2022-06-07 Planning & Environment Meeting Agenda

PLANNING & ENVIRONMENT COMMITTEE MEETING AGENDA                                         7 JUNE 2022


with assessment of the PMF event taking place. The assessment found that the lots within the
development would become isolated in extreme events such as the PMF, with the internal access
road having flow conditions that would not be trafficable. An assessment was completed as to the
length of time the internal access would be considered not trafficable (flow depths > 0.2m) along
the warning time between the start of the rain event to when the internal crossing becomes not
trafficable. The assessment was completed for the PMF event for a number of durations and is
highlighted below (table 4.6 (sic)). From this, it is considered that a site flood warning system would
not be appropriate in this situation due to the short response times for flood waters to rise. It would
be safer outcome for residents to seek refuge within their houses and communication about a
extreme weather system come from Council or State Disaster Management Services.




This assessment highlights that there would be a limited amount of time to warn / evacuate
residents due to the relatively short amount of time it takes for the internal road to become
inundated in an extreme (i.e PMF) event. This would be typical of all catchments where the
upstream catchment is relatively small. From this, the time of submergence is also relatively small
with times ranging from 2.0 to 8.0 hours depending on the event duration.
 From this, it is considered that the provision of suitable flood refuge or evacuation to higher ground
(i.e. to Glenridge Drive) would be the preferred risk management measure as opposed to
evacuating the area to a formal evacuation centre. Each lot is provided with an 1,100m2 building
envelope above the PMF level that would be suitable to act as a flood refuge until flood waters
recede.
The application of a flood refuge is recommended in similar circumstances as means to address
flood risk and potential burden by Gold Coast City Council (GCCC), who have large amount of
flood plain on which developments are managed.
…Further to this, that subsequent that Council Disaster Management Plan includes
communications to these future residents that their building envelopes serve as a flood refuge
should an event greater than the 1% AEP event occur. This information could also be conveyed
via property note.
The revised report has been reviewed by Switchback Consulting who supports the ‘shelter -in-
place’ strategy due to the short flood warning times.
As part of the preliminary approval recommendation, Councilors also requested that the access
road be raised to be above 1%AEP. The applicants engineering consulting has reviewed this
requirement and declined to increase the flood immunity of the internal road as the road design
meets Council and QUDM requirements as detailed below:
The Internal Road extension has been designed achieve flood immunity in the 10% AEP event,
whilst allowing some overtopping in the 1% AEP (major event). The high point of the road (which
acts as a weir) is at a minimum 12.81m AHD. This results in a flow depth of less 0.2m occurring
over the road pavement in the 1% AEP, with flow velocities less than 1.6 m/s and flow hazard less
than 0.3m2/s. These parameters are below the trafficability requirements of QUDM, which require
a maximum depth of 0.2m and hazard less than 0.3m2/s in the 1% AEP event.

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