How Autonomous and Anthropogenic Development of the Scheldt Estuary (Belgium) Affect Tidal Asymmetry and Sand Transport

The navigability of the upper part of the Scheldt estuary must be improved to accommodate ships up to Class Va. An integrated plan was made with different scenarios for enlargement of the navigation channel. Besides an improved navigability, flood protection and a sustainable natural system must be maintained at least at the current levels.

The enlargement scenarios of the upper part of the estuary take into account the autonomous development of the estuary and climate change. Different scenarios are evaluated by a model train starting from a 3D hydrodynamic model of the entire Scheldt estuary and Belgian coastal zone. TELEMAC-3D was used for this model. A sand transport model using SISYPHE and a cohesive sediment model in SEDI-3D (all modules of the open TELEMAC-MASCARET modelling suite) were coupled with the hydrodynamic 3D model. Results of these models will be used in an ecosystem model to assess the changes of light climate on the phytoplankton and zooplankton communities and on biogeochemical cycling of nutrients. Higher trophic levels, like fish and birds, are also taken into account.

The results of the hydrodynamic and sand transport model are subject of this paper. The sand transport model uses a single fraction of fine sand with a D50 value of 150 µm (based on field measurements). The Engelund and Hansen equation was chosen as total load transport equation. Sand transport rates of the model were in good agreement with field measurements using the Delft bottle; executed specially for the model validation. Furthermore the sand balance in the model is compared with an estimated sand balance based on the difference in bathymetric surveys of 2001 and 2010 and on lithological information and dredging activities.

The net sand transport over different transects along the Scheldt estuary is linked to the tidal asymmetry of the cross-sectionally averaged flow velocity to the power five, which explains the net transport direction. This is done for different scenarios and results are compared with each other. The model results showed that the autonomous development combined with a climate change boundary (sea level rise of 0.2 to 0.4 m) had a much larger effect on the sand transport rates and directions than the straightening of the channel in the upper part of the estuary.