Historical shoreline change and beach morphodynamics at Rapahoe Bay, West Coast, New Zealand

Abstract

This thesis utilises a range of methodologies to investigate the historical shoreline change and beach morphodynamics at Rapahoe Bay, West Coast, New Zealand. Rapahoe Bay is a small embayment located 15 km north of Greymouth, and contains a complex and dynamic environment under a dominant swell condition. The objectives of this thesis include the investigation the coastline history through aerial photographs and relevant literature, identify and quantify historical shoreline change and the processes that have induced change, examine the short term and seasonal changes in beach profile, identify and quantify wave and transport process and to test the applicability of the zeta shoreline curve on a composite beach. This combined approach investigates the dynamics and process drivers involved in coastline change. This thesis contributes to the research gap of understanding morphodynamic behaviour and controls of composite beach under a dominant swell. Composite beaches types are a variation from mixed sand and gravel beaches with distinct morphological differences. This thesis provides an insight in to the morphodynamic behaviour of composite beaches. The study area contains a small village based by the shoreline and the potential coastal hazard that threatens people, property and infrastructure. Therefore the results from this thesis have an important management implication towards mitigating coastal hazards. The historical coastline change was induced through a combination of wave processes and transport, composite beach morphodynamic behaviour, anthropogenic influence and planform shape. Results show that human infrastructure restricted the retreat of a small hapua landward of the gravel barrier. A combination of change in sediment supply, consistent sediment transport and a high wave energy environment resulted in rapid landward retreat through gravel rollover and coastal erosion. The gravel barrier morphodynamics include increase in crest elevation, steeper shore gradients as a response to high swells resulting in erosion or rollover. The wave environment includes a sediment transport hinge point due to a dominant wave refraction and changes in the shoreline orientation, which further induces coastal erosion. The valid applicability of the zeta planform shape concludes that the shoreline may further iii retreat due to geological controls, potential sediment transport and the transgressive nature of the composite beaches. The combination of methods and results provide both quantified historical change and also potential future scenarios of coastline reshaping. These methods and results are applicable not only to Rapahoe but along other West Coast composite beaches, and the validity of the combination of methods provides a greater understanding of the behaviour of morphodynamic composite beaches and provides quantified results of historical shoreline change and sediment transport at the field site.