Response to Stormwater Management Approval Conditions Letter

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    surface levels and contours of any potentially affected properties must be provided with any application for
    a Development Permit for Operational Works for earthworks and/or stormwater drainage works.

    This may require the design and construction of drainage pipes and/or an overland flow-path within the
    development site to cater for drainage from the adjoining sites for a 1% AEP design storm.


Response

    Section 3.2.6 of Report No. 16-001014-01A demonstrates that the proposed earthworks to be carried out on site will
    not adversely impact the pattern and severity of flooding downstream, as no filling is proposed within the regional
    flood extents (up to and including the regional 1% AEP climate change storm event).

Stormwater Drainage (Quantity & Quality)

61. A suitably qualified Registered Professional Engineer, specialising in hydraulics shall prepare engineering
    drawings and specifications for stormwater designs and shall be submitted in conjunction with an
    application for a Development Permit for Operational Works.

Response

    A suitably qualified Registered Professional Engineer, specialising in hydraulics, has prepared drawings and
    specifications for stormwater design and management. Refer to Operational Works Drawings for the proposed
    stormwater devices (Drawing Set No. 16-001014-400 Series) and Report No. 16-001014-01A.


62. Stormwater pipelines within the site shall cater for 10% AEP storm event and secondary flow paths shall be
    provided within the site to cater for 1% AEP storm event.

Response

    Stormwater pipelines have been designed to cater for the 10% AEP storm event and higher storm scenarios. Road
    network and secondary overland flowpaths have been designed to convey runoff from the 1% AEP storm event.
    Refer to Stage 2 Operational Works Drawing Set No. 16-001014-100 Series for details.


63. The development must not adversely impact on the existing drainage conditions on other properties. The
    applicant must demonstrate that all stormwater runoffs from the fully developed site must be maintained at
    predevelopment rates for all storm events up to and including the 1% AEP storm event.

Response

    Section 3.2.6 in Report No. 16-001014-01A demonstrates that peak flows from the site will be reduced back to or
    below existing levels for all recurrence interval storm events up to and including the 1% AEP storm. This
    demonstrates that the development will not adversely impact existing drainage conditions on other properties.

    While aerial imagery indicates that there appears to be no existing dams on downstream properties, water balance
    analysis demonstrates that the proposed Northern (Bioretention) and Central (Water Quality Control Pond) Basins
    will not deprive downstream dams of their natural stormwater runoff. Water balance analysis results indicate that the
    average annual runoff volume at each site discharge location has not been reduced. Refer to Section 3.2.6 in
    Report No. 16-001014-01A for details.

64. Stormwater runoff generated from the development must be treated to achieve the Water Quality Objectives
    as specified under Healthy Waterways Water Sensitive Urban Design – Technical Guidelines for South East
    Queensland (2006) as well as the State Planning Policy 2014 (SPP 2014) prior to release off site. For the
    purpose of this condition, treatment of waters from catchments B1 and C1 as shown on Drawing B14158.W-
    SK100 A by Calibre Consulting, dated 050/2/16 shall be considered as part of the development to determine
    mitigation requirements and load reduction targets.

    If detailed design demonstrates insufficient footprint is available for the water quality treatment devices to
    achieve the pollutant reduction targets of 80% reduction in Total Suspended Solids, 60% reduction in Total
    Phosphorous, and 45% reduction in Total Nitrogen compared to an unmitigated development, the applicant
    must increase available space to ensure the water quality objectives can be achieved.

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Response

   As indicated in Section 4.5 in SWMP Report No. 16-001014-01A, proposed stormwater management devices are
   adequate to treat stormwater runoff to meet Water Quality Objectives (WQOs) specified under the Healthy
   Waterways WSUD Technical Guidelines for SEQ (2006) and the SPP (2014) prior to release off site.

   Table 4-7 from SWMP Report No. 16-001014-01A has been provided below demonstrating that WQOs will be
   achieved.

   Table 4-7: MUSIC Modelling Results (Extract from SWMP Report No. 16-001014-01A)

                                                     TSS              TP             TN         Gross Pollutants
                  Description
                                                    (kg/yr)         (kg/yr)        (kg/yr)          (kg/yr)

    Developed Site Only (Without
                                                    29,700            65             425              4,270
    Mitigation)
    WQOs (Percentage Reduction)                      80%             60%            45%                90%

    Load Retention Target                           23,760            39             191              3,843

    Developed Site plus External
                                                    36,500            77             480              4,630
    Catchment (Without Mitigation)
    Developed Site plus External
                                                    12,700            33             289               488
    Catchment (With Mitigation)
    Load Retention Achieved                         23,800            44             191              4,142

    Load Retention Achieved as % of
                                                     80%             68%            45%                97%
    Developed Site Load

    Objective Achieved?                              YES             YES            YES                YES


   Two models have been provided to Council, the “Developed Site Only” model and the “Developed Site with External
   - Mitigated” model. The methodology for demonstrating that WQOs for the site have been met is summarised below.
       The Load Retention Target (in kg/yr) for the site was determined using the “Developed Site Only” model,
        estimating the pollutants generated from the developed site only.
       The Load Retention Achieved (in kg/yr) was determined using the “Developed Site with External - Mitigated”
        model, estimating the amount of pollutants removed from site and External Catchment B1 and C1 as a result of
        the stormwater quality improvement devices.
   A comparison between the targets and pollutant retention achieved demonstrate that WQO’s for the site have been
   met. This approach is consistent with past DA approvals (for the interim layout) and earlier discussions with
   Council.

   Catchment B1 and C1 as shown on Drawing No. B14158.W-SK102 are external and are not part of the site area.
   While External Catchment B1 and C1 have been included in the stormwater quality analysis (as runoff from these
   areas enter the site and into proposed stormwater quality treatment devices), these areas are not considered a “part
   of the development” or identified as ‘site area’. As such, determining the amount pollutant reduction required to meet
   objectives for the site (i.e. Load Retention Target) does include the pollutants generated from the external
   catchment areas. The Load Retention Achieved has been determined by the total pollutants removed by the
   proposed SQID’s prior to discharge from the site.

   The approach and methodology outlined above is in line with past approved documentation and is therefore
   considered to be appropriate as it demonstrates that WQO’s for the site will be achieved through the proposed
   stormwater quality improvement devices.

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65. Stormwater Quality Treatment must be designed in accordance with the Water by Design MUSIC modelling
    Guidelines. Any deviation away from the recommendations of the Water by Design guidelines may only be
    approved where:

   65.1. Letters signed by the authors of the Water by Design MUSIC modelling guidelines and/or eWater
         MUSIC modelling program are provided to confirm that the modelled design is expected to meet the
         required water quality outcomes at the discharge point of the site; and

Response

   As detailed in Section 4 of the SWMP Report No. 16-001014-01A, modelling of the site and design of stormwater
   quality treatment devices have been undertaken in accordance with Water By Design MUSIC Modelling Guidelines
   (2010) and other relevant Water By Design guidelines. The SWMP Report No. 16-001014-01A demonstrates that
   WQOs for the site are achieved without modelling of the Water Quality Control Pond (WQCP). This is conservative
   considering that the proposed pond will capture suspended solids through sedimentation and will treat some
   nutrients. Furthermore, the proposed Gross Pollutant Traps (GPTs) were excluded from the modelling, which are
   also expected to capture gross pollutants and suspended solids. The only treatment devices modelled are the
   Bioretention Basins, which have been configured in accordance with Water By Design’s MUSIC Modelling
   Guidelines (2010). Therefore, since there is no deviation from the recommendations specified in Water By Design
   Guidelines, a letter from the authors of the Water By Design guidelines or eWater MUSIC program is not required.
   For email correspondence with Water By Design and eWater regarding the modelling of the pond, refer to Calibre
   Consulting Letter No. B14158.W-10A dated 5 February 2016.


   65.2. A contingency plan to monitor for and to remove cyanobacterial and algal outbreaks from
         waterbodies, including costs, is provided for approval with the Operational Works application and the
         approved version is implemented throughout the lifetime of the development. All stormwater runoff
         treatment must be undertaken within the development areas of the site. Stormwater likely to contain
         grease and oil (i.e. from all roads and car parking areas) shall be managed via grease and oil
         arrestors prior to treatment within bioretention basins, wetlands and water quality control ponds.

Response

   As discussed above, modelling of the site and design of stormwater quality treatment devices have been undertaken
   in accordance with Water By Design MUSIC Modelling Guidelines (2010) and other relevant Water By Design
   guidelines. As detailed in Section 4 of the SWMP Report, all stormwater treatment is undertaken within development
   areas of the site. Modelling has demonstrated WQO’s can be achieved prior to discharge from the site.

   Stormwater from all significant roads and car parking areas are proposed to be treated with a HumeGard GPT.
   Council specify that runoff from all road and car parking areas is to be pre-treated for grease and oil prior to
   discharging to the downstream Bioretention systems. According to Water By Design Bioretention Technical Design
   Guidelines (2014) Bioretention systems are suitable for treating runoff from roof, ground and road areas. Table 14 of
   the guidelines outlines pre-treatment measures typically proposed upstream of Bioretention Basins based on
   catchment size (i.e. for catchments between 2ha and 5ha vegetated swales, coarse sediment forebays, inlet pond or
   GPT’s are recommended). The condition to pre-treat all runoff from road and car parking area specifically for oil and
   grease prior to discharge into Bioretention Basins is considered to be conservative given the nature of the
   Bioretention treatment device. There are also many examples of urban, commercial and industrial development that
   propose Bioretention Basins without pre-treatment of oil and grease.

   As per details provided in Section 3.1 of the Stormwater Management Construction & Maintenance Plan Report No.
   16-001014-02A, the HumeGard GPT proposed upstream of the proposed Bioretention Basins has the capability of
   trapping floating oil and grease pollutants. As demonstrated on Page 2 of the HumeGard GPT Technical Manual
   provided in Attachment 2, the application of a floating boom provides separation at the surface of the waters
   between the sediment storage chamber and the outlet chamber. This effectively captures pollutants such as oils and
   greases that float to the surface. Based on laboratory and field testing, Table 1 on Page 3 of the Technical Manual
   indicates 90% removal efficiency of Hydrocarbons during emergency spill events.

   Also, the proposed Bioretention Basins and Pond are not to become Council assets, rather this will be operated by
   the retirement community owner who is responsible for the maintenance and management of these treatment
   devices. Managing and replanting of the Bioretention Basins and Pond will be the responsibility of the retirement
   community owner.

   Based on the above, all areas of road and car parking, except for the road areas in the northern catchment
   (Catchment A1 & A2) will be treated by GPT’s. Since Catchment A is only 2.45ha with no significant areas for car
   parking a sediment forebay is appropriate.

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    Further to this a contingency plan (including costs) to monitor for and to remove cyanobacterial and algal outbreaks
    has been provided in Appendix A of the SMCMP Report No. 16-001014-02A. Monitoring of the stormwater quality
    treatment devices will be implemented throughout the lifetime of the development.

66. Wetlands, bioretention basins and other water quality treatment devices shall be designed and constructed
    in accordance with Healthy Waterways Water Sensitive Urban Design – Technical Guidelines for South East
    Queensland 2006, Water by Design Bioretention Technical Design Guidelines 2014, Healthy Waterways
    Water by Design Construction and Establishment Guidelines: Swales, Bioretention Systems and Wetlands
    February 2009. In the case of any inconsistencies between these documents, the most recent document
    shall prevail.

Response

    Design of stormwater quality treatment devices is in accordance with the above mentioned documents. Further
    details are provided in Section 2.1 and 3.1 of SMCMP Report No. 16-001014-02A.

67. Stormwater treatment strategy must be designed and maintained to achieve compliance with the
    approved version of B14158.W.07L.AMcP.ag.docx dated 8 October for the Stormwater Management
    Construction and Maintenance Plan Framework.

Response

    The stormwater treatment strategy has been designed and maintained in accordance with Calibre Consulting’s
    Letter No. B14158.W.07L.AMcP.ag.docx dated 8 October 2015. Refer to SMCMP Report No. 16-001014-02A for
    details.

68. The detailed Stormwater Management Construction and Maintenance Plan (SMCMP) must be designed and
    implemented in accordance with the framework detailed in the approved version of
    B14158.W.07L.AMcP.ag.docx dated 8 October 2015 and approved by the Council prior to issue of
    operational works approval.

Response

    The SMCMP Report No. 16-001014-02A is in accordance with the framework provided in Calibre Consulting’s Letter
    No. B14158.W.07L.AMcP.ag.docx dated 8 October 2015. Refer to the report for details.

69. The owner of the development is responsible for all costs associated with the construction and ongoing
    maintenance and repair of the treatment of stormwater on the site and for all catchments that enter the site.

Response

    As detailed in the SMCMP Report No. 16-001014-02A, the owner of the development will be responsible for the
    construction, maintenance and repair of all stormwater quality treatment devices on site.

70. The Operational Works Application must include calculations in accordance with the Queensland Urban
    Drainage Manual to demonstrate that the confluence of stormwater at the drainage easement will not cause
    scour and that any required dissipater/erosion control can be contained wholly within the applicant’s
    property.

Response

    Calculations have been provided in Appendix A of the SMCMP Report No. 16-001014-02A to demonstrate that the
    confluence of stormwater at the drainage easement will not cause scour and that adequate energy dissipation
    wholly within the site will be provided.

71. Any waters captured on site must be plumbed to be available for all external uses including gardening,
    irrigation, filling swimming pools and outdoor cleaning, as well as all ground floor toilet cisterns and
    washing machine cold taps.

Response

    The capture of all water for external uses was not proposed as part of the previously approved MCU submission.
    The capture of all water on site is not required for the site to meet both Stormwater Quality and Quantity Objectives
    to be achieved and is therefore not proposed or required.

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72. Operational Works Application must demonstrate that the peak 1-year ARI event discharge off site is limited
to the pre-development peak 1-year ARI event discharge flow.

Response
Refer to the response to Condition 63. Peak flows including the 63% AEP (1 year ARI) storm will be reduced back to
or below existing levels.

73. Prior to commencement of use, the applicant must provide Council with a performance bond to the value of
150% of the value of monitoring and maintenance of the water bodies for the first 5 years of full operation.
The bonds shall be incrementally released annually following completion of the development and upon
demonstration to the satisfaction of Council Officers that all aspects of the approved version of the detailed
Stormwater Management Construction and Maintenance Plan required by this approval have been
implemented to effectively prevent or reduce likelihood of cyanobacteria outbreaks for the site for the
preceding year.

Response
As specified in Appendix A of the SMCMP Report No. 16-001014-02A, a performance bond to the value of 150% of
the value of monitoring and maintenance of the water bodies for the first 5 years of full operation will be provided to
Council.

We trust the information provided in this letter is acceptable and satisfactorily addresses Council's Approval Conditions.
We respectfully request that the Operational Works designs for stormwater management be approved.

Should you require any further information please do not hesitate to contact the undersigned or Andrew McPhail on (07)
3895 3444.

Yours sincerely,
Calibre Consulting

Matthew Starr
Senior Engineer — Water & Environment