Super typhoon Haiyan struck the Philippines on 8 November 2013, marking one of the strongest typhoons at landfall in recorded history. Extreme storm waves attacked the Pacific coast of Eastern Samar where the violent typhoon first made landfall. Our field survey confirmed that storm overwash heights of 6–14 m above mean sea level were distributed along the southeastern coast and extensive inundation occurred in some coastal villages in spite of natural protection by wide fringing reefs. A wave model based on Boussinesq‐type equations is constructed to simulate wave transformation over shallow fringing reefs and validated against existing laboratory data. Wave propagation and runup on the Eastern Samar coast are then reproduced using offshore boundary conditions based on a wave hindcast. The model results suggest that extreme waves on the shore are characterized as a superposition of the infragravity wave and sea‐swell components. The balance of the two components is strongly affected by the reef width and beach slope through wave breaking, frictional dissipation, reef‐flat resonances, and resonant runup amplification. Therefore, flood characteristics significantly differ from site to site due to a large variation of the two topographic parameters on the hilly coast. Strong coupling of infragravity waves and sea swells produces extreme runup on steep beaches fronted by narrow reefs, whereas the infragravity waves become dominant over wide reefs and they evolve into bores on steep beaches.
A series of scale-model experiments investigated the scouring mechanisms associated with a tsunami impinging on a coastal cylindrical structure. Since scaling effects are significant in sediment transport, a large-scale sediment tank was used. Video images from inside the cylinder elucidated the vortex structures and the time development of scour around the cylinder. The scour development and mechanisms differed according to the sediment substrate – sand or gravel. For gravel, the most rapid scour coincided with the greatest flow velocities. On the other hand, for the sand substrate, the most rapid scour occurred at the end of drawdown – after flow velocities had subsided and shear stresses were presumed to have decreased. This behaviour can be explained in terms of pore pressure gradients. As the water level and velocity subside, the pressure on the sediment bed decreases, creating a vertical pressure gradient within the sand and decreasing the effective stress within the sand. Gravel is too porous to sustain this pressure gradient. During drawdown, the surface pressure decreases approximately linearly from a sustained peak at $\uDelta P$ to zero over time $\uDelta T$. The critical fraction $\Lambda $ of the buoyant weight of sediment supported by the pore pressure gradient can be estimated as \[ \Lambda = \frac{2}{\sqrt \pi} \frac{\uDelta P}{\gamma_b \sqrt {c_v \uDelta T}}, \] in which $\gamma_{b}$ is the buoyant specific weight of the saturated sediment and $c_{v}$ is the coefficient of consolidation. Much deeper scour was observed where $\Lambda $ exceeded one-half.
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