Dynamics of currents and sediment in estuaries with different density stratifications

Abstract

Estuaries are important geomorphic units of coastal zones, as they provide critical habitat and shelter for a large variety of living species and moreover, they have a high economic demand. In estuaries, different intensities of tidal currents at the estuarine mouth and river discharge from land result in large variations in density stratification, complex hydrodynamic and sediment processes. This thesis focuses on gaining fundamental knowledge about three key issues, i.e., tidal currents, residual currents and suspended sediment trapping in estuaries with different density stratifications. The first issue is inspired by field observations in bays and estuaries, which have revealed pronounced subsurface maxima in the vertical profiles of either the current amplitude of the principal tidal harmonic or of its vertical shear. In order to understand these phenomena, a semi-analytical model is designed and analysed. The new analytical solutions of the tidal current amplitude are used to explore their dependence on the degree of surface turbulence, the vertical shape of eddy viscosity in the upper part of the water column and on the density stratification. The second issue regards the residual currents generated by Eddy viscosity-Shear Covariance (ESCO) in a narrow estuary. To explicitly resolve the temporal and spatial structure of vertical eddy viscosity and tidal current at different locations, a numerical model is applied. New aspects concern the dependence of the spatial structure of ESCO current on longitudinal depth variation for fixed semi-diurnal tidal forcing, varying amplitude of the semi-diurnal tidal forcing, and mixed tidal forcing (an externally forced tide that contains one semi-diurnal and two diurnal constituents) for fixed stratification at the mouth. Regarding the third issue, particular focus is on the longitudinal variation in locations where lateral entrapment of suspended sediment occurs. In this study, field data from the North Passage of the Yangtze Estuary and results of a new three-dimensional exploratory model are compared and analysed.