AbstractForensic engineering field surveys conducted in the aftermath of large-scale tsunamis documented the presence of deep local scour holes around structures caused by extreme inundation occurring during such events. The mechanisms leading to scour in extreme flows are still not well understood, as several physical phenomena influencing the spatiotemporal extent of scour have not been adequately investigated. The authors have conducted an experimental test program that has employed a large square column in the Large Wave Flume of the Coastal Research Center, Germany, while they also used a state-of-the-art numerical model (FLOW-3D) to numerically reproduce the experimental results. An investigation of the turbulent flow structures observed around the impacted structure showed that these flow structures are largely responsible for the sediment transport during the runup phase, but the turbulent energy was far less intense during the drawdown phase. The weakness of the turbulent structures observed during drawdown indicates that a different physical phenomenon than the one corresponding to the inflow phase is responsible for the sediment transport experienced during inundation drawdown. Due to the rapid lowering of the flow depth during the drawdown phase of tsunami inundations, a loss of excess pressure occurs because of the upward pressure gradient forming within the soil. However, the pore pressure measurements taken inside the soil in the physical experiment indicate no sign of upward pressure gradient on the inshore side of the column, which is an observation that is incongruent with previous similar studies and previous theoretical concepts. This difference was explained by a layer of soil that remained with a low water content throughout the test because the column was installed on dry sand with low permeability, a condition never tested before for pore pressure change caused by tsunami-like waves.

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