The effects of dredging activities on the multi-channel network of the Western Scheldt

Abstract

Many shipping fairways need semi-continuous dredging to maintain access for ships to major ports, often located at river mouths, i.e. estuaries. One of the negative effects of dredging is the shift from a multi-channel system to a single-channel system. An increase in flow asymmetry between flood and ebb channels is regarded as undesirable for many user-functions, including ecological values. Our aim is to quantify how dredging and disposal affect the channel network composed of flood and ebb channels. Therefore, we analysed the bathymetry of the Western Scheldt (The Netherlands) since 1955 and used a Delft3D schematization of the Western Scheldt to isolate the effect of dredging and disposal strategies. We use a novel and mathematically rigorous network extraction method to characterise the channels in scale and topology. All model runs show that current dredging and disposal strategies are unsustainable for the multi-channel system because dredging (1) further disturbs the balance between high and lower bed levels of the flood and ebb channels and (2) increases bifurcation asymmetry, i.e. angles and elevation jumps. The model runs also suggest that disposal of dredged sediment in the scours of the main channel is economically feasible and contributes to the preservation of the multi-channel system. We argue that the disposal strategy of dredged material is as important as the dredging it-self in maintaining suitable conditions for the persistence of an ecologically valuable system.