W.M. van Dijk1*, J.R.F.W. Leuven1, P.S. Martens1, J. Vlaming1, J.R. Cox1, M.G. Kleinhans1
Estuaries are characterized by intertidal bars, i.e. shoals, that are encompassed by ebb and flood channels forming a multi-channel system. Division of the flow at channel junctions into the main and side channels indicates that the estuary consists of asymmetric bifurcations. While disturbances, such as shoal margin collapses (Van Dijk et al., 2019 – JGR-ES) only have a near-field effect on erosion and deposition at individual channel scale, disturbances have a far-field effect on elevation jumps and flow division at the channel junctions. Tidal asymmetry is produced by the distortion of the propagating tidal wave. We expect, therefore, that a disturbance will migrate differently through the flood- and ebb-dominated channels. Our aim is to determine how dredging and disposal affect the channel network, the tidal asymmetry, and the bifurcation asymmetry of channel junctions.
Bathymetry of the Western Scheldt (The Netherlands) since 1955 was analysed to determine tidal and bifurcation asymmetry. A Delft3D schematization of the Western Scheldt was used to isolate the effect of dredging and disposal strategies. A novel channel network extraction tool was applied to quantify the changes in the channel network, the tidal asymmetry, and the bifurcation asymmetry among the main, side and connecting channel scales.
The tidal asymmetry in the Western Scheldt, represented by peak velocity ratio and period of flood-ebb ratio, shows that the duration of the ebb flow increases with increased flow current. Disposal strategy determines the development of the tidal asymmetry. The ebb period becomes longer and stronger in case dredged sediment is disposed in the side channels. Model results show that dredging affects the bifurcation asymmetry. Dredging of the main channel leads to an increased elevation jump between the high-order channel and the bifurcating channel. We expected that the bifurcating channels would close off because the increase in elevation jump. However, both channels remained open because the bifurcation angle increased. Bifurcations become less stable with dredging, which is indicated by a decrease in the number of ebb and flood channels in the channel network. The decrease in channel numbers co-exist with the amalgamation of intertidal shoals (Leuven et al., 2016 – ESR). Side and connecting channels become shallower, increasing the elevation difference within the estuary.
We conclude that dredging and disposal affect the stability of bifurcations by increasing the ebb period and decreasing the peak velocity ratio. The stability of bifurcations is important as it is an indicator for future reduction in the number of channels and thus threatens the sustainability of the multi-channel system. This ultimately affects the area of tidal flat and the biodiversity in the Western Scheldt.
Figure 1 Extracted channel network and intertidal shoal volumes for the Western Scheldt. Right top corner shows how the bifurcation asymmetry is measured, the example indicates a symmetric bifurcation, in which the angle and bed elevation among branch 2 and 3 is equal.