Impact of cross-sectional area reduction by barrage placement on estuarine hydro- and morphodynamics

Abstract

Estuaries are morphological dynamic features, positioned on the edge between river and sea. Their morphodynamic evolution is the result of continuous interaction between the available sediment and the nonlinear tidal propagation giving rise to residual sediment transport patterns. There is an increased need for renewable energy and tidal energy generation in estuaries might be a viable option. Investments in the tidal resource fall behind to solar and wind energy because full exploitation of the tidal resource is hindered by the limited amount of knowledge on impacts of tidal barrages on estuarine hydro- and morphodynamics. Estuarine hydro- and morphodynamics are coupled to estuarine ecology which is considered valuable and is therefore protected in many of the potential tidal energy extraction sites. This study aims to investigate the impacts of a tidal barrage placement on estuarine hydro- and morphodynamics. To assess the impacts of this, a 2DH Delft3D idealized estuary model was used. A depthaveraged approach was used to reduce calculation time in the Delft3D flow module. The tidal barrages were modelled as a local width reduction in the estuary. Different barrage configurations (degree of width reduction and location) were used to assess the hydro- and morphodynamic responses of the estuary. Additional model runs were included with a fixed bottom layer in and around the barrage mouth, reducing the maximum (mouth) depth. Only tidal forcing (M2 with amplitude of 1.0m) was included in the model, in combination with a small river discharge from upstream. The morphology was modelled up to a period of 34 years. All estuary parameters were kept constant in the model runs. Only tidal barrage positions and relative barrage mouth widths were varied between model runs. The most important hydrodynamic responses included a decrease in tidal prisms of up to 3% and consequent decreases in water levels in the estuary. This led to a compression of the intertidal zone of 7 centimeter locally. The hydrodynamic response was immediate and decreased over time as the morphodynamic response grew. The morphodynamic response was predominantly visible in the area near the modelled barrages, up to a few kilometers away. The most important morphodynamic response was the increase of the depth in the barrage mouth (up to 8 meters) and the channels around the mouth. In the model runs with limited maximum mouth depth, the hydrodynamic response did not decrease considerably over time as the morphology around the barrage was not able to adapt to the new situation by channel deepening. The material eroded from the channels near the barrage was transported seaward and partly deposited in the intertidal zone (based on the offshore low- and high-water lines). This resulted in an increase in area in the intertidal zone and an increase in the Vs/Vc ratio (volume of intertidal storage/volume of channels at mean sea level). Combined with an observed decrease in the a/h ratio (tidal amplitude/channel depth) this led to the conclusion that the system became stronger ebb-dominated by the placement of a tidal barrage. Resulting in sediment starvation starting from the middle estuary.