University of Twente Student Theses


Mangrove dynamics in the Richmond River’s estuary

Hoogeveen, S.J.W. (2020) Mangrove dynamics in the Richmond River’s estuary.

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Abstract:Coastal ecosystems such as mangroves can reduce risk of ooding and wave damage to people and infrastructure from wave. The continued provision of these coastal defence services by mangroves is dependent on their capacity to adapt to long-term (anthropogenic) stresses such as sea level rise, decrease in sediment supply and coastal squeeze. Mangrove forest width is thought to be an important factor for the adaptation capacity of mangroves. Therefore, the aim of this research is to gain insight in the morphodynamic response of two transects of different mangrove forest widths, when variations in river discharge and uvial sediment concentrations are considered. In order to accurately assess the impact of mangrove width on the hydro- and morphodynamics, a case study of the South Ballina mangrove forest, which is evidently characterising for mangroves along the coast of New South Wales (NSW), has been executed. Quantitative field data regarding topography, vegetation and sediment characteristics were obtained. Field observations concluded that the effect of mangrove width on the biophysical properties and interactions is limited. This study concludes that the estuarine location, bio-geophysical settings and transect properties are the major causes for the observed differences within the mangrove forest, and not the mangrove width. In addition to the field campaign, two depth-averaged process-based numerical models were developed in Delft-3D Flexible Mesh (DFM), which provided decent estimations of ow velocities and deposition rates. The two models allowed for comparison of the impact of mangrove width on the morphodynamic response to variable river discharges and uvial sediment concentrations, by means of model simulations. The results from the model study regarding the effect of mangrove width on the biophysical properties and interactions remains inconclusive, since no clear comparison could be made regarding variations in mangrove width. During low discharge conditions, both models showed a turnaround from ebb-dominance towards ood-dominance throughout the spring-neap cycle. This could lead to an accumulation of water in the estuary, resulting in net accumulation of sediments in the river stream and foreshore. River ood conditions will lead to larger velocities within the forest at the 'long' transect compared to the 'short' transect, and is attributed to the width of the ow domain of the model. Furthermore, this study concludes that the Richmond River has a limited capacity for river ood discharge. When this capacity is exceeded, ooding of the adjoining mangroves as a traditional oodplain occurs. The mangroves then provide for oodwater storage and discharge as well as sediment deposition. The findings in this study strongly indicate the need for a-periodic ood events to occur, in order for higher elevated parts of the mangrove forest to inundate and accrete. This study investigated a minor ood with a return period of one in two years. It is however likely that there is a limit towards the magnitude of the ood on one hand, and the amount of deposition in the forest on the other hand. This inverse proportion between the ood magnitude and the sediment trapping capacity should be investigated in future research.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Programme:Civil Engineering and Management MSc (60026)
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