6.5.22 - Flooding Report 5 may

      4.3.2.7.

      4.3.2.7. Severe Storm Assessment
      A severe storm assessment was completed for the 1% (2100) AEP event assuming all of the
      stormwater culverts are 100% blocked to ensure the future dwellings within the development
      site do not become inundated and no new flow paths are formed that may impact
      adjoining properties.
      The assessment found that the building envelopes are not impacted by the 100% blocked
      scenario with a flood level increase of up to 0.08m noted, which is less than the minimum
      0.3m freeboard required. It is also noted that no new flow paths form as a result of the 100%
      blocked scenario and adjoining properties are not impacts.
      Whilst it is not a design requirement for the 100% blocked scenario to achieve QUDM design
      requirements, a comparison against the QUDM road drainage criteria has been made for
      some context. In the 100% blocked scenario, the depth of flow exceeds the 0.2m QUDM
      depth criteria in the upstream lane (with depths up to 0.25m noted), however, is less than
      0.2m on the downstream lane. Flow velocity is up to 1.75m/s. Approximately 80m of the
      road has a hazard greater than the 0.3m2/s QUDM hazard criteria (with hazard up to 0.5m2/s
      noted).
      A flood depth map for this scenario is provided in Appendix I.

      4.3.2.8. Flood Risk

      This development has been designed to provide flood free access to all lots via the new
      internal road in events up to the 10% AEP and will remain trafficable in events up to the 1%
      AEP event in accordance with the Queensland Urban Drainage Manual (QUDM).
      Further consideration of flood risk has been undertaken in events greater than the 1% AEP
      event, 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 in . 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. The Guideline developed by
      GCCC is presented in Appendix J, and in summary states “a shelter-in-place approach




21-0288-FS001-V5 | Flood Investigation | 60 Glenridge Drive, Cooroibah                                  | 16