Regime shifts in sediment concentrations in tide-dominated estuaries

Abstract

Within estuaries one can often observe areas where the concentration of fine suspended sediments is higher than in the surrounding waters, called estuarine turbidity maxima (ETM). ETM play an important role in the natural and socio-economic value of estuaries. The suspended sediments can for example greatly diminish the value of the estuarine ecosystem by negatively affecting the light climate and oxygen level, as well as the economic value by leading to increased dredging costs for maintaining the depth of shipping channels. In at least two tide-dominated estuaries, the Ems River (Netherlands, Germany) and Loire River (France), the suspended sediment concentration has increased dramatically over the course of several decades. This has resulted in a great decline of the ecosystem and increase in dredging costs. As it is not well understood why these so called regime shifts in suspended sediment concentrations occurred, it remains unclear whether similar regime shifts can occur in other tide-dominated estuaries. The current leading hypothesis states that the regime shifts in the Ems and Loire are a result of man-made deepening of the estuary in the preceding decades. In addition, the hypothesis states that a similar regime shift can also occur in other tide-dominated estuaries that are subject to deepening. In this thesis, this hypothesis is systematically investigated by investigating the main physical processes that drive the sediment dynamics in tide-dominated estuaries and their response to channel deepening. This is illustrated by taking examples of two estuaries: the Ems (Netherlands, Germany) and Scheldt (Netherlands, Belgium)…