Seasonal to Decadal Variability of Shoreline Position on a Multiple Sandbar Beach

Seasonal to decadal variability of shoreline position at a long multi-barred beach with gentle slopes was investigated using a dataset of biannual shoreline surveys of 23 years. An empirical orthogonal function (EOF) and spectral analysis on 15.2 km long stretch were conducted to describe characteristic patterns of shoreline variation at multiple scales in time and space. A relatively alongshore uniform shoreline migration toward offshore in summer and onshore in winter was dominant in the first EOF mode which secondarily exhibited a long-term trend of shoreline recession. Mid-term shoreline variations related to shoreline sand wave and longshore sandbars were represented by the combinations of the second to sixth EOF modes filtered through two bands of dominant frequency. The shoreline pattern reconstructed on a longer time scale represented a large scale shoreline undulation whose scale and migration speed were comparable to shoreline sand waves observed in various coasts. On the otherhand, the shoreline pattern reconstructed on a shorter time scale represented alongshore uniform shoreline migration toward onshore and offshore in about five-year cycles, which corresponded to the average cycle of net offshore sandbar migration observed at the site. The periodic shoreline recession-advance related to sandbar migration was estimated to be comparable to seasonal scale shoreline variation. A correlation analysis between seasonal scale shoreline variation and wave statistics exhibited that shoreline advance in summer increased with low wave heights. It was suggested that a moderate waves were responsible for seasonal scale shoreline advance in this coast