Seasonal variability of cohesive sediment aggregation in the Bach Dang Cam Estuary, Haiphong (Vietnam)

Transcription

1 Geo-Mar Lett (2012) 32: DOI /s 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 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, Toulouse, France jean-pierre.lefebvre@ird.fr 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, Marseille cedex 09, France J.-Y. Panché IRD, US 191 IMAGO, BP A5, Nouméa cedex, New Caledonia X. Mari : J.-P. Torréton IRD, Université Montpellier II, UMR 5119 ECOSYM, cc 093, Place Bataillon, Montpellier, France Present Address: X. Mari Institute of Biotechnology, Environmental Biotechnology Laboratory, 18 Hoang Quoc Viet Street, Cau Giay, Hanoi, Vietnam

2 104 Geo-Mar Lett (2012) 32: 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 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 D v is related to the product of the turbulence-induced shear rate (G) and SPMC through a power law D v 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 high tide slack water is revealed by a close relationship between SPMC and D v (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; Peters1997; 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 ( 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

3 Geo-Mar Lett (2012) 32: 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 D 75 in Guo and He 2011, where D is diameter), moderate variations in coarse microflocs (D 50 <D D 75 ), and a lack of significant variations in flocculi and fine microflocs (D D 25 ). Due to the 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, 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 N and E and has a total catchment area of 169,000 km 2. 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 ( 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 Fig. 1 The Red River delta, Vietnam

4 106 Geo-Mar Lett (2012) 32: sediments (Le et al. 2007). Data from long-term monitoring ( ) indicate that the annual suspended sediment flux was on average metric tons before dam construction, and since then has decreased to on average 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 , discharge averaged 1,200 and 14,000 m 3 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 O 1 and K 1 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. 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). Fig. 2 The Bach Dang Cam Estuary, with the locations of the three sampling stations

5 Geo-Mar Lett (2012) 32: 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 ¼ 1 t N t 1 X N 1 i¼1 Q i þ Q iþ1 2 ðt iþ1 t i Þ ð1þ 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): u av " ¼ k b h þ k w 3 s yh ð2þ where k b and k s are the bottom and surface drag coefficients respectively, u av the depth-averaged water velocity, w the wind velocity, and ψ the ratio between the water and air density. The Kolmogorov microscale (l k, μm) yields an estimate of the smallest turbulent eddies, and was calculated from the kinematic viscosity of water (ν) and from ε integrated over the water column (van Leussen 1997): 1=4 l k ¼ n3 ð3þ " The turbulence-induced shear rate G (s 1 ) is given by: G ¼ n ð4þ 2 l k 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: ϕðtþ ¼ g h X h z¼0 ½ρ w ðtþ ρ w ðz; tþšzδz ð5þ where ρ w is the water density, g the acceleration due to gravity, and h the water depth, ρ w being the value averaged over the water column (ρ w ðtþ ¼ 1 h P h z¼0 ρ w ðz; tþδz). 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 ml (depending on turbidity) subsamples through

6 108 Geo-Mar Lett (2012) 32: 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 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. 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 f s (z,t) (g m 2 s 1 ) was calculated as: f s ðz; t Þ ¼ uz; ð tþspmcðz; tþ 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: < f s >¼< ð< u > þu 0 Þ ð< SPMC > þspmc 0 Þ > ð8þ where the brackets < > indicate time-averaging over one tidal cycle, and the prime indicates the deviation from the tidally averaged value. Since << u > SPMC 0 > ¼ < u > < SPMC 0 > ¼ 0 and < u 0 < SPMC >>¼< u 0 > < SPMC >¼ 0, Eq. (8) becomes: < f s >¼< u > < SPMC > þ < u 0 SPMC 0 > ð9þ These two components are defined as (cf. Geyer et al. 2001) the advective component of the tidally averaged sediment flux < q a ðzþ >¼< uz; ð tþ > < SPMCðz; tþ >, and the tidally driven component of the tidally averaged flux < q p ðzþ >¼< u 0 ðz; tþspmc 0 ðz; tþ > 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) (m 3 s 1 ) and q s (t) (gs 1 ) respectively, were calculated as: qðtþ ¼ S h and q s ðtþ ¼ S h X h z¼0 X h z¼0 uðz; tþδz uðz; tþspmcðz; tþδz ð10þ ð11þ The total sediment load over the whole cross section of the river, Q s (t), was obtained by assuming that the ratio q s (t)/q(t) did not vary significantly across the whole cross section: Q s ðtþ QðtÞ ¼ q sðtþ qðtþ ð12þ The sediment load averaged over one tidal cycle was computed according to: hq s i ¼ 1 X N 1 Q sðiþ þ Q sðiþ1þ ðt iþ1 t i Þ t N t 1 2 i¼1 Floc size distribution ð13þ 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 D v 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

7 Geo-Mar Lett (2012) 32: 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 D v, 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). 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 (SPMC max SPMC min )/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 D v 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

8 110 Geo-Mar Lett (2012) 32: Table 1 Physical parameters per station and field campaign: discharge (Q, liquid), sediment load (Q s, solid), advective (q a ) and tidal pumping (q p ) sediment fluxes (calculated positively downstream), tidal asymmetry factor, potential energy anomaly (ϕ), salinity (Sal.), Kolmogorov microscale (l k ), suspended particulate matter concentration (SPMC), 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 Cam Bach Dang Dinv Vu Wet season Dry season Wet season Dry season Wet season Dry season <Q> (ms 3 ) <Q s > (metric tons day 1 ) 18,290 1,200 5, ,030 1,920 hq a i q p ð gm 2 s 1 Þ Tidal asymmetry (%) <ϕ> (Jm 3 ) 0.2 ( ) 3.5 ( ) 4.9 ( ) 31.9 ( ) 4.3 ( ) 23.4 ( ) Sal: 0.0 ( ) 1.2 ( ) 0.7 ( ) 5.8 ( ) 2.0 ( ) 15.0 ( ) <l k >(μm) 323 ( ) 390 ( ) 406 ( ) 482 ( ) 293 ( ) 438 ( ) SPMC mg L ( ) 70 (22 91) 128 (41 179) 50 (21 105) 214 (56 372) 45 (23 72) G SPMC 10 3 mg L 1 s 1 3 (0.7 7) 0.7 (0.04 1) 0.9 (0.2 2) 0.3 ( ) 4 (0.3 8) 0.4 ( ) ðμmþ D v RVC of microfloc mode (%) 9.0 ( ) 9.5 ( ) 12.4 ( ) 7.5 ( ) 23.4 ( ) 8.7 ( ) Dang station (Table 2). There was a consistent linear relationship (R ) between <l k > and <D v > (data not shown). During the wet season at the Cam station, D v values were rather homogeneous throughout the water column (Fig. 7). D v was larger at flood tide than at ebb tide. From the end of ebb tide until low tide slack water, the smallest D v values were found near the bed. The pattern was reversed at the Bach Dang station, with larger D v at ebb tide than at flood tide. Larger D v were recorded near the surface at high tide slack water and at late ebb tide. A decrease in D v was observed at mid-ebb tide. At the Dinh Vu station at flood tide, the water column was characterized by small D v in the upper layer and larger D v near the bed. At ebb tide, the distribution of moderate D v 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 D v values were smaller than in the upper part of the water column, with the exception of large D v being recorded at flood tide. Large D v were also found near the surface at mid-ebb tide. At the Bach Dang station, large D v were observed near the surface at low tide slack water, and near both the bed and the surface at high tide slack water. D v 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 D v 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 D v near the bed and small D v near the surface at slack tide, and homogeneous distribution of D v values in the water column at mid-tide. Fig. 5 Deflocculated size distributions of bed samples obtained during the dry season at the Cam, Bach Dang and Dinh Vu stations Table 2 Proportionality factor m obtained per station and campaign, where turbidity (FTU) 0 m SPMC (mg L 1 ) Wet season Dry season Cam Bach Dang Dinh Vu

9 Geo-Mar Lett (2012) 32: 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 D v of the four components obtained from the analysis of all measured FSDs combined were very consistent: <10 μm, μm, 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 D v during one tidal cycle at the Cam (top), Bach Dang (middle) and Dinh Vu (bottom) stations during wet (left) and dry (right) seasons

10 112 Geo-Mar Lett (2012) 32: 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 D v 09.2 μm, σ 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%). 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 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 b

11 Geo-Mar Lett (2012) 32:

12 114 Geo-Mar Lett (2012) 32: 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 (m 3 s 1 ) and b solid flux (metric tons day 1 ) during wet (black) and dry (white) seasons

13 Geo-Mar Lett (2012) 32: Table 3 TKE dissipation rates (10 5 m 2 s 3 ) due to wind (ε wind ) and discharge (ε discharge ) averaged over one tidal cycle (extreme values within brackets) Turbulence Wet season 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 l k averaged over a tidal cycle increased only slightly from the wet to the dry season, the variation in turbulence (l k max l k 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 Dry season ε wind ε dischage ε wind ε dischage Cam 1.8 ( ) 24.4 ( ) 0.1 ( ) 9.4 ( ) Bach Dang 0.7 ( ) 3.3 ( ) 0.0 ( ) 2.9 ( ) Dinh Vu 0.7 ( ) 28.9 ( ) 0.0 ( ) 8.6 ( ) Compared with the wet season, the tide was more symmetrical (tidal asymmetry closer to 0.5) during the dry season. 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 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 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

14 116 Geo-Mar Lett (2012) 32: Fig. 12 Linear relationship between mean SPMC at various depths in the water column and l k during wet (black) and dry seasons (white) at the Cam (squares), Bach Dang (triangles) and Dinh Vu (circles) stations. Dashed line Threshold for erosion (l k <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 l k (Fig. 12, white symbols). On the contrary, during the wet season there was a significant increase in SPMC with decreasing l k (Fig. 12, black symbols). This trend was enhanced for l k 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 D v 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 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 l k 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. l k 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 l k 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 l k 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, l k varied in the range μm. At flood tide, l k decreased to 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 D v of 60 μm. Macrofloc breakup at high turbulent energy generated an increase in microfloc RVCs (Fig. 16, black symbols). As most of the 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 l k during the wet season at the Cam (squares), Bach Dang (triangles) and Dinh Vu (circles) stations

15 Geo-Mar Lett (2012) 32: 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, l k 0423 μm, ϕ09.30 Jm 3 ), b mid-ebb tide (03:30, l k μm, ϕ00.06 Jm 3 ) macroflocs were broken up into fine microflocs, and some fine microflocs into particles/flocculi, the D v 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, l k 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 (l k μm at slack tides). 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 Fig. 15 Linear relationship between the depth-averaged median diameter D v of the microfloc mode and the Kolmogorov microscale l k below the threshold for major breakup (l k <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 D v (grey squares, right axis) of the microfloc mode vs. the Kolmogorov microscale l k during the wet season at the Cam station (linear regression for the nine volume ratio data points). Dashed line Threshold for major breakup (l k <235 μm)

16 118 Geo-Mar Lett (2012) 32: Fig. 17 a Logarithmic relationship between the D v 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 (l k <250 μm), and a threshold of major breakup (l k <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