Chapter 47
Morphology of Australia’s Eastern Continental
Slope and Related Tsunami Hazard
Samantha Clarke, Thomas Hubble, David Airey, Phyllis Yu, Ron Boyd,
John Keene, Neville Exon, James Gardner, Steven Ward,
and Shipboard Party SS12/2008
Abstract Morphologic characterisation of five distinct, eastern Australian upper
continental slope submarine landslides enabled modelling of their tsunami hazard.
Flow depth, run-up and inundation distance has been calculated for each of the
five landslides. Future submarine landslides with similar characteristics to these
could generate tsunami with maximum flow depths ranging 5–10 m at the coastline,
maximum run-up of 5 m and maximum inundation distances of 1 km.
Keywords Submarine landslides • Wave height • Southeastern Australia •
Upper slope • Flow depth • Run-up • Inundation distance
S. Clarke (�) • T. Hubble • D. Airey • P. Yu • J. Keene
Geocoastal Research Group, University of Sydney, Sydney, NSW, Australia
e-mail: [email protected]
R. Boyd
School of Environmental and Life Sciences, University of Newcastle, Newcastle, NSW, Australia
ConocoPhillips, Houston, TX, USA
N. Exon
Earth and Marine Sciences, Australian National University, Canberra, ACT, Australia
J. Gardner
CCOM, University of New Hampshire, Durham, NH, USA
S. Ward
Earth and Marine Science, University of California at Santa Crux, Cal. United States
Title of Team: Shipboard Party SS12/2008
S. Krastel et al. (eds.), Submarine Mass Movements and Their Consequences, Advances
in Natural and Technological Hazards Research 37, DOI 10.1007/978-3-319-00972-8 47,
© Springer International Publishing Switzerland 2014
529
530
S. Clarke et al.
47.1 Introduction and Aims
Submarine landslides can damage seabed infrastructure, cause subsidence of coastal
land, and generate tsunamis (Masson et al. 2006). Examples of large submarine
landslide generated tsunamis include the 1929 Grand Banks’ event (Fine et al.
2005), the 1946 Scotch Cap Alaska event (Fryer et al. 2004), and the 1998 Aitape
Papua New Guinea event (Tappin et al. 2001). Submarine landslide generated
tsunami are not as well understood as those associated with large earthquakes and
consequently present a significant but poorly-quantified hazard (Sue et al. 2011).
The eastern Australia (EA) coast is potentially vulnerable to tsunamis due to the
population concentration (""85 %) and critical infrastructure within 50 km of the
coast (Short and Woodroffe 2009). However, there has been little reason to suspect
a local source for the generation of tsunami on the EA coastline. The identification
of relatively recent, abundant submarine landslide scars has changed this perception
(Boyd et al. 2010; Clarke et al. 2012; Keene et al. 2008) and established that
submarine landsliding should be considered a common and ongoing characteristic
of this passive continental margin (Clarke et al. 2012; Hubble et al. 2012).
This study uses data collected during the RV Southern Surveyor (SS12/2008)
survey of the EA continental margin (Boyd et al. 2010) to (a) characterise slope
morphology of the margin between Noosa Heads and Yamba, (b) determine the
geometry of selected landslide features, and (c) quantify the size of tsunami
potentially generated by these lands.
The study area (Fig. 47.1) is located along the EA continental slope between
Noosa Heads to Yamba. We focus on five distinct landslide scars from the upper
continental slope identified by Boyd et al. (2010) and one adjacent potential
submarine landslide site. They are the: (1) Bribie Bowl Slide; (2) Coolangatta2
Slide; (3) Coolangatta1 Slide; (4) Cudgen Slide; (5) Byron Slide; and (6) Potential
Fig. 47.1 Digital elevation model (DEM) of the slope geometry of the EA continental margin.
Insets show details of actual (black lines) and potential landslides (red outline)
47 Morphology of Australia’s Eastern Continental Slope. . .
531
Table 47.1 Summary of the submarine landslide parameters
Canyon region
Average slope angle >3°
Landslide
name
Bribie Bowl
Slide
Bryon
Slide
Plateau region
Average slope angle <3°
Potential Slide (adjacent to
Bryon Slide)
Cudgen
Slide
Coolangatta1
Slide
Coolangatta2
Slide
Parameter
(m)
Thickness (t)
125
220
200
50
20
45
Length (L)
3,000
3,700
3,800
7,500
8,300
1,400
Width (W)
2,465
3,558
3,268
5,338
2,286
1,558
Water depth
(h0)
600
1,000
800
600
600
900
See Fig. 47.1 for landslide locations
Slide mass (c.f. Table 47.1). The five scars are representative of the failures that
occur in the two contrasting slope morphologies in the study area: (1) the relatively
steep (3–7ı) and canyon incised slope (Bribie Bowl Slide, Bryon Slide and Potential
Slide) and (2) the relatively gentle sloping (1–3ı ) Nerang plateau (Coolangatta1&2
Slides and Cudgen Slides). At least one gravity core was recovered from each of
these landslides.
47.2 Data and Methods
47.2.1 Bathymetry and Landslide Geometry
Approximately 13,000 km2 of bathymetric data was acquired using a 30-kHz
Kongsberg EM300 multibeam echosounder. The multibeam data was processed to
produce a 50 m gridded digital elevation model (DEM) (Boyd et al. 2010).
The DEM was used to examine the six individual sites using Fledermaus V7.3.3b
software (http://www.qps.nl/). Landslide thickness (t), length (L), width (W ), and
water depth at landslide centre of mass (h0 ), as well as distance from the adjacent
coastline to head of the landslide source (r) were determined for each feature
(Table 47.1). Landslide thickness is the maximum thickness within the landslide scar
assuming the surface is continuous without the apparent landslide feature (McAdoo
et al. 2000). Landslide length is the distance from landslide head to landslide toe.
Landslide width is the average of measurements taken every 500 m down the
landslide scar, perpendicular to the landslide axis. Water depth is taken from the
landslide centre of mass.
47.2.2 Tsunami Calculations
The size of tsunami generated by potential of slope failures identified here has
been assessed using empirical equations developed by Ward et al. (Chesley and
532
S. Clarke et al.
Ward 2006; Ward 2001; Ward 2011). Equations 47.1 and 47.2 use a landslide’s
geometric characteristics and an estimate of landslide mass velocity to generate
maximum flow depth at the coastline (Fd (0)) (i.e. tsunami height). Function 3
allows run-up (R(Xmax )) and inundation distance (Xmax ) to be estimated where
left and right sides are equal only at a particular Xmax values, determined from
site-specific coastline profiles perpendicular to the coastline and adjacent to each
landslide site.
Flow depth at coastline:
F
d
.0/ D ŒA 0 P .r /]4=5 h0 1=5
(47.1)
Propagation and beaching factor:
W
A 0 P .r / D 0:7345t
Lref
Lref
h0
h
0:36
e
-3:74
vs
vt
vt
i2
Lref
r
0:69
(47.2)
Run-up and Inundation depth:
ŒR.X max / - T .X 0 /] C
16:7n2
.X max - X 0 / D F
F d .0/
d
.0/
(47.3)
Where A0 is the initially generated surface elevation, P(r) is propagation factor P
at distance r from the source of the wave vs is landslide speed, vt is tsunami speed
p
at the landslide D (gh0 ), Lref is a reference length, taken here as 1 m, X is distance
inland from the coastline (X0 D 0 at coastline), T(X) is topographic elevation at
location X. Land surface roughness is represented by Manning’s coefficient n which
is taken as 0.015 for very smooth topography, 0.03 urbanized/built land, and 0.07
densely forested landscape (Gerardi et al. 2008).
47.3 Results
47.3.1 Morphometric Characteristics of Individual Landslides
The five landslides (Fig. 47.2) are U-shaped in cross-section (3–6 km wide and 20–
220 m deep) backed by an amphitheatre shaped crestal zone. In each case, landslide
morphology is similar to the classical circular failure profile described by Varnes
(1978), but elongated in the downslope, longitudinal profile.
The Bribie Bowl, Byron and Potential Slides are located within the steeper
canyon regions (average slope 3–7ı ) and are thicker (>100 m) compared with
landslides developed on the adjacent Nerang plateau. Both canyon landslides
present an average slope of approximately 12ı along the majority of the failure
plane, increasing to 33ı at the head scarp. In plan view the crown scarps present
47 Morphology of Australia’s Eastern Continental Slope. . .
533
Fig. 47.2 Detailed DEM for each landslide showing slope geometry and cross-section profiles
across slide (black line S-N) and down slide (black line W-E). Landslide outlines (black a-e and
red f) and cross-sections for the Potential Slide (dark blue) are shown, along with tension cracks at
the landslide head and slope break at the toe (dashed red). Reference profiles marked in light blue
distinctive semicircular shape, but the detached landslide slabs are essentially planar
blocks, as the exposed failure surface is planar with a declination roughly parallel
to the adjacent unfailed slope.
The Coolangatta1&2 and Cudgen Slides are located on the shallower Nerang
plateau region (average slope 2–3ı). These landslides are thinner (<50 m) and
representative of the numerous upper slope failures that occur on this very gently
dipping plateau (Boyd et al. 2010). All three landslides present a “hummocky”
texture within the failure region, a gently concave landslide shape, and average
slopes of approximately 3.5ı within the failure plane, up to 7.5ı at the head scarp.
The potential landslide mass identified adjacent to the Byron Slide protrudes
anomalously out from the shelf in the heavily incised southern canyon section
(Fig. 47.1). The extensive mass wasting surrounding this block and apparent tension
crack features at the head of the identified mass (Fig. 47.2f) suggests it’s possible
future failure. The tension crack represents the landslide head, while a break in slope
that presents as extant circular failures cresting at the 1,500 m contour defines the
expected toe.
534
S. Clarke et al.
Table 47.2 Summary of the maximum flow depths
Maximum flow depth at coastline Fd (0) (m)
Average slope angle >3ı
Average slope angle <3ı
Maximum landslide
velocity (vs ) (ms 1 )
Bribie
Bowl Slide
Byron
Slide
Potential Cudgen
Slide
Slide
Coolangatta1 Coolangatta2
Slide
Slide
10
20
30
40
2:7
5:0
8:5
13:0
6:2
10:3
16:3
24:0
6:2
10:9
17:8
26:7
0.6
1.2
2.0
3.0
3:0
5:6
9:5
14:5
0.7
1.2
1.8
2.8
Failures are assumed to occur as one complete landslide block, rather than
as a number of multiple landslides from the same site. This may generate an
overestimated volume of the landslide block.
47.3.2 Calculated Characteristics of Landslide-Generated
Tsunami
Table 47.2 summarises maximum flow depth at the coastline for the five landslides
and the potential landslide using a range of maximum landslide velocities (vs ).
The dimensions of the landslide mass, initial acceleration and maximum velocity
of the sliding mass are important when assessing expected tsunami size (Ward
2001). The range of velocities tested has been constrained by minimum and
maximum velocity values reported in the literature (Masson et al. 2006). The
velocity at which submarine landslides travel after failure is not well defined due
to a lack of direct measurements. The 1929 Grand Banks Slide was measured
travelling at 25 ms-1 (Fine et al. 2005), while the 2006 SW Taiwan event measured
turbidity current velocities between 17 and 20 ms-1 (Hsu 2008). Velocities are
based on cable breakages during failure and are measured from the gentle upper
slope, approximately 2ı and <0.5ı respectively. At the higher end, speeds of
up to 80 ms-1 have been inferred for some large landslides based on landslide
debris travel distances (Masson et al. 2006). We have tested a range of possible
landslide velocities; however we consider 20 ms-1 a reasonable and conservative
(i.e. minimum) value (Driscoll et al. 2000).
The calculations demonstrate that submarine failures along the EA continental
margin have the potential to generate tsunami with flow depths at the coast ranging
from 1.2 to 10.9 m for landslide velocities of 20 ms-1 . In particular, the Bribie Bowl,
Byron, Cudgen and the Potential Slides all generate flow depths greater than 5 m at
the coastline for landslide velocities of 20 ms-1 (Table 47.2).
Flow depth at the coastline directly relates to landslide thickness at a particular
location. The thinner landslides considered, Coolangatta1&2 Slides (thickness 20 m
and 45 m respectfully), produce smaller tsunami with inundation depths of 1 m. The
47 Morphology of Australia’s Eastern Continental Slope. . .
535
Table 47.3 Summary of the maximum inundation distance (Xmax ) and run-up (R(Xmax ))
Maximum
landslide
velocity (vs )
(ms 1 )
Inundation
ı
Average slope angle <3ı
distance (Xmax ) Average slope angle >3
(m) & Run-up Bribie
Byron Potential Cudgen Coolangatta1 Coolangatta2
(R(Xmax )) (m) Bowl Slide Slide Slide
Slide Slide
Slide
10
Xmax
R(Xmax )
Xmax
R(Xmax )
Xmax
R(Xmax )
Xmax
R(Xmax )
20
30
40
26
2.4
398
1.5
880
1.95
1,715
1.9
500
2.08
898
4.1
1,720
5.95
3,252
6.9
500
2.08
901
4.8
1,978
6.3
3,805
7.36
24
27
2:75
0:15
50
54
5:2
0:4
100
130
8:8
0:42
732
220
10
0:73
31
0:15
53
0:4
120
0:37
220
0:39
Cudgen Slide (50 m thick) and Bribie Bowl Slide (125 m thick) generated 5–6 m
inundations, while the Byron Slide (220 m thick) and Potential Slide (estimated
200 m thick) generated inundation depths of 10 m.
Maximum flow depth at the coastline is larger for the thicker canyon landslides
(e.g. Byron Slide and Potential Slide ""10 m; Bribie Bowl Slide ""5 m) which
occur on steeper slopes in comparison to shallow plateau landslides which generally
produce waves less than 1 m in height, except where landslide surface area was
particularly large (e.g. Cudgen Slide: surface area ""50 km2 , flow depth ""5 m).
Table 47.3 summarizes maximum expected inundation distance and run-up for
the identified landslides over a range of landslide velocities (vs ). Local topography
greatly affect the ability of a wave to inundate past the immediate coastline (Gerardi
et al. 2008) and these values are estimates which are less reliable than coastal
inundation depth.
The results show that submarine landslides along the EA continental margin
have the potential to generate inundation and run-up distances up to 1 km and 5 m
respectively for landslide velocities of 20 ms-1 . Using 20 ms-1 landslide velocity
as a benchmark, the two shallow Coolangatta1&2 Slides generate maximum
inundation distances around 50 m and run-up about <0.5 m. The Cudgen Slide
generates inundation distances around 50 m, however with much greater run-up at
around 5 m. The Bribie Bowl Slide produced inundation distances around 400 m and
run-up about 1.5 m, while the Byron and Potential Slides from the canyon regions
generates maximum inundation distances around 1 km and run-up about 4 m.
47.4 Discussion
This study demonstrates that blocks shed from submarine scars have the potential
to generate significant tsunami on the EA coast with flow depths of at least 10 m in
height, run-up of ""5 m and maximum inundation distances of 1 km. These estimates
536
S. Clarke et al.
are based on relatively conservative estimates of landslide velocity given (a) the
relatively steeply inclined slopes down which sliding material has moved and (b)
the lack of evidence for a depositional site where the landslide mass ceased moving
in mapped areas downslope of the landslide, indicating that the landslide material
has travelled at least 15–20 km from its site of origin.
Evidence for historical tsunamis that have inundated the EA coastline is limited.
A recently published tsunami catalogue (Dominey-Howes 2007) identifies historical
events on the EA coast, of which 16 (""30 %) have no identified cause (runups between <0.1 to 6 m asl). Nevertheless, our results show that upper slope
landslides similar to those investigated in this study are a plausible source for
the tsunami documented in catalogue (Dominey-Howes 2007). A growing body of
evidence (Hubble 2013) is strengthening the contention that shedding landslides
of the size investigated in this study should be considered to be a common,
ongoing characteristic such that future failures are very likely to occur. Constraining
estimates of landslide velocity is critical as coastal flow depth, inundation distance
and run-up all increase with landslide velocity. We have used conservative estimates
of landslide velocity and it is quite possible that these landslides can generate
significantly more destructive events than we have suggested.
Evaluating possible triggering conditions for submarine sliding is also critical,
as these conditions are not well understood (Masson et al. 2006). However, the
majority of documented twentieth century submarine landslide tsunami events are
related to moderate earthquakes (generally >M7). Such events are rare on the stable
Australian continent and suggested recurrence intervals for such events are at multimillennial timescales (Clark 2010).
47.5 Conclusions
The morphometric characterization of five distinct, geologically young, submarine
landslides on the eastern Australian upper continental slope has enabled an estimation of tsunami size their related landslide masses probably generated and an
insight into tsunami hazard that might be expected on the eastern Australian coast.
Flow depth (1.2–10.9 m), run-up (0.4–4.8 m) and inundation distance (50–901 m)
were calculated for five landslide sites assuming landslide velocities of 20 ms-1 . The
reoccurrence of submarine landslides with similar characteristics to those shed from
the margin in the geologically recent past would therefore be expected to generate
tsunami with maximum flow depths between 5 and 10 m at the coastline, run-up of
up to 5 m and inundation distances of up to 1 km. In particular, a potential landslide
mass adjacent to the Byron Slide has been identified. If it was to fail it could generate
maximum flow depths of ca. 10 m at the coastline, with inundation distances of ca.
1 km, for a conservative landslide velocity of 20 ms-1 . If these assumptions are
correct, a tsunami this size would cause significant damage and possibly loss of life.
47 Morphology of Australia’s Eastern Continental Slope. . .
537
Acknowledgments We would like to acknowledge the P&O crew and scientific crews of the RV
Southern Surveyor voyage (12/2008). Funding for this voyage was provided by ARC Australia and
ConocoPhillips Pty Ltd. This paper benefitted from reviews by Dr Geoffroy Lamarche and Dr Julie
Dickinson.
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