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Assessment of the potential interactions of an oil spill with sediments on the west coast of New Zealand

There are limited data available on the interaction of spilt oil and sediment commonly found on New Zealand beaches and the data available have been obtained for intermediate state east coast beaches. The west coast of New Zealand’s north island generally has higher energy dissipative to ultra-dissipative beaches. Physical mixing of oil with beach sediment depends on both the depth of penetration into sediment and surface elevation changes. This study assessed physical mixing depths on three contrasting beaches; a highly dissipative open coast beach, a tidally controlled beach and a sheltered estuarine beach. Estimated and measured forcing conditions were correlated with vertical maxima of disturbance. The use of spatially discrete, non-averaged measurements of the depth of disturbance allowed spatial variation to be interpreted. Surface elevation changes were evaluated in conjunction with depth of disturbance measurements which allowed morphological features to be correlated with mixing depths alongshore and crosshore. Measurement of large scale morphological change also allowed interpretation of maximum potential oil burial depths. Oil settling experiments were carried out to evaluate oil settling times and behaviours. Morphological response is a function of changing incident wave regimes, currents, pre-existing morphology and tidal range. Large-scale erosive events have been recorded and observed at Ngarunui Beach that change the bed elevation in excess of 5 m, while small bed level variation occurs on the scale of decimetres during each tidal cycle. It was found that disturbance depths varied substantially in the cross-shore and longshore during all experiments. Wave breaking was determined to be the main mechanism for sediment mixing. Hence, the significant variation across the beach is attributed to the complex morphology of the beach. The areas most exposed to wave breaking exhibited the most disturbance at Ngarunui Beach; larger values of disturbance in the mid intertidal zone at Ngarunui Beach correspond with the zone that is most exposed to wave breaking. Cross-shore bimodal distributions of mixing were not observed. Swash processes dominated in the high intertidal zone with accretion occurring during spring tides however swash processes have limited effects on this beach, with mixing values greatly reduced under these processes. A tidally controlled beach located within the estuary close to the harbour entrance experienced significantly larger mixing depth values when no waves were present due in part to stronger currents and greater inundation during spring tides. The sheltered estuarine beach within the harbour experienced minimal mixing depths. Values for the vertical limits of the mixing layer exceed 40 % of the breaking wave height, Hb, for reflective beaches, while on dissipative beaches, theory predicts that the values will be extremely reduced as wave energies are dispersed across wide surf zones. However, in this study, disturbance values were higher than those previously reported in the literature for dissipative beaches. Using parameters for wave obliquity and beach slope, the average mixing depths could be somewhat predicted using the method of Bertin et al. (2008). Significant variation between and within locations means that use of this model could significantly underestimate depths of disturbance and hence oil burial on the west coast of New Zealand’s North Island. Assessment of the interactions between oil and sediment in the laboratory indicated preferential bonding of oil with heavy minerals common on west coast New Zealand beaches. This implies that oil is more likely to form stable oil-mineral aggregates on west coast beaches compared to east coast beaches with low heavy mineral content.
Type of thesis
Park, J. A. (2016). Assessment of the potential interactions of an oil spill with sediments on the west coast of New Zealand (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/10804
University of Waikato
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