Morphological development after the July 2014 flow slide on the tidal flat of Walsoorden in the Western Scheldt

Abstract

The overall behaviour of the tide-dominated Western Scheldt is in general well known. Its multichannel-system consists of ebb and flood channels, separated by intertidal areas; the tidal flats. The estuary shows a dynamic behaviour of erosion and sedimentation of the tidal flats. A large flow slide occurred at the tidal flat of Walsoorden in July 2014. A flow slide occurs when the slope is sufficient steep and a trigger starts the process of liquefaction or breaching. In a few hours a huge amount of sediment slides away, causing loss of surface and endangering safety. During this flow slide approximately 800,000 m3 of sediment flowed to the navigation channel, leaving a large gap in the tidal flat. This flow slide is one of the largest known in this part of the Western Scheldt. A measurement campaign was initiated to measure the bathymetry around the tidal flat, starting a few months before the flow slide. After the event monthly measurements were taken to monitor the development of the area. Dunes were formed on top of the accumulation of sediment, which travelled in flood (eastern) direction. A large part of the sediment accumulation was transported as bed load in flood direction, which is unexpected since the Zuidergat channel is known as an ebb channel. The sediment volume in the channel is decreasing, suggesting suspended load transport. The channel dominance is determined based on three characteristics: the water level, velocities and sediment transport. Based on water level data flood dominant behaviour is expected, while velocity and sediment transport data result in ebb dominance. Based on the bathymetry measurements, the volume changes in the channel and in the gap were determined. Those showed an exponential change in time. The erosion rate in the channel was faster than the sedimentation rate in the gap. The recovery time of the system is estimated through extrapolation of this data. The recovery time for the channel is thus shorter than the recovery time for the gap, 9 months and 1.5 to 2 years respectively. An existing Delft3D model schematisation of the Western Scheldt was used to calculate the morphological development. The model was setup around the tidal flat of Walsoorden. The boundary conditions were generated by a model schematization which describes the full Scheldt estuary; the validation was performed with observations of water level and velocity along the tidal flat. The morphological changes determined by the model are different from the bathymetric changes found in the measurements. In the model the sediment accretion smoothens out; the sediment is deposited on the sides of the accumulation. Most of the sediment from the channel is transported in ebb direction. The model shows almost no sedimentation in the gap, which is contradictive with the measurements. The sensitivity of the model was tested to investigate the differences between the observations and the model. A varying sediment diameter in the model had effect on the amount of transported sediment only, not on the residual direction. The used model is a 2DH model, neglecting the secondary circulations. The sensitivity of these processes was tested using a 3D model. From this model it was found that the secondary circulations were not of significant importance concerning the areal development after the flow slide. Finally, a non-cohesive sediment fraction was implemented in the model. This showed sedimentation in the gap. Therefore, it is suggested that the sediment settling in the gap is a combination of cohesive and non-cohesive sediment, which is supported by the observations.