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Transactions of the Royal Society of South Australia
ISSN: 0372-1426 (Print) 2204-0293 (Online) Journal homepage: http://www.tandfonline.com/loi/trss20
Bank instability along a weir pool of the River
Murray
Martin Thoms
To cite this article: Martin Thoms (2017) Bank instability along a weir pool of the River
Murray, Transactions of the Royal Society of South Australia, 141:2, 151-168, DOI:
10.1080/03721426.2017.1374158
To link to this article: http://dx.doi.org/10.1080/03721426.2017.1374158
Published online: 08 Sep 2017.
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Date: 28 October 2017, At: 20:19
TRANSACTIONS OF THE ROYAL SOCIETY OF SOUTH AUSTRALIA, 2017
VOL. 141, NO. 2, 151–168
https://doi.org/10.1080/03721426.2017.1374158
Bank instability along a weir pool of the River Murray
Martin Thoms
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Riverine Landscapes Research Laboratory, University of New England, Armidale, Australia
ABSTRACT
ARTICLE HISTORY
River banks are an important transition zone between aquatic and
terrestrial environments. This ecotone is highly vulnerable to natural and anthropogenic disturbances. Bank erosion is a common
occurrence along the River Murray. Morphological features reflecting river bank instability form a near-continuous pattern along
large tracts of the river. This study investigates the character and
extent of bank instability along the Torrumbarry Weir Pool. Over
90% of the bank length in this weir pool was assessed as being
actively eroding. Notch development was the dominant instability
mechanism and it promoted other forms of bank instability.
Erosion notches result from stable water levels and inherent soil
instability of the river banks along the weir pool. The character
and extent of river bank erosion recorded along this section of the
River Murray is an artefact of flow regulation and not a reflection
of the natural occurrence of bank erosion in river networks. Water
level management in the upstream weir pool has significant implications for the management of river banks along regulated
reaches of the River Murray.
Received 14 June 2017
Accepted 29 August 2017
KEYWORDS
Bank erosion; notch
development; flow
regulation; ecotone; littoral
zone
Introduction
River banks are an important physical component of the riverine landscape and the
riparian zone in particular. Their geomorphic complexity plays a fundamental role in
various physical and biological processes. In large lowland river systems, the presence of
morphological features such as benches and bars located at multiple levels within the
bankfull channel contributes to channel roughness and in-channel flow conveyance
(Knight & Brown, 2001). The role of river bank complexity in creating differential
physical patterns, trapping organic matter and providing habitats for lower order
aquatic organisms in large lowland rivers has been demonstrated by Sheldon and
Thoms (2006) and Thoms and Olley (2004). River bank complexity plays a fundamental
role in the retention of organic matter, a key component of river ecosystem functioning
as suggested in the Riverine Productivity Model of Thorp and Delong (2002). Rivers
with high physical complexity retain more organic material than those with comparatively low complexity (Prochazka, Stewart, & Davies, 1991). Reductions in geomorphic
complexity could have repercussions for the overall health of river ecosystems. This is
especially significant for large lowland rivers, such as the River Murray, where river
CONTACT Martin Thoms
Martin.Thoms@une.edu.au
New England, Armidale, Australia
© 2017 Royal Society of South Australia
Riverine Landscapes Research Laboratory, University of
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M. THOMS
banks are the main component of river channel complexity that are subject to increasing anthropogenic pressures (cf. Thoms et al., 2000).
Bank erosion is a key geomorphological process of meander formation, lateral
channel migration and movement of sediment throughout a catchment. Three primary
mechanisms are involved in river bank erosion: subaerial processes, fluvial entrainment
and mass failure (Couper & Maddock, 2001). The dominance of each mechanism can
vary systematically throughout a river network creating distinct bank erosion process
zones (cf. the “Process Domain Model” of Lawler, 1992, 1995). In lowland river zones,
the Process Domain Model predicts that fluvial entrainment and attrition processes are
the dominant mechanisms promoting bank instability. Thus, regardless of the lateral
mobility of lowland rivers, bank erosion will be prevalent on the outer bend of meanders (Lawler, 1992, 1995; Thorne, 1992).
It has been hypothesised that bank erosion processes are significantly influenced by
human activities, a hypothesis that has yet to be fully tested. However, in the Upper
Nepean River, New South Wales, which is regulated by 11 weirs, Hubble (2004) showed
that the presence of weirs had both positive and negative effects on bank stability. Weir
operations promoted bank failure through erosion at the toe of the river banks at many
locations within the weir pools. However, proposed weir removal was modelled to
potentially increase the magnitude of bank failure. Despite the importance of bank
erosion on the riverine landscape, the relative efficacy of bank erosion processes and
anthropogenic influences on these are still poorly understood, especially for large
Australian lowland rivers (Saynor & Erskine, 2006).
Increased river channel instability is a feature of many rivers subject to flow regulation
(Petts, 1984). The flow and sediment regimes of the River Murray are modified by two
headwater dams and a series of low-level weirs, which has a significant impact on its
channel stability (cf. Thoms & Walker, 1989, 1992, 1993). In particular, the presence of the
low-level weirs along the River Murray has seen changes in water level variations (Walker,
Thoms, & Sheldon, 1992). In some respects, the studies by Keith Walker and his colleagues
have shown that water level variations, rather than discharges, are more appropriate
currency for studies of the environmental effects of weirs. Routine weir operations maintain
close control over upper pool levels but cause levels below each weir to fluctuate; rapid
water level changes in excess of 2 m have been recorded (Maheswahri et al., 1995). This
change in the water level regime of the River Murray, imposed by the weirs, has implications for bank stability. Preliminary studies by Thoms and Walker (1989) recorded rates of
bank erosion of up to 2 m a year, downstream of two weirs, because of the rapid draw down
in water levels. As a consequence, there have been significant adjustments in the channel
profile and cross-sectional dimensions in these reaches immediately downstream of the
weirs (Thoms & Walker, 1993). Repeated surveys of the river channel 0.5 km below 10
weirs in the lower River Murray recorded increases in bankfull cross-sectional areas by
285% in the years following weir construction (Thoms & Walker, 1992). Despite these
noticeable effects of weirs on river bank stability and the implications for river management
(cf. Thoms et al., 2000), there has been limited research on this aspect of the impact of flow
regulation along the River Murray.
In this manuscript, bank-instability mechanisms within a weir pool of the lowland
River Murray are investigated. It is hypothesised that the character and extent of bank
instability along the weir pool will reflect the impact of flow regulation, and not the
TRANSACTIONS OF THE ROYAL SOCIETY OF SOUTH AUSTRALIA
153
general processes associated with the lateral mobility of this lowland river channel. The
research presented here complements that which has focused on general river channel
adjustments between a series of weir pools on the lower River Murray by Thoms and
Walker (1989, 1992, 1993).
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Study area
This study focused on a section of the River Murray regulated by Torrumbarry Weir, an
area of extensive ongoing interdisciplinary research. The River Murray drains 420,000 km2
of the western slopes of the South Eastern highlands of Australia. Four river zones have
been identified along the River Murray (Figure 1(a)); these are the Headwater Tract,
Riverine Tract, Mallee Tract and Lower Murray Tract (Thoms et al., 2000), each with a
unique physical, hydrological and ecological character. The Torrumbarry Weir Pool is
located in the Riverine Tract. The River Murray within this zone has a highly sinuous
channel (sinuosities = 2.12–3.21; where sinuosity is measured as channel length over valley
length), very low river-bed slopes (average gradient = 6.4 cm km−1) thus low stream power
and contains river-bed sediments composed of medium-size sands and highly cohesive
river bank soils. Extensive floodplains are a feature of the River Murray in the Riverine
Tract, with floodplain widths exceeding 30 km. These floodplains contain an array of
physical features including contemporary and relict meanders, distributary and anabranch
channels along with numerous billabongs, deflation basins and shallow lakes. Thus, this
section of the River Murray has a relatively high geomorphic diversity. Several tributaries
enter immediately upstream of the Torrumbarry Weir Pool, including the Goulburn River,
which enters the River Murray several kilometres upstream of Echuca, and the Campaspe
River, which joins the River Murray further upstream.
Approximately 25,000 years ago, tectonic activity in the southern Murray–Darling
Basin resulted in a block uplift creating what is now known as the Cadell Fault (Brown &
Stephenson, 1991). The fault has a north/south orientation and is most notable in the
Echuca to Deniliquin region. The presence of the Cadell Fault resulted in the formation
of two large-scale alluvial fan complexes – the Barmah and Gunbower Fans (Figure 1(b)).
The apex of the Gunbower Fan is located between the Torrumbarry Weir and Echuca.
This reach of the River Murray between Echuca and Torrumbarry has a confined floodplain that is often less than 300 m in width but becomes unconfined as it flows further
downstream of the Gunbower Fan (Rutherfurd, 1991). In the confined sections of the
Murray, the river channels are typically wider and shallower, have sandier bank sediments
and follow a more sinuous path compared to the unconfined sections of the river. In
general, the planform of the River Murray has been relatively stable over the last 200 years
(Thoms et al., 2000). This lateral stability is considered to result from low stream powers
and the abundance of silts and clays in the channel perimeter (Rutherfurd, 1991).
River red gums (Eucalyptus camaldulensis) are the dominant riparian vegetation of
the River Murray in the Riverine Tract. These riparian trees are common along most
Australian inland water ways and occur in stands along the River Murray. The understorey vegetation of the riparian zone is generally dominated by herbaceous species
such as grasses, daisies, sedges and peas. Large stands of Phragmities australis, common
before European settlement, now occur in isolated stands along the River Murray
(Walker, Boulton, Thoms, & Sheldon, 1994).
M. THOMS
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154
Figure 1. The Torrumbarry Weir Pool study reach. (a) The location of the weir pool and the four
geomorphological tracts of the River Murray and (b) the Riverine Tract with the location of Barmah
and Gunbower fans.
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155
Flows in the River Murray within the Torrumbarry region are a combination of
unregulated flows, including flows from the upper River Murray through spills from
Hume Dam, the Kiewa, Ovens and Goulburn Rivers, as well as regulated releases from
Hume Dam. There are also regulated releases from Eildon Reservoir on the Goulburn
River. Unregulated flows are generally prominent during the winter – spring months
(June–October) with regulated flows occurring between September and April, depending on downstream water requirements. Flows in the River Murray have become
increasingly regulated since the 1920s. Hume Dam became operational in 1934, and
in 1961, the capacity of the reservoir was doubled to its present capacity of 3.07 million
ML by the addition of an earthen weir. Other important structures that regulate flows
along the River Murray are the Yarrawonga Weir, impounding Lake Mulwala (0.116
million ML) and Dartmouth Dam, a large headwater reservoir on the Mitta Mitta River
that impounds 4.0 million ML. Despite the presence of these dams and weirs, flows
along the River Murray are variable (Maheshwari et al., 1995). At Tocumwal, upstream
of Echuca, the coefficient of variation of long-term annual flows is 125%, compared
with an average 33% for similar-sized rivers elsewhere in the world (Finlayson &
McMahon, 1988). At Barham, 360 river-km downstream of Tocumwal, annual flows
vary from 25% to 214% of the mean, 150 m3 s−1 (69 years of record), while at
Blanchetown, 274 river-km from the Murray Mouth in South Australia, flows have
varied from 6% to 491% of the average daily flow of 318.2 m3 s−1 (average of 88 years).
In the Torrumbarry region, annual flows exceed channel capacity approximately
once in every 10 years, on average. Major floods (>2000 m3 s−1) occurred along the
River Murray in 1870, 1917, 1931, 1956 and 1974–75. Long-term patterns of runoff
have not varied markedly, although significant changes have occurred in other parts of
the Murray–Darling Basin (Riley 1988). In general, flow management along the River
Murray has resulted in reductions in flow variability as well as overall decreases in the
magnitude and frequency of certain flood events. Originally, the construction of weirs
on the River Murray (including Torrumbarry Weir) was designed to promote riverboat
navigation but has come to assume important roles as flow regulators. They are
operated to maintain a steady upper pool level (surface water level variations range
between 50 and 200 mm), except when flows exceed storage capacity. Stable water levels
are thought to have been associated with increased river bank instability, thereby
causing subsequent reductions in channel complexity and loss of channel habitat due
to increased erosion and sedimentation (Thoms et al., 2000).
Torrumbarry Weir is located ~83 km downstream of Echuca on the River Murray.
Completed in 1924, it was built for the River Murray Commission by the State Rivers
and Water Supply Commission of Victoria. This weir operated until 1992 when damage
to its foundations required the construction of a new weir. The current weir was
commissioned in 1997 and is 73.5 m wide, 13 m high, incorporates six radial gates
and has a capacity of 35,000 ML. At full supply, water levels are maintained at a height
of 86.05 m Australian Height Datum, primarily for irrigation purposes. The installation
of the new radial gates for water level management has reduced the range of water level
movements in the upstream pool. The weir pool is also used for recreation, including
tourism activities such as paddle steamers and house boats, fishing, water skiing and
wakeboarding. The current weir also has a fish ladder to allow migratory fish to pass the
structure.
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M. THOMS
Methods
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Physical survey
The physical character of river banks along an 85-km reach immediately upstream of the
Torrumbarry Weir (known as the Torrumbarry Weir Pool) was determined in 1 km
intervals during a period of low water levels. At five equidistant locations within each 1km interval, the height of the exposed river bank was recorded with a Vertex Laser
VL400. All bank heights were measured at bankfull capacity. Between one and four soil
samples were collected from each of these locations at sites near the low water level, and
at mid-bank and upper bank sites, with the number of samples collected being dependant
on the height of the river bank. Each soil sample was classified as per the International
Soil Standards protocol and its mechanical stability determined via the Emerson
Aggregate Test, which assesses aggregate soil stability (cf. Emerson, 1964). In addition,
the particle-size composition of each soil sample was measured by determining the clay,
silt and sand fractions using an ASTM 152H soil hydrometer (ASTM, 1985).
A survey of the extent of each bank-instability mechanism identified (Table 1) within
the 1-km interval was also undertaken along the Torrumbarry Weir Pool. The linear
extent of each bank-instability mechanism was surveyed on both the left and right banks
with a theodolite. In addition, the severity of notch development was recorded according
to the classes of “none” (no notch evident), “low” (0–20 cm), “moderate” (20-40 cm) and
“severe” (>40 cm) as recommended in the stream reconnaissance surveys of Thorne
(1998) and Parsons, Thoms and Norris (2004).
Vegetation survey
The abundance of emergence aquatic and bank-side vegetation was classified into five
groups. These being dominant, >70% of bank covered by vegetation; abundant, 50–70% of
bank covered by vegetation; frequent, 20–50% of bank covered by vegetation; occasional,
0–20% of bank covered by vegetation and absent, 0% of bank covered by vegetation.
Data analysis
Data collected in the field were digitised into ARC GIS and the corresponding bank
heights and linear lengths of the various instability mechanisms and vegetation classes
on both river banks were calculated for the individual 1-km intervals of the
Torrumbarry Weir Pool. These data were used to assess the extent of bank exposure
and bank instability as well as the diversity of instability mechanisms along the
Torrumbarry Weir Pool. The diversity of bank-instability mechanisms was calculated
via the Margalef Richness Index (DMg) as recommended by Magurran (2004). A
breakpoint analysis was undertaken on both bank height and the extent of bank erosion
in each 1-km interval to determine if systematic changes in bank instability occurred
along the weir pool. Significant zero crossings (SiZer) models were applied to both
datasets for each 1-km interval. For these SiZer models, a non-parametric, locally
weighted polynomial smoother was applied to each interval (predictor) and the primary
and secondary derivatives of the smoothed curves were used to identify possible
thresholds, and thus the locations in bank exposure and instability along the weir
Description
Mass failure by slumping of bank materials occurs as the result of reduced bank
material strength induced by saturation. Banks are most vulnerable to slumping
failures following draw down of water levels in the river channel. Draw down
can leave the bank saturated but without the support provided by the weight
of water against the bank
The direct removal of material from the face of a river bank by the action of
flowing water. Rates of attrition can be influenced by the coherence of the
bank material (in water), velocity of flow adjacent to the bank, as well as the
binding, protection and flow retarding effects of vegetation. This is a common
form of bank erosion in lowland rivers, like the River Murray
Notch development is material which is preferentially entrained at the water
surface. Notch development is usually related to wave action. This relationship
may be the direct result of fluctuations in water velocity against the bank or it
may result from weakening of bank material by repeated wetting and drying.
Bank erosion notches may form within the bank profile and lead to subsequent
block failure and collapse
Instability mechanism
Mass failure by bank
slumping
Attrition
Notch development
Table 1. Common river bank-instability mechanisms occurring along the River Murray.
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Schematic
(Continued )
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Rill erosion
Rill erosion is the result of rainfall or surface flow running down the face of river
banks and cutting small gullies or rills into the bank surface. Generally, rill
erosion occurs more frequently on river banks that are devoid of vegetation
Instability mechanism
Description
Block failure
Banks are also eroded by undermining and block collapse. Undermining may
result from the preferential removal of a weak layer of materials in the bank,
deepening of the channel causing basal undercutting or diverging secondary
currents in a bend attacking and removing the bank toe
Table 1. (Continued).
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Schematic
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M. THOMS
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pool. SiZer defines a threshold as a point where the first derivative changes significantly
(Clements, Vieira, & Sonderegger, 2010). Four bandwidths (h = 2, 4, 8 and 15 km),
equivalent to levels of smoothing, were applied to the model as recommended by
Sonderegger, Wang, Clements and Noon (2009). Advantages of SiZer include its ability
to handle multiple thresholds in one dataset and the examination of potential thresholds at different of levels resolution. Delong and Thoms (2016) applied the SiZer model
to a long-term isotope dataset for five rivers in the Mississippi River Basin subjected to
various levels of flow regulation and land use. SiZer analysis identified a significant
change in food web character in all rivers in the 5 years following flow regulation.
The character of bank instability along the Torrumbarry Weir Pool was further
examined through a range of multivariate statistical analyses. Initially, a similarity
matrix, using the Gower similarity coefficient, was calculated using the number and
linear extent of instability mechanisms identified in each 1-km unit. Differences in bank
instability between sub-reach differences identified by the SiZer analysis were undertaken using the analysis of similarity (ANOSIM; Clarke, 1993). This was followed by a
Semi-Strong-Hybrid Multidimensional Scaling ordination (cf. Belbin, 1993) in order to
represent the similarity matrix graphically. A stress level <0.2 indicated that this
ordination solution was not random. Relationships between the location of the 1-km
intervals in the ordination and the different bank-instability mechanisms were determined using Principal Axis Correlation (PCC; Belbin, 1993). For this, only those
variables with an R2 greater than 0.8 were considered significant.
Results
Physical bank character
River bank heights ranged from 0.1 to 10.2 m above the low weir pool water level
(Figure 2). There is a general trend for bank heights to increase with distance upstream
from Torrumbarry Weir; this is consistent for both banks. At approximately 37 river-
Figure 2. River bank heights along the Torrumbarry Weir Pool. Average heights above lower water
weir pool levels are given along with a moving average (as determined by a moving window
analysis, where the window gap = 2 km).
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M. THOMS
km upstream of Torrumbarry Weir, bank heights increased significantly, from an
average of 1.82 m above the water level to 4.96 m. A SiZer model for bank heights
along the weir pool confirmed that this location represents a significant change in the
character of bank heights along the weir pool. Thus, based on bank heights, the weir
pool can be separated into two zones, a lower zone from 0 to 37 km (mean bank heights
range: 0.1–3.8 m) and an upper zone from 37.1 to 83 km (mean bank heights range:
3.6–6.2 m) (Figure 2).
River bank sediments along the weir pool can be classed as either Silty Clay Loam
(41%) or Silty Loam (25%) (Figure 3), based on the international soil classification
(ASTM, 1985). The character of the river bank sediments differs between the upper
and lower weir pool zones. Riverbank sediments in the lower weir pool zone are
predominantly Silty Clay Loams (64%) and Silty Loams (25%) whilst in the upper
weir pool zone, they are composed of Silty Loam (24%), Silty Clay Loam (22%) and
Loam (20%) (Figure 3). In addition, soil character varied vertically from the water
level to the top of the river bank and this vertical variation differed between the lower
and upper weir pool zones (Figure 3). The character of the river bank soil in the lower
weir pool zone displayed relatively minor vertical variations compared to those in the
upper weir pool zone. Soils in the lower weir zone are predominantly Silty Clay
Loams or Silty Loams and are present mainly at mid-river bank levels and at the bank
top. In upper weir pool zone, soils vary from predominantly Loams, closer to the
water level to Loamy Sands in the mid-river bank levels, and then Sandy Loams at the
top of the river bank. These variations in river bank sediments reflect the coarser soils
of the Gunbower Fan sequence that influence the upper weir pool zone, while finer
soils of the lower weir pool zone are part of the general Riverine Tract floodplain
sediment complex.
The stability of river bank sediments, as assessed by the Emerson aggregate stability
test, also varied along the weir pool and vertically on the river bank. Stable sediment
aggregates do not slake, disperse or swell, whereas unstable soil aggregates do slake and
disperse when added to water. In the Torrumbarry Weir Pool, 56% of the soil samples
can be classed as being unstable or have the potential to become unstable when
saturated at field capacity (Figure 4). Only 19% of soils tested were classed as being
stable. In the lower weir pool zone, soils are either stable or moderately stable in
comparison to those in the upper weir pool zone, which are either unstable or unstable
when wet. Vertically, soil stability on the lower river bank sections is generally moderately stable and this grades to unstable soils further up the river bank. This vertical
transition in soil stability occurs in both weir pool zones. Thus, the river bank soils in
the Torrumbarry Weir Pool are inherently unstable although the degree of instability
varies along the weir pool.
River bank vegetation
The majority of the river banks along the Torrumbarry Weir Pool have no vegetation.
Only 40% of the river bank recorded a presence of vegetation. In addition, 16% of the 1km intervals recorded an abundance class of dominant or abundant reeds and sedges or
grasses. Therefore, approximately 60% of the banks are bare of reeds, sedges or grasses.
The general depauperate nature of the bank vegetation suggests that it would not have
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TRANSACTIONS OF THE ROYAL SOCIETY OF SOUTH AUSTRALIA
Figure 3. The character of the river bank sediment along the Torrumbarry Weir Pool. (a) The entire
reach, (b) the lower and upper Weir Pool sub-reaches and (c) examples of the longitudinal and
vertical variations in sediment character.
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M. THOMS
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Figure 4. Soil stability of the river bank sediments in the Torrumbarry Weir Pool.
an influence on the character or extent of river bank stability along the Torrumbarry
Weir Pool (Abernethy & Rutherfurd, 1998).
River bank stability
Instability is a feature of the river banks along the Torrumbarry Weir Pool with 90% of
the total bank length displaying some form of erosion (Figure 5(a)). The spatial distribution of bank-instability mechanisms was not uniform along the weir pool. Bank instability
was relatively more continuous along both banks with increasing distance upstream from
Figure 5. River bank instability along the Torrumbarry Weir Pool. (a) Different forms of river bank
instability and (b) the richness of instability mechanisms (as determined via the Margalef Richness Index).
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163
the weir (Figure 5(a)). Notch development is the dominant instability mechanism along
the Torrumbarry Weir Pool. It accounted for 57% of bank instability or 102 km of the
river bank in the weir pool (Figure 5(a)). Moderate notch development was the most
common form of notch development in the weir pool affecting 28% of the bank, followed
by low notch development (26%) and then severe notching (17%). Other bank-instability
mechanisms observed along the weir pool included block failure, cantilever failure,
rotational slipping and bank slump (Figure 5(a)) accounting for a total of 29% of
instability mechanisms influencing 31.3 km of river bank. General attrition accounted
for 4% or 7 km of the river bank, whilst the instability mechanisms of rill and gully
erosion accounted for 10% (17.5 km of river bank).
Multiple instability mechanisms were present on the river banks along the
Torrumbarry Weir Pool. Over 90% of the river banks displayed more than one
instability mechanism (Figure 5(a)). The composition of bank-instability mechanisms
changed along the weir pool. A SiZer model for bank instability detected a change of
37 km upstream of the weir (Figure 5(a)) – the identified boundary of the lower and
upper weir pool zones. There is a notable increase in the number of instability
mechanisms within each 1-km bank unit, in the upper weir pool zone compared to
that in the lower weir pool zone. The increase in the occurrence of different bankinstability mechanisms is associated with an increase in the mechanisms of mass failure,
attrition and rill gully erosion. Although the occurrence of notch development was
relatively constant along the weir pool, notching in the lower weir pool zone was
observed to be less severe. In the lower weir pool zone, notch development was
dominated by low and moderate notching compared to the moderate-to-severe notching of the upper weir pool zone (Figure 5(a)). The change in the composition of bankinstability mechanisms along the weir pool is reinforced by the spatial pattern of the
richness or diversity of the different bank-instability mechanisms (Figure 5(b)).
Margalef Richness Index (DMg) values, which is a measure of diversity and calculated
for each 1-km bank interval, range from 0.14 (10 km upstream of the weir) to 1.02
(71 km upstream of the weir). Overall, the median Margalef Richness value was 80%
higher in the upper weir pool zone compared to the lower weir pool zone; i.e., there is a
greater diversity of the types of bank erosion mechanisms in the upper weir pool.
Bank instability is significantly different between the upper and lower zones of the
Torrumbarry Weir Pool. Results of a pair-wise ANOSIM indicate statistical differences
between the lower and upper sections of Torrumbarry Weir Pool, in terms of the extent
and composition of bank-instability mechanisms (Global R = 0.691, p < 0.001). A PCC
analysis reveals that three bank-instability mechanisms, notch development, bank failure
and rill/gully erosion, had R2 > 0.60 and their positions in ordination space were strongly
associated with the upper weir pool zone. Thus, the increased incidence of notch
development, bank failure and rill/gully erosion in the upper weir pool was responsible
for differences in bank-instability between the upper and lower weir pool zones.
River bank-instability mechanisms also varied between different morphological subreaches of the weir pool. In highly meandering reaches, bank erosion and in particular
general bank scour or attrition are more prevalent on the outer bends (Figure 6). The
outer banks of meander bends in the weir pool have a higher proportion of their length
eroding compared to inner banks and straight sections of river channel, all of the outer
banks displayed instability compared to 52% of the inner meander bends and 63% on
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M. THOMS
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Figure 6. The composition of river-instability mechanisms between the inner, outer and straight
river sections.
straight river sections. Overall, notch development is the predominant instability
mechanism in all three sections of river bank accounting for 62%, 47% and 65% of
the various mechanisms observed along the outer, inner and straights sections of river
bank, respectively.
Discussion
The “Process Domain Model” (cf. Lawler, 1992, 1995) predicts the dominant processes
promoting bank instability within a river network. Accordingly, the upper reaches of
the river, where stream power is lower, subaerial processes tend to dominate. If the rate
of erosion increases downstream and the contribution of subaerial erosion remains
constant, the relative significance of these processes decreases. Maximum stream
powers commonly occur in the mid-reaches of a river network resulting in the
dominance of fluvial erosion or attrition processes. With increasing channel depth
downstream, Lawler (1992) suggests that there will be a point where the maximum
(“critical”) bank height for stability with respect to mass failure is exceeded. Mass failure
processes therefore tend to dominate bank erosion in the lower reaches of river systems.
Field evidence of the Process Domain Model along Australian river systems is provided
by Abernethy and Rutherfurd (1998) in the Latrobe River, Victoria and Saynor and
Erskine (2006) in Ngarradj River catchment, Northern Territory.
The character and extent of bank-instability mechanisms recorded in the Torrumbarry
Weir Pool does not support the Process Domain Model of Lawler (1992, 1995) for the River
Murray. According to this model, fluvial erosion or attrition processes should dominate
along the Torrumbarry Weir Pool. While attrition was identified, it accounted for only 4%
of the bank-instability mechanisms. Notch development was the dominant mechanism
promoting bank instability in this mid-reach section of the River Murray. Moreover, most
of the river banks (90%) displayed signs of instability along the Torrumbarry Weir Pool. In
addition, erosion notches were present on both outer and inner bends of meanders as well
as along straighter sections of the river. These results differ markedly from other studies of
channel morphologies and bank erosion in large lowland meandering river systems (cf.
Habersack, Haspel, & Schober, 2014). In a study of the channel morphology of the
Brahmaptura–Jamuna River, Sarker, Thorne, Aktar and Ferdous (2014) established a
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TRANSACTIONS OF THE ROYAL SOCIETY OF SOUTH AUSTRALIA
165
spatio-temporal sequence of river instability over a 200-year period. This highlighted bank
instability was significantly greater in the outer sections of meander bends compared to
other morphological sections of the river and that general scour or attrition was the
dominant erosional process. The generality of these results has been confirmed through a
systematic analysis of channel patterns for many large rivers across multiple continents by
Lewin and Ashworth (2014). The near-continuous spatial distribution of bank instability is
a geomorphological feature along this section of the River Murray and an artefact of flow
regulation.
River channel erosion and bank instability are natural processes of riverine landscapes. River channels adjust their morphology primarily in response to changes in
sediment and water regimes. However, the rate and type of river channel adjustment is
influenced by many factors, and human activities, such as flow regulation, can exacerbate these processes. The predominance of well-developed erosion notches on the river
bank along the regulated section of the Torrumbarry Weir Pool can be attributed to two
factors, the inherent instability of river bank soils and the relative stability of water
levels in the weir pool. The majority of the river bank soils (56%) along Torrumbarry
Weir Pool become unstable after periods of wetting; they become saturated, soften, lose
strength and become easily erodible. This inherent instability is associated with the
presence of well-developed notches at multiple bank elevations along the length of the
Torrumbarry Weir Pool. Stable water levels also exacerbate the effects of wave erosion
on river banks. Waves created by either wind or boating activities erode material from
riverbanks, and rates of this wave-induced erosion are enhanced if wave activity is
focused on one area of the river bank (Nanson, Von Krusenstierna, Bryant, & Renilson,
1994). Thus, stable water levels in the Torrumbarry Weir Pool narrow the areal extent
of wave-induced erosion on riverbanks. In a preliminary study of wave action along the
Torrumbarry Weir Pool, Southwell and Thoms (2004) recorded 3 cm increases in notch
depth, over a 24-h period, associated with boat-generated waves.
The presence of well-developed notches promotes other forms of river bank instability. Increases in the diversity of bank-instability mechanisms were recorded along the
Torrumbarry Weir Pool. This increase in diversity was the result of increases in the
presence of mass failure mechanisms. The influence of the presence of erosion notches
on the overall instability of riverbanks is dependent on the height of the river bank
above the eroded notch. With sufficient bank height and the exceedance of a critical
bank height, bank collapse through cantilever erosion and or block collapse will occur.
Differences in the extent, severity and diversity of instability mechanisms were recorded
between the upper and lower weir pool zones and this is directly associated with a
change in the bank height between these zones. River banks in the lower weir pool zone
(average 1.82 m) are not as high as those in the upper weir pool (average 4.76 m). The
prominence of erosion notches and the associated increases in other mass failure
mechanisms suggest that river bank instability is an artefact of the presence of
Torrumbarry Weir and its water level management regime in the upstream weir pool.
Conclusions
River banks or the aquatic river-edge environment are important ecotones within the
riverine landscape. As ecotones, they encompass sharp gradients of environmental factors,
166
M. THOMS
physical and ecological processes as well as displaying enhanced species diversity, elevated
rates of matter and energy exchange and providing important refuges for species from these
areas. The research of Keith Walker, in the River Murray, established the significance of this
littoral zone to be an outstanding example of an ecological boundary. In addition, he
demonstrated this ecotone to be an indicator of environmental change and one especially
sensitive to changes in water levels (cf. Walker et al., 1992). The research presented in this
study of the Torrumbarry Weir Pool further highlights the vulnerability of the aquatic
river-edge environment to water management. It reinforces the need for sympathetic weir
operations in the River Murray and the relaxation of the current 50-mm water level
tolerance in weir pools; a call made by Keith Walker over 30 years ago.
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Acknowledgements
The assistance of Erin Lenon and Mark Southwell in collecting much of the field data is gratefully
acknowledged. I acknowledge the mentorship of Keith Walker and his encouragement to challenge
those conventional and at times unsupported wisdoms of river science and management.
ORCID
Martin Thoms
http://orcid.org/0000-0002-8074-0476
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