Salt Intrusion and Mixing in Multi-connected Estuaries
Nguyen Hoang Duc, Graduate School of Urban Environmental Sciences, Department of Civil and Environmental Engineering
Figure 1. The Red River Delta
Salt intrusion is a physical phenomenon of which seawater flows into rivers or estuaries. We consider that this aspect is a major concern, since billions of people living in coastal area are constrained by freshwater supply for drinking and aqua-agricultural activities. The Red River Delta (RRD) in the northern part of Vietnam (Figure 1) is a lower-laying area that occupies approximately 23% of Vietnam’s population. Higher ocean level is already contaminating the water resources in the RRD. Despite the rapid development of the economy in the past decade, the RRD faces many difficulties and challenges related to water environment. The excessive extraction of freshwater, the decreasing of water release from reservoirs in the upstream, and the increasing of sea level have increased the intrusion length of saltwater. The need for managing and controlling salt intrusion is therefore very important for the sustainable management of water resources, as well as for the development of socio-economics.
Figure 2. Comparison of numerical results and measured data for water level (above), discharge (middle), and salinity (below) at the Dong Quy station in the Tra Ly estuary.
The objectives of this study aim to understand the physical mechanism of saline intrusion in the RRD by analyzing the measured data and to predict the saltwater distribution by using the numerical and analytical methods. We believe that the study results contribute suitable solutions that can help to solve the diverse water problems in the RRD.
To investigate the spatial distributions of salinity in the RRD, we used a 1-D numerical model, MIKE11. The values of water level, discharge, and salinity can be determined at certain location along each river (Figure 2). We have also developed an analytical model to simulate the longitudinal salinity profiles for different tidal conditions (Figure 3).
In addition, using the analytical model we have determined the freshwater discharge and its relationship to the salt-intrusion distribution for a complex estuary system. In this analysis, we have taken accout of the variation of river cross section due to the tidal range (Figure 4).
Figure 3. Comparisons of longitudinal salinity profiles at spring high water (HWS) and spring low water (SLW).
Figure 4. Description of cross-sectional profile and future sea levels in the RRD’s estuaries
Figure 5. Predictive salt intrusion length at high water in spring tide for four estuaries in the southern part of the RRD.
We also estimate the salt intrusion length at high water during spring tide for each estuary branch (Figure 5). With the aid of numerical and analytical models, we can predict the spatial salinity distribution in multi-connected estuaries in the Red River Delta.
Most of these study results are published in four international conferences.
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