Riverbank Collapse on the Lower River Murray: Recent Recent Phenomenon or Long-term Geomorphic Process? De Carli, E.V ; Hubble, T ; Jaksa, M; Clarke, S ; Airey, D; O’Toole, J; Carpetner, G and Scientic Parties MV Breakfree 2012-2013  Geocoastal Research Group, School of Geosciences, The University of Sydney, Australia 2006;  School of Civil Engineering, The University of Sydney, Australia 2006;  School of Civil, Environmental and Mining Engineering, The University of , 5005;  The Department of Environment, Water and Natural Resources (DEWNR), South Australia 5005. Email: [email protected] Poster ID 1380

A 2. B RESULTS

BLANCHETOWN Analysis of multi-beam bathymetric data demonstrates that slumps and slab slides from the Murray’s submerged banks are widespread and common. Failures are concentrated immediately downstream of bedrock intrusions into channel margins and association with channel incision. Three dierent types of bank-failure morphology have been identied. 3. These are: Type 1, ‘recent’ (2009-2011) 1. deep-seated rotational slumps cresting 5 to 10 -2 (i) metres inland from the waterline, -4 N -6 characterised by distinct, sharply-dened failure scars and associated debris elds of -8 (ii) -10 angular blocks shed from the failure site; -12 Type 2, ‘mature’ shallow planar-failures, with -14 Depth (m) less well-dened smoother failure scars and Walker Flat 100 metres associated debris elds of smoothed or rounded blocks; and Type 3, ‘relict’ shallow 70 (ii) 60 planar-failures characterised by subdued-relief 50 scars that do not present an associated debris 40 eld. It is suspected that successive high-ow 30 events progressively erode and redistribute 20 Slope Angle (%) Slope Angle material, smoothing the landslide scars and 3. Woodlane 10 0 redistributing the slide-debris deposits. 25 metres

Murray Bridge VE = 2

Type 1 Type 2 Type 3 White Sands (i) 4. 4. -2 ‘Recent’ Failure ‘Mature’ Failure ‘Relict’ Failure N -4 -6 Direction of Downslope Debris (ii) -8 ? Bedrock Ledge 2. -10 Fluvial Transport Direction

N -12 LAKE Depth (m) Record of Failure in DEWNR Incident Register 10 km -14 ALEXANDRINA 100 metres 1. The Murray-Daring Basin, Australia. 3. Woodlane Reserve, Mypolonga, South Australia. 2. Location of multi-beam bathymetric Image 3(i): Multi-beam bathymetry of the left-hand surveys undertaken from to (ii) channel margin of Woodlane Reserve, indicating Lake Alexandrina, Lower River -channel depth (m) and present bank failures. South Australia ( ). Inset 3. Location of Inset Image 3(ii): Morphology of bank failures, and Woodlane Reserve, and 4. White Sands. direction of downslope debris movement. 70 Scale bar indicates slope angle (%). 60 4. White Sands, Riverglen, South Australia. 50 Image 4(i): Multi-beam bathymetry of the left-hand 40 channel margin of White Sands indicating

INTRODUCTION (%) Slope Angle 30 river-channel depth (m) and present bank failures. 25 metres 20 Inset Image 4(ii): Morphology of bank failures, direction The lower River Murray connects the Murray-Darling Basin to the Southern Ocean and drains 10 of downslope debris movement and presence of bed- VE = 2 0 rock ledge. Scale bar indicates slope angle (%). 14% of Australia’s landmass (Image 1). During the peak of the Millennium Drought Nb. Digital representation of geological map features based on FGDC U.S. Geological Survey standards. (1997-2011) inows for the Murray-Darling Basin were the lowest on record and the lower River Murray received only 19% of its long-term average inow during 2008-2009. As a consequence the pool-level in the lower Murray fell to 1 meter below sea level. This event C triggered an episode of widespread mass failure in the alluvial river banks and ground CONCLUSIONS subsidence in river-adjacent oodplain deposits between Blanchetown and Lake Alexandrina. Bank-failure and the delivery of slide material with the amplication being a consequence of 1 meter below sea level during the Millennium into the channel is interpreted to be an ongoing anthropogenic modications of the river channel Drought. The deep cresting geometries of the Multi-beam surveying of the river channel and submerged banks was undertaken by the and long-term geomorphic characteristic of the and banks, e.g. levee and embankment Type 1 failures pose an increased risk to river Department of Environment, Water and Natural Resources (DEWNR) South Australia, in sections of lower River Murray. It is strongly suspected that construction on the channel margins, as well as the users and a challenge for the management of the river where large bank failures threatened private housing or public infrastructure (Image 2). the recent Type 1 failures are amplied construction of barrages at the river mouth, which this hazard. equivalents of the older Type 2 and 3 failures, enabled the recession of the pool-level to reach