Understanding the Driftage Behaviour of Multiple Debris Due to Tsunami-Induced Flows on a Sloped Bed

Post-disaster field surveys confirmed that impacts of floating objects entrained by tsunami flows with onshore structural facilities caused various levels of damage. This paper presents the findings of a series of hydraulic experiments conducted to understand the driftage of multiple cuboids and their interaction with the free-standing square-prism structure in terms of initiation of driftage, spreading over the plane, collision modes with the model structure and yielded impact forces placed in different configurations such as side by side, and stacked behind on other. Results indicated that the wakes generated around the objects caused them to drift in distinct trajectories and encountered an overall maximum spreading angle between ± 4° and ± 10° from the centre of geometry from the initial position. Simultaneous and non-simultaneous collisions were also observed. Objects placed side by side showed a gradual increment in collision forces with the increment in wave conditions. On the other hand, debris stacked behind others led to higher peak impact force for lower flows and reduced impact forces with increased flow conditions. It might be due to the increment in the number of debris, their movements and considerable wave energy losses during the driftage process. Overall, a 5–40% additional increment in collision forces was observed considering all cases.