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Modelling the hydrodynamics within the mangrove tidal flats in the Firth of Thames

Understanding how mangroves control shoreline stability, through altering hydrodynamic and sedimentation patterns, is important to predicting how resilient our coastline is to climate change. Recent work has shown that some mangrove ecosystems may modify the morphology, and allow the seabed to evolve upward with rising sea level, and thus alleviating the pressure on coastal adaptation. This study is focussed on improving the understanding of the hydrodynamics within the mangrove habitat in the Firth of Thames, New Zealand. The mangrove habitat within the Firth of Thames is a shallow, muddy and rapidly prograding environment. The main aim of this study was to determine whether or not, the mangroves influence the dynamics of the tidal wave propagation, which in turn can have implications for the sedimentation patterns and shoreline stability within the Firth of Thames. A hydrodynamic model for the Firth of Thames was developed using Delft3D. A coarse resolution grid was created to simulate the offshore tidal wave propagation and used to force a nested, fine resolution grid within the mangrove intertidal flats. A field deployment was completed during May 2016, where in the offshore region water levels, current velocities and suspended sediment measurements were taken, and within the mangrove forest, water levels, bed elevations and vegetation characteristics were measured. Field measurements were used to calibrate both the overall and nested models. Comparisons between model outputs, with and without vegetation included in the model, indicate that the presence of mangroves does influence the tidal wave dynamics across the intertidal flats. Two of the main effects of vegetation were reduced current velocity and tidal amplitude. Regardless of whether mangroves were present, the model showed flood dominance across the upper intertidal flat and ebb dominance at the seaward edge of the intertidal flat. However, with vegetation included this pattern of tidal asymmetry was enhanced, due to the nonlinear effect of friction. A series of numerical experiments were also performed to understand the control of sea bed roughness and vegetation characteristics (vegetation roughness, pneumatophore height, plus pneumatophore and tree density) on the size, spatial changes and timing of tidal currents. Pneumatophore density had the largest influence on model outputs, with increasing pneumatophore density causing a reduction in currents and a delay in the drainage of the ebb tide. Based on the hydrodynamic model outputs and measurements of offshore suspended sediment concentrations, rough calculations were completed to estimate the net flux of sediment into the forest over time. Estimated sediment fluxes were surprisingly similar to the estimated volume deposited sediment between the 2005 and 2016 elevation surveys. Physical processes, such as tidal asymmetry, are likely to have caused the initial accretion and tidal flat development, making it suitable for mangrove growth. However, now that there is a vast area of mangroves, they are altering the hydrodynamics and therefore are also contributing to the accretion and overall stability of the system.
Type of thesis
Haughey, R. R. (2017). Modelling the hydrodynamics within the mangrove tidal flats in the Firth of Thames (Thesis, Master of Science (Research) (MSc(Research))). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/11129
University of Waikato
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