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The control of wind and waves on sediment transport asymmetry and the long-term morphological development of estuaries

Abstract
Most theories on the equilibrium shape of estuarine basins highlight the dominance of tidal asymmetry in controlling the hydrodynamic and morphodynamic feedbacks that ultimately lead to a stable morphological state. This thesis uses field measurements, numerical modelling and sediment deposition records to investigate how waves interact with tidal processes and how these interactions influence sediment transport and the non-linear feedbacks between hydrodynamics and estuarine basin morphology. Numerical modelling experiments were conducted over a series of idealised estuaries representing different stages of infilling, which show that wind and waves are far more efficient than tides at shaping intertidal areas, with the effect being subtly dependent on the depth distribution in the intertidal. Moreover, the wind and wave climate can substantially alter the hydrodynamic regime over the entire estuary and in intertidal areas can exert a much greater control on asymmetry than tidal currents alone, which dominate in the deeper channels. Under the effect of tides alone, currents over intertidal flats are to found to remain flood-dominant as the estuary infills, thus promoting continued accretion until tidal currents become too weak to entrain sediment. Therefore, estuaries with only tidal currents are likely to evolve into in-filled areas of salt-marsh or mangrove with drainage channels, whereas fetch-aligned estuaries (in which wind-waves are common) have a greater probability of attaining deeper hydrodynamically-maintained stable intertidal areas. Although waves have little direct influence on hydrodynamics within the subtidal channel, the tidal asymmetry is controlled by the height and volume of the intertidal areas and therefore is indirectly influenced by wave activity. The higher-energy, infrequent storm-wave events can considerably modify estuarine morphology over short timescales whereas lower-energy but perpetual effects like the tides operate continuously and so the relative contribution of such events in shaping the long-term morphological evolution can be considered on a range of timescales. Within this thesis a combination of hydrodynamic measurements, sediment deposition records and numerical modelling are used to determine the conditions under which observed waves are morphologically significant. Morphological significance is defined as when waves influence tidal and suspended sediment flux asymmetry and subsequently infilling over geomorphological timescales. By comparing a fetch-aligned and a non fetch-aligned mesotidal basin, it is shown that for a sufficiently large fetch, even small and frequently occurring wind events are able to create waves that are morphologically significant. Conversely, in basins with a reduced fetch, wave events are less frequent and therefore of far less morphological significance. The role of tidal range in controlling sediment transport is investigated through consideration of its influence on bed shear stress, τmax. The decrease in bed shear stress associated with greater water depths is compared to increases owing to the larger fetch that accompanies the increased tidal range and the generally stronger currents associated with larger tidal range. Here, it is shown that during neap tides the reduction in water depths around high water and tidal currents are not offset by the reduction in fetch. Thus, it is shown that the basin-averaged τmax is similar during both spring and neap tides in addition to the shorter duration of slack water during neap tides. Consequently, as τmax is lower in the subtidal channel during neaps, the sediment gradient (and hence sediment transport potential) between tidal flats and channels is greater. Thus it is concluded that sediment deposition potential may actually be increased during a neap tide. This result is in sharp contrast to previous observations from microtidal wave-dominated environments, in which differences between spring and neap result in increased erosion during spring tides. Overall, this thesis shows that short period locally generated waves can be considered a morphologically significant hydrodynamic process within estuaries. With increased fetch, waves become more morphologically significant relative to tides as even low, frequent wind speeds are able to generate waves that are capable of controlling patterns of sediment transport.
Type
Thesis
Type of thesis
Series
Citation
Hunt, S. (2016). The control of wind and waves on sediment transport asymmetry and the long-term morphological development of estuaries (Thesis, Doctor of Philosophy (PhD)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/10684
Date
2016
Publisher
University of Waikato
Rights
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