Attachment 7 - Flood Impact Assessment - Covey · 132005.638.06

         Earthworks have been proposed to accommodate the development of the site and to facilitate efficient
         drainage. Appropriate drainage infrastructure has been proposed in the design, including overland flow
         swales to discharge local flows from the site and to convey runoff from external catchments around the
         site.

         3.0     FLOOD IMPACT ASSESSMENT

         3.1     Flood Modelling

         Flood modelling of the site was undertaken using TUFLOW, a 1D/2D hydraulic modelling software
         package, to ensure stormwater flows from upstream catchments external to the site can be conveyed
         through the site and to demonstrate that no unacceptable impacts will occur on surrounding properties
         as a result of the proposed development.

         Scenarios modelled as part of this investigation include:
            • Existing Case Scenario – modelling the undeveloped conditions.
            • Developed Case Scenario – modelling the site with an updated Digital Elevation Model (DEM) for
                 each design stage, upgraded hydraulic structure at the road crossing the large clean water drain
                 and updated Manning’s roughness coefficients and soil infiltration rates to represent the
                 proposed development layout.

         Modelling was undertaken for the 50%, 10%, and 1% and 1% climate change Annual Exceedance
         Probability (AEP) events for all existing and developed case scenarios. The current climate events rainfall
         depths were scaled up to consider 1.0 degree of warming up to the year 2025 (relative to 1961-1990
         rainfall depths). The climate change event rainfall depths were scaled up based on 3.6 degrees of
         warming, equivalent to SSP3-7.0 in the year 2100.

         Rare events such as 0.5%, 0.2% and 0.1% AEP storm events have also been modelled for the ultimate
         developed case scenario (Stage 6). The rare events were also scaled up to consider 1.0 degree of warming.

         3.2     Hydrology

         A “Rain on Grid” modelling approach was adopted for this overland flow study and as such, no traditional
         hydrological modelling was required. The total catchment area adopted for this assessment was
         approximately 287 ha. Refer to Figure 3-1 for the overall extent of the 2D flood model and the quadtree
         extent for the model. This is discussed further in Section 3.11.




Flood Impact Assessment                                                                                      Page 7 of 25
Project No 244035 | Ref No M25-0479Rpt – Issue A – January 2026