Spatial and temporal variations in wave characteristics across a reef platform, Warraber Island, Torres Strait, Australia

A field experiment was conducted at Warraber Island, Torres Strait, Australia to investigate spatial and temporal variations in wave characteristics and energy across a mesotidal coral reef platform. Measurements of water depth were obtained using five pressure sensors deployed across a 2.7-km section of reef flat from July 3–5, 2001. The reef surface was uneven and consisted of an outer reef flat, a central reef flat depression, an inner reef ramp, a palaeo-reef surface and the shoreline. Water levels decreased landward across the platform with tide ranges at the shoreline being almost 50% lower than at the outer reef flat. Rising and falling tides were characterised by a bimodal energy distribution with both short-period (0–3 s) and wind (3–8 s) waves present. Higher water levels were dominated by wind waves. The highest waves occurred at high tides associated with nocturnal tidal cycles with Hs decreasing from 0.5 to 0.2 m from the outer reef flat to the shoreline. Wave energy at swell (8–20 s) and infragravity (>20 s) frequencies was negligible across the reef platform although there was evidence of wave groups at higher water levels.Reef geometry and changes in water level determine the magnitude of wave energy on the reef platform. Up to 85–95% of incident wave energy is attenuated by the central reef flat depression at high and low tide, respectively, and strong linear relationships exist between Hs and h at all locations. Both wave height and wave type are strongly depth dependent. Critical reef rim depths required to produce Hs of a given size vary spatially across the reef rim due to variations in reef topography. A distinct depth related threshold exists at which short-period and wind wave dominance reverses. Over a 14-day spring-neap tidal cycle, the time of occurrence of wave action diminishes across the reef platform to the shoreline. Larger waves (Hs=0.2 m) occur for only 9% of time at the outer reef flat and for less than 0.5% over the remaining reef platform. This implies that on mesotidal reef platforms, sediment entrainment and transport are severely constrained under normal wave energy conditions and significant change is likely restricted to extreme events. [Copyright &y& Elsevier]