Risks to Murray-Darling Basin Water Resources Program
Project Summaries
Project code:
CS1
Project title:
A case study of risks to flows and floodplain ecosystems posed by
structures on the Macquarie Floodplain
Project timeline:
March – August 2010
Contractors:
Australian Wetlands and Rivers Centre, University of New South Wales
Report Authors:
L. Nairn, K. Brandis, R.T. Kingsford, C. Steinfeld, S. Ren, T. Rayner
Project objectives:
The effects of large river regulatory structures, particularly dams and diversions in reducing flows to river
floodplains and putting organisms and ecosystems at risk, is reasonably well known. The effects of
smaller structures, such as weirs, small in-stream structures and floodplain earthworks are less well
known, but may also affect organisms and ecosystems. There is relatively little understanding of the
location and potential impact that these structures may have on flows and ecosystems across the MurrayDarling Basin. The impact of these structures may affect the success of basin level management that does
not account for such structures, affecting the delivery of objectives of the Basin Plan. Such structures are
widespread on the floodplains of the MDB and there is a need to identify their location and assess their
impact on flows and dependent organisms and ecosystems. Using the Macquarie River Valley as a study
site, this project aimed to assess the risk to flow and floodplain ecosystems from structures and develop a
methodology applicable to other areas of the Basin. The objectives of this project were:
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Provide a literature review of methods used for preparing floodplain structure inventories
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Identify the locations of in-stream and floodplain structures in the Macquarie Valley
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Assess the accuracy of the methodology used
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Provide survey data
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Identify potential impacts of structures on flow, floodplain ecosystems and biotic responses in the
Macquarie Valley
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Discuss the implications of structures for water management
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Discuss applicability of the methods for other parts of the Murray Darling Basin
Methods:
Following a review of literature about the effects of floodplain structures on flow, vegetation and aquatic
ecosystems, the project undertook a detailed risk assessment of such structures within the Macquarie
valley. This included flood mapping based on Landsat imagery (MSS and TM) and 25m grid cells of
flood events during the period 1979 to 2006, and flow modelling to explore long-term changes to flow
and inundation extent of the Macquarie Marshes using built statistical models of 'unregulated' and
'regulated' (including all storages and abstractions) flow at the Dubbo, Warren and Oxley gauges on the
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CS1 Project Summary
Macquarie River. The models were based on the relationships between the flow gauges and rainfall data
at Bathurst, Orange, Mudgee, Wellington, Dubbo, Warren and Quambone stations over various
timescales. The ‘unregulated' and 'regulated' flow models for the Oxley gauge were extended for the
period 1879-2006 using the full rainfall record from Quambone and Warren, and a model developed to
integrate the 'unregulated' and 'regulated' flow models with mapped inundated areas. The ‘unregulated'
and 'regulated' flow models for the Oxley gauge were extended for the period 1879-2006 using the full
rainfall record from Quambone and Warren. Finally, spatial flood models that linked inundated area and
gridded flow data were developed and used to prepare maps of simulated ‘unregulated' and 'regulated'
inundations for the period 1879-2006.
To identify all in-stream structures in the Macquarie Valley, the project collated existing datasets and
conducted an aerial survey of the main creeks in the Macquarie Valley downstream from the township of
Warren. Using on-board avionics, the location of each barrier was mapped, and width and height of each
structure was estimated. Structures were classified into eight types: bridges, bund wall, causeway,
culvert, pipe, regulator, rock wall and weir. The exact locations of floodplain structures in the Macquarie
Valley were obtained from the results of satellite image analyses (SPOT imagery 2005). Levees and
channels were then classified and digitised using spectral properties, shape, texture and placement in
relation to surrounding cadastral features to aid correct identification. Roads were differentiated from
linear channels using a road map (1:10,000 scale). Results for both in-stream and floodplain structures
gained from interrogation of satellite images were checked for accuracy against aerial photographs.
The impact of in-stream barriers on flow was assessed at two levels:
1. the impact on within-bank flows (important for aquatic ecosystem processes and passage of
organisms) and
2. the impact on over-bank flows, or flooding (connection of channel with floodplain).
Monthly flow data (maximum stream height, minimum stream height and flow volume (ML)) was used
for this analysis. The spatial flood models were used to conduct a preliminary assessment of the risks to
flows posed by levees and channels in the Macquarie Marshes. The project used a combined map that
reflected the ‘actual’ flows in the Marshes for the past 128 years, i.e. the unregulated map for the years
prior to major dam construction and the regulated map for more recent years. This enabled the project to
fully explore the impact of the barriers by demonstrating how many of the floods that have occurred in
the past 128 years would have crossed the barriers that are now in place. Flood frequency was expressed
as the mean number of years flooded for each 50m buffer on either side of each levee and channel
calculated as the difference in mean flood frequency between the two sides.
Changes detected from a sequence of vegetation maps (1949 to 2008), and sampling the area of each
vegetation type occurring within a 200m buffers on either side of the floodplain barriers were used,
together with assessed changes in river red gum health, to evaluate the impacts of structures on floodplain
vegetation.
Summary of key results:
Comparison of structures identified by aerial survey and those identified using SPOT satellite imagery
found that only large (>40m wide) in-stream structures were identifiable e.g. Marebone Wier, and bridges
across creeks, and this was primarily due to the presence of roads. Smaller structures were either not
visible on the SPOT imagery or were not recognisable as an in-stream structure.
The project identified the location of 72 in-stream barriers in the Macquarie Valley. This total included
33 additional structures that were not identified by the NSW Department of Primary Industry Weirs
Review (2006). Fish communities in the Macquarie and Bogan Rivers have declined in health,
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CS1 Project Summary
abundance, native species richness and diversity. The native species in the catchment require a range of
habitats to complete their lifecycles, access to which has been compromised by the development of flow
regulation and water extraction infrastructure. More than 85% of the 72 in-stream structures identified
have the potential to impede fish movements through the river system, particularly during low flow
periods which are common. While these structures are mostly relatively small, their ecological and
geomorphic effects are likely to be cumulative and amplified by the extremely flat topography that
characterises this area and continued low-flow conditions.
There were 338km of levees, 1,648km of channels, 54 off-river storages and 664 tanks or farm dams on
the floodplain area of 4,793km2. A further 119km levees were classified uncertain, requiring further
ground truthing, making an upper estimate of 2,320km of earthworks. Most levees and channels (72%)
were in the southern part (max. density 3,400m km2) and two northern areas, where there was irrigation
development. Outside these areas, curvilinear levees and channels were built to move water around the
floodplain and also for erosion control. More than half (62%) of channels were between 0.5-1m, a third
were < 0.5m in height, and the rest (5%) were greater than 1m in height (n= 55). Most levees (65%) were
less than 0.5m high (n=20), except those that bounded off-river storages (1-5m in height).
Based on 12 randomly selected irrigated and non-irrigated quadrats (10x11km) tracked from 1949-2005,
most of these earthworks occurred between the 1980s and 1990s. Levees within the Macquarie were used
to enhance drying and alienated parts of the original floodplain, particularly in the Southern Section.
Floodplain structures have a considerable impact on flooding, reducing the area flooded even by
comparatively large floods. For example, the 1990 flood, which was one of the largest recorded, failed to
cross barriers in the southern section of the Macquarie Valley.
Nearly 70% of floodplain barriers (697 channels and 180 levees) impacted flooding in some way and
these effects translate to large changes in multi-scale ecosystem processes. Of these, over 35% of barriers
had a flow reduction (reduction in number of years flooded from one side of the structure to the other)
greater than 10%. The effects on floodplain vegetation of reduced flooding caused by barriers were
clearly demonstrated, with differences in flooding significantly linked to vegetation community
composition and to measures of vegetation health. The severe extent of vegetation habitat decline has
been associated with other biotic effects. A less than 10% difference in flood frequency had a noticeable
effect on River Redgum health and vegetation community composition was significantly affected by
impacts from barriers. The numbers of waterbirds and species of birds present has declined between 1983
and 1995, and there are now an estimated 100,000 fewer nests over a period of 11 years, linked to
reduced water flows and degraded river redgum habitat.
Flow and structure analyses identified the difficulty in assigning broad risk classifications to in-stream
structures without taking account of each specific situation. Comparison of in-stream structures 1 m high
in the Monkeygar and Gum Cowal Creeks showed different impacts on flow. One metre high structures
in the Monkeygar Creek were exceeded by flows 42% of the time, while structures of the same height in
the Gum Cowal were exceeded only 6% of the time. This has potentially different ecological
implications with regards to the movement of flows and fish, and management implications regarding the
delivery of water. Risk analyses for in-stream structures need to be undertaken on an individual basis,
taking into account hydrological characteristics, the presence and location of other in-stream structures
and the cumulative impact of additional structures.
Comparisons with other projects:
Results from this project broadly match the findings of other work that has examined the impacts of instream and floodplain structures on aquatic ecosystems and processes and on floodplain vegetation
(extent, community structure and health). The implications for management of these results matches
other findings from the Risks Program that legislation, regulatory frameworks, license conditions and
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CS1 Project Summary
inspection programs will all need review and strengthening if the objectives of the Water Act 2007 are to
be achieved.
Knowledge gaps identified:
Knowledge gaps were not discussed specifically in the project report, but further refinement and testing
of the methods in different catchments is a clear priority. The methods presented in this report for the
preparation of floodplain and in-stream structure inventories and the assessment of their impacts could be
applied to any area of interest. The detailed methods for detecting floodplain structures are transferable
between floodplains and provide rigorous, repeatable techniques that can be used where appropriate data
is available. The aerial survey technique presented in this report for detecting and mapping in-stream
structures provides a fast, cost effective method for collecting data across large areas. These methods
should now be tested through application elsewhere. However, it must be remembered that the risks
posed by structures to ecosystems are unique to the area of interest and are not transferrable.
Implications for policy:
In-stream structures and floodplain structures all affect connectivity of rivers and ecosystems that rely on
flow. In-stream structures primarily impact on longitudinal connectivity in the main river stem or
channel, while floodplain earthworks affect lateral and longitudinal connectivity on the floodplain. The
relative impact of a structure will largely be related to their size and orientation in relation to flows across
the floodplain. It is useful to consider the two types of structure separately as management and policy
differs.
In-stream structures
There is a well established body of work on the impacts of major structures in rivers and their effect on
flow and connectivity, particularly fish populations. As a result, large structures require licensing and
associated assessment of their impacts on connectivity and fish populations. There are reasonably well
advanced databases of weirs and other in-stream structures licensed by fish management authorities and
water management agencies. Unfortunately these databases are not complete and may not incorporate all
of the potential structures that require assessment and licensing; the project found a significant number of
structures that were not listed on the NSW weirs licensing database.
Following identification and assessment from an aerial survey, it is possible to identify the likely impacts
of such structures on flows down a river system. This can be done by assessing changes to flow in
relation to the height of the structure and its position in the river. Detailed ecological impact analysis will
probably only be required with major structures. Regulatory authorities are then in a position to either
license the structure as it is, or require adaptation measures to ensure little impact on connectivity, or
request removal of a structure.
The following steps can be implemented on any river to determine the extent and potential impact of instream structures on the connectivity and ecology of a large floodplain.
1. Identify all in-stream structures on the floodplain through an aerial survey that accurately locates
all structures, photographs them and then produces an objective assessment by a fish biologist of
their potential impact on connectivity and fish populations.
2. Determine whether structures are licensed and whether they require licensing on the basis of their
potential impacts on the environment and flow. This requires flow analyses as set out in this
report.
3. For in-stream structures identified to have ecological and hydrological impacts, follow up
management that requires mitigation of the impact of the structure or its removal.
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CS1 Project Summary
Floodplain structures
Few governments have developed adequate policy and legislative frameworks for the management of
floodplain structures. This largely reflects the inherited definitions in original water where rivers and
their management were primarily specified by their nominated ‘watercourses’. Floodplains were often
not part of this association and were left out of most management and regulation.
The five key steps required for the management of floodplain structures are:
1. Identification of location and type of structures on the floodplain;
2. Verification of when each structure was built;
3. Determination of the potential for each structure to harvest water from the floodplain and the
amount of water diverted;
4. Determination of the potential risk of structures to vegetation health and connectivity and;
5. Assessment of which structures require removal or adaptation to ensure that they do not affect
flows or ecosystems.
There are some important policy and management considerations for the floodplain structures which need
to be clarified to ensure that the Basin Plan is effective in its implementation. In particular, there needs to
be a clear definition of all structures capable of harvesting floodplain flows, for compliance and
regulation. There also needs to be similar legislative and policy instruments in each of the states to ensure
that all the rivers in the Murray-Darling Basin are managed with consistent policy. There is a need to
improve the legislative and regulatory frameworks in all states so that they adequately regulate structures
on floodplains and include these in the licensing process.
There five broad policy and management issues relevant to the steps above:
1. accounting for floodplain structures need in terms of diversions,
2. consistent license and regulatory framework,
3. assessment of potential impacts on environmental assets,
4. adequate resourcing and attention on addressing the issue, and
5. decommissioning of structures or mitigation of their effects.
Recommended Communication Approach:
The Final Report should be made widely available in electronic format as it contains a detailed
description of the methods developed and used in the Macquarie Valley, as well as valuable discussion
of the ecological effects of both in-stream and floodplain structures. The project team should be
encouraged to present their findings at workshops and conferences, as well as through journal
publications
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CS1 Project Summary