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The interaction of buoyant river plumes with vegetation and consequences for sediment transport and deposition in coastal regions

Mangrove forests are one of the most prominent vegetated coastal habits in tropical and subtropical areas. These wetland ecosystems typically inhabit estuaries and tidally influenced river banks. The presence of trees helps to protect coastal regions through the dissipation of tidal currents and wave energy, forming an essential barrier against coastal erosion. River plumes are the primary mechanism of sediment delivery to these vegetated coastlines. As the buoyant freshwater merges with saline ocean waters, horizontal advection of the freshwater establishes the shape of the river plume and influences the distribution of sediments along the coast. In this thesis, I investigate the principal dynamics associated with the interaction of buoyant river plumes with vegetation. In particular, we investigate the interplay between mangrove vegetation, hydrodynamics, and sediment dynamics, specifically: (1) the principal drivers of sediment deposition within a mangrove-lined river delta, (2) the influence of river forcing and winds on the sediment transport, and (3) sediment dynamics of two adjacent and interacting coastal river plumes. Understanding the hydro-morphodynamics of a river plume in a mangrove environment: We developed a 3-D idealized Delft3D morphological model to explore the effects of vegetation on the river plumes and associated sediment transport patterns. Using an idealized model based on the Firth of Thames (FoT) mangrove forest located on the North Island of Aotearoa-New Zealand, we observed that, while sediment deposition occurred in the forest and tidal flats region of the model domain, the fringe region (between the vegetation and mudflat) experienced erosion. Compared to the eastern side, sediment deposition in the western side of the model domain was more prominent owing to the influence of Coriolis (Southern hemisphere). By examining the momentum balances on the surface and bottom layers in different regions of the river plume, we found that the principal balance between the bottom shear stress (enhanced by the presence of vegetation) and baroclinic pressure gradient largely controlled the sediment deposition in the riverine sections of the domain. Additionally, we found that vertical advection and diffusion during flood tide enhanced erosion in the fringe region of the mangrove forest. The advection of suspended sediment into the forest was controlled by factors including longer duration of high water slack at the forest fringe region, pressure gradients, and inertial acceleration. In the near-field region (close to the river mouth) of the river plume, during ebb tide, the barotropic and baroclinic pressure gradients coupled with Coriolis accelerations deliver sediments into the forest and mudflat regions. In the mid-field regions of the river plume, the magnitude of changes in the bed elevation was smaller due to larger Coriolis acceleration which arrested the spreading of the river plume. Furthermore, the far-field region (away from the river mouth) of the river plume experienced erosion due to more substantial tidal influence. Conversely, in the shallow forested regions, reduced movement of sediment in the offshore direction leads to an overall flood dominance. Linking sediment transport within river plumes to varying river flows and winds: Subsequent modeling experiments were carried out to investigate how the forcing factors of riverine discharge and wind velocities influence sediment transport within an idealized moderate-sized freshwater river plume. We assessed total sediment transport along with the relative contributions of riverine and bed-sourced sediment into a mangrove forest. Instantaneous and tidally averaged sediment fluxes were evaluated to investigate the critical linkages between the riverine flows, tidal influence, and winds in the presence of vegetation. In the near-field region of the river plume, riverine and bed-sourced sediment were dominant and found to be directed into the forest, indicative of an accretionary environment. In the mid- and far-field regions of the river plume, bed-sourced sediment was found to be the dominant contributor to total sediment transport and was directed out of the forest, indicative of sediment erosion. While the mass loads (directed into the mangrove forest) increased with an increase in riverine discharge, mass loads were relatively similar for large flow events. The river sediment delivered into the mangrove-forested regions was found to push into and through the forest front for large river discharges. The presence of 5 m/s wind velocities could alter the sediment deposition patterns in the mid- and far-field regions of the river plume. Despite the significant river momentum, strong winds (10 m/s) were also able to alter the sediment transport in the near-field region of the river plume. Moreover, results showed that in the far-field region of the river plume, strong winds combined with the effects of riverine discharge, Coriolis, and bed-shear stress affected the overall mass movement of sediment. In the case of easterly winds, a combination of tidal effects and wind stress controlled the sediment deposition into the forest. In the case of westerly winds, as the plume was pushed away from the forest, tidal effects controlled the total sediment transport. Understanding the effects of coalescing buoyant river plumes on sediment transport: We investigated how the interaction between plumes from two mangrove-lined rivers in close proximity affected coastal sediment transport patterns. In particular, I examined the links between the river plume coalescence and the trapping of sediment by mangroves, using an idealized 3-dimensional numerical. Sediment fluxes and relative contributions of riverine- and bed-sourced sediments were quantified along the western, central (located between the rivers), and eastern mangrove forests. The two river plumes coalesced into a single river plume that flowed along the edge of the central mangrove forest. As the river plumes were deflected to the left by Coriolis (Southern hemisphere), the total sediment fluxes were largest for the central and the western forest, with only modest sediment deposition along the eastern mangrove forest. Also, modeled transport fluxes along the central mangrove forest were the largest close to the rivers, consistent with the satellite imagery of the FoT field site. Flow variations through either river resulted in changes to the sediment transport patterns into the mangrove forests. In particular, an increase in flow discharge through the western river resulted in reduced contributions of the eastern river towards the total sediment transport fluxes. Conversely, an increase in flow discharge through the eastern river inhibited the contribution of western river-associated sediment towards the total sediment transport flux through the central, eastern, and western mangrove forests. Furthermore, this study revealed that 5 m/s wind velocities altered the expansion, coalescence, and overall sediment transport fluxes of the river plumes. The total sediment fluxes through the central and western mangrove forests were greatest for easterly winds. However, the total transport into the eastern mangrove forest was largest for the southerly winds, as the bulge of the eastern river plume was pushed into mangrove forests. These modeling experiments help elucidate the interaction of river plumes with mangrove vegetation. The work presented in this thesis highlights the complex non-linear interactions of hydro-morphodynamics, sediment transport, the effects of river flows and winds within mangrove environments, and the fate of riverine sediments in the bay of the Firth of Thames. Finally, the results presented here provide insights into the sediment trapping capabilities of mangroves under variable conditions and thus may help to predict their ability to function as a coastal defense mechanism under future sea-level rise and sediment supply scenarios.
Type of thesis
The University of Waikato
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