Sediment response to tsunami loading: mechanisms and estimates

Tsunami effects on soil stability are evaluated by imposing hypothetical, but typical, tsunami loads on an idealised beach condition of a plane beach with homogeneous sediments. Under this model tsunami condition, the extent of sediment motion in the form of bed load and suspended load on the beach is demonstrated. Sediment motions are particularly significant in the nearshore area between the initial shoreline and the maximum tsunami drawdown. During the drawdown phase, pore-pressure gradients develop by rapid reduction in water pressure on the bed, creating soil instability, and triggering momentary liquefaction in some locations. Such effects of pore-pressure gradients are quantitatively evaluated with a proposed modified Shields parameter. The analysis used for the model tsunami case is further applied to laboratory data and real field data collected in Kesen-numa, Japan, during the 2011 East Japan tsunami.