Geoscience Reference
In-Depth Information
Table 4.1
Comparison of tsunami and storm wave heights required to transport the boulders mentioned to this point in the text
Location
Boulder width (m)
Height of tsunami at shore (m)
Breaking storm-wave height (m)
Jervis Bay
Mermaids Inlet
2.3
1.4
5.6
Little Beecroft Head
ramp
4.1
2.7
10.8
clifftop
1.1
0.7
2.8
Honeysuckle Point
2.8
1.8
7.2
Tuross Head
1.3
0.8
3.2
Bingie Bingie Point
2.8
1.8
7.2
O'Hara Headland
1.1
0.7
2.8
Samson Pt., WA
1.0
0.6
2.4
Flores, Indonesia
1.5
2.0
8.0
Source Based on Young et al. ( 1996a )
4.3.2
Movement of Boulders
Isolated boulders tossed by storms, however, are differ-
ent from the accumulations of boulders deposited by tsu-
nami. The analysis used in the previous chapter to define the
wave height of tsunami capable of moving boulders can be
applied to storm waves (Nott 1997 ). The velocity of a wind-
generated storm wave breaking at the shoreline can be
approximated as follows:
Tsunami and storm waves differ in the way that they
transport bouldery material. The forces of storm waves and
their ability to destroy stony breakwalls and move large
boulders on rock platforms are well documented. Under
exceptional circumstances, waves as high as 30 m have
been recorded in the world's oceans (Bryant 2005 ). A
boulder was tossed 25 m above sea level on the island of
Surtsey in Iceland by storm waves. On the west coast of
Ireland, boulders up to 78 tonnes in size have been moved
by historical storms up to elevations of 11 m above high
tide (Cox et al. 2012 ), although tsunami cannot be ruled out
for larger boulders piled higher and of an older age
(Scheffers et al. 2010 ). Waves with a force of 3 t m -2 in the
1800s moved blocks weighing 800 and 2,600 tonnes into
the harbor at Wick, Scotland (Bascom 1959 ). There are
many stories of pebbles and even cobbles being hurled
against lighthouse windows situated on cliff tops. Probably
one of the best-documented incidences showing the ability
of storms to transport coarse material to the tops of cliffs
occurred during Tropical Cyclone Ofa on the south coast of
Niue in the southwest Pacific on February 5, 1990 (Solomon
and Forbes 1999 ). Niue is a raised, relict coral atoll fringed
by limestone cliffs rising up to 70 m above sea level. A
platform reef, up to 120 m wide, has developed at the base
of the cliffs. Ofa generated winds of more than 170 km h -1
and waves with a maximum significant height of 8.1 m. As
it approached Niue, it produced waves 18 m high along the
coast. The effect of these waves along the cliffs was dra-
matic. At Alofi, waves broke above the roof of a hospital
situated on an 18 m high cliff. The lower floor of a hotel
was severely smashed by the impact of storm-tossed debris.
Coarse gravel and boulders 2-3 m in diameter were flung
inland over 100 m from the cliff line. Storms can move
boulders within the storm wave limit.
Þ 0 : 5
v ¼ gHb
ð
ð 4 : 1 Þ
This is half the velocity of an equivalent tsunami wave at
shore. When Eq. ( 4.1 ) is combined with Eqs. ( 3.3 )-( 3.5 )
and solved for wave height, the following relationship is
obtained:
H b 4H t
ð 4 : 2 Þ
This relationship holds for exposed and submerged boul-
ders, and those that have originated from bedrock surfaces.
Table 4.1 presents a comparison of the wave heights of
tsunami and storms necessary to transport the boulders
described so far in this topic. Clearly, tsunami waves are
more efficient than storm waves at transporting boulders
inland. This fact becomes more relevant knowing that storm
waves break in water depths 1.28 times their wave height.
Hence, the heights for storm waves shown in Table 4.1
require much larger waves offshore to overcome the effects
of wave breaking. Storm waves lose little energy only along
coasts where cliffs plunge into the ocean. Unless storm
waves reach a platform before breaking, they do not have
the capability to move boulders more than 2 m in diameter,
under traction, on most rocky coasts. While storm waves
under ideal conditions can transport boulders, as shown by
the effect of Cyclone Ofa on the island of Niue, they are
unlikely to transport boulders and deposit them in imbri-
cated piles at the top of cliffs. Boulder imbrication in con-
trast to pebble imbrication is rarely referenced in the coastal
 
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