Reference Document - Flood & Stormwater Management Plan 28_8_2025

                          In addition, the magnitude of the increases (change) in non-critical durations (pre-post) still remain
                          orders of magnitude below the 1% AEP peak flow for the regional event. For example, the 1% AEP
                          peak flow in the regional event is in the order of 300 m3/s and the greatest change across
                          equivalent duration within an AEP event is 0.22 m3/s, (approximately 0.07% of the regional 1% AEP
                          flow). Based on guidance provided in the Flooding and Stormwater Management
                          Guidelines (Sunshine Coast Council, 2020) policy this is argued to be an acceptable difference.
                          Therefore, the change in non-critical durations is considered not to be of any significant
                          consequence within the context of the overland flow path and will not results in nuisance or
                          damage the downstream property.

                          Based on the above, and that the proposed solution provides an improved performance outcome
                          above the current approved basin, it is considered that the proposed detention basin meets the
                          performance objectives of the Noosa Planning Scheme, the intent of the QUDM guidelines and is
                          analysed using ARR2019 methodology. As such it is considered that the basin design is sufficient to
                          meet the approval criteria of the application.




                          Figure 1-4: Moderate Flood Extent (Source: Noosa Council Disaster Mapping)


                     d)   Across some storm durations there is a minor difference in flow rates above existing. These
                          differences are largely unobservable above 90 min, and resolved once the duration is greater than
                          270 min (4.5 hours):
                     e)   Differences between external catchments have been corrected in Table 2-1 to reflect the existing
                          scenario WBNM model. There is no difference in the external catchment size modelled for the
                          existing and developed scenarios.
                          Differences between SUB3 and SUB4 reported in SMP are correct. This is due to grading within the
                          site adjusting catchment delineation. Refer to Figure 2-1 and Figure 2-2 for details.
                          The difference in catchment delineation between existing and developed scenarios does not
                          materially change the results of the model. This is because SUB3 is routed to SUB4, which connects
                          to the model outlet “SINK” in the existing scenario, bypassing sub-catchments SUB1 and SUB2. In
                          the proposed scenario SUB3 is routed to SUB1, where the basin structure is located, in alignment
                          with the engineering drawings for the proposed development (Appendix A). Refer Table 2-1 and
                          Table 2-5 for model routing.




                 23-0167SMP01-V4 | Stormwater Management Plan | 33 Pavilion Street, Pomona                              Page 16

Document Set ID: 24092538
Version: 1, Version Date: 02/09/2025