The effects of dredging and disposal activity on the resilience of estuary morphodynamics

Shipping fairways in estuaries are continuously dredged to maintain access to major ports for large ships. However, various estuaries worldwide show adverse side effects to dredging activities, including shifts from a multi-channel system to a single channel, loss of ecologically-valuable intertidal areas, and increased muddiness. The Western Scheldt (Netherlands) is an example of a system where several million m3 sediments are dredged annually and disposed of within the estuary. Here, the effects of dredging and disposal strategies on the channel-shoal morphodynamics, including the evolution of the multi-channel system and its ecological impact are studied using field observations, numerical and physical models. Time series of bathymetry, morphodynamic model runs and physical experiments in the Metronome tilting flume show for the first time that the deliberate dredging and disposal strategy increases the surface area of inter-tidal habitats, which decreases the connectivity between channels and destabilizes the multi-channel system. We quantify these changes in scale and topology using a novel and mathematically-rigorous network extraction, which identifies the partial switching of the main channel with the side channel in the past and that continuous disposal of sediment in the side channel results in siltation. The physical experiments indicate that the estuarine network is dramatically changed by dredging even for a long period following the halting of any dredging and dumping works. The morphodynamic model similarly shows that dredging of the sills is essential to maintain the current shipping fairway. A new disposal strategy targeting the deepest scours of the main channel appears to be the optimal solution for ecological value and sustains dynamic channel-shoal interactions and the multi-channel pattern. Sea level rise scenarios demonstrate the importance of keeping the sediment in the system for sustainable management, but present dredging and disposal techniques put current ecosystems under pressure.