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Tributary Inflow Augmentation Strategies in the
Murrumbidgee River
Craig Mackay, Sinclair Knight Merz
Marcus Wright, Murrumbidgee CMA
Ian Varley, Sinclair Knight Merz
Overview
> The Murrumbidgee River System and River Reach Project
> Flow Augmentation – the principles
> Assessing the potential for making releases
> Tools for augmentation releases
The Murrumbidgee River System
Murrumbidgee River and Floodplain
Typical Riverbank Wetland Features
> Channel cut-off features forming
lagoons
> Low lying banks
> Different wetlands have different
thresholds; each wetland has an
individual threshold level water has to
reach before wetland receives water
River Regulation and Wetland Flooding
> Construction of large dams on Murrumbidgee has reduced
the frequency and duration of floods in the lower river
> Many lower river wetlands now receive much less water
than they would under unregulated conditions, especially
during seasonal flooding periods July - November
Average Seasonal Flow in the
Murrumbidgee River at Balranald
(Green = ‘natural’ flow;
blue = ‘current river regulation’)
From CSIRO Sustainable Yields
Study ‘ Water Availability in the
Murrumbidgee’, 2008
Flow Augmentation - Overview
> An attempt to compensate for reduced flood frequency and
size in the lower river
> Dam releases to artificially increase small floods in the
tributaries downstream of the dams
> Additional water raises river levels, produces overbank
flows into riverbank wetlands, and extends length of time
flows stay above riverbank thresholds
> The extra water increases depth and duration of wetlands
flooding downstream; increasing flood duration to 3+
days and follow-up events to reconnect wetland to river
highly desirable
The Murrumbidgee River System
Study Objectives
> What the study considered:
o What can be learnt from past attempts?
o How often will a successful and beneficial release be possible?
o How to manage a specific flow augmentation event?
> Builds on substantial work by New South Wales government
ecologists and hydrologists
Large Scale Environmental Flow Releases
in the Murrumbidgee
> 3 previous trials of note
o 1998: 118 GL dam releases without tributary inflow– quickly
attenuated downstream
o 2000: 38.5 GL tributary augmentation– judged a success
o 2001: Release in addition to a large irrigation release – limited
success
> Environmental releases alone can’t reach most wetlands
unless they are very large
> Irrigation releases provide good baseflow but difficult to
create a sustained peak without higher peak flows
> Tributary events best but most unpredictable
The 2000 Environmental Flow Release
> Release of 38.5 GL from the dams
over five days to augment tributary
inflows
> Concluded afterwards that the
additional water (DLWC, 2001):
o didn’t increase peak flows
downstream
o did extend the duration of the
event, and the flow into riverside
wetlands by 2-4 days
o Considered successful in
producing benefits for river
connectivity and biodiversity
Assessing how often Flow Augmentation
could be useful
> Important to know how often tributary inflows suitable for
flow augmentation occur as:
o It can involve water purchased at significant cost
o Need to have realistic expectations about when it might
be useful
Flow Augmentation – Occurrence of
Suitable Inflows
> A number of events occur in tributaries downstream of the dams each year
> Suitable events clustered in wet periods - much larger volumes may have
already reached upper bank levels – additional benefit?
> Can’t create suitable events in dry periods
> Need criteria to say whether flow augmentation is possible AND provides
additional benefit
Criteria for Flow Augmentation
1. Seasonal Value: Most ecologically valuable watering July – October
2. Minimum Size: Minimum unaugmented tributary event of 30,000
ML/d at Wagga Wagga
3. Minimum Duration: Enough water to keep flows >20,000 ML/d for
several days at Wagga Wagga
4. Flood Constraints: Large events can’t be augmented due to flood
and infrastructure constraints
5. Relative Value - volume: Volume otherwise reaching upper river
banks should be taken into account
6. Relative Value – duration: Extending long events has little
additional impact
Ability to make releases, and potential benefits of releases
Flow Augmentation – Potential Reliability
> 18 years between 1928 and 2005 satisfied all the criteria
(approximately 1 year in 4)
> Suitable events clustered in wet periods when much larger volumes
have already reached upper bank levels, but also some in wet- dry
transitional periods
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Reliability - Conclusions
> Flow augmentation proven to be effective in the past
> The preferred option for use of environmental water releases,
> Wet / dry climatic cycles mean it can’t be carried out every year,
and may not always produce additional benefit
Decreasing environmental benefit relative to unregulated tributary flow
Increasing likelihood of suitable tributary event and successful routing
Increasing water availablity
Environmental Dam Release Impact
Calculation
> How to assess the potential benefit of an augmented tributary
inflow?
> Routing tool developed based on calibrated routing parameters
from NSW DECCW IQQM model to assess relative impact of
release
Scenario Assessment Tool
> Impact of a specific release volume
depends on range of factors,
including tributary inflow and river
baseflow
> Rapid assessment tool to identify:
o Which wetlands receive water for a
given volume of environmental
water
o What volume of water and river
conditions are required to reach a
specific wetland
Conclusions
> Augmentation of tributary inflows in July-October is the
preferred use of Environmental Water releases, however:
> Dry years – limited chances, far less certainty in tributary
response to rainfall
> Wet years – less relative benefit
> Ideally target drier than average years when tributary events
are still around but water is scarce
> Still very difficult operationally to carry out! River managers
need flow routing tools and guidance on what can be
achieved with water available
Acknowledgements
> The project was supported by the Murrumbidgee River
Reach program, which is exploring the potential for
obtaining environmental water through trade with irrigators http://www.environment.gov.au/water/policy-programs/water-
smart/projects/nsw16.html
> The project was carried out in conjunction with
Murrumbidgee CMA and Parsons Brinckerhoff
> Ilan Salbe from the New South Wales DECCW provided
technical advice on river routing
> Lawrence Lingam and State Water provided advice on river
operations and current flow management approaches