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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 prin

The development of a theoretical description of how sound interacts with flocculating sediments has been lacking and this deficiency has impeded sound being used to extract quantitative suspended sediment parameters in suspensions containing flocs. As a step towards theoretically examining this problem a relatively simple heuristic approach has been adopted to provide a description of the interaction of sound with suspensions that undergo flocculation. A model is presented for the interpretation of acoustic scattering from suspensions of fine sediments as they transition from primary particles, through an intermediate regime, to the case where low density flocs dominate the acoustic scattering. The approach is based on modified spherical elastic solid and elastic fluid scatterers and a combination of both. To evaluate the model the variation of density and compressional velocity within the flocs as they form and grow in size is required. The density can be estimated from previous studies; however, the velocity is unknown and is formulated here using a fluid mixture approach. Uncertainties in these parameters can have a significant effect on the predicted scattering characteristics and are therefore investigated in the present study. Furthermore, to assess the proposed model, outputs are compared with recently published laboratory observations of acoustic scattering by flocculating cohesive suspensions.

The development of a theoretical description of how sound interacts with flocculating sediments has been lacking and this deficiency has impeded sound being used to extract quantitative suspended sediment parameters in suspensions containing flocs. As a step towards theoretically examining this problem a relatively simple heuristic approach has been adopted to provide a description of the interaction of sound with suspensions that undergo flocculation. A model is presented for the interpretation of acoustic scattering from suspensions of fine sediments as they transition from primary particles, through an intermediate regime, to the case where low density flocs dominate the acoustic scattering. The approach is based on modified spherical elastic solid and elastic fluid scatterers and a combination of both. To evaluate the model the variation of density and compressional velocity within the flocs as they form and grow in size is required. The density can be estimated from previous studies; however, the velocity is unknown and is formulated here using a fluid mixture approach. Uncertainties in these parameters can have a significant effect on the predicted scattering characteristics and are therefore investigated in the present study. Furthermore, to assess the proposed model, outputs are compared with recently published laboratory observations of acoustic scattering by flocculating cohesive suspensions.

A series of controlled laboratory experiments have been conducted to investigate the backscatter of high frequency sound (3-5 MHz) from suspensions of fine sediment in its unflocculated (primary) state and at various levels of flocculation. The size and fall-velocity distribution of the flocs was determined using an optical system and a settling tube, thus allowing floc density to be determined. The measurements have conclusively demonstrated that the acoustic properties of the flocculated particles are not solely controlled by the primary particles; some aspect of the floc structure is influencing the scattering characteristics. The overall trend is for the form function (Ks) to increase as the degree of flocculation increase. This trend was also observed in the total scattering cross section (inline image) but this result is dependent on the assumption that viscous absorption for flocculated particles is negligible. The measured scattering properties are compared to the predicted values from two theoretical models, the elastic (ES) and fluid sphere (FS) models. While the results show that, in their current form, neither model is capable of adequately representing the scattering characteristics of a suspension of flocculated particles, the two models did provide upper (ES) and lower (FS) bounds to the measurements. In terms of the operational use of acoustics to measure the concentration of flocculated sediments, empirical relationships could be fitted to the observations but, until a better theoretical understanding of how sound interacts with flocculated particles is achieved, the fitting of such empirical relations may be somewhat premature.

A series of controlled laboratory experiments have been conducted to investigate the backscatter of high frequency sound (3-5 MHz) from suspensions of fine sediment in its unflocculated (primary) state and at various levels of flocculation. The size and fall-velocity distribution of the flocs was determined using an optical system and a settling tube, thus allowing floc density to be determined. The measurements have conclusively demonstrated that the acoustic properties of the flocculated particles are not solely controlled by the primary particles; some aspect of the floc structure is influencing the scattering characteristics. The overall trend is for the form function (Ks) to increase as the degree of flocculation increase. This trend was also observed in the total scattering cross section (inline image) but this result is dependent on the assumption that viscous absorption for flocculated particles is negligible. The measured scattering properties are compared to the predicted values from two theoretical models, the elastic (ES) and fluid sphere (FS) models. While the results show that, in their current form, neither model is capable of adequately representing the scattering characteristics of a suspension of flocculated particles, the two models did provide upper (ES) and lower (FS) bounds to the measurements. In terms of the operational use of acoustics to measure the concentration of flocculated sediments, empirical relationships could be fitted to the observations but, until a better theoretical understanding of how sound interacts with flocculated particles is achieved, the fitting of such empirical relations may be somewhat premature.