Numerical investigation of tsunami bore effects on structures, part I: drag coefficients

Recent tsunami disasters caused devastating damage to non-engineered as well as engineered coastal infrastructure. In fact, previous design guidelines containing provisions for tsunami loading did not provide accurate estimations of tsunami loads in order to design structures capable of withstanding tsunami impacts. The drag coefficient plays a significant role in the calculation of tsunami hydrodynamic forces. This coefficient is traditionally calculated based on a steady flow analogy. However, tsunami-induced coastal inundation is a typical case of rapidly varying unsteady flows. The present study aims at investigating the tsunami forces exerted on structures with different geometries in order to provide realistic guidelines to estimate drag coefficients for unsteady flows. In this paper, a dam-break wave is used to investigate the tsunami-induced bore interaction with structures. A three-dimensional multiphase numerical model is implemented to study the tsunami loading on rectangular-shaped structures with various aspect ratios (width/depth) and orientations. The numerical model results are validated using measured forces and bore surface elevations from physical experiments previously carried out by some of the authors of this paper. A scaled-up domain is then modeled in order to assess the performance of the model and the induced tsunami loading at prototype scale. The drag coefficient relations for various structural geometries and bore depths are further provided. The calculated hydrodynamic forces and associated drag coefficients are compared with data in the existing literature and current design codes.