Nearshore wave environments around a sandy cay on a platform reef, Torres Strait, Australia

Abstract

Waves are the primary factor affecting reef-island morphology. This study examines spatial and temporal variations of wave characteristics in the nearshore around Warraber Island, a sandy cay on a platform reef in Torres Strait Australia, based on field measurements during the predominant southeasterly wind season. Water pressure was recorded simultaneously, and transformed to water surface wave spectra, at a location close to the reef edge and across the nearshore at different locations around the island. Wave environments off the reef were estimated based on wave characteristics measured at the reef-edge location and found to be primarily dominated by sea. Low and high wave-energy events were identified, based on wave energy level at the reef-edge location. Analysis of measured data at locations around the island shows that in general, waves on the windward side were larger and shorter in comparison with those on the leeward side. This spatial pattern was found to be more evident during high wave-energy events. The spatial variation of wave characteristics is influenced by relative energy contribution from three main spectral components (incident short-period waves <2.5 s, incident wind waves <8 s and infragravity waves, the principal component >8 s). During low wave-energy conditions, incident short-period waves are dominant. During high wave-energy conditions, wave spectra clearly show triple peaks on the windward side, but have only two peaks at infragravity and incident wind wave components on the leeward side. Incident wind wave components around the island are primarily a function of incident waves in seas off the reef. Development of infragravity waves is directly related to that of the incident wind waves. Incident short-period waves are generated locally on the reef flat and increase towards the island, particularly on the broad windward reef flat, and their characteristics can be estimated using fetch-limited and depth-limited wave models. Strong influence of water depth on wave characteristics was observed. Wave height and energy of the three wave components at all locations varied directly with changing water level on the reef, whereas wave periods show no clear trends, depending on location around the island. Wave conditions around the island during the northwesterly wind season were also estimated and are likely to be dominated by incident wind wave components.