Levee overtopping creates large scoured region at the landside, which greatly affects the levee performance. Both 3D and 2D experiments were conducted to analyze the downstream scour profile after levee overtopping. For elucidating the difference of scour pattern under various overtopping water depth and width conditions, 3D scour profiles are investigated. In addition, vertical 2D scour profiles are also investigated to compare the 3D scour profiles with 2D experiment results. From the analysis, it has been observed that for overtopping width (W bank ) to levee height (h bank ) ratio of 6 to 8, multiple scour holes appear with overtopping height (d bank ) to levee height (h bank ) ratio of 0.1 to 0.2, while for same W bank /h bank , two scour holes are created when d bank /h bank =0.25 to 0.3. Comparison of 2D and 3D scour data sets at downstream scour profiles shows that 3D experiments tend to create larger scour depth and shorter scour length than 2D cases. This result indicates the importance of 3D experimental analysis for levee design..

Three dimensional experiments are conducted to analyze the downstream scour dimensions after levee overtopping. For elucidating the difference of scour dimension under various hydraulic (overtopping flow height at the levee top, d bank ) and geometric parameters (levee height, h bank , overtopping flow width along the levee, W bank ), 3D scour profiles are investigated. From the analysis, it has been observed that at equilibrium conditions, for the same d bank cases scour depth (S d ) is higher for greater h bank (10cm) compared with lower levee height (6cm). In contrast, scour length (S l ) is longer in case of lower h bank compared to higher h bank for same d bank . The stronger hydraulic jump that took place in case of smaller levee height cases appears to play a significant role in developing greater scour length along with shorter scour depth. Three separate equations for S d , S l and S w (scour width) have been proposed from multiple regression analysis. These results can be useful for the study of levee design.

When a tsunami reaches a shore, it propagates along river channels with tremendous energy and overtops river levees. It causes severe damage to the levee structures through scouring along levee slope. In the present study, gully scours created along the levee slopes of two rivers (Omotogawa and Sakarigawa Rivers) in Iwate Prefecture and three rivers (Nanakitagawa, New Kitakamigawa, and Abukumagawa Rivers) in Miyagi Prefecture were investigated after the 2011 Great East Japan tsunami. The energy head during levee overtopping for each location was calculated from post-tsunami surveys. The relationship between the energy head and gully scour dimensions (length, width, and depth) was analyzed. The analyses clarified that gully scour dimensions were greatly dominated by two important phenomena: (i) overflow type (free overflow or submerged overflow), and (ii) soil texture at the scoured region. The present study results can be utilized to obtain the overtopping flow height for post-tsunami analysis.

Scouring downstream of hydraulic structures is a complex phenomenon that causes the instability and even the failure of the structures. This experiment-based research was conducted to study the effect of the protection work length, L, along the embankment landside slope on the relationship between non-dimensional scour depth (S D /Lsinθ) and its location (S P /Lcosθ) under various overtopping flow depths (S D : maximum scour depth, S P : the location of S D , and θ: the landside slope angle). For evaluating the possibility of the fragility of the embankment landside slope, a schematic diagram based on the angle of repose of foundation behind embankment toe is proposed in this study. The result shows that the effect of the protection work length on the risk of the embankment landside slope fragility are classified into 3 types according to the variation of the overtopping flow depths.