Horizontally composite breakwaters,covered with wave-dissipating concrete blocks,are widely employed in Japan. The weight of these blocks is provided by the desgin formula,mainly by the Hudson formula(Hudson,1959).However,a large amount of failures have occurred in many ports in japan and each failure costs at least 10 million yen.Mostly,these failures are caused by the waves smaller than the design waves.To elucidate such a settlement mechanism,field investigation and large scale hydraulic experiment are conducted in a Large Hydro-Geo Flume.
　According to the field investigation,the settlement failure is caused by the waves much smaller than the design waves just after the set-up of wave dissipating blocks. The rubble mound under the wave dissipating blocks also settles down.The settlement of the rubble mound has not been seen before covered by the wave dissipating blocks.
　Large-scale experiment elucidates the followings. 1)The settlement of the wave dissipating blocks is caused by the scouring beneath the rubble mound just below the wave dissipating blocks. 2)Scouring is caused by the wave breaking acting on the wave dissipating blocks. 3)This scouring phenomenon is different form the L-type scouring and the fluidization caused by the standing waves. 4)Using Forheimer equation and hydraulic gradient,the flow velocity in the rubble mound is estimated well. 5)Some countermeasures such as Asphalt mat,geotextile,and small grains are examined. The weight of non-permeable mat has to be properly desiged. Using small grains,the construction cost becomes smaller.