UPDATED DA - Attachment 7 - Flood Impact Assessment

     3.11    Grid Cell Size

     A 16-metre hydraulic grid cell size was adopted for the model generally. A one metres hydraulic grid cell size was adopted
     for the area of work within the site and a four to eight-metre grid cell size for the surrounding areas using the TUFLOW
     Quadtree add on which precisely simulates hydraulics of stormwater runoff within the area of interest. Refer to Figure 3-1
     for the Quadtree extent.

     The adopted hydraulic grid cell sizes have been deemed sufficient to accurately represent flood flows and levels throughout
     the hydraulic model.

     3.12    Tailwater Conditions

     A tailwater condition with an average downstream slope of 0.5% has been adopted at the outlet of the hydraulic
     model. The downstream boundary of the model is approximately 500 m downstream from the outlet of the site. This
     ensures negligible influence from the downstream tailwater condition.

     3.13    Hydraulic Structures

     Hydraulic structures adopted for this assessment have been modelled as 1D elements in the hydraulic model. All 1D element
     locations are shown in Figure 3-3, Figure 3-4 and Figure 3-5.

     The existing culvert information (e.g., size, length, material and invert levels) was based on available survey information.
     However, the survey information for the culverts on Eumundi-Noosa Rd and Emu Mountain Rd only included downstream
     invert levels. Therefore, the upstream invert levels were estimated by assuming a culvert grade of approximately 0.5%. If
     there are any major discrepancies it is recommended that the culverts be surveyed and the information provided to Covey
     Associates for determination.

     The proposed culvert information was based on the engineer plans as shown in Appendix A. Refer to Table 3-2 for further
     details of the existing hydraulic structures. Refer to Table 3-3 for further details on the developed hydraulic structures.

     The 1D elements included a 30% blockage factor in accordance with QUDM (2017) and a Manning’s n of 0.013 was applied.
     The hydraulic model did not include on-site detention tanks.




                                          Figure 3-3 Hydraulic Structure Locations – North

Noosa Shire Council                                                                                        6
M25-0045Rpt – 244035– February 2025