Thesis defence Y.M. Dijkstra: estuaries

30 April 2019 15:00 - Location: Aula, TU Delft - By: webredactie

Regime shifts in sediment concentrations in tide-dominated estuaries. Promotor 1: Dr. H.M. Schuttelaars (EWI); Promotor 2: Prof.dr.ir. Z.B. Wang (CiTG).

River and coastal currents transport fine sediments. In the areas where rivers and seas meet, called estuaries, this sediment transport usually converges, leading to the formation of an area of particularly high sediment concentrations called estuarine turbidity maxima(ETM). ETM are relevant for the ecosystem, since they lead to reduced light penetration in the estuarine water and therefore less growth of algae and plants that form the basis of the food web. They are also important economically, as ETM are associated with sedimentation and therefore reduced navigability. In the Ems River Estuary (Germany and Netherlands), sediment concentrations in the ETM have increased by a factor 10-100 over the past decades. During the same period, the river has been deepened substantially to improve navigation. It has been suggested that the deepening is responsible for the increase in sediment concentration. Based on this suggestion it has been hypothesised that other strongly deepened estuaries, including the Scheldt River Estuary (Netherlands and Belgium), can also face such a dramatic increase in sediment concentration, also called regime shift in sediment concentration.

This thesis demonstrates for the first time that man-made deepening can indeed be responsible for the regime shift observed in the Ems. This thesis furthermore shows that the regime shift did not occur over a few decades but was concentration to a period of a few years starting from 1989. To explain how deepening is linked to increased sediment concentrations, the underlying physical feedback processes were systematically investigated. This was done by developing and using a new idealised model called iFlow and comparing model results to observations. The model was additionally used to investigate if a similar regime shift can occur in the Scheldt. It is shown that deepening in the Scheldt does not lead to a large increase in sediment concentrations due to similar processes as in the Ems. The differences between the Ems and Scheldt are explained by an analysis of the underlying physical processes.

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