1D morphodynamical modelling of macro-tidal sandy beaches under pure tidal currents

Huynh, Van Long (2019) 1D morphodynamical modelling of macro-tidal sandy beaches under pure tidal currents. PhD thesis, University of Nottingham.

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Abstract

This research aims to study bed level evolution of a sandy beach under tidal effects over a long-time scale, using a fully-coupled hydro-morphodynamical numerical model.

The proposed model originates from a system comprising nonlinear shallow water equations (NSWEs), an advection equation to describe suspended load and the Exner equation for bed change. Here non-flux conservative NSWEs are used, since no shock waves are formed by tidal motion. The moving boundary at the shoreline is treated by a coordinate transformation method (CTM) which can capture accurately the time variation of the shoreline. The absorbing-generating seaward boundary allows description of the outgoing hydrodynamic signals along with incoming signals. We also include the seaward boundary condition proposed by Incelli et al. [2015] to allow sediment to exit and enter the domain whilst maintaining the conservation. A fully coupled model with both bed-load and suspended-load transport is developed. The model performance is verified in both hydrodynamics and morphodynamics against available analytical and numerical solutions.

The morphodynamic evolution of a sandy beach under macro-tidal currents over a duration of 100 tidal cycles is investigated. We consider both bed load and suspended load transport, either in individual or combined mode. Furthermore, we also consider how the coupling could affect bed change by considering the fully coupled against the quasi coupled one.

Regarding suspended load only, erosion is observed offshore; while deposition always occurs at the intertidal zone. The non zero bed change under symmetrical tide due to settling lag effect, which is the difference in phase between the suspended sediment concentration and velocity. Bed change due to suspended load depends not only on the exchange parameter but also on the bed mobility parameter. While bed mobility parameter mainly affects the bed change rate, the exchange parameter affects both bed change rate and patterns. Particularly, in case of low exchange parameter value, erosion is also observed in the sub-tidal region.

Bed change due solely to bed load is only significant in the intertidal region. This is partly due to the shock condition applied in the shoreline boundary. Moreover, the shoreline velocity due to transient from initial still water is much higher than normal, resulting in higher bed change after the first cycle compared to the remaining simulation time. Although the hydrodynamic nonlinearity is small in tidal motion, it still exists and is higher in the intertidal region than further offshore. As a result, the spatial derivatives of bed load flux is not zero and is higher in the intertidal region. Bed change under suspended load transport is more dominant than under bed load transport. Moreover, bed change under bed load converges after a shorter time than that due to suspended load.

The results of fully-coupled and quasi-coupled models qualitatively agree in bed change magnitude and pattern. The difference is proportional to bed change. In particular, the higher the bed change rate is, the greater the difference between the two model is. The quasi-coupled model obtains total suspended load or bed load over a tide. Then bed change is only updated after every tide. The comparison results have shown that in cases of high bed mobility or low exchange parameter, the coupling effect is significant.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Dodd, Nicholas
Zhu, Fangfang
Keywords: Coastal Engineering, Tidal motion, Nonlinear shallow water equations, Hydro-morphodynamics numerical model
Subjects: G Geography. Anthropology. Recreation > GC Oceanography
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 55818
Depositing User: Huynh, Van
Date Deposited: 05 Apr 2019 10:08
Last Modified: 05 Apr 2019 10:08
URI: http://eprints.nottingham.ac.uk/id/eprint/55818

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