Geoscience Reference
In-Depth Information
Fig. 2.6
Unstructured, variable
grid of bathymetry around Banda
Aceh, Indonesia used to simulate
the effects of the Indian Ocean
Tsunami of December 26, Based
on Harig et al. (
2008
)
Pulau Nasi
Grid spacing
< 100 m
Banda Aceh
Grid
spacing
< 200 m
0
2
4 km
is now possible to provide real time simulations of most
tsunami simply by matching an event to one in a database.
July 9, 1958 by an earthquake-triggered landslide in Lituya
Bay, Alaska (Miller
1960
). Water swept 524 m above sea
level up the slope on the opposite side of the bay, and a
30-50 m high tsunami propagated down the bay.
Wind-generated waves are limited in Stokes wave theory
by depth. A Stokes wave will break when the height-to-
water depth ratio exceeds 0.78. Thus, on flat coasts storm
waves break in a surf zone and dissipate most of their
energy before reaching shore. On the other hand, 75 % of
tsunami reach shore without breaking, bringing tremendous
power to bear on the coastline, and surging landward at
speeds of 5 s
-1
-8 m s
-1
(Fig.
2.9
). The opposite occurs on
steep coasts such as those dominated by rocky headlands.
Here, storm waves surge onto shore without breaking,
whereas a tsunami wave is more likely to break. The pop-
ular media often portray this latter aspect as a plunging
tsunami wave breaking over the coast. Under tsunami
waves, significant water motion occurs throughout the
whole water column. Close to the coast, this aspect is best
described by a solitary wave (Fig.
2.4
) (Geist
1997
). A
solitary wave maintains its form into shallowing water, and,
because the kinetic energy of the tsunami is evenly dis-
tributed throughout the water column, little energy is dis-
sipated, especially on steep coasts. Synolakis (
1987
)
approximated the maximum run-up height of a solitary
wave using the following formula:
2.5
Run-Up and Inundation
2.5.1
Run-Up
Tsunami are known for their dramatic run-up heights, which
commonly are greater than the height of the tsunami
approaching shore by a factor of 2 or more times. The
National Geophysical Data Center (
2013
) catalogue lists 32
events with a run-up of 30 m or more. For example, in the
Pacific Ocean region, 44 tsunami have generated wave run-
up heights in excess of 10 m since 1900. The largest run-up
produced by a volcano was 90 m on August 29, 1741 on the
west coasts of Oshima and Hokkaido Islands, Japan. The
eruption of Krakatau in 1883 generated a wave that reached
elevations up to 40 m high along the surrounding coastline
(Blong
1984
). The largest tsunami run-up generated by an
earthquake was 100 m on Ambon Island, Indonesia, on
February 17, 1674. In recent times, the tsunami that struck
Flores Island on December 12, 1992 had a run-up of 26.2 m
at Riang-Kroko, the Alaskan Tsunami of April 1, 1946
overtopped cliffs on Unimak Island and wiped out a radio
mast standing 35 m above sea level (Fig.
2.8
), and the
T ¯hoku Tsunami of 2011 produced run-up of 38.9 m. By
far the largest run-up height recorded was that produced on
Þ
0
:
5
H
1
:
25
t
H
rmax
¼
2
:
83 cot b
ð
ð
2
:
11
Þ
