It is assumed that treatment and detention of rainfall events across the development will occur via retention and/or detention within WSUD measures. Section 6.4 discusses how WSUD measures can be utilised across the SP to demonstrate that sufficient POS has been proposed for the purposes of managing drainage. Indicative locations and sizes of WSUD measures are shown in Figure 9 based upon calculations provided in Appendix I and Appendix K. Further, conceptual landscape designs for these POS areas are provided in Appendix J.
Implementation of WSUD measures will be further detailed in the future UWMP where appropriate.
The use of WSUD measures across the SP will assist in achieving Criteria SW1, SW2 and SW5.
Development WSUD measures proposed are detailed in the following sections.
6.2.1 Bottomless gully/side entry pits
Bottomless pits (e.g. side-entry pits, gully pits) can be utilised within road reserves to assist in retaining the first 15 mm of rainfall within the road reserve. The invert of the pits will need to achieve appropriate clearances to MGL (as agreed with the CoS) or the separation distances required by Institute of Public Works Engineering Australasia (IPWEA 2016) where groundwater is controlled (i.e. through use of subsoils). Pits will be underlain by appropriate filtration media to provide treatment of runoff during infiltration (Chang et al. 2010; Lucas et al. 2013; Reddy et al. 2014).
Runoff beyond the capacity of these structures will flow as overland flow along the road reserve.
6.2.2 Tree pits
Tree pits can be located within road reserves and assist in achieving treatment higher in the catchment. Tree pits can receive runoff via direct sheet flow and/or via online connection to the stormwater treatment train. It is understood the CoS are generally supportive of low maintenance measures that increase canopy cover and incorporate drainage.
6.2.3 Bio-pockets
Bio-pockets can be located within road reserves and assist in achieving treatment higher in the catchment. It is understood the CoS are supportive of these when the CoS is engaged in determining their location(s). Bio-pockets can receive runoff via direct sheet flow and/or via online connection to the stormwater treatment train. An example of a bio-pocket is provided in Plate 1.
Bio-pockets will be planted with native vegetation to encourage biological nutrient uptake, consistent with the Vegetation guidelines for stormwater biofilters in the south-west of Western Australia (Monash University 2014). The species chosen should have extensive and fine root systems, be relatively fast growing, be able to withstand temporary and regular inundation, and have long growing seasons. A sufficient density of plants/m2 is recommended to provide adequate initial coverage and room for growth. The actual density will comply with relevant overarching biofiltration adoption guidelines (Payne et al. 2015) and is to be confirmed and approved in future detailed landscape designs.
An appropriate soil medium (either amended soil with phosphorus retention index (PRI)>10 or media that meets relevant guidelines) will be used to line the bio-pocket and increase nutrient removal from runoff during infiltration. This will be underlain by a coarse sand layer to prevent washout of the above soil medium and a gravel layer to collect the treated runoff and encourage it to infiltrate.
Imported fill can be used within the subgrade to ensure that bio-pockets dry out between rainfall events. WSUD measures that are designed to treat runoff via infiltration, such as bio-pockets, should achieve a minimum clearance of 300 mm from the underlying MGL or phreatic surface (IPWEA 2016).
Overflow from the bio-pockets will be via surface overflow and/or appropriate outlet structure (e.g.
gully pit within the bio-pocket).
Plate 1: Example of a bio-pocket
6.2.4 Swales
Swales can be located within road reserves (in the median or roadside verge) or POS to provide at- source treatment for runoff from an adjacent road catchment. The CoS prefer the use of verge swales compared to median swales, and that these be located adjacent to POS areas. Swales will accept runoff directly from the adjacent road pavement and/or via appropriate inlet structures (e.g.
bubble-out pit to convey runoff from lots < 300 m2 in size towards a WSUD measure).
Vegetation, soil medium and clearance requirements are consistent with the design of bio-pockets (Section 6.2.3).
Overflow from the swales will be via surface flow over a broad and shallow weir structure or similar (i.e. a low point that can be designed to avoid erosion), and/or appropriate outlet structure (e.g. gully pit within a median swale). Depending on the location of the swales, this runoff can be directed onto the road reserve and pipe network, a downstream WSUD measure or the living stream (i.e. Saint Leonards Creek foreshore area).
6.2.5 Bio-retention areas
BRAs can be located within downstream POS areas to provide at-source treatment for runoff from an adjacent road catchment and/or end-of-pipe treatment. No BRAs will be located in the living stream (i.e. Saint Leonards Creek foreshore area). An example of a BRA is provided in Plate 2.
Vegetation, soil medium and clearance requirements are consistent with the design of bio-pockets (Section 6.2.3). Similarly, overflow from BRAs will be consistent with that described for swales above (Section 6.2.4).
Plate 2: Example of a BRA
6.2.6 Subsurface storage
Subsurface soakage structures can be utilised beneath road verges to provide at-source retention of runoff from an adjacent road catchment or beneath turf / groundcovers to provide end-of-pipe retention for runoff from the upstream catchment. There are a number of proprietary subsurface infiltration cells available that could potentially be used (e.g. Ecoaid, City Green, Stormtech, Atlantis, etc.).
Soakage structures will be underlain by a layer of construction aggregate for structural purposes and a layer of appropriate filtration media to provide treatment of runoff during infiltration (Chang et al.
2010; Lucas et al. 2013; Reddy et al. 2014). The invert of underground infiltration systems can be set at MGL or the phreatic surface where a groundwater control is proposed (IPWEA 2016). Soakage structures may be underlain by a subsoil pipe, which will set the phreatic surface and ensure drainage of the structure.
6.2.7 Flood storage areas
Runoff beyond the capacity of the previous WSUD measures will be conveyed towards FSAs located within downstream POS areas. FSAs will be sized to detain minor and major event runoff (i.e. up to the 1% AEP event) and ensure post-development peak discharge rates meet pre-development peak discharge rates.
FSAs will be turfed or vegetated depending on the detailed landscape design and whether BRAs (which are required to be vegetated) are proposed to be collocated within FSAs. Imported fill can be used to ensure FSAs dry out between rainfall events and should achieve a minimum clearance of 300 mm from the underlying MGL or phreatic surface where groundwater control is proposed (IPWEA 2016).
6.2.8 Temporary FSA
Temporary FSAs will be required to manage runoff from future subdivision areas given the
fragmented ownership across the site. Some may also need to serve a treatment function, which will require the inclusion of vegetation and an appropriate soil medium. Detailed design of temporary FSAs should be included within future detailed civil designs and UWMPs for approval by the CoS.
6.2.9 Saint Leonards Creek foreshore area
Conceptual landscape design of the living stream (i.e. the foreshore area) and POS encompassing the foreshore area were described in Section 5.3.2.2 and detailed in Appendix I. The use of appropriate erosion controls will need to be considered when detailed design of the main channel occurs during subdivision (DWER 2018). Due to the fragmented land ownership of the site, detailed design of the Creek should be addressed within the first UWMP to include the Creek. This is discussed further in Section 9.3.