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Salt intrusion in the Ghent-Terneuzen channel : assessing Salinity Dynamics with Delft3D FM due to planned lock modifications

Noordman, B. (2016) Salt intrusion in the Ghent-Terneuzen channel : assessing Salinity Dynamics with Delft3D FM due to planned lock modifications.

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Abstract:Saltwater intrusion is a natural phenomenon which primarily exists because of the spatial density differences and it occurs in the fresh-saline transition zone. The Dutch Terneuzen locks, separating the brackish Scheldt estuary and the Ghent-Terneuzen canal [GTC], is an example where salt water intrusion currently occurs. The freshwater inflow from Flanders and the exchange volumes of the locks are the most important variables that influence the salt intrusion in the GTC. Plans have already been agreed and assessed to replace one of the existing locks with a large sea lock, fitting within the current port layout. An environmental impact assessment [EIA] was carried out with the 1D numerical model to estimate changes in the salinity dynamics, however the salinization process in the GTC has much more of a 3D character in view of a salt wedge migration over the seasons. For this study, a 3D numerical model was built with the new generation hydro software Delft3D Flexible Mesh [Delft3D FM] in order to assess the impact of the planned lock modification on the salinity dynamics in the GTC. The study focused on density flows and density driven salinity transport, approximating the physical flow and transport processes on a more realistic scale. The downstream model conditions were adopted from the EIA, per lock the estimated average tidal exchange of fresh-saline water was explicitly imposed as lock exchange flows, using 3D sinks and sources. With the necessary scaling of the implicitly imposed fresh-saline lock exchange flows, the Delft3D FM model was successfully calibrated and validated with a limited amount of verification data. The seasonal impacts of the lock modifications were assessed, by simulating various scenarios with the current lock exchange flows and similar for the new situation, with historical inflow series that led to three representative salt intrusion scenarios in the GTC. Longitudinal salt profiles were plotted to visualize the changes in the salinity dynamics. The results of the analysis show that due to the planned lock modifications, the salt intrusion will affect salinity concentrations over the full length of the channel. In winter, the salinity concentrations over the GTC will approximately be twice as high and remain constant over this period. During the low flow period in summer, salinity concentrations increase at least by a factor of 1.8 following the planned lock modifications. Over the complete annual cycle, the relative increase is estimated to be greater upstream. For this specific case, the 3D modelling gave clearer insights in the density driven salinity transport in the GTC. However, the general conclusions agree with the results of the 1D model used in the EIA. The dynamic behaviour of salinity in the GTC will most likely change, as a salt wedge draws inland and then the gradual salinization of the entire water column follows quickly, even under highly unfavourable freshwater inflows. It can be concluded that the current water quality standards in the GTC are no longer achievable in the near future. Because of the future increase in the salt intrusion, mitigating measures in the new lock will most likely be the best solution, but inherently the local users must be aware and adapt to a more brackish state of the GTC. Further research should clarify the effects of other factors for a comprehensive analysis, like influences of climate change and economic developments in the GTC, on the tempo-spatial salinity dynamics in the GTC.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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