Geoscience Reference
In-Depth Information
Fig. 7.19 Inverted toothbrush-
shaped promontory at Ball Rock
near Sully Island, south Wales
Table 7.2
Boulder dimensions and inferred tsunami flow characteristics at various sites around the Bristol Channel and Severn Estuary, U.K
Area
Site
A-
axis
(m)
B-
axis
(m)
C-
axis
(m)
Volume
(m 3 )
Weight
(tonnes)
Velocity of
run-up
(m s -1 )
Distance
inland
(km)
Height of
Tsunami at
shore (m)
Breaking
storm-wave
height (m)
Severn
Estuary
Sudbrook
4.5
3.6
0.7
11.2
25.6
16.2
4.9
6.1
24.4
Portishead
2.6
2.5
0.2
1.5
3.4
14.9
4.0
5.7
22.7
Inner
Bristol
channel
Brean
Down
5.2
4.8
2.1
51.4
132.0
14.5
3.7
5.3
21.3
Sully
Island
2.2
1.6
0.3
0.9
2.3
11.8
2.1
3.7
14.9
Dunraven
Bay
2.9
2.8
0.6
5.2
13.3
13.8
3.2
4.8
19.3
Ogmore
4.9
3.1
1.1
15.8
40.6
13.5
3.1
4.6
18.6
Sker Point
4.4
4.1
0.9
15.3
41.5
18.1
6.7
8.4
33.6
Outer
Bristol
channel
Tears
Point
2.1
2.1
0.4
1.8
4.7
12.3
2.4
3.8
15.4
Croyde
3.0
2.3
0.7
4.5
11.8
12.4
2.5
3.9
15.7
Source Bryant and Haslett ( 2007 )
expected as a tsunami travelled up a funnel-shaped channel.
Maximum run-up velocities reach 16.2 m s -1 at the
entrance to the Severn Estuary. These velocities are in the
range needed for bedrock sculpturing. When the tsunami
heights derived from boulders are inserted into Eq. 2.14 , the
wave could have penetrated 2.5 and 6.7 km across the
lowlands of Outer and Inner Bristol Channel respectively.
In the Severn Estuary upstream from Sudbrook, the wave
could have reached 4.9 km inland. These values agree with
the observations of the extent of flooding (Bryant and
Haslett 2003 ; Haslett and Bryant 2005 )
There are other characteristics besides hydrodynamic
ones that implicate tsunami as the mode of transport.
Table 7.3 catalogues each site according to the ten char-
acteristics of boulder deposits indicative of tsunami trans-
port (Bryant and Haslett 2007 ). Three locations: Dunraven,
Tears Point, and Brean Down have nine of these ten char-
acteristics. The fact that boulders are deposited in groups,
have not been flicked into position by storm waves, show
signs of lateral transport and have imbrications that match
the approach of the tsunami wave are strong indicators that
a tsunami entered the channel and began to move large
boulders. One of the best sites showing evidence for a
tsunami occurs at Dunraven. Here, the boulders occur in a
train, are clearly imbricated against each other, and do not
show evidence of percussion at contacts—all features
indicative of suspension transport (Fig. 7.20 ). Hydrody-
namically, the largest boulder here requires a storm wave of
19.3 m in height, but a tsunami wave only 4.8 m high. In
addition, two rock types are mixed together showing that
boulders have not simply fallen from the cliffs, but have
been moved laterally inland. This site is more intriguing in
that isolated boulders, deposited on the adjacent beach
through cliff retreat, all begin shoreward of the deposit as if
the beach were swept clean of boulders at one point in time
commensurate with deposition of the boulder train. Rates of
 
Search WWH ::




Custom Search