Risk and Avoidance - Tsunami - page 192

Fig. 10.2 The world's
coastlines having historical
records of tsunami in 1500 and
1750

AD 1500

AD 1750

Area with possible historical record of tsunami

topography or coastal settings are particularly prone to
tsunami. First and most obvious are exposed ocean beaches.
Figure 8.1 , which is an artist's impression of the tsunami
generated by the eruption of Krakatau in 1883 hitting the
coast of Anjer Lor, shows this clearly. If you live by the
seaside, you are at risk from tsunami. This fact is clearly
recognized for earthquakes by the United States National
Oceanic and Atmospheric Administration ( 2012 ). In its
publication Tsunami! The Great Waves, it states, ''If you are
at the beach or near the ocean and you feel the earth shake,
move immediately to higher ground. DO NOT wait for a
tsunami warning to be announced''. Sometimes a tsunami
causes the water near the shore to recede, exposing the
ocean floor. Anyone who frequents the ocean should be
aware that a rapid withdrawal of water from the shore is
overwhelmingly a clear signature of the impending arrival
of a tsunami wave crest. The time until arrival may be less
than a minute or, in the case of the coast near Concepción,
Chile, following the Great Chilean Earthquake of May 22,
1960, up to 50 min later.

Second, tsunami travel best across cleared land because
frictional dissipation is lowest. This is shown mathemati-
cally by Eq. 2.14 where the distance of inland penetration is
controlled by the value of Manning's n, which is lower for
smooth topography such as pastured floodplains, paved
urban landscapes dominated by parking lots, and wide
roads. The residents of Hilo, Hawaii, were dramatically
made aware of this fact following the Alaskan earthquake of
April 1, 1946 (Fig. 10.4 ) and the Chilean earthquake of
May 22, 1960 (Fig. 6.8 ). On many flat coastlines that have
been cleared for agriculture or development, authorities are
now planting stands of trees to minimize the landward
penetration of tsunami. The effect is clearly shown in Fig.
10.5 at Riang-Kroko, on the island of Flores, following the
December 12, 1992 Indonesian Tsunami (Yeh et al. 1993 ).
The tsunami bore had sufficient energy to move large coral
boulders; but these were deposited once the wave pene-
trated the forest and rapidly lost its energy through dissi-
pation. Greenbelts have been constructed using this premise
as a measure to protect a coastline from tsunami. The drag