Appendix N Namoi River Flood Impact Assessment CONTINUATION OF BOGGABRI COAL MINE – NAMOI RIVER FLOOD IMPACT ASSESSMENT FOR HANSEN BAILEY 22 December 2009 REPORT TITLE: Continuation of Boggabri Coal Mine – Namoi River Flood Impact Assessment CLIENT: Hansen Bailey REPORT NUMBER: 0590-01-B (Rev 3) Revision Number Report Date Report Author Reviewer 0 9 September 2009 AH GR 1 23 November 2009 GR SM 2 16 December 2009 AH GR 3 22 December 2009 AH GR For and on behalf of WRM Water & Environment Pty Ltd Greg Roads Director COPYRIGHT: The concepts and information contained in this document are the property of WRM Water & Environment Pty Ltd. Use or copying of this document in whole or in part without the written permission of WRM Water & Environment Pty Ltd constitutes an infringement of copyright. LIMITATION: This report has been prepared on behalf of and for the exclusive use of WRM Water & Environment Pty Ltd’s Client, and is subject to and issued in connection with the provisions of the agreement between WRM Water & Environment Pty Ltd and its Client. WRM Water & Environment Pty Ltd accepts no liability or responsibility whatsoever for or in respect of any use of or reliance upon this report by any third party. 0590-01-B (Rev 3) 22 December 2009 TABLE OF CONTENTS Page 1 INTRODUCTION 4 2 BACKGROUND 6 2.1 CATCHMENT CHARACTERISTICS 6 2.2 NAMOI RIVER STREAM FLOWS 6 2.3 NAMOI RIVER CHANNEL AND FLOODPLAIN CHARACTERISTICS 7 2.4 EXISTING HAUL ROAD 7 2.5 PROPOSED THERRIBRI ROAD OVERPASS 9 2.6 PROPOSED RAILWAY BRIDGE CROSSING 9 3 NAMOI RIVER DESIGN DISCHARGES 10 3.1 GENERAL 10 3.2 PRE-FEASIBILITY STUDY ESTIMATES 10 3.3 FLOOD FREQUENCY ANALYSIS 11 3.4 ADOPTED DISCHARGES 11 4 HYDRAULIC MODELLING 13 4.1 GENERAL 13 4.2 MODEL SELECTION 13 4.3 TUFLOW MODEL CONFIGURATION 13 4.3.1 Spatial Extent 13 4.3.2 Available Survey Data 15 4.3.3 Adopted Manning’s ‘n’ Values 15 4.3.4 Inflow and Outflow Boundaries 15 4.4 PRE-MINE CONDITIONS MODEL VERIFICATION 16 4.5 EXISTING CONDITIONS MODEL 16 4.6 PROPOSED CONDITIONS MODEL 18 4.7 HYDRAULIC MODEL RESULTS 20 4.7.1 Existing Conditions 20 4.7.2 Proposed Conditions 20 4.7.3 Impact on Flood Levels and Flood Velocities 21 4.8 DISCUSSION OF IMPACTS AND MITIGATION MEASURES 21 5 SUMMARY OF FINDINGS 26 6 REFERENCES 28 APPENDIX A EXISTING HAUL ROAD BRIDGE ARRANGEMENT 29 APPENDIX B PROPOSED RAIL BRIDGE ARRANGEMENT 31 i 0590-01-B (Rev 3) 22 December 2009 APPENDIX C FLOOD DEPTHS 33 APPENDIX D VELOCITIES 43 ii 0590-01-B (Rev 3) 22 December 2009 LIST OF TABLES Page Table 2.1 Streamflow Gauges 7 Table 3.1 Namoi River Design Discharges and historical design flows adopted by PB (2008) 10 Table 3.2 Flood Frequency Analysis Results, Namoi River at Boggabri 11 Table 4.1 Adopted Manning’s ‘n’ values 15 Table 4.2 Recorded and Predicted Peak Water Level at Flood Mark, 1955 flood event. 16 Table 4.3 Existing Haul Road Bridge Configuration Data 16 Table 4.4 Proposed Rail Bridge Configuration Data 18 LIST OF FIGURES Page Figure 1.1 Boggabri Coal Mine Project Locality 5 Figure 2.1 Namoi River channel in the vicinity of the proposed rail bridge crossing 8 Figure 2.2 Boggabri Coal Haul Road Bridge during the Namoi River December 2008 Flood 8 Figure 3.1 Comparison of PB (2008) Study Discharges and Flood Frequency Distribution, Namoi River at Boggabri 12 Figure 4.1 TUFLOW Model Configuration for the Pre-Mine Conditions Topography 14 Figure 4.2 Existing Conditions Topography including the Haul Road 17 Figure 4.3 Proposed Conditions Topography including the Proposed Rail Line and Therribri Rd Overpass 19 Figure 4.4 Namoi River Existing Conditions Flood Extent, 5 year, 20 Year and 100 Year ARI Events 23 Figure 4.5 Namoi River Proposed Conditions Flood Extent, 5 Year, 20 Year and 100 Year ARI Events 24 Figure 4.6 Namoi River Design Flood Depth Difference, 100 Year ARI 25 iii 0590-01-B (Rev 3) 22 December 2009 1 INTRODUCTION Boggabri Coal Pty Limited (Boggabri Coal) is seeking a Project Approval under Part 3A of the Environmental Planning & Assessment Act 1979 (EP&A Act) to continue its open cut mining and associated activities largely consistent with its existing operation for a further 21 years (the Project). As part of this approval, Boggabri Coal proposes to construct a 17km rail line to haul coal from the mine across the Namoi River floodplain to the Werris Creek Mungundi Railway, approximately 9 km north of Boggabri. The existing haul road across the Namoi River floodplain will be widened and Therribri Road will be upgraded to overpass the haul road. The locations of the mining areas, the rail line and haul road are shown in Figure 1.1. WRM Water & Environment Pty Ltd was requested to prepare a flood study to be incorporated in the Environmental Assessment (EA) to support the Project Approval application. The study was to determine whether the proposed rail bridge, haul road upgrade and Therribri Road overpass on the Namoi River floodplain would constrict flows sufficiently to cause increased flooding or potentially cause erosion along the river and floodplain. To undertake the study, a TUFLOW two dimensional hydraulic model was developed of the Namoi River and its floodplain for existing conditions and with the proposed rail bridge and road upgrades in place. The 5 year, 20 year and 100 year average recurrence interval (ARI) design discharges were used for the impact assessment. This report is structured as follows. Section 2 describes the characteristics of the Namoi River catchment and the drainage characteristics of the river channel and floodplain in the vicinity of the two roads and rail bridge crossing. Descriptions of the haul road, Therribri Road upgrades and the proposed rail bridge are also provided. Section 3 presents the Namoi River design discharges. Section 4 presents the methodology and results of the hydraulic modelling undertaken to determine design flood levels, flood extents and flood velocities for existing and proposed conditions. Section 5 summarises the findings of the study. Section 6 is a list of references. Appendix A provides the technical drawings of the existing mine haul road crossing of the Namoi River. Appendix B provides the technical drawings of the proposed rail line and crossing of the Namoi River. Appendix C provides maps that show the existing and proposed conditions flood depths and changes in flood depths in the study area for the range of flood events investigated. Appendix D provides maps that show the existing and proposed conditions flood velocities and changes in flow velocities in the study area for the range of flood events investigated. 4 BoggabriBoggabri SydneySydney 0590-01-B (Rev 3) 22 December 2009 NamoiNamoi RiverRiver 041901704190170419017 --- BoggabriBoggabriBoggabri WeirWeirWeir KKaammiiiillllaarrrooiiii HHwwyy Nam oi R iv er 041901204190120419012 --- BoggabriBoggabriBoggabri Project Boundary Mine Haul Road Namoi River Study Area Mine Infrastructure Area BoggabriBoggabri Town LEGEND Road Waterway WL Station Proposed Rail Line 5 kilometres 02.5Figure 1.1 Boggabri Coal Mine Project5 Locality 0590-01-B (Rev 3) 22 December 2009 2 BACKGROUND 2.1 CATCHMENT CHARACTERISTICS The Namoi River begins in the Great Dividing Range and extends for over 350km west where it discharges into the Barwon River near Walgett. It has a total catchment area of approximately 42,000km2. The major tributaries of the Namoi River include the Peel River, Mooki River, Manilla River, Coxs Creek, Baradine Creek and Pian Creek. There are a number of major storages in the catchment, namely Keepit, Chaffey and Split Rock Dams located on the Namoi, Peel and Manilla Rivers respectively. Keepit Dam, the main water storage in the catchment, is located approximately 70km upstream of the study area and was constructed in 1960. The Namoi River has a catchment of about 22,700km2 to the proposed rail bridge crossing, which is located approximately 9km downstream of the township of Boggabri. The Namoi River flows in a northerly direction through the study area. 2.2 NAMOI RIVER STREAM FLOWS Figure 1.1 shows the locations of two stream flow gauges on the Namoi River operated by the NSW Office of Water (previously Department of Water and Energy (DWE)) in the vicinity of the Project. Details of the two stations are given in Table 2.1. The flow recorded at both gauges would be representative of the flow at the proposed rail bridge crossing because the catchment areas are relatively similar. However, the Boggabri gauge has a much longer period of record than the Boggabri Weir gauge and the rating curve, which is used to convert recorded water level to flow, is also more reliable because it has been gauged up to a much higher flood level. The largest recorded flood event at the Boggabri Gauge occurred in February 1955. This flood had a peak discharge of 4,300m3/s and caused widespread inundation and damage. At Narrabri, over 1000 homes were affected by floodwater, some inundated to their rooftops during this flood. Other large floods have occurred in January 1971, February 1956, January 1976 and February 1984. The Namoi River water levels at Boggabri are heavily regulated by the upstream dams. Releases are made from Keepit Dam, the most downstream dam, during the summer months to supply irrigators, most of which are located downstream of Boggabri to the west of Narrabri. Keepit Dam releases are much lower through the winter months when there is less irrigation demand. The unregulated sections of the river downstream of Keepit Dam, including from the lower catchment of the Peel River, Mooki River and Coxs Creek are characterised by long periods of low to no flows with intermittent bursts of higher flows following storm events.