EL NIÑO/SOUTHERN OSCILLATION (ENSO), WAVE CLIMATE, AND BEACH VARIATION- TWO CASE STUDIES FROM AUSTRALIA

Twenty-four years of monthly measurements and ten years of daily measurements from two sites in Australia, form one of the longest beach databases in the world. The datasets allow beach morphology to be analyzed in relation to long-term wave climate. This study provides definitive support for a theoretical model linking El Nino/Southern Oscillation (ENSO) to beach change via wave climate. Spectral analysis of beach profile volume time series data was conducted. Significant spectral power was found in the annual and inter-annual (4-7 year) range. The first site, Narrabeen Beach, New South Wales exhibited volume changes of +/- 200m3/m over a 24 year period. The second site, Gold Coast, Queensland exhibited +/- 70m3/m over a ten year period. The dataset from Narrabeen Beach exhibits a long-term pattern of erosion and accretion with an approximately twelve cycle. The Gold Coast profile volume data exhibits both strong annual and medium-term (3-5 year) cycles of beach change. Both beaches show a sharp reduction in volume beginning around late 1995 and early 1996. The beach volume changes at both sites are significantly correlated to each other (r^2=0.50, p=0.000) over the period 1990-2000 with the Gold Coast lagging Narrabeen Beach by approximately one year. Wavelet analysis was used to relate the forces (wave climate) to the responses (beach morphology). Daily wave measurements (height, period, and direction) were compiled into wave power estimates. Periods of above average wave power correlated with decreases in beach volume. Next, the wave power and profile volume data, were correlated with Southern Oscillation Index (SOI). Results displayed inter-annual coherence, with beach response lagging the SOI by approximately 1 year. The results suggest that during positive ENSO (La Niña) cyclones bring powerful waves inducing beach erosion. Conversely, during negative ENSO (El Niño) cyclones are less common and the waves less powerful producing beach accretion. It is anticipated that such long-term morphologic changes may affect many other, as yet unstudied beaches.