CHARACTERISTICS BOUNDARY LAYER AND BOTTOM SHEAR STRESS BENEATH TSUNAMI WAVES

Abstract

 Using tsunami-induced velocity data measured at the mouth of Monterey Bay in US during the 2010 Chilean Earthquakes Tsunami, an investigation of bottom boundary layer characteristics beaneath tsunami was carried out. The observed velocity profile is highly similar to that in a wave boundary layer, rather than depth-limited velocity distribution as normally assumed under long waves. In order to distinguish a depth-limited boundary layer from wave boundary layer, theoretical criterion is obtained by using an empirical formula for wave boundary layer thickness, and the effectiveness of this criterion is examined for sea bottom boundary layer under shoaling hypothetical tsunami. It is found that the entire region of shoaling from tsunami source area with 4000m depth to shallow area with 10m depth is classified into wave friction zone. This result coincides with the measurement result in Monterey Bay. Furthermore, condition for the predominace of bottom frictiona term in shallow water equation is obtained from the ratio to local accerlations term, which is epressed in terms of two dimensionless parameters.