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Morphodynamics of Ebb-Tidal Deltas

The purpose of this study was to learn more about the significant drivers of ebb-tidal delta morphology using observational methods and fundamental physical relationships between forcing conditions and morphological response. Two techniques were adapted in a novel way to study the dynamics of geomorphic features at an ebb-tidal delta. A 5-year long record of video imagery was used to observe a natural mixed-energy ebb-tidal delta in the field. A semi-analytical ebb-jet model, which was coupled to an exploratory morphological model, was developed and used to explore the interactions between tidal currents, waves, and morphology, and to test the sensitivity of morphological development and response to changes in forcing conditions. A detailed video-based observational record was used to identify and track geomorphic features over 5 years at an ebb-tidal delta on the energetic west coast of New Zealand at Raglan by using depth-limited wave-breaking patterns as a proxy for the position of shallow sandbars. Oblique 20-minute averaged time-exposure images were geo-rectified to provide detailed spatial measurements of ebb-tidal delta features such as the terminal lobe, mouth bar, channel margin linear bars (or levees), and swash bars over the 5-year duration. Movements of these features were quantified and related to wave and tidal forcing, including seasonal and interannual trends in wave climate. In general, the low-energy restorative summer waves led to a more cuspate terminal lobe, while in the high-energy erosive winter waves straightened the terminal lobe and moved it further seaward than the long-term average. Movements throughout the delta were intermittent between less active periods, with the fastest swash bar migrations occurring during the transition between seasons, namely winter to spring and summer to autumn. A semi-analytical model for ebbing tidal jet flow was developed based on the balance of momentum between inertia, bed friction, turbulent mixing, and wave effects. Previous analytical jet models (Özsoy and Ünlüata, 1982; Joshi, 1982) were extended to include the effects of directly opposing breaking waves. The model was calibrated and compared with scaled laboratory measurements (Ismail and Wiegel, 1983) and numerical simulations (Nardin et al., 2013) of river jets flowing over flat bathymetry with non-breaking waves, and with detailed field measurements of jet flow and wave dissipation at New River Inlet, North Carolina (e.g. Wargula et al., 2014). The jet model demonstrated the influence of opposing breaking waves on ebb-jet currents and jet width, along with the emergence of a point of flow convergence. The contribution of wave effects to the momentum balance were shown to impact the rapid slowing of jet flow, overall extent of an ebb-jet, and increase the jet width agreeing with previous studies (Nardin and Fagherazzi 2012; Nardin et al., 2013; Olabarrietta et al., 2014). Using a channelization parameter to emulate the ability of channel levees to constrain jet spreading, the model was calibrated over nine and validated over sixty-one ebb-events, respectively, to flow and wave measurements at New River Inlet. The model predicted the jet conditions well, receiving an overall excellent skill score. However, the model over-predicted the dissipative effects to jet flow of depth-limited wave breaking over the shallow ebb-shoal. The calibrated friction coefficient was roughly an order of magnitude higher than measured in the field, suggesting that the friction term was absorbing underrepresented processes. Pressure gradient was a neglected process identified as potentially significant, but was effectively included in the jet spreading term. The jet model was coupled to sediment transport formulae to form an exploratory type morphological model for exploring the sensitivities of ebb-tidal shoal and channel morphology to changes in forcing conditions and sediment characteristics. Equilibrium ebb-shoal morphology was formed over many model iterations and shown to be morphologically dependent on forcing conditions. Wave-dominant conditions developed ebb-shoals in closer proximity to the tidal inlet, with wider channels at their seaward end, than jet-dominant conditions. Increases in jet velocity increased the rate of development of the delta more than did increases in wave height. Sediment characteristics had very little influence on the equilibrium morphology, but did influence the rate of development. Short-term morphological responses of established morphology were sensitive to the initial channel width, with wide channels being most susceptible to the effects of waves and jet interaction. A double-barred ebb-shoal was shown to develop under mixed-energy conditions, with a shoreward bar being influenced by jet-flow and the seaward bar being influenced by wave breaking. The observational data and modelling tools are used to explore ebb-tidal delta morphodynamics. Themes include the dependence on environmental conditions, transitions from equilibrium, and the competing influence of jet currents and opposing waves on ebb-shoal and channel morphology. The work asserts that ebb-tidal delta morphology forms as a result of dynamic balance between the dominant forces responsible for sediment transport, namely the ebbing tidal jet currents and opposing waves.
Type of thesis
Harrison, S. R. (2015). Morphodynamics of Ebb-Tidal Deltas (Thesis, Doctor of Philosophy (PhD)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/9810
University of Waikato
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