16-001014-02A (Ultimate SCMMP)

3         BIORETENTION DEVICES - DESIGN, CONSTRUCTION,
          OPERATIONAL MANAGEMENT & MAINTENANCE
The following sections discuss the design, construction and operational management and maintenance measures for the
proposed Bioretention Devices and upstream proprietary Gross Pollutant Trap devices. This is in accordance with the
approved Framework Letter Report (No. B14158.W.07L dated 8 October 2015), as per the approval conditions indicated
in Council’s Decision Notice (dated 22 July 2016). While the structure of the report may vary slightly from the Framework
Letter Report (2015), all design features and management measures identified in the Framework Letter Report (2015)
has been implemented in this report.

The following stormwater treatment devices have been reviewed in this section:
 Northern Bioretention Device – Refer to Drawing No. 16-001014-415.
 Central Bioretention Device – Refer to Drawing No. 16-001014-405.
 Southern Bioretention Device – Refer to Drawing No. 16-001014-400.
 Bioretention Pod D – Refer to Drawing No. B14158.W-SK113.

Detailed design drawings for the above Bioretention Devices are provided in Appendix D. Bioretention Devices are
proposed as secondary treatment devices within the proposed treatment train. Refer to details provided in the SWMP
Report (No. 16-001014-02A dated September 2016) for the stormwater quality management strategy for the site.
Proprietary Gross Pollutant Trap devices (GPT’s) are proposed upstream of each Bioretention Device as a first measure
of treatment or primary treatment. The design, construction and operational management and maintenance measures for
the proposed Proprietary GPT’s are also discussed in this section.

Since each Bioretention Device has been designed as an unsaturated device to drain within a 24 hour period, an
outbreak of mosquito, let alone cyanobacterial blooms are highly unlikely in these systems (cyanobacterial blooms form
in deep water zones). Occurrence of mosquitoes in the Bioretention Device may be possible in the very unlikely event
that the Biorention filter system remains clogged for an extended period (causing extended period of stagnant water) or if
the batters are severely scoured such that isolated depressions and mounds are formed that prevent draining of the
basin for an extended period. The design, construction and operational monitoring and maintenance procedures detailed
in this section, provides effective design measures and counter-actions to avoid the very unlikely event of an extended
period stagnant water occurring in the Bioretention Device. While not specifically mentioned, the standard Bioretention
measures detailed below are considered to be appropriate for avoiding risk of mosquitos in the Bioretention Devices
proposed.

3.1       DESIGN PHASE
Design objectives for the Bioretention Devices are detailed in the Framework Letter Report (No. B14158.W.07L dated 8
October 2015). Design details are indicated on Drawing No. 16-001014-400 Series provided in Appendix D.

   A Design Check and Summary from Table 2.2 of Water By Design’s Bioretention Technical Design Guidelines
    (October 2012) has been undertaken for each Bioretention Device proposed. Refer to Appendix C. Relevant
    calculations and supporting material has also been provided in this appendix.

   Proprietary GPT will be designed in accordance with manufacturers specifications (refer to HumeGard GPT
    Technical Manual in Appendix C or approved equivalent for design details).

As per Condition 65.2, 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 Devices. According to WBD Bioretention Technical Design
Guidelines (2014), Bioretention Devices 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 Devices 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 Devices is considered to be conservative given the nature of the Bioretention treatment
device. There are also many examples of urban, commercial and industrial developments that propose Bioretention
Devices without pre-treatment of oil and grease.



16-001014-02A | SMCMP | PALM LAKE RESORTS                        COOROY RETIREMENT COMMUNITY, TRADING POST
                                                                                             ROAD, COOROY                | 11