. When
the flocs begin to touch each other and interact hydrodynamically the settling velocity is reduced.
This phenomenon is known as hindered settling and may lead to high concentration suspensions
or fluid mud layers. Fluid mud is a concentration of fine-grained material in which settling is
substantially hindered. It forms when the rate of settling exceeds the capacity of dewatering9.
The process forms a very concentrated suspension that acts neither as a Newtonian fluid nor as a
sediment bed. The lower concentration limit of naturally occurring fluid mud layers is often
given as about 10 kg m-3. This concentration can often be recognized as a lutocline and it is
around this concentration that the suspension transits to become framework supported and much
less mobile than the suspension10. Fluid mud layers are thus layers with extreme concentrations
of sediment. The layer is moveable but moves as a gel rather than as a Newtonian fluid. Fluid
mud layers accomplish a significant challenge for fine-grained sediment modelling.
Estuarine biology
One often overseen feature in cohesive sediment dynamics is the effect of biological
components. This effect is however in the majority of estuaries of significant influence. The field
of estuarine biology is extremely complex and only a brief introduction to biology on mudflats is
given here. For further information see e.g. Cadée11 or Madsen12.
7
Estuarine species are tolerant when it comes to changes in temperature, salinity, sediment
concentrations and to a certain degree current velocities. The biota affecting the sediment
dynamics ranges from the smallest diatoms on the intertidal sediment bed to fish filtering the
sediment and birds causing turbation of the deposited sediment.
Some important biological issues in intertidal cohesive sediment dynamics are:
Many intertidal organisms produce sticky mucus as part of their life cycle. This mucus
enhances floc formation and limits floc-breakup7.
Intertidal microphytobentos can create biofilms or algal mats. These formations can
increase the erosion strength by orders of magnitude13
Suspension feeders as e.g. clamps and mud snails catch suspended matter in collecting
food. The matter passes through the ingestion system and is excreted as fecal pellets or
pseudofeces14. This process can be regarded as a very effective flocculation process.
Fecal pellets can reach several millimeters in diameter15.
Burrowing organisms as well as larger animals feeding on mud flat might cause
bioturbation e.g. mixing of the sediment, making it more prone to erosion16.
Sediment Transport Equations for Cohesive Sediments.
Sediment transport of cohesive sediment is in general described by equation for conservation of
mass. This can be expressed by the three dimensional equation for advection and dispersion of a
suspended particle with inclusion of the interaction with the bed, see e.g. Teisson17:
8
ci
t
uci
x
vc i
y
wci
z
ws c i
z
x
Tx
i
Tx
ci
x
y
Ty
i
Ty
ci
y
z
Tz
i
Tz
ci
z
Si
where
t is time
x, y, z are cartesian coordinates
u,v,w are velocity components
ci is the i’th scalar component (defined as the mass concentration)
wsi is the settling velocity of the sediment particles under the influence of flocculation
Tx
Tx
i
is the turbulent Schmidt number
is the anisotropic eddy viscosity
Si is a source term
The term
ws C i
z
in the equation takes into account the settling of particles with a settling velocity
wsi. The interaction with the bed is described through the source terms that can either be negative
for sedimentation (loss of material from the water column) or positive for erosion (gain of
material in the water column).
When sediment concentrations are high, density differences develops and the viscosity increases
and the sediment laden flow can exhibits non-Newtonian behavior also known as fluid mud. For
adequately low concentrations of mud, these effects can be neglected.
The partial differential equation above cannot be solved analytically and must be solved
numerically. The solution requires an accurate description of the advective terms on the left hand
side of the equation in order to avoid numerical diffusion.
9
From a sediment transport point of view it is the source terms that describes deposition and
erosion that are interesting. These equations are empirical and based on laboratory and field
experiments. Figure 1 illustrates the interaction between the water column and the bed (modified
after Mehta et al.18 and Mehta19). The sediment particles form flocs that settle at the bed. Initially
the freshly deposited sediment is a loose layer or in very turbid environments even a fluid mud
layer. If the bed sediment is undisturbed, the sediment gradually consolidates and the resistance
towards erosion increases with depth. This is conveniently described by including two or more
individual bed layers with different characteristics with respect to resistance to erosion and other
bed specific parameters. With increasing current speed, fluid mud is re-suspended by
entrainment followed by floc erosion of the lower bed layers. In the presence of short surface
waves the effective bed shear stress increases the erosion. This is typical for intertidal flats in
shallow estuaries.
10
Figure 1
Suspended sediment forms flocs that deposits at the bed. With increasing currents the loose
mud layer is re-entrained. Bed shear stresses can be enhanced by short surface waves and
during spring tides or storms the lower sediment layers are eroded. Modified after Mehta et
al.18 and Mehta19.
The deposition of sediments in suspension depends on the turbulence level in the flowing water
and the settling velocity. From a set of laboratory experiments, it was found that deposition
occurs when the bed shear stress τb is less than a critical shear stress value for deposition τcd
(Krone20). This value is assumed constant at a particular location. The rate of sediment
deposition in kg/m2/s is then modeled as
SD = ws cb (1
b
/
cd
), when
b
cd
Where ws is the settling velocity (m/s), cb is the near bed concentration and pd
termed the probability of deposition.
(1
b
/
cd
) is
11
From field observations of settling velocity and flocculation equations for the settling velocity is
as follows:
cfloc is the concentration above which flocculation becomes important. chindered is the
concentration beyond which the suspended sediments will experience hindered settling. In order
to make the exponent m independent of units the sediment concentration is normalized with
s
(grain size sediment density). If hindered settling is included, the settling velocity reduces with
increasing concentration until it reaches a value of zero at the gelling point where the individual
flocs touches each other and the entire suspended sediments acts as a non Newtonian fluid. It
should be noted that the salinity flocculation is only active at salinities above ~2 ppt. The settling
velocity reflections can as such not be used in pure freshwater environments.
For a 3D model the near bed concentration is the result of the simulation of the vertical sediment
profile. For a 2D depth integrated model the near bed concentration must be calculated based on
the depth averaged concentration and an assumed sediment profile. The most common example
of sediment profile is the Rouse profile. The rouse profile is determined by equating the diffusive
upward flux of sediments due to turbulence and the downward settling of sediments. With the
assumption of a logarithmic velocity profile and a parabolic distribution of the eddy viscosity an
equilibrium concentration profile can be determined, commonly used for non cohesive sediments
in steady currents. Its applicability for cohesive sediments is, however, in doubt as increasingly
more sediment is resuspended as long as the bed shear stress exceeds the critical shear stress for
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erosion (τce). Teeter21 developed a sediment profile based on an approximate solution to the
balance of the vertical sediment fluxes during deposition. The near bed concentration is
calculated as a function of the depth averaged concentration, the Peclet Number (Pe) (ratio
between the convective transport and average diffusive transport in the vertical) and the
probability of deposition.
Pe =
ws h
Dz
ws
1
u*
6
In which κ is von Karmans constant and u* is the friction velocity.
The source terms for erosion are as suggested from laboratory experiments by Ariathurai22 for a
hard bed subject to erosion (as parameterized by Partheniades23):
Hard bed:
S E = Eo ( b /
ce
1) n , when
b
>
ce
Parchure and Mehta24 proposed for a soft newly deposited bed (partly consolidated) the
following equation:
S E = Eo exp( (
b
ce
)½ ), when
b
>
ce
Where Eo is the floc erosion rate (kg m-2 s-1) and n,α are erosion coefficients.
A detailed model will often consist of two or more bed layers. τce, E0, , and n is then adjusted in
order to mirror the rheological changes that can be observed with increasing depth.
13
In Figure 2, a diagram that shows how the sediment transport equations and coefficients are
linked is presented. The diagram is shown for illustration of the depth integrated 2D equations.
The diagram summarizes the calibration parameters (user defined), in total eight parameters. In
addition the bed layers thickness and number of bed layers is often part of the calibration.
Figure 2
Diagram showing the 2D cohesive sediment transport equations and the user defined
calibration parameters. Modified from Lumborg and Pejrup 25
The challenge in modeling of cohesive sediments is that the sediment dynamics is described by a
mixture of theoretical and empirical equations. This is further complicated by the fact that
14
important calibration parameters such as settling velocity, critical shear stresses for erosion and
deposition and erosion rates are difficult to measure in field and laboratory.
In practice the calibration therefore becomes a search for a reasonably combination of
parameters that can reproduce the observed behavior of the suspended sediment concentration.
However, this is no guarantee that other combinations of parameter settings cannot provide an
equally good calibration. It is therefore important that a fundamental understanding of the
expected behavior of the sediment dynamics is developed prior to modeling in order to assess if
the simulation results are a reliable representation of reality. In practice this is indeed based on
experience and the skills of the individual modeler and of the amount of data available for
calibration.
In conclusion a model for cohesive sediment dynamics cannot be developed without a thorough
understanding of the expected behavior. This understanding is most conveniently developed
through a formulation of a conceptual model. This starts with analysis of all available data in
conjunction with the objective of the model study. Combination of sediment properties and
hydrodynamics helps to determine the relevant time scales for the sediment transport that defines
the conceptual model. The development of a conceptual model is illustrated in Figure 3.
15
Figure 3
A conceptual model starts with analysis of measurements, sediment properties, and
hydrodynamics in order to formulate the in relation to scope of work most important sediment
transport processes.
Time Scales and Sediment Budget
For development of a conceptual model it is useful to investigate the length and time scales of
the sediment processes and establish a rough sediment balance for the area to be modeled.
The sediment transport is the result of local processes governed by short time and length scales
and longer scales representative for the entire estuary system including seasonal cycles. The
processes and thereby scales interact and are not independent, for example, a seasonal storm may
induce strong erosion due to surface waves that increases the bed shear stresses. For all scales
advection dispersion are important either for carrying away sediment resuspended by waves or
for export or import of sediment on tidal and season scales. Figure 4 illustrates how the scales
and sediment transport processes interact. Short time scales (seconds) are characteristic for
16
surface wave induced erosion. With the tidal cycle (semi-diurnal or diurnal) the currents rise and
fall and a boundary layer flow develops which can both erode and advect suspended sediments.
The tidal flow typically exhibits neap and spring fortnightly variations. Time scales for
deposition depends on the settling velocity and thereby flocculation and occurs at scales from
hours to longer for very fine particulates. The deposited sediments typically consolidate with a
time scale of weeks and more. Seasonal variations are important for floods and storms that can
impact the sediment balance.
Figure 4
Illustration of time scales of estuarine sediment transport processes.
As part of the development of the conceptual model and understand the following scaling
parameters or non-dimensional numbers are useful.
17
The Rouse number is defined as:
Ro =
ws
u*
The inverse of the Ro is called the suspension parameter. If it is larger than unity the sediment
transport is dominated by suspended load as expected for cohesive sediment transport. The
sedimentation time for a sediment particle at a water depth h is:
Ts = h / ws
The relative sedimentation time Ts/T is a measure for how quickly sediment will deposit at a
given water depth relative to the tidal period.
The sedimentation depth hs can be estimated as:
hs =
cwsT
cbed
Where T is the time for deposition and cbed is the concentration of the deposited sediments. Thus
hs/h is a measure for the sediment that is available in the water column for the formation of fluid
mud or high bed concentrations.
As an example assume ws=0.005m/s, U=1m/s or u* ≈0.2m/s, h=4m, c=1kg/m3, T=2hours
(corresponding to the duration of slack water and cbed=80kg/m3. 1/Ro=16 i.e. suspended
sediment transport dominates, Ts=800seconds and we find hs=0.45m and hs/h=0.11. This shows
that for the given values one can expect suspended sediment transport and high bed
concentrations around slack water.
The next step is to understand the overall behavior of the sediment transport system and the
importance of the boundary conditions. This is most conveniently done by establishment of a
simple box model and a sediment balance. In an estuary conservation of salt can be applied to
18
determine the volume fluxes of water26, 3. This can be applied to estimate the sediment input and
export.
Basically the sediment balance can be written as the increment of the change of sediment stock
Mt from one time step to the next:
Mt
t
= Mt
Mt
Where
M t = ( Si , fluvial Si ,marine Si ,erosion Si ,disposal ) ( Se, fluvial Se,marine Se,deposition Se,dredging )
is the difference between sediment input and export. See also Figure 5.
Figure 5
Sediment balance for a sediment stock Mt is a balance of input from the open sea, rivers,
internal erosion, and disposal of dredged sediments and export to the ocean, as deposition on
tidal flats/marshes and dredging (off shore and land disposal).
Assessment of the input of marine sediment is important. This input is often dominant and
density driven flow due to salinity gradients can be responsible for upriver sediment transport
along the bottom.
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When the formulation of a conceptual model and analysis of the sediment transport system is
accomplished a sound basis exists for the application of the numerical model as shown in the
examples.
Examples from the European Wadden Sea
The European Wadden Sea is located on the west coast of northern Europe. It reaches from den
Helder in Holland and continues through the west coast of Germany and ends at the Skallingen
peninsula in southern Denmark. In the northern part the tidal range is only about 1.6 m whereas
it gradually increases to more than 4 m in the south. The Wadden Sea represents a continuum of
grain sizes from very fine grained sediment to coarse sand. The area has also very diverse flora
and fauna and the area is protected by a trilateral conservation agreement.
Study of disposal of harbor sediments in Northern Wadden Sea
As an example of a practical model study of cohesive sediment transport considers the northern
Wadden Sea called Grådyb, located in Denmark. The tide is semi-diurnal with a spring – neap
variation and the average tide is about 1.5m. It has a surface area of approximately 135 km2 and
a tidal volume of 160×106 m3. The sediment concentration of fines is 0.02–0.1 kg m-3 and there
is a net deposition of 90 000 tonnes per year. The port of Esbjerg is located in the area and the
navigation channel and the port facilities are maintained at a water depth of 10.3 m relative to
chart datum. The material from the maintenance dredging is disposed at two locations denoted E
(Ebb) and F (Flood) and is about 100 000 tonnes per year. The area outside the port is
characterized by large inter tidal areas and natural ebb and flood channels. Several studies have
been prepared in order to apply for environmental permits that require assessment of the impact
on the environment of the maintenance dredging. As part of these studies a 2D MIKE 21 FM
20
model has been set-up for the area and applied to simulate the behavior of the fine sediments.
The model mesh is shown in Figure 6.
E
F
Figure 6
MIKE 21 model schematization of the Northern Wadden Sea. The zoom (lower panel) shows
the port areas, navigation channel and location of disposal sites E and F.
The model was applied to determine transport paths of fine sediments and the final deposition of
sediment disposed at the existing disposal sites E and F. The consequence of off shore disposal
21
and the effect on the intertidal flats was also investigated. A conceptual model was prepared that
showed that the sediment transport was governed by a combination of calm and windy periods. It
was therefore required to sequence the simulations in periods that represented different wind
conditions and simulate the corresponding waves by application of MIKE 21 SW (Short Waves)
and combine these with the sediment transport equations. A long term sediment budget has been
prepared36 that shows that approximately 60% of the sediment input to the area origins from the
North Sea while the remains stems from internal sources from rivers and bank erosion. Thus a
model should be build that was able to import sediment into the Grådyb tidal area. Field data of
currents, water levels and sediment concentrations were collected near to the disposal sites E and
F for about two neap spring periods (28 days) in order to be able to verify that the model could
reproduce the short term fluctuations in suspended sediment concentrations. Figure 7 shows the
measured and simulated current speeds for a week period near disposal site E. The current speed
reaches about 1.0 m s-1 at ebb and 0.8 m s-1 at flood.
Figure 7
Comparison between simulated and measured current speed near disposal site E
The simulated and measured sediment concentration is shown in Figure 8. The measurements
vary between less than 0.1 kg m-3 and up to about 0.5 kg m-3. This variation is due to influence of
wind through the action of waves that enhances re-suspension and disposal of sediment from
22
maintenance dredging of the port. The actual disposal records from the maintenance dredging
were incorporated in the simulations.
Figure 8
Comparison between simulated and measured average suspended sediment concentrations over
the depths near disposal site E (upper) and F (lower) and the wind speed and direction. Notice
the period before the 1 May with strong westerly winds that favors high sediment
concentrations.
23
As discussed above flood and ebb dominance is a useful indication of the behavior of the
different parts of the estuary system. Flood dominance is typical responsible for accumulation of
fine sediments. A simple measure is to calculate the ration between the average maximum
current speed at falling and rising tide. When this ratio is less than one it is flood dominated.
Figure 9 shows this ratio computed from the hydrodynamic simulations in comparison with a bed
type sediment map. An excellent agreement between areas with mud and mixed sand-mud flats
and flood dominance is observed.
Figure 9
Ratiobetween max average ebb and flood currents as a measure of flood dominance to the right.
Blue and black collars indicate fine sediments. This ratio is correlates to a high degree with the
grain sizes observed in the sediment bed27
In a previous study in 1991 a sediment cloud disposed at site F was followed and measured for as
many hours as possible before it was fully dispersed and mixed with the in situ sediments. It was
observed that the sediment cloud had a tendency to split up in two due to the diverging of the
flood current in two separate channels. Although the actual conditions could not be precisely
reproduced this field experiment was simulated and it was indeed seen that sediment cloud had a
tendency to disperse laterally as if to split into two after some hours (Figure 10).
24
Figure 10
In 1991 a sediment cloud was followed for some hours, upper panel. An attempt to reproduce
the dispersion of the sediment cloud is shown in the lower panel.
The calibrated sediment transport model was subsequently applied to assess the transport paths
and final deposition of the disposed sediments. It was concluded that the disposed sediment
remained within the tidal area. The impact of disposal of off-shore disposal was also
25
investigated. The simulations showed that only a small percentage, depending on the actual
disposal site, of the disposed sediment would return to the tidal area. In the long term this could
potentially lead to a reduced sedimentation rate at inter tidal mud flats.
Detailed study of sediment budgets: Rømø Dyb
In a study investigating the sediment budget in Rømø Dyb tidal area in the Danish Wadden Sea
(Figure 11) MIKE 21 MT model was applied25. The study area is geo-morphologically an
enclosed coastal lagoon with one connection to the open sea and with only to minor rivers
discharging into the bay comprising less than 0.2% of the average tidal prism28. The area is
enclosed to the north and south by causeways build in the middle of the 20th century.
26
Figure 11
Distribution of intertidal sediments in the Lister Dyb tidal area in the Danish Wadden Sea.
The map is modified from Pejrup et al.29
The lagoon covers an area of about 400 km2 of which 45% is intertidal flats. The suspended
sediments in the area are generally fine-grained and the intertidal flats located in sheltered area
are mainly comprised of fine-grained sediment30. In the deeper parts of the tidal channels and on
27
wind and wave exposed intertidal flats less than 5% of the sediment is in the mud fraction29. The
area is generally well investigated and a large number of field studies have been reported 31, 32, 33,
34, 30, 35
. The present study focused on establishment of a yearly budget of cohesive sediment
computed as input from the open sea using a numerical modelling tool MIKE 21 MT. The model
was applied with the formulations as given above and calibration parameters were obtained from
the field studies. The model proved able to reproduce measured suspended sediment
concentrations with a fairly high degree of reliability due to the intense use of measured
sediment parameters from the site (Figure 12).
1000
Suspended sediment concentration (mg l-1)
800
Simulated SSC
Observed SSC
600
400
200
Jun 01
May 31
May 30
May 29
May 28
May 27
May 26
May 25
May 24
0
2002
Figure 12
Comparison of measured and modelled SSC in Rømø Dyb. Modified from25
28
After the set up and calibration of the model it was applied for estimation of the sediment input
from the open sea during a full year (2002) setup.
The gross annual transport of cohesive sediment to the lagoon through the tidal inlet was about 1
million tons (Figure 12). Of this high number about 45 000 tons where permanently deposited in
the lagoon, corresponding to about 4.5% of the total input This means that during every tide
about 20 tons of sediment is deposited in the area. This amount is however not uniformly
distributed over the year. The analysis showed that during calm weather conditions the sediment
is brought in at a slow rate showing an almost uniform import. The tide comes in with about
1390 tons and it leaves again with about 1370 tons corresponding to a drop in concentration of
about 0.1 mg l-1. This very delicate balance causes an accumulation in the sediment amount in
the lagoon. The balance is disturbed by events with strong wind. In such cases sediment
deposited on the intertidal flats may become mobilized leading to export events. In most cases
such events are compensated for fairly quickly with a higher import rate during the next tidal
periods.
During periods with storms over more than a couple of days however, the sediment already
deposited on the intertidal flats may be mobilized and transported out of the lagoon. As an
example it was shown that the strongest gale in the investigated year happened in the end of
January. During this gale a large amount of deposited sediment was mobilized and during one
single ebb period a total of about 40 000 tons was exported from the lagoon. This loss of
sediment was so substantial that it was not compensated for until after about 2.5 months.
29
Figure 13
Cumulated sediment transport through Lister Dyb during 2002. It is clearly seen that the
estuary experiences a slow constant import interrupted by few extreme export events. The
figure is modified from Lumborg and Pejrup25
This shows that even though large amounts of sediment are exported from the lagoon during
storms, the estuary still catches more sediment than it looses. In the present study the estuary was
found to catch about 4% of the total incoming sediment. This is in agreement with other studies
that has used other methods to estimate the catchment capacity (e.g. Postma36).
General cook-book/flow chart
The following flow chart illustrates three steps involved in the preparation, calibration and
application of a sediment transport model for cohesive sediments.
30
Step 1: Prepare a conceptual model that defines the requirements for the model set-up
31
Step 2: Calibrate the model with respect to global data (turbidity distribution, sedimentation
patterns and characteristics), local data (time series) and check overall sediment budgets are
consistent with the conceptual model expectations.
Step 3: Apply the model and re-evaluate calibration if results does not agree with expected
results or are not consistent with respect to local and global parameters (both time series results
in selected locations and overall patterns must be evaluated).
32
References:
1 Kranck, K. "Particulate matter grain-size characteristics and flocculation in a partially
mixed estuary" Sedimentology 28 (1981): 107–114.
2 Eisma, D. "Particle-Size of Suspended Matter in Estuaries" Geo-Marine Letters 11 no. 3-4
(1991): 147–153.
3 Dyer, K., "Coastal and Estuarine Sediment Dynamics" (1986). John Wiley & Sons.
4 Postma, H. "Hydrography of the Dutch Wadden Sea. A study of the relations between
water movement, the transport of suspended materials and the production of organic matter"
Archives Neerlandaises de Zoologie 10 (1954): 405–511.
5 Van Straaten, L. M. J. U. and Ph. H. Kuenen "Tidal action as a cause of clay accumulation"
Journal of Sedimentary Petrology 28 no. 4 (1958): 406–413.
6 Eisma, D. "Flocculation and de-flocculation of suspended matter in estuaries" Netherlands
Journal of Sea Research 20 no. 2/3 (1986): 183–199.
7 Kiørboe, T., K. P. Andersen, and H. G. Dam "Coagulation efficiency and aggregate
formation in marine phytoplankton" Marine biology 107 (1990): 235–245.
8 van Leussen, W., "Estuarine macroflocs and their role in fine-grained sediment transport"
(1994):Ph.D. Thesis, Utrecht University
9 Ross, M. A. and A. J. Mehta "On the Mechanics of Lutoclines and Fluid Mud" Journal of
Coastal Research Special Issue 5 (1989): 51–61.
10 McAnally, W. H., C. Friedrichs, D. Hamilton, E. Hayter, P. Shrestha, H. Rodriguez, A.
Sheremet, and A. T. A. T. C. o. M. o. F. Mud "Management of Fluid Mud in Estuaries, Bays, and
Lakes. I: Present State of Understanding on Character and Behavior" Journal of Hydraulic
Engineering 133 no. 1 (2007): 9–22.
11 Cadée, G. C., "Intertidal fauna and vegetation" no. 9 (1998): 383–438.
12 Madsen, J. D., P. A. Chambers, W. F. James, E. W. Koch, and D. K. Westlake "The
interaction between water movement, sediment dynamics and submersed macrophytes"
Hydrobiologia 444 (2001): 71–84.
13 Andersen, T. J. "Seasonal variation in erodibility of two temperate, microtidal mudflats"
Estuarine, Coastal and Shelf Science 53 no. 1 (2001): 1–12.
14 Andersen, T. J., "The role of fecal pellets in sediment settling at an intertidal mudflat, the
Danish Wadden Sea" (2001): 387–401.
15 Haven, D. S. and R. Morales-Alamo "Occurence and transport of faecal pellets in
suspension in a tidal estuary" Sedimentary Geology 2 no. 2 (1968): 141–151.
33
16 Paarlberg, A. J., M. A. F. Knaapen, M. B. de Vries, S. J. M. H. Hulscher, and Z. B. Wang
"Biological influences on morphology and bed composition of an intertidal flat" Estuarine,
Coastal and Shelf Science 64 no. 4 (2005): 577–590.
17 Teisson, C. "Cohesive suspended sediment transport: feasibility and limitations of
numerical modelling" Journal of Hydraulic Research 29 no. 6 (1991): 755–769.
18 Mehta, A. J., T. M. Parchure, J. G. Dixit, and R. Ariathurai, "Resuspension Potential of
Deposited Cohesive Sediment Beds" (1982): 591–609.
19 Mehta, A. J. "On estuarine cohesive sediment behavior" Journal of Geophysical Research
94 no. C10 (1989): 14303–14314.
20 Krone, R. B., "Flume Studies of the Transport of Sediment in Estuarine Shoaling
Processes. Final Report to San Francisco District U. S. Army Corps of Engineers, Washington
D.C." (1962).
21 Teeter, A. M., "Vertical transport in fine-grained suspension and newly deposited
sediment" no. 9 (1986): 170–191.
22 Ariathurai, R., "A finite element model for sediment transport in estuaries" (1974):Ph.D.
thesis, University of California
23 Partheniades, E. "Erosion and deposition of cohesive soils" Journal of the hydraulics
division.Proceedings of the ASCE 91 no. HY1 (1965): 105–139.
24 Parchure, T. M. and A. J. Mehta "Erosion of soft cohesive sediment deposits" Journal of
Hydraulic Engineering – ASCE 111 no. 10 (1985): 1308–1326.
25 Lumborg, U. and M. Pejrup "Modelling of cohesive sediment transport in a tidal lagoon –
An annual budget" Marine Geology 218 no. 1-4 (2005): 1–16.
26 Hearn, C. J., "The Dynamics of Coastal Models" (2008): 1–512. Cambridge University
Press.
27 Pedersen, J., "Fine-grained sediment budgets for the Grådyb, Knudedyb, and Juvredyb tidal
areas, the Danish Wadden Sea." (2004). Prisopgave fra Geografisk Institut, Københavns
Universitet.
28 Lumborg, U. and A. Windelin "Hydrography and cohesive sediment modelling: application
to the Rømø Dyb tidal area" Journal of Marine Systems 38 no. 3-4 (2003): 287–303.
29 Pejrup, M., M. Larsen, and K. Edelvang "A fine-grained sediment budget for the SyltRømø tidal basin" Helgoländer Meeresuntersuchungen 51 no. 3 (1997): 253–268.
30 Andersen, T. J. and M. Pejrup "Suspended sediment transport on a temperate, microtidal
mudflat, the Danish Wadden Sea" Marine Geology 173 no. 1-4 (2001): 69–85.
34
31 Edelvang, K. "The significance of particle aggregation in an estuarine environment. Case
Studies from the Lister Dyb Tidal area." Geographica Hafniensia A5 (1996): 1–105.
32 Austen, I. "Temporal and spatial variations of biodeposits - a preliminary investigation of
the role of fecal pellets in the Sylt-Rømø tidal area" Helgoländer Meeresuntersuchungen 51 no.
3 (1997): 281–294.
33 Riethmüller, R., A. Behrens, G. Gassenmaier, G. Gayer, H. Günther, W. Puls, W.
Rosenthal, P. Rybaczok, G. Witte, F. Ziemer, D. Herbers, and H. Baumert, "The PROMISE/SyltRømø data set" GKSS 99/E/53(1999): 1–47.
34 Andersen, T. J., O. A. Mikkelsen, A. L. Møller, and M. Pejrup "Deposition and mixing
depths on some European intertidal mudflats based on 210Pb and 137Cs activities" Continental
Shelf Research 20 no. 12-13 (2000): 1569–1591.
35 Andersen, T. J., M. Pejrup, and A. A. Nielsen "Long-term and high-resolution
measurements of bed level changes in a temperate, microtidal coastal lagoon" Marine Geology
226 no. 1-2 (2006): 115–125.
36 Postma, H. "Exchange of materials between the North Sea and The Wadden Sea" Marine
Geology 40 (1981): 199–213.