Signatures of Tsunami in the Coastal Landscape - Tsunami - page 47

Fig. 3.15 Boulder ripples deposited by tsunami on the rock platform
of Jibbon, New South Wales, Australia. The theorized flow with
standing waves is shown by the white path. The boulders were

deposited under the crest of standing waves generated in the flow,
while vortices bored pools into the rock surface in the troughs. Person
circled for scale

unidirectional flow. This was recognized early in the fluvial
research where calculations on groups of boulders were
favored over those for individual ones (Costa 1983 ;
Williams 1983 ). More specifically, the largest boulders in a
deposit were identified as being more significant in deter-
mining minimum flow velocities. Many attempts have been
made to prove or refine Nott's equations with mixed results
(Shah-Hosseini et al. 2013 ). There are just as many studies
that have found that they overestimate the height of a tsu-
nami as underestimate it. One of the more unique meth-
odologies based upon Nott's equations was developed by
Lorang ( 2011 ) to distinguish between the wave period of
storm waves and those in the tsunami window, which differ
by an order of magnitude. In addition to the variables in
Eq. 3.5 , Lorang used the height of a boulder above sea-
level, the slope of the beach and the maximum velocity of
flow to make this distinction. The simplicity of this
approach may overcome the problem of distinguishing
storm waves and tsunami based upon wave height calcu-
lations alone.
Equation 3.3 does have practical application for deter-
mining the height of a tsunami wave based upon the size of
debris found afterwards. For instance, the largest boulder
transported by the Flores Tsunami shown in Fig. 3.6
required a tsunami wave of about 2 m in height to move.
Note that these flow depths are minimum values, because
the tsunami wave would have been much higher at the
shoreline than where the boulder was dumped. Calculations
of the mean tsunami wave heights required to move boul-
ders in the Jervis Bay region are included in Table 3.2 .
Boulders in the Jervis Bay region only required a tsunami
wave 3 m high to be moved, even though waves higher than
this must have been involved in order to transport boulders
in suspension up cliffs. These examples illustrate how effi-
cient tsunami waves are at initiating movement of bouldery
material. The height of storm waves required to move the
same size material is not nearly as small—a point that will
be discussed in Chap. 4 .
If tsunami wave height at shore equates with flow
depth—Eq. ( 2.15 ) —the height of the tsunami can also be

calculated using the spacing between boulder bedforms as
follows (Moore and Moore 1988 ):

H t ¼ 0 : 5 L s p 1 ð 3 : 6 Þ

where L s = bedform wavelength (m).

However, boulder ripples or dunes are rare in nature. In
the Jervis Bay region, boulder piles suggestive of mega-
ripples exist at Honeysuckle Point and have a spacing of
60 m (Young et al. 1996 ). The alignment of individual
boulders in one pile, at an elevation of 16 m above sea
level, shows foreset and topset bedding characteristic of a
ripple. The minimum tsunami wave height for these features
using Eq. 3.6 is 9.5 m. This agrees with the flow depth
(7.5-12.0 m) required to construct the giant mega-ripples in
dunes located nearby at Crocodile Head and described
earlier (Bryant et al. 1997 ). By far the best site with boulder
ripples occurs at Jibbon in the Royal National Park imme-
diately south of metropolitan Sydney. Here at least four
ripples are evident over a distance of 100 m (Fig. 3.15 ).
Flow swept over the platform at an oblique angle from the
south and set up standing waves as velocities accelerated
over the platform surface about 2-4 m above sea level.
Boulders were deposited against each other under the peaks
of the standing waves in piles that show clear foreset and
topset bedding. In the troughs, where flow impinged upon
the rock platform, vortices were generated that carved small
pools 40-50 cm in diameter and depth into the bedrock. The
latter features are termed bedrock sculpturing and will be
described in more detail later.
The following scenario can account for the formation and
transport of boulders along rocky coasts (Young et al. 1996 ).
On headlands or rock platforms where there is a cliff or ledge
facing the sea, waves have to drown the cliff face before
overtopping them. Tsunami do this by jetting across the top
of the cliffs and developing a roller vortex in front of the cliff
edge. Flow is thus thin (depths of 2.5-3.5 m) and violent
(minimum velocities typically between 5 and 10 m s -1 ). In
jets, tsunami flow velocities may exceed 20 m s -1 . High-
velocity flow first strips weathered bedrock surfaces of