Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 130

Table 5.2 Conditions required for boulder transport by storm waves and tsunamis

Situation/process Storm Tsunami

Wave height at
shore

Breaks and dissipates energy in
water depths 1.2 times wave height

Notrestricted by offshore water
depth, can surge across dry shore

gh 0.5 (0.25 that of tsunami) only
when wave breaks at shore, less
when wave breaks offshore

Velocity at shore 2 gh (4 times greater than storm)

Work
performance

Wave height needs to be
approximately 4 times larger than
tsunami to move given size block

One-quarter height of storm wave
needed

Boulder
deposition on
cliff tops

Must have very deep water
immediately offshore, lower
velocity restricts ability of wave
(fluid flow, not spray and projected
water) to climb cliffs

Can climb cliffs at least 5 times
wave height at cliff base
irrespective of water depths
offshore

Need for
accompanying
events

Where offshore waters are shallow,
large waves need storm surge to
support breaking at shore, surge
heights have physical constraints
(often restricted to less than
4--5 m)

Do not need storm surge

Source of waves Largest waves restricted to

mid-latitude westerly belts and
tropical cyclone prone regions

Most common near convergent
tectonic plate boundaries, but
possibility of submarine
landslides, bolide impact and
meteotsunamis (in this sense
probably have more extensive
source areas when considering
geological time frame)

thelarge boulder was overturned during transport, slid or was projected to its
present position.
Noormets et al .(2002)undertook a study of boulder movement on the shore
platform at Sunset Beach, Oahu (Hawaii), a location renowned by surfers for
big waves. They studied a series of aerial photographs of the site dating from
AD 1928 to 1996 and noted the position of boulders on the platform both before
and after known tsunamis and big storm wave events. They observed that the
largest lithic clast (96 t) was most likely emplaced as a solitary feature during
the 1946 Aleutian tsunami which had a wave run-up of between 3.3 and 10.7 m
at various locations on the island of Oahu, but 4.3 m near Waimea Bay (closest