Tooleybuc Bridge Detailed Hydrology and Hydraulics Study

Home , Murray Valley Highway

Tooleybuc Bridge

Detailed Hydrology and Hydraulics Study

59914544

Prepared for Road and Maritime Services

25 September 2015

25 September 2015 Cardno i Tooleybuc Bridge Detailed Hydrology and Hydraulics Study

Contact Information Document Information

Cardno (NSW/ACT) Pty Ltd Prepared for Road and Maritime Services ABN 95 001 145 035 Project Name Detailed Hydrology and Hydraulics Study

File Reference R001_Tooleybuc_Bridge_Detailed_Hydrology_Rev005.docm Level 9, The Forum, 203 Pacific Highway Job Reference 59914544 St Leonards NSW 2065 Date 25 September 2015 PO Box 19 St Leonards NSW 1590 Version Number Rev005

Telephone: 02 9496 7700 Facsimile: 02 9439 5170 Effective Date July 2014 International: +61 2 9496 7700 Date Approved: July 2014

sydney@cardno.com.au www.cardno.com

Document History

Version Effective Description of Revision Prepared by: Reviewed by: Date Rev001 July 2014 Initial report structure and data Heath Sommerville Rob Swan and survey requirements Rev002 Sept 2014 Hydrology and design events Heath Sommerville Rob Swan added Rev003 Oct 2014 Hydraulic model details and Heath Sommerville Rob Swan calibration added Rev004 Dec 2014 Draft Final Heath Sommerville Rob Swan Rev005 Jan 2015 Final Heath Sommerville Rob Swan

© Cardno. Copyright in the whole and every part of this document belongs to Cardno and may not be used, sold, transferred, copied or reproduced in whole or in part in any manner or form or in or on any media to any person other than by agreement with Cardno. This document is produced by Cardno solely for the benefit and use by the client in accordance with the terms of the engagement. Cardno does not and shall not assume any responsibility or liability whatsoever to any third party arising out of any use or reliance by any third party on the content of this document.

25 September 2015 Cardno ii Tooleybuc Bridge Detailed Hydrology and Hydraulics Study

Executive Summary

Roads and Maritime Services (Roads and Maritime), in partnership with VicRoads, are currently assessing the proposed replacement bridge options for the single lane lift span bridge crossing the Murray River at Tooleybuc. This detailed hydrology and hydraulic study assessed three proposed replacement options to determine their suitability in relation to the floodplain function of the area. This investigation required assessment of the hydrology of the region and development of a hydraulic model to perform the assessment. Flooding at Tooleybuc is largely controlled by tributaries upstream and flood scenarios have been established using the long term record at Swan Hill. The model has been verified using known flood levels through the region for historic events. The Blue and Purple options are a similar replacement option to the existing structure. The options are a similar length, height and contain a lift span to allow water traffic to pass. The Blue option is located immediately upstream of the existing bridge and the Purple option is about 100 m upstream. The Yellow option is a high level crossing and this spans the Murray River about 100 m downstream of the existing bridge. Details of the structures are summarised in Table i.

Table i Option parameters Alignment Length (abutment Soffit level (m Deck level (m Bridge Girder to abutment) AHD) AHD) Depth (m) Existing bridge 100 m 63.45 64.6 1.15 Blue option 108 m 63.45 64.95 1.5 Purple option 116 m 63.45 64.95 1.5 Yellow option 116 m High level crossing High level crossing 1.8

The peak flood levels and peak velocities recorded at each of the structures is shown in Table ii. It should be noted that each of the options did not change the existing flood levels and the differences in peak flood levels is due to the location of the option within the floodplain. For the peak velocity the peak recorded was for the Yellow option. This option has a higher velocity due to the proximity of the option the bend in the Murray River.

Table ii Peak flood levels and peak velocity for each option Peak flood level (m AHD) Peak velocity (m/s) Alignment 20y ARI 50y ARI 100y ARI 100y ARI Existing bridge 61.88 61.92 61.95 0.94 Blue option 61.89 61.92 61.95 0.82 Purple option 61.84 61.88 61.94 0.78 Yellow option 61.84 61.88 61.91 1.03

The assessment indicated none of the options required additional mitigation structures. For each of the options similar scour protection is required and this has been recommended to have a minimum D50 rock size of 150 mm. Overall, each proposed option met the objectives to not increase or adversely change flooding behaviour in the area due to changes within the floodplain. From a flood perspective, each option is viable and as there are no additional mitigation measures required there is no preferred option. Each option requires similar erosion and scour protection.

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Table of Contents

Glossary vii Abbreviations ix 1 Introduction 10 1.1 Study area 10 1.2 Flooding at Tooleybuc 12 1.3 Study objectives 15 1.4 Key outcomes 15 1.5 Relevant legislation, policy and guidelines 15 2 Data collection and review 19 2.1 Available data 19 2.1.2 Previous reports 21 2.2 Required survey 21 3 Hydrology 23 3.1 Flooding behaviour 23 3.2 Available data 23 3.3 Flood frequency assessment 26 3.4 Probable maximum flood 28 3.5 Summary 29 4 Hydraulic model 30 4.1 Hydraulic model development 30 4.1.1 Channel and structure systems (1D) 30 4.1.2 Topography (2D) 30 4.1.3 Roughness 31 4.2 Model calibration 34 4.3 Model validation 36 5 Existing conditions 38 5.1 Design events 38 5.2 Probable maximum flood 43 5.3 Sensitivity assessment 45 5.3.1 Flow rate sensitivity 45 5.3.2 Downstream boundary levels 45 5.3.3 Roughness 48 5.4 Summary 51 6 Proposed options 52 6.1 Proposed options 52 6.2 Performance criteria 53 6.3 Option analysis 55 6.3.1 Blue option 55 6.3.2 Purple option 57 6.3.3 Yellow option 59 6.4 Summary 61 7 Option analysis 62 7.1 Constraints, levels and structure types 62 7.2 Scour assessment 63

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7.2.1 Existing geology and soil types 63 7.2.2 Peak velocity 64 7.2.3 Abutment protection 64 7.2.4 Pier protection 65 7.3 Mitigation requirements 65 7.4 Costing 65 7.5 Climate change 65 8 Conclusions 66 9 References 67

Tables

Table i Option parameters iii Table ii Peak flood levels and peak velocity for each option iii Table 1-1 Relevant policies and framework 16 Table 2-1 Bridge option details 19 Table 3-1 Gauge comparison Swan Hill against Piangil 24 Table 3-2 Flood Frequency Assessment fitted distribution results 27 Table 3-3 Design flow rate comparison at Swan Hill against previous studies 27 Table 3-4 Summary of final design flow estimates 29 Table 4-1 Calibration for the 1975 event 34 Table 5-1 Peak flood levels for the study area 38 Table 5-2 Peak flood levels for the study area 45 Table 5-3 Manning’s Roughness sensitivity values 48 Table 6-1 Specific legislation and policy directives relating to flooding 53 Table 7-1 Peak flood levels at proposed option locations 62 Table 7-2 Peak velocity at the existing and proposed options 64 Table 7-3 Estimation of rip rap median particle size 64 Table 8-1 Summary of design flow rates and peak levels 66

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Figures

Figure 1-1 Study area for the full hydraulic model 11 Figure 1-2 Murray River floodplain characteristics for Tooleybuc (GHD, 2014) 13 Figure 1-3 Local features, gauges and levees (GHD, 2014) 14 Figure 2-1 Levee survey data location 20 Figure 2-2 Details of the cross section data collection 22 Figure 3-1 Murray River at Piangil (from data.water.vic.gov.au/monitoring.htm) 24 Figure 3-2 Regression relationship between the streamflow gauge peaks 25 Figure 3-3 Comparison of gauged records between Swan Hill and Piangil (1967 to 1980) 26 Figure 3-4 Flood frequency curve for the Murray River at Swan Hill 28 Figure 4-1 Hydraulic model extent and topography 32 Figure 4-2 Manning’s roughness for the model area 33 Figure 4-3 Calibration for the 1975 event 35 Figure 4-4 Validation of the 100 year ARI event against the GHD levels (GHD, 2014) 37 Figure 5-1 Existing conditions 20 year ARI peak depths and extent 39 Figure 5-2 Existing conditions 50 year ARI peak depths and extent 40 Figure 5-3 Existing conditions 100 year ARI peak depths and extent 41 Figure 5-4 Existing conditions 2,000 year ARI peak depths and extent 42 Figure 5-5 Existing conditions PMF peak depths and extent 44 Figure 5-6 Sensitivity of the downstream boundary – High boundary case 46 Figure 5-7 Sensitivity of the downstream boundary – Low boundary case 47 Figure 5-8 Sensitivity of the roughness – High roughness case 49 Figure 5-9 Sensitivity of the roughness – Low roughness case 50 Figure 6-1 Proposed options for the replacement Tooleybuc Bridge 52 Figure 6-2 Peak flood difference for the Blue option (Developed less Existing, 100 year ARI) 56 Figure 6-3 Peak flood difference for the Purple option (Developed less Existing, 100 year ARI) 58 Figure 6-4 Peak flood difference for the Yellow option (Developed less Existing, 100 year ARI) 60 Figure 7-1 Flow distribution description 63

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Glossary

Annual Exceedence Refers to the probability or risk of a flood of a given size Probability (AEP) occurring or being exceeded in any given year. A 90 per cent AEP flood has a high probability of occurring or being exceeded; it would occur quite often and would be relatively small. A 1 per cent AEP flood has a low probability of occurrence or being exceeded; it would be fairly rare but it would be relatively large.

Australian Height Datum A common national surface level datum corresponding to (AHD) mean sea level.

Average Recurrence The average or expected value of the period between Interval (ARI) exceedences of a given discharge or event. A 1 in 100 year ARI event would occur, on average, once every 100 years.

Catchment The area draining to a site. It always relates to a particular location and may include the catchments of tributary streams as well as the main stream.

Design flood A significant event to be considered in the design process; various work within the floodplain may have different design events e.g. some roads may be designed to be overtopped in the 1 in 1 year or 100 per cent AEP flood event.

Development The erection of a building or the carrying out of work; or the use of land or of a building or work; or the subdivision of land.

Discharge The rate of flow of water measured in terms of volume over time. It is to be distinguished from the speed or velocity of flow, which is a measure of how fast the water is moving rather than how much is moving.

Flood Relatively high stream flow which overtops the natural or artificial banks in any part of a stream, river, estuary, lake or dam, and/or overland runoff before entering a watercourse and/or coastal inundation resulting from super elevated sea levels and/or waves overtopping coastline defences.

Floodplain Area of land which is subject to inundation by floods up to the probable maximum flood event, i.e. flood prone land.

Freeboard A factor of safety above design flood levels, typically used in relation to the setting of floor levels, and levee crest heights. It is usually expressed as a height above the design flood level. Freeboard tends to compensate for flood prediction uncertainties and for factors which increase flood levels, such as a wave action, localised hydraulic effects, settlement of levees.

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Geographical information A system of software and procedures designed to support systems (GIS) the management, manipulation, analysis and display of spatially referenced data.

Hydraulics The term given to the study of water flow in a river, channel or pipe, in particular, the evaluation of flow parameters such as stage and velocity.

Hydrograph A graph showing how the discharge changes with time at any particular location.

Hydrology The term given to the study of the rainfall and runoff process as it relates to the derivation of hydrographs for given floods.

Mathematical/computer The mathematical representation of the physical models processes involved in runoff and stream flow. These models are often run on computers due to the complexity of the mathematical relationships. In this report, the models referred to are mainly involved with rainfall, runoff, pipe and overland stream flow.

Probability A statistical measure of the expected frequency or occurrence of flooding. For a more detailed explanation see Annual Exceedence Probability.

Probable Maximum Flood The largest flood conceivably occurring at a particular (PMF) location. This flood defines the maximum extent of land liable to flooding. The extent, nature and potential consequences of flooding associated with the PMF event should be assessed in a Flood Study.

Risk Chance of something happening having an impact. It is measured in terms of consequences and likelihood. For this study, it is the likelihood of consequences arising from the interaction of floods, communities and the environment.

Runoff The amount of rainfall ending up as stream or pipe flow, also known as rainfall excess.

Topography A surface which defines the ground level of a chosen area.

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Abbreviations

Abbreviation Full Description

1D One dimensional 2D Two dimensional AEP Annual Exceedence Probability ARI Average Recurrence Interval BOM Bureau of Meteorology Council Wakool Shire Council CMA Catchment Management Authority DEM Digital Elevation Model DTM Digital Terrain Model FFA Flood Frequency Assessment GEV Generalised Extreme Value LEP Local Environmental Plan LGA Local Government Authority LPIII Log Pearson Type III LiDAR Light Detection and Ranging Mallee CMA Mallee Catchment Management Authority ML/day Mega Litres per day MSS Municipal Strategic Statement NSW New South Wales PMF Probable maximum flood Roads and Maritime Roads and Maritime Services RWC Rural Water Commission SPPF State Planning Policy Framework TIN Triangular Irregular Network Vic Victoria

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1 Introduction

1.1 Study area The township of Tooleybuc is located on the New South Wales (NSW) side of the Murray River 150km east of Mildura. Tooleybuc forms part of the Wakool Shire Council Local Government Area (LGA). The Victorian side of the Murray falls under the LGA of the Swan Hill Rural City Council and within the Mallee Catchment Management Authority (Mallee CMA). The immediate study area is based on the location of the replacement bridge structures which extends 200m upstream of the existing bridge and 500m downstream for the three proposed options. The larger study area for the hydraulic assessment extends approximate 6km upstream and downstream of the site. The hydraulic assessment includes the full width of the Murray floodplain within this area. Figure 1-1 shows the full model area for the study.

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Victoria NSW

Lake Coomeroop

Figure 1-1 Study area for the full hydraulic model

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1.2 Flooding at Tooleybuc Tooleybuc sits on the Murray River and is located 50 km downstream of Swan Hill. The catchment area upstream of Tooleybuc is large and encompasses many river systems. However, Tooleybuc does not receive all floodwaters from the upstream areas due to the interactions of the Murray River floodplain with the Edwards, Wakool and Niemur Rivers to the north of this area. During flood events a large portion of the floodwaters from the Murray River flow through these systems and re-enter the Murray River downstream of Tooleybuc. This interaction is shown in Figure 1-2 (taken from the draft Tooleybuc Flood Study (GHD, 2014)). This interaction is important as it limits the possible peak flows passing Tooleybuc during extreme flood events. The flow rates at Tooleybuc are very similar to Swan Hill due to the absence of any major inflows between the two townships. Tooleybuc is located on the NSW side of the Murray River and is elevated to generally above the 100 year ARI flood levels. There are three levees on the NSW side of the Murray River (GHD, 2014) and these levees were surveyed as part of the recent Tooleybuc Flood Study. The township does not rely on these levees for flood protection but rather the levees are used for flood prevention for the more frequent flood events. The locations of the levees are shown in Figure 1-3.

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Figure 1-2 Murray River floodplain characteristics for Tooleybuc (GHD, 2014)

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Figure 1-3 Local features, gauges and levees (GHD, 2014)

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1.3 Study objectives The primary study objective is to assess the three proposed options for the option of the replacement Tooleybuc Bridge. The assessment focusses on the impact of the replacement structures on the 20, 50, 100 and 2,000 year ARI events. This assessment will form part of the overall detailed hydrology and hydraulics study and help Roads and Maritime in deciding on the preferred option. Key objectives include:  Literature and previous study review  Assess required survey and undertake additional survey  Examine legislation and required approvals  Estimate design events and assess floodplain behaviour  Establish assessment criteria (hydraulic criteria)  Develop the hydraulic model for the study and assess the proposed bridge options  Review potential structure types and any constraints  Assess geotechnical aspects for the design  Qualitative climate change assessment  Ensure the options have appropriate mitigation  Provide detailed reporting for the approvals.

1.4 Key outcomes The assessment identifies the predicted impact of the three proposed bridge options. The key outcomes for the study include:  Report outlining the background and outcomes of the study  Mapping key impact on the flood regime at Tooleybuc  Hydraulic model for assessing proposed structures.

1.5 Relevant legislation, policy and guidelines The legislation, policy and guidelines have been considered during this study:  Victorian Flood Management Strategy (State Flood Policy Committee, 1998)  State Planning Policy Framework as part of Victoria Planning Provisions  A planning guide for land liable to flooding in rural Victoria (RWC, 1989)  A guide to Floodplain Management in Country Victoria (RWC, 1987)  Water Act 1989 (Vic)  The Water Act (1917) and the water Management Act 2000 (NSW)  Floodplain Development Manual (NSW Government, 2005)  Water Management Act 2000 (NSW)  Environment Protection Act 1970  Wakool Local Environmental Plan (LEP) (2013). A detailed outline of the planning process for the Tooleybuc Bridge Replacement is outlined in the Environmental Constraints Analysis (Roads and Maritime Services, 2014). This document outlines the detailed statutory planning considerations for the process. A more detailed outline of the legislation and relevant policy framework relevant to flooding is outlined in Table 1-1.

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Table 1-1 Relevant policies and framework

Policy/Strategy Clause Objective

State Planning 15.01 Protection of catchments, To assist in the protection and, where Policy Framework waterways and groundwater. possible, restoration of catchments, (SPPF) waterways, water bodies, groundwater, and the marine environment.

15.02 Floodplain Management To assist the protection of:  Life, property and community infrastructure from flood hazard  The natural flood carrying capacity of rivers, streams and floodways  The flood storage function of floodplains and waterways  Floodplain areas of environmental significance.

15.09 Conservation of native flora To assist the protection and conservation of and fauna biodiversity, including native vegetation retention and provision of habitats for native plants and animals and control of pest plants and animals.

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Policy/Strategy Clause Objective

Wakool Local 6.2 Flood Planning (1) The objectives of this clause are as Environmental follows: Plan 2013 (a) To minimise the flood risk to life and Swan Hill Planning property associated with the use of land Scheme & MSS (b) To allow development on land that is compatible with the land’s flood hazard, taking into account projected changes as a result of climate change (c) To avoid significant adverse impacts on flood behaviour and the environment (2) This clause applies to flood liable land (3) Development consent must not be granted to development on land to which this clause applies unless the consent authority is satisfied that the development:: (a) Is compatible with the flood hazard of the land (b) Will not significantly adversely affect flood behaviour resulting in detrimental increases in the potential flood affectation of other development or properties (c) Incorporates appropriate measures to manage risk to life from flood (d) Will not significantly adversely affect the environment or cause avoidable erosion, siltation, destruction of riparian vegetation or a reduction in the stability of river banks or watercourses (e) Is not likely to result in unsustainable social and economic costs to the community as a consequence of flooding.

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Policy/Strategy Clause Objective

22.01 Flooding & Clause 21.05-4 The objective of these clauses are to: from MSS  To maintain the unobstructed passage of floodwaters  To recognise appropriate public and community based flood management organisations and their role in maintenance and development of existing flood protection levee banks  To provide suitable flood plain management which will ensure that any new development is suitably designed to ensure that development is compatible with the identified flood hazard and local drainage characteristics  To protect and encourage the rural and riverine character of the area  To recognise the agricultural value of land within levee protected areas and to support the continued use of these areas for agricultural production. The objectives of this plan are: Murray Regional Reg 3 Environmental Plan (a) To ensure that appropriate No 2 - Riverine consideration is given to development Land with the potential to adversely affect the riverine environment of the River Murray (b) To establish a consistent and co- ordinated approach to environmental planning and assessment along the River Murray (c) To conserve and promote the better management of the natural and cultural heritage values of the riverine environment of the River Murray.

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2 Data collection and review

After the project inception meeting and site visit, Cardno have collated the available data and reviewed this data for the use within the project.

2.1 Available data Data was collated as part of this study:  Concept design sketches for proposed bridge options based on 12da files received on the 19th September 2014. Details of the bridge spans are shown in Table 2-1.

Table 2-1 Bridge option details

Option Description Details Length (abutment to abutment)

Blue Option Lift span 2m diameter circular piers, 1.5m 108 m superstructure depth and 25-32m Purple Option Lift span span lengths (except for lift span) 116 m

Yellow Option Raised structure 2m diameter circular piers, 1.8m 116 m superstructure depth and 30-36m span lengths

 Photos from the site inspection  Digital Terrain Model (DTM) for the study area was supplied by Roads and Maritime  Triangular Irregular Networks (TINs) for the proposed bridge options  Streamflow data for Murray River gauges at Swan Hill and Piangil  Aerial imagery  NSW levee top of bank survey (captured 2013) and covers three levees on the NSW side of the Murray River (see Figure 2-1), namely the: o Ring Levee o Southern Levee o Golf Course Levee  Six (6) cross sections from the 1982 Rural Water Commission survey  Flood height data (limited to a small set of 1975 and 1956 flood points)  Existing Tooleybuc Bridge dimensions and survey.

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Figure 2-1 Levee survey data location

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2.1.2 Previous reports Previous reports have been reviewed and utilised as part of this investigation:  Tooleybuc Flood Study (draft) – GHD (2014)  Hydraulic Issues for the Swan Hill Planning Study – LJ5525 / RM2140 – Cardno Lawson Treloar (2007)  Detailed Hydrology Study for the Swan Hill Bridge Planning Study – LJ5573 / RM2180 – Cardno Lawson Treloar (2009)  Swan Hill Tyntynder Flats Floodplain Management Study – Binnie and Partners (1992)  Swan Hill Regional Flood Strategy reports (1990s)  Flood Data Transfer Project Flood mapping Report for Swan Hill – Egis Consulting (2000)

2.2 Required survey Due to the lack of available cross section information, hydrographic survey was required to capture the bathymetry of the Murray River for the model area. The bathymetry of the river is important for the study area as this determines the carrying capacity of the main channel, but also the location of the main flow path for positioning of piers and bridge open spans for navigation. Cross sections were captured at intervals as outlined in Figure 2-2. Overall 41 cross sections were captured, the sections extended from bank to bank as defined by the high water line at the time of capture. The accuracy of the data captured was +/- 0.05 m for both vertical and horizontal accuracy. The survey was verified against multiple Permanent Survey Marks and accuracy was found to match to within 3 mm of recorded.

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Figure 2-2 Details of the cross section data collection

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3 Hydrology

The design events at Tooleybuc have been developed using a flood frequency assessment method similar to previous studies. This method is the most suitable approach due to the available long term streamflow records and the absence of inflows between the long term gauge at Swan Hill and Tooleybuc.

3.1 Flooding behaviour As discussed in Section 1.2, flooding at Tooleybuc is significantly impacted by flows bypassing this section of the Murray River to the north via the Wakool, Edwards and Niemur River system. These bypassing flows re- enter the Murray River downstream of Tooleybuc and do not impact the township. This limits the range of possible flows occurring during floods at Tooleybuc and this is the primary reason the Swan Hill gauge can be utilised to develop the design inflows at Tooleybuc. Between Swan Hill and Tooleybuc there are no major inflows or tributaries and local catchment inflows are insufficient to impact the large peak flow rates of the Murray River. The impact of the distributary flows to the north at Swan Hill results in reduced range of peak flow rates for extreme flood events as the distributary flows place an upper limit on the flows passing through Swan Hill and along the Murray River to Tooleybuc. The absence of any significant inflows means peak flow rates and flood hydrographs at Tooleybuc will closely resemble those at Swan Hill.

3.2 Available data For the Murray River at Tooleybuc there are two main streamflow gauges:  Murray River at Swan Hill (409204)  Murray River at Piangil (409213). The Swan Hill gauge has an extensive record extending from 1909 and is currently still operating. The gauge at Piangil is more limited and was only operated from 1967 to 1980. The gauge at Piangil is located upstream of Tooleybuc next to the intersection of the Murray Valley Highway and the Mallee Highway at Piangil. This is shown in Figure 3-1. This is the closest gauge to Tooleybuc. There is insufficient record to undertake a flood frequency assessment (FFA) at this location.

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Figure 3-1 Murray River at Piangil (from data.water.vic.gov.au/monitoring.htm)

The next gauge upstream is the Swan Hill gauge. This gauge has a long record at more than 100 years and is suitable for a FFA to determine peak design flow rates. From previous studies for the Murray River it has been noted the flows recorded at Swan Hill are likely to be a suitable for estimating the flows at Tooleybuc. A comparison of the peak flow rates extracted for each year of the concurrent record at Piangil and Swan Hill has been summarised in Table 3-1. The concurrent period is from 1967 to 1980 (note 1980 data only extended to July for the Piangil gauge). The comparison shows the peak flows at Piangil are within + 6 per cent to - 12 per cent of the peak flow at Swan Hill.

Table 3-1 Gauge comparison Swan Hill against Piangil Peak flow at Piangil Peak flow at Swan Difference (Piangil Year (ML/day) Hill (ML/day) relative to Swan Hill) 1967 20,800 21,600 -4% 1968 25,900 27,800 -7% 1969 22,600 24,300 -8% 1970 27,600 26,400 4% 1971 25,100 26,100 -4% 1972 16,600 18,000 -8% 1973 30,200 32,200 -7% 1974 30,000 32,000 -7% 1975 31,300 35,100 -12% 1976 23,800 22,300 6% 1977 17,100 17,000 1% 1978 24,100 24,900 -3% 1979 27,600 27,800 -1% 1980* 11,900 11,400 4%

* 1980 data is only a partial year dataset from January to July.

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Each of the peak flow pairs have been presented in the form of a regression relationship in Figure 3-2. The regression relationship is another method for determining the relationship between the peak flows. In this figure the relationship between the peaks shows a small average difference of 4 per cent with the Swan Hill gauge recording on average marginally higher peak flow rates than at Piangil. The root mean square error of the fitted linear regression line is 0.96 which shows a very strong correlation between the gauges.

Figure 3-2 Regression relationship between the streamflow gauge peaks

In addition to the peak flow assessment a visual assessment of the concurrent gauged records has been presented in Figure 3-3. The figure shows the strong correlation between the flows and the well matched peaks through the 1967 to 1980 period. The hydrographs are well matched on both the rising and falling limb of events and for both high and low flow periods. Overall the Swan Hill gauge is a suitable proxy for estimating the design events at Tooleybuc. It is important to note the peaks are marginally higher at Swan Hill and as such the use of this gauge to derive the design events will include an element of conservatism in the peak flood estimate.

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Figure 3-3 Comparison of gauged records between Swan Hill and Piangil (1967 to 1980)

3.3 Flood frequency assessment The flood frequency assessment (FFA) involves the calculation of the statistical probability a flood of a certain magnitude will occur in a certain period of time. Each flood is recorded and ranked in order of magnitude with the highest rank being assigned to the largest flood. The return period is the likely time interval between floods of a given magnitude and is a product of the length of streamflow record and the rank of the event. Ultimately, the FFA assigns a recurrence interval to the annual peaks of the recorded time series and uses this information to develop a relationship linking recurrence interval with flood magnitude. The FFA for the Swan Hill gauge has been carried out using the TUFLOW FLIKE software. This software facilitates the fitting of the various distributions to annualised peak flows extracted from the gauge. The two primary distributions fitted were the Log Pearson Type III (LPIII) and General Extreme value. Due to the long record available the fitted distributions represented the annualised peak events well with high confidence (low range for the 90 per cent confidence limits). The summary of the results is shown in Table 3-2. The two fitted distributions match well. For the purpose of the estimate of the design peak flow rates the LPIII fitted distribution has been adopted as this generated higher design flow estimates.

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Table 3-2 Flood Frequency Assessment fitted distribution results Log Pearson Type III Distribution Gen. Extreme Value Distribution Flow (ML/d) 90% Confidence Flow (ML/d) 90% Confidence ARI (years) limits limits 10 31,100 30,200 – 32,100 31,200 30,400 – 32,100 20 32,500 31,500 – 33,700 32,500 31,700 – 33,500 50 34,000 32,900 – 35,400 33,800 33,000 – 35,000 100 35,000 33,700 – 36,500 34,500 33,600 – 35,900 200 35,700 34,400 – 37,600 35,000 34,100 – 36,700 500 36,500 35,100 – 38,800 35,500 34,500 – 37,500 2000 37,600 36,000 – 40,400 36,100 34,900 – 38,400 ~PMF (100,000) 39,600 37,600 – 44,000 36,700 35,300 – 39,700

To confirm the current estimated design peak flow rates are appropriate a comparison has been carried out against previous studies. The estimated design flow rates for the available previous studies are summarised in Table 3-3. Each study source is referenced below the table. The current estimated peak flow rates match the recent Flood Study for Tooleybuc and are appropriate for the use within this assessment. It should be noted the PMF estimate has been derived from upper bound of the 90 per cent confidence interval estimate of the 100,000 year ARI event. The upper bound has been used for this estimate to provide a conservative estimate for the PMF from the FFA. This event is discussed further in Section 3.4.

Table 3-3 Design flow rate comparison at Swan Hill against previous studies Peak Design Flow (ML/day) Current 2014 Year 1992 Study1 1999 Study2 2009 Study3 2014 Study4 estimate 10 33,600 - - 31,200 31,100 20 34,800 - 33,800 32,600 32,500 50 35,800 - 35,500 34,000 34,000 100 36,200 42,100 36,400 35,000 35,000 200 - - - 35,700 35,700 500 - - - - 36,500 2000 - - 39,200 - 37,600 PMF - - - - ~44,0005

Note: 1 1992 Study – Binnie & Partners – Swan Hill / Tyntynder Flats Floodplain Management Study 2 1999 Study – SKM – Swan Hill regional Flood Study 3 2009 Study – Cardno Lawson and Treloar – Detailed Hydrology Study for the Swan Hill Bridge Planning 4 2014 Study (draft) – GHD – Tooleybuc Flood Study 5 Probable maximum flood has been estimated by the upper range of the LPIII confidence intervals for the 100,000 year ARI event.

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3.4 Probable maximum flood The Probable Maximum Flood (PMF) is difficult to estimate for Tooleybuc due to the fact the Murray River is a detailed and complex system covering more than 50,000 km2. This is further complicated by the observation that most of the high flows divert upstream of Swan Hill to the north via the Niemur, Wakool and Edward River systems before the flows rejoin the Murray River upstream of Robinvale. A detailed estimate of the PMF is not possible however some methods can be applied to quantify the likely maximum flood through the area. The Flood Frequency Analysis (FFA) from the Murray River at Swan Hill gauge can be used to extrapolate this data to a sufficiently rare event. Figure 3-4 shows a fitted Log Pearson Type III distribution to the annual peak event data at Swan Hill (see Section 3.3). The curve is well fitted to the data and has been extrapolated out to the 1 in 100,000 year ARI event (0.001 per cent AEP). The fitted curve shows the peak flow rates do not increase significantly as the flood events become rarer. This is due to the diversion of the majority of flood waters to the north of Swan Hill in large events. The predicted peak flow rate for the 100,000 year ARI event based on this curve is 460 m3/s (39,600 ML/day).

533

Gauged Expected quantile 90% limit Expected prob quantile 461

388

Peak flow m^3 316

243

170 0 1.5 2 5 10 20 50 100 200 500 1000 2000 5000 10000 20000 50000 100000 12 ARI (years) Figure 3-4 Flood frequency curve for the Murray River at Swan Hill

The recent GHD study applied a method of applying a peak flow rate of three times the 100 year ARI peak flow rate to the system. In this case equates to a peak flow rate of 105,000 ML/day (1,215 m3/s). It is stated in the GHD assessment they believe this to be a very conservative estimate of the PMF and the peak flow rate would be expected to be lower than this in reality. Cardno agree with this approach as is a very conservative method for assessing a complicated system. This is likely to produce extents and depths which are above the likely maximum for the area. This allows for the critical assessment of infrastructure, such as bridge soffit levels, with a degree of confidence the peak levels modelled across the floodplain from the PMF have a factor of safety applied and are unlikely to be exceeded.

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3.5 Summary Overall the use of the Swan Hill gauge is noted to be the most appropriate method for developing the design flow rates for this assessment. The final design flow rates are summarised in Table 3-4. The key observations from this assessment demonstrate the suitability of the design estimates include  High correlation of the peak flows and recorded hydrographs at Swan Hill and Piangil  The long duration of the flood events leads to the peak flows being sustained for long periods of time and as such the events can be represented by Swan Hill  Flood Frequency Analysis produced low uncertainty in the flow estimates  Estimated design peak flows matched previous studies closely.

Table 3-4 Summary of final design flow estimates ARI (years) Design Flow 90% Confidence limits (ML/day) 10 31,100 30,200 – 32,100 20 32,500 31,500 – 33,700 50 34,000 32,900 – 35,400 100 35,000 33,700 – 36,500 200 35,700 34,400 – 37,600 500 36,500 35,100 – 38,800 2000 37,600 36,000 – 40,400 ~PMF (100,000) 105,0001 N/A

1 A conservative factor of 3 times the 100 year ARI has been applied

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4 Hydraulic model

The assessment of the proposed Tooleybuc Replacement Bridge structure is facilitated by the use of a detailed 1D/2D hydraulic model. For this assessment Cardno have developed the model using SOBEK. This section outlines the model development and calibration to demonstrate the suitability of the model for the assessment of the proposed options. The model has been calibrated to known points recorded during the 1975 flood event and validated against the recent GHD hydraulic modelling as part of the Tooleybuc Flood Study (GHD, 2014).

4.1 Hydraulic model development The hydraulic model has been developed using a nested grid approach which is shown in Figure 4-1. The model extends 6 km upstream and downstream of the site in a direct line, the Murray River meanders a significantly longer distance through the floodplain (more than 20 km of river length). The hydraulic model covers a large area to ensure the hydraulic model results are not influenced by the boundary conditions of the model. The hydraulic model is a coupled 1D and 2D system and the development of each component are discussed in the following sections.

4.1.1 Channel and structure systems (1D) Due to the constant standing water level in the Murray River and the inability of LiDAR to penetrate water, the available LiDAR (see Section 4.1.2) could not capture the cross section information of the Murray River. As part of this study hydrographic survey was captured to represent the Murray River cross sections below the standing water level. The details of the survey captured are discussed in Section 2.2 and Figure 2-2. These survey cross sections were then imported into the hydraulic model at 1D cross sections (the cross sections are 1D because flow can only move in one direction through this component of the model. SOBEK then facilitates connectivity between the 2D Digital Elevation Model (DEM) and the 1D system. For the areas upstream and downstream of the captured river cross sections the most upstream and downstream cross section was assumed to be representative of the cross section upstream and downstream of the captured data. The slope of the river was estimated based on the captured hydrographic data and the LiDAR. The Tooleybuc Bridge is represented as a 1D bridge structure to ensure it is accurately modelled in the system. Critical culverts under the Murray Valley Highway have also been included as thee impact the flood behaviour of the system. The culvert sizes were estimated based on aerial imagery and Google Street View. The exact dimensions of the culverts were not required as the model was being run using a steady state approach.

4.1.2 Topography (2D) The surface of the hydraulic model was derived using a number of data sources including:  Murray Darling Basin Authority (MDBA) LiDAR information (captured in 2001)  Survey of the NSW levee systems (GHD, 2014). The hydraulic model included a nested grid approach which allows for a large grid cell size to be modelled to reduce model runtimes and increase efficiency, the nested grid has a smaller grid cell size to allow for more detail to be added to the area of interest. For this system the broad floodplain was modelled at a 24m x 24m resolution with a nested 6m x 6m resolution. The nested grid covers the full area of the proposed Tooleybuc replacement bridge investigations. Key features for the floodplain were manually added to the topography to ensure critical hydraulic controls (such as the levee survey and the major roads within the floodplain) were represented accurately in the hydraulic grid. It should be noted on the Victorian side of the Murray River it can be observed from aerial imagery there are some rural levee systems in place the LiDAR may not have completely defined. GHD noted the same

25 September 2015 Cardno 30 Tooleybuc Bridge Detailed Hydrology and Hydraulics Study limitations on the LiDAR. These rural levees are not expected to impact or change the flood behaviour in and around the replacement Tooleybuc Bridge, however the flood extent on the Victorian side of the model should not be used for any other purpose than this bridge assessment.

4.1.3 Roughness The roughness for the model has been defined using Manning’s Roughness parameters. The roughness parameters used for the model are shown in Figure 4-2 and range from 0.02 for the roads up to 0.10 for the township areas. The main floodplain has a roughness of 0.05 which is consistent with farmland. The roughness layer is in-line with the roughness used for the GHD model (2014) which set the main channel roughness at 0.045.

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Figure 4-1 Hydraulic model extent and topography

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Figure 4-2 Manning’s roughness for the model area

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4.2 Model calibration For the hydraulic model there is very little calibration data available. The primary calibration event is the 1975 event which had a peak flow captured at the Murray River at Piangil streamflow gauge. The peak flow rate for the 1975 event was 362 m3/s (31,300 ML/day) which was 9 per cent lower than the peak recorded at Swan Hill. There are four peak levels captured through the floodplain and these were taken from the GHD Flood Study (2014). Due to the duration of the flood events within the system the flows were entered as a steady state flow rate at the upstream boundary. The downstream boundary was set at 61.19 m AHD which corresponds to the level recorded at 75-1. The results of the calibration are summarised in Table 4-1 and are shown graphically with the peak flood depths in Figure 4-3.

Table 4-1 Calibration for the 1975 event

Event Flood Height Recorded Modelled Difference (m) Number flood height flood height (m AHD) (m AHD)

Nov 1975 75-1 61.19 61.20 +0.01

75-2 62.05 61.91 -0.14

75-3 62.49 62.56 +0.07

75-4 62.90 63.00 +0.10

There are four calibration locations, however location 75-1 is located at the downstream end of the model and was used to set the downstream boundary level for this event. The calibration at the remaining points was acceptable at all locations. It should be noted during this event the heights and breach conditions of the NSW levees was unknown and as such there is some uncertainty on the levee impact on the flood conditions.

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Figure 4-3 Calibration for the 1975 event

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4.3 Model validation In addition to the calibration of the 1975 event, the recent completion of the GHD Flood Study (2014) allowed for a comparison of the 100 year ARI flood levels to those produced as part of the GHD study. The 100 year ARI levels from the GHD study were taken from the Flood Study report in the form of a depth plot with water surface elevation contours. The current 100 year ARI results is shown overlaid on the GHD 100 year ARI results with comparison points through the system in Figure 4-4. The comparison shows a good validation of the levels, particularly through the main Murray River floodplain where levels are within +/- 0.08 m. The flood extents match well through the main floodplain. The main difference between the two sets of results is to do with the Murray Valley Highway. As part of this project Cardno used the LiDAR to capture the peak height along the road surface and to force this to be captured within the topography. This allows for the road to behave as expected as a hydraulic barrier to floodwaters. From the GHD report it is difficult to tell where the Murray Valley Highway is overtopped and it is unclear whether there are culverts under the road within the floodplain. As such there are some differences in extent to the east of the Highway. A significant difference is the area near the intersection of the Mallee Highway and Murray Valley Highway Cardno shows as not being flooded in the 100 year ARI. The Murray Valley Highway in this location is above the 100 year ARI levels and no culverts could be identified in this area. There are some irrigation connections but it is not clear whether these are fitted with one way valves to stop floodwaters. The extent to the east of the Murray River was flagged within the GHD Flood Study as unreliable due to the large range of rural levees and irrigation systems present. To improve this a detailed survey of levees (assessing their state of repair) would be required. This is beyond the scope of this investigation and the extent in these areas do not impact the flood behaviour in and around the study area of Tooleybuc.

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Figure 4-4 Validation of the 100 year ARI event against the GHD levels (GHD, 2014)

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5 Existing conditions

The calibrated and validated hydraulic model was used to assess the existing conditions for the study area using the design events generated in Section 3. The details of the model and results are discussed in this section.

5.1 Design events The design events have been simulated for the 20, 50, 100 and 2000 year ARI events for the study area. These events form the base case for the flood assessment. Each of the events has been simulated as a steady state simulation, the upstream boundary has a constant inflow and the downstream boundary is set at a constant level. The model has then been simulated for a sufficient period so levels within the system stabilises and the peak flood information can be extracted. For the design events information is generated in a gridded format:  Peak depths (m)  Peak water surface elevations (m AHD)  Velocity (m/s)  Hazard (based on depth and velocity x depth metrics). Each of the design events has been presented as a peak depth plot in the figures:  20 year ARI - Figure 5-1  50 year ARI - Figure 5-2  100 year ARI - Figure 5-3  2,000 year ARI - Figure 5-4. For the Murray River at Tooleybuc the peak levels at the existing bridge and at the Piangil gauge are summarised in Table 5-1. The difference in peak levels at the Tooleybuc Bridge are very low due to the width of the floodplain and the relatively small differences in peak flow rates for the design events. The current bridge soffit level of 63.45 m AHD is above the Probable Maximum Flood level. During large events the floodwaters overtop the Murray Valley Highway to the east of the existing bridge and the majority of the volume of the floodwaters bypass the existing bridge across the Murray Valley Highway.

Table 5-1 Peak flood levels for the study area

Event Peak Level at Peak level at the Difference (relative Piangil gauge (m existing Bridge (m to the modelled AHD) AHD) 100 year ARI)

20 year ARI 62.57 61.88 - 0.07

50 year ARI 62.61 61.92 - 0.03

100 year ARI 62.64 61.95 0

2,000 year ARI 62.70 62.00 + 0.05

PMF 63.35 62.68 + 0.73

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Figure 5-1 Existing conditions 20 year ARI peak depths and extent

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Figure 5-2 Existing conditions 50 year ARI peak depths and extent

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Figure 5-3 Existing conditions 100 year ARI peak depths and extent

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Figure 5-4 Existing conditions 2,000 year ARI peak depths and extent

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5.2 Probable maximum flood The PMF event has been simulated using a flow rate which is set at 105,000 ML/day which is three time larger than the 100 year ARI event. This is likely to be a conservative estimate of the flow rate as during extreme events flows above 35,000 ML/day at Swan Hill are naturally diverted north into NSW. The PMF levels are summarised in Table 5-1 and are below the current bridge structure. The peak depths are shown in Figure 5-5.

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Figure 5-5 Existing conditions PMF peak depths and extent

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5.3 Sensitivity assessment In order to test the sensitivity of the model to assumptions required as part of the modelling process a range of sensitivity runs were carried out. Sensitivity was carried out on:  Flow rates  Downstream boundary level  Roughness.

5.3.1 Flow rate sensitivity The impact of the peak flow rate on the model results inherently accounted for by the assessment involving the full range of design events. The model is not sensitive to the peak flow rates with levels at Tooleybuc Bridge only varying by 0.8 m for a range of flows from 32,500 ML/day to 105,000 ML/day.

Table 5-2 Peak flood levels for the study area

Event Peak flow (ML/day) Peak level at the Difference (relative existing Bridge (m to the modelled AHD) 100 year ARI)

20 year ARI 32,500 61.88 - 0.07

50 year ARI 34,000 61.92 - 0.03

100 year ARI 35,000 61.95 0

2,000 year ARI 37,600 62.00 + 0.05

PMF 105,000 62.68 + 0.73

5.3.2 Downstream boundary levels The downstream boundary levels have been assessed for the 100 year ARI event. This event has a downstream boundary set at 61.3 m AHD. To test the sensitivity of the floodplain to these conditions the boundary was set at a lower level of 61.1 m AHD and a high level of 61.6 m AHD. The results of the assessment are summarised in Figure 5-6 and Figure 5-7 for the high and low boundary conditions respectively. The results show the model is sensitive to the downstream boundary conditions with changes to the flood levels noted as far as 6 km upstream from the model boundary for the high downstream level scenario. At the Tooleybuc Bridge site the high level case increased the flood levels by around + 9 cm. The low boundary conditions scenario impacted the peak flood levels by - 4 cm. Although there were differences induced by the downstream boundary conditions at the Tooleybuc Bridge site, the range of difference of - 4 to + 9 cm is not significant to the assessment of the proposed replacement bridge structures.

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Figure 5-6 Sensitivity of the downstream boundary – High boundary case

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Figure 5-7 Sensitivity of the downstream boundary – Low boundary case

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5.3.3 Roughness An important part of the hydraulic model is the assumptions associated with the roughness of the floodplain. The roughness impact has been assessed by running a high and low roughness case. The Manning’s roughness values used are summarised in Table 5-3.

Table 5-3 Manning’s Roughness sensitivity values

Design Value Low Roughness High Roughness

Roads 0.02 0.015 0.025

River channel 0.045 0.03 0.06

Rural area 0.05 0.04 0.065

Treed Area 0.055 0.045 0.07

Township 0.1 0.07 0.15

The high roughness scenario increases the peak levels across the floodplain. Figure 5-8 shows the difference against the current 100 year ARI results. The areas shown in magenta are additional flooding due to the increased flood levels. Across the floodplain near the Tooleybuc Bridge the levels increased by around + 10 cm. For the low roughness scenario levels were reduced throughout the majority of the floodplain. Figure 5-9 shows the difference against the existing 100 year ARI results. The levels near the existing Tooleybuc Bridge decreased by around - 10 cm. Overall by changing the roughness across the floodplain there is a change in flood level of +/- 10 cm at the Tooleybuc Bridge.

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Figure 5-8 Sensitivity of the roughness – High roughness case

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Figure 5-9 Sensitivity of the roughness – Low roughness case

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5.4 Summary Overall the hydraulic model is calibrated and validated against the 1975 event and previous flood studies. The key outcomes from the sensitivity assessment include  The model is not sensitive to flow rate. Ranging from the 20 year ARI event up to the extreme event simulation there was only a change of +/- 80 cm across the floodplain. This is due to the extensive width of the floodplain and the low variability in peak flow rates (due to distributary systems upstream and no large local inflows)  The model showed some sensitivity to the downstream boundary conditions due to the flat nature of the floodplain. Changing the downstream boundary from 61.1 to 61.6 m AHD showed impact to the flood levels extending several kilometres upstream. However the impact at the Tooleybuc Bridge site were limited to less than +/- 10 cm of change  The model was also not sensitive to the roughness used. Changing the roughness by around +/- 30 per cent resulted in changes in peak flood levels by +/- 10 cm across the floodplain. In the context of the assessment of the replacement Tooleybuc Bridge, impact on levels of +/- 10 cm are not significant.

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6 Proposed options

The existing conditions and calibrated hydraulic model has been used to carry out an assessment of proposed options to determine their feasibility based on hydraulic criteria. This section of the report outlines the proposed options and the initial hydraulic performance.

6.1 Proposed options Three replacement bridge options have been proposed by Roads and Maritime. The three proposed options include:  Blue option – this option would replace the existing bridge with a similar structure including a lift span for river traffic. The replacement structure would be parallel to the existing bridge directly upstream  Purple option – this option would replace the bridge with a similar structure including a lift span for river traffic. Within Tooleybuc there would be a connection to Grant Street. This option is located 100 m upstream of the existing bridge location  Yellow option – This option is a clear span option set at a high level to remove the need for a lift span to be included in the structure. The abutment footprint for this option is larger than Blue and Purple options. The option connects directly to the Mallee Highway on the NSW side of the Murray River. Each of the options are presented in Figure 6-1. The nomenclature for the options within this report will be Blue, Purple and Yellow as specified in the definitions.

Figure 6-1 Proposed options for the replacement Tooleybuc Bridge

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6.2 Performance criteria The performance criteria and assessment of the proposed options is required to be assessed with due regard to the specific legislation, policies or guidelines apply to this area. Section 1.5 outlines the details of the details of the guidelines. The specific guidelines include:  Victorian Flood Management Strategy (State Flood Policy Committee, 1998)  State Planning Policy Framework as part of Victoria Planning Provisions  A planning guide for land liable to flooding in rural Victoria (RWC, 1989)  A guide to Floodplain Management in Country Victoria (RWC, 1987)  Water Act 1989 (Vic)  The Water Act (1917) and the water Management Act 2000 (NSW)  Floodplain Development Manual (NSW Government, 2005)  Water Management Act 2000 (NSW)  Environment Protection Act 1970  Wakool Local Environmental Plan (LEP) (2013). The legislation is both state and locally based and outlines the main directive of this assessment which is to assess the impact each of the proposed options have on the existing floodplain behaviour. The overriding directive is any changes to the floodplain should not increase or adversely change flooding behaviour in the area. The overriding control through each of the relevant legislation and policy is as summarised in Table 6- 1. As per Section 1.5 this is not the only overall objective but for the purpose of hydraulic assessment this is the principal guideline.

Table 6-1 Specific legislation and policy directives relating to flooding

Legislation, Policy or Guideline Issue Implication for Project

Water Act 1917 & Water Avoid intensifying the impact of Ensure the preferred option does Management Act 2000 – Division flooding through development of not increase or adversely change 5 Floodplain Management (NSW) structures in land subject to flooding behaviour. flooding.

Water Act 1989 – Section 208 Avoid intensifying the impact of Ensure the preferred option does (Vic) flooding through development of not increase or adversely change structures in land subject to flooding behaviour. flooding.

State Planning Policy Framework Avoid intensifying the impact of Ensure the preferred option does (SPPF) 13.02-1 Floodplain flooding through inappropriately not increase or adversely change Management located uses and developments. flooding behaviour.

Wakool Local Environment Plan Avoid adverse impact to flood Ensure the preferred option does 2013 – Section 6.2 behaviour and is not likely to not increase or adversely change significantly adversely affect flood flooding behaviour. behaviour resulting in detrimental increases in the potential flood affectation of other development or properties.

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The planning and policy directives do not supply specific assessment criteria for the change in peak flood levels due to a development and as such the approval for the impact a development has on the peak flood levels is at the discretion of the local planning authorities. For this location these authorities are the Wakool Shire Council on the NSW side and Mallee CMA on the Victorian side of the Murray River. From discussions with the authorities it is evident that the mandate of no increase in impact to properties and buildings implies no increase in flood levels due to the development. For this assessment the criteria has been set at limiting increases to + 2.5 cm which is in line with guidance of acceptable increases in flood levels for projects on the Murray River floodplain. The Mallee CMA and Wakool Shire Council are required to approve the final options before they can progress to the development phase.

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6.3 Option analysis Each of the options has been assessed for the 20, 50 and 100 year ARI flood events and subsequently directly compared to the existing conditions to determine any changes to peak flood water surface elevations or flood behaviour. Each of the options has been presented with the comparison of the developed scenario peak water surface against the existing conditions. For the report only the 100 year ARI event results have been shown as these are the critical event for the assessment. This allows for a direct assessment for any changes to the floodplain behaviour. For these assessments the existing structure has been removed.

6.3.1 Blue option The Blue option is the closest option to the existing bridge structure with a proposed similar option, as such it is expected this proposed option would perform a similar function to the existing structure. Figure 6-2 shows the difference in peak water surface elevations for the 100 year ARI between the proposed option and the existing conditions. The figure shows there is very little change between the existing and developed conditions with only minor localised changes in peak flood levels. These changes are within the main channel only and are limited to around + 2 cm. Downstream of the Blue option structure there is an area shown as a large change (red), this is the existing structure which has been removed for the developed scenario. As there is no change in flood conditions there are no required mitigation structures to be developed as part of this option. The Blue option was also assessed using the 20 and 50 year ARI events and no change was found between the existing and developed conditions.

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Figure 6-2 Peak flood difference for the Blue option (Developed less Existing, 100 year ARI)

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6.3.2 Purple option The Purple option is located 100 m upstream of the existing structure and is also a similar structure to the existing Tooleybuc Bridge. Again it is expected this structure would have a similar capacity as the existing structure, however as the opening is wider than the existing bridge (116 m compared to 108 m for the existing opening) it may have increased capacity to pass flood waters. Figure 6-3 shows the 100 year ARI results for the Purple option compared to the existing conditions. The peak water surface elevation difference plot shows the Purple option having a reduction in peak flood levels of between – 2 to – 3 cm. Downstream of the proposed structure there are minor increases in flood depths of less than + 2 cm. These changes in peak water level are a direct result of the proposed structure allowing a peak flow rate though the bridge of 375 m3/s which exceeds the peak flow through the existing structure of 370 m3/s for the 100 year ARI. This change in structure capacity is due to the orientation and location of the structure in the floodplain and due to the main channel cross sections through the area. Overall the changes to the peak flood levels are minor in relation to the peak depths in the area (at around 10 m deep in the main channel). The changes in the peak water surface levels are a reduction and as such there is no worsening of the peak flood levels. Downstream the increases in flood depths are limited to below 2 cm. For the 20 and 50 year ARI events the difference in peak flood levels shows a similar pattern to the 100 year ARI event. These results show the sensitivity of the floodplain to changes in structure capacity.

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Figure 6-3 Peak flood difference for the Purple option (Developed less Existing, 100 year ARI)

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6.3.3 Yellow option The Yellow option is an elevated structure option and as a result requires additional area for the abutments of the structure. The option crosses a section of the Murray River downstream of the existing bridge. Figure 6-4 shows the change in peak flood levels due to the proposed option for the 100 year ARI event. The option has some localised increases in flood depths at around + 0.025 cm due to the additional fill associated with the proposed option, however the majority of the floodplain remains unchanged. The 20 and 50 year ARI events show similar changes to the peak flood levels for the floodplain and maintain the existing flood conditions within +/- 2 cm from the existing levels.

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Figure 6-4 Peak flood difference for the Yellow option (Developed less Existing, 100 year ARI)

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6.4 Summary Overall all three proposed options meet the requirements of maintaining the peak flood conditions within acceptable limits of existing conditions. The Blue and Yellow options maintain the flood levels closely to existing conditions for the 20, 50 and 100 year ARI conditions. The Purple option reduces peak flood levels upstream due to providing a slightly more efficient peak flow rate though the structure (due to increased structure width). It should be noted the changes in peak flood levels are small at – 2 to – 3 cm only. From this stage of the assessment each of the proposed options are acceptable from a hydraulic performance basis, however this is subject to approval from the relevant authorities from both Victoria and NSW.

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7 Option analysis

Each of the proposed options meets the functional objective of maintaining the peak flood conditions in line with existing (subject to approval) and this section examines some of the constraints, details associated with the structures, scour protection and provides guidance on the most appropriate option.

7.1 Constraints, levels and structure types Each of the proposed designs meet the functional objectives of the hydraulic investigation (Section 6). Each option also does not require any additional mitigation structures to maintain peak flood levels in the area. The peak flood levels have been presented in Table 7-1 for the existing conditions and for the proposed options. The 2000 year ARI and PMF levels are summarised to show the maximum levels predicted to be reached in the bridge location.

Table 7-1 Peak flood levels at proposed option locations

Option Peak level at bridge (m AHD)

20 year ARI 50 year ARI 100 year ARI 2000 year ARI PMF

Existing Bridge 61.87 61.91 61.94 61.99 62.67

Blue option 61.89 61.92 61.95

Purple option 61.84 61.88 61.94

Yellow option 61.84 61.88 61.91

Overall the variability in peak flood level ranging from the 20 year ARI up to the PMF is predicted to be only 0.8 m with the PMF level set at 62.67 m AHD. The low variability in peak flood levels is due to a number of reasons:  The primary reason is due to the limited variability in the hydrology due to the distributary flows upstream of Swan Hill flowing north into NSW (Section 3)  Once the bridge capacity is reached the Mallee Highway on the Victorian side is overtopped and this takes a proportion of the flows overland down an old anabranch of the Murray River (see Figure 7-1). The existing bridge structure has a soffit level of 63.45 m AHD which is 0.78 m above the predicted PMF event. Given this is the theoretical maximum flood depth to be experienced in the bridge location the proposed options would only need to be at this level. For the Blue and Purple options the substructure depth is anticipated to be 1.5 m. If the soffit level is set at 63.45 m then the deck would be set at 64.95 m. This would be sufficient to protect the structure from being impacted during any event through the area. Both structures are planned to have a 16 m lift span and 25 – 32 m spans for the remaining sections which will require a minimum of four clear spans and three sets of piers. The span across the Murray River is around 110 m for both each option. For the Yellow option the substructure is estimated to be deeper at 1.8 m, however as the structure is elevated the deck and soffit are to be set well above the peak flood levels in the area. The soffit levels are driven by the clearance requirements for river traffic. Overall, due to the limited peak flood levels reached in the study area as long as the bridges maintain the existing soffit levels (or even slightly lower) then there will be sufficient freeboard from the PMF event.

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Figure 7-1 Flow distribution description

7.2 Scour assessment The assessment for each of the options has been completed on a conceptual level based on the preliminary guidance for the option orientations and concept designs. Once a detailed design has been determined this should be assessed to determine the required scour protection. This assessment provides preliminary advice and details of the peak velocity to be used for these calculations.

7.2.1 Existing geology and soil types Tooleybuc is situated within the Mallee dune field system of the Murray Geological Basin. For the study area the key sedimentary layers identified in the Geotechnical Constraints Report (Appendix E of Tooleybuc Bridge Replacement report (Vicroads/Roads and Maritime, 2014) include the upper Coonambidgal Formation and the deeper formation of the Woorinen Formation. The upper formation is characterised by clays, sands and clayey sands have collected over time. The Coonambidgal Formation is characterised by unconsolidated gray or red-brown silt, silty clay, poorly sorted sand and gravel have been alluvially deposited over time. Since the bridge was constructed in 1924 there has been no significant migration of the Murray River flow path. Downstream of the bridge there is some evidence of minor scouring, however this is not in the immediate vicinity of the current bridge structure. The Murray River banks next to the current bridge site are un-vegetated muddy banks around 1 metre above the typical water level oriented at around 20 to 30 degrees from horizontal. Beyond the muddy bank section there is existing vegetation including grasses and trees. In some areas there is isolated incidences of slumping and in some instances there are tree roots exposed as a result. A key area where this has occurred is the outside of the bend immediately downstream of the current bridge site (near the proposed Yellow option location).

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7.2.2 Peak velocity A critical factor in the assessment of scour is the peak velocity of floodwaters during large events. For the flows under the existing bridge the velocity ranges from 1 m/s up to 1.2 m/s for large flood events. Velocity in this range is relatively low and is consistent with previous findings from studies along the Murray River. The primary reason the velocity is in this range is due to the large widths and cross section of the Murray River main channel in this location. Table 7-2 shows the peak velocity reached at each of the proposed options, please note the 2000 year ARI and PMF events were only simulated for existing conditions.

Table 7-2 Peak velocity at the existing and proposed options

Location Peak Velocity (m/s)

100 year ARI 2000 year ARI PMF

Existing bridge 0.94 0.97 1.21

Blue option 0.82

Purple option 0.78

Yellow option 1.03

7.2.3 Abutment protection The current bridge structure has a timber vertical wall abutment on the Victorian side of the Murray River and a gentle sloping earth abutment on the NSW bank. There is no evidence of erosion on the existing abutments. For the proposed options the abutment setback is as per the supplied design orientations which are shown in Figure 6-1. For the Blue, Purple and Yellow options the setback from the main river channel is such the abutments can be facilitated by sloping earthen embankments at a slope of no greater than 2:1 (h:v). From the abutment setback it is anticipated that there will be a minimum of 10-20 m between the bank of the main channel and the bridge abutments. During the 100 and 2000 year ARI flood the peak depths in the overbank area are likely to be in the range of 1 – 3 metres deep and flowing at a peak velocity of 1 m/s.

In determining the appropriate D50 rock sizing for rip-rap protection for the use of open abutments some assumptions must be made. The assumptions and calculated rip rap median size is summarised in Table 7- 3. The preliminary calculation shows that a rip rap with D50 of 150 mm would be appropriate.

Table 7-3 Estimation of rip rap median particle size Parameter Assumption Value Bank Angle Slope 2:1 (h:v) 26.6 degrees Rock specific gravity Sandstone rock used 2.2 Rock angle of repose 40 degrees Peak velocity 2000 year ARI 1.0 m/s Depth of Flow Average depth next to abutment 2.5 m *Calculated using: [Croad, 1989] Refer: Melville & Coleman pp369~374 (2000)

Rip rap median size (m) D50 0.10 m Factor of Safety Due to the preliminary nature of the assessment 1.5 this has been set high Recommended Rip rap D 0.15 m median size (m) 50

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This assessment assumes that the abutments will be open abutments with a slope no greater than 2:1 (h:v). Should the design be required to be different to that specified this assessment should be revisited. For the area between the bank and the abutment stabilisation work should be carried out after disturbance due to major work. Revegetation should be in place to stabilise the surface, particularly next to the abutment. This will prevent any erosion beginning to form and undermine any rip rap put in place.

7.2.4 Pier protection Due to the near surface instability associated with the main river bed for the three options it is likely that piers will require suitable foundations stabilised using bored piles. Driven piles will be required to a depth where they interact with the deeper dense sands and gravels in the area. The foundation design, pile depth and number will form part of the detailed bridge design process after detailed geotechnical assessment along the selected option. The nature of the proposed options is such that all pier structures will be located in-bank and as such no assessment has been carried out for protection of exposed piers and foundations.

7.3 Mitigation requirements Each of the proposed mitigation options maintained the existing conditions within acceptable levels without the requirement for any mitigation structures to be included in the design. As such there is no need to consider mitigation measures.

7.4 Costing Costing for each of the options with relation to the mitigation options is not required as each option did not require any additional mitigation beyond the currently designed bridge spans and options. Each of the options are expected to have varying costs due to the varying requirements for construction of the approaches, type of structure and potential property acquisition. These costs form part of the decision criteria for selecting the preferred option however from a mitigation perspective there is no difference between the three proposed options.

7.5 Climate change Climate change is likely to change the rainfall patterns, rainfall intensity and distribution across the Murray River catchment over time. Numerous investigations and studies have been carried out discussing this issue over time. The current Murray Darling Basin Authority outlines that the predicted impact will be varied throughout the many regions of the river system. CSIRO outline that over time it is likely that temperatures will rise and rainfall variability will be impacted. In some cases the rainfall could decrease by as much as 60 per cent or increase by up to 40 per cent. As a result of these investigations it is evident that on a whole the Murray River system is likely to have unpredictable influences throughout the catchment of more than 300,000 km2. In any case, due to the high regulation of the system it is unlikely that peak flood events are going to increase over time. The sensitivity assessment also demonstrated that the floodplain is sufficiently wide that increases in flow rates only generate small increases in peak flood levels (see Section 5.3). Rather the result of climate change is more likely to reduce reliability of water supply for irrigation needs. Overall climate change will change the behaviour of the system, however due to the nature of the Murray River system this is unlikely to change the peak flood events at Tooleybuc. The Murray River has a natural high flow bypass at Swan Hill that will alleviate any increased peak flow rated via the distributaries into NSW. In addition the system is regulated and as such the large dams upstream on the Murray River are likely to help mitigate any increase flow rates.

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8 Conclusions

The Tooleybuc Bridge Replacement report has investigated three proposed options using detailed hydrology and a calibrated hydraulic model. The primary objective of this assessment was to develop the three options such that they met the primary legislative requirements of not increasing or adversely change flooding behaviour in the area due to changes within the floodplain. The Tooleybuc reach of the Murray River has a low change in peak flood levels through the site primarily due to low variability in peak flow rates for large flood events. Peak flow rates are controlled by upstream behaviour at Swan Hill. At this location during large flood events excess flows split from the system and flood north into the distributary systems within NSW. The floodwaters that flow via the northern distributaries re- enter the Murray River system downstream of Tooleybuc and do not influence flood levels at this location. Therefore, the peak flood events have a low range of peak flows, similar peak velocities and a low variability of peak flood levels (see Table 8-1).

Table 8-1 Summary of design flow rates and peak levels

Design event Peak Flow (ML/day) Peak velocity at the Peak level at the existing bridge (m/s) existing bridge

20 year ARI 32,500 0.91 61.87

50 year ARI 34,000 0.93 61.91

100 year ARI 35,000 0.94 61.94

2000 year ARI 37,600 0.97 61.99

PMF 105,000 1.21 62.67

As a result of low variability in flood events, and similarities of proposed options to the current bridge, the proposed options maintained the flood conditions within acceptable levels for the design events. Therefore no mitigation structures will be required for any of the alignment options. This assessment also considered required bridge soffit levels. The Blue and Purple options are proposed to be similar structures to the existing bridge with a lift structure allowing river traffic, these structures are required to be at a similar soffit level to the existing bridge (63.45 m AHD). This level is 0.8 m above the PMF event and provides enough freeboard for any flood event through the area. The Yellow option is a high level option and has a requirement to have a soffit level high enough to allow river traffic to pass under the bridge safely. Scour protection was assessed for each option. Assessment was completed using the hydraulic model to determine the peak velocity at the site and geotechnical information from previous Roads and Maritime investigations. This assessment was a preliminary assessment only as no detailed design has been completed at this stage of the project. Based on an understanding of abutments setback, each option can include open abutments which will require scour protection. Preliminary rock sizing for riprap protection was set with a D50 of 150 mm (a factor of safety of 1.5 applied). Any areas next to the abutments should be revegetated to stabilise the surface between the abutment and the river bank. Piers within the main Murray River channel are expected to be built with suitably designed foundations with piles to stabilise them. Piles should be driven to deeper gravel and compacted layers. Detailed geotechnical information should be used to design the piers to ensure the local geology is taken into account. Overall, each of the three proposed options meet the objectives to not increase or adversely change flooding behaviour in the area due to changes within the floodplain. From a flood perspective, each option is viable and as there are no additional mitigation measures required there is no preferred option. Each option requires similar erosion and scour protection.

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9 References

Binnie and Partners (1992), Swan Hill Tyntynder Flats Floodplain Management Study, Australia.

Cardno Lawson Treloar (2007), Hydraulic Issues for the Swan Hill Planning Study – LJ5525 / RM2140, Australia.

Cardno Lawson Treloar (2009), Detailed Hydrology Study for the Swan Hill Bridge Planning Study – LJ5573 / RM2180, Australia.

Egis Consulting (2000), Flood Data Transfer Project Flood mapping Report for Swan Hill, Australia

GHD (2014), Tooleybuc Flood Study – Draft, Australia.

Melville and Coleman (2000), Bridge Scour Water Resource Publications, LLC., Highlands Ranch, Colorado, U.S.A.

Roads and Maritime Services (2014), Tooleybuc Bridge Replacement – Environmental Constraints Analysis, Australia.

Vicroads/Roads and Maritime (2014), Tooleybuc Bridge Replacement – Appendix E, Geotechnical Constraints Report, Australia.

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