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