Beach recovery and studies in accretive sediment transport

Shimamoto, Tomoko (2016). Beach recovery and studies in accretive sediment transport PhD Thesis, School of Civil Engineering, The University of Queensland. doi:10.14264/uql.2016.301

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Author Shimamoto, Tomoko
Thesis Title Beach recovery and studies in accretive sediment transport
School, Centre or Institute School of Civil Engineering
Institution The University of Queensland
DOI 10.14264/uql.2016.301
Publication date 2016-06-03
Thesis type PhD Thesis
Supervisor Tom Baldock
Peter Nielsen
Total pages 166
Language eng
Subjects 090599 Civil Engineering not elsewhere classified
Formatted abstract
Erosion has been a focal point in sediment transport research while sediment accretion has been neglected until recent times. This thesis addresses some components of accretive sediment transport through modelling and new experimental measurements.

The existing Grab and Dump model (Nielsen, 1988), a simple sediment transport model over rippled beds, is generalised to reflect a wider range of flow conditions, including acceleration skewed flows, boundary layer streaming, sheet flows, and cases with superimposed current. The revised model is compared to another semi-empirical sediment transport model and is found to have similar predictive abilities for small scale laboratory data. The results show that there are limitations to the model and more work is required to accommodate flows with no acceleration skewness and under the sheet flow regime.

The local approximation method for non-linear irregular waves (Nielsen, 1989) is updated for converting surface elevation data into velocity, as many sediment transport models require velocity time series as input while surface elevation data are more readily measured and available. Data from three experiments are used to investigate this method, which was previously used to convert pressure measurements to surface elevation. The results are compared to the spectral transfer method, and found to perform similarly in most locations apart from near the breakpoint, where it performs better due to an additional multiplier which accounts for the increasingly negative time-mean velocities closer to shore.

New beach profile experiments were undertaken with an emphasis on the beach profile at equilibrium under random waves. The beach profiles were subjected to waves of various heights, with each case run until the profile appeared to be at equilibrium, for both accretive and erosive conditions. It was observed that the profile under accretive conditions behaves in a cyclic manner, where the outer bar is destroyed and recreated from the inner bar moving offshore, a process which also has a much longer time scale compared to the profile evolution under erosive conditions.

The experimental profile measurements are used to evaluate an equilibrium type total sediment transport model utilising the relationship between the dimensionless fall velocity parameter and cross-shore bulk sediment transport proposed by Baldock et al. (2011). The model is applied to both laboratory and field measurements, and compared to an existing equilibrium shoreline model. The model is found to perform well under laboratory conditions where the profiles reach equilibrium but faces limitations with field measurements, which is an area for future work.
Keyword Sediment transport
Acceleration skewness
Bed shear stress
Equilibrium beach profile
Beach accretion
Beach morphodynamics

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Created: Wed, 25 May 2016, 11:30:10 EST by Tomoko Shimamoto on behalf of Learning and Research Services (UQ Library)