The Millennium Drought Riverbank Failures

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.

The Millennium Drought Riverbank Failures - Lower Murray River || 3

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


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


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 processes of channel change.

The identification of failure prone river reaches and recognition of the factors that contributed

to the Millennium Drought Failures will assist management of this hazard on the Lower

Murray River in the future. In particular it is apparent that:

Pool level in the Lower Murray from Blanchetown to Wellington should be maintained

at or above the normal operating level of +0.75m AHD; and

Further bathymetric mapping of the channel should be undertaken as this will assist in

identifying those reaches of the channel where deep scour holes and protruding

bedrock blocks are present. It is at these locations where large mass failures are

more likely to occur during future low flow drought events that accompany extended

periods of drought.


GLOSSARY

Alluvial (or alluvial deposits): sedimentary material transported by a river and deposited adjacent

to the river’s channel or on the river’s floodplain

AHD: Australian Height Datum, the base or reference level for Australian topographic maps. Height of

the ground surface is usually expressed as an elevation in metres, e.g. 33 m AHD, relative to mean

sea level. The reference point of 0 ma AHD, was determined as mean sea level measured at thirty

reference tide gauges in 1971.

Bank Toe: the toe of the bank slope; i.e. the lowermost portion of the river bank slope; in this case the

bank toe represents the lowest fifth of the bank slope.

Bedrock Pinch Points: in the case of the Murray River this term refers to those reaches of the river

channel that are laterally restricted or confined by a bedrock bank (or both banks) which protrude into

the channel and obstruct the river flow. The channel’s width is significantly narrower at these sites in

comparison to the average channel width upstream and downstream of the pinch point due to the

confinement of the channel by the bedrock walls. Typically, channel width is reduced to between 1/2

and 2/3 of the average channel width at these sites.

Cone Penetration Test (CPT): Standard geotechnical and civil engineering site investigation tool

that can be used to determine the type, compressibility, bearing capacity and strength of soils and

sediments. Typically used for determining loads that can be placed on cylindrical piles and piled

foundations because the device directly determines a) the vertical end-bearing load that the flat,

circular end of a pile can exert on a soil layer without deforming it; and b) the force required to drive

the cylindrical shaft of the pile down through the soil. Muddy sediments tend to generate a

proportionally higher shaft (or sleeve) resistance in comparison to their end bearing loads in

comparison to the ratio of end to sleeve resistance of sandy soils.

Cone Sleeve Resistance (or pressure) - see Sleeve Resistance (CPT)

Cone Tip Resistance or Cone Tip Pressure (qc): a measure of the force required to advance the

conical head of a cone penetrometer device vertically down through soil or weathered rock at 25 mm

per second. See also Cone Penetrometer Test, Sleeve Resistance (CPT).

CPTu: pore-water pressure measured at the advancing head of a Cone Penetrometer Device,

measured by an electronic transducer embedded in the head of the cone penetrometer device.

Cutbanks: outside curve or bank of a meandering (sinuous) stream. Generally expected to erode as

the stream evolves and migrates with time. The water flows faster around the outside of the curve and

slower on the inside of the curve. In a river system that is a net receiver of additional sediment over

time (i.e. accreting material or aggrading) the inside bank generally receives additional sediment and

is known as the accreting bank. On the Lower Murray, despite the meandering shape of the river, it is

not currently receiving sediment in the same way as text-book examples of meandering river systems

do (the Mississippi River).

Eddying, eddy: the process of forming an eddy or the downstream movement of an eddy; eddy:

water moving with a circular motion, small whirlpool.

Friction Ratio: (refer to CPT Test description, sleeve resistance, and cone tip resistance): ratio of the

sleeve resistance to the cone tip resistance. Usually expressed as a percentage. The friction ratio

tends to be high for muds and low for sands.


Glacial Maximum: refers to the period of time between approximately twenty-five thousand and

fifteen thousand years ago when global glacial conditions were at their peak. At this time the Northern

Hemisphere and Antarctic ice-sheets expanded to their maximum extent and mean sea-level was

lowered by approximately 120 metres beneath its current position. Both these ice-sheets shrank and

retreated to their current configurations and positions between fifteen thousand and five thousand

years and sea-level rose back to just above (about two metres) its current position. The current

position of sea-level has been relatively stable, with a slight recession since that time.

Glacioeustasy; glacioeustatic: a change in sea level due to the uptake or release of water from

glaciers and polar ice.

kPa: kilo-Pascals; Systeme Internationale unit for the measurement of stress (or moderate

pressure); pressure can be expressed as force per unit area, such that one kilo-Pascal is equal to one

mega-Newton per square meter. One kilo-Pascal is equivalent to the force exerted by 100 kilograms

placed on an area of one square metre. The development of this pressure on a surface can be

visualised by imagining a thin slab of concrete with one-metre square base that is four centimetres

high.

Levee: natural or constructed embankment adjacent to the water line on a river. Led Zeppelin’s

‘When the Levee Breaks’ (we’ve got no place to go), famously captured the drama and danger of

flood-compromised levees on the classic, 1971 album, Led Zeppelin IV.

LMR: Lower Murray River

MPa: mega-Pascals, or one million Pascals. Systeme Internationale unit for the measurement of

stress (or high pressure); pressure can be expressed as force per unit area, such that one mega-

Pascal is equal to one mega-Newton per square metre. One mega Pascal is equivalent to the force

exerted by 100 tonnes placed on an area of one square metre. The development of this pressure on a

surface can be visualised by imagining a column of a of concrete with one-metre square base that is

forty metres high

Porewater Pressure (u2): the pressure exerted by the water contained within the pores or voids

between sand or sediment grains in a soil or sediment layer. If the porewater pressure approaches

the pressure generated by the ambient load or self-imposed weight of the sediment column then the

material will be prone to failure or collapse. This condition can arise if the permeability of the sediment

is very low, in which case the water cannot seep out of the soil easily. The value of u2 is the

porewater pressure measured by the CPTu.

Planar-Failure: Type of landslide that has occurred on a flat or planar basal surface. The slide mass

or body is typically tabular in shape and the mass commonly moves as a single, contiguous mass,

which may break as the mass accelerates or collides with obstructions.

Scarp crest: the crest of a scarp, a scarp being a small cliff-like feature. In the case of the Murray

River bank failures, the crest or uppermost portion of the circular or tabular failure surfaces are

commonly one to two metres high although the scarps developed at Long Island Marina are slightly

larger at three or four metres.

Sleeve Resistance, or Cone Sleeve Resistance (fs): a measure of the force required to advance the

cylindrical sleeve or shaft of a cone penetrometer device vertically down through soil or weathered

rock at 25 mm per second. See also Cone Penetration Test (CPT).

http://en.wiktionary.org/wiki/sea_level

http://en.wiktionary.org/wiki/water

http://en.wiktionary.org/wiki/glacier


Shallow Planar Failure (see planar failure): a thin planar failure; with the term thin being relative to

the length of the slope from slide mass has been shed. In the case of the River Murray failures the

shallow planar slides are generally between one and four metres thick.

Sinuosity, sinuosities (plural): a measure of the degree to which something is sinuous, for example

a meandering stream. Sinuous, descriptive term for the shape of a type of regular undulating curve;

typically exemplified by the sine wave, or sine function (mathematics). Commonly determined from the

ratio of actual channel length to the straight-line distance between two points on the channel.

Subdued-relief scars: A scarp which has been smoothed off by erosive action or buried beneath a

thin mantle of redistributed channel sediment. That is, the relief or shape of the scar (or feature) has

been subdued.

Thalweg: a line connecting the deepest points of a river channel.

Toe scour: scour or erosion of the toe of a river bank by flowing, usually fast-flowing water.


1 INTRODUCTION

1.1 BACKGROUND

The Lower Murray River (LMR) between Blanchetown (Lock 1) and Lake Alexandrina within

South Australia is the lowermost reach of the Murray-Darling river system and drains 14% of

Australia’s landmass (Fig. 1). At the peak of the Millennium Drought (1997-2011) (Leblanc et

al. 2012) widespread mass failure of the alluvial banks occurred after the fall of the river’s

water surface (pool levels) from a normal operating range of +0.75m AHD (Australian Height

Datum) to -1.05 m AHD. The depressed pool levels occurred as a consequence of upstream

flow regulation and extraction, and evaporation due to the prolonged drought conditions.

During the Millennium Drought river flows were only 19% of the long-term averaged

regulated inflow for the 2008-2009 period (CSIRO 2008) (Fig. 2). The Goolwa barrages

during this period remained closed preventing seawater from entering the terminal lakes and

lower Murray channel, in order to maintain freshwater supplies in the lower Murray for

agricultural reasons. Historically, flow in the Murray River has been known to cease during

18th and early 19th century droughts with seawater ingress as far upstream as Murray Bridge

reported prior to barrage construction (SLSA 2010).

Between 2008 and 2011, 68 failures were recorded by the Department of Water,

Environment and Natural Resources (DEWNR) between Blanchetown and Wellington. The

largest of these failures occurred at Long Island Marina where at least five separate

slumping events occurred and approximately 300m of riverbank collapsed into the channel.

The duration of the largest of these slide events was less than a minute and a slightly

different timing of the inception of the event could well have resulted in a fatality.

Geotechnical investigations commissioned by DEWNR concluded that historically low river

levels and the presence of soft clays in the channel margins as the primary factors

contributing to riverbank collapse (ARUP 2008a, ARUP 2008b, SKM 2010a, SKM 2010b,

Coffey 2013), and identified this episode of failure as being atypical. Mass failure of

riverbanks during low flow conditions is unusual as most riverbanks fail during floods due to

removal of the bank toe by erosion or during the recession of floodwaters due to toe scour

and rapid drawdown (Brizga and Finlayson 1999, Schumm 2005, Hubble et al. 2010, Grove

et al. 2013).


Figure 1: A digital elevation model (DEM) of the Lower Murray River from Blanchetown (Lock 1) to Lake Alexandrina. Water bodies are shown in dark blue.

Figure 2: Pool levels (y-axis) at Blanchetown, Mannum and Murray Bridge during the peak of the Millennium Drought (x-axis) (DEWNR 2013), and the

occurrence of riverbank failures documented in geotechnical reports (red marker). Note an onset of failure events as pool levels dropped to -1.05m AHD from

January 2009 (nb. normal operating range +0.75m AHD), and a continuation of failures post July 2010 as pool levels peak with flood waters.

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1.2 AIM AND OBJECTIVES

The aims of this report are to:

Present and synthesize bathymetric, sedimentary and geotechnical data collected in

the 2013 and 2014 University of Sydney fieldwork programs;

Integrate these findings with those of the literature review report that was published

in 2013 (Jaksa et al. 2013);

Identify the primary causes of the riverbank failures in the LMR during the Millennium

Drought; and

Determine the reasons why large-scale failure events (such as the Long Island

Marina event) were restricted to a small number of particular sites rather than

occurring everywhere along the lower Murray.

Outcomes from this project will guide the development of management strategies that will

reduce the likelihood of future failures and/or help identify river reaches that are likely to be

at high risk of similar failures in the future.

Specifically this report will present:

a) A summary of the 2014 program of bathymetric mapping of several LMR reaches at

Mannum, Caloote and Murray Bridge;

b) A summary of the findings of the 2013 and 2014 program of riverbank sampling and

geotechnical testing;

c) A summary of the results of geotechnical modelling of the representative riverbank

failure mechanisms;

d) An interpretation of the findings which integrates the results of the three investigative

methods and identifies the probable causes and processes responsible for bank

failures on the LMR during the Millennium Drought;

e) Suggested management strategies and recommendations for future work.


2 NEW DATA ACQUISITION

2.1 MULTIBEAM BATHYMETRIC SURVEYS

Multibeam bathymetric surveys were undertaken at particular failed riverbank sites between

Blanchetown and Wellington by SA Water in 2009 and 2011 (See Appendix A - Table A1 for

survey locations and extent details). The purpose of these initial surveys was to identify

below surface navigational hazards, acquire three-dimensional images on the extent of the

failure debris and bank geometry data for geotechnical investigations. As such, these initial

surveys focussed only on mapping localised failed debris at reported riverbank failure sites,

e.g. survey of the right riverbank at Long Island Marina (approx. 400m in length).

As part of the 2014 data acquisition campaign, the University of Sydney commissioned SA

Water to acquire regional bathymetric maps of the LMR channel around Mannum, Caloote

and Murray Bridge (See Appendix A - Table A1 for survey locations and extent details). The

objective of these surveys was to supplement the failure-specific surveys previously

acquired, and provide the project with whole-of-channel information and the regional

geomorphic context for individual riverbank failures, e.g. bank to bank mapping from Avoca

Dell to Riverglen (approx. 19 river kilometres).

2.2 IN-RIVER SEDIMENT SAMPLING

In-river sediment cores and piezocone cone penetration tests (CPTus) were acquired during

the 2013 and 2014 fieldwork programs, in order to understand the geomorphic history and

sedimentology of the LMR, and provide geological context for the interpretation of

subsurface CPTu results. CPTus are commonly used in geotechnical investigations to

provide a guide to the soil behaviour type of subsurface materials. A CPTu includes pore

water pressure (u2) measurements, as well as cone resistance (qc) and sleeve friction (fs),

and enables for a reliable assessment of soil type when calibrated against physical samples

(e.g. sediment cores).

In-river sediment cores were collected at a total of nine sites of riverbank instability through

push coring techniques alongside a moored houseboat, the RV Breakfree. The cores were

recovered at points located approximately 2m, 12m and 20m distance from the water’s edge

on the riverbank. This allowed for the reconstruction of a representative subsurface profile of

the riverbank sediments, ground-truthing CPTu profiles, and supplementing boreholes

acquired during previous geotechnical investigations that sampled on-riverbank sediments.

See Appendix B for core location and recovery details.

In-river CPTu’s were undertaken at all coring sites and at additional locations at

approximately 5 to 10 km intervals between Wongulla and Wellington. The purpose of this

was to confirm the valley wide distribution of the Coonambidgal Mud’s ‘Soft Clay’ layer (cf

Coffey, 2013; Jaksa et al. 2013) which was identified by previous geotechnical investigations

as being a primary factor contributing to LMR riverbank collapse (ARUP 2008a, ARUP

2008b, SKM 2010a, SKM 2010b, Coffey 2013). See Appendix B for CPTu acquisition,

location and depth of penetration details.


Part 1 - Geomorphic Analysis

3 GEOLOGICAL SETTING AND SUBSURFACE MATERIALS

3.1 GEOLOGICAL SETTING

The LMR is located at the terminus of the Murray-Darling Basin drainage system which

covers 1.073 km2 or 14% of Australia’s landmass. The LMR channel has been structurally

controlled through uplift of the Murray Basin since the Pliocene, and the river itself

entrenched and incised within the LMR bedrock valley due to a combination of this uplift and

glacio-eustatic sea level fluctuations during the Pleistocene (Twidale et al 1978; Murray-

Wallace et al 2010). The valley is between 3 to 5 kilometres wide and its base reaches ca.

10 m below present-day sea level at Swan Reach and ca. 65 m at Murray Bridge (Twidale et

al., 1978). Sands of the Monoman Formation comprise the ‘lower valley fill’ deposited during

late Pleistocene deglaciations and marine transgressions. As sea level stabilized during the

Holocene, alluvial muds of the Coonambidgal Formation were deposited comprising the

‘upper valley fill’ or ‘Soft Clays’ referred to in previous geotechnical investigations.

The present-day LMR is a low sinuosity, suspended-load channel with cohesive bank

materials, low bed slopes and low stream power (Thoms and Walker 1989). It terminates at

Lakes Alexandrina and Albert (LA&A) before debouching into the Southern Ocean through

the Coorong Lagoon and Murray Mouth.

3.2 SUBSURFACE RIVERBANK SEDIMENTS

The in-river sediment cores obtained demonstrate the spatial and lateral consistency (or

valley wide extent) of the Soft Clay or Coonambidgal muds sediment, confirming the

uniformity observed at depth in CPTu results and supplementing findings by previous

geotechnical investigations (ARUP 2008a, ARUP 2008b, SKM 2010a, SKM 2010b, Coffey

2013).

The Soft Clay is underlain by stiffer Monoman Sands or near-surface bedrock, this interface

becoming deeper in the stratigraphic profile from Walkers Flat to Riverglen (approx. 10m

below the surface to greater than 20m). Generally the top 2 to 4 metres of the Soft Clay is

overlain by colluvium and fill associated with anthropogenically constructed levees and

riverbanks, reflected in the cores and CPTu results.

Photographs of typical cores taken at four in-channel sites at Younghusband, Woodlane

Reserve, Thiele Reserve, Riverglen and a single off-channel site at Wellington East Marina

are provided in Figure 3. CPTu soundings for the same sites showing the position of the

push core in the CPTu log are given in Figure 4.


The in-river CPTu profiles confirm the low strength nature of the Coonambidgal muds at

depth, with cone tip resistance (qc) typically increasing on average from just 0.2 MPa to 0.8

MPa over an approximate 20m depth increase. Typical undrained shear strengths for

materials with low cone tip pressures are 10 kPa to 25 kPa with a trend of increasing shear

strength with depth (cf USBR 2001).


Figure 3: In-river sediment cores acquired along the LMR from Younghusband to Wellington. For core locations and recovery details see Appendix B. Cores lengths a, b and c are in successive order.


Figure 4: In-river CPTu profiles acquired from the LMR between Younghusband and Wellington. For CPTu locations see Appendix B. The locations of cores represented in Figure 3 are displayed within each CPT profile

(dashed black line).


4 MULTIBEAM BATHYMETRIC RIVER CHANNEL MAPPING

Examination of the regional bathymetry acquired in 2014 has revealed previously unreported

and undocumented riverbank failures in the channel margins of the LMR between

Blanchetown and Wellington (Section 4.1). The failures can be separated based on differing

characteristics into low and high-risk categories (Section 4.2). A regional analysis has also

revealed several strong relationships between river channel width, depth, the location of

bedrock and the occurrence of riverbank failures (Section 4.3).

4.1 UNIDENTIFIED FAILURES

Many of the failures identified in our examination of the 2014 regional bathymetry surveys do

not appear in the DEWNR Incident Register (e.g. Figs. 5 and 6). These unreported failures

are located slightly lower down the riverbank slope, and visual inspections by the authors at

a number of these sites indicated that there were no obvious disturbances of the landscape

or infrastructure located on the bank adjacent to the failure. Similarly, many of these features

do not present obvious, tell-tale scarp crests that emerge above the water line and are,

therefore, much less likely to have been noticed by residents or professional and

recreational river users. The recognition of these features is also hampered by the murky

and turbid river water, obscuring the failures from view. A good example of co-located

reported and unreported bank failures are the failures located either side of the Thiele

Reserve boat ramp (Fig. 6). The smaller, riverbank failure (#55) presented a crest which

emerged above the waterline and caused subsidence and cracking of the adjacent bank.

The other much larger failure occurred on the foreshore of the public reserve and crested

below the water line and did not noticeably disturb the immediately adjacent land. The

angular debris field of the undocumented Thiele Reserve failure presented morphologic

characteristics similar to other failures that occurred during the Millennium Drought,

indicating that it probably occurred during the recent riverbank instability period (2008-2011).


Figure 5: Bathymetry at Whitesands showing incident #50 reported in the DEWNR register and

undocumented failures in front of riverfront properties as indicated by black outlines. Image Source:

Google Earth 2014.

Figure 6: Bathymetry at Thiele Reserve showing incident #55 reported in the DEWNR register

adjacent to boat ramp, next to an undocumented larger failure downstream on the foreshore of the

reserve. Image Source: Google Earth 2014.


4.2 TYPES OF RIVERBANK FAILURES

Examination of the bathymetric mapping enabled recognition of three different types of bank

failures apparent in the channel margins of the LMR (Jaksa et al., 2013) (Fig. 7). They are:

Type 1 – large-scale rotational failures cresting 5 to 10 m inland from the

waterline. They are characterised by distinct, sharply-defined failure scars and

associated debris fields of angular blocks shed from the failure site. These

failures occurred during the Millennium Drought from 2008 to 2011 and damaged

riverfront infrastructure in several instances (e.g. Long Island Marina, Woodlane

Reserve), and due to their size and destructive nature are considered high risk.

Type 2 – shallow planar-failures characterised by smoothed failure scars and

associated debris fields of smoothed or rounded blocks. These failures are

suspected to have occurred during the drawdown phase of large historic floods

such as those that occurred during the 1956 or 1974 events, or during the recent

2010 and 2011 flood pulse that alleviated Millennium Drought conditions.

Type 3 – shallow planar-failures characterised by smoothed failure scars,

associated debris fields are not present. Type 3 failures are considered ‘older’

versions of Type 2 failures, with the debris field having been removed by

numerous erosive high flow events. Type 2 & 3 failures are considered a lower

risk, as they present less of a hazard to the public and riverfront infrastructure

due to their shallow cresting nature and apparent lack of related riverbank

disturbances.


Figure 7: LMR Failure Types as presented in the right bank of the channel near Woodlane Reserve-Mypolonga. Type 1 rotational failure with large and angular debris field (blue shading, Incident #7 in DEWNR Register), Type 2 (green

shading) and Type 3 (brown shading) shallow-planar failures with a smoothed or eroded debris field (Jaksa et al., 2013).


4.3 RELATIONSHIP BETWEEN CHANNEL GEOMORPHOLOGY AND

RIVERBANK FAILURE

Detailed analysis of the high-resolution regional bathymetric surveys conducted around

Murray Bridge (Fig. 8), Caloote (Fig. 9) and Mannum (Fig. 10) revealed a strong relationship

between riverbank failure and channel geomorphology.

The LMR’s channel is on average 160m wide and 10 to 11m deep at the thalweg (i.e. the

channel’s deepest point). Concentrations of high risk rotational (Type 1) and low risk

shallow-planar failures (Type 2 & 3) commonly occur where the river channel narrows

(<160m wide) and/or deepens (<10.5m deep) due to the presence of bedrock channel

margins (e.g. Fig. 8 (A) Thiele Reserve, Fig. 8 (B) Bells Reserve, Fig. 9 (A) Woodlane

Reserve). Both low and high risk failures were also found to be concentrated at sites where

large bedrock blocks protruded prominently up from the floor of the channel in mid-channel

or side-channel locations (e.g. Fig. 8 (D) Sturt Reserve and Fig. 8 (E) Long Island Marina).

Deep erosional scour holes were observed in association with these conditions and are well

known to local fishing enthusiasts, commonly referred to as ‘cod holes’. They have been

observed elsewhere on the Murray River and are thought to act as aquifer recharge points

(Lawrie et al. 2012). It is worth noting that Type 2 and 3 failures are common throughout the

entire channel which suggests that they should considered to be normal, or a phenomenon

that is characteristic of the current geomorphic state of the LMR.


Figure 8: Murray Bridge 2014 bathymetry. Site (A) Thiele Reserve and (B) Bells Reserve depict areas of bedrock margins (dashed white line), channel width reduction, associated scour holes and bank instability. Site (D)

Sturt Reserve and (E) Long Island Marina represent areas with bedrock protrusions (n) and associated bank instability. Image source: Google Earth


Figure 9: Caloote 2014 bathymetric image. Note the numerous scour holes

depicted by the blue-purple colours in the depth legend. Site (A) Woodlane

Reserve shows a close up on the bedrock margin (white dashed line),

associated deep scour hole and downstream area of riverbank failure. Image

source: Google Earth


Figure 10: Mannum 2014 bathymetric image. East Front Rd which has

experienced numerous episodes of instability (subsidence, tension cracks,

pot-holes). Site (A) shows a close up on the bedrock margin (white

dashed line), and associated deep scour hole; note the obvious narrowing

of the channel which is due to the protrusion of a bedrock wall into the

channel (i.e. bedrock pinch-point). Image source: Google Earth


A reduction in river width (and channel volume) due to channel constriction by bedrock

margins leads to flow constrictions, resulting in increased water flow velocities, and in turn

increased erosive capacity of the flow (cf. Leopold and Wolman 1957, Schumm et al. 1972,

Schumm 1977, Summerfield 1991, Darby et al. 2010). Studies on mixed bedrock-alluvial

rivers have found that bedrock protrusions act to increase channel depth, flow velocities,

shear velocities and water surface slopes (Rennie et al. 2013), resulting in complex and

variable flow patterns on the inner and outer channel margins (Constantinescu et al. 2013). It

is believed that in the case of the LMR, a similar situation applies and the presence of

bedrock channel margins, constrictions or boulder-like protrusions affect the hydraulic flow

patterns during high flow conditions, promoting downstream hydraulic toe scour and

erosional flow eddies. These processes over-steepen the riverbank margins inherently

reduces bank stability. Along bedrock margins the deepening of the channel is most

pronounced alongside the bedrock and/or channel constriction, and gradually shallows with

distance downstream. Bedrock boulder-like protrusions apparently focus their erosional

effects immediately around or downstream of the protrusion. Determining or studying the in-

channel flow velocity distribution that erodes these scour holes is beyond the scope of this

study but it is strongly suspected that these effects are generally expressed during floods or

periods of relatively high flow events.

Figures 11 to 13 demonstrate the typical relationship between the presence of bedrock

margins and the downstream occurrence of a concentrated failure zone of low and high-risk

failures at Woodlane Reserve, Mypolonga (this failure zone is analysed in detail in Figure 7).

Figure 11 shows the river channel set within floodplain sediments and confined within the

bedrock valley, alternating between bedrock and alluvial river margins. This example at

Woodlane Reserve demonstrates that where the river channel encroaches upon the bedrock

margin (white dashed line in Fig. 12) over a short distance (~350m), the channel width

narrows from approx. 180m to 100m, and the channel bed incises from 10m deep to 22m

deep. This relationship is also portrayed through channel profiles (cross sections A and B)

represented in Figures 12 and 13. It is along the immediate downstream riverbank that over

steepening occurs due to hydraulic scour of the toe of slope, and low and high-risk failures

are found in concentration.


Figure 12: Aerial view of Woodlane Reserve, Mypolonga with colour coded bathymetry of the channel overlain on Google Earth Imagery of the site taken in 2010. Note the constriction of the channel (see cross sections A and B in Figure

13) imposed by the bedrock wall that protrudes into channel margins (dashed white in top right corner), the associated scour hole, and the immediate downstream failure zone.

-25

-20

-15

-10

-5

0

0 50 100 150 200

Ch

ann

el D

epth

(m

)

Channel Width (m)

Figure 11: Bedrock

confinement of the

LMR. The river channel

(colour scale for

riverbed depth) within

the light-grey coloured

floodplain sediments,

and grey-blue bedrock

gorge. Inset box shows

location of Woodlane

Reserve portrayed in

Figure 12.

Figure 13: Woodlane Reserve channel profiles taken from cross section

A (red line) and cross section B (blue line) from Figure 12. Profiles are

approximately 350 metres apart, and show a decrease in channel width

and depth.


4.4 PREDICTIONS OF FAILURE PRONE ZONES

Analysis of the newly acquired regional bathymetry shows that for the bathymetric surveys

that occupy the Mannum, Caloote and Murray Bridge reaches, low risk (T2-3) and high risk

(T1) risk failures were identified at over-steepened slopes downstream or adjacent to

bedrock margins or block protrusions in 94% of instances.

An example of this relationship is demonstrated at Woodlane Reserve in Fig. 14 below (See

Fig. 12 for image of bedrock margin and associated scour hole and riverbank failures). Here

it is evident that low risk (T2-3) and high risk (T1) failures occur in a section of the channel

where the river depth is below average (<10.5m). In addition, analysis of the bathymetry

confirms the presence of a deep scour holes adjacent to and downstream of a bedrock

margin at this site.

Figure 14: Concentration of low and high risk failure types in vicinity/immediately downstream of a

deep scour hole and channel width reduction at Woodlane Reserve, Mypolonga – a typical example

for the location of failure prone reaches of the LMR. Imagery of the site can be seen in Figs. 7 and 12.

Based on this relationship the authors suggest it possible to identify ‘failure prone’ river

reaches. Those reaches of the river where a river depth is greater than average has over

steepened riverbank slopes, making them prone to failure during extreme low flow

conditions. A first-pass assessment is presented here. Additional bathymetric mapping of the

channel is required to confirm this relationship.

The occurrence of in-channel bedrock block protrusions are also related to riverbank failure.

The presence and underwater extent of bedrock margins should also be investigated

through analysis of the ‘backscatter’ signal acquired in the bathymetric mapping of the

channel.

130 131 132 133 134 135 136

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-14

-12

-10

-8

-6

-4

-2

0

0

50

100

150

200

250

Riv

er

Depth

(m

)

Riv

er

Wid

th (

m)

River Kilometres

T1 Failure T2 Failure T3 Failure River Width (m) River Depth (m)

Flow Direction


Cross-sectional channel profiles that convey river depth and width data were acquired at

100m spacing by SA Water from Blanchetown to Wellington. Figure 15 shows the

relationship between river depth and width, with reaches of regional bathymetric mapping

marked by grey shading, and Type 1 riverbank failures identified from the bathymetry are

depicted also. It is evident that the identified higher risk failures occur where the river depth

is significantly deeper than average. It is also apparent from these graphs that river depths

between Mannum and Wellington are frequently greater than average. It follows that the

downstream Mannum to Wellington reach is likely to be more prone to high risk failures than

the upstream Mannum to Blanchetown reach. This is confirmed by the larger number of

riverbank failures reported during the Millennium Drought. It is also evident that a decrease

in river depth is often associated with a narrowing of channel width. It is emphasised that this

is a rough first –pass assessment of failure prone reaches limited by the patchy and

scattered bathymetric maps available for the entire length of the channel between

Blanchetown and Wellington. However, assuming the valley wide presence of the Soft Clay

muds and the confirmation of bedrock margins through geomorphic and bathymetric

analysis, this model provides the best available indicator of the likely riverbank ‘failure prone’

reaches to focus investigation and risk-assessment activities.


Figure 15: The relationship between river width (black line) and river depth (blue line) shown in river kilometres from Blanchetown to Mannum and Mannum to Wellington. Reaches that have been mapped with high-resolution

bathymetry are shaded grey. Type 1 higher risk failures identified from this bathymetry are represented by the brown circles. It is evident that the majority of these failures occur where the river thalweg (depth) is below

average (light-blue dashed line), and therefore this relationship/model could be used as a first past assessment for the identification of failure prone reaches on the LMR.


4.5 CONCLUSIONS

Several studies of the larger riverbank failures that occurred during the peak of the

Millennium Drought were undertaken soon after their occurrence in order to provide

DEWNR with advice and a context for appropriate management of the problem (ARUP

2008a, SKM 2010b, Coffey 2013). These studies combined conventional terrestrial site

investigations with site-specific bathymetric mapping and testing of physical properties to

constrain geotechnical models of the failures. This early work is summarised in the Stage

Two Literature Review and Knowledge Gap Report (Jaksa et al. 2013). The geotechnical

studies completed between 2010 and 2012 found that the following phenomena

characterized, or were probably responsible for, the Millennium Drought Failures (Coffey,

2013):

1) Unusually low river levels caused a large reduction in riverbank stability and

appeared to be the major precursor of the Millennium Drought failures;

2) Small variations in strength of the deposit of Soft Clay were likely to have a

significant effect on river bank stability

3) The construction of embankments/levees or the placement of fill on the riverbank

reduced stability at that site, however, some of the failures occurred at sites where

there is no fill or embankment/levee constructed;

4) The collapses that caused large regressions of the bank at the waterline were

probably the result of progressive failure (i.e. a rapid succession of collapses);

5) “It was reasonable to assume that riverbank collapse could occur during periods of

low river levels wherever the bank is underlain by Soft Clay.” (Coffey 2013);

The bathymetric and sedimentologic findings reported in Part 1- Sections Three and Four

(this report) extends this basic conceptual framework by providing additional findings:

1) The regional bathymetric mapping demonstrated that the high risk Millennium

Drought failures occurred in locations that were adjacent to deep scour holes

eroded in the river bed that cause an over steepening of bank slopes. These sites

are located adjacent to or downstream from bedrock margin narrowing of the

channel (i.e. bedrock pinch-points) or protruding in-river bedrock pinnacles;

2) formed at sites adjacent to or downstream from bedrock margins, channel width

reductions or protruding bedrock pinnacles that cause an over steepening of bank

slopes;

3) The Soft Clay of the Coonambidgal Formation is ubiquitously present regionally

within the LMR Valley (comprising the LMR’s alluvial floodplains), and varies little

with depth in the subsurface materials.


Part 2 - Failure Modelling

The findings concluded in Part 1 of this report will now be incorporated into slope stability

models that have been used to back-analyse the Millennium Drought Failures and extend

the previous geotechnical models using the protocols of Hubble et al. (2010). These

models have been applied to the Thiele Reserve failure, which is located adjacent to a

scour hole excavated in the river that has arisen from a nearby upstream lateral

constriction of the channel by intrusion of bedrock margin (See Fig 11A). This approach

provides general insight into how the various geomorphic and hydrologic factors identified

in Section 4.5 contributed to the failure events.

5 SLOPE STABILITY MODELLING

Geomechanical models for LMR banks have been generated and analysed in XSLOPE

(Balaam 1994) using Bishop's Slip Circle method (Bishop 1995). In the absence of detailed

information about the hydrogeological behaviour of the banks during the recession of pool

level the following simplifying assumption was made. The LMR was assumed to behave as

an effluent system where the ground water was recharged from the river requiring the peak

elevation of the water table to be located adjacent to the channel margin and creating a flat,

gently rising curve that peaked at the waterline of the river. The Coonambidgal muds are low

permeability materials and fluctuations in the ground water surface are unlikely to occur

rapidly (Jaksa et al. 2013).

Three types of bank geometry were considered for this analysis:

Original unmodified banks;

Banks upon which a fill (0.5m thick) had been placed that generated a surcharge

load (a common occurrence downstream of Mannum); and

Banks upon which a fill (0.5m thick) and an embankment (1.3m thick) had been

placed (immediately adjacent to the waterline of the river).

These three geometries have been investigated for three river pool conditions which span

and extend the range of pool levels that occurred during the Millennium Drought:

normal pool level (+0.75m AHD);

1m below normal pool level (–0.25m AHD); and

2m below normal pool level (–1.25m AHD).

These combinations replicate the variety of conditions that the major anthropogenic

modifications (i.e. river pool manipulation and embankment construction) have imposed on

the behavior of the banks. The models presented in this study are preliminary, simplified

representations of the site’s stratigraphy and materials and are based on the river profile at

Thiele Reserve. An inferred bedrock bench is included in the model as these have been

observed elsewhere when the river channel borders the bedrock valley walls. The sands of


the Monoman Formation which underlie the Coonambidgal Muds at a depth of 15 metres

below river level at this site are also included in the analyses.

The first modelling scenario considers the actual conditions at Thiele Reserve. The second

scenario considers the effect of a constructed embankment on the stability of the channel

margin at this site. The latter scenario was chosen since levees and roadways have been

built at many sites adjacent to the channel margin in the surrounding area and some of the

larger and more problematic failures have occurred where a levee or deep fill has been

constructed. An example of this is the bank instability and subsequent road closure at East

Front Rd, Mannum.

A five-layer geomechanical model has been used to represent banks and the adjacent

floodplain (Figure 16). It consists of a sandy surface fill and embankment (total 1.8m thick),

the Coonambidgal muds which is separated into a soft near-surface mud layer (undrained

strength 8kpa to 11.5kpa) approximately 5m thick, and a deeper and stronger mud layer

approximately 15m thick (undrained strength ~25 kPa) overlying the Monoman Sand and

then bedrock. Undrained analyses were selected after consultation with expert Geotechnical

Engineers (Airey 2014, Jaksa 2014) and are considered to be appropriate for modelling

these failures as the sediment is an impermeable mud (Lambe and Whitman 1968, Craig

2004).

Two key parameters were varied in order to understand their effect on the stability of the

banks. These parameter were:

a) Pool level, which was slowly lowered over a period of six-months to levels

between 1 to 2m below normal; and

b) The undrained strength of the near surface muds (low-medium-high values

indicated by CPTu results for the Coonambidgal mud unit).

The results of 27 model runs are summarized in Table 1. The generated Factor of Safety

(FoS) is a ratio that divides the restoring force acting on a slide mass by the disturbing force

acting on it. A FoS > 1 indicates stability, FoS <1 indicates instability, and a FoS=1 indicates

that the disturbing and restoring forces are critically balanced. However caution is normally

exercised in interpreting FoS values because natural materials vary laterally such that the

model parameters may not be completely representative of the actual situation.

Consequently, consideration of the change in FoS is often just as useful as the absolute FoS

value with increases indicating greater stability and decreases indicating lower stability (cf.

Hubble et al. 2010). Decreases of FoS values to near unity are commonly accepted to

indicate a high likelihood of failure when considering riverbank stability (cf Hubble, 2010).


Table 1. Factors of Safety1 calculated for Thiele Reserve for lowered pool levels and a range of

undrained shear strength values for the near surface mud layer. FoS>1 are shaded green, FoS=1 are

shaded blue and FoS<1 are shaded red.

XSLOPE MODEL

RUNS

Undrained shear

strength of near

surface muds

FoS at normal pool

level (+0.75m AHD)

FoS one metre

below normal pool

level (-0.25m AHD)

FoS two metres

below normal pool

level (-1.25m AHD)

Thiele Reserve -

natural profile

8.5 kPa 1.12 1.07 0.98

10 kPa 1.26 1.25 1.14

11.5 kPa 1.27 1.27 1.15

Thiele Reserve -

surface fill

8.5 kPa 1.06 0.99 0.88

10 KPa 1.23 1.13 0.99

11.5 kPa 1.25 1.14 1.03

Thiele Reserve

surface fill

& embankment

8.5 kPa 1.00 0.85 0.66

10 KPa 1.16 0.99 0.77

11.5 kPa 1.17 1.08 0.88

1 Factor of Safety = a ratio that divides the restoring force acting on a slide mass by the disturbing

force acting on it.

The model results demonstrated several characteristics that have been documented in the

geotechnical reports and studies produced during or just after the Millennium Drought (e.g.

SKM 2010b, Coffey 2013, Jaksa et al. 2013, Hubble et al. 2014). The models which most

accurately represent Thiele Reserve during the Millennium Drought are the natural profile

and surface-fill scenarios (Fig 16 – Rows A & B). These analyses produced critical circles

that are located in positions where actual failures occurred for situations similar to the

models (cf. SKM, 2010b; Coffey, 2013; Jaksa et al., 2013) and generate FoS consistent with

failure during lowered pool levels 1 (-0.25m AHD) and 2m (-1.25m AHD) below normal pool.

In particular the positioning of the shallow failure circles in the unmodified bank (Fig. 16 –

Row A) replicates very closely the shallow failure style and geometry described for

Whitesands (Jaksa et al. 2013) and the failures that occurred at Thiele Reserve (De Carli

and Hubble 2014, Hubble et al. 2014).


Key findings of this modelling are:

1) Lowered pool levels destabilise a riverbank with a natural profile and those

anthropogenically modified (i.e. banks with fill and/or an embankment). For

anthropogenically modified banks lowering the pool level by 1m (-0.25m AHD) produces

FoS indicative of failure. For unmodified natural banks failure would be expected at

lowered pool levels of 2m (–1.25m AHD).

2) The failure style is very sensitive to the undrained shear strength of the near surface

muds which form the banks. The lower end of the range of undrained shear-strengths

(8.5 kPa) indicated by CPTu tests favoured the formation of shallow, wide-diameter, slip

circles and produced a failure-surface geometry that approached a planar slide.

Stronger near-surface muds (11.5 kPa) favour the formation of more deeply located,

smaller-diameter, slip circles;

3) The location of the Monoman Sand layer strongly influenced the geometry and location

of deep-seated failure circles in model runs. For example, the occurrence of this layer at

shallower depths than depicted in the models shown here (e.g. above –15m) tended to

suppress the formation of deep-seated critical circles. The lowered location of Monoman

Sand in the sediment profile and slope stability models (e.g. –15m and below) enhanced

the formation of deep-seated critical circles and failure surfaces that emerged in the

floodplain (similar to the deep-seated circles shown in Fig. 16 – Row C). Geotechnical

investigations have shown that the Monoman Sand layer is located at shallow depths

(<15m) upstream of Mannum (Twidale et al., 1978). No examples of actual bank failure

with these characteristics was evident in the incident register upstream of Mannum;

4) The presence of a fill significantly reduced the FoS. The presence of such material

might be expected to compact and strengthen the underlying sediments (Foot and Ladd

1981). However, the fact that the larger and more problematic failures have occurred at

sites where fills have been placed (e.g. Thiele Reserve and Long Island Marina)

indicated that this effect must be overwhelmed by the head-loading of the potential slide;

5) The presence of fill and an embankment significantly reduced the FoS and tended to

induce rotational failures. Similarly it is expected that the presence of fill and an

embankment overwhelms the effects of compaction (Foot & Ladd 1981), and head-

loads the potential slide.

While the models presented above provide a general understanding of the processes and

contributing factors involved in the Millennium Drought failures, site specific modelling that

appropriately represents local conditions such as groundwater and anthropogenic

modifications is required on all failure prone reaches to enable appropriate management of

the problem in the future.


Figure 16: A selection of slope stability models representing a low undrained shear strength (8.5kpa) scenario for the near-surface Coonambidgal mud layer. Row (A) represents a natural profile bank and the critical slip circle for

scenario i. normal pool level (+0.75m AHD), scenario ii. One metre below normal pool level (-0.25m AHD), and scenario iii. Two metres below normal pool level (–1.25m AHD). Row (B) represents a bank with a surface fill (0.5m thick)

and the critical slip circle for scenario i. normal pool level (+0.75m AHD), scenario ii. One metre below normal pool level (–0.25m AHD), and scenario iii. Two metres below normal pool level (–1.25m AHD). Row (C) represents a bank

with a surface fill and an embankment (total 1.8m thick) and the critical slip circle for scenario i. normal pool level (+0.75m AHD), scenario ii. One metre below normal pool level (–0.25m AHD), and scenario iii. Two metres below normal

pool level (–1.25m AHD). The critical slip circles are coloured green (FoS>1), blue (FoS=1) and red (FoS<1) to indicate the Factor of Safety rating expressed in Table 1.


6 CONCLUSIONS

This report presents an integrated geomorphic, sedimentologic and geotechnical

investigation of the LMR’s riverbank failures during the Millennium Drought, providing a

regional interpretation of failure occurrence. This regional approach has resolved a critical

issue concerning the location of riverbank failures. Given the valley wide occurrence of the

Soft Clays and lowering of pool levels (and their impact as major contributing factors to

failure) it was not known what factor determined why the large-scale failures occurred at

particular sites. This report demonstrates that high risk Type 1 failures occur in close

proximity to deep scour holes in the river channel.

These deepened channel sections occur in the vicinity of bedrock margins protruding from

either side of the channel which cause the channel to narrow and increase the erosive

capacity of river flow, or around large protruding in-river bedrock blocks. Both situations

increase flow and shear velocities and induce hydraulic scour of the toe of the bank, over

steepening channel margin slopes which become unstable during low flow conditions.

Slope stability modelling in this report and previous geotechnical investigations demonstrate

that the presence of the near-surface Coonambidgal muds renders these particular over

steepened riverbanks vulnerable to collapse during periods of low flow. Those riverbanks

that have had levees and/or embankments constructed on them are also more likely to fail

than unmodified banks.

6.1 PROPOSED MECHANISMS OF FAILURE

The vast majority of riverbank failures documented in the Australian and international

scientific literature are caused by erosional scour (commonly induced by flood or high-flow

events); drawdown effects (commonly post-flood during the recession phase of the flood); or

a combination of these two processes. The information, modelling and interpretations

presented in this report demonstrate that the Murray’s Millennium Drought Failures are an

example of ‘normal’ but extraordinarily slow-motion riverbank failure. The very soft muds into

which the Murray has incised to form the channel margins presents a situation that favours

channel widening by mass failure processes. We have demonstrated that the dominant

processes which caused these bank failures were;

1) Erosion that produced channel margin slopes that were over steepened; and

2) Slow drawdown due to river pool lowering at the peak of the drought.

That is, the two common causes of riverbank failure are both presented in this case but they

acted as a slow and irregular set of events that occurred intermittently during a drought

rather than the expected, instant response of a river subjected to a large flood.


The geomorphic phenomena identified in the bathymetric analysis predispose the channel

margins of the LMR to rotational failures as well as shallow planar failures. These processes

are normally involved in the evolution of riverbank profiles through time and are expressed to

a greater or lesser extent on all rivers. The dominant mechanisms responsible for riverbank

failure are:

a) Erosion of the bank toe which removes lateral support and over-steepens the

bank slope (cf. Simon 1989, Hubble 2004, Darby et al. 2010, Hubble et al. 2010);

b) Drawdown effects (Morgenstern and Price 1965, Coffey 2013, Hubble et al.

2014); and

c) Buoyancy effects provided by the river pool. Here the submerged portion of the

bank supports and stabilises the emergent bank slope (cf. Hubble, 2004).

For the recent Type 1 failures, a combination of drought conditions and anthropogenic

interventions in the river channel allowed pool levels to drop below sea level over a 6-12

month period. This initiated a slow-motion drawdown problem due to the highly impermeable

character of the bank sediments and reduced the buoyancy support provided by the river

pool. This in turn increased the likelihood of mass failure of the LMR riverbanks but was

particularly evident in reaches where a deep thalweg was present. The absence of silt or

sand layers within lake muds of the Coonambidgal Formation suppressed drainage of the

muds as river levels fell and maintained relatively high water tables within the banks and

therefore high pore-pressure distributions within the river banks. This probably lowered the

effective stress within the bank soils and further promoted their failure. It is strongly

suspected Type 2 and 3 shallow planar failures are the consequence of earlier high flow

events, with toe scour occurring during the flood and post-flood drawdown acting as the

main trigger mechanisms.


7 RECOMMENDATIONS

Strategies to mitigate the risk of repeat Type 1 failures on the riverbanks of the LMR can be

drawn from this report, enabling management of this hazard during future low flow drought

conditions.

Recommendations arising from this study are:

1) Development of management protocols to prevent lowering of the river pool to the

levels experienced during the Millennium Drought. In particular the pool-level from

Blanchetown to Wellington should be maintained at or above the normal operating

level range +0.4m AHD to +0.75m AHD. Operations above the full supply level of

+0.75m AHD will not themselves increase the risk of river bank collapse, but it should

be noted that there is an increased risk of bank collapse due to rapid lowering of the

pool-level, particularly during periods of flood recession and post-flood drawdown;

2) Lowering pool-levels for the Lower Murray River below normal operating levels,

whether this be a case of rapid drawdown (hours or days) or over a prolonged period

(weeks), will replicate conditions extant during the Millennium Drought and increase

the likelihood and risk of bank failure. Therefore it follows that pool-levels should be

maintained above +0.4m AHD at all times, as pool levels below this triggered the

failures in 2009-2011; and

3) Acquisition of additional regional bathymetric mapping between Blanchetown and

Wellington so that complete coverage for the LMR is available. The identification and

assessment of river reaches with deepened channel scour holes and steepened

bank slopes enable the identification of failure prone zones and the categorisation of

low-medium-high risk riverbanks to focus hazard management strategies.


9 ACKNOWLEDGMENTS

This study was supported by a Goyder Institute for Water Research Program, Environmental

Water Grant (Project E.1.8) that is an initiative of the South Australian Government. The

many University of Sydney students and community volunteers, particularly David and Marie

Mitchell, who ensured the success of the field program on the RV Breakfree are thanked for

their enthusiasm and assistance. Also thanks to the many riverfront residents of the Lower

Murray towns who have allowed easy access to their properties and shared their wealth of

local knowledge with us. Richard Brown, Gareth Carpenter and Jai O’Toole of South

Australia Water are also thanked for their encouragement and support during the project.

The University of Adelaide’s Civil Engineering school and staff are thanked for their use of

the CPTu equipment, and Professor Mark Jaksa is thanked for his edits to this report. Nicole

and Doug Bergersen of Acoustic Imaging are thanked for their assistance with bathymetric

data post-processing. We thank reviewers Professor Ian Rutherfurd and Professor David

Petley for their helpful reviews and comments which have greatly improved this report. Danni

Oliver is also thanked for her contribution to this report.


10 REFERENCES

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ARUP 2008a Assessment of Soil Cracking & River Bank Slumping in the Lower Murray. Part 1 Report. The Department of Water Land and Biodiversity Conservation.

--- 2008b Assessment of Soil Cracking & River Bank Slumping in the Lower Murray. Part 2 Report. The Department of Water Land and Biodiversity Conservation.

BALAAM N. P. 1994 XSLOPE. Version 8 Users manual, Centre for Geotechnical Research. The University of Sydney.

BISHOP A. W. 1995 The use of slip circle in the stability of earth slopes, Geotechnique, vol. 5, pp. 7-17.

BRIZGA S. & FINLAYSON B. 1999 River management : the Australasian Experience. John Wiley Ltd, Chichester, UK.

COFFEY 2013 Review of Management Options for Four River Bank Collapse High Risk Sites. In MOON A. ed. Final Report. Coffey Geotechnics Pty Ltd.

CONSTANTINESCU G., KASHYAP S., TOKYAY T., RENNIE C. D. & TOWNSEND R. D. 2013 Hydrodynamic processes and sediment erosion mechanisms in an open channel bend of strong curvature with deformed bathymetry, Journal of Geophysical Research: Earth Surface, vol. 118, pp. 480-496.

CRAIG R. F. 2004 Craig's Soil Mechanics Spon Press, London.

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APPENDIX A

MULTIBEAM BATHYMETRIC DATA ACQUISITION

Multibeam bathymetric surveys were undertaken at failed sites between Blanchetown and

Wellington by SA Water in 2009, 2011 and 2014 using an R2sonic 2022 sonar with 300 kHz

frequency. Figure A1 shows the extent of bathymetric mapping sites numbered from

Blanchetown to Wellington. Details including the extent of the bathymetric map at all sites

are conveyed in Table A1. Figure A2 shows the SA Water survey boat used for data

acquisition.

All bathymetric soundings were collected and processed using Hypack Software. Position,

heading and velocity data were acquired using Trimble 332 High Precision GPS, CSI Vector

Pro and Reson SVP-40 respectively, and integrated with measurements for surface water

elevation (Trimble R7 RTK GPS) and heave, pitch and roll using a DMS 05 motion sensor.

Post-survey processing with Hypack Software included: (i) removal of multiples; (ii) sound

velocity corrections; (iii) water surface elevation corrections; (iv) integration with motion

sensor and navigation information; and (v) thinning soundings to a gridded surface using a

CUBE multibeam data processing algorithm. Riverbank bathymetry data were then gridded

using a 1 m2 cell size and exported to the 3-Dimensional imaging package Fledermaus V7.4

for examination of channel morphology.


Figure A1: Multibeam bathymetric extent between Blanchetown and Wellington, LMR South Australia from SA Water and University of Sydney surveys. Inset boxes A-E represent close-ups of individual surveys, which are numbered

starting from upstream to downstream. See Table A1 for survey details. Image Source: Google Earth 2014.


Table A1. Inventory of bathymetric surveys acquired by SA Water in 2009, 2011, 2014. The survey

reference number and section columns refer to the individual numbered surveys within the closeup

sections shown in Fig. A1.

Location Year

acquired

Extent in River km's (upstream to

downstream)

Total Survey Length

(km)

Survey Focus

(heading downsream)

Survey Reference

# Section

Ngaut Ngaut 2011 222.5 to 221.5 1 Left bank 1 A

Ngaut Ngaut 2009 222.3 to 222 0.3 Left bank 2 A

Wongulla 2009 217.5 to 217.2 0.3 Right bank 3 A

Sub Aruma 2009 212.7 to 212.2 0.5 Right bank 4 A

Walkers Flat 2011 210.5 to 210.1 0.4 Right bank 5 A

Walkers Flat 2009 209 to 206 3 Right bank 6 A

Scrubby Flat 2009 205 to 204.6 0.4 Left bank 7 A

Bowhill 2009 186.5 to 184.5 2 Left bank 8 B

Teal Flat 2009 179.3 to 178.2 1.1 Right bank 9 B

Piggy Flat 2009 177.1 to 173.8 3.3 Left/Right

bank 10 B

Lake Carlett 2009 172.9 to 170.7 2.2 Left bank 11 B

Rocky Point 2009 165.4 to 165.2 0.2 Right bank 12 B

East Front Rd Younghusband

2014 157.5 to 153.8 3.7 Full channel 13 C

East Front Rd Younghusband

2011 155.6 to 156.3 0.7 Left bank 14 C

Mannum 2009 151.2 to 149.2 2 Right bank 15 C

Caloote 2014 149.4 to 130.9 18.5 Full channel 16 C

Ponde 2009 146.1 to 145.7 0.4 Left bank 17 C

Caloote 2009 144.3 to 143.9 0.4 Right bank 18 C

Caloote 2011 144.3 143.9 0.4 Right bank 19 C

Zadows Landing

2009 142.7 142.4 0.3 Left bank 20 C

Zadows Landing

2009 141.5 to 141 0.5 Right bank 21 C

Wall Flat 2009 139.4 to 139.2 0.2 Right bank 22 C

Pompoota 2009 135.7 to 135.2 0.5 Left bank 23 C

Woodlane Reserve

2009 133.5 132.8 0.7 Left bank 24 C

Woodlane Reserve

2011 133.4 to 132.5 0.9 Right bank 25 C


Location Year

acquired

Extent in River km's (upstream to

downstream)

Total Survey Length

(km)

Survey Focus

(heading downsream)

Survey Reference

# Section

Mypolonga 2009 126 to 123.3 2.7 Left bank 26 D

Murrawong 2011 123.6 to 123.3 0.3 Left bank 27 D

Thalia 2009 121.2 to 119.5 1.7 Right bank 28 D

Avoca Dell 2009 118 to 117.2 0.8 Full channel 29 D

Murray Bridge 2014 118 to 99.2 18.8 Full channel 30 D

Murray Bridge 2009 114.4 to 110 4.4 Full channel 31 D

River Front Rd 2011 111.8 to 110.5 1.3 Left bank 32 D

LIM 2011 110.1 to 109.3 0.8 Right bank 33 D

LIM 2009 110 to 109.6 0.4 Right bank 34 D

MurrayBridge South

2009 109.6 to 108.9 0.7 Right bank 35 D

Swanport Bridge

2009 108.5 to 107.9 0.6 Left bank 36 D

Sailing Club Swanport

2009 107.4 to 107 0.4 Right bank 37 D

Bells Reserve 2009 105.2 to 104.6 0.6 Left bank 38 D

Bells Reserve 2011 105 to 104.6 0.4 Left bank 39 D

Riverglen 2009 102.5 to 100.8 1.7 Right bank 40 D

Riverglen 2011 102.1 to 101.4 0.7 Right bank 41 D

Tailem Bend 2009 89.8 to 84 5.8 Left bank 42 E

Dixon Reserve 2011 87.5 to 87 0.5 Left bank 43 E

Placid Estates 2009 84 to 79.4 4.6 Left bank 44 E

Freds Landing 2011 84 to 81.5 2.5 Full channel 45 E

Wellington 2009 76.2 to 74.8 1.4 Right bank 46 E


Figure A2: SA Water Survey boat with bathymetric setup.


APPENDIX B

IN-RIVER SEDIMENT SAMPLING

In-river sediment cores and piezocone cone penetration tests (CPTus) were acquired during

the 2013 and 2014 fieldwork programs.

In-river sediment cores were collected at a total of nine sites of riverbank instability through

push coring techniques alongside a moored RV Breakfree, spaced approximately 2m, 12m

and 20m distance from the edge of the riverbank. See Figure B1 and Table B1 for coring

location and recovery details. See Figure B2 for coring method.

In-river CPTus were undertaken at all coring sites and at additional locations at

approximately 5 to 10 km intervals between Wongulla and Wellington. A CPTu includes pore

water pressure (u2) measurements, as well as cone tip resistance (qc) and sleeve friction (fs),

allowing for a more reliable assessment of soil type. The University of Adelaide CPTu

instrument was deployed from the forward deck of the RV Breakfree using a purpose built

hydraulic drive frame (See Figure B3). See Figure B1 and Table B2 for CPTu acquisition,

location and depth of penetration details

.


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. See Appendix Table B1 for a

detailed list of locations and depths. Core and CPTu labels displayed here correlate to the those discussed

in Section 3.2, Figures 3 & 4. Image Source: Google Earth.


Table B1. Inventory of sediment cores and CPTu (cone penetrometers tests) acquired by Sydney

University on 2013 and 2014 field campaigns.

Date Location Type Easting Northing

Total Recovery/

Penetration (m)

5/02/2014 Avoca Dell CPTu Profile

345676 6115810 13.47

4/02/2014 Bells Reserve Monteith

CPTu Profile

346634 6106805 16.31

7/02/2014 East Front Rd CPTu Profile

349749 6137775 8.65

11/02/2014 Long Island Marina

CPTu Profile

345564 6110662 17.27

11/02/2014 Long Island Marina

CPTu Profile

345364 6110811 20.43

10/02/2014 Murray view Estates

CPTu Profile

357350 6093006 4.17

10/02/2014 Murray view Estates

CPTu Profile

357368 6093010 3.41

6/02/2014 Neeta Irrigation Area

CPTu Profile

341939 6129444 13.71

4/02/2014 Riverglen Marina

CPTu Profile

348576 6104196 1.56

11/02/2014 Riverglen Marina

CPTu Profile

348214 6104380 21.8

5/02/2014 Sturt Reserve, Murray Bridge

CPTu Profile

343532 6112394 2

5/02/2014 Thiele Reserve CPTu Profile

343129 6113921 16.91

6/02/2014 Wall Flat CPTu Profile

346193 6130131 15.85

10/02/2014 Wellington East Marina

CPTu Profile

353233 6089803 27.29

9/02/2014 Wellington CPTu Profile

353178 6089048 4.64

11/02/2014 Westbrook CPTu Profile

354771 6101444 1.62

6/02/2014 Woodlane Reserve

CPTu Profile

348350 6125995 11.37

7/02/2014 Younghusband CPTu Profile

363437 6139565 12.44

25/03/2014 BowHill CPTu Profile

372520 6137668 14.75

22/03/2014 Long Island CPTu Profile

344665 6111528 15

21/02/2014 Long Island Reserve

CPTu Profile

344634 6111322 4

25/03/2014 Purnong CPTu Profile

375342 6140438 6.32

24/03/2014 Scrubby Flat CPTu Profile

367778 6149959 9.12

23/03/2014 Walkers Flat CPTu Profile

368198 6153555 9.31

24/03/2014 Wongulla CPTu Profile

369210 6158135 7.55


360167 6140549 13.02


Date Location Type Easting Northing

Total Recovery/

Penetration (m)


358836 6141823 10.23

27/03/2014 Younghusband Gravity Core

362210 6139769 1


362213 6139788 2.7


362209 6139846 1

24/03/2014 Walkers Flat Push Core

368208 6153563 ~1

20/03/2014 Wellington East Marina

Push Core

353225 6089830 2.42

24/03/2014 Wongulla Push Core

369212 6158133 ~0.75

10/05/2013 East Front Rd Push Core

349758 6137808 2.3


349759 6137797 4.45


349383 6137634 1

1/05/2013 Riverglen Push Core

348283 6104393 2.28


348284 6104397 2.06


348284 6104400 1.79


348276 6104390 0.98

7/05/2013 Thiele Reserve Push Core

343122 6113931 0.8


343117 6113934 1.61


343108 6113941 2.69


343122 6113948 2.37


343129 6113943 1.31


343077 6113904 2.02


343089 6113898 0.72

3/05/2013 White Sands Push Core

346772 6104945 2.71

3/05/2013 White Sands Push Core

346777 6104950 1.92

6/05/2013 White Sands Failure

Push Core

347609 6104588 1.81


Push Core

347610 6104596 0.37


Push Core

347594 6104590 0.71


Push Core

348366 6126013 2.13


Push Core

348366 6126014 1.1


Push Core

348369 6126018 1.83


Push Core

348381 6126011 1.1


Figure B2: Volunteers percussion coring off side of RV BreakFree, May 2013.

Figure B3: Volunteers deploying CPTu gear from custom built frame installed on the front of the RV

Breakfree, February 2014.


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 Millennium Drought Riverbank Failures

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