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Rock chutes: a review of damage and failure mechanisms

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Abstract

Rock chutes (also known as rock ramps and rock riffles) are an important technique for controlling erosion, and have been widely used in Victorian streams. Occasionally, for a number of reasons, they are damaged or fail. Based on a survey of 170 rock chutes in north east Victoria and Gippsland, eight damage or failure modes have been identifi ed. These failure mechanisms include loss of rock from the face and crest, downstream erosion, undermining of the chute apron, stream bed instability, abutment damage, total loss of the chute and willow infestation. The greatest risk to rock chutes arises from three mechanisms, loss of rock from the face of the chute, willow infestation and abutment damage. It is important to consider the complete range of possible failure mechanisms when designing rock chutes. Improved design procedures are discussed which especially target the greatest risk to chutes, loss of rock from the chute face.
103
© Institution of Engineers, Australia 2006
Australian Journal of Water Resources, Vol 10 No 1
technical note
* Paper W25/737 submitted 03/05/05
Paper accepted for publishing 05/07/05.
Rock chutes: a review of damage and failure mechanisms *
Anthony R Ladson
Institute for Sustainable Water Resources
Cooperative Research Centre for Catchment Hydrology
Department of Civil Engineering, Monash University, Vic
Ross E Hardie
Earth Tech Engineering Pty Ltd, Australia
Robert J Keller
Institute for Sustainable Water Resources
Cooperative Research Centre for Catchment Hydrology
Department of Civil Engineering, Monash University, Vic
SUMMARY: Rock chutes (also known as rock ramps and rock riffl es) are an important technique
for controlling erosion, and have been widely used in Victorian streams. Occasionally, for a number
of reasons, they are damaged or fail. Based on a survey of 170 rock chutes in north east Victoria
and Gippsland, eight damage or failure modes have been identifi ed. These failure mechanisms
include loss of rock from the face and crest, downstream erosion, undermining of the chute apron,
stream bed instability, abutment damage, total loss of the chute and willow infestation. The
greatest risk to rock chutes arises from three mechanisms, loss of rock from the face of the chute,
willow infestation and abutment damage. It is important to consider the complete range of possible
failure mechanisms when designing rock chutes. Improved design procedures are discussed which
especially target the greatest risk to chutes, loss of rock from the chute face.
2 METHOD
Chute characteristics, failure modes and repair
costs were identifi ed from fi eld inspections of rock
chutes or from descriptions and/or photographs
in previously published assessments [ID & A P/L
1998; ID & A P/L 2001]. Information on 170 damaged
chutes was available and damage or failure modes
were grouped into 8 categories:
1. Rock lost from the face of the chute
2. Rock lost from the crest of the chute
3. Bank erosion downstream of the chute
4. Undermining of the chute apron
5. Ongoing instability in the stream bed
6. Damage to the chute abutments, or outfl anking
of the chute
7. Total loss of the chute crest, face and abutments
8. Willow infestation
The proportion of failures and the average cost of
repair within each group were calculated. Note that
a chute may be subject to more than one type of
damage mode.
1 INTRODUCTION
The stability of rivers and channels is often linked to
the stability of the channel bed. Channels may be de-
stabilised by catchment clearing, changes to stream
hydrology or modifi cation of the stream channel.
A rock chute is a relatively short and steep section of
the bed of a channel which is armoured with rock.
Chutes are normally intended to either stabilise an
erosion head, preventing it from moving upstream
in the channel, or to reduce the overall grade of a
channel by providing a weir.
Guidelines for the design of rock chutes are available
[Standing Committee on Rivers and Catchments
1993; Keller 2003], and there has been recent work
to develop a spreadsheet based design aid [Keller
2003].
The purpose of this paper is to highlight potential
failure modes of rock chutes identifi ed from a review
of a sample of rock chutes in North East Victoria and
Gippsland. Correct hydraulic design, in terms of rock
sizing, does not guarantee freedom from failure as
there are a range of possible failure modes that need
to be considered.
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Australian Journal of Water Resources Vol 10 No 1
“Rock chutes: a review of damage and failure mechanisms” – Ladson, Hardie & Keller
3 RESULTS
3.1 Chute characteristics
Our sample manly consists of small chutes on
ungauged streams. Characteristics are summarised
in Table 1.
Table 1: Chute characteristics.
3.3 Risk of damage or failure
A risk based approach [A/NZS 4360] has been
adopted to identify the failure modes that provide
the greatest risk to rock chutes. Risk can be described
as the combination of the consequence and likelihood
of an event impacting on an asset:
Median Maximum Minimum No.*
Width (m) 6 40 1 83
Length (m) 20 60 5 91
Year of Construction 30/06/1993 30/06/2001 30/06/1955 132
Cost of Construction (base year 2000) $10,000 $60,000.00 $1,500.00 135
* No. refers to the number of chutes where the listed information was available.
3.2 DAMAGE FREQUENCY AND COSTS
Damage frequencies, and repair costs, provide
information on those problems that are most likely
to occur, most costly to fi x, or provide the greatest
risk (Figure 1). From this review, the most common
damage mode was willow infestation with over 36%
of chutes affected; 25% had minor infestation and 11%
major. Minor willow infestation was one of the least
expensive problems to fi x at an average cost of $6,400
per chute (a base year of 2000 is used for all costs).
The highest cost problems were those associated with
outfl anking or failure of abutments or total loss of the
chute. These problems occurred least often but repair
costs can be equal to, or greater than the original
cost. The average repair cost for chutes damaged by
abutment failure was about $37,000 in our sample.
Figure 1: Frequency and cost of damage modes for rock chutes
Risk = consequence x likelihood
Considering consequence as equal to damage cost,
and likelihood as damage frequency, the risk of the
damage modes can be calculated. Three damage
modes stand out, in order:
1. Rock lost from the face of the chute;
2. Willow infestation; and
3. Damage to the chute abutments, or outfl anking
of the chute.
To manage these risks, we can reduce the cost of repair
(consequence) and/or the occurrence (likelihood) of
failure. This requires a combination of good design,
construction and maintenance.
The occurrence and frequency of the failure modes
listed here should be considered to be specifi c to the
105
“Rock chutes: a review of damage and failure mechanisms” – Ladson, Hardie & Keller
Australian Journal of Water Resources Vol 10 No 1
streams where this study was undertaken (Gippsland
and North East Victoria). Some of these failure modes
have been identifi ed by others [Frissell & Nawa 1992;
Roper et al. 1998; Schmetterling & Pierce 1999], but
they have also identifi ed other failure modes and
different failure frequencies. The comparison of
our work with these North American examples
suggest that some failures are uniquely related to
the characteristics of particular streams, for example,
high sediment loads and presence of invasive exotic
riparian vegetation. Other types of damage can
probably occur in any stream.
3.4 Loss of rock from the face of a chute
Our analysis showed that the greatest risk to chutes
is damage associated with the loss of rock from the
chute face.
Better design procedures can make this type of
damage less likely. A common approach is to design
chutes for a fl ow event of a particular recurrence
interval, usually a 10-year to 50-year event. This
design fl ow is converted into a discharge per unit
width and the rock sized accordingly. This design
approach should not be used as it will normally give
a misleading indication of the required rock size.
Rock chutes need to be designed for a worst-case
ow, not a fl ow based on a particular recurrence
interval. As fl ow increases the required rock size
will initially increase to a maximum but for further
increases in fl ow, the hydraulic stress on the chute is
reduced as it is downed out.
Figure 2 shows an example. In this case, the design fl ow
from waterway capacity was 4 m3s-1m-1 but the worst-
case fl ow rate for rock design is about 1.5 m3s-1m-1. The
selection of rock for a chute should always be based
on the maximum required rock size up to the design
ow event, not the size based on an arbitrary discharge
such as bankfull capacity or a fl ow selected on the
basis of a design average recurrence interval. Note
that some design procedures may be unnecessarily
conservative because they do not take into account
the fact that a chute can be drowned out at high fl ows.
Instead the required rock size is determined from the
maximum discharge per unit width [Robinson et al.
1997]. Best practice design procedures should be based
on an estimate of the actual forces on the chute which
may reduce at high fl ows depending on downstream
hydraulic conditions.
3.5 Willow infestation
Willows growing on a chute can cause a number of
problems:
Flow can be concentrated between the willow
trunks which may result in rock being removed
from the face or crest of the chute;
Increased fl ow resistance may mean that less
water will fl ow down the chute which could
increase the likelihood of outfl anking.
When willows topple, a large part of the chute
may be disturbed leading to damage in later
oods.
In problem areas, regular maintenance will be
required to remove willows from chutes. Willows
are only likely to be a problem for chutes where
willows are an exotic invasive species.
3.6 Damage to the chute abutments, or
out anking of the chute
Damage to chute abutments or outflanking of
the chute was found to be a costly problem as
any protection of the upstream reach against bed
instability will be lost until the chute is repaired
and it will usually have to be rebuilt much larger
than the original design.
This type of failure can occur where a chute is
threatened by lateral migration of a stream. Chutes
Figure 2: The maximum required rock size does not necessarily occur at the highest discharge per
unit width where the chute may be drowned out.
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Australian Journal of Water Resources Vol 10 No 1
“Rock chutes: a review of damage and failure mechanisms” – Ladson, Hardie & Keller
are designed to control vertical instability but issues
of lateral erosion must also be considered.
Another cause of this type of failure is when a chute
is constructed in a section of channel where fl ood
ows are constricted which can result in very high
shear stresses during fl ood fl ows. It is safer to build
chutes in wider reaches, but the initial costs may be
higher.
These types of extreme failures were rare,
representing less than 3% of our sample (and the
sample only consisted of damaged chutes).
4 IMPROVED DESIGN PROCEDURES
As discussed above, our results suggest that the
highest risk failure mode corresponds to movement
of rock from the face of a chute. We also suggest
that inappropriate selection of the worse-case
ow for a chute may exacerbate the risk of failure
(Figure 2). Rock chute design procedures need to
enable the straightforward calculation of these
critical conditions.
The authors have been involved with the
development of a computer program that can assist
with the design of rock chutes [Keller & Winston
2003; Keller 2004], which is available for free
download from www.toolkit.net.au. This program
allows the computation of a water surface profi le on
a proposed rock chute for given design conditions.
Hydraulic conditions can be analysed over a range of
ow rates (and associated range of tail-water levels)
from the lowest to the highest expected fl ow. The
worst-case fl ow rate with respect to rock size can
be identifi ed [Keller & Winston 2003]. Rock chute
design guidelines have also been prepared [Keller
& Winston 2003].
It is important to note that these new guidelines
[Keller 2003], and existing design guidelines
[Standing Committee on Rivers and Catchments
1993; Robinson et al. 1998; Olivier 1967] are mainly
directed at designing rock chutes so they do not fail
because of rock movement on the crest and face of
the chute (damage mode 1 and 2 above). There has
been less discussion of best practice design and
construction to protect against the other failure
modes. Designers need to use their own skills,
experience, and alternative information sources
[www.toolkit.net.au/chute; Biedenharn & Smith
1997; Hardie 1996; ID&A P/L 1996] to assist with
design of these aspects.
CONCLUSION
Rock chutes are an important technique for controlling
erosion, and have been widely used to control bed
instability in incising streams. Occasionally, for a
number of reasons, they are damaged or fail. On the
basis of this study of failed chutes, we have found
that the highest risk failure mode is rock lost from the
face of the chute. A new computer program, CHUTE,
has been specifi cally developed to analyse the risk
of this type of failure.
The other two risks that stand out are, damage to
chutes because of willow infestation, and damage
to abutments or outfl anking of the chute. Chute
designers need to consider these and the other
failure modes in the design, construction, operation,
monitoring and maintenance of rock chutes.
REFERENCES
A/NZS 4360 (Australian/New Zealand Standard
4360) Risk Management. Standards Australia.
Biedenharn DS, Smith JB. Design considerations for
grade control siting. In: Management of Landscapes
Disturbed by Channel Incision, S.S.Y Wang, E.J.
Langendoen, and F.D. Shields Jr., eds., University of
Mississippi Oxford, MS, 1997:229-239.
CHUTE: a hydraulic design program for the design
of rock chute structures used for stabilising river
and stream beds. Cooperative Research Centre for
Catchment Hydrology. www.toolkit.net.au/chute.
Hardie RE. Streambed longitudinal gradient and
unit stream power analysis of tributary streams of
North East Victoria, Australia. Proceedings of the
First National Conference on Stream Management
in Australia, Merrijig, Cooperative Research Centre
for Catchment Hydrology. 1996:57-62.
ID&A Pty Ltd. Improvements to the design and
construction of rock chutes. Melbourne, Department
of Natural Resources and Environment: 1996:34.
ID&A Pty Ltd. Flood Damage Assessment June 1998
Flooding. Final Report. East Gippsland Catchment
Management Authority, 1998.
ID&A Pty Ltd. Regional river health strategy for the
North East Catchments: register and assessment of
previous work. North East Catchment Management
Authority, 2001.
Frissell CA, Nawa R. Incidence and causes of physical
failure of artifi cial habitat structures in streams of
western Oregon and Washington. North American
Journal of Fisheries Management 1992:12: 182-197.
Keller RJ. Guidelines for the design of rock chutes
using CHUTE. Melbourne, Cooperative Research
Centre for Catchment Hydrology: 2003:33.
Keller RJ. Stabilising Channel Beds and Banks
Using Rock Chutes and Riprap. Proceedings of 2nd
International Conference on Scour and Erosion, Nov
14-17, Singapore, 2004:2:45-52.
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“Rock chutes: a review of damage and failure mechanisms” – Ladson, Hardie & Keller
Australian Journal of Water Resources Vol 10 No 1
Keller RJ, Winston F. Stabilisation of channel beds
using rock chutes. 28th International Hydrology and
Water Resources Symposium, Wollongong, NSW,
Institution of Engineers, Australia 2003:107-113.
Olivier H. Through and overfl ow rockfi ll dams - new
design techniques. Proceedings of the Institution of
Civil Engineers 1967:36: 433-471
Robinson KM, Rice CE, Kadavy KC. Design of rock
chutes. Transactions of the American Society of
Agricultural Engineers 1998:41(3): 621-626
Roper BB, Konnoff D, Heller D, Wieman K. Durability
of Pacifi c Northwest instream structures following
floods. North American Journal of Fisheries
Management 1998:18: 686-693
Robinson KM, Rice CE, Kadavy KC. (1997) Rock
chutes for grade control. In: Management of
Landscapes Disturbed by Channel Incision, S.S.Y
Wang, E.J. Langendoen, and F.D. Shields Jr., eds.,
University of Mississippi Oxford, MS, 1997.
Schmetterling DA, Pierce RW. Success of instream
habitat structures after a 50-year fl ood in Gold Creek,
Montana Restoration Ecology 1999:7(4): 369-375
Standing Committee on Rivers and Catchments
Guidelines for stabilising waterways. Melbourne,
Working Group on Waterway Management, 1993.
108
Australian Journal of Water Resources Vol 10 No 1
“Rock chutes: a review of damage and failure mechanisms” – Ladson, Hardie & Keller
TONY LADSON
Tony Ladson is a senior lecturer in the Department of Civil Engineering and a
member of the Institute for Sustainable Water Resources at Monash University.
He is also an honorary fellow in the School of Anthropology, Geography and
Environmental Studies at the University of Melbourne. His research interests
include developing and testing methods to assess the condition of streams and
approaches to their restoration. He has worked in the fi eld of environmental fl ows
and has a long-term interest in the management of the Barmah-Millewa Forest.
Tony teaches undergraduate and post graduate courses in hydrology, hydraulics,
ood management and water resources management.
ROSS HARDIE
Ross Hardie is one of Australia’s leading waterway management practitioners
with more than 18 years direct experience in the waterway management industry.
Ross has a particular interest in the investigation and management of incised
stream systems including the implementation of grade control and channel
roughness strategies incorporating rock chute style structures in combination
with vegetation establishment. This interest has also led to the review of failure
mechanisms for such programs and the development of parameters and criteria
for the hydraulic design of incised stream management works.
BOB KELLER
Bob Keller works in the fi elds of river engineering, hydraulic structures, and
computer and physical modelling of rivers and fl oodplains. Bob worked in the
river restoration program of the Cooperative Research Centre for Catchment
Hydrology where he developed CHUTE a computer based aid for the design
of rock chutes.
Article
Full-text available
The durability of 3,946 instream structures in 94 streams that had floods with return intervals exceeding 5 years were assessed. Overall structure durability (defined as the degree to which a structure remained at its original location) was high; less than 20% of the sampled structures had been removed from the site of original placement. The magnitude of flood events had a significant effect on structure durability with higher magnitude floods reducing durability. Stream order also affected structure durability; structures in large streams were 20 times more likely to have been removed from the site of original placement than structures in small streams. Other conditions that affected structure durability included location of the structure within the stream channel, whether the structure was anchored or not, structure material, and upslope landslide frequency. Instream structures are most appropriate when used as short-term tools to improve degraded stream conditions while activities that caused the habitat degradation are simultaneously modified. When instream structures are part of a properly sequenced watershed restoration strategy, they can improve habitat conditions through a range of flow conditions including large floods.
Article
Full-text available
In recent years an increasing share of fishery management resources has been committed to alteration offish habitat with artificial stream structures. We evaluated rates and causes of physical impairment or failure for 161 fish habitat structures in 15 streams in southwest Oregon and southwest Washington, following a flood of a magnitude that recurs every 2–10 years. The incidence of functional impairment and outright failure varied widely among streams; the median failure rate was 18.5% and the median damage rate (impairment plus failure) was 60%. Modes of failure were diverse and bore no simple relationship to structure design. Damage was frequent in low-gradient stream segments and widespread in streams with signs of recent watershed disturbance, high sediment loads, and unstable channels. Comparison of estimated 5–10-year damage rates from 46 projects throughout western Oregon and southwest Washington showed high but variable rates (median, 14%; range, 0–100%) in regions where peak discharge at 10-year recurrence intervals has exceeded 1.0 m·s·km. Results suggest that commonly prescribed structural modifications often are inappropriate and counterproductive in streams with high or elevated sediment loads, high peak flows, or highly erodible bank materials. Restoration of fourth-order and larger alluvial valley streams, which have the greatest potential for fish production in the Pacific Northwest, will require reestablishment of natural watershed and riparian processes over the long term.
Article
Rock chute design information is consolidated from several sources to provide a comprehensive design tool. The rock slope stability, boundary roughness, and outlet stability of rock chutes are each discussed. Tests were performed in three rectangular flumes and in two full size structures. Angular riprap with a median stone size ranging from 15 to 278 mm was examined on rock chutes with slopes ranging from 2 to 40%. The typical mode of channel failure is described. An empirical prediction equation is presented relating the highest stable discharge on a rock chute to the median stone size and the bed slope. A boundary roughness relationship is also presented that relates the Manning roughness coefficient to the median stone size and bed slope. These tests also suggest that the riprap size required for stability on the slope will remain stable in the outlet reach even with minimal tailwater. This article contains information needed to perform a rock chute design. ock chutes or loose-riprap-lined channels are used to safely convey water to a lower elevation. These structures provide an alternative method of protecting the soil surface to maintain a stable slope and to dissipate a portion of the flow energy. Watershed management applications for this type of structure are numerous such as channel stabilization, grade control, and embankment overtopping. Depending on the availability and quality of accessible rock materials, rock chutes may offer economic advantages over more traditional structures. Flow cascading down a rock chute is visually pleasing, and these structures offer aesthetic advantages for sensitive locations. Construction of these chutes can be performed with unskilled labor and a comparatively small amount of equipment. A typical rock chute profile is shown in figure 1.
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Gold Creek, in western Montana, lost complexity and diversity of fish habitat following riparian logging activities, removal of instream wood, and subsequent scouring. In the 4.8-km study area, the stream was almost totally void of large woody debris (4.2 pieces/km) and associated pools (1.3 pools/km). We constructed 66 structures made of natural materials (rock and wood) that resulted in 61 new pools in the study area in an attempt to restore salmonid habitat in the fall of 1996. An estimated 50-year recurrence interval flood occurred in the following spring. Of the original 66 structures, 55 (85%) remained intact and stable. Laterally confined reaches retained significantly more pools than laterally extended reaches. Owing to a history of anthropogenic impacts in forested streams in the intermountain west, restoration efforts are needed. If instream structures are tailored to specific morphologic channel types, fish habitat restoration can be successful and withstand major floods.
Design of rock chutes. Transactions of the American Society of Agricultural Engineers
  • Km Robinson
  • Ce Rice
  • Kc Kadavy
Robinson KM, Rice CE, Kadavy KC. Design of rock chutes. Transactions of the American Society of Agricultural Engineers 1998:41(3): 621-626
Rock chutes for grade control In: Management of Landscapes Disturbed by Channel Incision
  • Km Robinson
  • Ce Rice
  • Kc S S Kadavy
  • E J Wang
  • F D Langendoen
  • Shields Jr
Robinson KM, Rice CE, Kadavy KC. (1997) Rock chutes for grade control. In: Management of Landscapes Disturbed by Channel Incision, S.S.Y Wang, E.J. Langendoen, and F.D. Shields Jr., eds., University of Mississippi Oxford, MS, 1997.
Guidelines for the design of rock chutes using CHUTE. Melbourne, Cooperative Research Centre for Catchment Hydrology
  • R J Keller
Keller RJ. Guidelines for the design of rock chutes using CHUTE. Melbourne, Cooperative Research Centre for Catchment Hydrology: 2003:33.
Streambed longitudinal gradient and unit stream power analysis of tributary streams of North East Victoria
  • R E Hardie
Hardie RE. Streambed longitudinal gradient and unit stream power analysis of tributary streams of North East Victoria, Australia. Proceedings of the First National Conference on Stream Management in Australia, Merrijig, Cooperative Research Centre for Catchment Hydrology. 1996:57-62.
Stabilising Channel Beds and Banks Using Rock Chutes and Riprap
  • R J Keller
Keller RJ. Stabilising Channel Beds and Banks Using Rock Chutes and Riprap. Proceedings of 2nd International Conference on Scour and Erosion, Nov 14-17, Singapore, 2004:2:45-52.