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Hypsometric and geometric controls on hydrodynamics, tidal asymmetry, and sediment connectivity in shallow estuarine systems

Estuaries and tidal basins are highly dynamic coastal systems that serve as a transition zone between the river and the ocean. The morphological evolution of these diverse environments is modulated by non-linear feedbacks between tides, meteorological forcing, and sediment transport processes. This thesis focuses on the fundamental links between these physical processes, and the geomorphologic characteristics of shallow estuarine systems, specifically: (i) how shallow basin geometries and hypsometries affect hydrodynamics and tidal asymmetry, (ii) how wind-induced currents modify velocity asymmetry in shallow basins, and (iii) defining the relationships between geometry, hypsometry, and sediment connectivity inside shallow estuarine systems. Linking geomorphological characteristics and tidal processes in shallow tidal basins The links between tidal basin geometry and hypsometry, bed shear stress patterns, tidal velocity- and slack water asymmetry, and hypsometric profile shapes were explored for six shallow microtidal basins of Tauranga Harbour, New Zealand. Model results, obtained from a depth-averaged numerical model developed in Delft3D for the full estuarine system, indicated that tidal distortion increases with distance from basin entrance. A simple ratio between tidal basin width and entrance width was defined to describe the planform shape of the basin. This metric, termed the ‘basin dilation factor’ indicates whether a basin can be designated as a divergent or convergent geometry. Shallow basins with a constricted geometry and relatively deep entrance channels were found to be associated with small bed shear stress values and high rates of flood-directed tidal velocity asymmetry in the sheltered basin centres. These results suggest substantial potential for sediment deposition of larger particles. Moreover, slack water asymmetry within these basins was weakly ebb-directed, indicating a small potential for export of fine sediments. These divergent, depositional basins were found to be characterized by convex hypsometric profiles with elevated intertidal regions. Conversely, unconstricted, convergent basins were associated with larger bed shear stress values and more ebb-directed tidal velocity asymmetry within basin centres. Consequently, there was limited potential for overall sediment deposition inside these basins. The slack water asymmetry was weakly flood-dominant, suggesting limited potential for fine sediment input. The comparatively high-energy conditions within these exposed tidal basins were associated with a less convex hypsometric intertidal profile. This study highlights the impacts of specific geomorphologic basin characteristics on tidal processes in shallow estuarine systems. The ability to predict the links between tidal asymmetry and morphological changes in tide-dominated systems is beneficial for coastal management, as the morphological evolution of estuarine systems affects coastal ecosystem functioning, port and estuary navigability, and potential for coastal protection. Understanding the effects of wind-driven currents on velocity asymmetry in shallow tidal basins Numerical modelling experiments were conducted for a series of idealized basins in which planform shape and bathymetry were varied. The model results were used to examine how wind-generated currents modulate horizontal velocity asymmetry patterns in shallow tidal basins. This study revealed that wind-driven currents primarily influence mean and peak flow velocities inside the basins, with a limited effect on tidal harmonics. Faster wind speeds led to more extreme horizontal velocity asymmetry (larger velocity asymmetry values), without substantially modifying overall spatial patterns of velocity asymmetry. The velocity asymmetry was found to be strongly depth-dependent, with changes to asymmetry patterns being most evident for wind speeds of 6 m/s and greater, and for wind directions parallel to the main axes of the tidal channels in the basins. Shallow intertidal regions inside the basins were characterized by a downwind-directed increase in velocity asymmetry, whereas deeper subtidal channels experienced asymmetry changes in the opposite direction. Wind event duration and timing were also found to influence the velocity asymmetry patterns. The differences between the relative size of the peak flood- and ebb directed currents were most evident for wind events with a duration of 6 hours or less that coincide with flooding tides. The results of this study highlight that hydrodynamics, sediment transport, and morphological evolution in shallow estuaries are modulated by tidal processes as well as meteorological forcing. Since anthropogenically induced climate change is expected to increase the intensity of extreme meteorological events, the ability to predict future pathways of morphological change in shallow estuarine systems, based on specific meteorological conditions as well as well-defined local tidal regimes, is vital for the management of these dynamic systems. An examination of sediment connectivity in a shallow estuarine system The sediment connectivity framework was used to examine links between hydrodynamics, sediment transport pathways, and local hypsometry inside a shallow estuarine system (Tauranga Harbour, New Zealand). The estuary was divided into twenty geomorphic cells, representing tidal channels, intertidal flats, and shallow sub-basins. Depth-averaged numerical modelling simulations were carried out to quantify tide-driven sediment connectivity between the cells for five sediment grainsize classes. Connectivity matrices were developed for the different grainsize classes, based on modelled sediment mass loads. Sediment connectivity inside the estuary was found to be modulated by tidal energy, estuarine morphology (depth), sub-basin hypsometry and geometry (planform shape), and sediment characteristics. The connectivity matrices, combined with metrics such as link density and cell strength, illustrated that sediment mass loads, and hence connectivity, were largest in the high-energy environments of the deep tidal channels located in the main estuary. In the more sheltered upper estuary, and inside the shallow sub-basins, connectivity was reduced. For fine sediments (< 125 μm) connectivity was found to be substantial throughout the shallow estuarine system, with estuary-level connectivity (link density) being greater than 50%. Link density for coarser sediments (> 275 μm) was found to be ~20%, with transport pathways primarily confined to the deeper regions of the estuary. An in-depth analysis of sediment transport pathways between the shallow sub-basins emphasized that flood-dominant, divergent basins with a convex-shaped hypsometric profile mainly function as sediment sinks, whereas ebb-dominant convergent basins act as sediment sources. This thesis highlights the substantial dependence of tidal asymmetry, morphology, sediment transport and connectivity on hypsometry, geometry, and grainsize characteristics inside shallow estuaries. Additionally, the effects of wind-driven currents on the non-linear physical processes inside these highly dynamic environments are described. Overall, this work provides a novel elucidation of some of the relationships between geometric parameters and forcing mechanisms applicable to many shallow coastal systems.
Type of thesis
The University of Waikato
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