Appendix J - Civil Engineering Report and SBSMP

                                                                                    Noosa Residential Development
                                                                             Ref: 24553 C R002 REV03 10.08.21.docx
                                                                                                      August 2021
               ‘B1,B2&b3_Unmitigated’; and

             Three (3) Post-Development Mitigated Catchments B1, B2 & B3; ‘B1’, ‘B2’ and ‘B3’.

            A modelled representation of the bioretention basins were provided to capture storage that occurs
            over the bioretention extended detention zone;

            The bioretention basins were provided with separate outlets simulated using multi-links;

            Each node included runoff sub-catchments simulating the contributing areas;
            The sub-catchment areas were split into 0% impervious & 100% impervious areas;
            The models were run through all storms from 10min to 24-hour durations for all AR&R 2016
            generated storms to determine the critical storm events for each duration and AEP; and
            The model was run assuming both a free-outlet and a tailwater level at the outlet node to match
            the top of the downstream open-channel within the drainage covenant.

6.3.1    Pre-Development Case Model
         The pre-development scenario model consisted of nodes representing the pre-developed
         contributing catchments as described above. A link with negligible energy loss was added to the
         downstream end of the runoff node and connected to an outfall node as to provide a single point of
         comparison between the pre- and post-development cases.

6.3.2    Post-Development Case Model
         The Hydraulic mode of XP-Storm is used to assess the stormwater runoff calculated in the Runoff
         mode by introducing factors such as storage and outlet configurations. The developed model
         includes the nodes for the post-developed catchments shown in Table 2.

         Piped flows from catchments B1 and B2 are to be directed to flow diversion chambers, where low-
         flow outlet pipes will limit flows to the bioretention to an approximation of the 4EY event (Q3month
         event) to limit scouring of the bioretention. The balance of flows are to bypass the bioretention basins
         and flow directly to the LPD.

         A general link with negligible energy loss was added downstream of the post-developed model and
         connected to an outfall node to provide a single point of comparison between the pre and post-
         development cases.

         XP-STORM modelling indicated that 14.0m3 within the OSD tank is adequate to mitigate the
         discharged flows for peak events up to and including the 1% AEP event and ensure there is no
         increase in discharge for any and all storms for events up to and including the 24-hour, 1% AEP
         event to pre-development rates. This was tested assuming a limited flows in the open-channel at the
         LPD (free outlet) and assuming that the open-channel is running full (Fixed Backwater to 4.25m
         AHD).

         The proposed OSD outlet configurations are presented in Table 3.

                                        Table 3 – Detention Outlet Configuration

           Catchment / OSD            Low flow orifice                               Weir
                                                                  10m long weir 0.60m above OSD invert
                                                                   100mm above bioretention surface to
                   B1                         Nil
                                                                simulate combined length of top of 900x900
                                                                      pit in bio and overflow to swale
                                                                 3.6m long weir 0.25m above Bio retained
                                  100x500 low flow orifice
                                                                   water level to simulate length of top of
                   B2            at Bio retained water level
                                                                              900x900 pit in bio



BRISBANE / SYDNEY / MELBOURNE / GOLD COAST / SUNSHINE COAST / DARWIN / PERTH                              Page 13