Tsunami scour around a cylinder

Abstract

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.

SPH modeling of dynamic impact of tsunami bore on bridge piers

“…Scouring of bridge foundations is identified to be one of bridge failure modes (Kawashima and Buckle, 2013). Tsunami-like solitary wave has been used in physical experiments to investigate tsunami-induced scour around coastal structures and bridge piers (see, e.g., Tonkin et al, 2003;Chen et al, 2013;Yeh and Mason, 2013). It is seen that solitary waves (even a series of solitary waves) have a very limited impact duration on the bridge pier, but a strong tsunami bore as encountered in this study lasts for a much longer period.…”

On the solitary wave paradigm for tsunamis

“…As examples we mention Synolakis [1986Synolakis [ , 1987, , , Yeh et al [1994], Briggs et al [1995], Liu et al [1995], Li [2000], Li and Raichlen [2001, Tonkin et al [2003], Jensen et al [2003] and Kobayashi and Lawrence [2004].…”

“…Positive and negative solitary waves were generated in water depth of 0.8m with the initial wave height of approximately 0.18-0.22m and a beach slope of 1/12. Tonkin et al [2003] studied the scouring mechanism associated with tsunami runup in an experimental facility, and used solitary wave inputs with wave height over depth ratios H/h 0 in the interval 0.05-0.13. In both studies, the waves were remarkably short compared to typical tsunami scales.…”

“…Since the early 1970s, it has been frequently assumed that solitary (or cnoidal) waves can be used to model some of the important features of tsunamis approaching the beach and shoreline, and that these theories, originating from the KdV equation, can define the proper input waves for physical or mathematical models of tsunamis. Examples from the literature are numerous see, e.g., Goring [1978], Synolakis [1986Synolakis [ , 1987, , , Tadepalli and Synolakis [1994], Yeh et al [1994], , Liu et al [1995], Li [2000], Li and Raichlen [2001, Tonkin et al [2003], Jensen et al [2003], Kobayashi and Lawrence [2004], Craig [2006], Synolakis and Bernard [2006] and Lakshmanan [2007]. Another example of the popularity of this concept is the recent NOAA Technical Memorandum [Synolakis et al, 2007] discussing necessary analytical and experimental benchmarking for numerical tsunami models.…”

Direct numerical simulations of instability and boundary layer turbulence under a solitary wave

“…For example, to evaluate the structural stability of critical infrastructures, it becomes necessary to understand how the tsunami-induced nearbed/seabed processes can cause liquefaction, scour and deposition of sediment (e.g. Tonkin et al 2003). Only recently has the boundary layer structure and turbulence under solitary wave motion been studied in detail (Liu & Orfilia 2004;Liu, Park & Cowen 2007;Sumer et al 2010;Vittori & Blondeaux 2008Lin & Rodekopp 2011;.…”