STP22-0347 DA Engineering Report RFI Response Rev C

                                                                                                       STP CONSULTANTS



3.      Stormwater Management Plan
3.1      FLOOD STUDIES
        According to the Noosa Shire Council Flood Study, the site is not subject to flood inundation at the AEP 1% level.

3.2      STORMWATER DETENTION
        Comparison of the impervious areas between the existing layout and the ultimate layout following completion of
        Stage 3B, shows an overall increase from 12,370m2 (IF = 0.4952) to 12,374m2 (IF = 0.4954) or 0.03%. This change
        is considered to be negligible and will not result in a measurable increase in site runoff. As this is the case, there
        will be no requirement for on-site detention for the proposed development.

3.3      STORMWATER QUALITY
        In accordance with the Assessment Benchmarks outlined in State Planning Policy, a Stormwater Quality
        Management Plan will be required in order to reduce the impact of the proposed development on the quality of
        stormwater discharged from the site. The site area is greater than 2,500m2 and the proposed development will
        result in the net developed area being greater than 25%. A stand-alone Stormwater Quality Management Plan,
        analysing the effect of the newly developed areas only, is included in Appendix C to this report.

3.4      EXISTING STORMWATER INFRASTRUCTURE
        The site currently drains to the wetlands to the north-east, which is the Lawful Point of Discharge as illustrated in
        the site survey in Figures 3.1 and 3.2 below.

3.5      SITE STORMWATER DRAINAGE
        A preliminary stormwater drainage layout is illustrated in Figures 3.3 and 3.4. For the main development area, the
        roofwater will discharge either directly to ground via splash pads or to roofwater inlet pits. The roofwater outlets
        and the landscaped areas are to discharge directly to the open swales which will then outlet to the eastern reserve
        area via the two proposed causeway structures.

        Rock causeways are proposed where the stormwater drainage outlets from the site at the two locations indicated
        in Figure 3.3. Detailed design drawings are included in Appendix B to this report. Catchment areas and runoff
        calculations for the two major catchments are included in Figure 3.5 and Table 3.1 below.

        The northern maintenance area (Figure 3.4) is a partial demolition and extension of the existing paved area, with a
        consequent increase in impervious fraction for the area. Of the existing 249m2 impervious area, 73m2 is to be
        demolished and be revegetated, with 176m2 to be retained. The area is to be extended with an additional 182m2 of
        impervious pavement. The net impervious pavement area increases from 236m2 to 329m2. It is proposed to
        discharge runoff from this area to a gravel soakage pit on the eastern boundary.

3.6      SITE EARTHWORKS
        Existing site levels and grades are generally to be maintained. A minimal volume of fill may be required for building
        pad works within strip footings. Minor excavation will be necessary for the creation of drainage swales and the
        external fire track. A preliminary earthworks layout is shown in Figure 3.6.




Civil Engineering Report & Site Based Stormwater Management Plan | STP22-0347                                            5