Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 131

Table 5.3 Suggested sequence of events required to determine whether boulders were
transported by tsunamis orstormwaves
Best evidence
Direct observation of event, or observations and measurements before and after event
Theoretical evidence
1. Establish that boulders have been transported by waves (imbrication, axis alignment, clast
size grading, presence of marine shells adhered to boulder surface)
2. Assess whether boulders have been overturned during transport, slid to present position,
or been blasted or tossed by wave impact on rock (shore platform) face
3. Determine likely pre-transport source of boulders (subaerial, submarine, joint bounded
blocks, cliff face)
4. Determine relationship between boulder size and wave height needed for transport (also
depends on pre-transport position of boulder)
5. Determine observed and theoretical storm wave climate (using numerical wave models)
6. Determine maximum possible storm surge (tropical and temperate coasts)
7. Determine offshore bathymetry and distance from shore of maximum breaking storm
wave height
8. Determine whether tsunami possible (not just likely) from any source
9. Determine, where possible, age of deposition
location to study site). The boulder has moved twice since its initial emplace-
ment, both times by approximately 30 m in different directions. Noormets et al .
(2002)suggested that the first of these movements could have been due to a
subsequent tsunami and the latter movement was most likely by very large
storm waves in 1969 (the 'Big Wednesday' event). Any possible movements of
this boulder, or other smaller ones, on the shore platform during the January
1998 'Biggest Wednesday' swell in the same area, however, are not mentioned in
thestudy. The waves from the 'Biggest Wednesday' swell are regarded by surfers
in Hawaii as the biggest waves seen in modern times in this region, and were
probably larger than those occurring during the 1969 event.
Nott (2004)studied a 1 km long boulder ridge, at the rear of a beach, near the
entrance to Exmouth Gulf, Western Australia to see if one of the most powerful
tropical cyclones (TC Vance) to strike the area had any impact on the ridge. The
ridge is composed entirely of Quaternary sandstone 'beach-rock' clasts measuring
up to 3.0 × 2.1 × 0.7 m 3 (Fig. 5.11). The clasts are imbricated with seaward dips,
and the majority are orientated with their a axes parallel or subparallel to shore.
The ridge was completely overwashed by the inundation generated by TC Vance.
Surveys before and after TC Vance showed that this ridge was not modified
by thewaves and surge generated by this cyclone. This was despite the fact that