MACQUARIE RIVER FLOOD STUDY

The information below is a summary of the New/Updated Macquarie River Flood Study at Dubbo. The full report can be downloaded below:

2019 -  (2019 Report)

2019 - (2019 Report - Figures 1-13)

2019 (2019 Report - Figure 14)

2012 (2012 Report)

Click here to view interactive maps https://maps.dubbo.nsw.gov.au/portal/apps/MapSeries/index.html?appid=433026f4a5e24913a047bd4de9e5004c

Click here  to view Frequently asked Questions 

Background

Dubbo Regional Council has a history of undertaking investigations and publishing mainstream flood data. These prior studies were undertaken in accordance with the then current best practice. 

This Flood Study, undertaken by Cardno, supersedes a prior study undertaken in 2000 by PPK Consultants. Since that study there has been significant progress in aspects including digital terrain modelling, the availability of aerial photography, the development of Two-dimensional (2D) hydraulic modelling, vastly increased computing power and packages for the display of information. These have led in part to some differences in flood estimates over prior studies as will future studies as technology advances and more data becomes available.

In 2012, Cardno completed an updated Macquarie River Flood Study for Council and adjacent areas. The Flood Study was undertaken to define the nature and extent of flooding and flood hazard for a range of design rainfall events. This Flood Study did not include calibration against the 2010 flood.

From 2014 to 2018, additional modelling including calibration against the 2010 flood, and changes to the model grid was undertaken to improve the accuracy of the results. A key benefit of this was a reduction in the number of new flood affected properties from 287 to 165.

Flood Planning Process

The Flood Plain Risk Management Process as prescribed by the NSW Government is shown below:

The Flood Study

The undertaking of the Flood Study has been a technologically advanced process compared to prior studies. Council is cognisant of the fact that previous studies require integration, and there is a need to extend the area for which detailed flood information needs to be available. The Study itself is made complex by the junction of the Macquarie River and the Talbragar River, immediately downstream of Dubbo and the upstream presence of Burrendong Dam. The combination of natural events in two major catchments and management intervention in dam releases provides a near limitless number of available flooding scenarios.

In this Study, thirteen flood events have been modelled from significant minor events to extreme events covering both the Macquarie and Talbragar rivers. A feature of the dynamic modelling is the generation of a flood surface contour/level at every point on the floodplain for each event in lieu of an assumed water level surface as in past studies. Predicted flood levels for the full range of floods were provided at 659 floodplain locations.

A practical approach has been adopted by assuming that during a large flood, a lesser flood will occur on the other catchment. For example, a 1% AEP flood on the Macquarie River catchment has been combined with a 5% AEP flood on the Talbragar River catchment, and vice-versa. This assumption is reasonably consistent with the available data.

For smaller floods (10% AEP and below), it is considered more likely that the flood-producing rains may cover both catchments. It is also more likely that these magnitude floods will be stored at Burrendong Dam, so there would be no flood outflow.

Two scenarios were modelled for the 5%, 2%, 1%, and 0.5% Annual Exceedance Probability (AEP) floods, with a single scenario was modelled for the 10% AEP flood as shown below:

Based on available metrological data, it appears unlikely that a 1% AEP Talbragar River flood would coincide with a 1% Macquarie River flood. The two main catchments are distinct and geographically separate, and the construction of Burrendong Dam may have altered the probability of this occurring. Should such an event occur, it is more likely that the AEP would be in the range of 0.01% to 1%.

Therefore, it has been assumed that during a large storm in one catchment, a lesser storm will occur on the other catchment. For example, a 1% AEP storm on the Macquarie River catchment has been combined with a 5% AEP storm on the Talbragar River catchment, which is reasonably consistent with the available data.

Key Results

Peak Design Inflows (m3/s) for Macquarie River and Talbragar Rivers

Estimated gauge heights at Station 421001

Three extreme events were also modelled for the purposes of Emergency Management, and these are detailed in the table below:

Flood Level Determination

The approach used in setting design flood levels (for planning purposes) was to use an ‘envelope’ approach. This involved taking the higher of the two levels at each location for the modelled scenario.

For example:

The Flood Planning Level is usually adopted as 500mm above the 1% AEP flood level.

Flood Planning Level Changes

It is essential that Council provides the most up-to-date information available to the community and relies upon it in the assessment of development applications.

The 2012 modelling identified 287 former ‘flood free’ properties being added to the Flood Planning Area . In comparison, the 2019 Study has determined that this number is 165.

There was a reduction of 162 flood affected/flood free allotments in urban North Dubbo, but 40 allotments in areas of north-western Dubbo (i.e. Brocklehurst, Burraway Road area) were added to the Flood Planning Area. All other areas within the Study area remained consistent. The changes were due to improvements in flood modelling technology and additional calibration to the 2010 flood.

In general, results from the new study are higher flood levels than those currently adopted by Council, with a selection shown below:

Caution needs to be exercised in the literal interpretation of flood inundation mapping. At a broad scale, the inundation boundary shown is considered satisfactory however, at high levels of resolution there may be inconsistencies with available contour data.

Both contours and inundation boundaries have been sourced from aerial laser scanned (ALS) data. Inundation boundaries have been defined according to exposure above the flood surface and mapped with an algorithm including drawing element and smoothing constraint, which it seems primarily accounts for divergences.

Council has sought to define inundation in terms of natural surface rather than structural element such as a bridge deck or building roof.

In critical areas, a site survey should be undertaken to establish ground levels, which will enable comparison with flood surface model levels for a more definitive inundation extent.

Because of these minor amendments, there are slight inconsistencies in the mapping of some attributes. They may appear discontinuous or may be limited in their ability to be represented in other formats. These limitations do not detract from the value and content of the mapping for the discerning enquirer.

Submission – MACQUARIE RIVER FLOOD STUDY

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Last Edited: 11 Oct 2019

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