Geoscience Reference
In-Depth Information
Fig. 4.5
Model for smooth,
small-scale, bedrock surfaces
sculptured by tsunami. Model is
for headlands within 7 m of sea
level. From Bryant and Young
(
1996
)
muschelbrüche
sichelwanne
V-shaped groove
pothole
hummocks
flutes
2 m
VE = 2x
cavettos
0 m
trough
4 m
0 m
Approximate scale
sinuous grooves
landscape typically forms on headlands rising above 7-8 m
the small-scale model can be found in this landscape. The
large-scale model is characterized by ramps, whirlpools,
and canyons, forming toothbrush-shaped headlands (Fig.
4.
1
). Ramps can extend from modern sea level to heights of
30 m and can evince zones of evacuated bedrock depres-
Whirlpools up to 10-15 m deep (Fig.
3.26
) are found pri-
marily on the upflow side of the headlands, although they
can also form on steep lee slopes. The base of whirlpools
generally lies just above mean sea level, but some are
drowned, with the central plugs forming stacks that are
detached from the coastline. The base of whirlpools is
controlled by the depth of large-scale vortex formation
rather than by the level of the sea at the time of formation.
Smaller potholes are also found in these environments.
Generally, canyon features are inclined downflow. How-
ever, where the effects of more than one event can be
identified, earlier canyons provide conduits across the
headland for subsequent, concentrated erosive flow. Irreg-
ular, large-scale landscapes preserve a crude indication of
the direction of tsunami approach, although the effects of
wave refraction may complicate the pattern. Under extreme
conditions, the complete coastal landscape can bear the
imprint of catastrophic flow—headlands from 80-130 m
high may be overwashed with sheets of water carving
channels as they drained off the downflow side, headlands
rising over 40 m above sea level may have their seaward
ends truncated, talus and jagged bedrock may be stripped
from cliff faces, platforms may be planed smooth to heights
20 m above sea level, and whole promontories may be
sculptured into a fluted or drumlin-like shape. Finally,
coastal tsunami flow is usually repetitive during a single
erosive event, which consists of pulses of unidirectional,
high-velocity flow as individual waves making up a tsunami
wave train surge over bedrock promontories. In these
instances, erosive vortices last for no more than a few
minutes and the erosive event is completely over within a
few hours.
4.4.4
Atolls
The islands of the South Pacific are exposed to both tropical
cyclones and tsunami. One of the more notable features is
the occurrence of alternating mounds and channels (Bour-
rouilh-Le Jan and Talandier
1985
). Mounds called motu,
consisting of sands and gravels derived from the beach and
reef, separate the channels or hoa from each other (Fig.
4.6
).
The motu are drumlin-shaped with tails of debris trailing off
into the lagoon. In many respects, they could also be inter-
preted as molded dump deposits. Motu-hoa topography
appears to be restricted to atolls in the South Pacific. Many
descriptions of this topography consider them features of
storm waves. However, storm waves from tropical cyclones
appear to modify prior motu-hoa topography rather than
being responsible for it. Even under storm surge, bores
generated by storms tend to flow through pre-existing hoa
into the lagoon. Despite the steepness of nearshore topog-
raphy around atolls, storm waves dissipate much of their
energy by breaking on coral aprons fringing islands. The
effect of storms is primarily restricted to the accumulation of
coarse debris aprons in front of motu-hoa topography. For
example, Cyclone Bebe in 1972 built a ridge on Funafuti that
was 19 km long, 30 km wide and 4 m high (Scoffin
1993
).
The volume of sediment thrown into this ridge occupied
1.4 9 10
6
m
3
and weighed 2.8 9 10
6
tonnes. The ridge ran
