Influence of topography on saline intrusion in a deltaic estuarine system

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Motohiko Umeyama, Narumi Ishikawa

Thursday 2 july 2015

11:00 - 11:15h at Europe 1 & 2 (level 0)

Themes: (T) Managing deltas, (ST) Saline and freshwater interaction

Parallel session: 11K. Managing deltas - Saline

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Many researchers have studied independent issues for single-channel estuaries, but quantifying the discharge water is still an ongoing problem. It is even more difficult to shed light on the state of multi-channel estuaries that consist of one main channel and some branched channels in a tide-dominated delta. The tidal flux transports seawater into the estuary region where it mixes with freshwater. Thus, prediction of tidal flow and saline intrusion in a multi-branched system has come to the attention. The objectives of this study are (i) to develop a mathematical model, considering how the energy loss by the bottom friction affects the distribution of freshwater discharges in the branched channels, (ii) to derive an analytical solution for predicting tidal flux along a channel, and (iii) to develop a predictive saline intrusion model, considering the salt balance by diffusion and advection. The study area is located in the Red River Delta (RRD), where the Red River and many distributaries spread out to form a large alluvial plain. The Red River Estuary System (RRES) lies in the southeastern part of the RRD and includes four branches: the Tra Ly, Red River, Ninh Co, and Day. The RRES has been threatened by salt intrusion with the rapid growth of human occupancy for a few decades. Especially, in the dry season, the discharge of the Red River decreases and consequently, the saline water intrudes into the upstream reaches. A series of field measurements was conducted in the RRD where salinity was sampled every hour at 21 stations simultaneously. The tidal information was collected from the Ba Lat observatory, which is located near the river mouth. The topography data were acquired from the Government of Vietnam. The predicted result was compared with a set of observed data and with the numerical result by commercial software. The mean freshwater discharge rate was calculated accurately at junctions where the main flow separates into branches. Salinity distributions were estimated by making use of the salt intrusion model with the tidal-flux formula. Overall, the present model efficiently evaluates the mixing mechanism caused by the tidal flux in a multi-branched estuarine system, where hydraulic data are not always feasible to observe in the dry season.