The Darling River Floodplain (Bogan River confluence to Louth) study area is located within the Darling

River Valley floodplain. The study area stretches approximately 130 kilometres along the Darling River

Floodplain, from approximately 40 kilometres upstream of Bourke downstream to Louth (some 90km

downstream of Bourke). Bourke is located approximately 774 kilometres north-west of Sydney and 369km north-west of its nearest regional centre, Dubbo.

The study area encompasses a floodplain area of approximately 2,700 square kilometres within the Local

Government Area (LGA) of Bourke Shire, the shire covers an area of around 41,679 square kilometres

and the town of Bourke is the main residential centre in the Shire and accommodates around two-thirds of the Shire’s population. Essentially, the study area corresponds with the designated floodplain (6th February 1987, under Section 166 Part 8 of the Water Act 1912) shown in Appendix D.

The Barwon Darling catchment contains a large semi arid river system that drains the inland slopes of the Eastern Highlands. The total catchment area of 650,000 km2 (386,000 km2, at Bourke) is characterised by low relief, with elevations ranging from 1000m, above sea level, in the Eastern highlands to 50m in the south west of New South Wales. Sixty percent of the catchment has elevation less than 300m. The town of Bourke is situated 106 metres above sea level and the Darling River is Australia’s longest River.

The Barwon–Darling system receives major flow contributions from the following six main areas (the percentage of each sub-catchment input flow is also shown):

• The Border Rivers, which contribute approximately 35% of the total long term flow

• Gwydir River (10%)

• Namoi River (25%)

• The Macquarie and Castlereagh Rivers (5%)

• The Culgoa River system (20%)

• Local rainfall events and minor contributions from the Warrego and Paroo (downstream of study area) rivers (5%).

Ground Survey

All survey work is to be Geo-referenced to GDA94, MGA Zone 55 and elevations reduced to the

Australian Height Datum (AHD) via the Ausgeoid98 geoid model.

Accuracy

The accuracy of the survey shall be 50 mm in both the vertical and horizontal.

4.1 Survey

4.1.1 Airborne Laser Survey

In response to the topographical data requirements identified in the early stages of the Study, an Airborne Laser Survey (ALS) of the study area was commissioned in order to provide the data to develop a Digital Terrain Model (DTM) of the Darling River Floodplain. The survey area encompassed approximately 4,000 km2 and included Bourke Township and the village of Louth. ALS data was supplied:

With a stated vertical accuracy +/- 0.15m @ 67% confidence;

With a stated horizontal accuracy +/- 0.30m @ 67% confidence;

Geo-referenced to GDA94, MGA Zone 55; and

Elevations reduced to the Australian Height Datum (AHD).

A Digital Terrain Model (DTM) was created from the ALS data. In addition, a Triangular Irregular Network (TIN) was constructed covering the Darling River, its tributaries and a river buffer area that extends 100m either side of each river bank, ensuring sufficient detail of the important features of the river bank were captured. The TIN has coverage of approximately 120 km2 and includes break lines that define the edges of features of importance for flood modelling, such as the top of levees or other channel works on the flood plain.

At the same time, spatially geo-referenced aerial photographs (ortho-imagery) of the area were captured.

A Digital Elevation Model (DEM) was created from the TIN as a means of verifying the coherence of the data in the TIN. Figure 4-1 presents the DEM of the study area.

4.1.2 Ground Survey

Ground surveys were undertaken at the following four drainage structures:

Old Bourke Bridge at North Bourke;

Bourke Weir approximately 5km downstream of Bourke;

The main bridge at Louth; and

The Anabranch Bridge, which is northwest of the main bridge at Louth.

Details of the structures were surveyed, including bridge piers and railing, together with ground levels and crest levels. Photographs of each structure were taken to record important features. Details of the surveys can be found in the Compendium of Data.

Development of floodplains in the western rural areas of NSW is managed through Part 8 of the Water Act 1912. Part 8 was gazetted in 1984 and makes provisions concerning ‘flood control works’ that affect, or are likely to affect, flooding or floodplain functions. Part 8 was amended in 1999 to allow for more strategic control of flood control works through the preparation of rural Floodplain Management Plans (FMPs) and a more streamlined and resource efficient approval process. The amendments also required that rural FMPs be developed in accordance with the provisions and policies of the operative NSW Manual (currently, Floodplain Development Manual, NSW Government, 2005). The manual supports the primary objective of the New South Wales Government’s Flood Prone Land Policy to reduce the impact of flooding and flood liability on individual owners and occupiers of flood prone property, and to reduce private and public losses resulting from floods, utilising ecologically positive methods wherever possible. The formulation and implementation of FMPs is the cornerstone of the policy and the process for their preparation is described in the manual. Administrative orders published in April 2007, inter alia, divided the responsibilities under Part 8 of the Water Act 1912 between the Department of Environment and Climate Change (preparing rural FMPs) and the Department of Water and Energy (implementing the plans through licensing and compliance functions). On 1 July 2009, following a Reorganisation of Government Agencies, the licensing and compliance functions regarding Part 8 were transferred to the newly established Office of Water within the renamed Department of Environment, Climate Change and Water (DECCW). Please note that any reference in this document to the Department of Environment and Climate Change, and Department of Water and Energy, is to be construed as a reference to DECCW. The development of a rural FMP progresses through three (3) key stages:  Data Collection & Flood Study (FS) - in general, includes data collection for hydrology, hydraulic and environmental aspects; defines the nature and extent of flooding, including development of a computer-based hydraulic model of flood behaviour;  Floodplain Risk Management Study (FRMS)– in general evaluates management options based on hydraulic modelling, analysis of the floodplain environment and social and economic considerations, to address existing and future floodplain risk management issues; and  Floodplain Management Plan (FMP) – presents the recommended management measures for the floodplain, as determined in the Floodplain Risk Management Study, and includes an implementation program with priorities. This current project, overseen by the Darling River Floodplain (Bogan River confluence to Louth) Floodplain Management Committee (the Committee) and prepared for DECCW, includes only the first of the above three elements of the Floodplain Management Plan process, and is structured as follows: Stage 1: Data Compendium and Digital Terrain Model All information and knowledge of the flood regime in the study area are included in a Compendium of Data (COD), as a permanent record, and for later use in Stage 2 of the FS. A Digital Terrain Model was prepared for hydraulic modelling use in Stage 2 of the FS and the results presented on an appropriate Geographic Information System (GIS). Stage 2: Calibrated Hydraulic Model and Final Flood Study Report A calibrated computer based hydraulic model was developed to quantify the distribution of floodwaters during historical flood events and for later use in designing a floodway network (as part of the FRMS) in order to evaluate management options. The Compendium of Data was completed in July 2008. This Report covers Stage 2: Calibrated Hydraulic Model and Final Flood Study Report. The Commonwealth Government’s Natural Disaster Mitigation Program (NDMP) and the New South Wales State Government through the NSW State Emergency Management Committee (SEMC) have jointly provided funding for the project.

The Darling River Floodplain (Bogan River confluence to Louth) study area is located within the Darling

River Valley floodplain. The study area stretches approximately 130 kilometres along the Darling River

Floodplain, from approximately 40 kilometres upstream of Bourke downstream to Louth (some 90km

downstream of Bourke). Bourke is located approximately 774 kilometres north-west of Sydney and 369km north-west of its nearest regional centre, Dubbo.

The study area encompasses a floodplain area of approximately 2,700 square kilometres within the Local

Government Area (LGA) of Bourke Shire, the shire covers an area of around 41,679 square kilometres

and the town of Bourke is the main residential centre in the Shire and accommodates around two-thirds of the Shire’s population. Essentially, the study area corresponds with the designated floodplain (6th February 1987, under Section 166 Part 8 of the Water Act 1912) shown in Appendix D.

The Barwon Darling catchment contains a large semi arid river system that drains the inland slopes of the Eastern Highlands. The total catchment area of 650,000 km2 (386,000 km2, at Bourke) is characterised by low relief, with elevations ranging from 1000m, above sea level, in the Eastern highlands to 50m in the south west of New South Wales. Sixty percent of the catchment has elevation less than 300m. The town of Bourke is situated 106 metres above sea level and the Darling River is Australia’s longest River.

The Barwon–Darling system receives major flow contributions from the following six main areas (the percentage of each sub-catchment input flow is also shown):

• The Border Rivers, which contribute approximately 35% of the total long term flow

• Gwydir River (10%)

• Namoi River (25%)

• The Macquarie and Castlereagh Rivers (5%)

• The Culgoa River system (20%)

• Local rainfall events and minor contributions from the Warrego and Paroo (downstream of study area) rivers (5%).

Ground Survey

All survey work is to be Geo-referenced to GDA94, MGA Zone 55 and elevations reduced to the

Australian Height Datum (AHD) via the Ausgeoid98 geoid model.

Accuracy

The accuracy of the survey shall be 50 mm in both the vertical and horizontal.

4.1 Survey

4.1.1 Airborne Laser Survey

In response to the topographical data requirements identified in the early stages of the Study, an Airborne Laser Survey (ALS) of the study area was commissioned in order to provide the data to develop a Digital Terrain Model (DTM) of the Darling River Floodplain. The survey area encompassed approximately 4,000 km2 and included Bourke Township and the village of Louth. ALS data was supplied:

With a stated vertical accuracy +/- 0.15m @ 67% confidence;

With a stated horizontal accuracy +/- 0.30m @ 67% confidence;

Geo-referenced to GDA94, MGA Zone 55; and

Elevations reduced to the Australian Height Datum (AHD).

A Digital Terrain Model (DTM) was created from the ALS data. In addition, a Triangular Irregular Network (TIN) was constructed covering the Darling River, its tributaries and a river buffer area that extends 100m either side of each river bank, ensuring sufficient detail of the important features of the river bank were captured. The TIN has coverage of approximately 120 km2 and includes break lines that define the edges of features of importance for flood modelling, such as the top of levees or other channel works on the flood plain.

At the same time, spatially geo-referenced aerial photographs (ortho-imagery) of the area were captured.

A Digital Elevation Model (DEM) was created from the TIN as a means of verifying the coherence of the data in the TIN. Figure 4-1 presents the DEM of the study area.

4.1.2 Ground Survey

Ground surveys were undertaken at the following four drainage structures:

Old Bourke Bridge at North Bourke;

Bourke Weir approximately 5km downstream of Bourke;

The main bridge at Louth; and

The Anabranch Bridge, which is northwest of the main bridge at Louth.

Details of the structures were surveyed, including bridge piers and railing, together with ground levels and crest levels. Photographs of each structure were taken to record important features. Details of the surveys can be found in the Compendium of Data.

Development of floodplains in the western rural areas of NSW is managed through Part 8 of the Water Act 1912. Part 8 was gazetted in 1984 and makes provisions concerning ‘flood control works’ that affect, or are likely to affect, flooding or floodplain functions. Part 8 was amended in 1999 to allow for more strategic control of flood control works through the preparation of rural Floodplain Management Plans (FMPs) and a more streamlined and resource efficient approval process. The amendments also required that rural FMPs be developed in accordance with the provisions and policies of the operative NSW Manual (currently, Floodplain Development Manual, NSW Government, 2005). The manual supports the primary objective of the New South Wales Government’s Flood Prone Land Policy to reduce the impact of flooding and flood liability on individual owners and occupiers of flood prone property, and to reduce private and public losses resulting from floods, utilising ecologically positive methods wherever possible. The formulation and implementation of FMPs is the cornerstone of the policy and the process for their preparation is described in the manual. Administrative orders published in April 2007, inter alia, divided the responsibilities under Part 8 of the Water Act 1912 between the Department of Environment and Climate Change (preparing rural FMPs) and the Department of Water and Energy (implementing the plans through licensing and compliance functions). On 1 July 2009, following a Reorganisation of Government Agencies, the licensing and compliance functions regarding Part 8 were transferred to the newly established Office of Water within the renamed Department of Environment, Climate Change and Water (DECCW). Please note that any reference in this document to the Department of Environment and Climate Change, and Department of Water and Energy, is to be construed as a reference to DECCW. The development of a rural FMP progresses through three (3) key stages:  Data Collection & Flood Study (FS) - in general, includes data collection for hydrology, hydraulic and environmental aspects; defines the nature and extent of flooding, including development of a computer-based hydraulic model of flood behaviour;  Floodplain Risk Management Study (FRMS)– in general evaluates management options based on hydraulic modelling, analysis of the floodplain environment and social and economic considerations, to address existing and future floodplain risk management issues; and  Floodplain Management Plan (FMP) – presents the recommended management measures for the floodplain, as determined in the Floodplain Risk Management Study, and includes an implementation program with priorities. This current project, overseen by the Darling River Floodplain (Bogan River confluence to Louth) Floodplain Management Committee (the Committee) and prepared for DECCW, includes only the first of the above three elements of the Floodplain Management Plan process, and is structured as follows: Stage 1: Data Compendium and Digital Terrain Model All information and knowledge of the flood regime in the study area are included in a Compendium of Data (COD), as a permanent record, and for later use in Stage 2 of the FS. A Digital Terrain Model was prepared for hydraulic modelling use in Stage 2 of the FS and the results presented on an appropriate Geographic Information System (GIS). Stage 2: Calibrated Hydraulic Model and Final Flood Study Report A calibrated computer based hydraulic model was developed to quantify the distribution of floodwaters during historical flood events and for later use in designing a floodway network (as part of the FRMS) in order to evaluate management options. The Compendium of Data was completed in July 2008. This Report covers Stage 2: Calibrated Hydraulic Model and Final Flood Study Report. The Commonwealth Government’s Natural Disaster Mitigation Program (NDMP) and the New South Wales State Government through the NSW State Emergency Management Committee (SEMC) have jointly provided funding for the project.