Geoscience Reference
In-Depth Information
decide it is a good time to have lunch at your favorite
quayside café. Do not do what the residents of San Fran-
cisco did during the Alaskan Tsunami of 1964 and flock to
the coast to see such a rare event. And do not do what the
residents of Hilo, Hawaii, did during the Alaskan Tsunami
event of April 1, 1946 (Fig.
10.4
), and hurry back to the
coast following the arrival of the first couple of tsunami
waves. Here, people returned to the coastal business area to
see what damage had occurred, only to be swamped by the
third and biggest wave. Big waves later in a wave train are
more common than generally believed. For example, the
eighth wave during the April 1 event was the biggest along
the north shore of Oahu.
Most people can escape to safety with as little as 10-min
warning of a tsunami. Along the northern coastline of Papua
New Guinea, where the July 1998 Tsunami had such an
impact, people have been encouraged to adopt a tree. In
substantial number of trees withstood the impact of the tsu-
nami even though it was 15 m high and moved at a velocity of
10-15 m s
-1
. Notches can be cut into trees as toeholds, and
people can easily climb a tree and lash themselves to the trunk
in a matter of minutes. Urban dwellers may not have the
opportunity to be as resourceful because of the lack of trees
(Fig.
10.1
). It is an interesting exercise to stand with a group
of people on an urban beach and say, ''Where would you go if
an earthquake just occurred and a tsunami will arrive in ten
minutes?'' Most people soon realise that they should run to the
nearest hill, preferably to the sides of the beach and away from
the coast. However, in a suburb such as that shown in Fig.
10.
1
, this option may be neither obvious nor feasible. The only
choice may be to seek safety in buildings. Personally I would
look for the closest and tallest concrete building, preferably an
office building (apartment buildings have secured access), run
to the lobby, push the elevator button, and go to the top floor.
Hopefully, the tsunami would not repeat the scene of the
Scotch Cap lighthouse, which the April 1, 1946 Tsunami
Researchers have investigated the ability of buildings to
withstand the force of a tsunami (Wiegel
1970
; Shuto
1993
;
Murata et al.
2010
). Damage to structures by tsunami results
from five effects (Wiegel
1970
; Camfield
1994
). First, water
pressure exerts a buoyant or lift force wherever water par-
tially or totally submerges an object. This force tends to lift
objects off their foundations. It is also responsible for
entraining individual boulders. Second, the initial impact of
the wave carries objects forward. The impact forces can be
aided by debris entrained in the flow or, in temperate lati-
tudes, by floating ice. For these reasons, litter often defines
the swash limit of tsunami waves. Third, surging at the
leading edge of a wave can exert a rapidly increasing force
that can dislodge any object initially resisting movement.
Fourth, if the object still resists movement, then drag forces
can be generated by high velocities around the edge of the
object, leading to scouring. Finally, hydrostatic forces are
produced on partially submerged objects. These forces can
crush buildings and collapse walls. All of these forces are
enhanced
by
backwash
that
tends
to
channelise
water,
moving it faster seaward.
Various building types and their ability to withstand
tsunami are summarized in Fig.
10.13
(Shuto
1993
). The
data come from the 1883 Krakatau, 1908 Messina, 1933
Sanriku, 1946 Alaskan, and 1960 Chilean Tsunami. Lines
on this figure separate undamaged, damaged, and destroyed
buildings. Wood buildings offer no refuge from tsunami.
Fast-moving water greater than 1 m in depth will destroy
any such structures unless they are perched on cross-linked
iron struts sunk into the ground. Stone, brick, or concrete
block buildings will withstand flow depths of 1-2 m. They
are destroyed by greater flows. The Nicaraguan Tsunami of
September 2, 1992 destroyed all such buildings wherever
pads that require significant force to be moved can be swept
away by such flows. The National Oceanic and Atmospheric
Administration (
2012
) in its publication Tsunami! The
Great Waves states, ''Homes and small buildings located in
low-lying coastal areas are not designed to withstand tsu-
nami impacts. Do not stay in these structures should there
be a tsunami warning''. However, clusters of buildings also
increase friction and decrease damage due to tsunami. Even
wood houses have withstood tsunami inundation up to 3.
0 m depth if they lie landward of a few rows of similarly
constructed buildings (Murata et al.
2010
). Reinforced
concrete buildings will withstand flow depths of up to 5 m.
Such depths have only occurred during the severest tsunami,
and then only along isolated sections of coastline. If there is
no escape, the safest option is to shelter in a reinforced
concrete building, preferably in the first instance above the
ground floor level. One of the most poignant videos of the
Indian Ocean Tsunami of December 26, 2004 was taken in
Banda Aceh from just such a vantage point as a raging
torrent of water destroyed every other surrounding structure
(Fig.
6.14
). The NOAA publication also states, ''High,
multi-story, reinforced concrete hotels are located in many
low-lying coastal areas. The upper floors of these hotels can
provide a safe place to find refuge should there be a tsunami
warning and you cannot move quickly inland to higher
ground''. However, before fleeing to a multi-storied con-
crete building, know your vulnerability. Residents along the
Sanriku coast of Japan thought they were safe evacuating to
the top of three-story buildings as the T ¯hoku Tsunami of
March 11, 2011 approached. Unfortunately, despite plenty
of evidence that the coastline was subject to tsunami flows
10 m or more deep, the incoming tsunami swamped these
low level structures (Fig.
10.11
).
