Intertidal Flats in Engineered Estuaries: On the Hydrodynamics, Morphodynamics, and Implications for Ecology and System Management

Abstract

Intertidal flats — regions of estuaries that emerge every tide from the water — form unique ecosystems. Benthic communities living in the bed are a valuable food source for wading birds. Salt marshes present on these flats further enhance the biodiversity. Through the damping of waves, intertidal flats also contribute to the safety of the hinterland against flooding. In engineered estuaries, human interventions such as storm surge barriers, navigation channels, dams, and levees affect these ecologically valuable intertidal flats and may even threaten their existence. Therefore, these systems should be managed with care, requiring a thorough understanding of the mechanisms shaping intertidal flats. This dissertation aims to identify and quantify the natural and anthropogenic processes driving hydrodynamics and morphodynamics of intertidal flats, and to reveal the implications for ecology and system management. The Eastern Scheldt and Western Scheldt estuaries (the Netherlands) were selected for this study. These were chosen because of the extensive datasets measured in both estuaries and the different types of human interventions affecting these systems. In the Eastern Scheldt, a storm surge barrier closes during storm conditions and reduces tidal flow velocities inside the estuary at normal conditions. Tidal velocities are also reduced by dams in the branches of this estuary. In the Western Scheldt, sediment is being relocated from too shallow parts of the navigation channel to other parts of the estuary, enabling navigation to economically important harbors. In this dissertation it is shown that it is the aggregated system of natural forces and human interventions that drives the eco-morphological evolution of intertidal flats in estuaries. Intertidal flats respond to local as well as to system-wide changes in sediment availability and hydrodynamics due to human interventions. Even under major human interventions, the natural forces remain relevant. Due to many spatial and temporal scales involved in the eco-morphological response of intertidal flats to changing natural and anthropogenic forces, estuaries require adaptive management strategies.