LONG-TERM BEACH CHANGES AND THEIR CAUSES AT AJIGAURA BEACH, IBARAKI PREFECTURE, JAPAN

Abstract

Long-term beach changes and the topographic responses to the wave climate were analyzed based on the field data of the wave characteristics, grain size of the bed materials and beach profiles measured every week in a 11-year period between 1976 and 1986 at Ajigaura Beach in Ibaraki Prefecture. For the wave characteristics wave direction, wave height and wave period were observed. Eigenfunction analysis was carried out using the profile data. It was found that distinctive changes were observed before and after a period of 1982-83 in the moving average value of one year regarding shoreline position, wave climate and foreshore slope, and the periodic fluctuations of wave direction and shoreline position had totally changed since this time. It was concluded that the changes in shoreline position and foreshore slope were caused by the offshore sediment transport associated with the extraordinary high waves. In addition, the abrupt changes of the periodic fluctuations in wave direction and shoreline position closely correlated to the long-term changes of the oceanic and meteorological changes associated with the climate anomalies such as El Nino.