Characteristics of the shoreline change of Narugashima Beach

Narugashima Island is located at the west side of the entrance of Osaka Bay extending from north to south. Narugashima Beach is on the east side of the Narugashima Island facing to the Osaka Bay. The grain size of bed material of the beach distributes in the wide range from fine sand to pebble. The beach is relatively stable without remarkable erosion or accretion for the last four years. To comprehend the characteristics of the stable beach like this, we have measured bottom topography, crossshore and vertical distributions of the grain size of the bed material at Narugashima Beach. Besides the field surveys, long-term shoreline changes are analyzed by seven aerial photos. We carried out numerical simulation to reproduce the measured responses of the beach profile to the high waves. It is found that the nearshore current and sediment transportation near the shoreline were feeble for a normal tidal level because waves break far from the shoreline at the offshore bars. It is also found that strong current directed to northeast was occurred along contour line of -3m from the south of Awaji Island located in the southwestern part of Narugashima Island, and sediment transport to Narugashima Island occurred. The one of the reasons why the beach is stable against large waves is the supply of sediment from Awaji Island.