Multiple sand bar dynamics in the macrotidal Shinduri beach, west coast of Korea

Extensive tidal flats and sandy beaches are a typical coastal landform along the macrotidal west coast of Korea. Macrotidal beaches typically with long stretches and the gentle slope, contain often a series of sand bar, parallel to the coastline. Shinduri beach under macrotidal condition in a semi-closed embayment, western coast of Korea, has three to four lines of sand bars on about 500 m stretches of intertidal zone. Interesting feature is the dynamic behaviour of the multiple bars. These bars appear only in summer and disappear dramatically all in winter, which is opposite pattern in common beaches. In order to understand the seasonal dynamics of multiple bars on a macrotidal beach, three years of topographic survey using a VRS-GPS system have been conducted at three-months interval on six transects placed on the beach. In addition, an ADV/ADCP was deployed to collect wave data on a bar. Shinduri beach is a 4 km in length and about 500 m in width. Topographic survey reveals that three to four bars occur only during summer, and disappear suddenly during winter. In response to bar growth and destruction, beach slopes become steeper in winter and gentler in summer. Mean grain sizes show generally shoreward coarsening trend, ranging from 2.0 phi to 2.75 phi. Sediments get coarser in summer, but finer in winter, which are opposite compared to other beaches in the west coast of Korea. Wave data show strong seasonality, high waves in winter and much gentler waves in summer, suggesting the study area experienced by monsoon climate. The opposite pattern of multiple bar dynamics, growth in summer and destruction in winter, is likely associated with strong winter waves, destroying the bars and hence filling the trough of bars, thereby the beaches becoming flat in topography. From spring the bars start to form under normal wave condition. This signifies that local wave condition is more important for maintaining patterns of multiple bars.