2019-02-12 Planning & Environment Committee Agenda

PLANNING & ENVIRONMENT COMMITTEE MEETING AGENDA                                  12 FEBRUARY 2019


    The critical time for concentration during 1% flood event is 33 minutes. This means it takes
      33 minutes to reach peak depth and velocity and then gradually subsides within
      approximately 60-65 minutes back to ‘dry’. Therefore, the peak (worst case) flows across the
      driveway and the site are only happening for 10-15 minutes as per typical flood hydrograph
      below shown as Figure 4.




Figure 4 - Hydrograph

Council’s hydrology consultant has reviewed the applicant’s Flood Hazard Assessment Report and
provides the following advice:
    The flood report suggests that only 30% of the total catchment flow will drain through the site
      but there is no detail of how this has been determined. A review of the LiDAR ground level
      DEM of the area suggests most, if not all, of the total catchment runoff is likely to drain
      through the site.
    The modelling makes no allowance for fence blockages and appears to be too localised to
      provide meaningful results. While I have not inspected the site, aerial imagery shows that a
      tennis court is situated immediately downstream. Given the bushland catchment, there is a
      high potential for a chain wire fence to become highly blocked with debris and this will
      increase flood levels through the site.
    No comments have been provided in regards to the flood resilience of the dwelling. It is
      situated within a natural bushland flow path and it will be subject to flooding from time to time
      (certainly below floor level). It would need to be demonstrated that the house is resilient to
      flooding and give consideration to the high levels of natural debris that is likely to be mobilised
      during a large flood.
    Flood hazard risk associated with the proposed access should be assessed in more detail
      and account for full catchment flows and debris.
    Overall I would not support this proposal. Ultimately it will place more people within an area
      which has a clear risk of flooding.
    There is a discrepancy between the house extension architectural drawings by Void Building
      Design and the Building Pad and Site Works Plan (ST03-1) (2) by Barlow Shelley on the
      exact location of the driveway and associated culvert crossing
    Flood level and velocity impact plots are required to demonstrate no significant adverse
      flooding impacts will result on adjacent and downstream properties, and to demonstrate that
      a lawful point of discharge will be achieved
    A number of deficiencies in the information supplied to date, as well as discrepancies in this
      data, have been identified.
The proposal is not consistent with the Reconfiguring a Lot Code requirements for 100% of the
allotment to be above the defined flood level of 1% AEP water surface levels, with lot 1 located
almost fully below the 1% AEP and lot 2 in part. Lot 1 clearly does not contain a suitable house
site area being subject to flooding from an overland drainage flow path.


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