Geoscience Reference
In-Depth Information
Table 3.2
Velocities and wave heights of tsunami as determined from boulders found on headlands along the New South Wales coast
Size
Mean velocity
Lowest velocity
Wave height
(m s
-1
)
(m s
-1
)
Location
(m)
(m)
Jervis Bay
Mermaids Inlet
2.3
7.8
3.1
1.4
Little Beecroft Head
ramp
4.1
10.3
4.2
2.7
clifftop
1.2
5.6
2.2
0.7
Honeysuckle Point
2.8
8.6
3.4
1.8
Tuross Head
1.3
5.9
2.3
0.8
Bingie Bingie Point
2.8
8.6
3.4
1.8
O'Hara Headland
1.1
5.5
2.2
0.7
Note Sediment size refers to the mean width of the five largest boulders
Source Based on Young et al. (
1996
)
Fig. 3.14
Illustration of the
forces necessary to entrain two
boulders having the same length
and width, but different
thicknesses
length = 8 m
width = 4 m
thickness = 4 m
mass = 346 tonnes
volume = 128 m
3
wave height = 2.6 m
length = 8 m
width = 4 m
thickness = 1 m
mass = 86 tonnes
volume = 32 m
3
wave height = 8.5 m
= density of a boulder (usually 2.7 g cm
-3
)
q
s
The tsunami wave height used in Fig.
3.14
is derived from
Nott (
1997
,
2003
). Boulders occupy three different environ-
ments in the coastal zone. They can exist exposed singly
along the coast on dry land such as on a rock platform (termed
sub-aerial), be submerged, or be part of a bedrock surface. In
the latter case, boulders have to be ripped from this surface,
usually along joints, before they can be transported. The
boulders are thus joint bound. In this topic we have assumed
the commonest case, namely that boulders preexist and are
moved by a tsunami from a submerged offshore environment.
The height of a tsunami wave at the shoreline, H
t
, can be
determined using the following relationship:
= density of sea water (usually 1.024 g cm
-3
)
q
w
C
d
= the coefficient of drag (typically 1.2 on dry land)
C
l
= the coefficient of lift, typically 0.178
Note that the tsunami height estimated by this equation is
conservative because the equation uses the velocity of a
tsunami wave in shallow water—Eq. (
2.2
)—rather than the
higher velocity that is possible across dry land—Eq. (
2.17
).
This equation can be simplified if the boulders being
transported are nearly spherical. These simplifications are
expressed as follows:
1
C
d
ð
acb
2
Þþ
C
l
1
H
t
0
:
82a 1
:
2acb
2
þ
0
:
178
Simplified:
ð
3
:
4
Þ
Þ
q
1
w
H
t
0
:
5a q
s
q
w
ð
ð
3
:
3
Þ
For spheres: Ht
0
:
6b
ð
3
:
5
Þ
where
Since Eq.
3.3
was proposed, very few field measure-
ments have shown it to be accurate. These discrepancies are
due to turbulence and the myriad of processes involved in
H
t
= wave height at shore or the toe of a beach
a
= boulder length (m)
c
= boulder thickness (m)
