Appendix J - Civil Engineering Report and SBSMP

                                                                                  Noosa Residential Development
                                                                            Ref: 24553 C R002 REV03 10.08.21.docx
                                                                                                     August 2021


6        STORMWATER QUANTITY ASSESSMENT
         The aim of the stormwater quantity assessment is to ensure that the development shall impose no
         adverse effects on downstream properties or receiving water bodies and that the conveyance of
         flows will be in a safe manner with minimal risk of human endangerment as well as the following
         objectives:

            Address the need for stormwater quantity control measures;
            Ensure there is no actional impacts from the subject site for events up to and including the 1%
            AEP event; and
            Ensure proposed quantity control measures detain and convey flows in accordance with QUDM
            (2017) minimum freeboard recommendations.

6.1      Proposed Development and Associated Issues
         As noted in Section 2.1 the existing site has no impervious areas and as noted in Table 2 the
         proposed development is 56% impervious. One of the major implications of an increase in impervious
         area is that the total volume and flow rate of stormwater run-off from the catchment will generally
         increase.
         It is essential that any increases are mitigated such that post-developed peak flows and volumes do
         not exceed those for the pre-developed case.

6.2      Stormwater Quantity Modelling – XP-STORM
         An XP-STORM Hydraulic and Runoff model was created to analyse the pre- and post-developed
         scenarios. The model includes a typical node-link connectivity identifying the catchments and
         hydraulic parameters. The proposed model was designed to reduce the increase of developed runoff
         to match pre-developed levels.
         The ARR Storm Generator was used based on the site coordinates to run all design storm events
         for events from the 10-minute storm to the 24-hour storm events.

6.3      XP- Modelling Assumptions and Methodology
         The following modelling assumptions were used to create the XP-STORM Models:

            Laurenson’s method was used for catchment routing with an ‘n’ value of -0.285 and ‘B’ value
            calculated by XP-STORM.

            Infiltration was modelled as ‘uniform loss’ with the following parameters:

             Impervious Areas
               ○ Initial loss of 1.0mm;
               ○ Continuing loss of 0.1mm/hr; and
             Pervious Areas
               ○ Initial loss of 27.0mm;
               ○ Continuing loss of 2.6mm/hr; and
             Note: initial and continuing losses were determined based on the AR&R 2016 storm generator
              values, which were calculated based on the site coordinates. Appropriate Manning’s ‘n’ values
              were used to suit the respective surfaces.
            Five (5) separate catchments were generated for the site, which were:

             One (1) Pre-Development Catchment; ‘A1’;
             One (1) Combined Post-Development Unmitigated Catchments B1, B2 and B3;


BRISBANE / SYDNEY / MELBOURNE / GOLD COAST / SUNSHINE COAST / DARWIN / PERTH                             Page 12