The Southern Oscillation Index, wave climate, and beach rotation

Abstract

Short embayed beaches bounded by headlands are a common feature along the southern and central coastline of New South Wales, Australia. Many of these embayed beaches have experienced severe erosion at their southern end over the last decade. Previous studies have suggested that this erosion may be the result of an oscillatory medium-term phenomenon known as beach rotation. The present study was undertaken with the objectives of: (1) establishing definitive links between the Southern Oscillation Index (SOI), wave climate, and beach rotation, and (2) determining the physical processes governing beach rotation. Data from two similar New South Wales beaches were analysed using time series analysis techniques and image processing techniques (using ARGUS video images) in this study. The results indicate that the northern end of this type of beach accretes during El Niño phases, while the southern end erodes, resulting in a net clockwise rotation of the beach. The opposite occurs during La Niña phases resulting in a net anti-clockwise rotation of the beach. The beach response at the northern and southern ends lags SOI trend shifts by 3 and 17 months, respectively. Waves are predominantly incident from the southeast during both El Niño and La Niña phases. Offshore wave height is positively correlated with the SOI while offshore wave direction is negatively correlated with the SOI. On average, the number of storms per year doubles from El Niño to La Niña. Based on these links between beach width fluctuations, SOI, and incident wave conditions, a conceptual model of beach rotation is presented. The model describes the combinations of cross-shore and longshore sediment transport and hydrodynamic processes that are expected to result in the observed clockwise and anti-clockwise beach rotation during El Niño and La Niña phases, respectively.