2014-10-14 Planning & Organisation Committee Agenda - Pages 1 - 50

PLANNING & ORGANISATION COMMITTEE MEETING AGENDA                                14 OCTOBER 2014


In our opinion, the applicant has considered the potential risks to existing properties and potential
adverse impacts associated with providing additional flood immunity to Garnet Street. The
solution provided represents a reasonable compromise between trafficability and flood impacts
on existing properties.
The flooding characteristics of Garnet Street do not satisfy Condition 65.1.2. Nor do they satisfy
the requirements of QUDM for the minor storm event (i.e. all flow be passed under the roadway
by the culverts) or major storm event.
    However, the proposed upgrade does provide a significant improvement to the flooding
      characteristics of Garnet Street, and greatly reduces the time of inundation at the creek
      crossing.
    It is agreed with the applicants that to provide the flood immunity requested by the Council
      would adversely impact the upstream properties due to an increase in peak water level.
In summary, the proposed Garnet Street culvert upgrade is considered to represent a reasonable
compromise between flood immunity for the roadway and damages associated with flooding for
the existing properties. While the upgrade does not satisfy Condition 65.1.2 of the Development
Approval or the requirements of QUDM for transverse flow over a roadway, the upgrade
represents a significant improvement in the existing flooding regime and greatly reduces the
period of time that the road is non-trafficable.
Council officers support the request to amend the condition, noting the following key findings:
    The revised design and modelling is based on Council’s recently completed Cooroy Creek
      Flood Model, which uses more contemporary landscape data and shows peak levels
      approximately 400mm higher than the previous model used for the previous culvert design.
      As such, the proposed immunity and trafficability levels are based on more onerous criteria
      and should provide an improved level of accuracy.
    The remodelling and redesign process has indicated that the original road upgrade
      condition cannot be achieved because raising the road to the suggested levels would
      significantly worsen upstream flooding.
    The modelling shows that the road and culvert formation provides an important local
      detention function restricting flood flows and velocities. Provision of a bridge, rather than
      culverts, would increase flows and worsen downstream flooding. Furthermore, a bridge
      has other prohibitive factors and would not be a reasonable imposition on the development.
    The applicant’s engineers also indicate that works in other locations, specifically upgrades
      to Mary River Road, would not provide a simple alternative to those proposed here as that
      road is subject to inundation over approximately 620m of its length.
    The amended road upgrade design proposed represents the best compromise in terms of
      improving the flood immunity and trafficability of the road, while avoiding non-worsening of
      upstream and downstream flooding.
    The proposed road design will significantly reduce inundation times and achieve:
      o     2 year ARI flood immunity
      o     10 year ARI flood event trafficability for passenger vehicles
      o     100 year ARI flood event trafficability for emergency vehicles




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