North Mackay, West Mackay and Andergrove/Beaconsfield drainage studies

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Council has prepared drainage studies for the North Mackay, West Mackay, and Andergrove/Beaconsfield sub-catchments. The studies were prepared for Council by engineering consultants WRM and AECOM.

Prior to the development of the drainage studies, there was significant effort dedicated to the collection and review of data. This data included detail survey of the stormwater drainage pits and pipes, historic flood data and the review of complaints. This provided a fit for purpose base to establish the stormwater models which included use of the 2015 lidar (ground levels) and the 2017 terrestrial lidar (floor levels and building information). Each study involved developing a 2D hydraulic model to understand local flood behaviour and assess potential options to reduce existing and future flood risk for the community. The purpose of the study and mitigation options assessment is to inform Council’s flood hazard mapping, capital works planning, budgetary processes, residual risk and assist in emergency management.

The studies provide Council with the ability to:

  • Determine existing flood levels and hazards for the purpose of setting minimum habitable floor levels and overlays for incorporation into the Mackay Region Planning Scheme .
  • Review areas impacted by over-floor flooding and cost damage impacts for the range of flood events.
  • Establish structural and non-structural mitigation options within each sub-catchment which provide some benefit in reducing flood extents and depths and the relative benefit cost ratios (BCRs) for such options.
  • Investigate and determine appropriate trunk stormwater infrastructure required for the study area for the purpose of catering for the ultimate catchment development conditions in accordance with Planning Scheme.

North Mackay

The North Mackay Drainage Study (WRM 2019) was undertaken using the direct rainfall method which involves the application of rainfall directly to the two-dimensional model domain. The North Mackay model extends to the levees bounding the study area adjacent to Goosepond Creek to the west, Vines Creek to the north and east, and the Pioneer River and Barnes Creek to the south.

Key findings of the North Mackay drainage study are:

  • Minor flooding occurs along most of the roadways with ponding greater than 0.3 m occurring on:
    • the Howard and Parr street intersection.
    • Ungerer Street to the east of Grendon Street.
    • King Street.
    • Forgan Street between Hamilton and Strang Street.
    • Strang Street between Forgan and Grendon streets; and
    • Grendon Street to the north of Strang Street.
  • Extensive ponding occurs in residential allotments between Hamilton and Ernest streets to the west of Gaylard Street with peak flood depths up to 0.75 m.
  • Ponding also occurs in several other residential allotments, which appears to be due to water not being able to freely drain to the stormwater drainage system.
  • For the larger events, the flood extents and depths increase across the study area.
  • New modelling of the Pioneer River identifies that the Pioneer River overtops the levees for events that exceed the 0.5% (1 in 200) AEP event and these flood levels dominate peak flood levels for these events.
  • The majority of the properties flooded above floor level are located in the topographical depression between Hamilton, Gaylard and Ernest streets. Four options were investigated to mitigate the above floor flooding within the depression.

Figure 1 – North Mackay 1% AEP flood event plus climate change factor

West Mackay

The West Mackay Drainage Study (AECOM 2019) was undertaken using the direct rainfall method which involves the application of rainfall directly to the two-dimensional model domain. The catchment was divided into two parts: Study Area 1 and Study Area 2. Proposed mitigation and flood damages assessment were only undertaken in Study Area 1 (refer Figure2).

Figure 2 – West Mackay Study Area 1 (red) and Study Area 2 (green)

Key findings of the study are:

  • TUFLOW hydraulic modelling undertaken shown that the critical duration for the most constraining area of the West Mackay catchment is the 120 minute (2 hour) storm.
  • No calibration of the model was undertaken as the model was updated based upon an already calibrated model.
  • Structural mitigations options were focused on the installation of a stormwater network on parts of Douglas, Holland, Mackenzie, Chataway, Donaldson, Barton and Mogford streets which connects to the existing network in Lagoon St and discharging to the Lagoons area.
  • The estimated cost for building the new stormwater network was indicatively$6,500,000.
  • Detail design plans have been generated for the proposed mitigation projects.
  • A flood damages assessment was undertaken for the 1EY to PMF storms to assess the proposed structural mitigation options to residential and commercial buildings. Average annual damages were reduced by 80%, with over 60% of the reduction occurring for low order events (≥5% AEP).

The 1% Annual Exceedance probability (AEP) event for the catchment is shown in Figure 3.

Figure 3 – West Mackay 1% AEP flood event plus climate change factor

Andergrove Beaconsfield

The Andergrove Beaconsfield Drainage Study (AECOM 2020) was undertaken using the direct rainfall method which involves the application of rainfall directly to the two-dimensional model domain. Key findings of the study are:

  • Tidal impacts are minimal up to the 1% AEP design storm.
  • The critical storm duration ranges from 30 mins to 6 hrs across the study area and range of events.
  • The majority of over floor flooding is around Farquhar Street and structural mitigation options provide some benefit in reducing flood extents and depths in Farquhar Street.
  • It is recommended that the study be used as a basis for further investigations, setting development levels and assisting in emergency management and planning.
  • The impacts of climate change are likely to increase the flooding experienced around the Apsley Creek area.

The 1% Annual Exceedance probability (AEP) event plus climate change factor for the catchment is shown in Figure 4.

Figure 4 - Andergrove Beaconsfield 1% AEP flood event plus climate change factor

Conclusion

Council is seeking submissions and comments from the public by November 23, 2020 at 11.59pm. Following consultation, Council will respond to submissions outlining how the submitters’ comments have been addressed. The studies will then be taken to Council for consideration. Following adoption, the studies will be used in Council’s development planning, flood hazard and emergency management systems and be provided to the Insurance Council of Australia.

Council has prepared drainage studies for the North Mackay, West Mackay, and Andergrove/Beaconsfield sub-catchments. The studies were prepared for Council by engineering consultants WRM and AECOM.

Prior to the development of the drainage studies, there was significant effort dedicated to the collection and review of data. This data included detail survey of the stormwater drainage pits and pipes, historic flood data and the review of complaints. This provided a fit for purpose base to establish the stormwater models which included use of the 2015 lidar (ground levels) and the 2017 terrestrial lidar (floor levels and building information). Each study involved developing a 2D hydraulic model to understand local flood behaviour and assess potential options to reduce existing and future flood risk for the community. The purpose of the study and mitigation options assessment is to inform Council’s flood hazard mapping, capital works planning, budgetary processes, residual risk and assist in emergency management.

The studies provide Council with the ability to:

  • Determine existing flood levels and hazards for the purpose of setting minimum habitable floor levels and overlays for incorporation into the Mackay Region Planning Scheme .
  • Review areas impacted by over-floor flooding and cost damage impacts for the range of flood events.
  • Establish structural and non-structural mitigation options within each sub-catchment which provide some benefit in reducing flood extents and depths and the relative benefit cost ratios (BCRs) for such options.
  • Investigate and determine appropriate trunk stormwater infrastructure required for the study area for the purpose of catering for the ultimate catchment development conditions in accordance with Planning Scheme.

North Mackay

The North Mackay Drainage Study (WRM 2019) was undertaken using the direct rainfall method which involves the application of rainfall directly to the two-dimensional model domain. The North Mackay model extends to the levees bounding the study area adjacent to Goosepond Creek to the west, Vines Creek to the north and east, and the Pioneer River and Barnes Creek to the south.

Key findings of the North Mackay drainage study are:

  • Minor flooding occurs along most of the roadways with ponding greater than 0.3 m occurring on:
    • the Howard and Parr street intersection.
    • Ungerer Street to the east of Grendon Street.
    • King Street.
    • Forgan Street between Hamilton and Strang Street.
    • Strang Street between Forgan and Grendon streets; and
    • Grendon Street to the north of Strang Street.
  • Extensive ponding occurs in residential allotments between Hamilton and Ernest streets to the west of Gaylard Street with peak flood depths up to 0.75 m.
  • Ponding also occurs in several other residential allotments, which appears to be due to water not being able to freely drain to the stormwater drainage system.
  • For the larger events, the flood extents and depths increase across the study area.
  • New modelling of the Pioneer River identifies that the Pioneer River overtops the levees for events that exceed the 0.5% (1 in 200) AEP event and these flood levels dominate peak flood levels for these events.
  • The majority of the properties flooded above floor level are located in the topographical depression between Hamilton, Gaylard and Ernest streets. Four options were investigated to mitigate the above floor flooding within the depression.

Figure 1 – North Mackay 1% AEP flood event plus climate change factor

West Mackay

The West Mackay Drainage Study (AECOM 2019) was undertaken using the direct rainfall method which involves the application of rainfall directly to the two-dimensional model domain. The catchment was divided into two parts: Study Area 1 and Study Area 2. Proposed mitigation and flood damages assessment were only undertaken in Study Area 1 (refer Figure2).

Figure 2 – West Mackay Study Area 1 (red) and Study Area 2 (green)

Key findings of the study are:

  • TUFLOW hydraulic modelling undertaken shown that the critical duration for the most constraining area of the West Mackay catchment is the 120 minute (2 hour) storm.
  • No calibration of the model was undertaken as the model was updated based upon an already calibrated model.
  • Structural mitigations options were focused on the installation of a stormwater network on parts of Douglas, Holland, Mackenzie, Chataway, Donaldson, Barton and Mogford streets which connects to the existing network in Lagoon St and discharging to the Lagoons area.
  • The estimated cost for building the new stormwater network was indicatively$6,500,000.
  • Detail design plans have been generated for the proposed mitigation projects.
  • A flood damages assessment was undertaken for the 1EY to PMF storms to assess the proposed structural mitigation options to residential and commercial buildings. Average annual damages were reduced by 80%, with over 60% of the reduction occurring for low order events (≥5% AEP).

The 1% Annual Exceedance probability (AEP) event for the catchment is shown in Figure 3.

Figure 3 – West Mackay 1% AEP flood event plus climate change factor

Andergrove Beaconsfield

The Andergrove Beaconsfield Drainage Study (AECOM 2020) was undertaken using the direct rainfall method which involves the application of rainfall directly to the two-dimensional model domain. Key findings of the study are:

  • Tidal impacts are minimal up to the 1% AEP design storm.
  • The critical storm duration ranges from 30 mins to 6 hrs across the study area and range of events.
  • The majority of over floor flooding is around Farquhar Street and structural mitigation options provide some benefit in reducing flood extents and depths in Farquhar Street.
  • It is recommended that the study be used as a basis for further investigations, setting development levels and assisting in emergency management and planning.
  • The impacts of climate change are likely to increase the flooding experienced around the Apsley Creek area.

The 1% Annual Exceedance probability (AEP) event plus climate change factor for the catchment is shown in Figure 4.

Figure 4 - Andergrove Beaconsfield 1% AEP flood event plus climate change factor

Conclusion

Council is seeking submissions and comments from the public by November 23, 2020 at 11.59pm. Following consultation, Council will respond to submissions outlining how the submitters’ comments have been addressed. The studies will then be taken to Council for consideration. Following adoption, the studies will be used in Council’s development planning, flood hazard and emergency management systems and be provided to the Insurance Council of Australia.

  • CLOSED: This survey has concluded.

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