Nigel P. Aird

Abstract

This contribution utilises a multifaceted approach to investigate the physical processes responsible for the onshore migration of ebb tidal sandbanks at an estuarine inlet dominated by low energy waves. A coastal video system was used to obtain two-weekly quantitative measurements of the position, plan form and crest depth of the landward migrating sandbanks over a five year period which encompassed four consecutive events. These data were supplemented with a 35 year photographic record of the inlet's development. Onshore sandbank migration was characterised by mean and maximum rates of I to 2 m. day"1 and 5 m. day" 1 respectively. The migration rate was found to be highly correlated (R2 = 0.7) with, and linearly related to the ratio of the incident wave height to the crest depth particularly prior to shore-attachment. The plan form, area and crest depth of the sandbanks are a function of the sediment availability. In the inlet's current morphologic mode two plan form geometries are typically observed. These are crescentic and elongate forms (high aspect ratios) where the latter have their major axis transversely-orientated with respect to the coast. Which of these forms develops is dependent on the chronology of wave energy and the crest depth in relation to the tidal water level variation. Elongate transverse morphologies are associated with low relief sandbanks which are synonymous with periods of relatively low sediment availability. Conversely the crescentic morphologies typically occur when sediment availability is higher and the depth to the sandbank crests is shallower. A unique set of in-situ measurements of waves, currents and sediment transport were obtained from four positions on a sandbank in the mid term of its onshore migration. Analysis of the data revealed that the sandbank was dominated by onshore directed sediment transport in the shallow surf zone where current maxima occurred. The gradients in transport were highest on the flood tide. An energetics analysis of the data revealed that short wave stirring and wave driven mean flows were the physical processes responsible for morphological change. The mean flows are in effect longshore currents since they are generated by strong refraction and focussing of the incident waves by the morphologies. These generate a zone of wave convergence over the sandbanks when in the submerged state and very oblique wave breaking along the flanks when the features are exposed by the tide. A numerical model (MIKE 21) was subsequently applied in order to simulate the wave driven patterns of erosion and deposition over the sandbanks using both idealised and measured boundary conditions. The numerical experiments determined that there exists a dichotomy in the patterns of erosion and deposition which are laterally constrained in the submerged state and divergent in the exposed state. Morphological evolution was therefore governed by the variable residence times of the causative wave driven processes at different tidal elevations. It was found that low energy waves when combined with neap tides promoted shoreward elongation through the dominance of the patterns of deposition in the shallow submerged state. Higher energy conditions were predicted to promote a degree of broadening in the longshore dimension and increased crest elevation. This was caused by the patterns of deposition being dominated by both the laterally constrained (high tide) and divergent (low tide) patterns due to their longer residence times.

Document Type

Thesis

Publication Date

2009

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