UPDATED DA - Attachment 7 - Flood Impact Assessment

     3.14   Developed Case Scenario

     The changes to the developed case scenario included:
         •      A design TIN to represent finished elevation levels for the proposed development site. Refer to Figure 3-6
                for developed case topography adopted for the model;
         •      Added a set of culverts for the road crossing at the existing clean water drain as detailed in Section 3.13;
         •      Update the impervious area to represent the proposed development site; and
         •      Updated Manning’s ‘n’ roughness coefficients and soil infiltration rates based on values shown in
                developed model Manning’s maps in Appendix B.




                                  Figure 3-6 Developed Site Topography with 0.5m contours

     3.15   Results

     Results from all modelled scenarios were exported and peak water surface levels, depths, velocities and hazards have
     been mapped to demonstrate conveyance within the overland flow paths. A comparison between the existing and
     developed peak water surface levels, velocities and hazards was undertaken and all scenarios were mapped to ensure
     no unacceptable offsite impacts were experienced and ensure risk and flood hazard will be appropriately managed.
     Refer to the Flood Maps in Appendix C for further detail.

     Generally, it was demonstrated that the existing and developed overland flow paths have an adequate conveyance of
     overland flow from upstream external catchments for all simulated events. In the developed scenario, during the 1%
     AEP climate change event, the water surface level on the western internal roadways increased between 50 – 100 mm.
     However, this flow is safely conveyed through the site without inundating the proposed buildings and discharges to
     the channel to the east (clean water channel). In addition, the hazard (depth x velocity) on the internal roads is
     acceptable (< 0.6 m2/s).

     An increase of water level greater than 100 mm is noted immediately upstream of the proposed culverts (DEV01). This
     is expected as the proposed road crossing the channel will cause head loss at this location as the flow enters the
     culverts. Further upstream in the channel the water surface level decreased by up to 50 mm due to the inclusion of
     the culverts. The results also demonstrated that the road crossing has an immunity up to the 1% AEP climate change
     event.



Noosa Shire Council                                                                                    9
M25-0045Rpt – 244035– February 2025