Gdh Raising Wivenhoe Final Report

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Department of Employment,Economic Development andInnovation

Report for Investigation of Options toincrease the flood mitigation

performance of Wivenhoe Dam

December 2011


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i41/24452/428742 Investigation of Options to increase the flood mitigation performance of Wivenhoe Dam

This Investigation of Options to increase the flood mitigation performance of Wivenhoe Dam(Report):

1. has been prepared by GHD Pty Ltd for the Queensland Department of Employment,Economic Development and Innovation

2. may only be used and relied on by the Department of Employment, Economic Developmentand Innovation

3. must not be copied, used by, or relied on by any person other than the Department ofEmployment, Economic Development and Innovation without the prior written consent ofGHD Pty Ltd

4. may only be used for the purpose as outlined in the Report (and must not be used for anyother purpose).

GHD Pty Ltd and its servants, employees and officers otherwise expressly disclaim responsibilityto any person other than the Department of Employment, Economic Development and

Innovation arising from or in connection with this Report.To the maximum extent permitted by law, all implied warranties and conditions in relation to theservices provided by GHD Pty Ltd and the Report are excluded unless they are expressly statedto apply in this Report.

The services undertaken by GHD in connection with preparing this Report were limited to thosespecifically detailed in the Terms of Reference and as detailed in this Report.

The opinions, conclusions and any recommendations in this Report are based on assumptionsmade by GHD Pty Ltd when undertaking services and preparing the Report (Assumptions),including, but not limited to those key assumptions noted in the Report, including reliance oninformation provided by others.

GHD Pty Ltd expressly disclaims responsibility for any error in, or omission from, this Reportarising from or in connection with any of the Assumptions being incorrect.

Subject to the paragraphs in this section of the Report, the opinions, conclusions and anyrecommendations in this Report are based on conditions encountered and information reviewedat the time of preparation and may be relied on for six months, after which time, GHD Pty Ltdexpressly disclaims responsibility for any error in, or omission from, this Report arising from or inconnection with those opinions, conclusions and any recommendations.


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ii41/24452/428742 Investigation of Options to increase the flood mitigation performance of Wivenhoe Dam

Contents1. Executive Summary 1

1.1 Overview 1

1.2 Next steps 3

2. Introduction 4

3. Reliance Statement 6

4. Background 7

4.1 Scope 7

4.2 Level of flood mitigation 9

4.3 Flooding and the Brisbane River catchment 9

4.4 Wivenhoe Dam 10

4.5 Brisbane River flooding 10

4.6 Historical flood events 10

4.7 Frequent Moderate Flood Events 17

4.8 Other flood influences 17

5. Dam Operation 20

5.1 Dam operating rules 20

5.2 Alternative Dam Operation Strategies 26

6. Dam Raising Options 32

6.1 Raise Dam crest 2 metres, maintain existing FSV 32

7. Hydrologic Modelling 39

7.1 Model description 39

7.2 Hydrologic modelling 42

8. Water Security 49

8.1 Level of Service (LOS) Objectives and LOS Yield 49

8.2 Mechanisms to defer new infrastructure requirements 50

8.3 Operation of desalination and purified recycled water 51

9. Downstream Flood Mitigation Options 52

9.1 Introduction 529.2 Expand Splityard Creek Dam storage 54


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iii41/24452/428742 Investigation of Options to increase the flood mitigation performance of Wivenhoe Dam

9.3 New flood retention dam 54

9.4 Bypass channel for Brisbane River 57

9.5 Channel upstream of Wivenhoe Dam to divert flow 57

9.6 Connect flood plains, new wetlands, other waterways 57

9.7 Watershed management 57

9.8 Backflow prevention 58

9.9 Levees and walls 58

9.10 Brisbane River 59

10. Economic Analysis 61

10.1 Overview 61

10.2 Hydrologic modelling 66

10.3 Benefits Flood Damage Analysis 66

10.4 Costs capital and operating 70

10.5 Economic evaluation 72

11. Summary and Recommendations 83

11.1 Summary 83

11.2 Recommendations 84

12. References 85

12.1 Queensland Water Commission 85

12.2 Seqwater 85

12.3 Department of Environment and Resource Management 85

12.4 Brisbane City Council 86

12.5 Other references 86

Table Index

Table 1 List of abbreviations 4

Table 2 Comparative Assessment Criteria 8

Table 3 Historical flood events peak flow and flood

volume comparison* 11

Table 5 Semidiurnal Tidal Planes 2011 (Maritime Safety

Queensland) 18

Table 6 Gate operating rules 22

Table 7 Increasing flood capacity of Wivenhoe Dam

adjustments to level 33

Table 8 Summary of potential upstream impacts 37


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Table 9 Adopted scaling factors for combined flows to the

Moggill gauge 40Table 10 Hydrologic model scenarios 43

Table 11 2011 Flood Brisbane River Hydrology Model 47

Table 12 1974 Flood Brisbane River Hydrology Model 47

Table 13 1893 Flood Brisbane River Hydrology Model, 47

Table 14 Probable Maximum Precipitation Design Flood

simulation results 48

Table 15 Summary of scenarios tested 50

Table 16 Probability of reaching Grid 12 Dam triggers for

desalinated and PRW 51Table 17 Assessment criteria 52

Table 18 Alternative Options Workshop Evaluation

Summary 53

Table 19 Influence of Lockyer Creek and Bremer River flows,

base case scenario (S0) 56

Table 20 Key costs and timing assumptions 71

Table 21 Economic analysis summary results 77

Figure IndexFigure 1 Brisbane River Catchment 15

Figure 2 1893 Flood Hydrograph 16



Figure 5 Gate Operating Rules Schematic 21

Figure 6 Comparison of 2007 and 2011 gate operating rules

(0 m3/s to 35,000 m3/s) 23

Figure 7 Comparison of 2007 and 2011 gate operating rules

(0 m3/s to 5,000 m3/s) 23

Figure 8 Wivenhoe lake level and flow 100 % Full Supply

Volume (0 m3/s to 35,000 m3/s) 25

Figure 9 Wivenhoe lake level and flow 100 % Full Supply

Volume (0 m3/s to 5,000 m3/s) 26

Figure 10 Existing gate operating rules Alternative FSVs (0

m3/s to 35,000 m3/s) 27

Figure 11 Existing gate operating rules Alternative FSVs (0

m3/s to 5,000 m3/s) 28

Figure 12 Existing gate Operating Rules 2 metre Damraising (0 m3/s to 35,000 m3/s) 29


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Figure 13 Existing gate Operating Rules 2 metre Dam

raising (0 m3

/s to 5,000 m3

/s) 29Figure 14 2011 Operating Rules against modified Operating

Rules (0 m3/s to 35,000 m3/s) 30

Figure 15 2011 Operating Rules against modified Operating

Rules (0 m3/s to 5,000 m3/s) 31

Figure 16 Wivenhoe Dam level, inflow and outflow January

2011 33

Figure 17 Water level and outflow comparison alternative

FSV 34

Figure 18 Target operating line for Wivenhoe Dam and

Somerset Dam (Seqwater, 2011) 41

Figure 19 Comparison of Seqwater simulated and recorded

hydrographs (Moggill gauge) 42

Figure 20 Estimated flood damage curve Brisbane City

Council* 63

Figure 21 Economic Analysis Methodology 65

Figure 22 Economic Modelling framework 66

Figure 23 Combined Stage Damage Curve 68

Figure 24 Loss probability curve Brisbane (Illustrative)* 70

Figure 25 Economic analysis and cost benefit analysis 73

Figure 26 NPV ($) for 1893 Flood Event options 74



Figure 29 Area of land inundated in Brisbane (hectares)

various flow rates 80

Figure 30 Length of road inundated in Brisbane various flow

rates 80

Appendices

A Hydrologic Model

B Economic Analysis - Cost Summary

C Economic Analysis - Sensitivity Analysis Results

D Queensland Water Commission Report

E Maps


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1. Executive Summary

1.1 Overview

In September 2011, as part of the overall Queensland Government response to the January 2011

Brisbane floods, the Department of Employment, Economic Development and Innovation (DEEDI)

commissioned GHD Pty Ltd (GHD) to carry out a rapid assessment of various options to improve

mitigation of floods downstream of Wivenhoe Dam. Subject to Government consideration, the Study

results may be used to inform more detailed investigations.

This Study report presents a preliminary rapid assessment and evaluation of potential options to improve

flood mitigation downstream of Wivenhoe Dam based on three larger historically recorded events (1893,

1974 and 2011). The variability of the characteristics of these historical events is reflected in theeffectiveness of Wivenhoe Dam to mitigate floods.

The Study includes consideration of the costs of increasing flood mitigation against the relative benefits

gained through reduced damages downstream. The costs associated with changing the timing of water

supply infrastructure required to maintain the current Level of Service (LoS) objectives were also

examined.

The modelling undertaken in the Study has been based on simplified hydrologic and economic modelling

techniques. While this provides some guidance and indicative outcomes, more sophisticated modelling

techniques will be required to verify and validate the findings of this study. The Study does not consider,

within its scope, modelling the full range of flood events including the impact of the more frequent and

moderate flooding events.

Caution should be exercised in the review of the options considered given the significant variability of

historical flood events and the potential for consequential flood risks. This variability was demonstrated in

the hydrologic modelling outputs where flood mitigation benefit may be apparent for one or two of the

historic flood events, but rarely all three showed significant benefit for all scenarios.

The Study considered scenarios involving a range of structural and non-structural options to assess the

potential improvements in flood mitigation and associated benefit, including:

lowering the Full Supply Volume (FSV) to 50% with no change to the Dam wall;

lowering the FSV to 75% with no change to the Dam wall;

raising the Dam wall by 2 metres while maintaining FSV;

raising the Dam wall by 2 metres with FSV at 75%; and

variation of the operating releases from the Dam.

The hydrologic modelling used to compare these options adopted The Manual of Operational Procedures

for Flood Mitigation at Wivenhoe Dam and Somerset Dam, Version 8, September 2011 (2011 FOM).

The 2011 (FOM) states that the selection of release rates at any point in time is a matter for the

professional engineering judgment of the Duty Flood Operations Engineer.... This allows each event to

be managed based on inflows, outflows, predicted stream flows and lake levels as well as other

operating rules stated in the manual.


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However, this manner of operating the dams would give any number of gate operating strategies for

each separate option and scenario that was investigated during the Study. Therefore, in order to identifya finite number of scenarios and be able to duplicate and consistently model the options to allow direct

comparisons to be easily made, our simplistic model used the more rigid set of progressive operating

rules within the manual called Loss of Communications procedures.

1.2 Findings

In general, the following broad findings were made:

A 2 metre increase in the crest level of the Dam provides approximately 437,000 ML (437 GL) of

additional flood mitigation capacity giving a total volume in the flood storage compartment of

approximately 2,404 GL and a subsequent total volume in Wivenhoe Dam of 3,596 GL;

Reducing the FSV to 75% and using the 2011 FOM (version 8) shows a decrease of the flood peak

at the Port Office for the three events;

Lowering the FSV levels permanently will require planned water infrastructure to be brought forward

(3 years for a permanent FSV of 75% and 11 years for a permanent FSV of 50%) at a significant

economic cost;

Two out of three of the flood events modelled showed that alternative operation of the Wivenhoe and

Somerset Dam may have the potential to reduce peak levels at the Port Office gauge. However, for

the modelled 1974 event, the early release strategies increased the peak flood level at the Port

Office gauge compared to the 1974 event (including Wivenhoe and Somerset operated as per the

2011 FOM); However, it has been found by other studies that early release strategy can have a greater impact on

the more frequent and moderate flood events including increased flows during smaller flood events.

This requires further detailed modelling in order to fully determine the impacts;

Additionally, previous hydrologic modelling has demonstrated that early release strategies worsen

downstream flood impacts associated with more frequent flood events. Five such events have been

recorded since Wivenhoe Dam was constructed. Accordingly, determining an optimum release

strategy will require extensive detailed modelling including a complete re-evaluation of the design

hydrology for the basin. This work is outside the scope of this study;

This outcome confirms the need for the Wivenhoe and Somerset Dam optimisation study

recommended by the Queensland Floods Commission of Inquiry which has already been initiated bySeqwater (long-term study). This study will allow Brisbane River system, including the Wivenhoe

and Somerset dams as a whole to be fully modelled in detail. GHD understands that although the

catchment areas of Bremer River and Lockyer Creek are not within the scope, their impacts on the

Brisbane River system will be included;

The cost of raising the dam crest by 2 metres is estimated to be approximately $400 million. This

cost does not include substantial modification to the gates (potentially up to $330 million), potential

upgrades associated with the requirement for Probable Maximum Flood (PMF) or fishways

(potentially greater than $100 million). The cost has been based on this rapid assessment and is

likely to increase during any preliminary design phase;

It is estimated that it would take approximately four years to raise the crest of the dam. However, this

is based on no delays in the design, approvals, consultation and construction of the project; and


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Based on the three events modelled the flood mitigation benefits were variable for many of the

options investigated across the three flood events. For example, while the options of alternativeoperation of the Wivenhoe and Somerset Dam and the 2 metre raise (100% FSV), had minimal

impacts during the 2011 flood event, these same options showed no benefit for the 1974 flood event,

with most benefit realised during the 1893 flood event.

While the above results provide an overview of performance of three larger historical floods in the

Brisbane River, caution should be applied in drawing conclusions from this information because of the

potential for significant variability in actual flood events in terms of flow patterns and flood risk. A more

detailed assessment of a broader range of scenarios using a more complex hydrologic model is

considered necessary before any detailed conclusions are reached.

The Study clearly demonstrates that there is no single solution to improving flood mitigation downstream

of Wivenhoe Dam given the significant variation in flood benefits realised across the nominated floodevents. However, before consideration is given to an additional infrastructure to improve flood mitigation,

opportunities may exist to improve performance through better use of existing assets and procedures.

These opportunities should be assessed in greater detail within the scope of the long-term study.

1.3 Next steps

Given the very preliminary nature of this study and the simplistic hydrologic and economic modelling, it is

recommended that further analysis be undertaken on options to optimise flood mitigation effects through

a detailed, integrated assessment of available storage and early releases from Wivenhoe and Somerset

dams with inflows from other tributaries (Lockyer and Bremer Rivers).

This would entail, but not limited to:

Further examination and development of damage curves, including identification and survey of

affected private and public property;

Optimisation of the operation of the Wivenhoe and Somerset dams;

Development of decision support tools for the operation of the Wivenhoe and Somerset dams,

Risk based assessment of release strategies; and

Integration of changes to Wivenhoe and Somerset dams operation as part of a catchment wide flood

management plan, incorporating early warning, land use planning and localised flood prevention

solutions.GHD understands that the long-term study which will investigate the system as a whole and will

investigate many of these issues is already underway.


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2. Introduction

As part of the overall Queensland Government response to the January 2011 Brisbane floods, DEEDI

commissioned GHD to carry out a high-level, rapid assessment of various options to improve the

mitigation of floods downstream of Wivenhoe Dam. The Study objective is to identify a number of high

level options to improve the flood mitigation performance of Wivenhoe Dam. It was not intended that this

study would provide one answer or to provide a definitive outcome. It was always intended that this rapid

assessment would provide a number of high level options that would enable further in-detail

investigation. It was also intended that a number of options would be identified which could be eliminated

from future investigation due to the indicative high cost or the construction period required or the

technical feasibility.

The Study objective is to provide a preliminary assessment of flood mitigation options and identify any

consequential water supply issues. The Study includes consideration of the costs of increasing the flood

mitigation against the relative benefits gained through reduced damages downstream together with

consideration of the impact on the timing for water supply infrastructure required to maintain the current

LoS objectives.

Table 1 List of abbreviations

Abbreviation Meaning

AAD Average Annual Damages ($)

AEP Annual Exceedance Probability

AHD Australian Height Datum

ARI Average Recurrence Interval

BCR Benefit Cost Ratio

Cumecs Cubic metres per second

EL Equivalent Level

2011 FOM Manual of Operational Procedures for Flood Mitigation at Wivenhoe Dam

and Somerset Dam, Version 8, September 2011

100% FSV 67 metres AHD (100% Full Supply Volume)



FSV Full Supply Volume

GIS Geographic Information System

GL Gigalitres (one billion litres)

GL/a Gigalitres per annumHAT Highest Astronomical Tide


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Abbreviation Meaning

LAT Lowest Astronomical Tide

LoS Level of Service

l/p/d Litres per person per day

MHWN Mean High Water Neaps

MHWS Mean High Water Springs

MLWN Mean Low Water Neaps

MLWS Mean Low Water Springs

ML Megalitres (one million litres)

ML/a Megalitres per annum

MSL Mean Sea Level

m3

Cubic metres

m3/s Cubic metres per second (flow rate)

NPV Net Present Value

Ogee crest Type of open spillway

PMF Probable Maximum Flood

PMP-DF Probable Maximum Precipitation Design Flood

PRW Purified recycled water

QWC Queensland Water Commission

TUFLOW Two-dimensional hydraulic modelling software platform

W2, W4 etc Reference predetermined stages of flow control from Wivenhoe Damprescribed by the Manual of Operational Procedures for Flood Mitigation atWivenhoe Dam and Somerset Dam, Version 8, September 2011


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3. Reliance Statement

Due to the limited time available to produce this Report, caution should be exercised on any reliance of

the accuracy of the simplistic economic and hydrological modelling results from the Study. It has not

been possible, within the limitations of the Study, to check the reliability of all information and data, which

for the purpose of this Study, has been presumed to be accurate.

While GHD has attempted to review the results of available modelling against previous studies and

documentation, no formal validation or independent checks have been undertaken.

The scope of this Study has been limited to three historical flood events, so will not represent more

extreme events or types of flooding such as storm tide. Similarly, options for flood mitigation considered

have not taken into account consequential impacts on other areas of the Wivenhoe, Somerset and

Brisbane River systems. Importantly, the Study does not include, within its scope, modelling of the full

range of flood events including the impact of the more frequent and moderate flood events.

Construction rates have been based on detailed cost estimates prepared for recent dam upgrade

projects in South East Queensland in 2011. At present there is considerable volatility in construction

pricing, so construction rates should be viewed with caution. In addition, risk pricing has not been

included, which may change the cost estimate further. Dam work is subject to stringent environmental

controls and geotechnical and other conditions can differ from what is expected based on existing data.

Likewise, the impact of wetter than average seasons on construction cost rates has not been assessed.

A detailed hydrology model of the Brisbane River catchment was not available for this study, and so

sophisticated routing of flows downstream of Wivenhoe Dam was not possible. The hydrologic modelling

conducted for this study is based on level-pool reservoir routing through the dams, and a lagging and

summation of hydrographs downstream of Wivenhoe Dam to estimate the peak flow rate at the Moggill

gauge. No hydraulic modelling has been undertaken for the purposes of this Study.


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4. Background

4.1 Scope

The scope of this Study presented in the project Terms of Reference has been developed and refined

during the initial project planning phase and through consultation with the Steering Committee.

The purpose of this Study was to undertake an eight week rapid assessment to identify preferred options

to improve mitigation of floods downstream of Wivenhoe Dam. There are a number of concurrent studies

being undertaken by various stakeholders, including Seqwater and Brisbane City Council, in response to

the recommendations of the Queensland Floods Commission of Inquiry Interim Report. As such, this

Study should be viewed within the context of the overall strategy for flood management including local

flood mitigation solutions, development zoning and community awareness.

Any relevant preliminary findings from the Wivenhoe Dam Study and the Preliminary Optimisation Study

may be incorporated into the Seqwater long term study activity.

This Study has considered potential mitigation options against three major historical events, namely:

1893 flood event;

1974 flood event; and

2011 flood event.

Relevant data for each of these events has been supplied by Seqwater. Since the Study does not

consider the full range of flood events, caution should be exercised in the review of options given thesignificant variability of three historical flood events and the potential for consequential flood risks.

The scope of this Study, limited by the time available, is for the identification and assessment of broad

options presenting alternatives to the present operation and functioning of Wivenhoe Dam. These broad

options are intended to be capable of mitigating the nominated historical events as closely as possible to

the dual control discharges of 1,900 m3/s and 3,500 m

3/s from the Dam.

The Study consisted of two principal elements for the assessment of options to improve the mitigation of

flooding impacts:

A Wivenhoe Dam Studyconsidering options to improve the flood mitigation potential of Wivenhoe

Dam; and

APreliminary Optimisation Studyseeking to identify various alternative options for flood mitigation.

Short-listed options for Wivenhoe Dam have been subjected to an economic assessment including

assessment of the reduced flood level in the Brisbane River and consequential benefits of each option.

Flood damage curves were used to determine the cost and therefore the economic benefit associated

with each option.

4.1.1 Wivenhoe Dam Study

A rapid assessment approach was adopted for this investigation to achieve the desired outcomes within

the timeframe. The investigation considered the following:

raising of the Dam crest by 2 metres (with FSV at current levels);


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raising of the Dam crest by 2 metres (with FSV at 75% of current levels);

lowering the FSV to 75% of current levels;

lowering the FSV to 50% of current levels; and

raising the downstream bridges and modifying the operational releases from the Dam.

A 2 metre increase in the crest level of the Dam provides approximately 437,000 ML (437 GL) additional

storage. For each option the Wivenhoe Dam Study considered:

the cost and time for implementation of the measures;

likely impacts upstream of the Dam wall including potential impact on existing infrastructure;

the likely benefit in terms of reduced flood damages (based on the 1893, 1974 and 2011 flood

events); and consequential impact on the Water Supply Infrastructure Program.

The development of these options used Manual of Operational Procedures for Flood Mitigation at

Wivenhoe Dam and Somerset Dam, Version 8, September 2011. GHD understands that there is now a

Manual of Operational Procedures for Flood Mitigation at Wivenhoe Dam and Somerset Dam Version 9

November 2011. However, this manual was released after the modelling for this study was completed so

Revision 9 has not been assessed nor the impact on the outcomes of this Study.

All options for the raising of the Dam wall have been considered against the required capacity to pass the

latest estimate of Probable Maximum Flood (PMF) as required by 2035 under the Guidelines on

Acceptable Flood Capacity for Dams.

For each option a comparative assessment of the following was made:

Table 2 Comparative Assessment Criteria

Potential benefits Costs

Net reduction in damage costs Capital and operational costs for construction of newinfrastructure

The need to bring forward water supply infrastructureinvestment to maintain levels of service

The hydrological and hydraulic nature of the contributing catchments within the Brisbane River system is

complex and subject to extreme variability between historic flood events. Management of flooding in the

system is equally complex and a single solution is unlikely to be sufficient.

Due to the timing and the short timeframe for the report to be undertaken and finalised, only existing

available data was used. For example, the flood damage curves from 2006 were used with an escalation

factor as much of the information from 2011 is not currently available.

4.1.2 Preliminary Optimisation Study

The Preliminary Optimisation Study required the consideration of various flood mitigation and water

supply scenarios comprised of infrastructure and non-infrastructure measures.


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Infrastructure options include:

raising of bridge crossings immediately downstream from Wivenhoe Dam;

flood retention storages in tributary river systems discharging downstream of Wivenhoe Dam

(Lockyer Creek and Bremer River);

levees; and

backflow prevention devices.

Non-infrastructure options were also considered, such as modifications to the operational procedures for

Wivenhoe Dam. This preliminary study is limited in its scope, time frame and this is reflected in the use of

the simplified economic and hydrologic models. GHD understands that the long term Wivenhoe and

Somerset Dam optimisation study which will investigate the system as a whole was a recommendation

of the recent Flood Inquiry and is already underway.

4.2 Level of flood mitigation

This Study does not consider options that attempt to achieve complete non-flooding of the downstream

catchment, given the corresponding impracticalities associated with Brisbanes position within the flood

plain of the Brisbane River. The other complicating factor in defining flood mitigation options is that every

flood behaves differently, and is dependent on where the flood originates, where the rainfall occurs, and

what areas are flooded.

The options that have been investigated have been ones that either achieve a level of flood mitigation or

are based on economic cost/benefit analysis. For example, when costs were identified as very high and

there was limited flood mitigation potential, then the option was rejected.

4.3 Flooding and the Brisbane River catchment

The lower extent of the Brisbane River, including the passage through Brisbane City, is highly flood-

prone given its low-lying position in the large Brisbane River Basin[1]. The catchment itself, combined with

the Bremer River catchment, includes a total area of 13,222 km2. Approximately half of this area is

downstream of the Wivenhoe Dam.[2]

Thousands of residential, commercial and industrial properties

have been constructed on the flood plain in the Brisbane metropolitan and Ipswich areas.

Changes to the hydraulic character of the Brisbane River, including periodic dredging and the

construction of Somerset Dam (commissioned in 1953), Cressbrook Dam (commissioned in 1982) andWivenhoe Dam (commissioned in 1984), have significantly altered the flood effects of heavy rainfall in

the Upper Brisbane River catchment. Consequently, comparative analysis of historical flood events is

contingent on an allowance for alterations to river bathymetry, and more significantly, increased

catchment storage and flow attenuation.

Despite changes to the river itself, the 1893 and 2011 floods were of a reasonably similar magnitude in

terms of total catchment rainfall, peak river flows, and volumetric outflow from the Brisbane River, while

the 1974 flood involved significantly less catchment rainfall and a lower peak river flows [3].

The 1893 and 2011 floods are characterised by more significant rainfall in the Upper Brisbane River and

Stanley River catchments, while the 1974 flood experienced more significant rainfall in the Bremer River

catchment and Brisbane metropolitan areas.


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4.4 Wivenhoe Dam

Construction of Wivenhoe Dam was formally proposed in 1971 by the Queensland Government with thedual purposes of providing a suitable urban water source and as a flood mitigation strategy.

An optimum flood storage volume was determined using the benefit-cost relationship between the cost of

Dam storage and assumed flood damages in 1974. Preliminary design considerations demonstrated that

even with the introduction of significant flood storage in Wivenhoe Dam, major flooding can still occur in

the Lower Brisbane River[4]

.

The Wivenhoe Dam has previously been classified using the ANCOLD Guidelines as an Extreme Hazard

Dam, as the design flood is the PMF which currently exceeds the spillway capacity. The Dam was

upgraded in 2005 to provide for the spillways to pass the 1 in 100,000 Annual Exceedence Probability

(AEP) flood. The AEP is simply the probability that a particular flood will occur in any one year. This

upgrade included construction of a three-bay fuse plug spillway on the right abutment [5]. A secondary

fuse plug and auxiliary spillway on the left abutment, intended to improve the capacity of the spillways to

the PMF, is due for completion by 2035.

Approximately half of the Brisbane River catchment (6,232 km2of 13,222 km

2) is located downstream of

the Wivenhoe Dam[6]

, and the flooding effects of rainfall in the lower catchment are therefore unaffected

by operation of the Dam.

4.5 Brisbane River flooding

4.5.1 Riverine floodingRiverine flooding was the primary cause of the floods in 1893, 1974 and 2011. Extended periods of

heavy rainfall in the catchment increase flow volumes within the Brisbane River to the point that the

capacity of the River was exceeded.

The Brisbane City (Port Office) gauge is a useful datum to compare peak flood levels during historical

flood events, and gauge heights are identified for each of the historical flood events identified[7]

.

4.5.2 Backwater flooding

All three major historical floods involved the effects of backwater flooding, whereby the raised river level

of the Brisbane River during the flood event caused water to back up into Brisbane River tributaries, with

flood run-off from the tributaries compounding backwater flood levels.

4.5.3 Storm surge

Storm surge involves an increase in water levels within the receiving body of water (Moreton Bay) due to

atmospheric pressure reductions and wind fetch. Storm surge can increase the extent of flooding in the

Brisbane River through a backwater effect on the primary flows, particularly when a storm surge

coincides with the high tide[8]

.

4.6 Historical flood events

The three most significant historical flood events are summarised in the following section. A comparisonof the peak flow and flood volume characteristics of the significant historical flood events is shown in


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Table 3. The summary flows and volumes include all catchment areas downstream to Moggill, west of

Brisbane City (95% of the total catchment area). The estimates in Table 3 were supplied by Seqwaterfor the purposes of this Study, and are based on Seqwater calibrated hydrology models.

Table 3 Historical flood events peak flow and flood volume comparison*

1893 1974 2011

Catchment Peak flow (m3/s) Volume (ML) Peak flow (m

3/s) Volume (ML) Peak flow (m

3/s) Volume (ML)

Stanley River 6,0531,080,976 3,961 551,381 5,519 705,262

Upper BrisbaneRiver

8,987 1,718,258 6,121 1,173,460 8,709 1,675,134

Lockyer Creek 2,460501,754 4,423 765,024 5,461 701,832

Mount Crosby 624176,593 1,360 197,306 2,289 201,299

Bremer River 856 196,825 4,223631,195 2,446 374,989

TOTAL 3,674,406 3,318,366 4,159,049

*Source: Seqwater Simulated Historical Flood Event Hydrographs

4.6.1 1893 Flood

In 1893, major flooding devastated parts of Brisbane.[2]

The flood represents one of the largest floods on

record in terms of both total rainfall and area inundated.[9]

The 1893 flood is similar in terms of rainfall

distribution and peak flows within the river system to the 2011 flood.

Flows into the area now occupied by the Wivenhoe Dam during the 1893 event were similar to the

2011 flood at approximately 2,650,000 mega-litres (ML).[9]

The Port Office gauge recorded a level of approximately 8.3 metres in the 1893 flood.[7]

4.6.1.1 Rainfall

Bureau of Meteorology records of the four-day peak of the 1893 flood suggested:

939 mm of rainfall was recorded in the Stanley River catchment; and

358 mm of rainfall in the Upper Brisbane River catchment.[8].

A maximum total of 1,026 mm of rainfall was recorded in the Brisbane River catchment during the monthof the 1893 flood. However, rainfall records for the time are largely inadequate for detailed analysis.

4.6.2 1974 Flood

The 1974 floods caused around $200 million in damage and resulted in 14 deaths.[8]

The flood was

mitigated to some extent by the construction of Somerset Dam in 1953, and the associated flow

attenuation.

The primary characteristic of the 1974 flood was heavy rainfall and outflows in the Bremer River and

Lower Brisbane River areas, with comparatively less rainfall and flows in the Upper Brisbane and Stanley

River catchments.10


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Flows into the Wivenhoe Dam area during the 1974 event were approximately half that of the 1893

and 2011 floods; and[9]


Rainfall

A monsoonal trough was present over South East Queensland in January 1974, south of normal latitude,

saturating the Brisbane River catchment.

Over a five-day period between January 24 and 29, a minimum of 300 mm of rainfall was

experienced in all parts of the Brisbane River catchment;

Between 500 mm and 900 mm of rain was recorded within the metropolitan area;

Over 1,300 mm of rain fell at Mount Glorious; and The most intensive rainfall occurred during a 24 hour period on Friday January 25, with between 300

mm to 500 mm recorded.

The 1974 flood saw more significant rainfall within the Lower Brisbane River and Bremer River

catchments relative to the 1893 flood, but less significant rainfall in the upper reaches of the Brisbane

River catchment.

4.6.3 2011 Flood

Flood inundation within Brisbane City in 2011 was remarkably similar to the 1974 flood[1]

, although river

flow volume during the January 2011 event was almost double the 1974 flood and rivals the 1893 flood,

exceeding a 1 in 100 AEP.[4]

Flooding from the January 2011 event resulted in 35 deaths in South East Queensland, in part reflecting

the increased urbanisation in and around riverine flood plains[11]

. Approximately 2,000 people were

relocated to emergency accommodation. The central business district was shut down for approximately

five-days at significant financial and commercial expense. Substantial damage was inflicted on services

and utilities in the Brisbane metropolitan and Ipswich areas.[11]

Operation of the Somerset and Wivenhoe dams attenuated the peak flow discharges into the Lower

Brisbane River (similar to the 1893 flood at approximately 2,650,000 ML[9]

), thus reducing flood levels

and consequently damage to urban areas and infrastructure.[3]


Rainfall

The following, unusually heavy falls were recorded in the catchment areas upstream of the Wivenhoe

and Somerset dams.[12]

rainfalls between 600 mm and 1,000 mm were recorded in parts of the Brisbane River Catchment in

December 2010 and January 2011;[13]

total daily rainfall of between 150 mm and 250 mm was experienced on average in the Brisbane

River catchment between January 9 and 11; and[1]

Seqwater records of the three-day 2011 flood suggested 412 mm of rainfall was recorded in the

Stanley River catchment, and 307 mm of rainfall in the Upper Brisbane River catchment, producing

the most significant peak flow rates.


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Sinclair Knight Merz concluded, based on review of overall data collected at alert stations, that the AEP

of the rainfalls for the whole [Wivenhoe] Dam catchment is likely to be between a 1 in 100 and 1 in 200year event.

[8]

Flood characteristics

Characteristics of the January 2011 flood include:[3]

a long duration (and lengthy associated rainfall period);

a large total flow volume;

a double peak in flow rate; and

under-forecast rainfall intensity prior to the peak of the flood up to 65% less than those actually

recorded.


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Figure 1 is a schematic representation of the Brisbane River catchment and associated tributaries,

demonstrating peak flows at flood volumes for each of the three historical flood events.

Figure 2, Figure 3 and Figure 4 demonstrate the simulated hydrographs for the historical floods

considered in the Study (including discrete hydrographs for each river catchment within the larger

Brisbane River catchment) as provided by Seqwater.

Key information regarding the benefits of Wivenhoe Dam and the 2011 FOM on these three major flood

events is presented in Table 4.

For the 1893 and 1974 flood events (prior to construction of Wivenhoe Dam), recorded flood levels

are significantly higher than the modelled flood levels which we undertaken assuming Wivenhoe

Dam had been in place; and

For the 2011 flood event (following construction of Wivenhoe Dam) the recorded flood level is2 metres lower than the level modelled assuming Wivenhoe Dam was not constructed.

Table 4 Summary of Peak Flood level at the Port Office for the scenarios tested at low tide

Flood Event (year)Peak Recorded Level at Port Office withoutWivenhoe Dam (metres AHD)

Wivenhoe Dam present Sep 2011FOM (metres AHD)

1893 8.3*[actual][7] 4.9 [modelled]

1974 5.5 [actual][7]

3.5 [modelled]

2011 6.46 [modelled][9]

4.46 [actual][7]

*Note: Somerset Dam was not present during the 1893 flood. This is the recorded levels at the Port Off ice without the benefit of

Somerset or Wivenhoe Dams.


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Figure 2 1893 Flood Hydrograph



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4.7 Frequent Moderate Flood Events

While this Study focusses on the three large historical events noted in Section 4.6 above, it is importantto acknowledge the moderate flood events which occur more frequently. Five moderate flood events

have occurred since Wivenhoe Dam was constructed, including the following approximate peak flows

recorded at the Moggill gauge[15]

:

Early April 1989 (1,700 m3/s);

Late April 1989 (1,700 m3/s);

February 1999 (1,900 m3/s);

October 2010 (1,500 m3/s); and

December 2010 (1,700 m3/s).

While these five events were not part of this Study, the long-term study should investigate moderate

events when considering changes into operating rules and other infrastructure solutions.

4.8 Other flood influences

While not part of the scope of this Study it is worth noting that tidal and coastal effects can have a

significant bearing on the extent of flooding in the lower Brisbane River reaches.

Flood level variations of over 1 metre can be expected for the range of flows anticipated (between

6,000 m3/s and 10,000 m

3/s) as a result of tidal, storm surge or local flooding influences.


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4.8.1 Tidal influences

Tidal conditions have a significant effect on the height of flooding at the Port Office gauge, downstreamin the Brisbane River catchment. This is most clearly illustrated by the difference between the 1974 flood

and the 2011 flood. Although the flows were of similar magnitude (between 9,500 m3/s and 10,500 m

3/s),

there was a 1 metre difference in flood level at the Port Office gauge (4.46 metres versus 5.46 metres).

This variation in flood levels primarily relates to the following tidal conditions:

the 2011 flood occurred during a small tidal range with high tides of under 1.0 metre; and

the 1974 flood coincided with king tides in excess of 2.2 metres.

The scope of this project has not considered mitigation of tidal backwater flow due to extreme high tides.

Table 5 indicates the Semidiurnal Tidal Planes for 2011. The numbers represent height (in metres)

above Lowest Astronomical Tide (LAT).

Table 5 Semidiurnal Tidal Planes 2011 (Maritime Safety Queensland)

Location Brisbane Bar

Mean High Water Springs (MHWS) 2.17

Mean High Water Neaps (MHWN) 1.78

Mean Low Water Neaps (MLWN) 0.76

Mean Low Water Springs (MLWS) 0.37

Australian Height Datum (AHD) 1.243

Mean Sea Level (MSL) 1.27

Highest Astronomical Tide (HAT) 2.73

The highest high tide or king tide for Brisbane was 2.71 metres (above LAT) on January 21 and May 17,

20111.

4.8.2 Storm surge

The effects of storm surge and local flooding should also be considered. Storm surge is the natural rise

in sea level as a result of low atmospheric pressure. Many of the most flood prone areas in the Brisbanemetropolitan area are around local creeks such as Oxley, Breakfast and Norman creeks. These creeks

carry run-off from the local area and the timing of the local peaks may coincide with the major flood peak

in the Brisbane River. In 2011, there was very little local flooding, which was extremely beneficial in

reducing flood inundation in these creeks. In 1974 local flooding was significantly worse.

1Maritime Safety Queensland 2010


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A storm surge is an atmospherically forced ocean response caused by extreme surface winds and low

surface pressure associated with severe and/or persistent offshore weather systems. The term stormtide refers to the rise of water associated with a storm, plus tide, wave set-up, and freshwater flooding.

Brisbane City Council does not have a contemporary storm tide assessment, however, Moreton Bay

Regional Council to the north and Gold Coast City Council to the south have recently undertaken storm

tide studies.


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5. Dam Operation

5.1 Dam operating rules

Wivenhoe Dam is a mixed-use structure containing:

1,165 Gigalitres (GL) of water supply storage for the Brisbane metropolitan area; and

an additional 1,967 GL of flood storage to mitigate flood inundation downstream.

5.1.1 Flood mitigation

The Dams flood storage capacity was increased from 1,450 GL to 1,967 GL during the Dam upgrade in

2005 via reconstruction of the wave wall on the crest into a water retaining structure. The main spillway isfitted with five 12 metre wide by 16 metre high radial gates. An auxiliary spillway was installed in 2005 on

the right abutment of the Dam. The auxiliary spillway includes three bays of fuse plug embankments,

which are designed to wash away in sequence during extreme flood events, in order to provide additional

emergency spillway capacity. The three fuse plugs trigger at different reservoir levels so that there is a

staged discharge. The three fuse plugs trigger progressively as the Dam water level reaches 75.7

metres AHD, 76.2 metres AHD and 76.7 metres AHD respectively.

Flood mitigation is provided by keeping the spillway gates closed, or nearly closed, and thereby retaining

incoming flows into the reservoir. Flood mitigation is implemented via the operating rules for the gates,

which have a number of decision points relating to expected inflows, lake level and the flows occurring

downstream of the Dam.

5.1.2 Gate operating rules

The current operating rules are published in the 2011 FOM. This Study has adopted these rules for the

purpose of hydrologic modelling and demonstrating the downstream effects of the various release rates

(see Figure 5). The 2011 (FOM) states that the selection of release rates at any point in time is a matter

for the professional engineering judgment of the Duty Flood Operations Engineer... This allows each

event to be managed based on inflows, outflows, predicted stream flows and lake levels as well as other

operating rules stated in the manual.

However, this manner of operating the dams would give any number of gate operating strategies for

each separate option and scenario that was investigated during the Study. Therefore, in order to identifya finite number of scenarios and be able to duplicate and consistently model the options to allow direct

comparisons to be easily made, our simplistic model used the more rigid set of progressive operating

rules within the manual called Loss of Communications procedures.

These are provided in the event of total communications failure to ensure the Dam operators open the

gates in a manner to allow the extreme flood that the Dam is designed for to pass safely. This approach

was taken as it provides for a repeatable basis for the comparison of the options. However it is

recognised that inclusion of more flexible operational rules to match the actual procedures within the

current operations manual as well as those rules implemented during the actual 2011 flood event are

likely to provide different results. It is highly recommended that further more detailed modelling is

undertaken and it is understood that this will occur in the long-term optimisation study.


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21

41/24452/428742

InvestigationofOptionstoincreasethefloodmitigationperformanceofWivenhoeDam

Figure

5

GateOperatingRulesSchematic


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The gate operating rules incorporate a number of procedures, which provide a different priority in

decision making for gate releases, and therefore downstream flows, based on the inundation of

community infrastructure downstream. These procedures were updated and approved in September

2011. The 2011 FOM procedures are summarised in Table 6 and the Flood Rules (Ver 7) have been

provided for comparison. The 2011 operating rules use a flowchart decision tool for the management of

flood waters within the W1 category. In contrast, the 2007 operating rules have defined flow rates for the

categories 1A-E as described below.

Note: the 2011 FOM and the simplistic model accounts for the interaction between Wivenhoe Dam and Somerset Dam through the

target operating line provided in the manual. In the following sections, when Wivenhoe Dam operation is described, it is in the

context of both Wivenhoe and Somerset dam operating together.

Table 6 Gate operating rules

2011 FOM Ver 8 2007 FOM Ver 7

Min lake level(metres AHD)

Max lake level(metres AHD)

Other limits (m3/s) Flow Limit (m

3/s)

W0 67 (FSV) 67.25 no release 0

W1A 67.25 67.5 Colleges Crossing


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Figure 6 Comparison of 2007 and 2011 gate operating rules (0 m3/s to 35,000 m

3/s)

Figure 7 Comparison of 2007 and 2011 gate operating rules (0 m3/s to 5,000 m

3/s)


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5.1.4 W1 operating protocol

For lake levels from 67.0 metres to 68.5 metres Australian Height Datum (AHD), floodwaters are stored

in the Dam in order to restrict the flow rates in the Brisbane River downstream so that key bridges at

Colleges Crossing, Burtons and Kholo remain open for as long as possible. This is intended to minimise

disruption to the community that use these bridges.

5.1.5 W2 and W3 operating protocol

The W2 and W3 procedures restrict the flow rates in the Brisbane River downstream of Wivenhoe Dam

to flow limits for Brisbane urban areas whereby damage is minimised. These flow rates are achieved

through the storage of the flood waters behind the partially open gates at the Dam. The W2 flow is

defined as 3,500 m3/s maximum release from the Dam with targets of 3,500 m

3/s at Lowood and

4,000 m3/s at the Moggill gauge. The W3 flow is defined as 4,000 m3/s maximum release from the Dam

with targets of 4,000 m3/s at the Moggill gauge.

5.1.6 W4 operating protocol

The current Wivenhoe Dam operating protocols switch to Dam safety mode at W4, or when the water

level reaches 74 metres AHD.

The W4 procedure defines the point at which the flood waters are no longer stored behind the gates. The

W4 procedure is primarily concerned with protecting the safety and structural integrity of the Dam by

ensuring the flood waters are released in a managed fashion to avoid overtopping the main Dam crest at

80 metres AHD. The 2011 FOM curves defined provide for this managed release. For this procedure, thegates are rapidly opened to discharge the flood and bring the lake level under control.

It is at the start of the W4 procedure that significant damage to community infrastructure occurs

downstream of the Dam. As such, any adjustments to flood mitigation needs to have an associated

change to the start of procedure W4.

At this point, the gates are opened rapidly and progressively until the water level begins to drop;

If the water level continues to rise, the gates must be opened fully before the fuse plugs trigger at

75.7 metres AHD to ensure the increase in Dam outflow caused by the fuse plug triggering is small

relative to the total flow. This is achieved by having the gates fully open with a large flow though the

spillway and therefore the incremental increase in flow as a result of the fuse plug embankments

washing away is smaller;

There is no opportunity to adjust the fuse plug embankment flow as this is controlled by the level of

the channel and the width of the fuse plug section;

When operated as designed, the fuse plugs act in the same manner as low cost gates and when

triggered result in a small increase in flow of 2,250 m3/s which is less than a 20% increase in flow

downstream of the Dam;

In accordance with the Wivenhoe Minimum Gate Opening operating curve, activation of the fuse

plugs in conjunction with fully opened gates comprises an increase in flow from 10,250 m3/s to

12,500 m3/s;

In the case of triggering the fuse plugs, flow can still be partially regulated by the gates tocompensate for the additional flow from the fuse plugs;


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The transition to the fully opened gate can be seen in Figure 17, as the graph diverges sharply at a

Dam level of 73 metres AHD; and

If the fuse plugs are triggered during a flood event, they can be reinstated as the concrete base of

the plugs is at FSV of 67 metres AHD.

Figure 8 and Figure 9 demonstrate the relationship between the Wivenhoe Dam lake level (AHD) and

flow discharge from the Dam during a significant flood event under 2011 FOM.

At the commencement of W4 at 74 metres AHD there is an increase in the flow from approximately

4,000 m3/s to 10,000 m

3/s. This corresponds to a change in the reservoir level of only 1.5 metres. This

demonstrates the aggressive release strategy once W4 is reached.

The peak spillway capacity is reached at the current Dam crest level of 80 metres AHD, with a

corresponding outflow of nearly 30,000 m3/s. This is needed to meet the design standard for the Dam

flood capacity.

Figure 8 Wivenhoe lake level and flow 100 % Full Supply Volume (0 m3/s to 35,000 m

3/s)


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Figure 9 Wivenhoe lake level and flow 100 % Full Supply Volume (0 m3/s to 5,000 m

3/s)

5.2 Alternative Dam Operation Strategies

Four key options have been considered as part of this Study:

2011 FOM combined with alternative FSVs;

2011 FOM combined with Dam raising;

2011 FOM combined with Dam raising and alternative FSVs; and

modified gate operating rules.

Many of the options considered as part of this Study focus on modifications to Wivenhoe Dam operation.

A number of alternative operations curves have been analysed. Alternative strategies to those developed

could be derived that would result in different flood levels. Further, the full range of design events have

not been considered for assessment of alternative Dam operation curves, as this assessment is basedon the expected performance for three key historical floods (1893, 1974 and 2011), and as each flood

has different characteristics.

In particular, it is important to have regard to more frequent and moderate flood events in relation to any

changes in gate operating rules. Previous hydrological modelling has demonstrated that early release

strategies worsen downstream flood impacts associated with more frequent flood events.

It is therefore recommended that further study is given to operational changes which show the potential

to reduce the downstream effects of different flood events.


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A number of different operating rules have been investigated to assess their impact on peak flows and

water levels for the 1893, 1974 and 2011 floods. Each flood has different characteristics which make it

practical to use as a test for the effectiveness of the option:

1893 flow centred on the Somerset Dam area, highest flows into the Wivenhoe Dam;

1974 flow centred on the Bremer River and local Brisbane creeks, a scenario for which the

Wivenhoe Dam has the least mitigation potential; and

2011 flow centred on Wivenhoe Dam, Lockyer Creek and the stretch of Brisbane River between

the Dam and Mt Crosby Weir with very little flow in the local Brisbane creeks.

5.2.1 2011 Operating Rules Alternative FSVs

The Study considered the same operating curve to the W4 limit as shown in Figure 10 and Figure 11,

with alternative FSVs (50%, 75% and 100%). These options investigate the change in flood impact as a

result of the additional storage within the Dam. However, water security is affected by lowering the FSV,

therefore additional infrastructure is needed to be brought forward in order to maintain the LoS

objectives. As demonstrated, the operating curves differ only at the very low flows.

Figure 10 Existing gate operating rules Alternative FSVs (0 m3/s to 35,000 m

3/s)


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Figure 11 Existing gate operating rules Alternative FSVs (0 m3/s to 5,000 m

3/s)

The lowered FSV options use the same operating rules at higher flows. The benefit relies solely on the

additional flood storage volume available within the Dam by starting the flood event with a lower FSV in

the reservoir.

5.2.2 2011 Operating Rules Dam raising

An alternative to lowering the FSV is to raise the Dam by 2 metres thereby expanding the flood storage

available and not impacting on water security. For this option the W4 trigger level and fuse plugs are

raised accordingly. An additional option was also considered of raising the Dam by 2 metres in

conjunction with a lowering to 75% FSV. Indicative operating curves for both of these options are shown

in Figure 12 and Figure 13.


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Figure 12 Existing gate Operating Rules 2 metre Dam raising (0 m3/s to 35,000 m

3/s)

Figure 13 Existing gate Operating Rules 2 metre Dam raising (0 m3/s to 5,000 m

3/s)


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The 2011 FOM is shown in blue below the new gate curves in Figure 12. The option of raising Wivenhoe

Dam by 2 metres needs to have an operating curve that discharges 10,000 m3/s from the Dam at 77.7

metres AHD to match the location of the new fuse plugs which have also been raised by 2 metres.

At this point the gates must be fully open to allow the increase in flow in the Brisbane River downstream

of the Dam due to the triggering of the fuse plugs to be approximately 20%.

If the 2 metre raising was considered with the lowered FSV, the transition to the W4 limit allows

regulation of the flow to an appropriate limit. In this case, 2,000 m3/s has been adopted to minimise

closure of the Brisbane Valley highway bridges. Alternatives might include setting this limit at 3,500 m3/s

to match flow limits for Brisbane urban areas whereby damage is minimised. This operating curve still

has to ensure the gates are fully open at the new W4 level of 76 metres AHD. It is assumed that the fuse

plugs will trigger at 77.7 metres AHD with the 2 metre raise.The cases where the fuse plugs are triggered have been rejected because they do not provide a

mitigation of floods (which is the primary objective of the study). When the fuse plug is triggered, there is

a move to a zone where the outflows increase in all cases modelled.

5.2.3 Modified gate operating rules

An alternative to Dam raising, or lowering the reservoir FSV, is an adjustment to the early stages of gate

operation to maximise flood storage at the expense of inundation of downstream bridges at low flows.

Two alternatives have been assessed as shown in the curves in Figure 14 and Figure 15.

Figure 14 2011 Operating Rules against modified Operating Rules (0 m3

/s to 35,000 m3

/s)


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Figure 15 2011 Operating Rules against modified Operating Rules (0 m3/s to 5,000 m

3/s)

These curves show alternatives as follows:

Starting at 75% of FSV with a more rapid increase in flows than the existing gate operations rules.This still allows for progressive closure of bridges but increases flow up marginally faster than the

existing W1 rules. This makes maximum use of both the increased flood storage by having a lower

FSV level and the increased available discharge range in the lake by having the FSV 3 metres lower;

and.

The alternative is to remove the W1 operating rule altogether, and within 1 metre of lake level rise

increase the flow to the flow limit for the protection from inundation of Brisbane urban areas of 3,500

m3/s. While flood damage occurs below this release rate, flows above this rate increase damages as

flood waters affect a much larger number of properties. This alternative strategy assumes that

bridges are either raised to maintain access, or that the inundation of the bridges is tolerable.


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6. Dam Raising Options

The following section considers the options for raising Wivenhoe Dam in relation to the effects on flood

mitigation.

6.1 Raise Dam crest 2 metres, maintain existing FSV

This option examines the relative effectiveness, in terms of flood mitigation, of increasing the available

flood storage capacity through raising the Dam crest and keeping the FSV at its current level. A 2 metre

increase in the crest level of the Dam provides approximately 437,000 ML (437 GL) of additional storage.

The storage of flood waters during the nominated flood events must be balanced with managing extremeflood events, such as the PMF, which are likely to cause a rise to the overall the Dam crest level.

The fuse plugs installed at Wivenhoe Dam in 2005 provide the capacity required to discharge the

extreme flood events, but are not required to be used for smaller floods, as occurred in January 2011.

The operating rules require that where fuse plug initiation cannot be avoided (75.7 metres AHD) the

radial gates are to be fully raised prior to the initiation of the first fuse plug.

Any raise to the Dam crest must therefore be coupled with an increase in the fuse plug trigger level, and

adjustment to the corresponding flood rules in the operating protocols. This is particularly important for

the level at which W4 is initiated, at which point flood damage to Brisbane becomes secondary to

maintaining the safety of the Dam.

Figure 16 demonstrates the operation of Wivenhoe Dam during the January 2011 Flood Event.


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Figure 16 Wivenhoe Dam level, inflow and outflow January 2011

It should be noted that recent reports on the raising of Wivenhoe Dam consider increasing the FSV from

67 metres AHD to 69 metres AHD, with a corresponding raise to the Dam crest (from 80 metres AHD to

82 metres AHD) in order to maintain the flood capacity of the Dam. These reports are not applicable in

the context of this Study, as increasing the Dams ability to mitigate floods requires that the FSV be

maintained at its current level, with an increase to the overall Dam crest level, thereby improving the

flood capacity rather than water supply volume.

The primary level changes considered in this Study are summarised in Table 7.

Table 7 Increasing flood capacity of Wivenhoe Dam adjustments to level

Existing (metres AHD) 2 metre raised Dam option (metres AHD)

Ogee crest level 57 57

FSV 67 67

W4 trigger limit 74 76

Fuse plug trigger level 75.7 77.7

Dam crest flood level 80 82

W4 Release


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Figure 17 Water level and outflow comparison alternative FSV

The proposed 2 metre raising of the Dam crest level permits the W4 level to shift to level of 76 metres

AHD. The fuse plugs would subsequently need to be raised to 77.7 metres AHD. However, the operating

requirement that the gates fully open at 77.7 metres AHD means that the gates need to be progressively

opened as outlined in the operating rules.

The elevation-discharge curve in Figure 17 shows flows throttled back until 76 metres AHD with very

similar outflows to the current arrangement for flows below 5,000 m3/s (due to the progressive gate

opening). The results demonstrated in Figure 17 would be refined in a more detailed assessment of the

Dam raising. The current operating curve triggers W4 at 74 metres AHD. This level corresponds with a

4,750 m3

/s outflow, which represents a 1 in 500 AEP event according to the outflow AEP curve.

Of note is the impact of the Probable Maximum Precipitation Design Flood (extreme design flood) on

the raised structure. The current arrangement only passes the 1 in 100,000 AEP event, which does not

meet current design guidelines. Raising the Dam by 2 metres allows the Probable Maximum Precipitation

Design Flood to pass through the structure with a peak water level 81.3 metres AHD. This indicates

that the Dam crest could be 0.7 metres lower, or further changes to the W4 trigger level could be

implemented, to further increase flood mitigation.

It should be noted that there is a significant level of technical uncertainty associated with the 2 metre

dam raise. Elements of the structure may, or may not need upgrading to cope with the increased water

levels during extreme flood events. The most significant of these relates to the inability of the spillway

gates to be lifted completely above the water flowing through the spillway. Currently a substantialdeflector beam is provided below the spillway bridge to deflect the water away from the gate. The 2

64

66

68

70

72

74

76

78

80

82

5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000

Elevation(mAHD)

Discharge (m3/s)

Existing Dam FSL 67m

(100% Capacity)

Dam FSL 64m (75%

Capacity)

Dam FSL 60m (50%

Capacity)

2m Raise to W4 (100%

Capacity)


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metre dam raise increases the water load on this deflector beam by 2 metre. Preliminary assessments

show that this wall can be strengthened to withstand the increased load but these issues need to be fully

considered within the scope of a more detailed study. This study will need to include hydraulic modelling

to better quantify the design loading and outside the scope of this preliminary assessment. If the

subsequent detail investigation shows that the wall is not capable of being strengthened then a new

spillway structure will be required at significant additional cost in the order of $330 million.

There are elements of dam raising option that are outside the scope of this current Study and may result

in significant uncertainty in overall project cost. These elements include aspects such as the necessity

for a fish passage to meet current environmental approvals. If required, this structure would comprise

substantial fish lifting system similar to that being used at Paradise Dam. As fish passage is also

required during flood periods, this structure would be very substantial and may cost greater than $100

million. Due to the uncertainties surrounding the requirement of this structure, these additional costs

have not been included in the economic assessment.

6.1.1 Required infrastructure

The infrastructure required to raise the Wivenhoe Dam was reviewed in detail in the year 2000 by GHD.

The Study is contained in the report Engineering Study in the Augmentation of the Flood Passing

Capacity of Wivenhoe Dam (GHD February 2001). This analysis laid the foundation for the design and

construction of the Auxiliary Spillway in 2005.

The Engineering Study also looked into various Dam crest level raising scenarios, and provides a

feasibility-level analysis of infrastructure required to raise the Dam crest level by either 2 metres or

4 metres. The Engineering Study provides material quantities and feasibility-level cost estimates for therequired infrastructure.

Key infrastructure and modifications required to raise the Dam include:

the addition of fill on the downstream Dam wall face to raise the Dam crest level while maintaining

stable slopes;

relocation of the crest road during the upgrade as the downstream fill allows a wider crest during

construction, it may be feasible to retain the crest road in the existing location during general

construction works, but be shifted laterally as necessary during key construction milestones;

extension of the existing core zone and filter zones to the new crest;

construction of a new upstream wave wall tied into the core zone;

construction of raised training walls into the spillway;

provision of removable bulkheads for the water quality control room;

modifications to the bulkhead lifting equipment and control room window to ensure suitability for the

raising of water levels by 2 metres;

raising the spillway bridges;

modifications to the approach roads to connect the new Dam centreline (downstream of the existing

centre) to the spillway bridge;

extension and modification of the existing flow deflector bulkhead below the spillway bridge(necessary to avoid the upper nappe surface of the spill flow affecting operation of the gate);


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construction of a new saddle dam in the Coominya saddle;

raising the existing saddle dams (1 and 2;)

raising the dividing walls in the fuse plug spillway

raising the fuse plug embankments through upstream and downstream addition of fill; and

modifications to and raising of dam monitoring and water control systems.

An indicative implementation schedule for the raising of the Dam is presented below.

The approximate volume of earth fill required (including filter zones, rip rap and bulk fill) is of the order of

750,000 m3for the main Dam, and an additional 110,000 m

3for the saddle dams. On this basis, the

following implementation times might be expected:

Year 1 geotechnical investigations, detailed design, tender documentation and relevant approvals

(minimum 12 month duration expected);

Year 2 first six months, procurement;

Year 2 second six months, mobilisation, site establishment, miscellaneous concrete works,

development of borrow sources for fill material; and

Year 3 earthworks and spillway and bridge works following wet season.

6.1.2 Fuse plug upgrades

During development of this Study an additional option was presented where the existing earth fuse plugs

could be replaced with collapsible gates such as Hydroplus gates with the potential to utilise the total

available flood storage volume. This option has not been considered in detail.

Hydroplus fuse gates do not give any increased functionality compared to the existing fuse

embankments. Both gates and the fuse plugs are designed to breach, or tip over, when the water level in

the Dam reaches the trigger point. This provides an additional spillway capacity for the highest reservoir

levels during extreme flood events.

However, use of the fuse plugs are included in the Dam operating curves and for very large floods should

be used as part of the Dam operation. The storage volume within the Dam should be used for flood

mitigation up to the trigger points for the fuse plugs, as long as the spillway gates are fully opened. With

the gates fully opened the percentage increase in flow downstream of the Dam due to the fuse plugtriggering is minimised to less than 20% additional flow.

The selection of earth fuse plugs in 2005 was based on these being the most economic system of

auxiliary spillway at the time.

If it was decided that the existing fuse plugs should be upgraded, steel and concrete Hydroplus fuse

plug gates could be used to directly replace the existing embankments.

In addition to the gates themselves, the existing concrete ogee crest buried beneath the existing

embankment would require reconstruction into a flat crest. Sufficient reinforcing would be required in

order to withstand the hydraulic gradient imposed by the fuse plugs. The estimated capital cost of such a

solution would be in the order of $80 million.


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If the fuse plugs where to be replaced with a radial or similar gate structure to facilitate early release of

water, the cost of the new works would be in the order of $300 million.

6.1.3 Storage impacts upstream

Raising of Dam and flood storage levels will impact on the existing upstream infrastructure and

environment. A cost allowance for these impacts has been included in the economic assessment of the

options for raising Wivenhoe Dam. A key point to note is the reservoir level versus flow does not change

until the 1 in 500 event. Consequently, for floods relevant to downstream roads and other non-critical

infrastructure, the flood risk does not change within the Wivenhoe Reservoir basin. Accordingly, minor

local and rural roads including the Wivenhoe Somerset Road should not need to be modified.

However, for floods of 1 in 1,000 to 1 in 100,000 the inundation level within the reservoir basin rises.

Significant capital works to upgrade these facilities at these extreme flood conditions are not warranted.

The likely extent of the impact is noted in Table 8 and Appendix E.

Table 8 Summary of potential upstream impacts

Area of impact Impact

Roads As the FSV remains unchanged the impact on roads will be minimal and then onlyduring extreme events. The existing road network is currently not designed forextreme flood events and as such, any temporary disruption to the highwaynetwork around Wivenhoe Dam during extreme events would be tolerable.However, some cost impacts should be expected as some roads will be subject to

more frequent flooding.

Environment Environmental impacts would be minimal and temporary during extreme events.Appendix E shows known sites of environmental, recreational and cultural interestthat would be affected during extreme events by the raising of the Dam.

Landresumption

Land impacts may be considered within the bounds of the increased floodenvelope. While there is no know policy to determine the extent of impacted land,for the purpose of this Study GHD has assumed the land area between 75 metresAHD and 77 metres AHD levels as a result of the 2 metre Dam raising.

The area of land between 75 metres AHD and 77 metres AHD levels forWivenhoe Dam is approximately 2,378 hectares. This is comprised of roadparcels, easement parcels and lot parcels.

There is already 16,424 hectares that is currently classed as Lake Wivenhoe onthe cadastral database, which suggests that it is already owned by Seqwater.

Slityard Creekpower station

Initial advice gained from the power station operators suggest that the station cantolerate water levels in Wivenhoe up to a level of 77 metres AHD without damageor effect on the operation of the power station. A 2 m dam raising takes theannual exceedance probability of a water level of 77 metres AHD from about 1 in7,000 to 1 in 5,000. This is a small change in probability for an extremely rareevent. This may be better managed as an insurable risk rather than any upgradesto capital works. However the effect of a 2 metre raising of the dam on the powerstation, including a review on any potential impact on electricity supply, should beassessed during the long-term study. Further consultation with the power station

owners should also occur.

Esk and The new Toowoomba raw water supply off take from Wivenhoe Dam was recently


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Area of impact Impact

Toowoombawater supply

constructed as part of the LinkWater Toowoomba Alliance. It is understood thatthe pump station at the Dam was damaged during the January 2011 flood as aresult of rising water levels in the Dam. Increasing the flood level in the Dam willclearly have an impact on the pumping facility and this would need to beconsidered in any raising option.

Similarly, the adjacent Esk raw water supply off take would also need to beconsidered.

Commercialoperations(gravel)

While not considered in detail as part of this Study some consideration should begiven to the impact on any commercial gravel and sand operations withinWivenhoe Dam.


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7. Hydrologic Modelling

7.1 Model description

Given the restricted timeframe for this rapid assessment project, a simplified Brisbane River hydrology

model was created. It uses the estimated run-off hydrographs from the principal sub-catchments across

the Brisbane River catchment. These sub-catchments include:

Stanley River (upstream of Somerset Dam);

Upper Brisbane River (upstream of Wivenhoe Dam and excluding Somerset Dam outflows);

Lockyer Creek;

Bremer River; and

Mount Crosby local area (the area along the Brisbane River downstream of Lockyer Creek and

upstream of the Bremer River).

There is a stream gauge downstream of the Brisbane River and Bremer River confluence at Moggill

(Moggill gauge). While there are a number of contributing sub-catchments further downstream of Moggill,

those catchment areas are modest in comparison to those listed above, and the run-off from these areas

were not considered for the purposes of this Study.

Three historical floods (1893, 1974, and 2011) were considered to assess potential flood mitigation

options at Wivenhoe Dam, or on Lockyer Creek, or the Bremer River. These flood events represent the

largest floods within the recorded history for the Brisbane River catchment, and each flood has a different

behaviour in terms of the rainfall distribution and variability that make them suitable for an assessment of

options for a rapid appraisal assessment of this nature.

It was assumed that the peak flow rate at the Port Office gauge is approximately the same as the peak

flow rate at the Moggill gauge based on the hydrology from the 2004 study undertaken by Sinclair Knight

Merz.

The hydrologic model has two components:

an estimation of peak flow rates at the Moggill gauge; and

flood routing through Wivenhoe Dam and Somerset Dam.

The estimation of the peak flow rate at the Moggill gauge was based on the Seqwater supplied

hydrographs and lag times for the Lockyer Creek catchment, the Mount Crosby local area catchment, the

Bremer River catchment, and the outflows from Wivenhoe Dam. As a simplified model was developed for

this Study, flood routing along the Brisbane River downstream of the Dam was not explicitly performed.

Instead, the attenuation of peak flow rates along the river was estimated based on the provided lag

times. The combined hydrographs were then scaled to estimate the peak flow rate at Moggill gauge. That

is, the lagged hydrographs from the Lockyer Creek catchment, the Mt Crosby local area, and Wivenhoe

Dam outflows were added together and then scaled to

Documents

Gdh Raising Wivenhoe Final Report