Attachment 7 - Flood Impact Assessment - Covey · 132005.638.06

         3.11    Grid Cell Size

         A 16-metre hydraulic grid cell size was adopted for the model generally. A one metre 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 and Figure 3-4. 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. Refer to Table 3-2 for
         further details of the existing hydraulic structures. Refer to Table 3-3 for further details on the proposed
         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.




Flood Impact Assessment                                                                                      Page 11 of 25
Project No 244035 | Ref No M25-0479Rpt – Issue A – January 2026