The inundation process caused by the penetration of swells and long-period waves into the mouth was investigated through field observation and numerical analysis, taking the Segami River flowing into Hitachi Port in Ibaraki Prefecture as the example. Due to the river improvement for flood control, the riverbed was deepened, enhancing vulnerability against wave penetration. The effectiveness of the plan constructing a detached breakwater off the river mouth and a reduction pond in the river was confirmed by the numerical analysis.
