The Sediment Dynamics of Ahuriri Estuary, Napier, New Zealand
Eyre, T. M. (2009). The Sediment Dynamics of Ahuriri Estuary, Napier, New Zealand (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from http://hdl.handle.net/10289/4290
Permanent Research Commons link: http://hdl.handle.net/10289/4290
Ahuriri Estuary is a shallow, microtidal estuary located in Hawke's Bay, New Zealand. Sediment transport pathways and areas of potential erosion and accretion of Ahuriri Estuary are identified to enhance the understanding of the fate of contaminants entering the estuary and to provide a basis for future large-scale modelling applications. Fine sediments have a high adsorption capacity for heavy metals and nutrients, which may result in the accumulation of contaminants in estuarine environments. Extensive land claim of previously inundated areas uplifted by the 1931 Napier earthquake for industrial, urban and agricultural expansion has led to an increase in heavy metal and nutrient runoff into the estuary. Characterising the hydrodynamics and sediment transport of the estuary is imperative for the prediction and mitigation of sedimentation and contaminant accumulation. A combination of hydrodynamic and sediment transport modelling using the DHI MIKE 21 suite was used to run a series of scenarios of sediment release into the estuary and subsequent dispersal. The MIKE 21 Flow model was calibrated and validated using field data from two field deployments conducted during a spring and neap tide. The hydrodynamic model predicted current direction and velocity patterns in concurrence with measured data. Residual circulation patterns in Ahuriri Estuary were ebb-dominated, indicating a likely net downstream movement in sediment. Flow was highly channelised within the middle - lower estuary, with a low energy regime in the upper estuary. This suggested that sediment introduced into the estuary from the Taipo Stream is likely to be flushed from the estuary if it reached the middle estuary. However, flow velocities and lack of residual currents in the upper estuary resulted in a large proportion of sediment introduced during storm runoff settling out of suspension, resulting in net deposition in close proximity to the source. Sediment grain-size and consequently the settling velocity were found to be the main contributing factor to sediment dispersal. Fine silts were found to be transported the furthest downstream with a large proportion being transported from the estuary mouth and the model domain, respectively. A large proportion of coarser silts and fine sands were found to settle out of suspension in close proximity to the sediment source. An increased settling velocity resulted in an increased influence of the simulation conditions at the time of sediment release, as it was the initial transport of coarser grain-sizes which determined their ultimate depositional location. Sediment deposition was greatest in close proximity to the source and in upper intertidal regions, which are a major sink for suspended sediments due to their sheltered nature. This was correlated with greater organic carbon content and finer grain-sizes in these areas, suggesting a depositional environment. A general trend of fining of grain-size from high energy to low energy environments was identified. In these areas, sediment reworking due to tidal action and wave-related processes was limited.
The University of Waikato
All items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.
- Masters Degree Theses