Crimson Publishers-Experimental Study on the Settling Velocity of Coastal Mud in Quiescent Water: The Case of Huangmaohai Estuary, South China Sea

Dong XU*, Yuchuan BAI and Chunning JI
Tianjin University, China
*Corresponding author: Dong XU, State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University, 92 Weijin Rd, Tianjin
300072, China
Submission: November 14, 2017; Published: February 21, 2018
Experimental Study on the Settling Velocity of
Coastal Mud in Quiescent Water: The Case of
Huangmaohai Estuary, South China Sea
Introduction
The settling and erosion feature of the bottom sediment in
coastal areas has long been an interest for numerous scientists
and engineers with motivations ranging from coastal engineering
applications, to long-term environmental and ecological problems
[1,2]. From the aspect of engineering, the hydraulic feature of
the bottom sediment under the action of currents and waves is
usually a main concern during the design of harbors and navigation
waterways [3], planning of waterway dredging schemes, as well as
siting of water intakes for coastal power plants [4-6]. For instance, a
total amount of about 57×106m3 of fine sediment has to be dredged
in the Yangtze Estuary during the 3rd phase (2006-2008) of the
Deep-Water Channel Project in China due to siltation requirement
[7]. With the ever-developing advancement of hydrodynamics
and computational mathematics, great progress has been made in
understanding the flow conditions in coastal environment, and two
or three dimensional flow simulations were extensively put to use
[8-10]. However, the modeling of the sediment transport remains
a challenge regarding the reliability of the simulation output
[4,11]. Therefore, the mechanical behavior of the bottom sediment,
especially the cohesive sediment or mud, need to be further studied.
From the environmental aspect, many pollutants, such as heavy
metals, pesticides, and nutrients are easily absorbed by sediment in
coastal areas [7,12-15] thus these sediment serve as carriers for the
transportation of both the pollutants and the nutrients. This will
eventually affects the living and growth of the bentonic organisms
[16]. In this context, research on coastal sediment behavior is of
great importance both environmentally and ecologically.
It has been greatly accepted that the settling behavior of
coastal sediment plays a crucial role in the seabed evolution,
sediment suspension and deposition in navigational channels
[17]. The settling velocity of the sediment is one of the most
important parameters required during the modeling of the
sediment transport [18]. However, the settling velocity is extremely
difficult to determine for cohesive sediment/mud, which is the
common composition in most coastal areas of the world. The lack
of understanding on coastal mud behavior is mainly owing to
the diversity of its physical and chemical properties. Particle size
grading and water content are usually believed to be the two major
Case Report
1/11Copyright © All rights are reserved by Dong XU.
Volume 1 - Issue - 3
Abstract
A series of laboratory experiments were carried out to investigate the settling behavior of cohesive sediment/mud from Huangmaohai Estuary
of the South China Sea. Based on the experimental observations and measurements, the influence of the flocculation phenomenon, tube geometries,
initial mud concentration and salinities on the sediment settling velocities were investigated systematically. Experiments showed complex time-varying
settling processes of the cohesive sediment in quiescent water, which were divided into two successive stages:
i) The stage of the flocculated sediment settling.
ii) The stage of sediment packing and consolidation.
During the transition between these two stages, there was an abrupt change of the settling velocity. Time-dependent settling velocity was then
describedwithfunctionsobtainedfromregressionanalysis,whichshowedalinearlydecreasingsettlingvelocityduringthefirststageandanexponential
decreasing trend during the second stage. The influence of tube diameter variation was negligible on the mud settling process, while the tube length
had significant influence on the transition pattern between two stages. Mud concentration played an important role in the flocculation formulation, and
finally dominated the settling velocity of the cohesive sediment. It led to a distinguished peak value of the settling velocity corresponding to an initial
sediment concentration of 20kg/m3
. Salinity also exhibited obvious impact at the beginning stage of the settling, which presented a lowest settling
velocity corresponding to a salinity of 15% for 50kg/m3
mud samples.
Keywords: Coastal mud; Settling velocity; Flocculation
Examines in Marine
Biology and Oceanography: Open AccessC CRIMSON PUBLISHERS
Wings to the Research
ISSN 2578-031X

Examines Mar Biol Oceanogr Copyright © Dong XU
2/11How to cite this article: Dong X, Yuchuan B, Chunning J. Experimental Study on the Settling Velocity of Coastal Mud in Quiescent Water: The Case of
Huangmaohai Estuary, South China Sea. Examines Mar Biol Oceanogr. 1(3). EIMBO.000511.2018. DOI: 10.31031/EIMBO.2018.01.000511
attributes dominating the mechanical behaviors of the coastal mud
[19], including the settling velocity and the incipient motion under
the action of currents and waves. However, there are still many
other factors may complicate coastal mud behaviors. For instance,
when the flocculation concentration approached, the turbulence-
induced aggregation and flocculation breakage may alter the
settling velocity of mud. This has been widely verified by both
laboratory experiments [20] and field surveys [21], and successfully
simulated by some mathematical models [22]. Furthermore, the
existence of benthic organisms or communities can also add to the
uncertainty of coastal mud settling, resuspension and transport
[15,16,23]. The salinity of water [24], flow shearing conditions [25]
or even temperature [26] may have influence on the flocculation
formation and consequently the settling velocity of the cohesive
sediment. Spatial and temporal variation in cohesive sediment
settling velocities were widely demonstrated [27]. Considering the
diversity of coastal mud properties, the knowledge of the above
mentioned factors, including their association with the generation,
transport, and dewatering of fluid muds, remains incomplete
[28]. More laboratory and field measurements of coastal mud
settling from diverse areas are still necessary to get to a further
and comprehensive understanding. The present research focuses
on a systematically investigation on the settling behavior of the
cohesive coastal sediment/mud from Huangmaohai Estuary, South
China Sea through a series of laboratory experiments. During these
experiments, the time varying sediment settling velocity were
measured and analyzed and the influence of the tube geometry,
mud concentration and water salinity on the settling velocities
were also systematically investigated.
Materials
Figure 1: Location of mud sample in Huangmaohai Estuary, South China Sea.
Figure 2: Particle grading curve of the mud sample used in
the experiments.
Mud samples were taken from the coastal area of Huangmaohai
Estuary of the South China Sea, which is a bell-mouthed bay on
the west of Pearl River Estuary, China, (Figure 1). A grab dragged
by steel wires was used to take the mud sample from the seafloor.
The grab has jaws that will close when they hit the sea floor, which
makes it very effective for taking muddy materials. The core of
the sample with a volume of around 0.5m3 was taken and kept
undisturbed to make it close to its original field status. (Figure 1)
The gradation of the mud sample was tested with a laser particle
size analyzer and the results were shown Figure 2. The median
size is 0.007mm, which shall be classified as “silt” according to the
Wentworth scale (or Udden-Wentworth) used in the United States
[29] and “medium silt” according to the ISO 14688-1 [3]. According
to the grading curve, the content of the silty sand with a medium
diameter between 0.005mm and 0.05mm is around 50% and
the clay content with a medium diameter between 0.002mm and

0.005mm is around 25%. The rest 25% are colloidal particles with
diameter smaller than 0.002mm. The sample is mainly clay mud
with trace amount of organic matters from the cultch (Figure 2).
Measurement Methods and Experimental Cases
The experiments were carried out with a series of transparent
Lucite tubes (or settling column) with diameters ranging from
3.0cm to 8.6cm and tube heights ranging from 30cm to 200cm. Scale
indicators were prepared and attached to each tube from the top to
the bottom to read the mud level with a reading accuracy of 0.1mm.
The density of the original mud, ρo, was measured with volumetric
method and the sediment concentration, Co, were computed as:

1
o w
o
w
s
C
ρ ρ
ρ
ρ
−
=
−
(1)
where wρ and sρ are the densities of the water and the
sediment, which are 1000kg/m3
and 2650kg/m3
respectively. At
the beginning of the experiments, the mass of the field mud was
computed according the tube volume and the objective sediment
concentration. Then the mud was prepared and weighted with a
balance. The mud was then put into a bucket, together with water
to achieve the designed volume. The mud and water were stirred
for 3 minutes in a bucket by hand to destroy the aggregation and
flocculation structures. When finished, the fully mixed fluid was
quickly poured into the tubes and a timer was started at the same
time. The mud levels were read at certain time intervals. At the
beginning of the experiment (around the first one hour), the time
interval of mud level reading was 2 minutes. With the decrease
in settling velocity, the time interval of mud level reading was
increased to 5 minutes, 10 minutes, 20 minutes or even as long as
severalhoursforsomecases.Watertemperaturewasalsomeasured
at certain time intervals. It was within 22 o
C to 25 o
C throughout all
of the experiments owing to the air conditioning in the laboratory.
Although it was reported that water temperature may have
influence on the settling velocity of cohesive mud [26,30] this
influence was ignored in our experiments because the temperature
fluctuation was of fairly small magnitude. The experiments were
categorized into four different cases: case I-IV, in order to compare
the results in a straightforward way, (Table 1).
Table 1: Configuration of all experimental cases.
Case
Tube
Number
Tube Height /Cm
Tube
Diameter/Cm
Initial
Concentration/
Kg/m3
Salinity/%
Median Size D50
/µm
Duration /Min
Case I:
Long-term
1 200 8.6 50 0 7 23834
Case II: Tube
effect
4 30; 200
3.0; 5.1; 6.8;
8.6
50 0 7
20000;
23834
Case III:
Concentration
effect
15 30 3.0; 5.1; 6.8
1; 3; 6; 10; 20;
30; 40; 50; 60;
70; 80; 90; 100;
200; 500;
0 7 2752
Case IV: Salinity
effect
14 30 3.0; 5.1; 6.8 50
0; 1; 2; 4; 8; 15; 20;
25; 30; 35; 40; 50;
70; 100
7 2571
Experimental Results
Case I: Long time sediment settling in a 200cm-long tube
Figure 3: Schematic of cohesive sediment settling
A. Initial status
B. An interface developed
C. Two interfaces, a-a and b-b developed
D. Both interfaces descended with time
E. Two interfaces merged into one interface
F. Slow settling of the finial interface
a) Settling of sediment particles.
b) Group settling of flocculated sediments.
c) Packing and consolidation of sediments.
In order to investigate the long time sediment settling
behaviour, mud settling was conducted in a 200cm-long tube for
over 14 days. The initial mud concentration was 50kg/m3. The
overall sediment settling process observed in the experiment can
be basically described in the schematics shown in Figure 3. At
the beginning, the fluid inside the entire tube was heavily turbid,
(Figure 3A). After about 2 minutes, an interface developed from
inside the tube and the fluid stratified into two layers: a lightly
turbid layer on the top and a heavily turbid layer at the bottom
(Figure 3B). With the settling of the interface, the turbidity of the
top layer decreased and there was another interface developed on
the top (Figure 3C& 3D). The second interface, b-b was not as clear
as the first interface (a-a) and the settling velocity of the second
interface was much lower, which led to a combination of the two
interfaces finally (Figure 3E). The combined interface deposited
slowly to form consolidated mud at the bottom of the tube (Figure
3F). The settling process indicated in Figure 3A-3F was dominated
by three major mechanisms: sediment particle settling, flocculation
and sediment packing (Figure 3a-3c). At the beginning of the
experiment, sediments settled downwards as individual particles
because they are mechanically well-dispersed during preparation.

The settling velocity at this stage can be estimated with the Stokes
flow theory and coarser particles settled faster compared to finer
ones. However, this status cannot maintain for a long time because
the flocculation developed rapidly. The flocculated sediments
usually present in the form of interconnected clusters, blocks or
networks with equivalent diameters much larger than individual
particle [25,31] (Figure 3b). Figure 3c shows the packing of the
sediment when most the particles settled to the bottom. This
process was very slow and it finally led to a consolidated status of
bottom sediment [32,33]. Notice that all of these three mechanisms
co-existedthroughouttheexperiment,butwithdifferentproportion
during different stages. For instance, the settling of the flocculated
sediment dominated the most of the deposition process inside the
tube, while the dispersed particle settling process and the sediment
packing mechanisms were only significant at the beginning and
the final stage of the experiment. Because the dispersed particles
settling process lasted just several minutes, it was quite short when
compared with the flocculation settling process and the sediment
packing and consolidation stage. Therefore, mainly the latter two
stages will be discussed in the following sections (Figure 3).
The settling process of the fully mixed sediments in a 200cm-
long tube is shown in Figure 4. Notice that only the lower mud
interface level was recorded, namely interface a-a in Figure 3C,
because the upper interface, interface b-b was not clear enough
for an accurate reading. The mud interface descended very fast
from the beginning of the experiment and over 80% of the entire
settling process occurred within the beginning 10hours; (Figure
4a). The average mud interface settling velocity was 0.4cm/min for
the first 1 hour of the experiment and it decreased to 0.25cm/min
and 0.15cm/min after 4 hours and 10 hours, respectively. There
are two possible mechanisms may interpret this tendency of time
decreasing settling velocity: firstly, the flocculation structures of
the cohesive sediments have different sizes, consequently those
with larger equivalent diameters fall faster, leaving the smaller
ones on the top showing the interface we observed. In this case,
the flocculation size is spatially varying [22] and shall be non-
uniform along the tube height. Another possible cause is that the
flocculation structures are temporally varying [34] and developing
into stronger networks or more complex fractural shapes [20,35]
The existence of such shapes leads to a decrease in the settling
velocity. However, notice must be paid to the fact such inferences
or hypothesizes have not been fully verified in the present study
and observation with high-power microscopes is possible to
interpret this process by looking into the micro-scale flocculation
structures [30,36,37]. After 10 hours from the beginning of the
experiment, the sediment settling finally changed into a status of
particle packing and consolidation. The settling velocity dropped
dramatically to 3×10-4cm/min, which was over 1000 times slower
than the flocculated settling within the beginning hours. After
around 2 weeks, the settling process almost finished and the
sediments turned into a relatively consolidated bed. At this stage,
the settled bed exhibited solid phase properties and the bottom
mud was supposed to gain certain strength to resist flow shearing
[33,38,39]. Small magnitude of unevenness of the mud interface
was observed close to the tube wall. During the settling, the mud
concentration was increasing with time, as well as the mud density,
which grew from 1031kg/m3 to 1280kg/m3. The time-varying
mud interface settling process was also expressed in a logarithmic
coordinates for the time duration, (Figure 4b). An interesting
phenomenon observed was that there was an abrupt change of
the settling velocity at around 600 minutes after the start of the
experiment. This is the very key time separating the flocculated
sediment settling stage from the final consolidation stage, which
in return implies that the second hypothesis on the non-uniform
settling of flocculated sediment should be more reasonable because
if the non-uniformity of settling velocity was due to the flocculation
size effect, the settling process should be smoother and the abrupt
changing of the mud level may not appear (Figure 4).
Figure 4: Settling process of sediments in a 200cm-long tube indicated with mud interface level and mud density. The
initial mud concentration was 50kg/m3. (Mud density was estimated according to the initial density and the transient
mud height).

Figure 5: History of settling velocity of the mud interface in a 200cm-long tube with initial mud concentration of 50kg/m3
.
The time-varying settling velocity of the mud interface is
shown in Figure 5. Due to the unavoidable reading error, the
data show a certain noises. These scatters were not smoothed to
keep the confidence of the original measurement. It can be seen
from the figure that the settling velocity has decreased with time,
which is the same as described in the discussions for (Figure 4),
the history of the settling velocity can be divided into two stages:
the flocculation settling stage and the consolidation stage, which
were separated at a time of around 600minutes. According to the
data regression results shown in Figure 5, the settling velocity has
a linearly decreasing relationship with time during the first stage
and a nonlinear relationship during the second stage, which can be
expressed with:
4
1 2
3
,
,
c
c
c
Sr c c t for t t
Sr c t for t t
=+ <
≥
(2)
where 1 2 3 4, , ,c c c c is the settling velocity; are experimental
coefficients and ct is the critical time for switching between two
different settling stages. In this experiment,
1 0.44;c = 4
2 5.94 10 ;c −
=− ×
3 2118.36;c = 4 1.65c = − and 600minct = (Figure 5).
The settling velocity of individual sediment particles can
be predicted with the Stokes’ law when the sediment is fine and
particle Reynolds number Re<<1. According to the Stokes’ law, the
settling velocity of a sphere, Sr, can be calculated by:
( ) 22
9
p f
Sr gR
ρ ρ
µ
−
=
(3)
where Sr is the particles’ settling velocity; g is the
gravitational acceleration; µ is the dynamic viscosity of the
fluid; pρ is the mass density of the particles, and fρ is the mass
density of the fluid. All of the above variables have SI units. Here
we take g =9.81m/s2
; for sediment particles, pρ =2650kg/m3
;
and for water at temperature 20 0
C, µ =1.002x10-3
Ns/m2
and
fρ =1000kg/m3. Therefore, the settling velocities of the particle
with an median diameter d50
=7x10-3
mm, the largest diameter of
dmax
=0.3mm and the smallest diameter of dmin
=10-4
mm are: 50dSr
=7.33x10-5
cm/min; =1.35x10-1
cm/min; =1.5x10-8
cm/min (4).
According to the calculation results shown in Equation(4), the
settling velocities measured in the experiment were much higher
than the one calculated for the sphere with a diameter d50
=7×10-
3
mm or even dmax
=0.3mm. The flocculation greatly augmented the
equivalent diameter of the sediment and accelerated the settling
[20,22,24]. At the settling velocity of 0.4cm/min, the equivalent
diameter (assuming that flocculations are roughly spherical [24]
computed using Stokes’ theory is 0.517mm, which is 74 times of
the median diameter d50
. From this point of view, one flocculation
cluster or block may have 743
= 405224 sediment particles from
a rough estimate. Considering that the flocculation structure may
not have a spherical shape [35], more particles may be required to
achieve the final group settling velocity. Therefore, it makes sense
to estimate that each flocculation cluster shall be composed of
sediment particles with a number at the order of one million (106
).
Case II: Tube effect on sediment settling
In order to investigate the influence of the tube/settling
column geometry on the sediment settling process, laboratory
experiments were carried out in 4 tubes with different lengths (L,
in cm) and diameters (Φ, in cm): L200 Φ8.6, L30 Φ6.8, L30 Φ5.1
and L30 Φ3.0. The time histories of the mud interface level and the
mud density are shown in Figure 6. The tube walls has enforced
non-slip (or nearly non-slip) boundaries for the fluid, which may

have introduced upward dragging on the sediment particles or the
mud interface of the flocculated sediment. Besides, tube diameter
may suppress the development of large flocculation structures.
Experimental measurements show that the tube diameter variation
from 3.0cm to 8.6cm has little influence on the settling process of
the cohesive mud, (Figure 6). This implies that even for the tube
with a diameter as small as 3.0cm, the tube diameter is large
enough compared with the size of flocculation structures and the
boundary effect from the tube wall on settling process is negligible.
However, (Figure 6) also shows that the length of the tube exhibited
significant influence on the settling process of the cohesive
sediment. The settling velocity of the mud interface decreases
much faster in the 30cm-long tubes compared with the 200cm-long
tube. Another interesting phenomenon is that the experiment in
the 200cm-long tube showed an abrupt change of mud interface
settling velocity which never appeared in the 30cm-long tubes;
(Figure 6a). As mentioned above, this abrupt change corresponds
to the transition from the stage of the flocculated settling to the
stage of sediment packing and consolidation. The smoothness of
the settling curve in the 30cm-long tubes implies that the packing
and consolidation process occurred at the same time of flocculation
settling. Consequently, there was no obvious transition between
these two settling stages. Another inference based on this is that
the longer the tube, the sharper of the transition between different
settling stages. In most of the coastal environments, for instance
the navigational waterways, the water depth usually ranges from
10m to 30m. The storm surge induced sediment suspension should
settle to seabed with a clear distinction between the settling stage
and the consolidation stage because the water depth is much larger
than 200cm. This feature shall be fully considered for the modeling
and computing of seabed evolution after storm surge. The mud
density history is shown in Figure 6b. It shows that mud density
increases faster in the 30cm-long compared with the 200cm-long
tube for most of the settling process. However, after a sufficiently
long time, the final density in the 200cm-long tube tends to exhibit
higher packed density, which implies that the water pressure upon
the mud interface may play a role on promoting the consolidation
process of the settled bed [32,39]. It is worth noting that although
the tube length has significant influence on the mud settling
process, it shows little influence on the highest settling velocity
of the mud interface, which usually appears within the beginning
60minutes, (Figure 6a). Therefore, 30cm-long tubes will be used for
all of the following experimental cases to obtain the highest settling
velocity (Figure 6).
Figure 6: Settling process of mud in various tubes:
a) Time history of the mud interface settling height.
b) Mud density history.
Case III: Initial mud concentration influence on sediment
settling
Mud concentration plays an important role in the flocculation
formulation, which finally dominates the settling velocity of the
cohesive sediment [20,40,41]. In order to systematically investigate
this influence, 15 mud samples with different initial concentrations,
ranging from 1kg/m3
to 500kg/m3
, were prepared and experiments
were carried out in the 30cm-long tubes. The settling process is
shown in Figure 7. The settling behaviors can be basically classified
into three groups according to the feature of the mud interface
history in Figure 7a Group I includes the initial mud concentration
of 20kg/m3
and 500kg/m3
; Group II includes the initial mud
concentration ranging from 20kg/m3
to 100kg/m3
and Group III
includes the initial mud concentration equal or lower than 10kg/
m3
. The settling velocities in Group I are much lower than that
of Group II mainly because of the high concentration feature of
the mud mixture, which brings forward the transition from the
flocculation settling stage to the packing and consolidation process.
Mud mixtures with such high concentrations can usually be found
near seabed [42] or after sediment re-suspension during very
strong storm surges [43,44]. Group II covers the mud concentration
of most of the coastal engineering applications [3,31,45,46,]. The
muds with initial concentration ranging from 20kg/m3
to 100kg/
m3
show similar settling processes, which have a smooth transition
between the two different settling stages. Flocculation plays an

important role during the settling in this group. For settling in
Group III, the flocculation of the sediment particles seems difficult
to form because of the extremely low sediment concentration and
the mixture kept turbid for a quite long time after the beginning of
the experiment. No clear mud interface was observed until the final
stage (Figure 7b-7c). According to the experimental observation,
mud concentration is an important factor determining both the
formation of flocculation and the visibility of the mud interface at
the top (Figure 7).
Figure 7: The settling process of sediment with different initial concentrations:
a) Time history of mud interface level.
b) Snapshot of the experiment at t=240min, the initial mud concentrations from the left to the right are 100kg/m3
, 90kg/m3
, 80kg/m3
, 70kg/
m3
, 60kg/m3
, 50kg/m3
, 40kg/m3
, 30kg/m3
, 20kg/m3
, 10kg/m3
, 6kg/m3
, 3kg/m3
and 1kg/m3
respectively.
c) Snapshot of the experiment at t=1800min.
d) The initial mud concentrations are the same as.
Table 2: The highest settling velocity of mud with different initial concentrations.
C0
kg/m3
ρ0 kg/m3 Sr cm/
min
Observed
Phenomenon
C0 kg/m3
ρ0 kg/m3
Sr cm/min Observed Phenomenon
1 1.001 0.005 no clear interface 60 1.037 0.26 with clear interface; clear upper fluid
3 1.002 0.012
no clear interface;
deposit at the bottom
70 1.044 0.23 with clear interface; clear upper fluid
6 1.004 0.022
no clear interface;
10 1.006 0. 16
no clear interface;
20 1.012 0.88
interface appears at
6cm; turbid upper
fluid
30 1.019 0.702
relatively clear
interface; light turbid
upper fluid
40 1.025 0.447
relatively clear
upper fluid
50 1.031 0.323
relatively clear
upper fluid
During the experiments, the highest settling velocity, Sr
, of the
mud with different initial concentrations, C0
, or initial mud density,
ρ0
, was calculated, see Table 2. The visibility of the mud interface
and the turbidity of the fluid upon the interface observed during
experiments were also described Table 2. Experimental results
show that the mud interface settling velocity does not change

monotonously with the mud concentration, see Figure 8. There
is an obvious peak value of the settling velocity corresponding
to an initial sediment concentration of 20kg/m3
. For the mud
concentration lower than 20kg/m3
, the particles were too sparse
to form large flocculation structures and particles settled slowly
and individually. On the contrary, in experiments with higher
concentrations,theflocculationstructuresweredenselydistributed
and the network were too strong to let the sediment settle down
quickly. The peak value of the settling velocity represents a balance
between these two mechanisms. Note that for the measurement
data for mud concentration lower than 10kg/m3, the mud interface
didn’t appear at the beginning stage of the experiment and the
settling velocities were calculated from the appearance of a later
readable mud interfaces. Because of the low concentration, the
collision of the particles was rare and the settling was dominated
by Brownian movement and single particle settling motion. The
flocculation failed to form at the beginning of the experiment. In the
experiments with initial sediment concentration between 10kg/m3
and 100kg/m3
, the highest settling velocities of the mud interfaces
ranged from 0.2cm/min to 0.9cm/min. For other cases with lower
or higher initial sediment concentrations, the settling velocity was
10-100 times lower. Therefore, the sediment concentration is a
key factor affecting the settling velocity of the cohesive sediment
(Figure 8).
Figure 8: Influence of initial sediment concentration
on the highest mud interface settling velocity.
Case IV: Salinity influence on sediment settling
The influence of salinity on the settling of cohesive sediment has
long been identified [24] and such influence has wide applications
in water treatment industry [47]. The saline matter dissolved
in water provides icons promoting the formation of flocculation
structures and consequently increase the settling velocity of
the sediment. On average, seawater in the world’s oceans has a
salinity of about 3.5% (35g/L). This means that every kilogram
(roughly one litre by volume) of seawater has approximately 35g
of dissolved salts (predominantly sodium (Na+
) and chloride (Cl-)
ions. To investigate the influence of salinity on the settling velocity
quantitatively, experiments were carried out under the same initial
mud concentration (50kg/m3
) but with different salinities. The
water salinity was adjusted with salt (NaCl) to 13 different values
ranging from 0 to 100% (mass percentage). The time history of the
sediment settling is shown with the mud density in Figure 9a. It
shows that the general settling tendencies under different salinities
are quite similar. This is due to the fact that for mud with initial
concentration of 50kg/m3, flocculation formed even with zero
salinity. Although the salinity didn’t show significant influence
on the final settling of the mud (Figure 9c), it exhibited obvious
influences at the beginning stage of the settling process (Figure 9b).
Such influences include the turbidity of the upper fluid, the clarity
of the mud interface and the sediment settling velocity as well. As
shown in Figure 9b, for sediments with salinity higher than 20%,
the experiments exhibited an easily recognizable mud interface
with a clear fluid upon, which means the ions provided by the salt
greatly enhanced the formation of flocculation [24,47] (Figure 9).
The highest settling velocity, Sr, of the mud with different salinities,
Cs was shown in Table 3, as well as descriptions on the visibility
of the mud interface and the turbidity of the fluid upon. Notice
that the settling velocities have relatively larger reading errors for
the cases with salinities less than 10% because the mud interface
was relatively obscure under the weak flocculation effect at the
beginning stage of the settling (Table 3).
The salinity can change the settling velocity (highest value
in time history) of the mud interface by a magnitude of nearly 2
times (Figure 10). The interesting phenomenon is that the mud
interface settling velocity does not change monotonously with
the salinity, which is quite similar as the influence from the mud
concentration shown in Figure 8. There is a lowest value of the
settling velocity corresponding to a salinity of 15%. For salinities
higher than 15%, the increase in salinity tends to increase the mud
interface settling velocity. This part agrees with the commonsense
reasoning based on the fact that ions provided by salt promote
the flocculation formation [47]. The settling velocity reached a
peak value of 4.9cm/min at a salinity of 70% and further increase
in salinity didn’t cause increase in settling velocity any more.
However, for the experiment with salinities ranging from 0 to 15%,
the increase in salinities tends to decrease the interface settling
velocity. The decrease in settling velocity with increasing salinity
is quite interesting because it implies that the role of added ions
on settling of cohesive sediment should be much more complex
than our original understandings. Our preliminary inference
here is that for experiments with salinities lower than 15%, the
flocculation effect is not intensive enough to capture all sediment
particles into the flocculation cluster and coarser particles tends to
settle firstly with a relatively high velocity, leaving part of the finer
sediments on the top. With the increase in salinity, the cohesive
sediments have to take time to form stronger flocculation clusters
at the very beginning of the experiment [48]. On the other hand,
the salinity promotes flocculation and accelerates the settling.
The counterbalance between these two mechanisms leads to the
appearance of a valley value at salinity of 15%. Of course, this is
only a rough deduction and the fundamental mechanism need to
be further investigated and hopefully, verified with experimental
observations under microscopes. Another fact to be noticed here is
that only one kind of substance (NaCl) was used to provide salinity
and ions, and the mud is also specific regarding its grain sizes and
chemical components. Much more experiments shall be carried out
to reproduce such phenomenon (Figure 10).

Figure 9: The settling process of sediment with different salinities and the same initial mud concentration (50kg/m3
) (a) time history of mud
density; (b) snapshot of the experiment at t=120min, the salinities from the left to the right are 100%, 70%, 50%, 40%, 35%, 30%, 25%, 20%,
15%, 8%, 4%, 2%, 1% and 0% respectively, the mark* represent the average seawater salinity; (c) snapshot of the experiment at t=2400min, the
salinities are the same as (b).
Table 3: The highest settling velocity of mud with different salinities.
Cs ‰ Sr cm/min Nxote Cs % Sr cm/min Note
0 0.447 no clear interface; turbid upper fluid 25 0.373 clear interface; clear upper fluid
1 0.343 relatively clear interface; turbid upper fluid 30 0.373 clear interface; clear upper fluid
2 0.327 relatively clear interface; turbid upper fluid 35 0.403 clear interface; clear upper fluid
4 0.32 relatively clear interface; light turbid upper fluid 40 0.41 clear interface; clear upper fluid
8 0.303 relatively clear interface; light turbid upper fluid 50 0.45 clear interface; clear upper fluid
15 0.287 clear interface; light turbid upper fluid 70 0.49 clear interface; clear upper fluid
20 0.323 clear interface; clear upper fluid 100 0.49 clear interface; clear upper fluid
Figure 10: Influence of salinity on the highest mud
interface settling velocity.
Conclusion
In order to investigate the settling of the cohesive sediment
in stationary water, a series of laboratory experiments were
carried out. The cohesive sediment/mud was sampled from the
coastal area of Huangmaohai Estuary of South China Sea and
settling processes were observed in settling tubes/columns with
the recorded time varying positions of the mud interfaces. Based
on the interface, the settling velocities and the mud densities for
each case were calculated and analyzed. The influence from the
flocculation phenomenon, the tube geometries, the initial mud
concentration and the salinities of the water were investigated
systematically [49]. The settling processes of the cohesive sediment
in quiescent water showed complex time-varying phenomena
during the laboratory experiments, which were divided into two
stages: the stage of the settling of the flocculated sediment and the
stage of the sediment packing and consolidation. Flocculation effect
accelerated the sediment settling by an order of 1,000 compared

with the settling velocity of individual sediment particles predicted
with the Stokes’ law. For sediment with initial concentration of
50kg/m3, the settling velocity of the mud interface reached as high
as 0.4cm/min. However, the settling velocity dropped dramatically
to 3×10-4cm/min during the final consolidation stage. There is an
abrupt change on the mud interface settling curve, which implies a
transition between these two stages [50]. Time-dependent settling
velocity was described with simple functions based on regression
analysis, which shows a linearly decreasing settling velocity with
time during the first stage and an exponential decreasing rate
during the second stage.
The tube diameter variation from 3.0cm to 8.6cm has little
influence on the settling process of the cohesive mud. However,
the length of the tube exhibited significant influence, except for
the beginning stage of the experiment. Experiments in tubes with
a length of 30cm showed much smoother transitions between the
two settling stages compared with the experiments in a 200cm-
long tube, which implied the packing and consolidation process
occurred at the same time of flocculation structure settling.
Mud concentration plays an important role in the flocculation
formulation, which finally dominating the settling velocity of the
cohesivesediment.Thereisaclearpeak value ofthesettling velocity
corresponding to an initial sediment concentration of 20kg/m3
.
Salinity exhibited obvious influences at the beginning stage of the
settling process, which include the turbidity of the upper fluid,
the clarity of the mud interface and the sediment settling velocity.
There is a lowest value of the settling velocity corresponding to a
salinity of 15% for mud with an initial concentration of 50kg/m3
.
Acknowledgement
We thank the support by the National Natural Science
Foundation of China (Grant no. 51009105), the Science Fund
for Creative Research Groups of the National Natural Science
Foundation of China (Grant no.51021004), the Natural Science
Foundation of Tianjin (Grant no. 12JCQNJC05600) and the Open
ResearchFundsfromStateKeyLaboratoryofHydraulicEngineering
Simulation and Safety (2012).
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