Wave Propagation and Scour Failure of Coastal Structures due to Tsunamis

The extensive research on the influence of sea dike geometry on tsunami-induced
scour indicates the necessity for a suitable model that can forecast the scour depth
behind coastal structures and thus minimize the damaging effect of a high category
tsunami occurrence. Current attempts to study this phenomenon have not gained
worldwide acceptance and have been limited to unproven analytical methods, and
either laboratory or numerical methods. These indicate the need for a verifiable
predictive expression that links both the sediment characteristics and coastal defence
geometry.
A comprehensive laboratory experiment was conducted to better understand the
propagation mechanism of tsunami waves around four varying dikes sizes; two of
which were modelled after dikes in Iwanuma and Soma cities in Japan that were
affected by the 2011 Great Eastern Japan Earthquake and Tsunami. This investigation
contributed to the understanding of the influence of dike dimensions on the flow
variables. Afterwards, the mechanism of the induced local scour at the landward
region was studied in detail because of the destructive impact it could have on the
stability of the structures. These led to the development of a predictive scour model,
from which a tsunami-induced scour resilient structure can be proposed. Also, using
the particle size analysis, the median grain size and permeability properties of the
sediment grain were determined and used to form the boundary condition of the
expression.
The two-dimensional CFD analyses of the wave propagation and associated induced
scour were investigated using ANSYS Fluent software package and sedFoam-2.0
solver respectively, and the results validated against the laboratory data showed good
agreement. The Reynolds-Averaged Navier-Stokes (RANS) modelling approach with
Realizable k-ε turbulence model and scalable wall function for wall modelling were
employed for the wave flow, while turbulence-averaged Eulerian two-phase flow
equations with the kinetic theory of granular flows for intergranular stress models and
a k-ε turbulence model were used for the numerical implementation of the sediment
scour.