Signatures of Tsunami in the Coastal Landscape - Tsunami - page 54

Fig. 3.24 Inverted keel-like
forms at Cathedral Rocks, 90 km
south of Sydney, Australia. Flow
came from the right. The stacks
formed between horizontal,
helical (eggbeater) vortices. A
sea cave is bored into the cliff
downflow to the left

3.3.3 Flow Dynamics

Any model of the flow dynamics responsible for tsunami-
sculptured bedrock terrain must be able to explain a range
of features varying from sinuous cavitation marks several
centimeters wide to whirlpools over 10 m in diameter
(Bryant and Young 1996 ). One of the controlling variables
for the spatial distribution of these features is bed slope that
can be higher than 10 at the front of promontories. Even a
slight change in angle can initiate a change in sculptured
form. For instance, sinuous cavitation marks can form
quickly, simply by steepening slope by 1-2. Similarly,
flutes can develop with the same increase in bed slope. This
suggests that new vortex formation or flow disturbance
through vortex stretching is required to initiate an organized
pattern of flow vortices able to sculpture bedrock. Because
bedrock-sculpturing features rarely appear close to the edge
of a platform or headland, vortices did not exist in the flow
before the leading edge of the tsunami wave struck the
coastline.
Bedrock-sculptured features are created by six flow
phenomena: Mach-Stem waves, jetting, flow reattachment,
vortex impingement, horseshoe or hairpin vortices, and
multiple-vortex formation. Mach-Stem wave formation was
described in Chap. 2 (Bryant and Young 1996 ). It occurs
when waves travel obliquely along a cliff line. The wave
height can increase at the boundary by a factor of 2-4 times
(Wiegel 1970 ). The process is insensitive to irregularities in
the cliff face and is one of the mechanisms allowing tsunami
waves to overtop cliffs up to 80 m high.
Tsunami, because of their long wavelength, behave like
surging waves as they approach normal to a shoreline.
Jetting is caused by the sudden interruption of the forward
progress of a surging breaker by a rocky promontory
(Bryant and Young 1996 ). The immediate effect is twofold.

Fig. 3.25 An arch at Narooma on the south coast of New South
Wales, Australia. The tsunami wave swept towards the viewer along
the platform in the background. A vortex on the seaward side eroded
most of the arch. Joints in the rocks do not control the arch, nor does
the rock face show evidence of major chemical weathering

of a headland. The overall whirlpool is 10 m wide and
8-9 m high. The central plug stands 5 m high and is sur-
rounded by four 3 m diameter potholes, one of which bores
another 3 m below the floor of the pit into resistant basalt.
The counterclockwise rotation of the overall vortex pro-
duces downward-eroded helical spirals that undercut the
sides of the pit, forming spiral benches. Circular or sickle-
shaped holes were drilled, by cavitation, horizontally into
the sides of the pothole and into the wall of the plug. Under
exceptional circumstances, the whirlpool can be completely
eroded, leaving only the plug behind. Figure 3.1 at the
beginning of this chapter shows an example of this cut into
aplite (granite) at Cape Woolamai on the south coast of
Victoria Australia.