The Millennium Drought Riverbank Failures | Lower Murray River – South Australia |
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The Millennium Drought Riverbank Failures | Lower Murray River – South Australia | Tom Hubble and Elyssa De Carli Goyder Institute for Water Research Technical Report Series No. 15/5 www.goyderinstitute.org Goyder Institute for Water Research Technical Report Series ISSN: 1839-2725 The Goyder Institute for Water Research is a partnership between the South Australian Government through the Department of Environment, Water and Natural Resources, CSIRO, Flinders University, the University of Adelaide and the University of South Australia. The Institute will enhance the South Australian Government’s capacity to develop and deliver science-based policy solutions in water management. It brings together the best scientists and researchers across Australia to provide expert and independent scientific advice to inform good government water policy and identify future threats and opportunities to water security. The following organisation contributed to this report: Enquires should be addressed to: Goyder Institute for Water Research Level 1, Torrens Building 220 Victoria Square, Adelaide, SA, 5000 tel: 08-8303 8952 e-mail: [email protected] Citation Hubble, T., and E. De Carli. (2015) Mechanisms and Processes of the Millennium Drought River Bank Failures: Lower Murray River, South Australia, Goyder Institute for Water Research Technical Report Series No. 15/5, Adelaide, South Australia Copyright © 2015 The University of Sydney. To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of The University of Sydney. Disclaimer The Participants advise that the information contained in this publication comprises general statements based on scientific research and does not warrant or represent the completeness of any information or material in this publication. CONTENTS List of Figures List of Tables SUMMARY ........................................................................................................................... 5 GLOSSARY .......................................................................................................................... 6 1 INTRODUCTION ........................................................................................................... 9 1.1 BACKGROUND ...................................................................................................... 9 1.2 AIM AND OBJECTIVES ........................................................................................ 11 2 NEW DATA ACQUISITION ......................................................................................... 12 2.1 MULTIBEAM BATHYMETRIC SURVEYS ............................................................. 12 2.2 IN-RIVER SEDIMENT SAMPLING ....................................................................... 12 Part 1 - Geomorphic Analysis ...................................................................................... 13 3 GEOLOGICAL SETTING AND SUBSURFACE MATERIALS ..................................... 13 3.1 GEOLOGICAL SETTING ...................................................................................... 13 3.2 SUBSURFACE RIVERBANK SEDIMENTS .......................................................... 13 4 MULTIBEAM BATHYMETRIC RIVER CHANNEL MAPPING ..................................... 17 4.1 UNIDENTIFIED FAILURES .................................................................................. 17 4.2 TYPES OF RIVERBANK FAILURES .................................................................... 19 4.3 RELATIONSHIP BETWEEN CHANNEL GEOMORPHOLOGY AND RIVERBANK FAILURE ......................................................................................................................... 21 4.4 PREDICTIONS OF FAILURE PRONE ZONES ..................................................... 27 4.5 CONCLUSIONS ................................................................................................... 30 Part 2 - Failure Modelling .............................................................................................. 31 5 SLOPE STABILITY MODELLING ............................................................................... 31 6 CONCLUSIONS .......................................................................................................... 36 6.1 PROPOSED MECHANISMS OF FAILURE ........................................................... 36 7 RECOMMENDATIONS ................................................................................................ 38 9 ACKNOWLEDGMENTS .............................................................................................. 39 10 REFERENCES ......................................................................................................... 40 APPENDIX A ...................................................................................................................... 42 APPENDIX B ...................................................................................................................... 47 The Millennium Drought Riverbank Failures - Lower Murray River || 3 List of Figures Figure 1: A digital elevation model (DEM) of the Lower Murray River from Blanchetown (Lock 1) to Lake Alexandrina. Pg. 10 Figure 2: Pool levels at Blanchetown, Mannum and Murray Bridge during the peak of the Millennium Drought, and the occurrence of riverbank collapse events documented in geotechnical reports. Pg. 10 Figure 3: In-river sediment cores acquired along the LMR from Younghusband to Wellington. Pg. 15 Figure 4: In-river CPTu profiles acquired from the LMR between Younghusband and Wellington. Pg. 16 Figure 5: Bathymetry at Whitesands showing incident #50 reported in the DEWNR register and undocumented failures. Pg. 18 Figure 6: Bathymetry at Thiele Reserve showing incident #55 reported in the DEWNR register next to an undocumented larger failure. Pg. 18 Figure 7: LMR Failure Types as presented in the right bank of the channel near Woodlane Reserve- Mypolonga. Pg. 20 Figure 8: Murray Bridge 2014 regional bathymetric map. Pg. 22 Figure 9: Caloote 2014 regional bathymetric map. Pg. 23 Figure 10: Mannum 2014 regional bathymetric map. Pg. 24 Figure 11: Bedrock confinement of the LMR. Pg. 26 Figure 12: Aerial view of Woodlane Reserve, Mypolonga with colour coded bathymetric map. Pg. 26 Figure 13: Woodlane Reserve channel cross section profiles. Pg. 26 Figure 14: Concentration of low and high risk failure at Woodlane Reserve, Mypolonga. Pg. 27 Figure 15: The relationship between river width and river depth from Blanchetown to Wellington. Pg. 29 Figure 16: A selection of slope stability models of Thiele Reserve representing a low undrained shear strength (8.5kpa) scenario for the near-surface Coonambidgal mud layer. Pg. 35 Appendix Figure A1: Bathymetric survey extent between Blanchetown and Wellington, LMR South Australia. Pg. 43 Figure A2: SA Water Survey boat with bathymetric setup. Pg. 46 Figure B1: Location of Cone Penetrometer Tests (CPTu) and sediment cores acquired on 2013 and 2014 field campaigns between Wongulla and Wellington, LMR South Australia. Pg. 48 Figure B2: Volunteers percussion coring off side of RV BreakFree, May 2013. Pg. 51 Figure B3: Volunteers deploying CPTu gear from custom built frame installed on the front of the RV Breakfree, February 2014. Pg. 51 List of Tables Table 1. Factors of Safety calculated for Thiele Reserve for a range of lowered pool level undrained shear strength scenarios. Pg. 33 Appendix Table A1. Inventory of bathymetric surveys acquired by SA Water in 2009, 2011, 2014. Pg. 44 Table B1. Inventory of sediment cores and CPTu (cone penetrometers tests) acquired by Sydney University on 2013 and 2014 field campaigns. Pg. 49 The Millennium Drought Riverbank Failures - Lower Murray River || 4 SUMMARY At the peak of the Millennium Drought pool levels of the Lower River Murray fell 1.8 metres below the normal operating range (+0.75m AHD) and triggered at least sixty mass failures of the alluvial riverbanks (e.g. the Long-Island Marina Failure). Ground subsidence also occurred in some river-adjacent floodplain deposits (e.g. the Caloote Landing Event) between Blanchetown and Lake Alexandrina, South Australia. Bathymetric mapping of the river channel indicates that the majority of the larger bank failure features are associated with deep scour holes that have been eroded into the channel floor due to either: a) bedrock margin constriction and pronounced narrowing of the channel cross-section or b) large outcrops of bedrock which protrude up from the floor of the channel. This has generated erosive flow patterns during periods of higher flow that have scoured deep holes and eroded the toe of the downstream riverbank and over-steepened the channel margins. Riverbanks located adjacent or near to deep scour holes in the channel are over-steepened and more likely to fail during periods of lowered pool level. Slope stability modelling indicates that the bank failures during the Millennium Drought were caused by the lowered river levels that were extant during this time. In addition, the presence of Soft Clay within the bank materials and local anthropogenic modifications of the banks increase the likelihood of failure. In particular, riverbanks that have been modified by the placement of fill or the construction of an embankment adjacent to the waterline are more likely to fail than unmodified banks. It is probable that these modifications have accelerated and amplified natural