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Estuarine Turbidity Maxima and Variations of Aggregate Parameters in the Cam-Nam Trieu Estuary, North Vietnam, in Early Wet Season

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Estuarine Turbidity Maxima and Variations of Aggregate Parameters in the Cam-Nam Trieu Estuary, North Vietnam, in Early Wet Season water Article Estuarine Turbidity Maxima and Variations of Aggregate Parameters in the Cam-Nam Trieu Estuary, North Vietnam, in Early Wet Season Vu Duy Vinh 1,* ID , Sylvain Ouillon 2,3 ID and Dinh Van Uu 4 1 Institute of Marine Environment and Resources, VAST, 246 Danang Street, Haiphong City 180000, Vietnam 2 UMR LEGOS, Université de Toulouse, IRD, CNES, CNRS, UPS, 14 avenue Edouard Belin, 31400 Toulouse, France; [email protected] 3 Department Water-Environment-Oceanography, University of Science and Technology of Hanoi, 18 Hoang Quoc Viet, Hanoi 100000, Vietnam 4 Hanoi-VNU University of Science, Vietnam National University, 334 Nguyen Trai, Hanoi 100000, Vietnam; [email protected] * Correspondence: [email protected]; Tel.: +84-912-799-629 Received: 28 November 2017; Accepted: 11 January 2018; Published: 13 January 2018 Abstract: This study aims at exploring the characteristic parameters of the Estuarine Turbidity Maxima (ETM) and at investigating their tidal variations within the Cam-Nam Trieu estuary (North Vietnam) during the early wet season. Six longitudinal river transects were performed at spring tide. Two types of ETM were observed: an upper well mixed ETM with high Suspended Particulate Matter (SPM) concentrations up to the surface at low salinity (0.11 to <1 psu), and a lower ETM confined in a bottom layer over stratified waters at salinities between ~1 psu and 15 psu. Their length depended on the longitudinal salinity gradient and was highest at low tide than at high tide. D50 of the flocs varied between 35 and 90 µm, their excess of density between 60 and 300 kg m−3 and their settling velocity ranged from 0.07 to 0.55 mm s−1 with values between 0.12 and 0.40 mm s−1 in the core of ETMs. The average fractal dimension of flocs was estimated to vary between 1.93 (at high tide) to 2.04 (at low tide). Keywords: estuarine turbidity maximum; flocculation; aggregation; excess of density; fractal dimension; settling velocity; suspended sediment; Red River; Vietnam; Hai Phong 1. Introduction Estuarine Turbidity Maxima (ETMs) are zones of elevated suspended particulate matter (SPM) concentration at the interface between the river and the sea. Because of the strong influences of marine water, tide and river discharge, the occurrence of ETMs is complex and difficult to predict [1,2]. They are trapping a lot of suspended matter and encompass a huge range of SPM concentrations with maximum values from less than 100 mg L−1 like in the Kennebec Estuary, USA [3,4] to >200 g L−1 like in the Severn Estuary, UK [5]. The settling of SPM in an ETM has been well documented in the Gironde Estuary [6–8], the Hudson estuary [9–11], the Columbia River estuary [12], the Seine estuary [13], the upper Chesapeake Bay [14,15], and the Humber Estuary [16], amongst others. For instance, Olsen et al. [9] showed that the deposition rate reached up to 30 cm year−1 in certain parts of the ETM in the Hudson estuary on timescales of 5–10 years. The convergence at the saltfront of landward sediment flux by the gravitational circulation and seaward sediment flux by the river flow is considered as a fundamental mechanism contributing to ETM formation [17–19]. In most estuaries, tidal resuspension has been recognized as a key factor in maintaining high SPM concentration [13,20–22]. Tidal asymmetry also contributes to the ETM Water 2018, 10, 68; doi:10.3390/w10010068 www.mdpi.com/journal/water Water 2018, 10, 68 2 of 33 Water 2018, 10, 68 2 of 35 formation.formation. Tidal currents generally are surface surface-intensified-intensified at ebb but ar aree bottom bottom-intensified-intensified at flood. flood. VerticalVertical mixing mixing may may also also be suppressedbe suppressed at ebb at andebb enhancedand enhanced at flood, at dueflood, to thedue interaction to the interaction of vertically of shearedvertically tidal sheared currents tidal with currents the along with channelthe along salinity channel gradients salinity gradients [23]. The position[23]. The ofposition the turbidity of the maximumturbidity maximum sometimes sometimes corresponds corresponds to the boundary to the betweenboundary river between water river and seawater water and [ 6sea,10, 14water,24]. The[6,10 suppression,14,24]. The suppression of turbulence of byturbulence density stratificationby density stratification tends to keep tends the to particles keep the near particles bed at near the convergencebed at the convergence zone [25,26 ].zone Lateral [25,26 interactions]. Lateral interactions between currents between and currents topography and alsotopography have the a potentiallso have tothe enhance potential asymmetric to enhance sediment asymmetric trapping sediment [27– 29trapping]. [27–29]. The Cam Cam-Nam-Nam Trieu Trieu estuary estuary (Figure (Figure 1a),1a), located located in inHai Hai Phong Phong city city (Northeast (Northeast Vietnam), Vietnam), is an isinteresting an interesting site to sitestudy to estuary study estuary dynamics dynamics because becausethis meso this- to meso-macrotidal to macrotidal estuary is both estuary influenced is both influencedby a strong by seasonal a strong river seasonal signal river and signala monsoon and a regime. monsoon This regime. area is This also area located is also in locatedthe Northern in the NorthernVietnam key Vietnam economic key region economic and regionHai Phong and ports Hai Phong system ports has defined system as has a main defined gate as to a connect main gate the toNorth connect Vietnam the North to the Vietnamworld market. to the Harbors world market. in Hai Phong Harbors city in are Hai located Phong in city the are Cam located-Nam inTrieu the Cam-Namestuary and Trieu create estuary the third and createlargest the port third system largest in port Vietnam. system In in recent Vietnam. years, In recentdue to years, the rapidly due to thegrowing rapidly of growingsocial-economics of social-economics development development of the region, of the the region, total the cargo total through cargo through the ports the portshave × 6 × 6 haveincreased increased very fast, very from fast, 10.3 from × 10.3106 tons10 in 2002tons into 78.1 2002 × to 10 78.16 tons in10 2016tons [30 in]. 2016However, [30]. Haiphong However, Haiphongharbor has harborbeen affected has been by affected increasing by increasingsiltation. The siltation. sediment The volume sediment that volume had to thatbe dredged had to beto × 6 3 dredgedmaintain toa minimum maintain adepth minimum of the depth navigation of the c navigationhannels was channels about 0.78 was × about106 m3 0.78in 201310 andm 1.17in 2013× 106 × 6 3 andm3 in 1.17 2015 with10 m a highin 2015 induced with a cost high (e.g. induced, 6.6 million cost (e.g., USD 6.6 in million 2013) [ USD31]. Many in 2013) studies [31]. Manywere recently studies weredevoted recently to find devoted the cause to of find siltation the cause in the of navigation siltation in channel the navigation deposition channel and to depositionpropose solutions and to, proposefocusing solutions,on geomorphology focusingon [32 geomorphology], geology [33], [hydrodynamics32], geology [33 ],and hydrodynamics sediment transport and sediment [34–38]. transportLefebvre [et34 al.–38 [].38 Lefebvre] showed et in al. particular [38] showed that in tidal particular pumping that tidalcreated pumping a net createdupstream a net transport upstream of transportsediments of which sediments settled which in a settled near-bed in a layer near-bed at low layer turbulent at low turbulent energy, causing energy, causingthe silting the siltingup of the up ofchannels the channels of Hai of Phong Hai Phong port, port, particularly particularly in the in thedry dry season. season. Mari Mari et etal. al. [39 [39] ]also also reported reported that thethe aggregation/sedimentation processesprocesses may may be be enhanced enhanced at salinity at salinity around around 10–15 1 psu0–1 due5 psu to thedue sticking to the propertiessticking properties of transparent of transparent exopolymeric exopolymeric particles (TEP), particles which (TEP) were, which shown were to be shown salinity-dependent to be salinity in- thedependent Cam-Nam in the Trieu Cam estuary.-Nam Trieu estuary. (a) Figure 1. Cont. Water 2018, 10, 68 3 of 33 Water 2018, 10, 68 3 of 35 (b) Figure 1. The Cam Cam-Nam-Nam Trieu estuary ( a) general location location–transects–transects were mostly performed between stations A and B, C is the hydrological station, D is the tide gauge at Hon Dau; ( b) example of survey (stations of Transect 4)4) alongalong thethe A-BA-B transect.transect. Based on new in situ data, this study focuse focusess on the maximum turbidity zones in the Cam Cam-Nam-Nam Trieu estuaryestuary under various fluvialfluvial and tidal regimes. Several parameters of the ETM or driving the formation ofof thethe turbidityturbidity maximum maximum such such as as SPM SPM concentration, concentration, floc floc size, size, settling settling velocity, velocity, salinity salinity and andhydrodynamics hydrodynamics condition condition were analyzed.were analyzed. The results The provideresults provide understanding understanding about position, about scaleposition, and patternsscale and of patterns the ETM of inthe the ETM Cam-Nam in the Cam Trieu-Nam estuary. Trieu This estuary. paper This focuses paper on focuses the analysis on the ofanalysis the ETM of thecharacteristics ETM characteristics during the dur earlying flood the period,early flood i.e., duringperiod, the i.e. transitional, during the regime transitional which occursregime between which occurslow discharge between and low floods discharge at the and beginning floods at of the the beginning summer monsoon. of the summer monsoon. 2. The Cam Cam-Nam-Nam Trieu EstuaryEstuary The Cam Cam-Nam-Nam Trieu estuary is located in Hai Phong city city (the (the third third largest largest ci cityty of of Vietnam), Vietnam), northeastern ofof VietnamVietnam (Figure (Figure1a).
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