Tsunami-like flow induced force on the structure: Prediction formulae for the horizontal force in quasi-steady flow phase

Destruction of the structures in coastal areas due to an extreme coastal event like a tsunami necessitates the deeper understanding of flow behaviour to improve the design guidelines. The characteristics of inland propagating tsunami surge or bore consist of an initial aerated surge or bore tip followed by the gradual increase in water depth (quasi-steady flow phase). During the bore interaction, the structure initially experiences an impulsive pressure, bore pressure followed by quasi-static pressure (quasi-steady flow phase) depending on the ratio of the structure obstruction width to flow channel width (b/W), incoming bore Froude number (Fr), the shape of the structure, and the orientation of structure to the direction of flow. From field surveys and video observations during the 2004 Indian Ocean Tsunami and the 2011 Tohoku tsunami, the structure failure during the quasi-steady state of tsunami flow is found to be predominant. Also, most of the literature in the past focused on the interaction of bore on a single structure in which the flow channelling effect (b/W) is not considered. Thus, the present experimental study gives a detailed insight into the b/W and Fr effect in the force acting on the rectangular structure during the quasi-steady flow of tsunami-like events. To estimate the hydrodynamic force in the quasi-steady flow phase, we have adopted the hydrostatic force equation by incorporating bore height at the structure front (hf) and bore height at the structure back (hr). A simplified semi-analytical method is proposed based on conservation of mass and momentum to estimate the force on the rectangular structures. This approach of force estimation is showed to reasonably predict experimental force-time history. Since design guidelines use the hydrodynamic drag force equation for force estimation, the present study also provides the drag force coefficient (Cd) for b/W > 0.2 and Fr between 0.6 and 2. Along with the consideration of Fr and b/W, the study attempted to provide a closed-form set of equations to the quasi-static force, which helps designers with a convenient force determination method.