Role of Surface Roughness on Tsunami Bore Overtopping of Coastal Dykes - IAHR World Congress, 2019 Water Connecting the World

Miguel Esteban ¹ Jochem Roubos ² Jorrit Salet ² Hidenori Ishii ¹ Go Hamano ¹ Kotaro Iimura ¹ Tomoyuki Takabatake ¹ Bas Hofland ² Jeremy Bricker ² Mark Bowen ¹ Hiroshi Takagi ³ Ioan Nistor ⁴ Jacob Stolle ⁴ Tomoya Shibayama ¹

¹Dept. of Civil and Environmental Engineering, Waseda University, Japan
²Dept. of Civil Engineering and Geosciences, TU Delft, The Netherlands
³School of Environment and Society, Tokyo Institute of Technology, Japan
⁴Dept. of Civil Engineering, University of Ottawa, Canada

Following the 2011 Tohoku Earthquake and Tsunami, Japan is reconstructing coastal defences along hundreds of kilometres of coastline in an attempt to make settlements more resilient against such future extreme events. Coastal risk management philosophy was also rethought, and current guidelines establish that defences should prevent coastal communities against floods generated by by Level 1 events (with a return period in the order of about 100 years). Thus, substantial research has been conducted in recent years to improve the understanding on how to design protection structures against overtopping, and ways to their survival, even when overtopped by higher inundation levels (Level 2 tsunamis with return periods in the order of 1000 years). However, it is unclear to what extent traditional countermeasures could slow down high energy bores (such as those generated by some tsunamis), as these could result in substantial overtopping.

Thus, to ascertain the overtopping mechanism and leeward inundation heights of high energy bores as they hit coastal structures, the authors conducted physical experiments in the Waseda University Wave Flume using a dam-break mechanism to generate bores. Three different types of structures were considered, namely a wall of “infinite” height, a coastal dyke, and a vertical tsunami wall. The impounded volume of water was varied to generate different types of bores, and the roughness of the false bed was changed (to understand how the this can affect the incoming bore velocity.

The results show that, depth of the incoming bore aside, the magnitude of the front velocity of the tsunami-induced bore is crucial in determining whether the structure will be overtopped or not. It is thus imperative to move away from only considering the tsunami inundation height at the beach. Finally, the authors also developed an empirical equation to help practicing engineers to easily evaluate the overtopping risk for dikes subjected to such extreme flows.