On the Damage to Houses due to Tsunamis

1933年三陸・1960年チリ津波において,三陸沿岸で津波の高さが平均海面上3m以下のとき,家屋の破壊率は10～20%であったが,4mをこえると破壊率は急激に増加し,50%以上に達した.家屋の破損は津波の流速にかなり左右され,水流圧力が5～9m3/sec2になると,家屋の破壊率は50%をこえた.1983年日本海中部地震津波における秋田県八森では,家屋の破損は地面から1.5mの浸水高で半壊し,2.0m以上に達したところが全壊していた.近年,家屋構造の変化や海岸保全施設の整備にともない,浸水高が1～2mの津波では流失家屋は目立って減少している.On the Sanriku coast which was hit by the 1933 Sanriku and the 1960 Chile tsunamis, 10～20% of the wooden houses were damaged when inundation heights were about 3 meters (above M.S.L.). The damage percentages suddenly increased to 50% or more when the inundation heights were over 4 meters. Damage to houses is controlled directly by the hydraulic pressure dur to the water flow. The hydraulic pressure that acts upon a house is expressed as p∝hv2, where h is the inundation height above ground and υ the water velocity. The percentage of damage to houses reached 50% or more when the hydraulic pressure became 5～9m3/sec2. In many cases of tsunamis, the water velocity was 3～6m/sec at the mouth of a river, 1～3m/sec in the street area and 0.5～1m/sec near the foot of a hill. According to the report of the field investigation of the 1983 Nihonkai-Chubu tsunami at Hachimori, Akita Prefecture, most of the houses inundatated floor when the water level were 0.5～1.0 meters (above ground). The wooden houses were partially damaged when the inundation heights were about 1.5 meters and destroyed 2.0 meters or more, but the houses hardly washed away. In recent years, bolts have been set in the bottoms of wooden houses and powerful sea walls have been constructed, so there is a strong tendency for the number of houses washed away to decrease