Modelling the long-term morphological evolution of tidal embayments
van Maanen, B. (2011). Modelling the long-term morphological evolution of tidal embayments (Thesis, Doctor of Philosophy (PhD)). University of Waikato, Hamilton, New Zealand. Retrieved from http://hdl.handle.net/10289/5580
Permanent Research Commons link: http://hdl.handle.net/10289/5580
A numerical model was developed to improve the understanding of the long-term morphological evolution of tidal embayments. Morphological change was simulated as a result of the interactions between hydrodynamics, sediment transport, and the evolving topography. Numerical simulations indicate that these morphodynamic interactions can lead to the initiation of tidal channels and potentially give rise to large scale channel pattern development. The tidal range and the depth of the initially unchannelized tidal basin determined the time scale over which the channel network developed. Channels and intertidal areas rapidly formed when the basin was shallow and the tidal range large. For a large tidal range and a deep tidal basin, the tidal flow imported large volumes of sediment. The large water depths inhibited the formation of channels and the imported sediment formed a flood-tidal delta. The flood-tidal delta grew and became shallower over time until it became incised by channels. Ultimately, a complete channel network developed. Changes in the morphology of a deep basin were slowed down when the tidal range was small and the channel network then remained underdeveloped over long time scales. All the simulated morphologies, with different combinations of the tidal range and depth of the basin, evolved toward a state of less morphodynamic activity and obtained a hypsometry which resembles those of natural systems. Basins with well-developed channel networks were used to explore the response of tidal embayments to sea level rise. During sea level rise, the intertidal geometry adjusted to the changing environmental forcing conditions. Tidal channels became larger and more widely-spaced and expanded landward because of headward erosion. This landward shift of the channel network can be accompanied by a change in the asymmetry between the flood and ebb tidal currents. Sea level rise can even lead to a transition from exporting to importing sediment. These findings indicate that morphodynamic interactions need to be included in the study of sea level rise impacts on tidal systems. The morphodynamic model was extended to account for the interactions between mangroves and physical processes. Mangroves affected hydrodynamics and sediment dynamics in a variety of ways. In turn, hydrodynamic conditions controlled the colonization, growth, and dying of mangroves. Mangroves influenced channel network evolution by enhancing the branching of channels because the extra flow resistance in mangrove forests drove flow concentration and thus sediment erosion in between vegetated areas. On the other hand, mangroves hindered the landward expansion of channels. When the sea level was rising, mangroves increased the ability of areas to maintain an elevation above mid tide. Channel network expansion, induced by the rise in sea level, occurred differently when mangroves were present because they hindered both the branching and headward erosion of the expanding channels.
University of Waikato
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