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Based on post-tsunami surveys following the Japan Tsunami 2011, the most prominent seawall damage was due to scours that was formed behind the seawall. The downstream of a seawall is an essential part of the seawall stability which must be securely protected from the tsunami overflow in order to maintain seawall stability. The scouring process on boulder bed behind the seawall and the method to reduce the scour was investigated in a laboratory experiment. The scouring process and scour depth are assumed to be closely related with the tsunami hydrograph. Hence, the tsunami hydrograph was also modelled to imitate an existing reported tsunami hydrograph. The results showed that both the maximum scour depth and scour length are increased with the length of tsunami overflow or the hydrograph. The bed material behind the seawall was transported downstream. The transport of boulder can be blocked by a set of vertical screens installed in between the boulder bed. This method is found to be effective in reducing both the scour depth and scour length. The reduction of maximum scour depths are approximately 50%, 43%, and 34% for dimensionless screen distance of 5.33, 6.67, and 8.0, respectively, while the scours length for all various screen distance is reduced for about 25%. Other than that, it is important to note that the scour depth exactly behind the seawall reduced significantly. This signify that, the screen also effective in keeping in place the bed material that directly support the seawall from sliding force and turning moment and thus secures the stability of the seawall. [ABSTRACT FROM AUTHOR]

Herein, scour depth measurements are compared with theoretical and empirical estimates obtained at four sites in Chiba and Fukushima Prefectures impacted by the 2011 Tohoku Tsunami. At sites where scour was caused by either floodwall overtopping or acceleration of flow around building foundation footings, results of published scour experiments are compared with measured scour depths. [ABSTRACT FROM PUBLISHER]

The 2011 Tohoku Tsunami induced sediment deposition of 334,000 m3 mainly in the basin and 9 m erosion at the tips of a breakwater and a groin in Oarai Port, Japan. The tsunami-induced bathymetry change was simulated using a numerical model and the simulated bathymetry change was compared with the measured one. The comparison showed that the simulated bathymetry change quantitatively agreed with the measured one including the sediment deposition and the erosion mentioned above although the erosion measured along the breakwater was not well reproduced by the model. The time series of simulated bathymetry change, water level, current and suspended sediment concentration showed that the sediment deposition in the center of the basin was mostly induced by the sediments that moved into the basin through its entrance between the breakwater and the groin and were redistributed by counterclockwise vortices. The severe erosion at the tip of the breakwater was mainly induced by the strong southward currents. Unlabelled Image • The simulation qualitatively reproduced tsunami-induced bathymetry change in a port. • Sediment deposition in basin was caused by sediments moved through port entrance. • The sediments moved into basin were redistributed by counterclockwise vorticities. • Severe erosion at the tip of breakwater was induced by strong currents. [ABSTRACT FROM AUTHOR]