A 30-year bathymetric data set obtained during and after construction of jetties at an entrance of Imagireguchi Inlet was analyzed (Figures 1 and 2). After construction of the jetties, the tidal channel deepened (Figure 3, Polygon D). The increase in water depth in the tidal channel is probably caused by the stabilization of the location of the channel, which had the area including the channel always exposed to the tidal current, and focusing of the ebb jet between the jetties.

Although accumulation occurred on the up-drift side of the jetties, the accumulation rate decreased with time (Figure 3, Polygons A, B and C). However, the amount of sediment supplied to the down-drift beach did not increase because some amount of sediment transported around the jetties was trapped to develop the ebb-tidal delta (Figure 3, Polygons H and G). The volume stored in the ebb-tidal delta was estimated, and compared with the equilibrium one calculated using the relationship between the equilibrium volume and the tidal prism proposed by WALTON and ADAMS (1976). The comparison showed that the volume almost reached the equilibrium.

Complex empirical orthogonal eigenfunction analysis showed that, during and just after construction of the jetties, shoals formed on both sides of the jetties and moved shoreward. Several years after the completion of construction, however, although the shoal on the down-drift side moved still shoreward, the shoal on the up-drift side moved seaward. The seaward movement of the shoal was assumed to be caused by the return flow and the wave-generated longshore current.