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Sediment transport in the Firth of Thames mangrove forest, New Zealand

Mangroves provide a unique habitat for many species. In addition to being biologically significant ecosystems, mangroves help protect coastal areas through the dissipation of tidal currents and wave energy. However, mangroves are unpopular with some communities in New Zealand and removal of them is often considered in management strategies. In order to effectively manage these vegetated coastal areas and predict growth and/or decline of mangrove systems, it is necessary to have a detailed understanding of how the hydrodynamic processes within forests influence sediment transport and deposition. A 6-day long experiment was conducted in the Firth of Thames mangrove forest to examine hydrodynamic processes that influence sediment transport from the intertidal mudflat across the fringing region of the mangrove forest into the forest itself. A unique deployment system was utilised to minimise environmental disturbance during the data collection process at 5 sites. Site 1 was located on the mudflat and site 2 was in the fringe area (i.e. the boundary between flat and forest). Sites 3 to 5 were located at increasing distances inside the forest. Results indicate wave activity causes a net offshore flux of sediment at site 1 (approximately 850 kg/m2 over the duration of the experiment), contrary to expectations. However, at all other sites, there was a net onshore flux of sediment from the fringe into the forest (maximum at site 4 of 113.3 kg/m2). Minimal scour in the experiment area indicates that sediment is primarily supplied by advection from other regions of the system. Flocculation is a key process in cohesive sediment transport. One of the aims of the study was to understand the controls on flocculation within this vegetated system. Despite difficulties inherent in measuring flocs, floc images were successfully collected from some sites. An initial qualitative analysis performed on images from 2 sites (1 and 3) over 2 tides revealed a greater quantity of large flocs on the mudflat than in the forest and that in general, flocs were larger on the ebb tide. These results imply that settling velocities on the mudflat are larger than within the forest and settling velocities are larger during ebb than flood tides.
Type of thesis
Lovett, N. J. (2017). Sediment transport in the Firth of Thames mangrove forest, New Zealand (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/11568
The University of Waikato
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