THE INFLUENCE OF HYDRODYNAMIC FORCING ON SEDIMENT TRANSPORT PATHWAYS AND SHORELINE EVOLUTION IN A CORAL REEF ENVIRONMENT

Fringing coral reef systems shape the dominant hydrodynamic processes that transport sediment within these environments which, over long time scales, can lead to the formation of various major planform (horizontal) morphological features. In this study, the influence of reef morphology on hydrodynamic processes within a coral reef environment was investigated in a field experiment conducted at Coral Bay (Ningaloo Reef, Western Australia) with the aim to identify sediment transport paths that are likely to affect the evolution of the adjacent shoreline. The shoreline position at the site was analyzed to identify net shoreline movements over an ~60 year period of digitized aerial photography. The sediment characteristics varied throughout the site with extensive regions of relatively coarser (1.1 – 1.3 Ф) sediment generally associated with areas with high coral coverage, some isolated regions of moderately coarse sediment and three large regions of relatively finer sediment (2.2 – 1.8 Ф) that were typically associated with large, un-colonized sandy patches. Whilst the position of coastline was largely stable (< 0 – 10 m movement) over the ~60 years of shoreline data available, substantial migrations ( ~ 30 – 150 m) were observed along isolated sections of coastline. The numerical model XBeach was used to model the reef and lagoon hydrodynamics from which two types of flow were identified: (1) several hydrodynamic circulation cells that are isolated from adjacent cells and (2) an interconnected cell system whereby an alongshore current propagates along the coastline and between cells before it empties into a large bay to the north of the site. Acceleration of flow adjacent to cuspate forelands and an absence of feature development suggest that some of these features are in a state of equilibrium, while the flow separation around Point Maud and the presence of a persistent eddy at this location is likely to be associated with the continual accretion of this feature over the study period