The double structure of the Estuarine Turbidity Maximum in the Cam-Nam Trieu mesotidal tropical estuary, Vietnam

The characteristic parameters of the Estuarine Turbidity Maxima (ETM), their aggregates and their tidal variations were investigated within the Cam-Nam Trieu mesotidal estuary (Red River and Thai Binh River system, North Vietnam) at spring tides during the early wet season, wet season and dry season. The structure observed repeatedly highlights the separation of two distinct zones of high turbidity around the salt wedge, separated by a zone of lower turbidity. The upstream part developed at very low salinity (from 0.1 psu) on a quasi-homogeneous water column (at Simpson parameter between similar to 0.65 and 7-10); tidal pumping associated with salinity-induced flocculation are likely responsible for its formation. A second maximum was observed in the lower layer of stratified waters (at Simpson parameters >10), i.e. in the salt wedge, at higher salinities up to similar to 12-15 psu; in this downstream part, governed by the gravitational circulation, settling is also seasonally enhanced by the presence of transparent exopolymeric particles (TEP) for salinity >10 psu. Turbidity was higher in the upper part of ETM in the dry season and higher in the lower part of ETM in the wet season. Floc size, excess of density and settling velocity were the highest during dry season. Stronger ETMs occur in dry season than in early wet season and wet season, when the tidal-induced ETM is maximum. The locations of both ETM parts changed seasonally, moving upstream in the dry season and downstream in the wet season. Their length depended on the longitudinal salinity gradient and was highest at low tide than at high tide. The intermediate zone of lower turbidity between these two parts was longer in the wet season.