Sediment exchange between flats and channels in tidal inlets

Sediment exchange between flats and channels in tidal inlets

Van Marion, B.B.

Stelling, G.S. (mentor)
Wang, Z.B. (mentor)
De Vriend, H.J. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

1997-12-01

The subject of this thesis is the sediment exchange between channels and flats in tidal inlets. This topic is part of a research scheme which must yield more insight into the dynamic equilibrium of tidal inlets. An important aspect of the sediment exchange is the drying and flooding of the flats during the tide. The oscillating water level gives rise to velocity variations which cause stirring and settling of sediment. In nature, tidal inlets are large and complex systems, the dimensions of which vary continuously due to natural effects (tide, storms, sea level rise). For the purpose of this study a schematised tidal inlet is considered, with dimensions as occurring in the Dutch Wadden Sea. In this schematisation, the bottom ofthe channel and the tidal flats is fixed. The schematisation is used as a starting point to model the water and sediment motion during a few tidal cycles. Subsequently the output of each model run, i.e. velocities, water depths and concentrations, is used to estimate the sediment transport during ebb and flood, respectively. The transport is calculated in points along a line perpendicular to the channel axis. Model runs are executed in one and two dimensions. Sediment transport rates according to power law formula (l-D and 2-D cases) and according to a suspended sediment equation (l-D cases only) are compared. The suspended sediment fluxes at the flat agree well with the sediment transport rates determined by the power law formula. In some of the I-D runs, it appears that problems occur around the transition from the flat to the channel. Very large values of the transport rate, much larger than those in the adjacent cells, result from a thin water film with relatively high velocities. Therefore, the sediment transport model is provided with a water depth threshold preventing this. The threshold hardly influences the sediment transport rates during flood, either suspended or by power law, because the front of the wave is rather steep. The sediment transport rates during ebb, however, for which the threshold was meant, are reduced, such that the transport rate approaches the values in the surrounding points. The parameters which have been chosen to vary in the I-D model runs are: the slope of the tidal flat, in order to investigate whether an equilibrium profile of the tidal flat is possible, and to investigate the net transport over the landward edge of the flat if the flat is relatively steep, the (a)symmetry ofthe imposed water level in order to compare the results of a symmetric boundary condition to the results of a flood-dominant condition; the water level in the channel changes with time towards the end ofthe channel, the tidal amplitude and the length ofthe tidal flat, in order to assess how they are related to the sediment transport.

tidal inlet
morphodynamics
Wadden Sea

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Student theses

master thesis

(c) 1997 Van Marion, B.B.