Flood Study For Mungindi - mpsc.nsw.gov.au

Flood Study For Mungindi

Report Prepared For

Moree Plains Shire Council

Report #J7834/R2 March, 2004

Lawson and Treloar Pty Ltd First Floor 40-44 Blackwood Street Mitchelton QLD 4053 Australia Telephone: 61 7 3855 1301 Facsimile: 61 7 3855 1904 A.C.N. 001 882 873 ABN: 55 001 882 873

Flood Study For Mungindi

Report Prepared For

Moree Plains Shire Council

Report #J7834/R2 March, 2004

Lawson and Treloar Pty Ltd First Floor 40-44 Blackwood Street Mitchelton QLD 4053 Australia Telephone: 61 7 3855 1301 Facsimile: 61 7 3855 1904 A.C.N. 001 882 873 ABN: 55 001 882 873

REPORT STATUS

Version

Date Status Prepared by: Reviewed by: Approved by:

1

18/03/04 Final RES JMCA NIC

It is the responsibility of the reader to verify the currency of the version number of this report. All subsequent releases will be made directly to the Client.

The information, including the intellectual property, contained in this report is confidential and

proprietary to Lawson and Treloar Pty Ltd (L&T). It may only be used by the person to whom it is

provided for the stated purpose for which it is provided, and must not be imparted to any third person

without the prior written approval of L&T. Lawson and Treloar Pty Ltd reserves all legal rights and

remedies in relation to any infringement of its rights in respect of its confidential information.

©

Lawson & Treloar Pty Ltd 2004

TABLE OF CONTENTS Page

1. INTRODUCTION ............................................................................................. 1

2. STUDY APPROACH........................................................................................ 2

3. HISTORICAL FLOOD DATA............................................................................ 3

4. HYDROLOGY .................................................................................................. 4

4.1 Historic Events ...................................................................................... 4 4.2 Design Events ....................................................................................... 4

5. HYDRAULIC MODELLING .............................................................................. 5

5.1 Modelling Software................................................................................ 5 5.2 Model Construction................................................................................ 6 5.3 Model Calibration................................................................................... 7 5.4 Design Events ....................................................................................... 7 5.5 Probable Maximum Flood (PMF)........................................................... 8

6. CONCLUSIONS & RECOMMENDATIONS ..................................................... 9

7. LIMITATIONS & QUALIFICATIONS .............................................................. 10

8. REFERENCES .............................................................................................. 11

FIGURES

APPENDICES Appendix A – Flood Frequency Curve Appendix B – Levee and Flood Profile

Lawson and Treloar Pty Ltd

Moree Plains Shire Council J7834/R2

Flood Study for Mungindi 18 March 2004 Page 1

1. INTRODUCTION New South Wales has in place a Flood Prone Land Policy, with the primary objective of the Policy being to ensure appropriate planning is in place to reduce the impact of flooding and associated flood liability on both existing and future property development. To support this Policy and provide guidelines on sustainable use of the floodplain, a Floodplain Management Manual has been established (January 2001) which identifies the four main stages in the floodplain risk management process. These are: • Flood Study : Determines the nature and extent of flooding behaviour • Floodplain Risk Management Study: Identifies, develops and compares

various floodplain management options utilising the results of the Flood Study as well as assessment of social, economic, ecological and cultural issues.

• Floodplain Risk Management Plan: Formalises outcomes of the previous

Studies and present the necessary information to enable relevant Authorities to plan for the future.

• Plan Implementation: Includes construction of structural floodplain

management measures as well as incorporation of non-structural measures into existing Local Authority Environmental and Development Control Plans.

This report provides the Flood Study component of the floodplain risk management process and details the investigation of flood behaviour associated with the ‘urban’ area of Mungindi. The general area of Mungindi is located on the Barwon River floodplain and has been subjected to various degrees of flooding over the years. The town itself lies on the Carnarvon Highway, 120Km Northwest of Moree, straddling the Queensland and New South Wales border. The population is approximately 650. The location of the township within the Moree Plains Shire Council boundaries is shown on Figure 1. The township supports agricultural (cotton, wheat) and cattle farming industries and has a road link to Moree. Over the years farming levees have been constructed in the Barwon River floodplain with some located within 1.5km of the town. These levees have impacted on the distribution of breakout flows from the river systems, both upstream and downstream of Mungindi. This study has involved the establishment of historic and design storm flows using flood frequency analysis and the set up of a two-dimensional, dynamic hydraulic model.




2. STUDY APPROACH

The approach undertaken for this flooding investigation has included the following key work items: (i) Identification of areas requiring additional ground survey and collation of

this survey with existing photogrammetric data; (ii) Site Inspections and Historic Flood Data Gathering; (iii) Extraction of discharges and water levels from the existing Border Rivers

Mike 11 model for use as boundary conditions in the 2D hydrodynamic modelling of historic events used for calibration purposes.

(v) Flood frequency analysis to determine design flows for the 20%, 10% and

1% AEP. (vi) Detailed hydraulic modelling of historic flood events for calibration

purposes followed by modelling of the design flood events for the AEPs mentioned above.

(vii) Estimation of the Probable Maximum Flood (PMF) if practical.




3. HISTORICAL FLOOD DATA

As part of this study historical flood information was sought to gain an understanding of previous flood history and flood patterns as well as for model calibration purposes. The flood events of January 1996 and September 1998 were used for calibration purposes. Although the February 1976 event is the largest on record, this flood occurred prior to the Mungindi levee being constructed. Flood level and flow information associated with the Mungindi stream gauge (Station Number 416001) located immediately downstream of the township was extracted from the previous Border River Study and augmented with more recent information by the Department of Infrastructure, Planning and Natural Resources (DIPNR).




4. HYDROLOGY 4.1 Historic Events

Flood flows associated with the 1996 and 1998 Barwon River events have been obtained from the rated stream gauging station at Mungindi (Station Number 416001). 4.2 Design Events

The Mungindi stream gauge has 112 years of height and rated flow records. A flood frequency analysis based on peak annual floods and using a Log Pearson III distribution was undertaken. The distribution is shown in Appendix A and design event flows presented below.

Table 4.1: Mungindi Peak Discharges – Flood Events

Event Type Flood Event Peak Discharge * (m3/s) 1996 638 Historic 1998 724

20% AEP 191 10% AEP 307 Design 1% AEP 1204

*through the gauge only The estimated peak flow for the 1976 event was 1816m3/s (i.e. larger than the 1% AEP).




5. HYDRAULIC MODELLING 5.1 Modelling Software

Modelling has been performed using the Nested Grid feature of the two dimensional (2D) finite difference model Sobek, produced by Delft Hydraulics in The Netherlands. The model can simulate steady and unsteady hydrodynamic flow in two directions on a rectilinear grid. It is based on a robust finite difference scheme able to compute both subcritical and supercritical flow regimes. The Sobek model is suited to simulation of dynamic hydraulic behaviour of overland flow over initially dry land. Based on this and Sobeks supercritical solving ability as well as its available Nested Grid feature the Sobek modelling system was considered the most appropriate investigative tool for the characteristics of Mungindi township flooding. Major advantages of the fully two-dimensional approach over traditional one dimensional approaches are: • Full topographic survey terrain models are used, rather than selected,

discrete cross sections;

• Flow patterns are dictated by the influence of topography and surface roughness conditions, rather than by ‘forced’ flow paths, as used in quasi-two-dimensional networks;

• Flow directions and paths can vary with stage and flow conditions (c/f ‘rigid’ networks forcing flow paths in quasi-two-dimensional models); and

• Production of detailed output of flow patterns, and output suitable for ready GIS interfacing. This allows production of accurate depth of flooding, velocity and hazard maps as well as inundation extent maps.

The nested grid models developed consist of a fine 10m ‘child’ grid nested on a coarser ‘parent’ grid. Benefits of this type of schematisation include; • The fine “child” grid allows for a better representation of features and flow

mechanisms in the urbanised area; and

• The larger grid size in the “parent” grid enables regional influences to be modelled without an excessive increase in computational requirements.




As the primary purpose of the flood study is to establish existing flood behaviour, the use of a nested grid provided an appropriate tool to define the nature and extent of flood risk on existing properties and road systems the town. 5.2 Model Construction

The 2D model topography was derived from detailed survey conducted by SMK Irrigation Consultants and photogrammetry utilised in the Border River Floodplain Hydraulic Analysis. Two topographic cases have been used in the analysis, one for both of the calibration events and the other for the design events. The topographic case used for the design event modelling includes the new Queensland town levee and one small farm levee just north of the Queensland township. These features are not included in the calibration case. Around the Mungindi township topography was discretised into 10m square grid elements associated with the child grid. The fine 10m child grid allows for a good representation of topographic features in the township and allows for flooding assessments on a lot by lot basis. The child grid is surrounded by 30m square elements of the parent grid. Inflow boundaries for the calibration events have been extracted from the existing Border Rivers Mike 11 Model. Design event discharges have been estimated using a flood frequency analysis. The tailwater conditions used in the analysis have been based on normal depth rating curves with the downstream boundary of the 2D model set a distance which will not influence flood levels in the vicinity of Mungindi. The spatial variation of roughness was incorporated into the model by assigning each grid element an individual Manning’s ‘n’ roughness value. Typical roughness values used in the modelling are as follows:

Table 5.1 – Typical Roughness Values Adopted

Land Use Manning’s ‘n’ Value Flood Plain 0.05 – 0.06

Creeks / Rivers 0.045 Built up area (buildings) 0.15

Roads 0.03




5.3 Model Calibration

Calibration has been undertaken using 2 historic flood events, the major event in 1998 and 1996, as discussed in Section 4. Models have been calibrated to peak flood levels. The 1976 event was not used for calibration purposes as the township levees had not been constructed. The current levees will have some influence on stream gauge levels. A satisfactory calibration has been achieved by adjusting tailwater conditions and roughness values. Table 5.2 shows the comparison of peak flood levels for each of the calibrated events compared with available historic data. The model has been calibrated at the Mungindi stream gauge, the location of which can be found in Figure 3.

Table 5.2 - Calibration Summary

Flooding Event

Recorded Level (m AHD)

Predicted Level (m AHD)

Difference (m)

1996 160.83 161.02 +0.19 1998 160.86 161.07 +0.21

Figure 4.1 and 4.2 show the peak water depths and peak velocity vectors estimated for the 1996 and 1998 historic events respectively. 5.4 Design Events

Design event hydraulic modelling was run using the calibrated Sobek models in conjunction with the calculated design discharges as discussed in section 4.2. Predicted peak flood depths through and around the township for the 20% to 1% AEP events are presented in Figures 5.1 to 5.3. Provisional Hydraulic Hazard Categories – as defined in the Figure G2 (Appendix G) of NSW Floodplain Management Manual, have been calculated through and around the township for each design event. Areas of high hazard indicate possible danger to personal safety as wading and evacuation by trucks becomes difficult. There will also be the potential for structural damage to buildings. Areas of low hazard indicate that able bodied adults should be able to wade without difficulty and evacuations by trucks should be possible. In areas defined as the transition zone the degree of hydraulic hazard is dependent on its landuse (NSW Floodplain Management Manual).




Hazard mapping for the 20% to 1% AEP events are presented in Figures 6.1 to 6.3. As a comparison of the predicted design events flood levels with levels found from historic event flood modelling, modelled flood levels at the Mungindi gauge are as follows.

Table 5.3 – Predicted Flood Levels at Mungindi Gauge

Event Peak Flood Level (m AHD) 1996 Historic Event 161.02 1998 Historic Event 161.07

20% AEP Event 160.55 10% AEP Event 160.79 1% AEP Event 161.45

The location of this stream gauge is shown in Figure 3. 5.5 Probable Maximum Flood (PMF)

As the primary area of interest is the township itself, model extents reflect the floodplain in the immediate vicinity (within 7.5km). For a flood based on the probable maximum precipitation (PMP) these extents are unlikely to be sufficient and in a wide floodplain such as the Macintyre / Barwon River, modelling results will be conservatively high. On this basis, a flood equivalent to three (3) times the 1% AEP event has been modelled. The resultant peak flood level at the stream gauge location for this extreme event is 162.48 mAHD which is 1.03 m higher than the 1% AEP event.




6. CONCLUSIONS & RECOMMENDATIONS If the flood resulting from a PMP in the Macintyre River catchment upstream of Mungindi is to be considered further, model extents will need to increase considerably to ensure the distribution of breakout flows from the Macintyre /Barwon River system upstream is correctly modelled. This would require additional survey and with current computer processing speeds would require that modelling be undertaken using a quasi – 2D modelling system like MIKE 11 rather than the full 2D modelling system used in this Study. Consideration could also be given to assessing the impact of a 1976 size flood with the existing township levees in place. The flood frequency analysis indicates the 1976 event was larger than a 1% AEP event and could be an appropriate flood event against which to set township levee levels.




7. LIMITATIONS & QUALIFICATIONS

This report has been prepared by Lawson and Treloar (L&T) specifically for Moree Plains Shire Council (MPSC) and specifically to provide advice on the flooding characteristics of the Barwon River at the Town of Mungindi. Our analysis and overall approach has been specifically catered for the particular requirements of MPSC and may not be applicable beyond this scope. For this reason any other third parties are not authorised to utilise this report without further input and advice from L&T. This report is based on the following studies and information prepared by others: • L&T has relied on 3D terrain information and survey provided by SMK.

• L&T has relied upon historic flood levels at the Departments of Infrastructure, Planning & Natural Resources (DIPNR) Mungindi stream gauge.

• Photogrammetry carried out for the Border Rivers Floodplain Hydraulic Analysis (BRFHA) Study.

• L&T has relied on historical and design flood level and flow estimates from previous studies for use as boundary conditions.

The accuracy of the report is dependent on the accuracy of this information. Future observed flood levels may vary from the predicted levels in this report depending on the characteristics of the catchments, rainfall, channels and floodplain.




8. REFERENCES NSW Government, Jan 2001, ‘Floodplain Management Manual: the management of flood liable land’ Institution of Engineers, Australia, 1987, ‘Australian Rainfall and Runoff’, Volume 2. Institution of Engineers, Australia, 1987, ‘Australian Rainfall and Runoff, Book 6, Estimation of Large and Extreme Floods’ Cameron McNamara, 1982, ‘NSW Inland Rivers Floodplain Management Studies, Summary Report, Gwydir Valley’ Cameron McNamara, 1982, ‘NSW Inland Rivers Floodplain Management Studies, Gwydir Valley’ Lawson and Treloar, October 1998, ‘Border Rivers Floodplain Hydraulic Analysis Main Report’ J7145/R6, for Waggamba Shire Council. Lawson and Treloar, January 2001, ‘Macintyre River – Boggabilla to Dingo Creek Flooding Analysis’ J7484/R1, for Waggamba Shire Council. Lawson and Treloar, March 2002, ‘Refinement of Border Rivers Mike 11 Flood Model and Cumulative Effects Checking’ J7876/R2, for Waggamba Shire Council. Water Resources Commision – NSW ‘Guidelines for Macintyre River and Whalan Creek Flood Plain Development, Boggabilla to Mungindi’, 1981. Public Works, NSW Dams Section. ‘Audit of Flood Levels for NSW, Town of Mungindi’, May 1992

Mungindi Flood Study Appendix B March 2004

MungindiDesign Flood Levels and Levee Profile

A

DC

B

A

160.0

160.5

161.0

161.5

162.0

162.5

163.0

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500

Distance Around Levee (m)

Z (m

AH

D)

Levee Profile1% AEP Flood Level10% AEP Flood Level20% AEP Flood Level

Documents

Flood Study For Mungindi - mpsc.nsw.gov.au