Geoscience Reference
In-Depth Information
Table 7.3
Proportion of boulders showing tsunami-transport characteristics, Bristol Channel, U.K
Characteristic of
tsunami transported
boulders
Sudbrook
Portishead
Brean
down
Sully
Island
Dunraven
Ogmore
Sker
point
Tears
point
Croyde
Number of sites
showing
characteristic (out
of 9)
Deposited in groups 4
4
4
4
4
4
4
4
4
9
Only boulders
4
4
4
4
4
4
4
4
7
Imbricated and
contact-
supported
44 44 4
Evidence of
suspension
transport
44 44 4
Evidence of lateral
transport
4
4
4
4
4
4
4
7
Above the storm
wave limit
4
4
4
4
4
Not flicked by
storm waves
4
4
4
4
4
4
4
4
4
9
Hydrodynamic
determinations
exclude storms
4
4
4
4
4
4
4
7
Imbrication
matches
direction of
Tsunami
approach
4
4
4
4
4
4
4
4
8
Other nearby
signatures of
Tsunami
44 4 4 4
Total number of
characteristics
(out of 10)
6
7
5
9
9
4
9
9
5
Source Bryant and Haslett (
2007
)
cliff retreat along this coast range from 0.30 to 34 m yr
-1
(Williams and Davies
1987
). The coast, in general, has
retreated 120 m between 1590 and 1990. At Dunraven, the
cliff has retreated 111 m from the seaward margin of the
scattered boulders on the beach (Bryant and Haslett
2007
).
Based on these data, it appears that the cliff in Dunraven
Bay was positioned at the current seaward margin of the
boulder lag around AD 1634-1672 and that the imbricated
boulders in Fig.
7.20
were transported by a high-energy
event prior to this time, but not earlier than 1590. The 1607
flood event fits within these temporal constraints.
A possible trigger for a tsunami in this region is most
likely an earthquake or submarine slide, or a combination of
both. Indeed, there is at least one historic account of an
earth tremor on the morning of January 30, 1607 (Disney
2005
). The steep continental slope offshore of Ireland has a
number of locations where large slope failures have
occurred—such as the Celtic Margin, Goban Spur, Sole
Bank, Porcupine Bank and Porcupine Bight—that could
have generated tsunami that could have reached the Bristol
Channel at this time (Kenyon
1987
). Recently, large failures
have been studied in the Rockall Trough (Øvrebø et al.
2005
), Porcupine Bight (Huvenne et al.
2002
) and else-
where along the continental slope (Evans et al.
2005
). The
quest is to map and date these slides.
7.7
The Risk in the World's Oceans
7.7.1
Other Volcanic Islands
Large debris avalanches are now known to be associated
with at least two other mid-ocean volcano complexes, La
Réunion in the West Indian Ocean and Tristan da Cunha in
the South Atlantic Ocean (Fig.
7.2
). On Réunion, the Grand
Brule slide fans out from the east coast. It consists of four
nested submarine landslides that have reached depths of 0.5,
1.1, 2.2, and 4.4 km below sea level (Whelan and Kelletat
2003
). The Tristan da Cunha complex consists of three
volcanoes rising 3,500 m from the sea floor (Holcomb and
