Tyhoon No. 9918 caused serious damage to the coastgal and harbor facilities in Kyushu and Yamaguchi district. Especially seawalls in the Suou-nada Bay suffered from severe damage due to high waves with an extremely high tide by the typhoon. This report describes the damage to the coastal facilities focussing on the seawall failures in the district with the result from series of wave flume experiments to reproduce the failures.
　The damaged seawalls were mainly of the rubble mound type with deformed concrete blocks for their armor layers. The armor concrete blocks，however, did not sufficiently cover the crown wall and the top of the armor blocks was near the still water surface when the maximum storm surge occurred. Consequently the storm waves were broken on the top of armor blocks causing severe impact forces on the crown walls（parapet walls）of seawalls. A nmber of the parapet walls were broken resulting in fatal increase in wave overtopping.
　Three series of model experiments were coducted in a wave flume，where the failures of the seawalls were reproduced，the wave overtopping rates for several types of seawalls including the ones to reduce the rate were measured，and the impact pressutes on several cross sections of the seawalls were measured. A new formula to evaluate the impact wave pressure in such a case was proposed from the experiments.