Development Application part 7 of 10 Stormwater Management Report - MCU13/0092

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4.5 Selection of Stormwater Treatment Options


Based on the understanding of Sunshine Coast Regional Council’s requirements for stormwater quality treatment for
commercial sites Bioretention Basins have been utilised as the SQID’s for this site.


4.5.1     Bioretention Basins


Bioretention basins are vegetated areas where runoff is filtered through a filter media layer as it percolates
downwards. It is then collected via perforated under-drains and flows to downstream waterways or to storages for
reuse. Bioretention basins often use temporary ponding above the filter media surface to increase the volume of

runoff treated through the filter media. They treat stormwater in the same way as bioretention swales; however,
’above design’ flows are conveyed through overflow pits or bypass paths rather than over the filter media. This has
the advantage of protecting the filter media surface from high velocities that can dislodge collected pollutants or
scour   vegetation.

Bioretention basins operate by filtering stormwater runoff through densely planted surface vegetation and then
percolating runoff through a prescribed filter media. During percolation, pollutants are retained through fine filtration,
adsorption and some biological uptake. The vegetation in a bioretention system is a vital functional element of the
system providing a substrate for biofilm growth within the upper layer of the filter media. Vegetation facilitates the
transport of oxygen to the soil and enhances soil microbial communities which enhance biological transformation of
pollutants.

Plants with extensive fibrous or a spreading, rhizomatous root system able to withstand periods of inundation are
preferred in bioretention systems.

A sandy loam filtration media with a hydraulic conductivity of 200mm/hr will ensure that filtration media does not
become waterlogged (Australian Runoff Quality (ARQ),2004, Section 9.4). The ARQ states that bioretention systems
are efficient in treating, TSS, TN, TP and Hydrocarbons (where attached to fine sediments). Refer to drawings in

Appendix A for bioretention basin details.

The uPVC slotted pipe will collect the treated Q3 Month flow after it has filtered through a layer of sandy loam. The
treated runoff water in the slotted pipe will connect into the side of the field inlet. The treated flow will be piped from
the field inlet together with the collected minor flow via the sites stormwater drainage network. The MUSIC
parameters used for the Bioretention Basins are shown on Table 5.

                                         Table 5: Bioretention Basin MUSIC Parameters


                                               Parameter                                 Value
                  Extended detention depth                                                0.30 m
                  Seepage Loss (mrnlhr)                                                  o mm/hr
                  Filter Depth (m)                                                        0.5 m
                  Particle Diameter (mm)                                                 0.45 mm
                  Saturated Hydraulic Conductivity (mm/hr)                              200 mm/hr
                  Depth below underdrain pipe (%of Filter Depth)                           0%
                  Lining Properties: Is basin lined                                        Yes
                  Vegetation Properties                                      Vegetated with effective nutrient
                                                                                     removal plants
                  Surface Area   -
                                     Basin A                                              16 m2
                  Filter Area    -   Basin A                                              16 m2




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