INTERACTIONS BETWEEN A SAND BARRIER AND FLOOD TERRACE AT THE ABUKUMA RIVER MOUTH

摘要

The sand barrier at the Abukuma River mouth flushed by floods in autumn firmly recovered within a half year by stretching its tip to the cross river direction. However, the direction suddenly changed upstream since 1993. This paper analysis the mechanism of the change. The river has a large terrace formed by the flood in 1986, the maximum in the observed floods over fifty years. Bathymetric data sounded once a year reveal gradual development of a channel on the terrace off the river axis. From wave ray analyses, it is shown the development accompanies change of the refraction characteristics of waves. The channel becomes to function as a half mirror; reflecting waves incoming from one direction and transmitting from the other direction. This is shown to modify direction of dominant waves near the barrier and to turn the stretch direction of the barrier. The channel evolution is also shown to correlate with the flood outflow.