Soil conservation guidelines for Queensland
Chapter 11 Stream stability
Chapter 11
Stream stability
Key points
• It is natural for the bed and banks of streams to erode, for sediment to
be transported by streams, and for stream channels to move. However,
excessive stream bank erosion can damage land and infrastructure and
pollute water supplies.
• In-stream environments are naturally variable. A single reach of a stream
may feature clumps of vegetation and debris interspersed with bare gravel,
mud or sandy beds, a mixture of bends, straights, pools and riffles, and fastflowing channels and slow backwaters. Natural variability is important for
stream stability—simplifying, straightening and/or de-snagging streams can
lead to problem erosion.
• Most streambank erosion occurs in occasional severe flood events. Both
the streambank and/or streambed can be effected by erosion. Engineered
structures such as rock walls can be useful in protecting against erosion;
however, the long-term stability of streams requires the establishment and
maintenance of appropriate vegetation in the stream, on the banks, in the
riparian strip and throughout the catchment.
• Stream systems vary considerably across Queensland, the response of
streams to stabilisation works can be unpredictable, and they may have farreaching consequences. Specialist advice should always be sought from the
appropriate authorities before undertaking works in or around streams.
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Contents
11.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
11.2 Erosion processes in streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
11.2.1
Overbank erosion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
11.2.2
Streambed erosion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
11.2.3 Bank scour or fluvial erosion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.2.4
Bank collapse or slumping. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.2.5
Channel widening. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
11.2.6
Channel avulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
11.2.7
Soil cracking and crumbling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
11.3 Sediment movement in streams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.4 Stream management strategies and treatments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
11.5 11.6 11.4.1
Planning a stream stabilisation project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.4.2
Acquiring data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.4.3
Working with landholders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
11.4.4
Legal issues. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
The role of vegetation in stream stabilisation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
11.5.1
Types of vegetation in riparian areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.5.2
The impact of vegetation on flooding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
11.5.3
Riparian vegetation succession. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
11.5.4
Riparian zone widths required for stabilising streambanks. . . . . . . . . . . . . . . 27
11.5.5 Establishing vegetation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Engineering approaches to stream stabilisation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
11.6.1
Controlling streambed erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
11.6.2
Bank stabilisation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
11.7Legislation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
11.8 Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
11.9
Further information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Soil conservation guidelines for Queensland
Chapter 11 Stream stability
Glossary
bank collapse/slumping (mass-failure): an erosion
process in which gravity is the primary force acting to
dislodge and transport land surface materials.
incised channel: a channel on the land surface having
distinct bed and banks which carries perennial or
ephemeral water flows.
capillary action: the ability of a liquid to flow in narrow
spaces without the assistance of, and in opposition to,
external forces like gravity.
levee bank: a long linear rise bordering a watercourse
comprising part of the floodplain formed by deposition of
sediment from overbank flow during floods.
channel avulsion: abandonment of a part or the whole of
a channel belt by a stream in favour of a new course. The
old channel may be abandoned either instantaneously or
gradually.
macrophyte: an aquatic plant that grows in or near water
and is either emergent, submergent, or floating.
crib wall: a form of prefabricated wall construction,
usually concrete or timber, used to shore-up unstable
batters.
desnagging: the practice of deliberately removing large
woody debris and living riparian vegetation from natural
waterways which has been widely practiced throughout
Australia in the past with the aim of reducing flooding and
improve drainage.
dispersive soil: a structurally unstable soil which readily
disperses into its constituent particles (clay, silt, sand) in
water.
drop structure: a hydraulic structure for allowing water to
fall to a lower level. The drop is typically vertical.
engineered log jam (ELJ): An emerging stream restoration
technique which involves reintroduction of large woody
debris into a stream.
gabion: a rectangular wire-mesh cage filled with rock,
brick, or similar material, usually assembled on site to
construct retaining walls and anti-erosion structures.
meander: the natural winding of channels which
results from a complex geomorphic process involving
streambank erosion and alluvial deposition.
revetment: a protective layer of erosion resistant material
either permanently or temporarily placed along the edge
of a stream channel or shoreline to stabilise the bank and
protect it from the erosive action of water.
riffle: a short, relatively shallow and coarse-bedded
length of stream over which the stream flows at lower
velocity but greater turbulence than it does through a
pool.
chute: the steeply inclined section of a flume or other
similar hydraulic structure between the inlet and outlet
that conveys the flows directly from one level to another.
A chute is typically angled (unlike a drop structure)
sodic clay: clays with a high amount of exchangeable
sodium (ESP). Excess exchangeable sodium adversely
affects soil stability, plant growth and/or land use. A soil
with more than 15% ESP is considered strongly sodic.
thalweg: the line of lowest elevation within a valley or
watercourse.
geofabrics: permeable fabrics, typically made from
synthetic materials, which when used in association with
soil have the ability to separate, filter, reinforce, protect,
or drain.
groyne: a wall usually built perpendicular to the
shoreline to trap material moved by littoral drift and/or
to retard erosion of the shoreline. Groynes are commonly
constructed of large rocks, concrete, piles or other
relatively permanent materials.
head cut or ‘knick point’: an abrupt change of gradient in
the profile of a stream or river, typically due to a change in
the rate of erosion.
Hjulström curve: a graph used by hydrologists and
geologists to determine whether a river will erode,
transport, or deposit sediment.
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11.1 Introduction
Streams are a key component of the hydrologic cycle (Figure 11.1). They collect
excess water from the land and safely channel it to the sea. Streams are also
important for biology, providing aquatic habitats within the stream itself, as
well as supporting terrestrial habitats in adjacent riparian areas. Water enters
streams as runoff from the land surface or as ground water replenished by
infiltration through permeable soils and rocks.
Figure 11.1: Hydrologic cycle
It is natural for the bed and banks of streams to erode, for sediment to be
transported by streams, and for stream channels to move. These processes
acting together help to form the floodplains and alluvial terraces that are
common in the middle and lower reaches of many of Australia’s river systems
(refer to Chapter 12) and which contribute so much of Australia's agricultural
productivity. For long periods the rate of change in a stream may be slow
and imperceptible. However, flood events which can occur comparatively
infrequently can cause sudden dramatic changes which may affect the stability
of a stream for decades.
An important function of streams and floodplains is to dissipate the energy of
flows by slowing them down. Landscape features such as channels, vegetation,
pools, riffles, meanders, wetlands, lakes, billabongs and lagoons all contribute
to this function. Interventions to ‘streamline’ flows, such as by straightening
streams, constructing levee banks or removing vegetation, counteract this
function by increasing stream velocities, and can result in more extensive
flooding and erosion downstream.
Stream erosion can destroy valuable agricultural and recreational land and
threaten infrastructure, such as roads, bridges and buildings. It can also greatly
increase sediment and nutrient loads leading to decreased water quality. A
decline in water quality can adversely impact on stream biology and it can affect
human health. For example, for several days during a flood in January 2013,
the amount of sediment coming from Lockyer Creek was so great that the Mt
Crosby water treatment plant on the Brisbane River was barely able to maintain
the quality of its water supply to Brisbane. Most of the sediment causing this
problem came from eroding streambeds, streambanks and gullies throughout
the catchment (‘Urgent meetings over water sediment threat,’ Brisbane Times, 22
March 2013).
Rutherfurd et al. (2000) describe the natural conditions within a stream. These
conditions can vary greatly: streambeds can contain woody debris or vegetation
of a range of different types; streambanks can slope gently or be undercut; whilst
the velocity of flow in the stream can vary from very high, in zones over rocks and
around logs, to almost calm in deep pools.
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Soil conservation guidelines for Queensland
Chapter 11 Stream stability
Stream channels often meander across the floodplain, and vary in shape from
a deep, narrow trench where they are hard up against the floodplain wall to a
broad, shallow stretch between meander bends.
Management of land throughout the catchment and within the stream can have a
major effect on the stream and its behaviour. Removing snags and vegetation to
make streams less ‘messy’ reduces channel complexity, makes stream habitats
more uniform, and can actually increase sediment loads within the stream. When
rehabilitating streams it is usual to aim to restore the ‘messiness’ of streams.
This involves rebuilding in-stream structures by reintroducing vegetation or
woody debris, which creates habitat complexity and increases the variability of
the velocity throughout the stream.
A number of practices and activities can increases the amount and/or rate of
runoff and make a stream unstable. These practices may include the following:
• changes in land management and land use such as clearing forest to expand
agriculture or develop urban areas
• removal or disturbance of protective streambank vegetation through grazing,
clearing or fire
• lowering streambeds by extracting excessive amounts of sand or gravel or by
dredging to deepen channels
• cultivating too close to streams
• Concentrating runoff on streambanks by building road cuttings in the wrong
place or by allowing livestock unrestricted access
• Straightening streams
• Redirecting and accelerating flow by introducing infrastructure, obstructions,
debris or vegetation within the stream channel
• Release or retention of water from storages causing sudden unnatural
changes in stream level
• ‘De-snagging’, or removal of large, woody debris from stream channels
• Constructing levee banks that divert and concentrate flows.
This chapter provides introductory information about stream stabilisation.
Implementing stream stabilisation projects is an uncertain business. It can
require a high level of technical expertise and experience and the cost can be
significant. It often involves the use of both vegetation and physical structures
in sites that can be difficult to access. Stream stabilisation also requires a lot
of patience. Vegetation may take many years before its roots and branches
develop sufficiently to perform their intended function. This means that stream
stabilisation projects that may initially appear to have been successful when
tested in a minor flood may fail in a major flood.
Stream stabilisation techniques need to take into account the local conditions
such as the type of stream, the erosion processes that are occurring, the climate
and the soil type. Stream systems vary considerably across an area as large as
Queensland. This means that what works well in one area may not be appropriate
for another. Specialist advice from fluvial geomorphologists, river engineers
and stream ecologists can be very helpful in avoiding costly failures on stream
stabilisation. Some of the variability that occurs in Queensland streams and
rivers was described in Chapter 10 Land management on floodplains.
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11.2 Erosion processes in streams
Stream erosion comprises a number of different processes that can occur singly
or in combination. These processes include:
• overbank erosion
• bed erosion
• bank scouring (fluvial erosion)
• bank collapse/slumping (mass failure)
• channel widening
• channel avulsion (the development of a new or additional course for a stream)
• soil cracking and crumbling during dry periods.
The following sections contain information about how to manage or prevent each
of these different forms of erosion.
11.2.1 Overbank erosion
Streambanks and adjacent riparian areas (often referred to as ‘frontage’ land)
are susceptible to erosion if protective groundcover is removed and they are
then exposed to raindrop impact and runoff. In severe cases, sheet erosion can
completely remove topsoil from these areas to form a ‘scald’ which can be very
difficult to rehabilitate. Groundcover removal could be by mechanical clearing of
vegetation, grazing, cultivation, recreational use, vehicular traffic, weed control,
fire or the actions of exotic animals such as feral pigs.
Where runoff from local rainfall concentrates and then flows over a streambank,
rills and gullies can form. This process is also referred to as lateral bank erosion.
The sharp increase in slope at the point where runoff flows over the bank is
known as the ‘knick point’. It is comparable to the head of a cascading waterfall.
Runoff is typically concentrated by a road, track, stock pad or an earthen bank or
levee built to divert runoff from adjacent areas (Figure. 11.2). The risk of overbank
erosion is increased when the stream is heavily incised or if stream levels are low
as this creates a substantial drop and increases the energy of the overbank flow.
Overbank erosion can also occur as a result of outflow from a flooded stream
(see Chapter 10).
Figure 11.2: Potential for erosion when a levee bank is directed towards a stream
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Soil conservation guidelines for Queensland
Chapter 11 Stream stability
Dispersive clay soils are especially susceptible to overbank erosion. For
example, sinkholes can develop in dispersive soils adjacent to a streambank,
greatly accelerating the rate of erosion as they collapse and expand. Dispersive
clay soils are usually sodic. Sodic clays have sodium ions attached to the clay
particles. When exposed to water, the size of the sodium ions increases and the
links between individual clay platelets are broken, causing the soil to disperse
and remain suspended in water. Soils with a high proportion of silt can also
disperse in a similar way to sodic soils. Dispersive soils are common in many
Queensland catchments and their floodplains. The particles resulting from
dispersion are a primary cause of turbidity in dams and in creeks and rivers
flowing into coastal waters and inland lakes.
11.2.2 Streambed erosion
Streambed erosion occurs when material is directly removed from the bed of
a stream and washed downstream. It can be the result of either high velocity
flows (causing uniform scour along the bed) or the formation of a head cut
or knick point. Head cuts such as this can be initiated as a scour hole in the
streambed. The head cut can then move upstream due to the continual removal
of bed material due to high velocities flowing over it (Figure 11.3). The scour
will continue to move upstream until equilibrium of the sediment and hydraulic
regimes is reached or until it encounters a scour-resistant section of the bed
such as a rock bar.
Streambed erosion often appears to be innocuous and is frequently ignored.
However, lowering of the bed level can have serious repercussions. For instance,
it can destabilise streambanks. Because streambank erosion is more obvious,
it is commonly given attention, when in fact erosion of the streambed may be
the real culprit. Bed erosion in a low-order ‘stream’ that is ephemeral and only
carries runoff two or three times a year could in fact be really considered to be an
example of gully erosion (see Chapter 13).
Figure 11.3: A head cut resulting from streambed erosion (modified from Kapitzke et al. 1998)
Streambed erosion can be a natural process associated with down-cutting in a
floodplain (refer to Chapter 10). Falling ocean levels during past geological eras
have been a significant factor contributing to the erosion of streambeds in some
parts of Queensland.
11–7
Other processes and activities that may contribute to bed erosion include the
following:
• a major flood following many years of low flows and sediment deposition
• an increase in bed slope due to:
—— the stream being straightened
—— a bed control such as a rock bar, weir or crossing being removed or reed
beds being damaged by grazing stock
—— excavation of the bed of a stream by extractive industries, or for recreation,
construction of large pump holes or dredging to deepen channels in
streams and their estuaries (see Figure 11.4)
• an increase in stream discharge due to:
—— runoff increasing due to clearing or development in the catchment
—— water being diverted from one catchment into another
—— prolonged releases from in-stream pondages e.g. to deliver water for
irrigation
• an increase in flow velocity caused by:
—— constriction of the channel by debris, fill or vegetation accumulating on the
streambed
—— excessive de-snagging or removal of vegetation from the channel
—— construction of bridge abutments and culverts
—— construction of levees
• a decrease in the supply of sediment from upstream resulting from new dams,
weirs, catchment erosion control measures or excavations in the streambed.
Figure 11.4: Excavating the bed of a stream can lead to bed erosion (Queensland Government 2009)
Rutherford et al. (2000) discuss the impact that sand and gravel extraction can
have on bed erosion. They suggest that such extraction can sometimes be used
strategically to rehabilitate streams, for example, when they are affected by
sediment slugs.
Lowering of the streambed can often have serious ‘knock-on’ impacts. These can
include the following:
• creating new gullies or deepening of existing gullies in the overland flow
paths leading towards the stream
• increased susceptibility of banks to collapse because their height relative to
the bed has been increased
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Soil conservation guidelines for Queensland
Chapter 11 Stream stability
• lowered ground water level in the adjacent floodplain which can adversely
affect the aquifer and deny water to bores
• drainage of chains of ponds in streams
• exposure of saline ground water aquifers
• restricted access to water for pumps for irrigation and/or domestic supplies
• increased siltation downstream, resulting in destruction of aquatic habitats
and adverse impacts on water quality, water availability, flooding, navigation
and recreational pursuits
• damage to infrastructure including bridges, crossings and pumps
• reduced habitat for in-stream fauna such as fish and platypus.
The following physical features may indicate that bed erosion is occurring in a
stream:
• vertical head cuts (knick points) in the streambed
• steep and mobile riffles
• extensive bank erosion on both sides of the stream
• loss of pools
• remnant evidence indicating that past attempts to stabilise streambanks are
well above current stream height
• marks on bridge pylons that indicate previous bed levels well above the
current
• lateral erosion indicated by head cuts (hanging valleys) on tributaries and
gullies
• different channel widths between disturbed and undisturbed reaches
• exposure of ‘ancient’ logs and rock bars in the streambed
• reaches downstream of a head cut that are wider and shallower and have
fewer deep holes
• complete relocation the channel
• high levels of sedimentation in the channel downstream of the head cut
• declining watertables in nearby wells and bores.
11.2.3 Bank scour or fluvial erosion
Bank scour, or fluvial erosion, is the direct removal of bank materials by
the physical action of flowing water. Bank scour is the result of flooding or
persistent, low flows against saturated banks resulting in the undercutting of the
bank toe as shown in Figure 11.5. It can occur in any section of a stream.
Figure 11.5: A streambank undermined by scouring of the toe-slope
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The outside meander of a stream can be especially susceptible to bank scour.
Figure 11.6 shows how the highest flow velocities occur on the outside curve of
meanders in streams. Much of the sediment from this erosion can be deposited
in a point bar on the next bend where the flow velocity reduces. The pattern of
alternating bars resulting from this process establishes a helical flow pattern in
the current (Figure 11.6).
Figure 11.6: Helical flow currents acting on the outside of a meander bend (Kapitzke et al. 1998)
Bank scouring can occur as a result of flows being diverted towards a bank
by debris or vegetation in the streambed or by watercraft. The streambank
is normally rougher than the streambed. However, an excessive amount of
vegetation growing in a streambed can increase roughness and divert flows
into the streambank causing it to erode. For this reason woody plants growing
in the middle of a streambed are not necessarily beneficial for the long-term
functioning of the stream. Wave erosion caused by wind or boat traffic can also
scour the bank through a process known as fretting.
Streambanks are most susceptible to erosion when they are saturated. When
soils are wet, the amount of lubrication and pore water pressure between soil
particles increases and soils become less cohesive. Sandy soils are loosely
bound in all circumstances and have very little resistance to flowing water. This
high erodibility of unconsolidated sand is illustrated by the rapid loss of sand
from beaches when exposed to high seas. Clay soils take longer to wet up and,
provided they are not dispersive, are more cohesive than sandy soils. Where
soils contain a lot of clay, bank scouring can be increased by the process of
slaking. Slaking is where soil aggregates break down when placed in water due
to trapped air being expelled and replaced with water sucked into pore spaces by
capillary action.
11.2.4 Bank collapse or slumping
Bank collapse or slumping is where large chunks of bank material become
unstable and topple into the stream under the influence of gravity. This is
comparable to a landslide. It is also referred to as mass failure, rotational failure
or toppling failure. Bank slumping is often the dominant form of erosion in the
lower reaches of large streams where riparian vegetation can be sparse. It can be
the major source of sediment in flood flows. As slumping occurs, the streambank
may eventually become stable by gradually battering itself back into the adjacent
riparian land. Streambank stabilisation measures such as bank reshaping,
rock armouring and revegetation will protect the treated sections of bank from
collapse.
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Soil conservation guidelines for Queensland
Chapter 11 Stream stability
Streambanks containing high proportions of sand and silt are most susceptible
to slumping. This is because sand and silt have lower cohesion and sheer
strength than clays. If the bed of a stream is naturally armoured with rock which
prevents bed cutting and deepening of the stream, serious bank erosion can
occur during floods as the only direction in which the stream can expand to
accommodate the increased flows is laterally. The formation of tensile cracks on
a streambank during a dry period (Figure 11.7a) is a precursor to bank slumping.
Typically these cracks might be 10 cm to 20 cm wide at the surface and 1 cm to
2 m in depth. Another precursor to bank slumping can be erosion of the toe of a
streambank due to scouring (Figure 11.7b).
Figure 11.7: Precursors to bank slumping: a) tension cracks in a streambank, or b) erosion of the toe of a streambank by
scouring
(a)
(b)
High stream levels during a flood can cause water to infiltrate into dry
streambanks. Capillary action can extend the saturated zone created by this
infiltration by up to 2 m (Figure 11.8). Whilst the stream remains in flood, the
pressure of water in the channel supports the bank. However, when the flood
level recedes, slumping can occur because of the excessive weight of the
saturated soil of the streambank. The shear strength of soils is reduced when
they are saturated. This means that slumping is more likely to occur along the
interface between soil that has been wetted and that which is still dry. This
type of slumping is more likely to occur if the stream level drops quickly as for
instance would occur if floodgates were closed in a dam wall upstream.
Figure 11.8: Saturated streambanks susceptible to slumping
11–11
Seepage through the streambank can also contribute to bank slumping. For
example, seepage is believed to have been a significant factor in the 2011
Lockyer valley floods, contributing to the loss of an estimated 10 000 cubic
metres of soil from several sites (Peter Pearce personal communication).
Seepage occurs when there is a vein of very permeable sandy alluvium at depth
in a streambank. This vein provides a path for water to seep into and through the
soil profile to exit at this point and undermine the bank. Such seepage flows can
be the result of old, deep sand deposits from past flooding, septic tanks set too
close to the bank, or overflow from domestic rainwater tank outlets.
Tensile cracks, erosion of the toe-slope and saturation of the streambank can
act in combination to cause bank slumping (Figure 11.9). Slumping often occurs
a day or two after a flood has receded. Freshly exposed faces on sections of
streambank that were not subject to flood flows are clear evidence that slumping
has occurred. The slumped soil may remain on the edge of the stream against the
bank until it is removed as sediment during the next flood. If the slumped soil
remains in situ for long enough, vegetation that was growing in the soil when it
was on the bank may re-establish in the streambed.
Figure 11.9: Tension cracks, toe-slope erosion and bank saturation causing bank slumping
Retrogressive bank movement (Figure 11.10) is a variation of shallow rotational
failure (‘slip-circle failure’) that occurs in fine sands and low cohesion silts,
often in conjunction with seepage from the bank (Kapitzke et al. 1998). The
retrogressive failure starts with a small rotational failure in the region of the
seepage face. This over-steepens the bank, initiating a succession of rotational
failures until a lower, stable slope is achieved, or until the soil above the most
recent slip has sufficient strength or cohesion to resist further failure. This
explains how some apparently deep-seated failures have developed in slopes
that otherwise appear too flat to be unstable.
Figure 11.10: Bank slumping arising as a result of a series of rotational failures (Kapitzke et al. 1998)
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Soil conservation guidelines for Queensland
Chapter 11 Stream stability
High and steep banks are most susceptible to slumping. Surveys of the Burnett
River carried out by the Burnett–Mary Regional Group following the floods
in 2011 and 2013 (Wilson 2013) found that approximately 80% of all slumps
occurred on banks >5 m high that had slopes >25o. Banks higher than 10 m and
with slopes >25o were at most risk. Very few slumps occurred on slopes <20o.
Alluvial soils were particularly prone to slumping. Slumps typically occurred
as the flood was receding or during the following several days. The floods in
2010–11 appeared to have destabilised lower banks, predisposing them to
severe damage from the much larger 2013 flood. In all cases of bank slumping,
land had been cleared to the top of the high bank and the slump had rotated
back some distance from the edge of the high bank into the adjacent cleared
flats, often resulting in significant loss of crops and productive land. Remnant
vegetation appeared to have been very effective at reducing the risk of slumping
where it had been retained.
In freely meandering alluvial rivers, individual bends on the same river will have
different erosion rates due to differences in their radius of curvature (Brooks
2006). The maximum bend erosion rates appear to occur when the radius of bend
curvature is approximately three times the mean channel width. While slumping
commonly occurs on the outside of channel bends, it can also occur where there
are sandy deposits on the inside of curves or between bends in the stream. This
was observed in the Bremer catchment in the floods of 2011 and 2013 (Geoff
Faulkner personal communication). Landslips can also occur on old river terraces
that are well above recent flood levels.
In some streams, bank collapse has been observed to occur more frequently
on banks facing south than on other aspects (Grant Witheridge personal
communication). It is believed that this is because south-facing banks receive
much less sunlight in winter. Vegetation in these areas is consequently far less
vigorous and provides much reduced bank strengthening. This phenomenon is
likely to be more significant in southern parts of Queensland where the angle
of the sun at midday in winter is low, than in other areas where it is closer to the
vertical.
Sidewall slumping is a common process contributing to streambank collapse and
gully formation in dispersive soils. Figure 11.11 shows how sidewall slumping
occurs when a shallow flow undermines and causes subsequent collapse of a
gully wall.
Figure 11.11: Gully widening by undercutting and slumping
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Where sidewall slumping is a major factor, gullies will have a distinct ‘U’ shape.
Such gullies can have very small catchments, and often they will have retreated
to the very top of their catchments. These gullies often only ever carry small
flows even in a major rainfall event. Dispersive subsoils are infertile and support
very little plant growth. They are consequently unprotected from raindrop impact
which can also remove significant quantities of soils from the sides and floor of
gullies in these circumstances. Structureless soils and soils that slake are also
vulnerable to sidewall slumping.
11.2.5 Channel widening
Channels will become wider due to channel incision and bank erosion. Brooks
(2006) describes the circumstances under which this will occur. These include:
• extreme floods
• excessive stock grazing, vegetation clearing or fire causing a reduction in
bank strength and/or roughness.
Where they are present, causal mechanisms such as those listed above should
be addressed before considering in-stream engineering approaches to treat
channel widening. Where widening is the result of occasional high-magnitude
flood events, an interventionist engineering ‘solution’ is unlikely to be the best
option. In situations such as this, providing assistance for regeneration of native
vegetation (such as by excluding grazing) may be all that is required.
11.2.6 Channel avulsion
Avulsion is a natural process occurring when a new or additional channel
develops on a different part of the floodplain separate from the existing
channel. As explained in Chapter 10, it is natural for streams on a floodplain to
periodically shift location as they carry out their function of spreading sediment
over the floodplain. Channel avulsion is most likely to occur during a major
flood following a period of significant sedimentation, and in locations where the
existing channel is constricted and the height of streambanks is reduced relative
to the bed, such as by a dense infestation of weed. Avulsion can also occur
where flows are concentrated or diverted out of the channel onto floodplains by a
structure such as a road or levee bank, or by an event such as the establishment
of crop rows in a direction that increases the amount and velocity of runoff and/
or increases the amount of sediment in runoff.
11.2.7 Soil cracking and crumbling
When exposed streambanks which comprise clay soils with a high shrinkswell potential dry out, tension cracks may form. Tension cracks can lead to
slabs of soil falling from the bank into the streambed. In addition, under these
circumstances, a 3 cm to 4 cm layer of loose friable soil may fall away from the
bank and form a scree at the base of the bank (Howard et al. 1998). Soil lost from
a bank as a result of cracking and crumbling will be friable and very susceptible
to slaking (i.e. disintegrate in water). Soil in this condition can easily be removed
from the streambed by low-velocity base flows or during the next runoff event.
Such a process of erosion can occur along any exposed parts of a streambank
including straight sections.
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11.3 Sediment movement in streams
Movement of sediment is important in the development of floodplains (see
Chapter 10). While excessive amounts of sediment within a stream flow can
impact negatively on water quality and in-stream habitat, some sediment
is necessary for the normal functioning of a stream. A stable stream is in
equilibrium, continuously picking up and depositing sediment along its length.
There is a limit to the amount of sediment a stream can carry. This depends on
the size of the stream and on its velocity. Once the stream reaches the upper
limit of the amount of sediment it can transport, any sediment in excess of this
amount is deposited in the channel. Where flows in a stream are retarded by an
obstruction, such as by a weir or dam, a large proportion of the sediment may
be deposited over a short distance immediately behind the obstruction (Figure
11.12a). In this event, the flow below the dam will become more erosive as the
stream may have a higher velocity if not carrying sediment. Dams may be large
enough to accommodate all of the minor flood flows that they receive from
upstream. If this occurs, sediment deposited by tributaries in the channel below
the dam can remain there for a long period until a flood large enough to overtop
the dam ‘flushes out’ the sediment from sections downstream (Figure 11.12b).
Figure 11.12: In-stream obstruction such as a weir causing a) sediment to be deposited upstream of the obstruction, and
b) sediment to be scoured downstream of the obstruction
(a)
(b)
The term commonly used for material carried by streams is ‘load’. Stream ‘load’
comes in the following three different types:
• Solute or dissolved load is chemicals, nutrients and contaminants that
are carried in solution. These solutes originate from natural, agricultural,
industrial, mining or urban processes within the catchment.
• Suspended load is small particles such as fine sand, silt and clay size which
are held up in water and transported over long distances. The amount and
distance these particles are carried depends on their size and on the velocity
of the water. Dispersed clay particles for instance remain in suspension even
in still water. These are responsible for the turbid appearance of streams and
water stored in dams.
• Bed-load is the sand, gravel, boulders or other debris transported by rolling,
sliding or saltating (hopping) along the bottom of a stream. The amount of
bedload depends on the stream depth and velocity. The physical impact of
larger particles colliding with the streambed or banks is an additional cause
of bed and bank erosion.
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The amount and type of load carried by a stream depends on the size of the
material and the velocity of the stream. A particle can vary between being bedload, part suspension and full suspension at different points along a stream as
the flow varies. The Hjulström curve (Figure 11.13) can help to determine whether
and under what conditions a stream will erode, transport or deposit sediment.
The Hjulström curve (Hjulström 1939) shows the relationship between sediment
particle size, stream flow velocity and particle movement. The upper curve
shows the velocity in cm/s at which particles will erode as a function of their size
in mm, while the lower curve shows the velocity at which particles will deposit as
a function of their size.
Figure 11.13: The Hjulström curve, explaining how stream velocity and particle size affects erosion, transport and
deposition of sediment in a stream (Hjulström 1939)
Points of interest 1 to 5 shown on Figure 11.13 highlight the different behaviours
of the various types of particle:
• Point 1: Due to their inherent cohesion non-dispersive clay soils resist erosion.
Dispersive clays are, on the other hand, highly erodible. While dispersive
clays readily break down into individual clay particles when placed in water,
non-dispersive clays can remain aggregated to some degree, and settle out
when the velocity is reduced.
• Point 2: Sand particles have no inherent cohesion and are the most
susceptible to erosion.
• Point 3: Boulders are obviously the form of sediment that most resist being
moved by a stream flow.
• Point 4: Individual clay particles tend to remain in suspension, even in still
water, whilst larger particles such as sand and silt can travel in suspension if
the stream velocity is high enough.
• Point 5: Large particles such as pebbles and boulders can be moved as bedload. When being moved in this way these particles may contribute to further
erosion by physically impacting (through projectile action) on the streambed
and banks.
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11.4 Stream management strategies and treatments
Streams can be stabilised using either revegetation or ‘hard’ engineering
approaches. Revegetation is generally cheaper although it can take years or
decades to become fully effective. Engineering approaches generally involve
greater up-front cost but should be fully effective as soon as they are installed.
The risk of failure associated with stream stabilisation projects can be high,
especially when a flood event occurs soon after the works are installed or
the flood exceeds that for which the project was designed. Where immediate
protection is required, revegetation should be combined with engineering
approaches. Stream stabilisation projects can be expensive and cost needs to
be balanced against the value of the asset being protected. High-cost projects
may be justifiable where streambank erosion is threatening a valuable piece
of infrastructure or an important environmental value (e.g. the habitat of a
threatened species). However, they will be too expensive where such values do
not exist or long stretches of a stream require stabilisation.
It is important to correctly diagnose the underlying causes of stream erosion
if the response is to be cost-effective and successful. Bank erosion can be a
natural process and an engineering solution may not be the best approach.
To determine whether a channel is undergoing accelerated erosion requires
an appreciation of the longer-term average and periodic rates of bank erosion
(Brooks 2006). This can be difficult. It requires a thorough historical analysis
coupled with detailed measurements of contemporary erosion rates (using either
a precise survey or erosion pins).
When deciding whether revegetation will be sufficient to protect a particular part
of a stream, it is worth first giving consideration to why the vegetation in that
section of the stream is currently inadequate. For instance, where the channel
has been significantly altered by erosion resulting from past stream clearing,
it may be difficult to return the stream to its pre-European condition through
revegetation alone. Also, sometimes native vegetation extends along a bank face
as far as the entrance to a bend, and then may stop. This can indicate that the
rate of erosion at the bend is too high for vegetation to survive. If this is the case,
revegetation efforts are likely to be unsuccessful unless they are complemented
with engineering solutions.
De-snagging of streams has been widely practised throughout Australia in the
past. However, the benefits of debris, such as wood, in streams is now well
recognised (Brooks 2006 and see below), and the practice of de-snagging has
largely ceased. Rutherford et al. (2000) point out that a complex in-stream
habitat is required if a high richness of fauna species is to be achieved in
a rehabilitated reach. Fish and other aquatic creatures require a variety of
conditions to ensure suitable habitat for feeding, reproduction and resting. One
indicator of complex habitat in a reach is that the range of flow velocities within
it is large at any one time. Constraining the channel, such as by constructing
a groyne part-way into the flow or placing a structure across the full width of
the bed, will decrease the cross-sectional area of a stream and consequently
increase the average velocity of that section. The opposite is true if the crosssectional area is expanded by dredging of the bed and construction of large
pools. In general, for every point in a stream where flow velocity is increased
there must be a corresponding decrease elsewhere.
When planning stream stabilisation it is important to consider the presence
of sediment slugs (sometimes referred to as waves). Sediment slugs are
aggregations of sediment that persist and generally migrate slowly downstream
between flood events (Brooks 2006). A streambed can become incised as the
sediment slug leaves the section, moving downstream.
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A rehabilitation strategy designed for such a reach that is based on the bed
level at the time the sediment slug is at its maximum, may be totally unsuitable
a few years later when the slug has moved on and the channel looks completely
different. Stream stabilisation structures installed under such circumstances
may become undermined, or may no longer function in the way they were
originally intended. In streams affected by a sediment slug, it is best to avoid
implementing any strategy based on bed level control until the sediment slug
has moved through. If the aim is to improve in-stream habitat (pools), a strategy
of channel constriction when the slug is at its peak is most likely to succeed.
11.4.1 Planning a stream stabilisation project
The type of stream and the erosion processes that are occurring need to be
considered before planning any stream stabilisation project. The approach
required to stabilise a small tributary will be quite different from that required for
a major river.
A detailed site plan should be prepared for any stream stabilisation project. This
plan should describe all the activities to be carried out whether they involve
the use of vegetation, engineering approaches or both. The plan should include
information such as the species to be used and the density and location in which
they are to be planted, weed control to be undertaken and any other ongoing
management that is required (e.g. fencing, feral animal control).
11.4.2 Acquiring data
Before starting stream stabilisation work on a specific site, it is worth checking
other sections of the stream for evidence of erosion. Areas at high risk of erosion,
especially those with steep and high banks, need to be identified. There may be
little point in investing a large amount to stabilise one short section of stream, if
extensive erosion is occurring elsewhere in the stream network. In some cases
money may be more effectively used in areas where the erosion problem is not
as severe rather than tackling the worst areas first up. Landowners should be
consulted throughout the district as they may have experienced similar issues,
and useful lessons can be learnt from the success (or otherwise) of attempts
they may have made to alleviate the problem in the past. Hydrographic data,
obtained by stream gauging stations, will provide useful information about the
height and duration of recent and past floods.
Aerial photography and satellite imagery provide a valuable overview of a stream
network. It is useful to review historical photographs to get an indication of
the rate at which erosion has been occurring and of changes in management
and condition of the catchment, floodplain and riparian area. The Department
of Natural Resources and Mines has aerial photography covering most of
Queensland, dating back to the mid-1950s. Images have been recaptured
for many areas every 10 years or less. It could also be worth checking with
organisations such as local historical societies for old photographs of streams
and bridges. Streams in flood create considerable interest and are frequently
photographed. Unmanned aerial vehicles (UAVs), which have rapidly developed
as platforms for aerial photography, may provide an excellent opportunity to
obtain photos of stream networks at a relatively low cost.
A thalweg survey of a stream (Brooks 2006) can provide useful information for
both the design of a rehabilitation strategy and for its subsequent monitoring. A
‘thalweg’ is a line connecting the lowest points along the length of a streambed.
It is usually the line of fastest flow in a stream. Where a streambed is scouring, a
thalweg profile of the streambed provides an estimate of the potential extent of
bed scour likely to occur within the reach.
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Chapter 11 Stream stability
11.4.3 Working with landholders
Most stream rehabilitation projects are initiated by landholders. Even if this isn’t
the case landholders obviously have a major stake in the project and need to be
closely involved in the planning, implementation and maintenance of the project.
Brooks (2006) lists a range of matters that should be discussed and agreed
upon with the landholder before the construction process can go ahead. These
include:
• vehicle access tracks
• storage of equipment and materials on site
• revegetation and stock management during and after the construction phase
• legal liability and permit requirements
• occupational health and safety considerations (stream stabilisation projects
can be a high risk, especially when carried out by inexperienced workers)
• ongoing maintenance.
11.4.4 Legal issues
Steep slopes adjacent to streams can be a high-risk work environment and the
safety of workers on any project needs to be a priority. Where the level of risk
is considered to be high it is best to engage experienced contractors. Before
commencing a project it is essential to check that workers are covered by
professional indemnity and liability insurance. This also applies to the persons
or organisation developing the designs for the rehabilitation strategy. If there is
uncertainty, seek legal advice.
Some of the strategies and actions recommended in these guidelines may
require authorisation from government environmental agencies before they
can legally be implemented. It is essential that all appropriate government and
environmental authorisations from the relevant agency are obtained. Failing
to do so may expose the persons and organisation undertaking the works to
serious penalty and legal liability. Relevant legislation is summarised later in
this chapter. If you are still uncertain about what authorisations are required, you
should seek further legal advice.
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11.5 The role of vegetation in stream stabilisation
Vegetation plays a vital role in stabilising streams (Figure 11.14). It also provides
key ecological functions such as biodiversity and habitat for both terrestrial and
aquatic organisms. Engineering works are often required to stabilise streams but
the most successful of these are nearly always in conjunction with revegetation.
Surveys of major watercourses in South East Queensland following the floods
in 2010 and 2013 highlighted the benefits of riparian vegetation in protecting
streams from flood damage (Wilson, 2013, Peter Pearce SEQ Catchments
personal communication).
Figure 11.14: Vegetation plays various roles in stabilising streams (Land & Water Australia 2004)
Before considering a program for riparian revegetation, the condition of the
existing vegetation should be assessed. Abernethy and Rutherford (1999)
provided a ‘traffic light’ (green, yellow, red) classification system suitable for
this purpose. To be effective, a continuous cover of a variety of plants is needed
over the whole bank. Isolated, single plants are not likely to be effective and in
fact can cause localised erosion.
Both the above and below ground parts of plants play important roles in stream
stabilisation (Figure 11.15). Branches and the foliage of trees, shrubs and reeds
reduce flood velocities in the same way that dense vegetation acts to abate wind
in a forest. Vegetation with flexible stems and branches will ‘flap’ around in a
flood to reduce the energy and velocity of damaging flows. This is especially
important on the outside curve of a meander where velocities are higher. The
roots of trees and shrubs interlock to form a mat which binds and strengthens
streambank soil in a manner similar to that of steel reinforcement in concrete.
Root systems can occupy a large volume of soil and after a flood has receded,
act to rapidly reduce the weight of streambanks and restore the tensile strength
of streambank soil by removing moisture through transpiration. When a root
dies, it leaves an airspace which improves soil porosity and helps to drain a bank
saturated during a flood.
Root systems of perennial plants are a combination of a few large, long-lived
structural roots and many small, ephemeral feeder roots. The larger roots form
the basic architecture of the system whilst the feeder roots (which despite
being only 1 mm or 2 mm in diameter constitute the major portion of the surface
area of a root system) are where water and nutrients are absorbed by the plant.
Feeder roots grow outward from the large roots and are generally more extensive
near the soil surface, where minerals, water and oxygen are most abundant.
Most diagrams illustrating tree roots only show the larger structural roots. This
gives us a distorted picture of the true nature of the system and the misleading
impression that it is the larger roots that contribute most to streambank stability.
In fact, the fine, dense root systems have higher tensile strength and provide
most protection (Andrew Brooks personal communication).
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Chapter 11 Stream stability
Figure 11.15: A range of vegetation types performing beneficial functions above and below ground on a streambank (Land
& Water Australia, 2004)
The greater the extent of the roots, the more protection they provide for
streambanks. The distance that roots extend downward and sidewards depends
on the species, age and size of the plant and on the properties of the soil.
Trees have much more extensive root systems than other types of plants such
as grasses. Evidence suggests that the influence of tree roots can extend
for a distance of 10–15 metres from the trunk and to a depth of at least 1.5
metres (Abernethy and Rutherfurd 1999). For the purposes of bank stability
applications, the roots of large trees are considered to extend to a depth of
about 3 m and laterally to the outer extent of the tree crown (Rutherfurd et al.
2000). Eucalyptus species which are by far the most common group of trees in
Queensland often have very deep (>5 m) root systems.
Root architecture varies considerably between different species. Some tree
species have a central ‘root ball’ which is about five times the diameter of the
trunk (Rutherford et al. 2000). For these species, individual roots spread out
from the root ball in all directions, the density declining sharply with distance
from the bank and with depth. Most of the roots outside the root ball are found in
the upper 0.5–1 m of soil, within the drip line of the tree. In deep, fertile alluvial
soils, root systems are likely to be most extensive, whilst in sodosols with a
shallow topsoil over an impermeable clay layer, root systems are likely to be
shallow (Peter Wilson personal communication).
Revegetation alone is not the solution to all stream erosion problems (Rutherford
et al. 2000). In this context the following points need to be considered:
• Streams are always evolving. Even with an intact cover of native vegetation,
significant erosion can occur at various stages of stream development.
• Clearing of vegetation and other modifications throughout the catchment
can greatly increase the ‘power’ of a stream. This may result in the stream
becoming incised. If this occurs, the stream will then be able to carry a
greater proportion of a flood flow in the channel and an equivalently reduced
proportion on the floodplain. The forces operating in channels modified in
this way are often too great for vegetation alone to address.
• Some areas are inherently difficult to revegetate. The effect of shade is an
important consideration in this respect. In winter, south-facing banks in
Queensland receive no direct sunlight and in southern parts of the state will
cast a shadow at midday that is as long as the bank is high. The negative
impact this shading has on vegetation growth may help explain why
11–21
streambank erosion has sometimes been observed as being most prevalent
on the north side of sections of streams running in an east–west direction.
(Grant Witheridge personal communication).
Erosion can sometimes remove vegetation from a previously well-vegetated
stream. This is most likely to occur in sections of bends where velocities are
highest. Loss of vegetation in these circumstances can indicate that revegetation
is not the best option to stabilise that stream. It could also indicate that the
vegetation species present may be unsuitable for those particular circumstances.
High banks that are subject to slumping are best protected by reinforcing with
deep root systems such as those of trees. However, surface erosion can occur
under the trees if, as is often the case, the soil under their canopy is bare. Soil
can become bare if stock access is unrestricted or if the growth of groundcover
species is inhibited by shading from the tree canopy or competition for water
and nutrients. Woodland thickening (Department of Environment and Resource
Management 2011) is a particular case where the growth of groundcover species
is inhibited. If bank slumping is not a problem but other forms of erosion are,
grasses may provide better protection than trees.
The term ‘soil bioengineering’ describes erosion control projects that combine
revegetation with ‘hard’ engineering. Willow, poplar and alder cuttings are
often used as the central feature of successful bioengineering designs in the
northern hemisphere (Rutherford et al. 2000). The cuttings are ‘woven’ into the
engineering structure and then grow quickly, producing a true bio-engineered
structure. The exotic species referred to above would not be recommended for
rehabilitation work in Australia, although there are a range of native species
that can be used in this way (see below and discussion of the concept of longstemmed planting).
11.5.1 Types of vegetation in riparian areas
To function fully and be self-sustaining, riparian vegetation needs to include
a mixture of overstorey, middle-storey, groundcover and macrophyte plant
species. Each plant group performs a different set of functions. In the correct
combination, the different plants complement each other to stabilise the stream
as well as to meet a diverse range of environmental needs. Riparian areas
contain zones with different moisture availability and with different exposure to
stream flows. This means that riparian vegetation needs to include some species
adapted to dry sites with seasonal wetness, as well as others that are adapted to
regular saturation and/or inundation. Trees and shrubs that grow naturally on the
water’s edge such as weeping lilly pilly trees, tea trees, weeping bottle brushes
and water gums are adapted to cope with fluctuating water levels by for instance
being sufficiently flexible to bend over during periods of high flow rather than
break.
Overstorey
Overstorey plant species help to stabilise streambanks in the following ways:
• Tree and shrub stems, branches and foliage knit together to help reduce the
velocity of flood flows against streambanks. Vegetation will only provide this
function when plants are grouped; isolated individual tree trunks provide
little protection.
• Tree roots strengthen streambanks by binding soil in a manner similar to that
of steel reinforcement in concrete. Fine and dense root systems with high
tensile strength provide the most protection. The greater the depth of the
roots, the more protection they provide.
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Chapter 11 Stream stability
• Tree roots can inhibit the development of tension cracks that run parallel to
the riverbank and which contribute to bank erosion.
• Trees help to reduce the weight of banks by removing excess moisture via
transpiration and soil drainage (although they may topple over and undermine
the bank if partly submerged and the soil is saturated).
• Overstorey vegetation on the face of the streambank helps to protect the bank
from collapse by supporting or buttressing the soil above it.
• Some trees (such as River Red Gum) can survive with their roots exposed.
This allows them to provide ongoing protection even when the bank is partly
eroded.
The contribution that trees make to controlling soil erosion has long been a
subject of debate. Tree roots in particular provide useful protection against soil
erosion where gravity is a key force driving erosion, such as on streambanks and
on steep slopes subject to landslip. However, where gravity is not a significant
influence, such as on land between the mountains and the streambanks, tree
roots provide little protection. In these parts of the landscape, surface cover
to protect soil against raindrop impact is the most effective strategy to reduce
erosion. Where livestock grazing is unrestricted and shade is limited, vegetation
under trees may experience high grazing pressure and groundcover may be
greatly reduced. This situation can occur in riparian areas. Without special
management, ‘frontage’ grazing country can become seriously eroded as a result
of overgrazing.
It is sometimes argued that the weight of streambank trees is responsible for
their collapse and that of the streambank. However, this is very unlikely, as
the weight of a tree is actually quite small in comparison to the weight of the
soil (especially when the soil is wet). Nevertheless, isolated trees on banks
(especially on the outside of bends) can be vulnerable to falling over. This is
because such trees can be destabilised through undermining by turbulence
around their roots and trunks and can then fall under their own weight,
especially in high wind conditions and when the tree is leaning towards the
stream. Instability and erosion in the bank around the site of a fallen tree may
occur after the tree has fallen. Trees that fall across the stream may divert flows
towards the bank, causing erosion elsewhere. Streams can undercut the roots
of trees and not necessarily be vulnerable to slumping. As the face of such an
undercut moves further and further back below the roots, the velocity of the
flow against the back of the undercut will decline. This means that a bank that is
undercut below a strong root plate can be quite stable and in fact provide quite
good habitat (Rutherfurd et al. 2000).
Middle storey
Middle-storey vegetation protects mid- and upper-bank sections of streams from
scour. Vegetation reduces scour by slowing the speed of water adjacent to the
streambank. It can also ‘ride down’ with a collapsed bank and protect the toe
of the bank from further scour. In-stream shrubs add roughness to the channel,
helping to reduce flow velocities. Depending on the species, middle-storey
vegetation may restrict the establishment of valuable groundcover (e.g. acacia
species).
Ground cover
Groundcover vegetation such as grass helps to protect streambanks from
erosion in the following ways:
• It absorbs the impact of raindrops falling on the streambanks and on the
riparian area.
11–23
• It controls scour by reducing flow velocities or folding down flat under stream
flow to provide a protective blanket.
• It stabilises the bank, although the contribution it makes in this respect is
limited by the shallow root system of most groundcover vegetation.
• It stabilises lateral inflow points.
The protection provided by groundcover is limited by the degree of shading by
overstorey canopy and the shade tolerance of the groundcover species present.
Grazing and fire management practices also impact significantly on the vigour
of groundcover species. The extent to which groundcover vegetation protects
streambanks from collapsing is limited because of their relatively shallow
root depth. Groundcover vegetation can act as a filter; however, as discussed
elsewhere, it is unusual for runoff from adjacent land to enter streams by flowing
over streambanks. Instead, runoff usually enters streams from catchments at
well-defined points that don’t involve passage through riparian vegetation. To
reduce sediment levels in runoff, the whole of a catchment and the watercourses
that drain it need to be protected by adequate levels of ground cover.
Macrophytes
Macrophytes are shallow-rooted herbaceous plants that grow at the margins of
the normal water level or in the stream. They protect lower bank sections which
remain wet throughout the year. Macrophytes help to protect streams from
erosion by:
• dissipating energy from the stream by flapping rapidly in the flow
• protecting trees from local turbulence-induced scour.
Macrophytes also assist fish passage during flood flows.
Woody debris
Large woody debris or snags can help control the grade of channel beds,
contribute to pool formation, and armour both bed and banks against erosion.
By encouraging greater storage in the streambed, they can also help to increase
the rate of replenishment of ground water aquifers. Woody debris also provides
important habitat for fish, algae and macro-invertebrates.
The belief that large woody debris causes channel erosion by restricting flows
and reducing channel capacity, leading to overflowing of banks during flood
events, is misguided (Lovett and Price 1999). This misunderstanding has led
to extensive past snag removal programs throughout Australia (Rutherford et
al. 2007). It is now accepted that de-snagging has significant negative impacts
on stream ecosystems. These impacts include loss of habitat for fish and other
aquatic and terrestrial organisms, to the point where some native species
are threatened or are locally extinct. Snag removal also impacts on channel
morphology. De-snagged rivers typically become uniform drainage channels,
with fewer channel features such as scour holes and bars to retain, or act as
substrates for, the processing of carbon and nutrients by in-stream organisms.
A hydrological and geomorphic assessment must be carried out before
undertaking any debris/snag removal project. The removal of a log-jam of
woody debris (sometimes known as beaver’s nests) blocking a stream in a flood
may be justifiable, especially if it is contributing to localised bank erosion and
threatening infrastructure. However, advice and permits to remove offending
debris from a watercourse from the Department of Natural Resources and Mines
will be required. If such work is carried out it may be useful to relocate debris
removed from across the creek to the toe of the bank (in the direction of the flow)
to assist in erosion control in that area.
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Where riparian areas have been cleared, they no longer provide the inputs of
woody debris to the stream that they would have in their natural state. Even
where trees have been replanted, it will take many decades before these
plantings begin acting as natural sources of wood recruitment. Artificial log-jams
can be created in stream stabilisation projects using ‘engineered’ log jams (ELJs)
(Brooks 2006).
Weeds
Weeds are often found on the bed and banks of drainage lines and streams,
particularly where natural vegetation has been disturbed and the overstorey
canopy has been opened up. The impact of weeds can vary greatly. Some weeds
can help control erosion, effectively trapping sediment from runoff flowing
towards a stream. Others can also in some instances provide habitat for wildlife.
Weeds of riparian areas such as cat’s claw (Macfadyena unguis-cati) and rubber
vine (Cryptostegia grandiflora) can be very aggressive, completely overrunning
all of the native vegetation along a stream. Weeds of wet areas such as Chinese
celtis (Celtis sinensis) and willows (Salix sp.) can invade and greatly reduce
the capacity of stream channels, increasing the frequency of flooding and
channel diversion. Floods can disperse weeds such as parthenium (Parthenium
hysterophorus) to downstream areas, hastening their spread onto floodplain
cropping and grazing land.
When removing weeds from a large area of streambank it is desirable to conduct
the work in stages. This avoids having an extensive area exposed to a high risk of
erosion all at once. Dead and sterile parts of weeds can be chipped and held in
place by a geotextile to act as a mulch, protecting the streambank from erosion
until native vegetation becomes established (Rutherfurd et al. 2000). Branches
and stems from some exotic vegetation may also be useful in providing woody
debris in the channel provided these do not come from a species that readily
reproduces vegetatively. When the aboveground parts of a weed are removed
leaving its roots in place it will continue to provide some stability to a bank until
replacement, non-weedy vegetation becomes established.
Lippia (Phyla canescens) is a serious weed of inland river systems of Queensland
and New South Wales. It smothers native vegetation and prevents regeneration,
causing erosion along streambanks and leading to a further loss of biodiversity
(Lovett et al. 2003). Lippia is a perennial, broadleaf, flat-growing plant, with
numerous branched stems of up to one metre long that can break up during
flooding and quickly re-establish as the water subsides. Lippia plants have the
ability to root at nodes along their length to create a dense mat of stems and
leaves which prevent the growth of other species. Under the dense mat of lippie,
the soil is bare and at high risk of erosion if the lippia should die back in drought,
or when flooding causes water levels to rise for a period and then subsequently
fall.
11.5.2 The impact of vegetation on flooding
Vegetation within the channel can contribute to localised flooding. It adds
roughness, slows flows and reduces the capacity of the stream. Vegetation in
the floodplain mitigates floods in downstream areas. It stores water, reducing
downstream flood peaks but increasing the duration of the flooding.
The impact of large woody debris in the channel depends on how much of the
stream cross-section it occupies and its orientation in relation to the flow in
the stream. A flood level may be hydraulically controlled by some downstream
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feature such as a bridge (Rutherfurd et al. 2007). Such a constriction will then
produce a backwater upstream. Under those circumstances an in-stream log will
have no hydraulic effect on the flood flow and, in fact, as the flood level falls, the
log may eventually produce its own shorter backwater.
Removing vegetation and large pieces of wood (snags) from streams allows
them to flow faster. While this may alleviate localised flooding, it causes stream
erosion and may lead to larger floods and more sediment downstream. In major
floods, riparian vegetation and snags in the channel will have minimal impact on
flood heights because they will be submerged and much of the flood flow occurs
on the floodplain outside of the channel (Rutherfurd et al. 2007).
Riparian vegetation on the floodplain of a creek near its junction with a river may
be aligned either parallel to, or at right angles to, the direction of a flood flow.
When the flood level in the creek is higher than the level in the river, the flow will
be parallel to the vegetation along the creek. However, when the river is flooding
over its floodplain, the vegetation along the creek will be at right angles to the
flood flow. How effective a particular area of vegetation is in protecting against
erosion will depend on which of these circumstances prevail.
11.5.3 Riparian vegetation succession
Vegetation that has established naturally in the bed or on the banks of a stream
may not necessarily be in the right place for long-term stability. A riparian plant
becoming established in a specific location may be the result of a random event
such as a seed being deposited by a bird defecating in mid-air. Such vegetation
may become well established in a stream over a long period of low or moderate
flows, only to be removed when a major flood occurs. Nature controls vegetation
by removing selected plants under stress or extreme conditions such as during
severe floods. Major floods, even in parts of a catchment that are otherwise
undisturbed, can sometimes result in the mass removal of riparian vegetation.
Succession is defined as the progressive change in species composition and/
or structure that occurs following disturbance of an ecosystem (Lovett and
Price 1999). Disturbances can be natural or human-induced and can include
flooding, regulation of flows, fire, clearing and fragmentation, competition from
introduced plant species, grazing, salinity or the rising or lowering of ground
water. A community, or species, that has a high capacity to recover after a
disturbance is considered to be resilient.
As described in Chapter 10 it is natural for a stream to change over time. The
plants selected for planting in a riparian area must suit the long-term needs
of the stream. For example, large trees such as eucalypts may need to be
established some distance from the edge of the bank for long-term stability. This
is because mature trees growing on streambanks that collapse during or after
a flood can end up growing in an inappropriate position such as in the toe of a
bank or the middle of the stream.
Vegetation can invade the river channel, especially during the sometimes
extended periods between floods. While channel vegetation can reduce scouring
by decreasing flow velocities, it can also direct flow towards the banks and
increase erosion. Lower-flow velocity in channels will reduce their capacity
and may increase the risk of localised flooding. However, this will increase the
storage of water on the local floodplain, and hence reduce the risk of flooding
further downstream.
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Chapter 11 Stream stability
11.5.4 Riparian zone widths required for stabilising streambanks
All of the functions of a riparian buffer need to be considered when deciding on
the width of vegetation required. While riparian vegetation has many important
functions, as discussed previously, its ability to filter runoff from adjacent land
can be overplayed. When considering the width of riparian vegetation required
for stabilising streambanks relevant factors include:
• the height and steepness of the streambank
• the erosion processes that have occurred in the past and those expected in
the future
• the amount of seepage
• the condition of the existing vegetation.
The minimum width of riparian vegetation generally recommended in the
literature is 20–30 metres from each bank for minor streams and at least 40
metres from each bank for major watercourses (Layden 2011). Riparian buffer
width recommendations have also been made for some specific industries such
as sugar (Lovett and Price 2001) and cotton (Lovett et al. 2003). When planting a
riparian buffer to stabilise a streambank, a suggested rule of thumb (Abernethy
and Rutherfurd 1999) is that the buffer width should be 5 metres from the
bank crest, plus a distance equal to the height of the bank (bank crest to bank
toe), plus the amount of erosion that would be expected before the vegetation
becomes effective (Figure 11.16).
Figure 11.16: Features used to describe stream dimensions (Abernethy and Rutherfurd 1999)
11.5.5 Establishing vegetation
A diversity of vegetation types is required to successfully stabilise a stream.
Under natural conditions, healthy riparian vegetation contains a mixture of
species and the aim should be to mimic this in restoration projects. Where
engineering approaches are used, vegetation needs to comprise species that
are suitable for combining with structures to help reduce flow velocities and to
increase the strength of the structure.
Vegetation planted on the slope and top of the bank should have a rooting depth
greater than the height of the bank. This is because if the plant roots do not cross
the potential slump area of the bank, they will have limited ability to reduce this
form of erosion. The influence of tree roots can extend for 10–15 metres from the
trunk and to a depth of at least 1.5 metres, depending on tree size (Abernethy
and Rutherfurd 1999). For the purposes of bank stability applications, the roots
of large trees are considered to have limits of about 3 metres deep and a lateral
extent equal to about that of the crown (Rutherfurd et al. 2000). Extending tree,
shrub and grass plantings over and beyond the bank top will provide additional
protection from bank collapse by reducing the growth of tension cracks on the
bank top.
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Rutherfurd et al. (2000) describe a technique of establishing a sacrificial
zone that is revegetated with fast-growing species that will slow the erosion
sufficiently for the larger, slower-growing species to establish further back from
the bank top (Figure 11.17).
Figure 11.17: A ‘sacrificial zone’ providing protection until slower-growing trees are large enough to stabilise the bank
(Rutherfurd et al. 2000)
Planting techniques
Streambanks are usually steep and difficult to access for revegetation. The bank
toe is the most difficult part of the bank to revegetate, especially with woody
species, and it is often best to start with water-edge grasses, sedges or similar
plants. One approach to achieve this can be to travel downstream by boat,
planting seedlings into the moist part of the bank by hand. Rutherfurd et al.
(2000) describe an example in Victoria where truckloads of Phragmites reeds
(Phragmites australis) were mixed with rock used to stabilise the eroding toe
of a streambank. In this example, the rhizomes of the reeds quickly became
established to the extent that the rock work was soon barely visible amongst the
reeds.
Local native plant species can be established in riparian areas by either:
• planting local native seedlings and spreading seed from local native stream
plants, or
• promoting natural regeneration by controlling stock access.
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Chapter 11 Stream stability
Either way, it is essential that weeds and fire be controlled. If it is necessary to
control weeds before planting, this should be carried out in stages. Removing all
of the weeds across an extensive area at the same time may create a high erosion
risk. Weeds can be controlled in stages by the strategic use of appropriate
herbicides. Spot spraying might be more appropriate than broadcast spraying a
large area. When controlling weeds very near to watercourses, woven weed mats
or mulch can be used to reduce competition for new plantings as an alternative
to spraying. Burning may be another option for weed control in some instances.
Water-jetting of long-stemmed tubestock native species is another useful
technique for streambank plantings. One of the main benefits of using the longstem method within the riparian context is that the roots of seedlings are planted
more deeply into the river bank, helping the seedling to resist being washed
away during a flood event. Long-stem planting also protects the root ball from
drying out and from temperature extremes such as frosts. Both of these can
damage seedlings planted using traditional methods.
A compost blanket provides an excellent medium for plant establishment. Also,
a mat of suitable geotextile placed on the streambank provides additional
protection from erosion until plants become well established. This will protect
planted batters against both raindrop impact and overland flow. Supplementary
watering should be minimised both for cost and for long-term plant hardiness.
The need for supplementary watering can be avoided by not planting during hot,
dry periods. In southern areas of Queensland, the autumn months when soil
moisture levels are good and temperatures are less extreme can be a good time
for planting. New plantings should also protected from browsing and physical
damage by livestock. Solar-powered electric fencing can be a cost-effective
option for this.
Plant selection
For a state as large and with as much variation in climate, soils and
aquatic environments as Queensland, it is not possible to make general
recommendations regarding selection of species for riparian revegetation.
Instead, it is recommended that advice be sought from experienced locals, such
as from regional natural resource management groups and/or from published
information. Localised planting guides have been published for many parts of
the state. A number are listed in the ‘Further information’ section at the end of
this chapter. A useful guide can also be just observing what is growing in similar
situations nearby, or noting the descriptions provided for vegetation in the area
from the Queensland Government’s regional ecosystem mapping program.
Particular attention is required when selecting plants to establish on
streambanks. Native grasses and smaller trees and shrubs are most suitable for
these areas. Larger trees should be planted away from the bank and well clear
of the stream edge so they are not likely to topple into the waterway, taking
part of the bank with them. It is best to use a mix of fast-growing ‘pioneer’
species (grasses and rushes such as Lomandra sp. and Dianella sp.) which will
quickly occupy the site, along with slower-growing ‘secondary’ species (such
as banksias, tea trees and melaleucas) that will provide longer-term protection.
Once the appropriate species have been selected, plants can be sourced through
local native plant nurseries or by propagation from locally collected seeds.
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11.6 Engineering approaches to stream stabilisation
This section describes ‘hard’ engineering approaches to stabilise the bed
and banks of streams. Such approaches would normally be combined with
revegetation. Professional engineering advice should be sought before
attempting to implement any hard engineering practices. Also, any relevant
legislation, such as the possible requirement for a Riverine Protection Permit
under the provisions of the Water Act 2000, should be checked before
commencing construction. Legislative requirements are discussed in detail later
in this chapter. The authors do not profess to be experts on this subject. They
have drawn heavily on other work referenced in the following sections.
Stream stabilisation projects need to consider both the streambed and the
streambank. Bed erosion is a common problem associated with incised streams.
Continued erosion and deepening of the stream is likely to threaten any projects
to stabilise the streambank. If a streambank is eroding and no action is taken,
it may eventually only reach stability by significantly increasing the width
of the stream, or even by changing the stream course. In either instance the
implications are likely to be serious.
When considering hard engineering solutions for stream stabilisation it is
also important to take into account the potential impact on other values.
Structures such as steep chutes and drop structures are not fish-friendly. It
may be necessary to design such structures to cater for fish passage. Another
consideration is erosion outside of the stream. Road culverts and floodways can
provide an effective bed erosion control structure by causing sediment to be
deposited above them. However, they can also cause erosion below their outlet.
11.6.1 Controlling streambed erosion
Where a stream is incised, it is important to determine whether the incision is
systemic (i.e. throughout the whole channel network) or restricted to a local
reach (Brooks 2006). In fully alluvial rivers without major bedrock controls in
the river bed channel, incision tends to be systemic. Under these circumstances
bed erosion can be more difficult to treat as it is often associated with upstream
migrating knick points, major bank erosion and downstream migrating sand or
gravel slugs resulting from the deposition of the liberated stored alluvium. It is
worth noting that a knick point can have passed through a reach at some stage in
the past but leave little evidence of current knick point activity.
Incision restricted to a reach is more likely to be related to a local disturbance,
such as de-snagging or local channel shortening associated with an artificial
cut-off. In fully alluvial rivers these types of disturbances can sometimes trigger
systemic channel incision. However, in rivers that have considerable bedrock
controls, such as rock bars which prevent longitudinal propagation of the
incision, the same disturbances normally result in local incision only.
The following are some of the practices that can contribute to streambed erosion:
• straightening of streams
• extraction of sand and gravel
• removal of bed control measures such as reeds
• excessive de-snagging and/or removal of vegetation from the channel.
Treating bed erosion requires that the energy of the stream be dissipated. This
involves measures that change friction and/or turbulence. The presence of a
scour hole in a streambed usually indicates that action is required to reduce
turbulent energy. Surface roughness provided by vegetation and some types
of structures will dissipate energy by increasing frictional losses. Alternatively
structures with plunge pools dissipate energy through turbulence.
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Chapter 11 Stream stability
Vegetation generally has a positive influence on bed erosion by increasing
channel roughness. However, dense vegetation in the middle of the channel
can divert flows towards the banks, thus increasing bank erosion. Midstream
vegetation can develop during a prolonged period of low flow or where sand or
gravel bars have built up. Under some circumstances selective clearing of this
regrowth can be desirable for river management purposes. Excessive clearing,
however, can trigger bed lowering and bank erosion. Advice should be sought
from the Department of Natural Resources and Mines, and a permit may be
required, for any clearing of native vegetation.
Rock chutes and drop structures
Rock chutes and drop structures are armoured drops in the beds of small
streams (Kapitzke et al. 1998). They are introduced to control the gradient in
the bed to dissipate energy and to prevent head cuts from moving upstream.
Rock chutes and drop structures are usually placed in straight sections of the
stream and are constructed with abutments to protect the streambanks. They
may be built as individual ‘stand-alone’ structures or as a series. Building
such a structure requires considerable excavation and disturbance which can
destabilise the stream. A stable bed profile can be reinstated after the structures
have been installed by deposition between the structures. The crest of a
structure is also sometimes curved downward toward the centre to direct energy
towards the centre of the downstream channel. A layer of suitable geotextile can
also be used to separate the soft streambed from the rock used in a chute or
gabion mattresses to reduce the amount of rock required and increase the life
span of the structure.
A series of rock chutes can be constructed to imitate the systems of pools and
riffles that occur naturally in streams (Figure 11.18). They provide benefits for
both stream stability and habitat. Under low flows a reasonable depth of cooler
water is stored behind the riffle. Shallow flows passing over riffles are aerated
where hydraulic jumps or whitewater plumes occur. When flows increase in
depth, sands and gravels accumulate in the pools. Under even higher flow
conditions, the riffles or rapids provide protected areas or resting places
for aquatic species in the dead zone behind boulders and other obstacles.
An increase in shear stress removes the fine sediments and debris that has
accumulated between floods. The increase in shear stress is less in the riffle
zone because the slope of the water surface decreases at high flows.
Figure 11.18: A series of pools and riffles in a stream (Newbury and Gaboury 1993)
Low weirs, riffles and rock chutes are generally constructed using quarried rock.
If the rock is well graded it packs well, which means it will resist interstitial water
flow and ‘plucking’ of rocks from the surface. When building a structure using
rock, the core of the base is usually constructed from smaller, well-packed rock,
with larger material used on the surface to provide roughness and to create
local hydraulic diversity (Rutherfurd et al. 2000). Rock boulder structures are
constructed by placing large boulders across the streambed and then packing
well-graded, smaller rocks between them.
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The row of well-packed boulders across the bed provides a low dam. Grouted
rock chutes can also be substituted when large rock is not available.
Constructing a chute with a flat crest allows better energy dissipation at the end
of the chute whilst including some large rocks along the crest with gaps between
them will provide passage for fish.
Drop structures are vertical or near-vertical drops that are usually constructed
from rock gabions (flexible) or concrete (rigid). Drop structures require a pool
for energy dissipation at the foot of the structure. Waterfalls are an example
of a natural drop structure. Rock gabions are wire mesh baskets that are filled
with local stone and rubble. This material can often be salvaged from a point bar
nearby. To prevent them being outflanked, gabions are riprapped immediately
upstream and downstream of where they are connected to the streambank. As
the life of the mesh is limited, gabions are best used where the material will be
relatively quickly stabilised by vegetation (Rutherfurd et al. 2000).
Backwater pools can form upstream of partial, or full-width structures during
flood flow because they constrict the channel cross-section. Figure 11.19 shows
this effect resulting from the depth of water flowing over a dam spillway. Note
that the extent of the backwater is greater than that predicted by extending a
horizontal line from the depth of flow over the spillway. Rutherfurd et al. (2000)
described methods of predicting backwater but also pointed out that it can be
difficult to determine accurately.
Figure 11.19: Water flowing over a dam spillway creates a backwater effect (Rutherfurd et al. 2000)
During periods of low flow, backwater pools are created only behind full-width
structures. In streams that transport substantial amounts of sediment low
velocity conditions in the backwater will encourage bedload to be deposited.
This will eventually fill the backwater pool. In Figure 11.20, Structure 1 creates a
backwater while Structure 2 produces no backwater because it is covered by the
backwater produced at the road crossing.
Figure 11.20: The backwater from a downstream structure can cover an upstream structure (Rutherfurd et al. 2000)
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Chapter 11 Stream stability
Any structure or object placed in a stream is likely to influence the direction
and velocity of flows in the stream. It is likely that scouring and subsequent
deposition around and below the obstruction (Figure 11.21) will occur as a result.
Figure 11.21: An in-stream structure affects flow depth and velocity as well as the streambed (Rutherfurd et al. 2000)
A sediment-laden stream can create a scour below a structure on the rising arm
of a flood (Figure 11.22). Deposition will fill the hole during the falling arm, as the
flood recedes.
Figure 11.22: Scour and sediment redeposition at the rise and fall of a flood (Rutherfurd et al. 2000)
Designing rock chutes and drop structures
When rehabilitating a stream it is useful to predict the shape, size and location
of scour holes. This information will allow designers to determine if a proposed
structure will be threatened by future bed scour and/or to determine by how
much a pool area is likely to increase after a structure is installed (Rutherfurd
et al. 2000). It is easiest to predict in rivers with gravelly beds. In rivers with a
sandy bed, scour pools which form at the height of a flood are often filled in as
the flood wanes or by smaller events shortly afterwards. In instances such as
this, the bed profile surveyed at a point in time between flood events is unlikely
to represent the maximum scour depth.
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The potential scouring caused by a proposed structure is related to factors such
as the following:
• type of bed channel (sand, gravel or cohesive). Streams with coarse bed
material will tend to armour the scour hole, limiting scour formation
• anticipated flow velocity, channel roughness and capacity
• critical tractive stress i.e. the shear stress at which the particles on the
channel bed and bank are on the verge of motion
• depth that a structure protrudes into a stream (e.g. a short groyne compared
with a sill across the full width of a stream).
• height of the structure relative to expected depths of flows
• extent to which the structure is submerged
• location of the structure in relation to bends in the stream
• angle of the structure in relation to the direction of flow
• downstream face angle of the structure. For example, a rock chute with a
gently sloping downstream batter will generally not produce a scour hole
because energy is dissipated on the face of the structure and not on the
downstream bed. This contrasts with structures such as log sills that have a
steep downstream angle often resulting in considerable scour
• likely incidence of backwater effects. Backwater effects behind full-width
structures cause suspended sediment to be deposited. A series of such
structures can starve downstream reaches of sediment, resulting in clearwater scour. In streams with high sediment loads, any backwaters are likely to
quickly fill with sediment allowing bed-load to subsequently pass over them.
11.6.2 Bank stabilisation
As discussed in the previous section, it is important to take into account the
condition of the bed of a stream when planning a streambank stabilisation
project. Ongoing erosion and deepening of the bed can threaten bank
stabilisation projects. There are many examples where streambank stabilisation
works have ended up stranded well above the level of the stream because of bed
erosion following their construction (Peter McAdam personal communication).
High levels of bed roughness, as may result from excessive reed growth or other
vegetation, can lead to scouring of streambanks. The following approaches aim
to stabilise the bank whilst keeping the stream in its current course and reducing
the stream width.
Modifying the slope
When stabilising a steep streambank the first step is often to reduce the slope
of the bank. This involves reshaping the bank to a modified slope and then
stabilising it, usually by placing rock at the toe and on lower sections of the bank
and revegetating the upper sections. When reshaping the bank, a batter of 1:2
(vertical:horizontal) is suitable for moderately stable soils, 1:3 for less stable
soils and 1:4 for unstable soils as shown in Figure 11.23 (Wilson, 2013). When
constructing steeper slopes, cut-and-fill operations are required (as shown for
the 1:2 batter). In these instances filled areas will need to be compacted that,
given the steepness, will require specialised equipment. Where the streambank
comprises dispersive soils, special precautions will be required to avoid
exposing the soil to runoff and raindrop impact and potentially worsening the
erosion problems. The flatter the batter slope, the greater the width required
to reshape a bank and the greater the amount of soil and vegetation that will
need to be removed and relocated. Topsoil should be stripped separately and
stockpiled to be respread at completion of the earthworks prior to planting
vegetation. Excess fill can be used to stabilise other sections of the bank.
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Chapter 11 Stream stability
Figure 11.23: Different batter slopes used in reshaping a streambank
When reshaping a high bank (i.e. > 2 m high), it can be a good idea to construct a
bench, berm or terrace halfway up the slope (Figure 11.24). This provides easier
access to assist in the construction, to stabilise the toe of the bank and to
establish vegetation. Where modifying only the slope above the bench reduces
the weight sufficiently to stabilise the bank, it is possible to leave the balance of
the slope undisturbed (Figure 11.24b). If this is not the case then slopes will need
to be modified both above and below the bench (Figure 11.24c). Where the soil is
dispersive it is strongly recommended that water not be allowed to pond on the
bench. Ponding of water can be avoided by making the bench impermeable (e.g.
by surfacing it with imported material) or by providing surface and subsurface
drainage.
Figure 11.24: Use of a bench to modify the slope of a streambank
Where space is limited (e.g. the floodplain is narrow or infrastructure is in the
road) and it is necessary to use steep batters, an alternative is to construct one
or more retaining walls on the embankment and to backfill behind them. The
retaining wall could be constructed from rocks or gabions and include provision
to protect the toe of the bank as described earlier. This is a more expensive
approach and would normally only be used to protect high-value assets.
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Stabilising the toe and lower slopes
This section describes the use of rock protection and engineered log jams to
stabilise the toe and lower slopes of a streambank. If the toe is not protected it
may continue to scour, undercutting the base of any engineered protection works
such as rock riprap that may be in place further up the bank, causing them to
eventually fail.
Rock protection
The toe of a bank can be protected by armouring the bank with some form of rock
protection. This rock work is referred to as a revetment. Normally revetment is
only used to protect the lower one third to one half of the bank with vegetation
used to protect the upper section. Revetment on the toe of the bank needs to
extend down to the bed of the stream (Figure 11.25) to prevent the rock from
sliding away. A geotextile layer installed behind a rock wall helps to extend the
life of the wall by relieving hydrostatic pressure and preventing movement of the
bank soil.
Figure 11.25: Erosion control on a streambank using rock for toe protection (Witheridge 2010)
In preparation for bank stabilisation earthworks, a rock toe can be built in
advance in the streambed and extending some distance out from the bank
(Figure 11.26). The bank can then be battered off and stabilised with vegetation.
Figure 11.26: Building a rock toe in a streambed (Rutherfurd et al. 2000)
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Chapter 11 Stream stability
Rocks in revetments will be exposed to the greatest hydraulic forces on the
outside bends of streams where stream velocities are highest. If there are air
spaces or voids in rock walls, floodwaters rushing past can create a build-up of
pressure causing the rock to move. The movement of large rocks and boulders by
stream waters is aided by Archimedes’ principle. Archimedes’ principle (upthrust
= weight of fluid displaced) explains why rocks weigh less when immersed in
water. Also, rocks may vary in density depending on the type (e.g. pumice rock is
so light that it floats on water). Rocks also vary in durability depending on their
type and the extent to which they may be weathered.
To increase the strength of a wall, the voids between the rocks can be filled with
soil to allow the pocket planting of vegetation such as shrubs and persistent
herbaceous plants (such as Lomandra sp). Vegetation such as this increases
the roughness of the wall, reducing flow velocities. The root growth between
the rocks also provides additional strength. Whilst smaller rocks are more
likely to be moved by fast flows, this can be compensated by the fact that walls
constructed from smaller rocks contain more voids in which to plant vegetation
to provide additional strength and protection.
Gabion rock baskets or crib walls can be used as a revetment on the outside
bends of streams. However, if used without vegetation, they have very little
surface roughness. This leads to high velocities which are likely to cause erosion
when the flow reaches the end of the gabion wall. A further disadvantage of
gabions is that they are made with steel wire which can rust and eventually fail.
The longevity of the gabion baskets needs to be considered.
Rigid revetments such as concrete retaining walls, interlocking sheet piling and
grouted rock are not compatible with vegetation, and their low level of roughness
can increase flow velocity and lead to erosion downstream. The rigidity of these
structures also makes them susceptible to cracking if the bank moves and/or is
destabilised by tree roots. When constructing structures with these materials,
some form of subsurface drainage may be required as the structures are likely to
inhibit subsurface water seepage from the bank.
Construction of rock protection structures generally requires extensive
disturbance of the streambank. This can expose soil to erosion and damage
habitat. Disturbance of streambank vegetation and habitat may be minimised by
carrying out construction from barges, the streambed or from the opposite bank.
Dispersive soils
Unstable soils, such as those with high levels of sodicity, are very erodible and
difficult to stabilise with vegetation due to their dispersive nature and hostile
subsoil. This can be a particular problem when carrying out works that involve
disturbing streambanks comprising sodic soils. Dispersed soil particles are
extremely fine and can pass through a geofabric. When working on a streambank
comprising such soils they need to be covered with a 20 cm layer of stable, nondispersive or dispersive soil that has been treated (e.g. with gypsum) before any
filter cloth and rock protection is put in place and they are revegetated (Figure
11.27). Where soils are dispersive, a low batter slope of at least 1:4 should be
used or if a steeper gradient is unavoidable, a rock retaining wall.
11–37
Figure 11.27: Bank stabilisation on a dispersive soil
Engineered log jams
The important role that woody debris plays in stream stabilisation has been
discussed earlier in this chapter. Woody debris is often in short supply in streams
due to past de-snagging, vegetation clearing in the catchment, or altered
hydraulic regimes (e.g. construction of artificial impoundments). This has led
to the practice of deliberately introducing large woody debris into streams to
restore natural conditions. Where the debris is specifically designed to achieve
pre-determined goals, rather than just randomly placed within a stream, it is
called an engineered log jam (ELJ). Well-designed ELJs will deflect the low-flow
channel away from the base of the bank and reduce scour energy at the bank
face. They will also provide biodiversity benefits such as valuable fish habitat.
This contrasts with ‘hard’ structures such as rock revetment which generally
reduce friction, increasing flow velocity and maintaining the same flow path.
The practice of reintroducing woody debris as a river rehabilitation strategy
is still very much in its infancy. Often the biggest drawback when using this
technique is difficulty in finding suitable quantities of appropriate large woody
material (logs, branches and roots) close enough to the site where they are to
be used. The best material to use in Queensland is from local hardwood timber,
species such as Eucalyptus sp. which are physically strong and resist decay.
Where this type of material is used the timber structure should be functional over
timeframes of at least 50 years.
Under natural conditions, log jams are stabilised by tree-root wads buried into
the riverbed and interlocked with logs and large branches. Accumulation of the
ballast from sediment deposition then provides a substrate for vegetation to
colonise the whole structure. Where the same principles for structural stability
have been applied in design of the engineered versions, they have proven to be
effective as a bank erosion-control device and fish habitat structure, particularly
in moderate to high energy gravel-bed river settings (Brooks 2006). However,
there are far fewer successful examples of the use of ELJs in sand-bed settings
where high energy flows are experienced. At the very least, additional anchorage
provided by driven piles or some alternative measure would be required in such
situations.
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Soil conservation guidelines for Queensland
Chapter 11 Stream stability
Engineered log jams are normally placed in a series on the outside bends of
curves. Logs with intact root wads placed to face upstream should be used
as the primary structural elements of the engineered log jam (Figure 11.28).
This is because logs with root wads anchor themselves into the bed of a river
in much the same way that boat anchors dig themselves into the seabed with
a force applied to the anchor chain; and like anchors, once buried, they are
difficult to dislodge. Burying a significant proportion of the structure into the
streambed and banks to a depth greater than the predicted scour depth, will
help ensure that it is adequately ballasted. This will reduce the risk of failure
if some undercutting occurs during a flood. Backfilling the completed jam with
gravel will provide additional ballast. The overall size of the log jam required will
vary with its location. However, where the primary role is bank protection it is
recommended that the structure should extend to at least half the height of the
most effective flood stage (Abbe et al. 1997).
Figure 11.28: Engineered log jam: a) section view looking towards bank, b) plan view (Brooks 2006)
(a)
(b)
In some instances cables have been used to strengthen ELJs. Using cables in this
way has advantages and disadvantages (Brooks 2006). Experience has shown
that cables add little to the stability of ELJ structures in rivers that have gravel
beds. This is believed to be because a flexible medium such as a cable will not
prevent wood from moving up and down, or side to side, with fluctuating stage or
turbulence. This movement of the wood will cause the cable to also move, setting
up an oscillation or vibration that will tend to cut away the material within which
it is set. As a result, the cable can become exposed to create an entanglement
hazard, or simply fail, liberating the log that it was intended to secure. It is
considered that a cable should only be used to secure logs tightly to one another
(such as when securing the top layer of logs in place) or directly to rock ballast so
that all the components act as one unified structure.
Following the first series of floods, after some sediment has been deposited, it
should be possible to start planting vegetation in and around the engineered log
jams and along the base of the protected cut bank. This will help ensure the longterm stability of the structure. Also, with time, a bench of deposited sediment is
likely to form along the bank downstream of a log jam.
Stream alignment training and bank protection
Revegetation strategies and bank protection works are commonly integrated
with alignment training structures. Alignment training structures can take the
form of rock groynes, pile groynes, retards, embayments, gabions, reinforced
concrete walls, randomly stacked concrete blocks or coconut fibre rolls.
Construction of these structures is also commonly preceded by earthworks to
construct a berm or terrace to provide a stable base to work on.
11–39
Alignment training structures are ‘partial width’ bank erosion control structures
in that they project from a bank into the stream to reduce flow velocity adjacent
to the bank.
Alignment training structures are typically located in the low flow channel on the
outside of bends of meandering streams where deeper water abuts an eroding
bank (Kapitzke et al. 1998). They function by diverting currents to protect an
eroding bank or a piece of infrastructure such as a bridge. Because they reduce
flow velocities, alignment training structures encourage sediment deposition
which in turn encourages vegetation to establish near parts of the bank and
provides habitat for aquatic fauna (such as small molluscs, crustaceans and
insects). The structures also reduce the rate at which the stream width increases
and induce channel deepening or constriction.
The terms retard and groyne are often confused (Rutherford et al. 2000). ‘Groyne’
refers to solid deflection structures that extend to almost the full height of the
bank whilst ‘retard’ refers to permeable structures that usually extend only part
way up the bank. Brush groynes and pile groynes are in fact retards rather than
groynes.
Alignment training structures can cause erosion by inducing local scour
downstream and at the outer end of the structure. Groynes are more likely to
cause scour damage as they are impermeable. A permeable structure such as a
‘field’ of piles is less likely to cause such problems. A solid groyne is also less
suited to very deep water situations because it requires much more material and
is more demanding to construct. However, permeable structures can become
impermeable over time, for example, when debris lodges on piles during
flooding, which can lead to bed and bank erosion.
Groynes (dykes)
The main function of a groyne is to reduce water velocities and shear stress
in the vicinity of the eroding bank. To achieve this they are usually located on
an eroding bend and aligned at an obtuse angle (e.g. 120 degrees) to the bank
to extend into the stream channel but not cause excessive turbulence and
direct flows into the bank. Rock groynes are impermeable alignment training
structures. They are similar to gabions, reinforced concrete walls and randomly
stacked concrete blocks. Because they are impermeable, vegetation is not
readily established as part of a groyne. Pile groynes can be used to fulfil a similar
function. Pile groynes are permeable. They can be constructed of timber, steel or
concrete. If timber is used it should be rot-resistant and where treated, non-toxic
chemicals should have been used.
Groynes should be constructed to a height above the low-flow levels but below
the design flood level and the top of the bank. This is in order to minimise the
risk of break-out. They normally slope upward, abutting the bank at a greater
height than the in-stream end. Rock groynes can be combined with pile groynes
by constructing the rock section adjacent to the bank and the pile section in the
deeper water farther from the bank.
Piles are commonly used for securing structures in riverine and marine projects.
Piles are most effective when supporting a vertically applied load. However, they
can also be used for securing lateral loads such as woody debris in a stream
stabilisation structure. When using piles to stabilise a stream, the specifications
of the piles is usually somewhat arbitrary. This is because in practice the
size of piles and the depth to which they are driven is usually determined by
the available machinery, piles and depth of sediment to bedrock rather than
the engineering requirements. If an engineering specification is required, it
is necessary to first estimate the maximum depth that the bed is likely to be
scoured at the design discharge. Once this is established, the ideal depth that
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Soil conservation guidelines for Queensland
Chapter 11 Stream stability
piles should be driven is determined as the depth required to prevent rotational
displacement of the pile under a given lateral load.
Retards
Retards are fence-like structures with posts and rails that extend from the bank
towards the centre of the channel. The gaps can be filled with timbers or brush.
Brush retards are low-cost structures built from locally available materials. They
comprise tree and shrub branches secured between timber posts. The retard
should be embedded into the bank and the bank protected against erosion
where the structure abuts it with rock. Retards are sometimes used as temporary
structures to stabilise the stream long enough to allow revegetation. Vegetation
is normally encouraged between the retards to increase sediment deposition.
Velocities can be increased around the base of retards and at abutments.
Retards placed in a grid pattern form embayments. Vegetation should be
established within the grid to encourage sediment deposition before the fences
rot away. Embayments can comprise mesh fences, timber fences and jacks. Local
scour may occur under embayment fences and at abutments. Because they are
permeable, embayment fences may catch debris during flooding, leading to bed
and bank erosion and damage to the fences.
Jacks are inexpensive structures that are usually constructed of three pieces
of timber fastened at the midpoint so that each piece is perpendicular to the
other two (Rutherford et al. 2000). Two jacks are sometimes joined together by
a longer piece of timber to form a structure resembling a hurdle or horse jump.
Jacks are normally placed in an array along an artificial bench at the toe of an
eroding bank. The jacks within the array are then fastened together by cable, and
anchored individually to the bank by cable attached to ‘dead men’ (buried logs).
Jacks work by increasing channel roughness. They are more porous than
traditional retards and are spaced more closely to achieve the low-velocity
conditions required behind the structures. They are used on sediment-starved
streams to reduce the flow velocity, allowing vegetation to establish. Unlike
other types of retards, jacks are not attached directly to the bed which means
they are useful in areas prone to bed scour. In such situations the jack simply
rides the scour down to the new level of the scour hole. This versatility makes
jacks applicable in streams with highly mobile beds, such as those with sand
beds or those which are incising.
Coconut fibre rolls
Coconut fibre rolls (also called coir logs) are cylindrical structures composed
of coconut husk fibres bound together with twine. They can be stacked and are
staked in place at the toe to protect the stream bank. The flexibility of coconut
fibre rolls allows them to conform to the shape of the bank. They commonly
come in sizes of 200–300 mm diameter and 3 m in length. They provide a good
environment for plants to grow and to provide bank protection once the roll has
decayed.
Streambank drainage
The additional weight contributed by the water contained within the soil of a wet
streambank is a key factor contributing to slumping. Streambanks are most likely
to become saturated as a result of high flood levels. In some cases, the slumping
will occur a day or two after the flood level has receded. High watertables in
the adjacent aquifer are less likely to be the source of soil moisture causing
slumping in a streambank than in other situations, because most watertables on
floodplains are below the level of base flows in the adjacent stream. However,
under sustained flood conditions aquifer levels may rise and then drain back
11–41
into the stream as the flood recedes. Where there are alluvial soils with good
drainage properties, streambanks should drain reasonably quickly because the
bank is exposed. Vegetation on a bank will also help to remove soil moisture
through transpiration, and their fine root systems will create voids and improve
drainage.
Where drainage is considered inadequate and slumping is a risk, consideration
should be given to installing subsurface drainage as an adjunct to other stream
stabilisation work. Subsurface drainage will increase the speed and efficiency
with which moisture leaves the streambank (Kapitzke et al. 1998). If the slope
of a streambank has been modified by the construction of a berm, it may be
necessary to provide subsurface drainage in the lower section of the bank (Figure
11.29). This involves laying slotted drainage pipes wrapped in a geotextile filter
sock in a trench filled with sand or gravel. These drainage pipes can be laid in
a longitudinal direction parallel to the bank or laterally (as in Figure 11.29). It
should be noted that this diagram is purely hypothetical and is not based on any
real experience of the authors.
Figure 11.29: Streambank drainage installed laterally
When installing drainage, outlet pipes should be laid above base stream-flow
height and protected from scour damage. Entry of animals should also be
restricted. Pipes should be laid on an incline to drain naturally and aligned
slightly downstream to prevent streamflow forcing water into the pipes. Drainage
pipes laid laterally may require collars to prevent subsurface flow of water in the
trench alongside the pipe. It may also be useful to include provision to flush the
pipes to increase the effective life of the drainage system. Berms and modified
slopes need to be revegetated but in selecting and placing plants it is important
to take into account the risk that plant roots could block drainage pipes. This risk
is greater when water hungry species (such as some trees) are planted close to
the pipes.
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Soil conservation guidelines for Queensland
Chapter 11 Stream stability
11.7Legislation
Many pieces of legislation may contain provisions that potentially impact on
stream stabilisation work. At the time this material was being prepared the
following legislation (including amendments) was considered to be relevant:
• Water Act 2000
• Vegetation Management Act 1999
• Sustainable Planning Act 2009
• Land Act 1994
• River Improvement Trust Act 1940
• Land Protection (Pest and Stock Route Management) Act 2002
• Fisheries Act 1994
• Nature Conservation Act 1992
• Environmental Protection Act 1994
• Chemical Usage (Agriculture and Veterinary) Control Act 1988.
Governments make frequent changes to legislation. It is not possible in a set of
guidelines such as these to maintain an up-to-date description of all the relevant
provisions. Accordingly it is recommended that before undertaking any stream
stabilisation works advice should be sought from the appropriate authorities
regarding the most current applicable legislative provisions. At the time of
writing the most appropriate authorities include the Department of Natural
Resources and Mines, the Department of Environment and Heritage Protection,
and the relevant local council.
11–43
11.8 Maintenance
In-stream and riparian environments can vary greatly throughout the year
and from year to year. Vegetation and structures in streams and/or riparian
environments can be exposed to extreme conditions such as a flood or an
extended drought. It is very important that stream stabilisation be monitored
and maintained regularly.
Planted areas can soon become overrun by weeds so weed control is essential,
especially in the early years, until plants become established. Plants may
sometimes require strategic watering depending on seasonal conditions. The
aim with any revegetation is to achieve a largely self-sustaining balance of trees,
shrubs and groundcover species to protect the surface, reduce water velocity
and ‘hold’ the soil. Ongoing monitoring and management is critical to maintain
this balance.
Engineered structures need to be monitored regularly, especially after significant
streamflow events, to ensure that they are still functioning as designed.
Maintenance should include:
• removing any debris that becomes entangled in the erosion control material
and could damage the bank
• replacing missing or damaged erosion control materials during times of low
stream flow
• weed and pest animal control
• restricting livestock from steep banks and from areas containing the erosion
control measures that are sensitive to disturbance.
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Soil conservation guidelines for Queensland
Chapter 11 Stream stability
11.9
Further information
References
Abbe, TB, Montgomery, DR and Petroff, C (1997) ‘Design
of stable in-channel wood debris structures for bank
protection and habitat restoration: an example from the
Cowlitz River, WA’, in: SSY Wang, E. Langendoen and
FD Shields, Jr (Eds), Proceedings of the conference on
management of landscapes disturbed by channel incision,
University of Mississippi, pp. 809–16.
Abernethy, B and Rutherfurd, I (1999) Guidelines for
stabilising streambanks with riparian vegetation,
Technical Report 99/10, Cooperative Research Centre for
Catchment Hydrology, Canberra.
Brooks, A [with contributions from Abbe, T, Cohen,
T, Marsh, N, Mika, S, Boulton, A, Broderick, T, Borg,
D and Rutherfurd, I] (2006) Design guideline for the
reintroduction of wood into Australian streams, Land and
Water Australia, Canberra.
Department of Environment and Resource Management
(2011) Managing grazing lands in Queensland,
Queensland Department of Environment and Resource
Management, Brisbane.
Howard, A, Raine, SR and Titmarsh, G (1998) The
contribution of stream bank erosion to sediment loads
in Gowrie Creek, Toowoomba, ASSI National Soils
Conference, Brisbane, volume 2004, pp. 491–493.
Hjulström, F (1939) Transportation of detritus by moving
water: Part 1. Transportation, pp. 5–31 in: P. D. Trask
(Ed.), Recent marine sediments, American Association of
Petroleum Geology, Tulsa, Oklahoma.
Kapitzke, IR, Pearson, RG, Smithers, SG, Crees, MR,
Sands, LB Skull, SD and Johnston, AJ (1998) Stream
stabilisation for rehabilitation in north-east Queensland,
Land and Water Resources Research and Development
Corporation, Occasional Paper 05/98, Canberra.
Land and Water Australia (2004) Stream stability, Fact
sheet 2, Riparian Management Series, Land & Water
Australia, Canberra.
Layden, I (2011) Wetland management handbook: Farm
management systems (FMS) guidelines for managing
wetlands in intensive agriculture, Department of
Employment, Economic Development and Innovation,
Queensland Wetlands Program, Brisbane.
Lovett, S and Price, P (Eds) (1999) Riparian land
management technical guidelines: Volume 1, Principles of
sound management; Volume 2: On-ground management
tools and techniques, Land and Water Resources Research
and Development Corporation, Canberra.
Lovett, S and Price, P (2001) Managing riparian lands in
the sugar industry: A guide to principles and practices,
Sugar Research and Development Corporation/Land and
Water Australia, Brisbane.
Lovett, S, Price, P and Lovett, J (2003) Managing riparian
lands in the cotton industry, Cotton Research and
Development Corporation, Narrabri, New South Wales.
Newbury, R and Gaboury, M (1993) Stream analysis and
fish habitat design—a field manual, Newbury Hydraulics
Ltd, British Columbia, Canada.
Queensland Government (2009) What causes streambed
erosion? Fact sheet R20, searchable at <www.dnrm.qld.
gov.au> (accessed 5 July 2015).
Rutherfurd, ID, Jerie, K and Marsh, N (2000) A
rehabilitation manual for Australian streams Volumes
1 and 2, Cooperative Research Centre for Catchment
Hydrology, Land and Water Resources Research and
Development Corporation, Canberra.
Rutherfurd, I, Anderson, B and Ladson, A (2007)
Managing the effects of riparian vegetation on flooding,
in Lovett S and Price P (Eds), Principles for riparian lands
management, Land and Water Australia, Canberra.
Wilson, PR (2013) Floodplain management in the Burnett
Catchment, Burnett Mary Regional Group <bmrg.org.au/
files/9713/7827/4601/Flood_plain_managementa.pdf>.
Witheridge, GW (2010) Watercourse erosion, Part 1, Fact
sheet, Catchments and Creeks Pty Ltd, Brisbane.
Other information
Specific information on the role of riparian and floodplain
vegetation can be found in:
• Chapter 5 (Managing the effects of riparian vegetation
on flooding) in: Lovett, S. and Price P. (Eds) (2007)
Principles for riparian lands management, Land and
Water Australia, Canberra.
• A rehabilitation manual for Australian streams
(Rutherford et al. 2000, listed in References above).
• Stream stabilisation for rehabilitation in north-east
Queensland (Kapitzke et al. 1998, listed in References
above).
• The technical report, Guidelines for stabilising
streambanks with riparian vegetation (Abernethy and
Rutherfurd 1999, listed in References above), was
prepared for the Queensland Government and should
be consulted for more detailed information on this
topic.
11–45
Before considering a program for riparian revegetation,
the condition of the existing vegetation should be
assessed. Information on this topic can be found in:
• Queensland Government fact sheet R34 How healthy is
your watercourse—Assessing stream bank vegetation
(see ‘River series fact sheets’ below for link).
Other publications related to this chapter
• Abernethy and Rutherford (1999) (listed in references
above) provided a ‘traffic light’ (green, yellow, red)
classification system in Guidelines for stabilising
streambanks with riparian vegetation.
Boulter, SL, Wilson, BA, Westrup, J, Anderson, ER,
Turner, EJ and Scanlan, JC (Eds) (2000) Native vegetation
management in Queensland: Background, science and
values, Department of Natural Resources, Brisbane.
The following resources provide advice related to the
choice of species and methods for revegetating streams
in Queensland:
Brisbane City Council (1997) Erosion treatment for
urban creeks—Guidelines for selecting remedial works,
Department of Works, Brisbane City Council.
• O’Donnell, S. (1996) Common riparian plants of southeast Queensland. Queensland Department of Natural
Resources, Brisbane.
• O’Donnell, S. (1998) Management of river and creek
bank plantings in sub-tropical coastal riparian
rainforest, Queensland Department of Natural
Resources, Brisbane.
• Section 8.5 in Volume 1 and Guideline in Volume 2 of
Lovett and Price (1999) (listed in References).
• Lovett and Price (2001) (listed in References).
• Lovett et al. (2003) (listed in References).
• Rural Industries Research and Development
Corporation (1999) Growing trees on cotton farms: A
guide to assist cotton farmers to decide how, when,
where and why to plant trees, RIRDC, Canberra.
• WetlandCare Australia (2008). Wetland rehabilitation
guidelines for the Great Barrier Reef catchment,
compiled for Department of the Environment, Water,
Heritage and the Arts, Canberra.
Information on the design of rock revetments, rock
chutes and drop structures for the control of bed erosion
is provided in:
• Standing Committee on Rivers and Catchments (1991)
Guidelines for stabilising waterways, prepared by the
Working Group on Waterway Management, Melbourne.
• Chapter 16, ‘Streambank and shoreline protection’,
in: United States Department of Agriculture (1996),
Engineering field handbook, USDA, Washington DC.
Design information on groynes is also included in
the publication Guidelines for stabilising waterways,
published by the Standing Committee on Rivers and
Catchments in 1991.
11–46
A detailed literature review on woody debris can be
found in Chapter 7 of the publication Principles for
riparian lands management [Lovett, S and Price, P (2007),
published by Land and Water Australia in Canberra].
Claridge, G (Ed.) (2005) Getting the process right:
Designing and implementing community-based riparian
rehabilitation projects in South East Queensland, Moreton
Bay Waterways and Catchment Partnership, Brisbane.
Coughlin, T, O’Reagain, P, Nelson, B, Butler, B and
Burrows, D, (2008) Managing for water quality within
grazing lands of the Burdekin Catchment—Guidelines for
land managers, Burdekin Dry Tropics NRM Solutions Ltd,
Townsville.
Department of Sustainability and Environment (2007)
Technical guidelines for waterway management,
Department of Sustainability and Environment,
Melbourne.
Hicks, W (2014) Longstem tubestock, see
<longstemtubestock.com> (accessed 6 July 2015).
McAdam, P (2013) Streambed erosion is a key threatening
process, presentation to 24th annual Queensland State
Landcare conference, 27–29 September, 2013.
Rural Industries Research and Development Corporation
(1999) Growing trees on cotton farms: A guide to assist
cotton farmers to decide how, when, where and why to
plant trees, RIRDC, Canberra.
Shellberg, J, and Brooks, A (2007) A fluvial audit of
the Brisbane River: A basis for assessing catchment
disturbance, sediment protection, and rehabilitation
potential, Australian Rivers Institute, Griffith University,
Queensland.
WetlandCare Australia (2008). Wetland rehabilitation
guidelines for the Great Barrier Reef catchment, compiled
for Department of the Environment, Water, Heritage and
the Arts, Canberra.
Soil conservation guidelines for Queensland
Chapter 11 Stream stability
River series fact sheets
The river series fact sheets produced by the Department
of Natural Resources and Mines are searchable at
Queensland Government website (accessed 5 July 2015).
In particular:
• R02 What causes bank erosion
<qld.gov.au/dsitia/assets/soil/what-causes-bankerosion.pdf>
• R20 What causes streambed erosion?
<qld.gov.au/dsitia/assets/soil/what-causes-streambed-erosion.pdf>
• R30 Stream bank vegetation is valuable
<qld.gov.au/dsitia/assets/soil/streambank-vegetationis-valuable.pdf>
• R31 Stream bank planting guidelines and hints
<qld.gov.au/dsitia/assets/soil/streambank-plantingguidelines.pdf>
• R33 Managing stock in and around waterways
<qld.gov.au/dsitia/assets/soil/managing-stockwaterways.pdf>
• R34 How healthy is your watercourse? Assessing
stream bank vegetation
<qld.gov.au/dsitia/assets/soil/how-healthy-is-yourwatercourse.pdf>
11–47