BOTTOM BOUNDARY LAYER CHARACTERISTICS UNDER THE MEASURED 2011 TSUNAMI WAVEFORM

Abstract

 In the present study, the bottom boundary layer characteristics under the 2011 actual tsunami conditions are investigated. The linear long wave theory is utilized to estimate the flow velocity from the measured offshore water level by the GPS buoy stations during the 2011 Great East Japan Earthquake tsunami. Based on this result, the characteristics of the bottom boundary layer such as the boundary layer thickness, flow velocity distribution, and bottom shear force were numerically examined by the k-ω model, which is a typical two-equation model. It was found that the thickness of the tsunami-induced bottom boundary layer is much thinner than the water depth. The bottom boundary layer characteristics are not a steady flow type but rather similar to the wave boundary layer type. Furthermore, we proposed a method for evaluating the tsunami-induced bottom shear stress under an irregular tsunami waveform by using the wave friction coefficient, instead of using the common Manning formula. As a result, the estimated bed shear stress by the new simple method was the most agreement with the k-ω model results among other methods.