The Seasonal Changes in the Beach Topography in Karatsu Coast, Japan

The Higashino Beach of Karatsu Coast in Japan has been prone to erosion since 2013. As a part of the coastal protection, regular monitoring of the beach’s topography plays an important role to determine the factors influencing the changing of beach topography. The elevation of 10 certain points was measured on a selected cross-shore profile. The monitoring was carried out every month in the winter was characterized by high waves and every two months in the summer was characterized by low waves. The wave height of extreme wave events and sediment budget of the Higashino Beach were taken into account for evaluating the changing of the beach topography. The monitoring results showed that the beach was eroded in the cases of the extreme wave events with wave height exceeding 3m and accreted in the other cases. The wave data analysis showed that the magnitude and frequency of the extreme wave events with the wave height over 3m in a year has been increased since 2011. The dominant wave directions in the Karatsu Coast are N and NNE that cause the longshore sediment transport from the east to the Higashino Beach. However, the annual sediment discharge from Tamashima River has been decreased since 1993. In addition, the long shore sediment transport rate was reduced due to the construction of detached breakwater on the east of Karatsu Coast. It can be concluded that the erosion in the Higashino Beach caused by increment of the magnitude and frequency of the extreme wave events and the reduction of sediment discharge from the Tamashima River. It was also found that the number of the extreme wave events with the wave height exceeding 3m should be taken into account for evaluating the long term variation of the beach topography in the Higashino Beach.