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.

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.