Impoundment depth effects on bore hydrodynamics and scour around a berm-mounted structure during tsunami-like inundation and drawdown

This study investigated the influence of varying impoundment depths on dam-break bore–induced local hydrodynamics and scour, as well as the effect of partially-returning bore, around a single structure positioned on a wet berm. Numerical simulations were performed using SedInterFoam, a Eulerian two-phase model for sediment transport extended with the capability to resolve air-water interfaces. Using the Reynolds-averaged formulation with a k−ω closure, the model was first validated against laboratory dam-break experiments. The experimental setup consisted of a fixed structure embedded in a sandy berm (median grain size of 0.28 mm) located at the end of a sloping beach. The validated model was then applied to examine how different impoundment depths (0.6 m, 0.8 m, and 1.0 m) influence bore evolution, local hydrodynamics, and scour development around the structure. These impoundment conditions generated three distinct tsunami-flow types: a non-breaking tsunami bore (0.6 m), a plunging tsunami bore impacting the structure (0.8 m), and a broken tsunami bore propagating over the berm (1.0 m). These flows exhibit an average Keulegan–Carpenter number higher than 10 and an average mobility number exceeding 200, indicating sheet-flow conditions that help reduce potential scale effects on sediment transport. The results show that maximum scour consistently occurs at the front edge of the structure during the inundation and incoming bore phase. The highest impoundment depth (1.0 m) produces the largest front–back edge contrast in scour depth (>60%), whereas the lowest impoundment depth (0.6 m) results in a much smaller difference (<10%). The maximum scour depth for the 1.0 m case is approximately four times greater than that of the 0.6 m scenario, yet only about 1.2 times larger than the 0.8 m case, highlighting the nonlinear relationship between impoundment depth, bore evolution, local hydrodynamics, and scour development. We identify and quantify the critical role of partially returning bore, which significantly increases scour at the back edge, up to three times greater than during the incoming bore (inundation) phase, while simultaneously reducing the front-edge scour depth by as much as half of its maximum value. These observations emphasize the importance of returning bore processes and suggest that post-event field measurements, which typically capture only the final scour state, may substantially underestimate the true maximum scour experienced during tsunami-induced flooding, especially for the front edge.