Medium-term variations of bar properties and their linkages with environmental factors at Hasaki, Japan

Beach profile data obtained every weekday for 15 years along a 427-meter-long pier were analysed by Complex Empirical Orthogonal Function analysis. The seaward bar migration was represented by the first mode, and the migration frequency and the temporal variation of the bar amplitude were estimated with the complex time coefficient of the first mode. Although the seaward bar migration frequency, which is the reciprocal of the duration time of the bar migration cycle, was relatively constant before 1993 and ranged between 1.3 to 0.8 cycles/year (0.7 to 1.25 years/cycle), it gradually decreased after 1994 and reached 0.1 cycle/year (10 years/cycle) at the end of 1996. Subsequently, it gradually recovered to the frequency before 1993. The bar amplitude had a similar long period fluctuation. It was large before 1993, but small from 1995 to 1997, and then returned to the pre-1993 value. The bar migration frequency was weakly correlated with the bar amplitude and the offshore wave energy flux.