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: samantha.clarke@sydney.edu.au

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

Landslide name

Parameter (m)

Canyon region Average slope angle >3° Plateau region

Average slope angle <3° Bribie Bowl Bryon Potential Slide (adjacent to

Slide Slide Bryon Slide) Cudgen Coolangatta1 Coolangatta2 Slide Slide Slide

Table 47.1 Summary of the submarine landslide parameters

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.

.0/ F

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 ŒA0 P .r /]4=5 h0 1=5 (47.1)

Propagation and beaching factor:

W

Lref 0:36 h

vs vt i2

Lref 0:69

A0 P .r / D 0:7345t Lref h0

e-3:74 vt

r (47.2)

Run-up and Inundation depth:

16:7n2

ŒR.X max / - T .X 0 /] C d

.X max - X 0 / D F 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

at the landslide D p

(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 Slide

Cudgen Slide

Coolangatta1 Slide

Coolangatta2 Slide

10 2:7 6:2 6:2 3:0 0.6 0.7 20 5:0 10:3 10:9 5:6 1.2 1.2 30 8:5 16:3 17:8 9:5 2.0 1.8 40 13:0 24:0 26:7 14:5 3.0 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

Inundation distance (Xmax )

Average slope angle >3ı Average slope angle <3ı

velocity (vs ) (m) & Run-up Bribie Byron Potential Cudgen Coolangatta1 Coolangatta2 (ms 1 ) (R(Xmax )) (m) Bowl Slide Slide Slide Slide Slide Slide

10 Xmax 26 500 500 24 27 31 R(Xmax ) 2.4 2.08 2.08 2:75 0:15 0:15 20 Xmax 398 898 901 50 54 53 R(Xmax ) 1.5 4.1 4.8 5:2 0:4 0:4 30 Xmax 880 1,720 1,978 100 130 120 R(Xmax ) 1.95 5.95 6.3 8:8 0:42 0:37 40 Xmax 1,715 3,252 3,805 732 220 220 R(Xmax ) 1.9 6.9 7.36 10 0:73 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 (run- ups 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 multi- millennial 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 esti- mation 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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