Geoscience Reference
In-Depth Information
Incorporating equations (5.4)--(5.7)into equation (5.11)gives
·
2a
−
C
m
(
a
/
b
)(
u
/
g
)
/
[
C
d
(
ac
/
b
2
)
+
C
l
]
H
t
≥
0
.
25 (ρ
s
−
ρ
w
)
/
ρ
w
(5.12)
for tsunamis where
H
t
=
height of tsunami, and,
·
2
a
−
4
C
m
(
a
/
b
)(
u
/
g
)
/
[(
C
d
(
ac
/
b
2
)
+
C
l
]
H
s
≥
(ρ
s
−
ρ
w
)
/
ρ
w
(5.13)
For storm waves where
H
s
height of storm wave at breaking point.
To initiate motion of a joint bounded block (where the block is located in a
shore platform but is separated from other blocks by joints), the lift force must
overcome the force of restraint less buoyancy providing the block has weathered
completely free from its substrate. The equation for tsunamis in this situation
is
=
H
t
≥
[0
.
25 (ρ
s
−
ρ
w
)
/
ρ
w
·
a
]
/
C
l
(5.14)
where
H
t
=
height of tsunami, and for storm waves,
H
s
≥
[(ρ
s
−
ρ
w
)
/
ρ
w
·
a
]
/
C
l
(5.15)
where
H
s
=
height of storm wave at breaking point.
Nott (2004)applied these equations to help resolve the origin of the boulders
composing the ridge at North West Cape, Western Australia. Numerical storm
surge models show that the highest storm surge attainable here is about 2.5 m.
The crestoftheridgesitsatapproximately3maboveAustralianHeight Datum
(AHD), which is about mid-tide level. The tide range at Exmouth is around 2 m,
so even at the highest tide the surge could only be 0.5 m above the crest of the
ridge during the most extreme surge. The storm waves required to move the
larger boulders onto this ridge range between 4 and 6 m for boulders in either
asubmerged or subaerial position prior to transport (Table 5.4). Storm waves
of much greater heights (>10 m) are required if the boulders were positioned
as joint bounded blocks prior to transport, as many of these boulders probably
were, for they have been derived from the 'beach rock' shore platform seaward
of the ridge. It seems unlikely, therefore, that even the most extreme intensity
tropical cyclones are able to produce a surge of sufficient height to support the
necessary height storm waves to transport and deposit many of the boulders on
this ridge.
Other forms of evidence for palaeotsunami
Large waves can also leave other fingerprints in the coastal landscape
besides sand sheets and boulder accumulations. These include deposits of shells
and corals atop high-elevation headlands, fluid carved channels also on head-
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