Seasonal variability of cohesive sediment aggregation in the bach dang–cam estuary, haiphong (vietnam

Geo-Mar Lett (2012) 32:103–121
DOI 10.1007/s00367-011-0273-8

ORIGINAL

Seasonal variability of cohesive sediment aggregation
in the Bach Dang–Cam Estuary, Haiphong (Vietnam)
Jean-Pierre Lefebvre & Sylvain Ouillon & Vu Duy Vinh &
Robert Arfi & Jean-Yves Panché & Xavier Mari &
Chu Van Thuoc & Jean-Pascal Torréton

Received: 6 April 2011 / Accepted: 19 December 2011 / Published online: 15 January 2012
# Springer-Verlag 2012

Abstract In the Bach Dang–Cam Estuary, northern Vietnam,
mechanisms governing cohesive sediment aggregation were
investigated in situ in 2008–2009. As part of the Red River
delta, this estuary exhibits a marked contrast in hydrological
conditions between the monsoon and dry seasons. The impact
on flocculation processes was assessed by means of surveys of
water discharge, suspended particulate matter concentration
and floc size distributions (FSDs) conducted during a tidal
cycle at three selected sites along the estuary. A method was
developed for calculating the relative volume concentration
for the modes of various size classes from FSDs provided by
the LISST 100X (Sequoia Scientific Inc.). It was found that all
FSDs comprised four modes identified as particles/flocculi,
fine and coarse microflocs, and macroflocs. Under the influence of the instantaneous turbulent kinetic energy, their proportions varied but without significant modification of their
median diameters. In particular, when the turbulence level
corresponded to a Kolmogorov microscale of less than

∼235 μm, a major breakup of flocs resulted in the formation
of particles/flocculi and fine microflocs. Fluctuations in turbulence level were governed by seasonal variations in freshwater discharge and by the tidal cycle. During the wet season,
strong freshwater input induced a high turbulent energy level
that tended to generate sediment transfer from the coarser size
classes (macroflocs, coarse microflocs) to finer ones
(particles/flocculi and fine microflocs), and to promote a
transport of sediment seawards. During the dry season, the
influence of tides predominated. The turbulent energy level
was then only episodically sufficiently high to generate
transfer of sediment between floc size classes. At low turbulent
energy, modifications in the proportions of floc size classes
were due to differential settling. Tidal pumping produced a net
upstream transport of sediment. Associated with the settling of
sediment trapped in a near-bed layer at low turbulent energy,
this causes the silting up of the waterways leading to the
harbour of Haiphong.

Responsible guest editor: D. Doxaran
J.-P. Lefebvre (*) : S. Ouillon
IRD, Université de Toulouse, UPS (OMP), UMR 5566 LEGOS,
14 av. Edouard Belin,
31400 Toulouse, France
e-mail:
V. D. Vinh : C. Van Thuoc
Institute of Marine Environment and Resources (IMER),
Vietnam Academy of Science and Technology (VAST),
246 Danang Street,
Haiphong City, Vietnam
R. Arfi
IRD, Université Aix-Marseille 2, UMR 6535 LOPB,

Centre d’Océanologie de Marseille,
Luminy,
13288 Marseille cedex 09, France

J.-Y. Panché
IRD, US 191 IMAGO,
BP A5,
98848 Nouméa cedex, New Caledonia
X. Mari : J.-P. Torréton
IRD, Université Montpellier II, UMR 5119 ECOSYM,
cc 093, Place Bataillon,
34095 Montpellier, France
Present Address:
X. Mari
Institute of Biotechnology, Environmental Biotechnology
Laboratory,
18 Hoang Quoc Viet Street, Cau Giay,
Hanoi, Vietnam


104

Introduction
In Vietnam, the silting up of the Red River delta constitutes
a main concern for the authorities due to its particularly
negative impact on traffic in the country’s second biggest
harbour, Haiphong. During the 1980s, the construction of
two dams across the Red River induced a reduction of
sediment inputs to the coast, which caused fast and locally
intense erosion in the bay of Haiphong. Although the impact

of dam constructions has long been investigated worldwide
in terms of total suspended particulate matter concentration
(SPMC; e.g. Uncles et al. 1988; Vörösmarty et al. 2003;
Mantovanelli et al. 2004; Wolanski et al. 2006; Scully and
Friedrichs 2007; Winterwerp 2011), less is known about
interrelations with floc size distributions (FSDs).
Flocculation processes depend on various factors including the electric charge on particles (ζ potential), organic
matter content, suspended matter availability, and turbulent
shear rate (e.g. Lunven and Gentien 2000; Lunau et al.
2006; Mietta et al. 2009). In estuaries, the high variability
in forcing (Verney et al. 2009) and the impact of biological
factors make the behaviour of cohesive sediments even
more complex and still not well understood (Winterwerp
2011). Organic bindings such as those due to transparent
exopolymeric particles (TEPs; e.g. Passow et al. 2001; Wetz
et al. 2009; Mari et al. 2011) or dissolved exopolymeric
substances (e.g. Bhaskar et al. 2005) can generate macroflocs of various sizes and strengths. The abundance of
mineral particles can affect the process of aggregation, except for SPMC in the range 50–250 mg L−1 (cf. Milligan
and Hill 1998). Turbulence affects flocculation processes by
increasing the collision frequency between aggregates, and
also by generating a shear stress at the surface of aggregates
that limits their size increase in the same order of magnitude
as the smallest turbulent eddies (van Leussen 1997; Jarvis et
al. 2005). Sudden disaggregation of flocs beyond a threshold of turbulent intensity was found by Chen et al. (2005) in
the Scheldt Estuary.
Turbulence level and differential settling of aggregates of
various sizes and densities are usually thought to dominate
the aggregation and breakup processes (e.g. van Leussen
1994; Winterwerp 2006; Kumar et al. 2010). Lick et al.
(1993) proposed a model where the median apparent diameter of aggregates Dv is related to the product of the

turbulence-induced shear rate (G) and SPMC through a
power law Dv 0 α (G·SPMC)β, where α and β are constants.
Manning and Dyer (1999) compared this equation to a more
sophisticated formula and proved that it was reasonably
accurate.
The governing action of tides on flocculation and differential settling has been identified in numerous estuaries of
temperate regions. In the Tamar Estuary (UK), for example,
the balancing of re-suspension and differential settling at

Geo-Mar Lett (2012) 32:103–121

high tide slack water is revealed by a close relationship
between SPMC and Dv (Uncles et al. 2010). In this estuary,
moderate levels of turbulence promote collisions between
flocs, and transfers from microflocs to macroflocs enhanced
by organic bindings. During spring tide, a highly concentrated benthic suspension layer is generated near the bed that
contributes to dampening the turbulence within that layer.
The generation of coarser macroflocs results in a marked
bimodality in floc size distribution (Manning 2004).
In shallow estuaries, an asymmetry between ebb tide and
flood tide caused by nonlinear tidal interaction and by
astronomic tides can be observed. Since the celerity of the
tidal wave increases with increasing water depth, the tide
propagates faster at high water than at low water. This
causes the shape of the tidal wave to distort as it moves
landwards; the rise of the tide becomes faster than its fall
and, consequently, the peak current is faster at flood tide
than at ebb tide. An increase in tidal level counterbalances
the liquid budget, so that the overall balance of the tidal flow
is essentially nil. This asymmetry results in a larger sediment transport upstream at flood tide than downstream at

ebb tide (Dronkers 1986). This landward transport of sediment caused by ‘tidal pumping’ has been described by
Geyer et al. (2001) for the Hudson River Estuary during
spring tides. At the site of the present study near Haiphong,
the Bach Dang–Cam Estuary is under the influence of a
diurnal tidal regime; therefore, this mechanism is likely to
be enhanced (Hoitink et al. 2003).
In monsoon-dominated rivers, the freshwater discharge
exhibits a marked seasonality that affects turbulence, salt
stratification in the water column and tidal wave propagation in the estuary. SPMC can be related to turbulence
through bed erosion, and to inputs from the catchment
basins. The balance between tidal propagation and freshwater discharge determines the amplitude and direction of the
current flow, and the level and advection of turbulence in the
water column, both impacting on the transport and settling
of sediment (Dyer 1995). Water column stratification can
constitute an impediment to the vertical advection of turbulence (Geyer 1993; Uncles and Stephens 1993; Jay and
Musiak 1994; Peters 1997; Scully and Friedrichs 2003),
and can prevent the advection of sediment across the freshwater–saltwater interface, resulting in the trapping of sediment in the upper freshwater or lower saltwater layers. As an
example, the Mekong and Red River estuaries experience
similar seasonal forcing. Solid discharge by the Mekong
River (170×106 metric tons year−1) is similar to that of the
Red River (Milliman and Meade 1983), both being characterized by silt-dominated material. In the southern branch of
the Mekong delta, the salt wedge is observed near the mouth
of the estuary during the wet season. During the dry season,
the tidal asymmetry increases and the salt wedge propagates
further into the estuary. The sediment load budget indicates


Geo-Mar Lett (2012) 32:103–121

a tidally averaged flux to the sea of at least 95% during the

wet season (Wolanski et al. 1996), and tidal pumping from
the coastal area to the estuary during the dry season, coupled
with a balancing of settling out at slack tide and reentrainment at higher current speeds. In the Mekong River
delta, non-biological flocs are found only in brackish water
and remain non-flocculated in the freshwater layer. This
results in a seaward transport of particles as ‘wash load’ in
the near-surface freshwater layer, and a tidally varying
transport of near-bed flocculated sediments. No significant
difference in floc size was observed between the wet and dry
seasons, and examination of the aggregates confirmed their
non-biological origins (Wolanski et al. 1998).
In the silt-dominated Fly River Estuary, Papua New
Guinea, smaller flocs have been found in the near-surface
layer and larger flocs in the near-bed saltwater layer.
Nevertheless, the impact of turbulence on floc size was
identical in the freshwater and saltwater layers (Wolanski
and Gibbs 1995). In the Yangtze River Estuary, flocculation
triggered by biological processes has been observed in
freshwater and brackish water (Guo and He 2011). The
varying hydrodynamics in the estuary generated strong
spatiotemporal fluctuations in FSDs, associated with strong
variations in macroflocs (defined as D≥D75 in Guo and He
2011, where D is diameter), moderate variations in coarse
microflocs (D50 complexity of flow in the Yangtze River Estuary, no clear
relation was obtained between vertical variations in current
flow and FSDs. In contrast to the Mekong delta, however,

Fig. 1 The Red River delta,
Vietnam

105

larger flocs were found episodically near the surface, where
the turbulence was weak.
The present study aimed at filling the gap in existing
knowledge on the hydrosedimentary functioning of the
northern tributary of the Red River delta. Special attention
was paid to the mechanisms responsible for the ongoing
silting up of the waterways leading to the harbour of
Haiphong. Because the size and settling velocity of suspended
aggregates are salient parameters for sediment transport, the
focus was on the response of floc size distributions to various
controlling factors characteristic of this monsoon estuarine
setting, notably the impact of the seasonally fluctuating
freshwater discharge on the tidal propagation in terms of
turbulence level, the presence/absence of a saltwater layer,
and tidal pumping.

Study area and physical setting
The Red River (Song Hong) is situated at 20–25°30′N and
100–107°10′E and has a total catchment area of
169,000 km2. It bifurcates into numerous distributaries feeding the Red River delta, and enters the Gulf of Tonkin
through six main mouths (Fig. 1). Milliman and Meade
(1983) estimated that the Red River brought approx. 1%
of the world’s solid discharge to the ocean (160×106 metric
tons year−1), ranking it as 9th worldwide. However, the
authors recognized that this estimate was based on an incomplete database. Moreover, the Hoa Binh dam constructed in the late 1980s has proved to efficiently trap

106

sediments (Le et al. 2007). Data from long-term monitoring
(1960–2008) indicate that the annual suspended sediment flux
was on average 113×106 metric tons before dam construction,
and since then has decreased to on average 49×106 metric
tons (Dang et al. 2010). This sediment reaches the Gulf of
Tonkin through several distributaries, including the Cam
River that is connected to the easternmost branch of the delta.
The Red River delta is under the influence of a tropical
monsoon climate. Annual rainfall in the region is close to
200 cm, of which nearly 90% falls during the summer
monsoon. The wind direction is dominantly from the south
in April–September (wet season), and from the northeast in
October–March (dry season). Typical of a monsoondominated river, the Red River discharge is highly seasonal.
Based on data for the years 1956–1998, discharge averaged
1,200 and 14,000 m3 s−1 at the Son Tay station (Fig. 1) in
the dry and wet seasons respectively (van Maren and
Hoekstra 2004). Approximately 90% of the total sediment
load is transported during the wet season (June–October).
Nowadays, inter-annual suspended sediment transport (Son
Tay) varies by as much as factor 4 (van Houwelingen 2000).
The tides in the Gulf of Tonkin are largely diurnal, due to
resonance of the O1 and K1 diurnal components. In the
vicinity of the Bach Dang–Cam Estuary, the diurnal tidal
regime shows a maximum amplitude of 4 m. Because of
sheltering by the island of Tonkin (Hainan), wave action is
reduced in the northern coastal sector of the Red River delta,
more under the influence of tidal currents.
The Bach Dang–Cam Estuary is located on the easternmost branch of the Red River, and fed by two relatively

wide main tributaries with shallow lateral shoals and deep
narrow channels: the Bach Dang and Cam.
Fig. 2 The Bach Dang–Cam
Estuary, with the locations of
the three sampling stations

Geo-Mar Lett (2012) 32:103–121

Materials and methods
Surveys
Two field campaigns were conducted during the wet season
of 2008 (July) and the dry season of 2009 (March). Three
stations situated at key spots of the estuarine system were
monitored during 24-h surveys corresponding to one spring
tidal cycle: one station was located upstream on the Cam
River, another was upstream on the Bach Dang River, and
yet another was close to the mouth of the estuary near the
Dinh Vu industrial area. These are hereafter referred to as
the ‘Cam’, ‘Bach Dang’ and ‘Dinh Vu’ stations respectively
(Fig. 2). The tidal amplitude was approx. 2 m during both
campaigns, similar to the mean annual tidal amplitude.
During the dry season, bed samples were taken at
each of the three stations by means of a clamshell-style
dredge. The deflocculated grain size distributions were
assessed in the laboratory by use of a laser particle size
analyzer in the range 0.05 to 878 μm (Mastersizer S,
MALVERN Instruments).
During each survey, key physical parameters were monitored every 3 h from aboard a 12-m flat-bottom vessel.
Instantaneous cross-sectional velocity profiles were
assessed using a 600 kHz acoustic Doppler current profiler

(ADCP RDI Workhorse in bottom tracking mode) configured for a 0.5 m bin size. Immediately after completing a
cross section, the ship was anchored at the location
corresponding to the maximum depth of the cross section
(determined by echo sounding), for vertical profiling and
sampling (cf. below). Discharge at each cross section was
estimated by WinRiver II software (RD Instruments).


Geo-Mar Lett (2012) 32:103–121

107

Water discharge and hydrodynamic parameters
The averaged river flow <Q> over a tidal cycle was estimated
from the integration of the instantaneous discharge Q(t) in a
24-h series of N measurements (N09) according to:
hQi ẳ

N
1
X
1
Qi ỵ Qiỵ1
tiỵ1 ti ị
tN t1 iẳ1
2

1ị

This steady component is defined as the fluvial component of the discharge, and the fluctuating part, Q′0Q–<Q>,

as the tidal component. The tidal asymmetry was defined as
the ratio of the duration of the observed flood tide (Q<0) to
the duration of the entire tidal cycle, expressed in percentage. In order to precisely estimate the moments for observed
slack tides, the data were interpolated by cubic spline
(Fig. 3).
During the wet season, the wind speed was obtained from
hourly recordings by the Vietnamese Meteorological Service in the city of Haiphong. During the dry season field
campaign, a Davis weather station was deployed on the roof
of the marine station of the Institute of Marine Environment
and Resources (IMER) at Do Son (Fig. 2), at the entrance of
the bay. Air temperature (under shelter), as well as wind
speed and direction were recorded every 30 minutes.
The turbulent kinetic energy (TKE) dissipation rate
(ε, m 2 s −3 ) integrated over the water column was
expressed as a function of wind and averaged current
velocity (van der Lee et al. 2009):
" ẳ kb

uav
w3
ỵ ks
h
yh

where kb and ks are the bottom and surface drag coefficients respectively, uav the depth-averaged water velocity, w the wind velocity, and ψ the ratio between the
water and air density.
The Kolmogorov microscale (lk, μm) yields an estimate
of the smallest turbulent eddies, and was calculated from the
kinematic viscosity of water (ν) and from integrated over
the water column (van Leussen 1997):

lk ẳ



n3
"

1=4

3ị

The turbulence-induced shear rate G (s1) is given by:
Gẳ

n
lk 2

4ị

At each station, two vertical profiles of temperature,
salinity and turbidity (optical backscattering sensor at
l0880 nm, Seapoint turbidimeter) were recorded by
means of a Seabird SBE19+ CTD probe. Due to strong
variations in salinity, a precise synchronization of the
different sensors and pressure-to-depth conversion was
carefully implemented.
Following Simpson et al. (1990), water column stratification was estimated in terms of the potential energy anomaly ϕ (J m−3), which represents the amount of energy needed
to mix a unit volume of water column. This parameter
accounts not only for saltwater input but also for other
factors (e.g. heat flux, wind, rain) commonly influencing

stratification:

2ị
tị ẳ

h
g X
ẵ tị w z; tịzz
h zẳ0 w

5ị

where ρw is the water density, g the acceleration due to
gravity, and h the water depth, ρw being the value averaged
h
P
over the water column (w tị ẳ 1h
w z; tịz).
zẳ0

Suspended particulate matter concentration

Fig. 3 Tidal asymmetry calculated as the ratio of the flood tide
duration and the tidal cycle duration (durations estimated from the
intercept of extrapolated discharge (line) with the zero ordinate; extrapolated discharge obtained by fitting a spline curve to measurements
of discharge, circles)

Optical backscattering sensors have been widely used to
assess total SPMC based on turbidity measurements (e.g.
Creed et al. 2001; Fugate and Friedrichs 2002; Hoitink

2004; Voulgaris and Meyers 2004; Jouon et al. 2008). In
this study, each turbidity depth profile was measured in the
main channel a few minutes after the velocity cross section;
this slight delay is negligible compared to the tidal cycle of
24 h and the slow variations in river discharge.
Water samples were collected at 3-h intervals 1.5 m below the surface and 1.5 m above the bed using Niskin
bottles. SPMCs (mg L−1) were determined by filtering
150–500 mL (depending on turbidity) subsamples through


108

Geo-Mar Lett (2012) 32:103–121

pre-weighed polycarbonate Nucleopore filters (porosity
0.4 μm), as recommended by Fargion and Mueller (2000).
Filters were rinsed three times with 5.0 mL distilled
water, dried for 24 h at 75°C in an oven, and then
stored in a desiccator until weighing on a high-precision
(5 μg) electrobalance. Data for duplicate or triplicate
samples were averaged in each case.
The voltage delivered by the turbidity sensor was
converted by a Seapoint routine into turbidity (in FTU) using
laboratory-determined calibration parameters (Wass et al.
1997; Bunt et al. 1999). Since this conversion assumes that,
in the absence of reflecting particles, turbidity is equal to zero,
SPMC (mg L−1) was calculated from the relationship:
turbidity ¼ mSPMC

These two components are defined as (cf. Geyer et al.

2001) the advective component of the tidally averaged
sediment flux < qa zị >ẳ< uz; t ị > < SPMCðz; t Þ > ,
and the tidally driven component of the tidally averaged flux
< qp zị >ẳ< u0 z; t Þ Á SPMC0 ðz; t Þ > in the vicinity of the
deepest location of the channel.
The discharge and sediment transport per unit area S at
the sampling station, q(t) (m3 s1) and qs(t) (g s1) respectively, were calculated as:
qtị ẳ

qs tị ẳ

Suspended particulate matter discharge
In each case, the velocity profile corresponding to the location of the turbidity profile was extracted from the crosssectional set. It was assumed that the CTD profile was
representative of the same location, i.e. any drift of the ship
was considered negligible, and the two scale depths were
matched between the surface and the bottom. The velocity
profiles achieved with a bin width of 0.5 m were interpolated at the depths of CTD profiling. Sediment flux fs(z,t)
(g m−2 s−1) was calculated as:
fs z; t ị ẳ uz; t ị SPMCðz; t Þ

ð7Þ

This comprises the advective sediment flux and the tidal
pumping of sediment. By expressing both SPMC(z,t) and u
(z,t) as the sum of their tidally averaged components and the
deviation from the tidally averaged values, the tidally averaged sediment flux becomes:
< fs >ẳ< < u > ỵu0 ị < SPMC > ỵSPMC0 ị >

8ị

where the brackets < > indicate time-averaging over one
tidal cycle, and the prime indicates the deviation from the
tidally averaged value.
Since << u > ÁSPMC0 > ¼ < u > Á < SPMC0 > ¼ 0
a n d < u0 Á < SPMC >>¼< u0 > Á < SPMC >¼ 0 , E q .
(8) becomes:
< fs >¼< u > < SPMC > ỵ < u0 SPMC0 >

ð9Þ

ð10Þ

and

ð6Þ

where m is a proportionality factor.
At each station and separately for each campaign,
second-order linear regression analyses were conducted
on datasets for near-surface and near-bottom layers.
Pooling these data per station and season revealed coefficients of determination that were sufficiently high to
justify using one average conversion factor per station
and campaign.

h
S X
uðz; tÞΔz
h zẳ0

h

S X
uz; tịSPMCz; tịz
h zẳ0

11ị

The total sediment load over the whole cross section of the
river, Qs(t), was obtained by assuming that the ratio qs(t)/q(t)
did not vary significantly across the whole cross section:
Qs tị qs tị
ẳ
Qtị
qtị

12ị

The sediment load averaged over one tidal cycle was
computed according to:
hQs i ẳ

N 1
X
Qsiị ỵ Qsiỵ1ị
1
tiỵ1 ti ị
tN t1 iẳ1
2

13ị

Floc size distribution
Immediately after each CTD profile, a depth profile of FSD
and concentration was conducted using an in situ laser
scattering and transmissometry instrument (LISST 100X,
Sequoia Scientific Inc.; e.g. Traykovski et al. 1999; Agrawal
and Pottsmith 2000; Mikkelsen and Pejrup 2000; Jouon et
al. 2008). The LISST of type C enables measurement of
volumetric particulate concentration in 32 logarithmically
spaced size classes ranging from 2.5 to 500 μm, with attenuation at l0660 nm. In view of the high turbidity in the
study area, an optical path reduction module of 90% was
employed, and the measurements corrected accordingly.
The mean apparent diameter Dv was calculated for every
FSD. D v was determined as the apparent diameter
corresponding to 50% of the cumulative volume concentration
of aggregates.
Expressed on a log normal scale for the apparent diameter, each FSD was decomposed into a mixture of 25 irregularly spaced Gaussian curves, using the expectationmaximization (EM) algorithm of Tsui (2009) based on a
maximum likelihood criterion. A non-supervised spectrum
analysis was applied: the Gaussian curves were sorted by


Geo-Mar Lett (2012) 32:103–121

increasing modal diameter, and they were then progressively
merged as partial components until the mid-height position
met the boundary condition for a given component: individual clay/silt particles and flocculi (<30 μm), fine (<105 μm)
and coarse (<300 μm) microflocs, and macroflocs
(≥300 μm). As the size distribution range of the LISST
100X (type C) is truncated at 500 μm, the macrofloc mode
is not fully defined and the windowed mode is used as an
indicator (Fig. 4). For each mode, two parameters were

calculated: its apparent median diameter Dv, and its relative
volume concentration (RVC, %) defined as the ratio of its
volume concentration to the cumulative volume concentration of all modes.

Results
The main results obtained during the wet and dry seasons at
the three stations and averaged/integrated over the tidal
cycle and water column are summarized in Table 1.
Sediments
Bottom sediments
Grain size distributions of bottom sediments were similar at
the two upstream stations (Bach Dang River and Cam
River), characterized by relatively high clay to fine silt
contents (Fig. 5). By contrast, bottom sediments at the
coastal Dinh Vu station were considerably more enriched
in fine sand (Wentworth scale).

109

Suspended particulate matter
The factor m used to convert turbidity into SPMC had an
average value (pooling all seasons and stations) of 1.54
(1.62 wet season, 1.47 dry season; Table 2). The normalized
bias for determination of SPMC averaged 2.8% and its
standard deviation 5.4% in the dry season, and less in the
wet season.
During the wet season, highest SPMCs averaged over the
tidal cycle reached 214 and 200 mg L−1 at the Dinh Vu and
Cam stations respectively, contrasting with only 128 mg L−1
at the Bach Dang station. During the dry season, the

corresponding values showed reductions by factors 2.6 and
2.9 at the Bach Dang and Cam stations respectively, and by
factor 4.8 at the Dinh Vu station. The relative variation in
SPMC, defined as (SPMCmax−SPMCmin)/mean SPMC, did
not differ markedly between the two seasons, but always
remained minimal at the Cam station.
At the Cam station during the wet season, SPMCs were
homogeneous throughout the water column. Similarly low
SPMCs were recorded at the beginning of ebb tide and at
flood tide. They increased slightly at flood tide and more
significantly at ebb tide. Maximum SPMC was attained in
the second half of ebb tide (Fig. 6). During the dry season,
SPMCs were lower at the beginning of flood tide than at ebb
tide. At flood tide and during the first half of ebb tide, a low
SPMC near-bed layer was observed.
At the Bach Dang station during the wet season, SPMCs
were homogeneous throughout the water column, except at
the end of ebb tide and beginning of flood tide when a low
SPMC near-bed layer appeared (Fig. 6). Lowest SPMC was
recorded at the beginning of ebb tide and highest SPMC
near the bed at the beginning of flood tide. During the dry
season SPMCs were very low, with a slight increase in the
second half of ebb tide and at flood tide, associated with the
formation of a turbid near-bed layer.
At the Dinh Vu station during the wet season, SPMC
reached its maximum value at mid-flood tide and, associated
with peak discharge, at ebb tide (Fig. 6). Thereafter, the
values decreased, associated with the formation of a turbid
near-bed layer that persisted until slack water of low tide.
During the dry season, the pattern was similar but more

marked than that at Bach Dang: the overall turbidity was
higher at flood tide than at ebb tide, accompanied by the
appearance of a high-turbidity near-bed layer at flood tide.
Minimum SPMC values were recorded at the beginning of
ebb tide.
Median apparent diameter

Fig. 4 Decomposition of an FSD into a particle/flocculus mode, a
microfloc mode comprising fine and coarse components, and part of a
macrofloc mode truncated by the LISST (dashed lines)

An increase in Dv averaged over one tidal cycle and over the
water column was observed between the wet and dry seasons at each station, although less markedly at the Bach


110

Geo-Mar Lett (2012) 32:103–121

Table 1 Physical parameters per station and field campaign: discharge
(Q, liquid), sediment load (Qs, solid), advective (qa) and tidal pumping
(qp) sediment fluxes (calculated positively downstream), tidal asymmetry factor, potential energy anomaly (ϕ), salinity (Sal.), Kolmogorov
microscale (lk), suspended particulate matter concentration (SPMC),
Cam

<Q> (m s−3)
<Qs> (metric tons day−1)
 
hqa i À qp ðg mÀ2 sÀ1 Þ

Tidal asymmetry (%)
<ϕ> (J m−3)


Sal:

<lk> (μm)

À
Á
SPMC mg LÀ1

À À3
Á
G Á SPMC 10 mg LÀ1 sÀ1
 
Dv ðμmÞ
RVC of microfloc mode (%)

G·SPMC scale and microfloc RVC (relative volume concentration).
The < > brackets indicate averaging over a tidal cycle, and the overlines integration along the water column. Minimum and maximum
values are indicated in some cases
Bach Dang

Dinv Vu

Wet season

Dry season

Wet season

Dry season

Wet season

Dry season

806
18,290
119.8–30.8
24
0.2 (0.1–0.4)
0.0 (0.0–0.2)
323 (201–483)
200 (133–287)
3 (0.7–7)
60
9.0 (5.1–13.1)

181
1,200
9.2–1.1
45
3.5 (0.2–17.8)
1.2 (0.1–7.0)
390 (266–779)
70 (22–91)
0.7 (0.04–1)
83

9.5 (5.9–19.4)

431
5,540
51.2–5.4
37
4.9 (0.0–28.3)
0.7 (0.1–3.0)
406 (276–551)
128 (41–179)
0.9 (0.2–2)
104
12.4 (3.7–52.3)

123
−720
12.4–1.8
45
31.9 (3.2–68.6)
5.8 (1.9–13.9)
482 (337–736)
50 (21–105)
0.3 (0.08–0.8)
106
7.5 (4.6–15.0)

988
23,030
7.9–3.4
37

4.3 (0.0–18.7)
2.0 (0.1–7.6)
293 (181–459)
214 (56–372)
4 (0.3–8)
67
23.4 (11.8–56.3)

175
−1,920
1.64–4.0
43
23.4 (1.8–59.1)
15.0 (7.0–23.7)
438 (256–950)
45 (23–72)
0.4 (0.05–0.7)
110
8.7 (5.6–16.1)

Dang station (Table 2). There was a consistent linear relationship (R2 00.82) between <lk> and <Dv> (data not
shown).
During the wet season at the Cam station, Dv values were
rather homogeneous throughout the water column (Fig. 7).
Dv was larger at flood tide than at ebb tide. From the end of
ebb tide until low tide slack water, the smallest Dv values
were found near the bed. The pattern was reversed at the
Bach Dang station, with larger Dv at ebb tide than at flood
tide. Larger Dv were recorded near the surface at high tide
slack water and at late ebb tide. A decrease in Dv was

observed at mid-ebb tide. At the Dinh Vu station at flood
tide, the water column was characterized by small Dv in the
upper layer and larger Dv near the bed. At ebb tide, the
distribution of moderate Dv values was more homogeneous
in the water column, with values decreasing at mid-ebb tide.
During the dry season at the Cam station, a near-bed
layer about 2 m thick was observed during the whole tidal
cycle, except at mid-ebb and mid-flood tide (Fig. 7). In this
layer, the Dv values were smaller than in the upper part of
the water column, with the exception of large Dv being
recorded at flood tide. Large Dv were also found near the
surface at mid-ebb tide. At the Bach Dang station, large Dv
were observed near the surface at low tide slack water, and
near both the bed and the surface at high tide slack water. Dv
values were larger at flood tide than at ebb tide, the smallest
values occurring in the lower part of the water column from
high tide slack water to mid-ebb tide. Water column distributions of Dv were most homogeneous at mid-ebb and midflood tide. At the Dinh Vu station, the patterns were rather
similar at ebb tide and flood tide, with large Dv near the bed
and small Dv near the surface at slack tide, and homogeneous
distribution of Dv values in the water column at mid-tide.

Table 2 Proportionality factor
m obtained per station and
campaign, where turbidity
(FTU) 0 m SPMC (mg L−1)

Fig. 5 Deflocculated size distributions of bed samples obtained during
the dry season at the Cam, Bach Dang and Dinh Vu stations

Cam
Bach Dang
Dinh Vu

Wet
season

Dry
season

1.60
1.59
1.67

1.43
1.51
1.46


Geo-Mar Lett (2012) 32:103–121

111

Fig. 6 SPMC during one tidal cycle at the Cam (top), Bach Dang (middle) and Dinh Vu (bottom) stations during wet (left) and dry (right) seasons

Floc size distribution
The Dv of the four components obtained from the analysis
of all measured FSDs combined were very consistent:

<10 μm, 30–50 μm, 130–170 μm, and >350 μm. The first

component is here defined as being constituted of individual
particles and flocculi. Following Dyer and Manning (1999),
the second and third components are defined as belonging to

Fig. 7 Dv during one tidal cycle at the Cam (top), Bach Dang (middle) and Dinh Vu (bottom) stations during wet (left) and dry (right) seasons


112

fine and coarse microflocs; the fact that fine aggregates are
likely to withstand higher levels of shear stress than are
larger aggregates legitimates the merging of these two components into a distinct microfloc mode. The last component
is defined as macroflocs, consistent with, for example,
Burban et al. (1990) who defined marine snow as being
larger than 500 μm.
The median apparent diameter for the particle/flocculus
mode exhibited a high stability at the three sites and during
the two seasons (mean Dv 09.2 μm, σ2 00.60 μm). In the
present paper, the focus on the microfloc mode circumvents
the bias associated with median size being calculated over a
range of multimodal LISST spectra, or with a major part of
the spectrum extending beyond the maximum size detected
by the LISST.
At the Cam station during the wet season, the RVC of coarse
microflocs exceeded that of fine microflocs at flood tide and at
the beginning of ebb tide, while the total RVC (fine+coarse
components of microflocs) remained almost constant (∼5%;
Fig. 8). The proportion was reversed for the remainder of ebb
tide, with microfloc RVC increasing from 5% to 11–13% at late
ebb tide. During the dry season, the coarse microflocs occupied

a larger volume than the fine microflocs throughout the tidal
cycle. Microfloc RVC initially increased and then decreased at
ebb tide, ranging from 5% to 19%. At flood tide, the values
remained constrained between 7% and 8%, exceeding those
recorded at flood tide during the wet season.
At the Bach Dang station during the wet season, the
coarse microfloc RVC always exceeded that of fine microflocs, except for one occasion at flood tide when the latter
reached 35%. At ebb tide, the weak variation in microfloc
RVC (6–8%) was due to slight fluctuations in the coarse
microfloc component, the fine microfloc RVC remaining
nearly constant. At flood tide the microfloc RVC increased,
whereby the fine component reached 35% and the coarse
component 18% at mid-flood tide. At this stage, the FSDs
differed strongly between the 3-m-thick near-surface freshwater layer and the near-bed saltwater layer, whereby fine
microflocs and particles/flocculi predominated in the former. Thereafter, both the fine and coarse microfloc components decreased to reach 1 and 5% respectively at the end of
flood tide. During the dry season, the volume of coarse
microflocs always exceeded that of fine microflocs. The
variation in microfloc RVC at ebb tide was similar to that
observed during the wet season, although more pronounced:
this encompassed an increase from 5% to 15%, followed by
a decrease to 6%, also mostly due to coarse microflocs. Near
low tide slack water, there appeared a 4-m-thick near-bed
layer in which the proportion of macroflocs was higher than
in the remainder of the water column. At flood tide, a similar
but less marked variation in microfloc RVC occurred, with a
maximum of approx. 9% at mid-flood tide. The fine microfloc RVC varied only weakly (approx. 2%).

Geo-Mar Lett (2012) 32:103–121

At the Dinh Vu station during the wet season, a strong

increase in particles/flocculi (5%) together with fine and
coarse microflocs (16 and 18% respectively) occurred at
the beginning of ebb tide. The particle/flocculus mode and
the fine microfloc component were more pronounced in the
2.5-m-thick near-surface freshwater layer, their RVCs
reached 3 and 38% respectively at mid-ebb tide. The coarse
microfloc RVC varied only slightly (14–18%) during most
of the ebb tide phase, with two exceptions: the values
decreased at maximum discharge (9%) and at the end of
ebb tide (9%). At flood tide there was an initial increase in
the RVCs of microflocs—from 4% to 9% and from 8% to
12% for the fine and coarse components respectively—then
a decrease of the two modes.
During the dry season, variations in microfloc RVC
remained limited (between 6 and 10%) at the Dinh Vu
station. Although one order of magnitude smaller, the RVCs
of particles/flocculi had patterns similar to those of fine and
coarse microflocs (Fig. 8). At ebb tide, the fine microfloc
RVC decreased steadily from 4% to 1%. A decrease in
coarse microflocs was recorded at flood tide simultaneously
with a maximum of discharge, followed by an increasing
and then decreasing pattern. At mid-ebb tide, the macrofloc
RVC decreased with depth, reaching a minimum at about
1 m above the bottom. Near low tide slack water, there was a
strong presence of particles/flocculi and microflocs; coarse
microflocs were found mainly in a 2.5-m-thick near-bed
saltwater layer. At flood tide, the fine microfloc RVC
remained constant (2%) but that of coarse microflocs decreased steadily from 8% to 5%. At high tide slack water, a
4-m-thick near-surface freshwater layer exhibited a high
macrofloc RVC. The difference between the RVCs of microflocs in the freshwater and saltwater layers was less marked

than at low tide slack water.
Hydrology
Discharge
Discharge exhibited a marked tidal influence at all stations
during both seasons (Fig. 9). The current flow averaged over
one tidal cycle showed high seasonal variations. The Cam
station is under a dominant riverine influence but is also
impacted by the tide, and would be classified as an upper
estuarine site in both seasons following the schemes of
Dionne (1963) and Dalrymple et al. (1992).

Fig. 8 Salinity (PSU) and volume concentration ratios for (from left to b
right) modes of particles/flocculi, fine and coarse components of
microflocs, and macroflocs at (from top to bottom) mid-ebb tide, low
tide slack water, mid-flood tide and high tide slack water at the coastal
Dinh Vu station during the dry season. Horizontal solid line Freshwater–saltwater interface


Geo-Mar Lett (2012) 32:103–121

113


114

Geo-Mar Lett (2012) 32:103–121

Fig. 9 Discharge measured (circles) and extrapolated (lines) during one tidal cycle at the Cam (top), Bach Dang (middle) and Dinh Vu (bottom)
stations during wet (left) and dry (right) seasons (for more information, see Fig. 3)

Discharge was about four times higher in the wet
season than in the dry season at all stations. The
budgets indicate water losses via channels, mangroves

and wetlands, amounting to 20 and 42% of the total
inputs in the wet and dry seasons respectively
(Fig. 10a).

Fig. 10 a Discharge budget Q (m3 s−1) and b solid flux (metric tons day−1) during wet (black) and dry (white) seasons


Geo-Mar Lett (2012) 32:103–121

115

Table 3 TKE dissipation rates (10−5 m2 s−3) due to wind (εwind) and
discharge (εdischarge) averaged over one tidal cycle (extreme values
within brackets)
Wet season
εwind

Dry season
εdischage

εwind

εdischage

Cam
1.8 (0.4–3.7) 24.4 (1.7–59.2) 0.1 (0.0–0.2) 9.4 (0.3–19.7)

Bach
0.7 (0.1–2.4) 3.3 (0.8–9.1) 0.0 (0.0–0.1) 2.9 (0.2–7.5)
Dang
Dinh Vu 0.7 (0.0–1.7) 28.9 (1.0–95.8) 0.0 (0.0–0.0) 8.6 (0.3–20.8)

Turbulence
At the Cam station, the contribution of wind to the TKE
dissipation rate was one order of magnitude lower than that
of discharge during the wet and dry seasons. There was
essentially no difference between the two contributions at
the Bach Dang station during the wet season. The TKE rate
generated by wind was two orders of magnitude lower at the
Dinh Vu station during the wet and dry seasons, and also at
the Bach Dang station during the dry season (Table 3).
Although lk averaged over a tidal cycle increased only
slightly from the wet to the dry season, the variation in
turbulence (lk max–lk min) increased significantly. In addition, the minimum instantaneous values were smaller during the wet season, indicating that, when river discharge
dominates the flow, a higher and more constant turbulence
level is generated in comparison to the dry season. The latter
is dominated by tidal intrusion, which is characterized by
moderate maxima and less variation in turbulence.
Tidal asymmetry
Compared with the wet season, the tide was more symmetrical (tidal asymmetry closer to 0.5) during the dry season.

Fig. 11 Relationships between a the steady (riverine) component of
discharge averaged over one tidal cycle and tidal asymmetry, and b
tidal asymmetry and the tidally averaged potential energy anomaly
during wet (black) and dry seasons (white) at the Cam (squares), Bach

This was due to the influence of the steady component of

discharge, which hindered the propagation of the tidal wave.
The impact of discharge on the asymmetry was similarly
high at the Cam and Bach Dang stations, where the channels
are relatively narrow and shallow, and less pronounced at
the Dinh Vu station where the channel is wider and deeper
(Fig. 11a).
Stratification
Compared to the wet season, water density stratification was
much higher during the dry season. For both seasons at all
stations, density stratification began to increase from midflood tide and reached its maximum at high tide slack water.
Then, it decreased quickly at ebb tide. At the Bach Dang
station during the dry season only, an increase in stratification was also observed at low tide slack water.
The measurements revealed that, during the dry season, the
tides were more symmetrical and associated with high density
stratification. The trend was reversed during the wet season.
The relationship between tidal asymmetry and density stratification showed that the potential energy anomaly decreased
exponentially as the tide became more asymmetrical (Fig. 11b).
Interactions between discharge and sediment load
Sediment transport budget
Sediment transport averaged over one tidal cycle showed high
seasonal variations. The estuary silted up during both the dry
and wet seasons, whereby approx. 2,400 metric tons per day
was deposited in the dry season, three times as much as that
deposited in the wet season. During this season, the massive
sediment load originating from the watershed was largely
transported to the bay, with only a small loss by deposition
in the estuary and leakage to outside the system. During the

Dang (triangles) and Dinh Vu (circles) stations. In a, the coefficient of
determination corresponds to the linear regression for the four data

points of the Cam and Bach Dang stations


116

Fig. 12 Linear relationship between mean SPMC at various depths in
the water column and lk during wet (black) and dry seasons (white) at
the Cam (squares), Bach Dang (triangles) and Dinh Vu (circles)
stations. Dashed line Threshold for erosion (lk <250 μm)

dry season, the sediment load originated mainly from the
coastal area and was transported upstream (Fig. 10b).
Turbulence and SPMC
During the dry season, there was no obvious relationship
between variations in SPMC and lk (Fig. 12, white symbols).
On the contrary, during the wet season there was a significant
increase in SPMC with decreasing lk (Fig. 12, black symbols).
This trend was enhanced for lk lower than approx. 250 μm.
Turbulence and floc size distribution
Because of its normalization with the total volume concentration of the FSD, the RVCs of the particle/flocculus and microfloc modes are related to the truncated macrofloc mode. Since
the Dv of the microfloc mode depends on the balance between
its fine (∼50 μm) and coarse (∼140 μm) components, its
variation is due to internal transfer between these two
Fig. 13 Power-law
relationships a between the
volume concentration ratios of
microfloc and particle/flocculus
modes during wet (black) and
dry seasons (white) at the Cam
(squares), Bach Dang

(triangles) and Dinh Vu
(circles) stations, and b between
the depth-averaged volume ratio of the microfloc mode and
the Kolmogorov microscale lk
during the wet season at the
Cam (squares), Bach Dang
(triangles) and Dinh Vu
(circles) stations

Geo-Mar Lett (2012) 32:103–121

components. Variations in the RVCs of particles/flocculi and
fine microflocs showed similar trends during the wet and dry
seasons at a given station (Fig. 13a). Breakup and recombination generated transfer between the particles/flocculi and
microflocs; reduced variation in the former can be explained
by the higher resistance to shear stress of smaller aggregates.
The wet season was characterized by higher turbidity associated with higher turbulence level that led to G·SPMC∼O(-3).
Turbulence overrode the influence of SPMC. At high
turbulent energy, breakup and recombination caused
transfers of material between modes. As more macroflocs were broken up into microflocs at increasing turbulent energy level, the RVCs of microflocs increased
(Fig. 13b). For a given level of turbulent energy, microfloc RVCs were always higher at the Dinh Vu station
than at the Cam and Bach Dang stations. Transfers also
occurred between microflocs and particles/flocculi.
Below a certain lk threshold, turbulence caused a major
breakup of macroflocs and coarse microflocs into particles/
flocculi and fine microflocs (Fig. 14). This threshold was
found to be approx. lk 0235 μm and, during the wet season,
the values were lower at the Cam and Dinh Vu stations. This
mechanism can explain why the median apparent diameter
of the microfloc mode diminished linearly with decreasing

lk lower than the threshold (Fig. 15).
Although the influence of the tide was overall limited
during the wet season, it was noticeable at these stations
because the average level of turbulence was close enough to
the lk threshold, so that the tidal contribution to turbulence
sufficed to at times attain values lower than this threshold.
At the Cam station at ebb tide, lk varied in the range 371–
484 μm. At flood tide, lk decreased to 201–235 μm, i.e.
close to or lower than the threshold for major breakup. Since
mean turbulence was high and close enough to the threshold, the microfloc mode was composed mostly of its fine
component and centred on a small Dv of 60 μm. Macrofloc
breakup at high turbulent energy generated an increase in
microfloc RVCs (Fig. 16, black symbols). As most of the


Geo-Mar Lett (2012) 32:103–121

117

Fig. 14 Examples of major
breakup for ca. 235 μm FSD
profiles at the Dinh Vu station
during the wet season: a near
high tide slack water (21:00,
lk 0423 μm, ϕ09.30 Jm−3), b
mid-ebb tide (03:30, lk 0
181 μm, ϕ00.06 Jm−3)

macroflocs were broken up into fine microflocs, and some
fine microflocs into particles/flocculi, the Dv of the microfloc mode varied insignificantly (Fig. 16, grey symbols).

The dry season was characterized by a balancing of tidally
controlled re-suspension and differential settling phases. Turbidity was always low (SPMC<100 mg L−1) and associated
with low turbulence levels, lk always being higher than the
threshold for major floc breakup. No erosion of the bed
occurred. For that range of G·SPMC∼O(-4), there was no
significant transfer between modes induced by breakup, and
differential settling was largely responsible for the variations
in volume concentration of the microfloc mode. Seeing that
the coarser flocs would have settled faster than the finer ones,
and that no erosion of the bed was generated for this range of
turbulence, the proportion of suspended fine aggregates
exceeded that of coarser aggregates as SPMC decreased.
Hence, a joint decrease in the median apparent diameter of
the microfloc mode and in SPMC was observed (Fig. 17a).
The dry season was characterized also by a low microfloc
RVC that varied only slightly for that range of G·SPMC

(Fig. 17b). Nevertheless, there was evidence that it tended to
increase somewhat with increasing G·SPMC. This suggests
that, during the dry season, turbulence was responsible for
re-suspension at moderate levels, and promoted differential
settling at lower levels (lk∼750–950 μm at slack tides).

Fig. 15 Linear relationship between the depth-averaged median diameter Dv of the microfloc mode and the Kolmogorov microscale lk
below the threshold for major breakup (lk <235 μm). This condition
was met only during the wet season at the Cam (squares) and Dinh Vu
(circles) stations

Fig. 16 Volume ratio (black squares, left axis) and Dv (grey squares,
right axis) of the microfloc mode vs. the Kolmogorov microscale lk

during the wet season at the Cam station (linear regression for the nine
volume ratio data points). Dashed line Threshold for major breakup
(lk <235 μm)

Tidal pumping
Advective and tidal sediment fluxes were measured at
the locations corresponding to the maximum depths of
the cross sections and averaged over a tidal cycle and
the water column. Because of the shape of the river
bed, which is characterized by large and very shallow
marginal shoals separated by narrow and deep central
channels, the calculated tidal sediment fluxes represent
the smallest landward transports along the cross sections
(Scully and Friedrichs 2007).
During the wet season, only the Dinh Vu station
experienced an upstream tidal sediment flux (Fig. 18).
Maximum values were recorded in the middle of the
water column. During the dry season when the tidal
sediment flux is upstream, the highest values occurred


118

Geo-Mar Lett (2012) 32:103–121

Fig. 17 a Logarithmic
relationship between the Dv of
the microfloc mode and SPMC,
and b evolution of the volume
ratio of the microfloc mode

with G·SPMC during the dry
season at the Cam (squares),
Bach Dang (triangles) and Dinh
Vu (circles) stations

near the bed where a dense layer of suspended matter
was present.
At the Cam station, the action of the tide was too damped to
generate an upstream tidal sediment flux whatever the season.
At the Bach Dang station, and although the advective sediment flux decreased only slightly between the wet and dry
seasons, the direction of tidal sediment flux reversed. At the
Dinh Vu station, the tide was only weakly damped during its
short propagation from the river mouth. Because the impact of
freshwater discharge was limited at this station, the upstream
tidal sediment flux remained almost constant during the wet
and dry seasons. During the latter, the advective component of
sediment flux was almost nil.

Discussion and conclusions
Floc size distributions recorded at three locations along the
Bach Dang–Cam Estuary during the wet and dry seasons
were constituted of four distinct components identified as
particles/flocculi, fine and coarse microflocs, and macroflocs. The median apparent diameter of each component
varied only slightly but their relative proportions were affected by turbulence, by promoting transfers between modes
or differential settling. In particular, two thresholds for turbulent energy were found with very similar values, although
not related: a threshold corresponding to an increase in
SPMC (lk <250 μm), and a threshold of major breakup
(lk <235 μm). The former corresponds to the turbulence
level necessary to exceed the critical shear stress and to
initiate bed erosion (e.g. Partheniades 1965; Brenon and

Le Hir 1999). The latter threshold involves the major breakup of coarse microflocs and macroflocs into particles/flocculi and fine microflocs. Because the values for the two
thresholds are so similar, eroded material was maintained
in suspension as particles/flocculi and fine microflocs,
which contributed to increasing the volume ratio of microflocs and to diminishing the median diameter of that mode.
During the wet season, increased freshwater input outweighed tidal influence. High SPMCs originating from the

catchment areas were enhanced by episodic erosion of the
bed. A steadily high level of turbulent kinetic energy influenced floc size distribution by promoting breakup/aggregation, accompanied by material transfer between size class
modes. Limited saltwater input prevented strong stratification in the water column. As the vertical advection component of turbulence was not hindered by a marked
freshwater–saltwater interface, the small aggregates were
transported throughout the water column. Thus, SPMC
was high with fine aggregates maintained in suspension by
turbulence throughout the water column, and transported
downstream with small losses due to some deposition in
the estuary and leakage out of the system through secondary
channels. Overall, mean TKE was the key controlling factor
in hydrosedimentary functioning during the wet season.
During the dry season, by contrast, it was only episodically that the TKE exceeded the threshold for major breakup. Freshwater input was reduced, and the discharge budget
dominated by tidal intrusion. The tidal cycle governed both
fluctuations in turbulence and the stratification of the water
column, turbulence being inhibited by the freshwater–
saltwater interface (cf. Geyer 1993). Tidally controlled
turbulence levels generated a balancing of differential
settling and re-suspension occurring mostly in the bottom saltwater layer, with macroflocs confined to a highturbidity near-bed layer.
It is to be expected that various factors are responsible for
variations in the conversion factor m with sampling location,
water depth, and season, including sediment type, turbidity,
salinity, turbulence rate, and organic matter. Although Mari
et al. (2011) demonstrated that, in the Bach Dang–Cam
Estuary, the sticking effect of TEPs increased at salinities

higher than 15 PSU, the impact of organic bindings on
aggregation in natural environments remains difficult to
quantify. Moreover, in the present study it was found that
the variable level of turbulence was largely sufficient to
explain the inferred aggregation and breakup processes
characterizing this estuary during the wet and dry seasons.
Thus, the balance between riverine and tidal forcing controlled the marked seasonality in hydrological functioning


Geo-Mar Lett (2012) 32:103–121

Fig. 18 Tidal (solid lines) and advective (dashed lines) sediment
fluxes during the wet (black) and dry (grey) seasons at the a Cam, b
Bach Dang and c Dinh Vu stations, averaged over one tidal cycle (mab
meters above bottom)

119

of the Bach Dang–Cam Estuary, in terms of turbulence
level and tidally driven saltwater intrusion. Mean turbulence levels averaged over one tidal cycle were of the
same order of magnitude during the wet and dry seasons but their variations with the tide more pronounced
during the dry season.
Although the tidal wave was less distorted during the dry
season, tidal asymmetry generated a pumping of suspended
particulate matter upstream. The landward transport of this
dense near-bed sediment layer occurred at flood tide. The
end of the flood tide was characterized by an increase in
density stratification. As this dampened the vertical exchange of momentum, it affected both SPMC and floc size
distribution, generating a high quantity of macroflocs settling faster under an overall lower level of turbulence near
high tide slack water. Although the total amount of sediment

entering the system corresponded to only 7.6% of that transported during the wet season, the quantity of sediment
deposited in the estuary was three times higher during the
dry season, and transport was generally landwards. Overall,
hydrosedimentary functioning was controlled by the balancing of re-suspension and differential settling during the dry
season.
These interpretations have certain constraints. The depths
of the cross sections varied strongly because of the presence
of a few dredged channels in the study area. This could have
induced some bias when extrapolating 1D vertical profiles
to entire cross sections. Moreover, the system proved to not
being closed, with major leakage taking place through lateral channels. This would have introduced some error in the
calculations of discharge and sediment load budgets in the
present case. Indeed, Dalrymple et al. (1992) distinguish
between net landward and net seaward sediment transport
in estuarine settings. Such calculations can be made only
over a period of several years, since estuarine dynamics
undergo long-term periodic variations (at the scale of years,
considering the lunar cycle of approx. 18.6 years) as well as
less predictable changes in, for example, turbulence and
river runoff (e.g. Chen et al. 2005). In the present case, this
would be even more difficult because of recent developments in anthropogenic activities involving dam building,
changes in land use, and maintenance dredging in the shipping channel and harbour (Vörösmarty et al. 2003; Luu et al.
2010).
Other drawbacks include the relatively long sampling
period of 3 h required for the additional monitoring of
biological and geochemical parameters. Furthermore, turbulence was averaged over the water column (Eqs. 2 and 3),
which probably yielded biased lk assessments for strongly
stratified flows during the dry season. Higher-resolution
spatiotemporal monitoring could make use of microstructure
turbulence profilers, and ADCP-based estimation of TKE

dissipation rate (Lorke and Wüest 2005).


120

Finally, floc size distribution profiles measured by the
LISST contain dense but truncated data that do not span
sufficiently large diameters to gain a more detailed insight
into the dynamics of macroflocs. Despite these drawbacks,
the method developed in the present study efficiently
assesses floc size distribution based on a limited number
of parameters. The information gained on the complex
transfers between flocculation modes should prove useful
in more in-depth research on seasonal hydrosedimentary
processes in the Bach Dang–Cam and similar estuaries.
Acknowledgements This paper is dedicated to the memory of our
colleague Do Trong Binh. This work was supported by grants from the
French program Ecosphère Continentale et Côtière (EC2CO-PNEC) to
the HAIPHONG project, the French Research Institute for Development (IRD), and the Vietnam Academy of Sciences and Technology
(VAST). We gratefully thank Gérard Chabaud and Alexandra Coynel
who performed the grain size analyses of bed sediments at the University of Bordeaux 1 (UMR EPOC), and Claire O’Donovan for her
helpful English corrections. This manuscript benefited from the
thoughtful comments of two anonymous reviewers.

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