Signatures of Tsunami in the Coastal Landscape - Tsunami - page 45

Fig. 3.12 Boulders transported
by tsunami down a ramp in the
lee of the headland at Haycock
Point, New South Wales,
Australia. This corner of the
headland is protected from storm
waves. The ramp descends from a
height of 7 m above sea level. Its
smooth undular nature is a
product of bedrock sculpturing

Fig. 3.13 Dumped and
imbricated sandstone boulders
deposited 33 m above sea level
along the cliffs at Mermaids Inlet,
Jervis Bay, Australia. Note the
eroded surface across which
waves flowed from left to right

fluvial (river) flow. Tsunami behave very much like this once
on shore. Table 3.2 presents the theorized velocities of tsu-
nami flow required to move the boulders found along the
New South Wales coast using these two equations. The
minimum theoretical flow velocity ranges between
2.2 m s -1 -4.2 m s -1 . Mean flows appear to have exceeded
5ms -1 with values of 7.8 m s -1 -10.3 m s -1 being obtained
on exposed ramps or at the top of cliffs.
It is more useful to be able to determine flow depth
because, as shown in the previous chapter, this equates to
the height of the tsunami wave at shore—Eq. ( 2.15 ). The
crucial parameter in the movement of boulders is the drag

force, and this is very sensitive to the thickness of the
boulder. The thinner the boulder, the greater the velocity of
flow required to initiate movement. Thickness is even more
important than mass or weight. This point is illustrated in
Fig. 3.14 for two boulders of equal length and width but
different thicknesses. Despite being four times larger in
volume and weight, the rectangular boulder only requires a
tsunami wave that is one third of the height of the wave
needed to move the platy one. This effect is also illustrated
in Fig. 3.12 . All of the boulders shown on the ramp required
the same depth of water to be moved, despite the spherical
ones being twice as large as the flatter ones.