Attachment 7 - Flood Impact Assessment - Covey · 132005.638.06

         There is an existing valley in this area which conveys overland flow through this easement. However, as
         the increase in water surface level is contained within this valley and the valley conveys flow in the existing
         case it is not considered to be an actionable nuisance.

         Increases in peak water surface levels also occurred just outside the south-eastern boundary within
         Walter Hay Drive (Location 2), a Transport and Main Roads (TMR) administered road reserve. Increases
         in water surface levels were relatively minor at this location, not exceeding 18mm, up until Stage 5. The
         impact was greater for Stage 6, with increases in water surface level of up to 82mm. In Stages 1 and 2,
         increases in peak water surface level were slightly more prominent for the 50% AEP whilst in Stage 3-5
         the increase was notably largest for the 1% AEP CC event. No inundation of Walter Hay Drive roadway
         occurs under any scenario and the impacts are contained within the vegetation and existing valley. It is
         expected that under the developed case scenario, the existing valley will continue to convey flows safely
         to the north-west back through the subject site, and would not pose an actionable nuisance.

         Similarly, in the vicinity of the road crossing at Hollett Road (Location 3), increases in water surface levels
         were also relatively minor up until Stage 5, not exceeding 12mm, with larger increases occurring after
         Stage 6. Increases in peak water surface levels ranged between 19-35 mm for the 50%, 10% and 1% AEP
         events. Notably at this location, the maximum increase occurred in the 50% AEP for all design stages as
         opposed to the 1%AEPCC event. In the existing scenario the maximum hazard is 2.26 m2/s, which has a
         hazard classification of H4 and is considered to be unsafe for vehicles and people (AIDR, 2017). In the
         developed case the hazard increases slightly to 2.31 m2/s however it remains in the same hazard category.
         Therefore, the impact is not considered to be an actionable nuisance as the road crossing maintains the
         same hazard category and is not safe to use for the simulated design events.

         Excluding the impacts noted above, the flood mapping demonstrated that there were no other offsite
         increases in peak water surface level, depth, velocity or hazard as a result of the proposed development.
         Further, as the proposed additional impervious areas are much smaller than the overall catchment, the
         minor increase in the fraction impervious has a negligible effect on the overall catchment peak discharges,
         as demonstrated by the flood mapping. Further, as the majority of the impervious areas of the proposed
         development are located closer to the downstream boundary (outlet) the peak discharge will arrive at
         the outlet much sooner than the overall catchment. Therefore, no detention is required for the proposed
         development.

         Based on a review of the results shown in the flood maps in Appendix C, it can be concluded that the
         proposed development will not create a nuisance or interfere with the use or enjoyment of any land
         surrounding the proposed development site.




Flood Impact Assessment                                                                                        Page 18 of 25
Project No 244035 | Ref No M25-0479Rpt – Issue A – January 2026