Five types of seawalls were tested with irregular waves equivalent to H'o =5.8m and T 1/3=10sec (model scale of 1/33.3). The ratios of crest heights of tested seawalls to those of upright seawalls which yield the same rate of overtopping at respective conditions were obtained from the measured relations between crest heights and overtopping rates.
　According to the tests, the seawall with a stepped slope located at the shoreline requires a crest higher than the upright seawall by 10 to 20% but lower than smooth slope. The crest height of seawall covered with a mound of artificial concrete blocks can be reduced with the increase in crest width: 40 to 50% of upright seawall for four rows of blocks. Installment of a permeable breakwater in the offshore of an upright seawall reduces the latter's crest height by 30 to 40%. The crest of uplight seawall can also be lowered by receding its parapet from the front position; reduction of 40% is obtained at the distance of four times the wave height. The seawall constructed with a vertical slit caisson having the optimum opening ratio of about 20% in the front and more than 40% in the top may have the crest height of 60% to 70% of upright seawall.