Modeling of Realistic
Scenarios for Tsunami Mitigation
Tsunamis in the open seas can
be well modeled by the shallow-depth wave theory. However, as
a tsunami nears the coastline and eventually makes landfall,
the interaction of the wave with the local topology and man-made
structures makes the flow three-dimensional. A fully 3D, viscous
solution covering a meaningful region with a reasonable grid
resolution requires a significant amount of computational time.
Thus, it is not currently practical to perform such detailed
computations in the relatively short time between the occurrence
of a tsunamigenic event and the eventual landfall of the tsunami.
It is hence necessary to envision the development of a knowledge
database that assimilates the outcomes of numerous tsunami scenarios
on representative coastal regions. With an understanding of
the potential devastation of vulnerable regions, urban designers,
builders, and decision-makers would be better equipped to make
hazard mitigation decisions. In any event, since not every tsunamigenic
ev ent results in significant waves on every stretch of populated
coastline, it is important to develop such a database since
it would enable a prediction of the potential level of devastation
from an intrinsic tsunami pattern on a particular region.
As an example of a mitigation
investigation, the problem of a wave impact with a structure
is revisited to evaluate different scenarios for impact reduction.
Five scenarios were simulated, differing only by the type of
"obstruction" positioned upstream of the tall structure.
The upper part of Figure 6 presents the initial
condition for the nominal case and each of the five scenarios.
The time histories of the net force and moment on the structure
in the flow direction are presented in the lower part of Figure
6. In addition, to the right of each of the time plots,
a table lists the maximum net forces and moments. It is interesting
to note the existence of two peaks in the force curve (and the
moment curve), even for the nominal case. The first peak is
associated with the initial impact of the wave with the base
of the structure. The second and less expected peak is associated
with the collapse of the wave column that has run-up the upstream
face of the structure. The moment arm of the second impact force
is much larger than the first and can yield a significant destructive
total moment, on par with the moment generated by the first
impact! In the absence of any obstruction, the structure experiences
a maximum force level of 157 N. Any of the obstructions studied
reduce the maximum force, with the porous dike providing the
highest level of reduction, namely 30%. However, as previously
alluded to, the moment results provide a better indication of
the mitigating effectiveness of the obstructions. Indeed, the
findings are important in terms of land use not only for the
frontline structures, but also for the buildings in their wakes.
Figure 6: Investigation
of protective properties
of different potential mitigation schemes
Based on the simulation results
of the five narrow structures and the dike, one can observe
that the front buildings can be destroyed from the back just
as much as, if not more than, from the front! Also, the findings
challenge the long-accepted concept of protecting a building
with a solid dike, which in these simulations is seen to project
the impact point toward a higher elevation, much like a ski
jump. While the magnitude of the impact force is slightly reduced
(134.9 N), the resulting maximum moment is increased by 36%.
The use of a porous dike on the other hand reduces the maximum
moment by 39%. The less obtrusive case of a ditch covered with
a porous medium (e.g., perforated metal bridge for beach access),
which was considered but not shown, yielded a similarly high
level of protection (maximum moment=2 mN). Given the accuracy
and versatility of the ELMMC method, these types of investigations
have become feasible. Even more complex and realistic scenarios
can be studied with the use of supercomputers.
