DSP East Wanneroo Appendix A Integrated Water Management ...

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INTEGRATED WATER MANAGEMENT FRAMEWORK East Wanneroo District Structure Plan

EWP72682.002-2 Integrated water management

framework Rev 0

23 July 2019

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EWP72682.002-2 | Integrated water management framework | Rev 0 | 23 July 2019

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Document status

Version Purpose of document Authored by Reviewed by Approved by Review date

Draft A Draft for client review MilBoz ShaMcS ShaMcS 06/07/2019

Draft B Draft for client review ShaMcS ShaMcS ShaMcS 15/07/2019

Rev0 Final for issue MilBoz ShaMcS ShaMcS 23/07/2019

Approval for issue

Shane McSweeney 23 July 2019

This report was prepared by RPS within the terms of RPS’ engagement with its client and in direct response to a scope of services. This report is supplied for the sole and specific purpose for use by RPS’ client. The report does not account for any changes relating the subject matter of the report, or any legislative or regulatory changes that have occurred since the report was produced and that may affect the report. RPS does not accept any responsibility or liability for loss whatsoever to any third party caused by, related to or arising out of any use or reliance on the report.

Prepared by: Prepared for:

RPS Department of Planning, Lands and Heritage

Shane McSweeney

Principal Water Management Consultant

Level 2, 27-31 Troode Street West Perth WA 6005

140 Williams Street Perth, WA 6000

T +61 8 9211 1111

E [email protected]

T +61 8 6551 8002

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Contents EXECUTIVE SUMMARY .................................................................................................................................... 1

Introduction ............................................................................................................................................... 1 Development and water planning background ......................................................................................... 1 Existing environmental setting .................................................................................................................. 1

Location and current land use ........................................................................................................ 1 Landform and topography .............................................................................................................. 1 Geology and soils ........................................................................................................................... 2 Contaminated sites ........................................................................................................................ 2 Flora and fauna .............................................................................................................................. 2

Pre-development surface water setting .................................................................................................... 2 Surface water and wetlands ........................................................................................................... 2 Surface water quality ...................................................................................................................... 2

Pre-development groundwater setting ..................................................................................................... 3 Groundwater systems, levels and flow .......................................................................................... 3 Groundwater quality ....................................................................................................................... 3

Drainage management ............................................................................................................................. 3 Drainage principles ........................................................................................................................ 3 Drainage strategy ........................................................................................................................... 3 Post-development catchments ....................................................................................................... 4 Future LSP areas and future water planning ................................................................................. 4

Groundwater management principles ...................................................................................................... 5 Future groundwater level rise ......................................................................................................... 5 Implications - Fill and subsoil drainage .......................................................................................... 5 Controlling groundwater level rise .................................................................................................. 5 Additional groundwater modelling .................................................................................................. 5

Groundwater management principles ...................................................................................................... 6 Groundwater quality ....................................................................................................................... 6

Water dependent ecosystems management principles ........................................................................... 6 Implementation and governance plan ...................................................................................................... 6 Acid sulfate soils ....................................................................................................................................... 6

1 INTRODUCTION ...................................................................................................................................... 8 1.1 IWMF objectives ............................................................................................................................. 8 1.2 Water management planning ......................................................................................................... 8

1.2.1 Next stage in water management planning ...................................................................... 9

2 EXISTING ENVIRONMENTAL SETTING..............................................................................................12 2.1 Location and current land use ......................................................................................................12 2.2 Climate .........................................................................................................................................12 2.3 Landform and topography ............................................................................................................14 2.4 Geology and soils .........................................................................................................................14

2.4.1 Regional mapping ...........................................................................................................14 2.4.2 Contaminated sites .........................................................................................................14

2.5 Vegetation and flora .....................................................................................................................17 2.5.1 State forest ......................................................................................................................17

3 PRE-DEVELOPMENT SURFACE WATER SETTING ..........................................................................19 3.1 Surface water and wetlands .........................................................................................................19

3.1.1 Surface water catchments ..............................................................................................19 3.1.2 Geomorphic wetland mapping ........................................................................................19 3.1.3 Wetlands .........................................................................................................................19 3.1.4 Surface water level monitoring .......................................................................................21 3.1.5 Surface water quality ......................................................................................................21

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4 PRE-DEVELOPMENT GROUNDWATER SETTING ............................................................................23 4.1 Groundwater system ....................................................................................................................23 4.2 Aquifers ........................................................................................................................................23 4.3 Groundwater levels ......................................................................................................................23 4.4 Groundwater flow .........................................................................................................................23 4.5 Groundwater quality .....................................................................................................................23

5 DRAINAGE MANAGEMENT PRINCIPLES ..........................................................................................26 5.1 Drainage principles ......................................................................................................................26 5.2 Drainage strategy overview ..........................................................................................................26

5.2.1 Minor drainage system including treatment of the first 15 mm .......................................26 5.2.2 Major drainage system ...................................................................................................27

5.3 Drainage characteristics ...............................................................................................................27 5.3.1 Post-development catchments .......................................................................................27 5.3.2 Flood storage areas in catchments with basins ..............................................................28 5.3.3 Flood storage in catchments with REWs or CCWs ........................................................28

5.4 Future Local Structure Plan Areas and Future Water Planning ...................................................30

6 GROUNDWATER MANAGEMENT .......................................................................................................36 6.1 Predicted future groundwater level rise .......................................................................................36 6.2 Implications on fill and infrastructure due to predicted groundwater level rise ............................36

6.2.1 Impact on infrastructure ..................................................................................................36 6.2.2 Fill and subsoil drainage .................................................................................................37

6.3 Controlling groundwater level rise ................................................................................................37 6.4 Additional groundwater modelling ................................................................................................37 6.5 Groundwater management principles ..........................................................................................38

6.5.1 Management principles - controlled groundwater levels ................................................38 6.5.2 Management principles - groundwater quality ................................................................38

7 WATER DEPENDENT ECOSYSTEMS MANAGEMENT PRINCIPLES ...............................................41 7.1 Protection of on-site wetlands ......................................................................................................41 7.2 Water sensitive urban design .......................................................................................................41

7.2.1 WDE protection through vegetation ................................................................................42 7.2.1 Other water quality control measures .............................................................................43

7.3 Mosquito management .................................................................................................................43

8 IMPLEMENTATION AND GOVERNANCE PLAN ................................................................................44 8.1 BUWM framework ........................................................................................................................44 8.2 Individual developer requirements ...............................................................................................44 8.3 DWER requirements ....................................................................................................................45 8.4 DPLH requirements ......................................................................................................................45 8.5 City of Wanneroo requirements ...................................................................................................45 8.6 Service provider requirements .....................................................................................................45

9 ACID SULFATE SOILS .........................................................................................................................46 9.1 Acid sulfate soils process .............................................................................................................46 9.2 ASS risk due to groundwater level rise ........................................................................................46 9.3 The role of controlling groundwater to reduce ASS risk ..............................................................46

10 REFERENCES .......................................................................................................................................48

Tables Table 1: Water Management and Drainage Proposed for East Wanneroo DSP area ..............................33

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Figures Figure 1: Site location ..................................................................................................................................10 Figure 2: Draft East Wanneroo District Structure Plan ................................................................................11 Figure 3: Water Corporation abstraction and public drinking water source protection areas .....................13 Figure 4: Potentially contaminated site .......................................................................................................14 Figure 5: Topography and geomorphic wetlands ........................................................................................15 Figure 6: Surficial geology ...........................................................................................................................16 Figure 7: Vegetation complexes and bush forever ......................................................................................18 Figure 8: Wetland mapping and PRAMS inundation areas .........................................................................22 Figure 9: Historic MGL contours ..................................................................................................................24 Figure 10: Historic AAMGL contours .............................................................................................................25 Figure 11: Probable post-development catchment types ..............................................................................29 Figure 12 Post-development precinct and catchment boundaries ...............................................................31 Figure 13 Post-development hydrological catchment and broad drainage management areas..................32 Figure 14: Change in groundwater levels (m) of the Superficial Aquifer by 2030, modelled using

PRAMS .........................................................................................................................................39 Figure 15: Groundwater levels (m AHD) of the Superficial Aquifer by 2030, modelled using PRAMS.........40 Figure 16: Typical vegetated swale (left) and bioretention basin (right) .......................................................42 Figure 17: ASS risk mapping .........................................................................................................................47

Graphs Graph 1: Mean monthly rainfall (BoM station 9105) ...................................................................................12

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EXECUTIVE SUMMARY

Introduction This Integrated water management framework (IWMF) has been prepared on behalf of the Department of Planning, Lands and Heritage (DPLH) as a guiding document to support future development of the East Wanneroo District Structure Plan (EWDSP) area. This IWMF aims to lay out the principles and responsibilities pertaining to water management.

Water management outcomes include stormwater management, groundwater management and water supply, recognising total water cycle management principles and objectives, guided by the Better Urban Water Management Framework (WAPC 2008).

The IWMF is intended to be used to inform water management and design objectives for the proposed land use, outlining a proposed drainage strategy for the future urban development which will guide the preparation and finalisation of future documents including the District Water Management Strategy (DWMS), Local Water Management Strategies (LWMS) and Urban Water Management Plans (UWMP).

Development and water planning background The Western Australian Planning Commission (WAPC) prepared a Structure Plan report for East Wanneroo (2011) to provide a framework to guide and inform future requests for amendments to the MRS. The current draft District Water Management Strategy (DWMS) will be refined and updated based on the results of an additional water modelling exercise. The DWMS will be finalised to support the future urbanisation of East Wanneroo. DPLH intends to conduct a more detailed groundwater modelling exercise with the aim of establishing an environmentally acceptable and sustainable controlled groundwater level across the DSP area in consultation with the key stakeholders. The intention is to conduct this groundwater modelling exercise and present the results as part of the finalised DWMS.

To facilitate urban and industrial development, the Public Drinking Water Source Area (PDWSA) within the EWDSP that is currently classified as P1 will need to be rezoned to urban and industrial, and a corresponding change to the P1 classification over the area will be required. This will require a rezoning application to be submitted to the WAPC. If this area is subsequently accepted by the WAPC to be rezoned then the Department of Water and Environmental Regulation (DWER) would reclassify the area, likely to a P3* protection zoning.

The EWDSP area also contains drinking water abstraction wells with associated wellhead protection zones. Further consideration will need to be given during future urban design and planning of these circular (500 m or 300 m in radius) protection zones surrounding each well.

Existing environmental setting

Location and current land use The EWDSP area is approximately 8,500 ha in size, extending from north of Neaves Road to Gnangara Road in the south, to Centre Way to the east and as far west as Pinjar Road. The site is located approximately 2 km east of the Wanneroo townsite, 6 km east of Joondalup and 25 km north of the Perth CBD. The site has numerous land zonings and a mixture of land uses including rural, rural – water protection, parks and recreation, waterways (wetlands) and reserved land for public purposes (state forest).

Landform and topography The landform of the EWDSP is comprised of undulating Spearwood and Bassendean dune systems running in a north-south direction, with a prominent dunal ridge east of Wanneroo Road and an inter-dunal swale interface between the Spearwood and Bassendean soil systems along which the low-lying east Wanneroo wetlands and seasonally inundated areas are found.

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Surface elevation, as shown by topographic contours, range from approximately 45 m Australian Height Datum (AHD) in low lying wetlands to 100 m AHD on the western boundary of the site. As the site consists of a dunal system, there are several high and low points throughout the site.

Geology and soils Regional surficial geological mapping (50 K map sheet) has identified that the western portion of the site typically consists of Spearwood sands, while the eastern portion of the site incorporates Bassendean sands. Typically, near the wetland depressions peaty clay is present.

Contaminated sites A search of the DWER contaminated sites database identified that there are no registered contaminated sites within the EWDSP area.

Flora and fauna Broad scale vegetation mapping has identified the following vegetation complexes over the site: Pinjar Complex, Bassendean Complex North, Bassendean Complex North/Transition Vegetation Complex, Bassendean Complex Central and South, Karrakatta Complex Central and South and the Herdsman Complex.

An Environmental Assessment Study completed by Emerge identified several key environmental values that should be protected, including TEC Banksia woodland, known occurrences of threatened priority flora, threatened and priority fauna habitat, groundwater dependent ecosystems associated with the Gnangara groundwater area and conservation category wetlands.

Some land use areas within the EWDSP are classified as state forests, much of which is pine plantations. The State Agreements have stipulated that they are to be harvested by approximately 2020.

Pre-development surface water setting

Surface water and wetlands The site is in the Swan Avon, Lower Swan Catchment within the Swan Coastal river basin. However, there are no defined streams, creeks or major drains mapped within the EWDSP boundary.

Due to the general high permeability of the Spearwood and Bassendean Sands, the pre-development hydrology of the site is characterised by high rates of infiltration in frequent rainfall events. Surface water runoff is likely to only occur during large storm events via overland flow to wetlands, localised low points or off-site areas.

Predevelopment catchment mapping identified 94 catchments, of which 41 catchments drain internally to Conservation Category Wetlands (CCWs) or Resource Enhancement Wetlands (REWs), 43 drain internally to localised depressions which do not contain CCWs or REWs, and 10 catchments were found to discharge off site.

Surface water quality The available surface water records for Lake Mariginiup, Jandabup Lake, Lake Adams, and Gnangara Lake show TN and ammonium concentrations to be highly elevated and some phosphorus concentrations above the ANZECC (2000) guidelines for slightly disturbed wetland ecosystems.

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Pre-development groundwater setting

Groundwater systems, levels and flow The site is located on the Gnangara Groundwater System which extends from Moore River and Gingin Brook in the north to the Swan River in the south. The Gnangara system incorporates the Superficial, Mirrabooka, Leederville and Yarragadee aquifer.

The site is located predominantly in the Wanneroo Groundwater Management area, covering five management subareas for the Superficial Aquifer including Mariginiup, Lake Gnangara, Wanneroo Wellfield, Adams, and Jandabup. A section of the site in the south-east is part of the Mirrabooka Groundwater Management Area, in the Plantation subarea.

Historic MGL and AAMGL contours were produced from existing monitoring data. MGL contours were calculated to range from 55 m AHD in the north east to 36 m AHD in the west, with the gradient increasing to the west. Contours created for the AAMGL determined levels to range from 52 m AHD in the north-east to 36 m AHD in the west of the site. Hydrographs from DWER observational bores indicate that groundwater typically flows in a west/south direction.

DWER have recently undertaken modelling using the Perth Region Aquifer Modelling Systems (PRAMS) to estimate future groundwater levels to the year 2030. The modelled identified several areas of the EWDSP which will be inundated by groundwater by 2030.

Geomorphic wetland mapping shows 23 conservation category wetlands (CCW), 24 resource enhancement wetlands (REW) and 28 multiple use wetlands (MUW) located within the site. These wetlands are typically groundwater dependent and PRAMS modelling shows that the overwhelming majority of CCWs and REWs will be inundated by groundwater by 2030. The three largest wetlands within the EWDSP are Lake Mariginiup, Jandabup Lake and Gnangara Lake.

Groundwater quality Overall, since 1995 the salinity of the groundwater of the Gnangara groundwater area has remained relatively constant, with groundwater salinity typically increasing closer to the coast. Salinity in the Superficial Aquifer ranges from fresh (<250 mg/L TDS) to marginal (1000 mg/L TDS). Elevated nutrient concentrations are common due to horticultural practices in the area, with nitrate concentrations frequently exceeding 20 mg/L and phosphorus concentrations frequently above 0.5 mg/L.

Drainage management

Drainage principles Several guidance documents including Decision process for stormwater management in Western Australia (DWER 2017a) which is a part of the Stormwater Management Manual (DoW 2004-2007), and the Gnangara Sustainability Strategy (DoW et al. 2009) were used to prepare the IWMF.

The site will effectively manage stormwater quantity and quality generated from minor and major events, incorporating best practice Water Sensitive Urban Design (WSUD) principles. The stormwater management criteria outlined in the IWMF and subsequent DWMS will inform the detailed drainage design for the EWDSP throughout the various stages of development.

Drainage strategy All stormwater will be treated and managed to ensure that there are no detrimental impacts to wetlands and public drinking water sources. Best management practice WSUD is to be implemented across the site to ensure groundwater resources including public drinking water source and the wetland receiving environments will be protected. Stormwater management will also ensure adequate flood protection is provided for the development.

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Minor drainage system including treatment of the first 15 mm Stormwater runoff from constructed impervious surfaces generated by the first 15 mm of rainfall will be managed, retained/detained and treated (if required) — at-source as much as practical. At-source means that lot runoff is managed within lots and road runoff is managed within road reserves and the stormwater has not entered a piped or lined channel conveyance system.

Where site conditions do not allow for the full runoff to be managed at-source, the runoff will be managed as much as practical at-source, subject to the pre-development hydrology. The remaining runoff will be conveyed from the lot or road reserve via overland flow wherever practical. Impervious areas which cannot be treat at-source will require treatment systems such as bioretention basins, raingardens or equivalent treatment systems.

• Lot drainage: lot generated runoff from the first 15 mm of rainfall will be retained and infiltrated within the lot boundary using soakwells. For lots with low clearance to groundwater, subsoil drainage may also be used to ensure sufficient clearance to groundwater is maintained.

• Road reserve: The road drainage network will ensure that roads will remain passable in the 20% AEP event. This is proposed to be achieved through pipeless systems where practical. The pipeless systems can include rain gardens, tree pits as well as roadside and median swales. Where pipeless systems are not practical, stormwater will be conveyed using a piped system and treated at downstream biofiltration areas. Biofiltration areas will be planted with reeds, sedges and other plant species that will be selected based on their ability to uptake nutrients.

Major drainage system For all rainfall events greater than the 20% AEP event up to and including the 1% AEP event, any runoff that does not infiltrate into the sandy soils will be conveyed via overland flow paths either via the road network or via drainage swales and living streams. This stormwater will be directed to either wetlands that have been pre-determined that they can receive flood waters or basins. Road centrelines should be designed to be considerably lower than the surrounding lots such that stormwater moving along the road can overtop without risk of flood damage to properties.

Post-development catchments RPS conducted a district-level hydrological assessment on the EWDSP area. This assessment determined that the DSP area will be split into the following three catchment categories, each of which will have distinct stormwater discharge points and storage types: catchments draining to CWWs and REWs, catchments draining internally to basins, catchments draining off site. The internal basins used to store the 1% AEP event volume will be sized accordingly, and where catchments are draining to wetlands, their capacity to store the 1% AEP volume will need to be confirmed.

Future LSP areas and future water planning To remove the risk of inconsistent drainage management, the DSP proposes to implement the principle that all surface and groundwater encountered within individual local structure plan (LSP) areas (referred to in the DSP as ‘precincts’) must be managed within that LSP area. The hydrological assessment indicates that this principle can be made to work in practice as the post-development hydrological catchments will be contained within these predetermined precincts or LSP areas.

The DSP area is to be divided into 27 precincts for which individual local structure plans and LWMSs will need to be prepared and approved before subdivision can proceed. Figure 13 shows the precinct boundaries proposed for the DSP. Figure 14 shows the post development catchments identified during the hydrological assessment and are overlain onto the proposed precincts and indicates the direction of overland flow and approximate infiltration basin locations where needed. Initial drainage considerations for each of the proposed precincts (LSP areas) is provided in Table 1.

By utilising the principles associated with the “self-management” of drainage for individual precinct areas, the risk of inconsistent or incoherent drainage design and associated arterial flood management as subdivision and development proceeds across the DSP area is removed.

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Groundwater management principles

Future groundwater level rise With the expected increase in recharge from the removal of the pines, lower evapotranspiration due to urbanisation and reduced abstraction from licenced groundwater users, groundwater levels in the Superficial aquifer are predicted to rise. The PRAMS model shows groundwater levels in the Superficial aquifer are estimated to increase by 3 to 4 m over most of the site. This will result in a significant proportion of the existing site being inundated with groundwater. The majority of inundated areas will occur where there are geomorphic wetlands.

Implications - Fill and subsoil drainage If groundwater levels are not controlled a substantial amount of fill will be required across the site to create sufficient clearance from currently predicted future groundwater levels (based on PRAMS). Due to increasing scarcity and cost of clean sand fill from the Swan Coastal Plain, the minimisation of fill through optimising earthwork design coupled with a strategic groundwater control mechanism to control rising groundwater levels will be required. This mechanism needs to be assessed at a district level and implemented uniformly across the low-lying areas of the DSP.

Based on current development and construction practices and standards, lot levels are required to have at least 1.2 m to 1.5 m clearance above groundwater level. Given this context, a large portion of the site will require significant amounts of imported sand fill if future groundwater levels are uncontrolled and expected to be similar to PRAMS groundwater levels.

Controlling groundwater level rise Controlling groundwater level rise post-development will significantly reduce development costs and lead to more affordable housing, as well as leading to a more environmentally sustainable outcome as the unnecessary use of basic raw materials could be reduced.

A critical design criterion that will need to be determined and presented in a future District Water Management Strategy (DWMS) will be a controlled groundwater level across the DSP area. Future groundwater modelling and assessment will need to assess historic, current and future predicted groundwater levels and provide guidance on a future controlled groundwater level across the DSP area. This future controlled groundwater level will need to be set based on Water resource considerations when controlling groundwater levels in urban development (DWER 2013) and giving considerations to the water dependent ecosystems and wetland and lakes with Ministerial water level criteria.

Additional groundwater modelling The current estimates of groundwater level rise post-development are based on DWER’s current PRAMS groundwater modelling (PRAMS). However, the PRAMS model is a coarse model used to determine groundwater allocation and groundwater resource management across a large scale (i.e. most of the Swan Coastal Plain). It is not suitable to be used to make engineering design decision that requires more site specific and granular assessment. It is acknowledged that additional groundwater assessments and the determination of a proposed controlled groundwater level criteria needs to be determined at a DSP scale to provide certainty to the local structure plans.

DPLH is, therefore, intending to conduct a more detailed groundwater modelling exercise with the aim of establishing an environmentally acceptable maximum groundwater level across the DSP area in consultation with the key stakeholders. The proposed district scale groundwater model would use the input parameters and future predicted climate change scenarios used by DWER to conduct the PRAMS modelling. The proposed district scale groundwater model will be more detailed (reduced cell sizes) than the PRAMS model and provide specific control groundwater levels for specific LSP areas. The model will also better quantify the change in water balance due to the change in land use and assess the options to control groundwater levels post-development. The intention is to conduct this groundwater modelling exercise and present the results in the finalised DWMS.

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Groundwater management principles

Groundwater quality Groundwater quality will be managed through the use of WSUD systems which will treat the first 15mm runoff from impervious areas prior to infiltration. Groundwater and drainage management practices will also need to give consideration to WQPN 38 and WQPN 25.

Water dependent ecosystems management principles The water dependant ecosystem (WDE) management of the site as part of future development will involve the protection and enhancement of onsite ecological systems and the treatment of water in order to protect downstream ecological systems. The protection of WDEs will involve the implementation of buffers around existing onsite ecological systems (e.g. wetlands), controlling groundwater levels and implementing WSUD systems to treat stormwater runoff.

The protection of wetlands will be achieved in part through the implementation of appropriate buffers. While further protection of wetland water quality, as well as the water quality of other WDEs will be achieved through the implementation of WSUD systems, which may include a combination of gross pollutant traps, swales, living streams, raingardens and bioretention basins, among others.

Mosquitos will be managed by ensuring drainage systems are free draining and do not hold standing water.

Implementation and governance plan The water management strategies outlined in this report will be implemented through a range of governance frameworks which will require the production of a range of studies and reports throughout the various stages of the development process. DWER with assistance from the DPLH, and in consultation with relevant stakeholders will guide the requirements for the water management framework as the planning of the development progresses.

The Better Urban Water Management (BUWM) framework (WAPC 2008) establishes a requirement for a DWMS to be prepared in support of a DSP. This is then followed by a LWMSs to support the subsequent LSPs for the area and UWMPs to support the subsequent subdivision approvals.

All involved stakeholders, including DWER, DPLH, City of Wanneroo and service providers will need to fulfil their various relevant requirements as part of the staged development process.

Acid sulfate soils ASS risk mapping has identified that the majority of the eastern third of the site has a moderate to low risk of ASS occurring within 3 m of the natural soil surface, with sections of the site, typically associated with wetlands and peaty clay soils, classified as having a high to moderate risk of ASS occurring within 3 m of the natural soil surface. The western portion of the site has no known risk of ASS occurring within 3 m of the natural soil surface.

The reduction in groundwater levels since the 1970s across the Gnangara Mound is well documented. The general lowering of groundwater levels cross the DSP area is also well noted and observed in long-term datasets. This reduction in groundwater levels may have resulted in the oxidation of soils that were previously saturated below the water table, which can result in the acidification of the water table and release of metals as described above.

The predicted groundwater level rise in a post-development scenario will likely re-saturate this potentially oxidised material and cause mobilisation of low pH water and elevated levels of acidity and metals in the shallow groundwater table. It could result in very acid groundwater as was the experience during the development of Ellenbrook. Given this site is largely within a trapped hydrogeological catchment and most local groundwater moves towards the wetlands which are the low point or groundwater sinks – there is a potential risk that these environmentally sensitive areas could be the receptors of acidified groundwater.

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There is no broad scale remediation technology that could be applied to reduce the mobilisation of acidity across an area this large. The most reasonable and practical approach would be to halt the groundwater level rise and reduce the potential for the re-saturation of potentially oxidised ASS and subsequent release of acidity and associated acidification products. Therefore, it is important that post-development groundwater level rise is controlled.

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1 INTRODUCTION

1.1 IWMF objectives This Integrated Water Management Framework (IWMF) has been prepared on behalf of the Department of Planning, Lands and Heritage (DPLH) with the purpose of acting as a guiding document to support future development of the East Wanneroo District Structure Plan (EWDSP) area shown in Figure 1. This IWMF aims to lay out the principles and responsibilities pertaining to water management.

Water management outcomes include stormwater management, groundwater management and water supply, recognising total water cycle management principles and objectives, guided by the Better Urban Water Management Framework (WAPC 2008).

A draft District Structure Plan (DSP) is to be submitted for the site, which will propose an amendment to the MRS to rezone sections of the EWDSP area to urban. The draft EWDSP area as proposed by the DPLH is shown in Figure 2.

The IWMF is intended to be used to inform water management and design objectives for the proposed land use, outlining a proposed drainage strategy for the future urban development which will guide the preparation of future documents including the District Water Management Strategy (DWMS), Local Water Management Strategies (LWMS) and Urban Water Management Plans (UWMP).

Criteria and land requirements for wetland protection, drainage and groundwater management are discussed in the IWMF for appropriate land allocation.

1.2 Water management planning This document has taken into consideration the Structure Plan report previously prepared by the Western Australian Planning Commission (WAPC) (2011) for the East Wanneroo site and has also been developed with reference to a number of guidance documents including (but not limited to):

• Water Quality Protection Note No. 38, Protecting Water Quality in P3* Areas (DWER 2018)

• Decision Process for Stormwater Management in WA (DWER 2017a)

• North-West Sub-Regional Water Management Strategy (Essential Environmental 2016a)

• Gnangara Sustainability Strategy, Summary of Land Use Planning Investigations (DoW et al. 2009a)

• Better Urban Water Management (WAPC 2008)

• Western Australian State Water Plan (Government of Western Australia 2007)

• Stormwater Management Manual for Western Australia (Department of Water 2004–2007)

• Gnangara Land use and water management strategy (WAPC 2001).

The Better Urban Water Management (BUWM) framework (WAPC 2008) integrates water management into the land use planning process to ensure planning strategies include total water cycle management and Water Sensitive Urban Design (WSUD). This IWMF outlines the integrated water management strategies that will be implemented at the site.

Consistent with State Planning Policy (SPP) 2.9, Water Resources (WAPC 2006), development of the EWDSP will aim to prevent or where appropriate ameliorate potential impacts such as a deterioration of water quality and quantity, and the removal of associated native vegetation important for long-term management of the water resource.

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1.2.1 Next stage in water management planning The current draft District Water Management Strategy (DWMS) will be refined and updated based on the results of additional water modelling exercise. The DWMS will be finalised to support the future urbanisation of East Wanneroo.

DPLH intends to conduct a more detailed groundwater modelling exercise with the aim of establishing an environmentally acceptable and sustainable controlled groundwater level across the DSP area in consultation with the key stakeholders. The intention is to conduct this groundwater modelling exercise and present the results as part of the finalised DWMS.

To facilitate urban and industrial development, the Public Drinking Water Source Area (PDWSA) within the EWDSP that is currently classified as P1 will need to be rezoned to urban and industrial, and a corresponding change to the P1 classification over the area will be required. This will require a rezoning application to be submitted to the WAPC. If this area is subsequently accepted by the WAPC to be rezoned then the Department of Water and Environmental Regulation (DWER) would reclassify the area, likely to a P3* protection zoning if the Water Corporation abstraction bores were to remain. This P3* is a variation of the P3 management approach, to address the increased water quality risks and cumulative impact resulting from the approved land use intensification.

The EWDSP area also contains drinking water abstraction points in the form of Water Corporation groundwater abstraction wells. These wells each have a protection zones surrounding them known as wellhead protection zones (WHPZs). WHPZs are generally circular with a 500 m radius around each drinking water production bore in P1 areas and a 300 m radius around each drinking water production bore in P2 and P3 areas (unless hydrogeological information is available to select a different size and shape). WHPZs do not extend beyond the boundary of the PDWSA in which they are defined. Further consideration will need to be given during future urban design and planning of future WQPNs, WHPZs and Water Corporation’s current and future planned assets during future LSP preparation.

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Figure 1: Site location

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Figure 2: Draft East Wanneroo District Structure Plan

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2 EXISTING ENVIRONMENTAL SETTING

2.1 Location and current land use The EWDSP area as shown in Figure 1 is approximately 8,500 ha in size, extending from north of Neaves Road to Gnangara Road in the south, to Centre Way to the east and as far west as Pinjar Road.

The site is located approximately 2 km east of the Wanneroo townsite, 6 km east of Joondalup and 25 km north of the Perth CBD. The site presently has numerous land zonings and a mixture of land uses including rural, rural – water protection, parks and recreation, waterways (wetlands) and reserved land for public purpose (State Forest).

Current land uses within the site include horticulture (predominantly market gardens), as well as turf farms and plant nurseries. Other land uses include pine plantations, horse training facilities, boarding kennels, poultry farms and rural residential dwellings.

A major point for consideration is that the site is located on the Gnangara groundwater area, which is a critical source of potable water for the Perth metropolitan area. There are 17 Water Corporation abstraction bores currently located within the site boundary with associated wellhead protection zones, as well as an addition three wellhead protection zones which also partially extend over the site. Large sections of the site are currently zoned as a P1 PDWSA and smaller sections are currently zoned as P2 (Figure 3).

2.2 Climate East Wanneroo has a Mediterranean climate with typically hot, dry summers and cool, wet winters. Most of the annual rainfall occurs between May and August (Graph 1). The closest rainfall station to the site is Wanneroo Station (BoM reference 9105), which has a moving average annual rainfall of 797 mm (BoM 2017). Since the 1970’s, the region has experienced increasing temperatures and declining rainfall, which is a trend that is predicted to continue.

Graph 1: Mean monthly rainfall (BoM station 9105)

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Figure 3: Water Corporation abstraction and public drinking water source protection areas

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2.3 Landform and topography The landform of the EWDSP is comprised of undulating Spearwood and Bassendean low dune systems running in a north-south direction, with a prominent dunal ridge east of Wanneroo Road and an inter-dunal swale interface between the Spearwood and Bassendean soil systems along which the low-lying east Wanneroo wetlands and seasonally inundated areas are found (WAPC 2005b).

Surface elevation, as shown by topographic contours, range from approximately 45 m Australian Height Datum (AHD) in low lying wetlands to 100 m AHD on the western boundary of the site (Figure 5). As the site consists of a dunal system, there are several high and low points throughout the site.

2.4 Geology and soils

2.4.1 Regional mapping Regional surficial geological mapping (50 K map sheet) has identified that the western portion of the site typically consists of Spearwood sands (S7) which are pale and olive yellow, medium to coarse grained, sub-angular quarts and trace feldspar of residual origin (Figure 6). The eastern portion of the site incorporates Bassendean sands (S8 and S10) which are very light grey at surface, yellow at depth, fine to medium grained sub-rounded quarts of eolian origin.

Typically, near the wetland depressions, there is peaty clay (Cps) which is dark grey and black with variable sand content of lacustrine origin.

2.4.2 Contaminated sites A search of the DWER contaminated sites database (DWER 2017c) identified that there are no registered contaminated sites within the EWDSP area. There is a registered contaminated site to the immediate north-west of the EWDSP (ID 70488, 70489 and 12882), which is classified for restricted use (Figure 4). This site has been identified to be contaminated with metals and nutrients as it was previously a municipal landfill site. As groundwater flows in a westerly direction, contamination from this site is not considered a risk to the site.

Figure 4: Potentially contaminated site

Some previous and current agricultural activities undertaken within the site such as poultry farms and intensive horticulture have the potential to result in contamination from the use of fertilisers, pesticides and herbicides. A Preliminary Site Investigation (PSI) to assess contamination needs to be undertaken at the LSP stage of development.

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Figure 5: Topography and geomorphic wetlands

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Figure 6: Surficial geology

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2.5 Vegetation and flora Broad scale vegetation mapping (Heddle et al. 1980) (Figure 7) has identified the following vegetation complexes over the site: Pinjar Complex, Bassendean Complex North, Bassendean Complex North/Transition Vegetation Complex, Bassendean Complex Central and South, Karrakatta Complex Central and South and the Herdsman Complex.

As part of the preparation of the DSP, Emerge completed an Environmental Assessment Study (Emerge 2018) to investigate the environmental values of the area and identify at a district level the areas of environmental significance that should be protected. Based on the desktop assessment, the key environmental values were identified as:

• Identified TEC including state listed SCP 20a “Banksia attenuate woodland over species rich dense shrublands” and Commonwealth listed “Banksia woodlands of the swan coastal plain”

• Known occurrences of threatened priority flora

• Threatened and priority fauna habitat, including potential foraging habitat for the threatened Carnaby’s Black-Cockatoo

• Groundwater dependent ecosystems associated with the Gnangara groundwater area

• Conservation category wetlands.

Remnant vegetation across the site is often associated with the large number of wetlands within the site including Lake Jandabup and Lake Mariginiup.

The assessment provided information on areas of vegetation that are already protected through existing mechanisms i.e. CCW and Bush Forever, as well as proposing additional areas >4 ha likely to contain SCP20A vegetation that should also be protected. The minimum 4 ha threshold is consistent criteria with the CoW local biodiversity strategy.

The Integrated Water Management Framework (herein) has assessed the water management objectives and principles based on the proposed development areas taking the proposed vegetation retention into consideration.

2.5.1 State forest Some land use areas within the EWDSP are classified as state forests, much of which is pine plantations. The State Agreements have stipulated that they are to be harvested by approximately 2020 (WAPC 2001).

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Figure 7: Vegetation complexes and bush forever

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3 PRE-DEVELOPMENT SURFACE WATER SETTING

3.1 Surface water and wetlands

3.1.1 Surface water catchments The site is in the Swan Avon, Lower Swan Catchment within the Swan Coastal river basin. However, there are no defined streams, creeks or major drains mapped within the EWDSP boundary.

Due to the general high permeability of the Spearwood and Bassendean Sands, the pre-development hydrology of the site is characterised by high rates of infiltration in frequent rainfall events. Surface water runoff is likely to only occur during large storm events via overland flow to wetlands, localised low points or off-site areas.

Predevelopment catchment mapping was undertaken using 1 m lidar natural surface contour data. 94 catchments were identified, of which 41 catchments drain internally to CCWs or REWs, 43 drain internally to localised depressions which do not contain CCWs or REWs, and 10 catchments were found to discharge off site.

DWER have recently undertaken modelling using the Perth Region Aquifer Modelling Systems (PRAMS) to estimate future groundwater levels to the year 2030. Most catchments which drain to CCWs or REWs and 13 internally draining catchments which do not have CCWs or REWs are predicted to be inundated with groundwater by 2030 according to PRAMS modelling.

Runoff in internally draining catchments which are not expected to be inundated with groundwater will likely infiltrate relatively quickly even after large events. However, infiltration in catchments which are expected to be inundated with groundwater will take considerably longer.

3.1.2 Geomorphic wetland mapping As previously identified in Section 2.3, the site consists of a series of sand dunes and interdunal depressions where there are frequently surface expressions of groundwater. Geomorphic wetland mapping (Figure 8) shows 23 conservation category wetlands (CCW), 24 resource enhancement wetlands (REW) and 28 multiple use wetlands (MUW) located within the site. These wetlands are typically groundwater dependent and PRAMS groundwater mapping shows that the overwhelming majority of CCWs and REWs will be inundated by groundwater by 2030 (Figure 8).

The three largest wetlands within the EWDSP are Lake Mariginiup, Jandabup Lake and Gnangara Lake. Lake Mariginiup and Jandabup Lake are also Ministerial criterial wetlands, which will be discussed further in the following sections. These wetlands are through-flow lakes, receiving groundwater from the east and discharging to the west (RPS 2009). Lake Jandabup receives and discharges water to and from the aquifer in all months, with Lake Mariginiup only receiving water in higher rainfall months (RPS 2009). Outflow from the lakes is considerably lower than inflow due to evapotranspiration. Hydrographs from Lake Mariginiup, Jandabup Lake and Gnangara Lake show declining water levels in recent years.

3.1.3 Wetlands As previously mentioned, Lake Mariginiup, Jandabup Lake and site MT3S located are included in the Ministerial Statement No. 819 and have environmental water provisions as water level criteria.

Lake Jandabup and Lake Mariginiup were first selected as significant wetlands for management in the Gnangara Mound Water Resources Environmental Review and Management Program in 1986 (DoW 2008a) and were revised in 1995 as part of the review of environmental conditions.

3.1.3.1 Lake Jandabup The water regime management objectives for Lake Jandabup include:

• Maintenance of current extent of wading bird habitat

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• No expansion in the areas of sedge vegetation, but maintenance of existing areas

• Removal of mosquito fish from the lake

• Maintenance of high species richness of aquatic macroinvertebrates, macrophytes and sedge vegetation.

The current ministerial criteria for Lake Jandabup are as follows:

• 44.3 m AHD absolute summer minimum

• 44.2 m AHD absolute spring minimum peak

• 44.7 m AHD preferred minimum spring peak

• Water level only allows between preferred and absolute minimum at a rate of two in every six years.

Lake Jandabup has been compliant with absolute spring peak criterion but non-compliant with absolute summer minimum criterion (DoW 2017a). The Water Corporation have artificially supplemented this lake with groundwater from the Leederville Aquifer since 1990 to meet the absolute spring peak water level criterion and to prevent the lake from acidifying. Lake Jandabup has also been supplemented using Superficial Aquifer water.

3.1.3.2 Lake Mariginiup The water regime management objectives for Lake Mariginiup include to:

• Maintain the existing areas of fringing sedge vegetation.

• Maintain deep, permanent water as a bird habitat and drought refuge and to protect aquatic invertebrates and fish dependent on permanent water.

• Maintain the existing extent of Baumea articulate fringe between typha stands and the fringing woodland.

• Provide some area of wading bird habitat at the end of summer, although it is recognised that this is limited by the shape of the wetland.

• Maintenance of the current extent of wading bird habitat.

The current ministerial criteria for Lake Mariginiup are:

• 42.1 m AHD spring preferred minimum peak

• 41.5 m AHD spring absolute minimum peak

• Water level only allowed between preferred and absolute spring peak minimum at a rate of two in every six years.

Lake Mariginiup has been non-compliant with absolute spring peak criterion. Water levels have not reached the preferred spring peak since 1994 and have not reached the absolute minimum spring peak since 2005 (DoW 2017a).

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3.1.3.3 MT3S While not a wetland, MT3S is a monitoring bore located north-west of Lake Jandabup. It was selected as a criterion bore to monitor water levels in an area of susceptible native vegetation (Pinjar vegetation complex in the Bush Forever Site 324) that may be affected by abstraction. The current ministerial criterion for the bore is:

• 43.0 m AHD absolute summer minimum.

Water levels measured in MT3S have been compliant with absolute summer minimum criterion. Water levels have generally declined since 1992 but appear to be stabilised since 2011 (DoW 2017a).

3.1.4 Surface water level monitoring As identified in Section 2.6.1, there are no streams, creeks or major drains located within the site. A search of the DWER Water Information Reporting database (DWER 2017b) identified that eight wetlands have had water levels monitored, as well as six forest sumps within the EWDSP area. Hydrographs of five of the main wetlands are presented in Appendix A, with Lake Adams, Lake Mariginiup, Jandabup Lake and Gnangara Lake all showing declining water levels.

3.1.5 Surface water quality A search of the DWER Water Information Reporting database (DWER 2017b) identified seven sites within the EWDSP that have surface water quality data recorded. These sites are in Lake Mariginiup, Jandabup Lake, Lake Adams, and Gnangara Lake (Figure 8).

TN and ammonium concentrations were found to be elevated and some phosphorus concentrations above the ANZECC (2000) guidelines for slightly disturbed wetland ecosystems. Gnangara Lake (AWRC no. 6162591) and Lake Mariginiup (AWRC no. 6164637) had TN and NH3-N concentrations that far exceeded the guideline values.

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EWDSPEWDSP

Figure 8: Wetland mapping and PRAMS inundation areas

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4 PRE-DEVELOPMENT GROUNDWATER SETTING The site is located predominantly in the Wanneroo Groundwater Management area, covering five management subareas for the Superficial Aquifer including Mariginiup, Lake Gnangara, Wanneroo Wellfield, Adams, and Jandabup. A section of the site in the south-east is part of the Mirrabooka Groundwater Management Area, in the Plantation subarea.

4.1 Groundwater system The site is located on the Gnangara Groundwater System which extends from Moore River and Gingin Brook in the north to the Swan River in the south (DoW 2017a). The Gnangara system incorporates the Superficial, Mirrabooka, Leederville and Yarragadee aquifer. The Gnangara groundwater system have an elevated water table and is located in the Perth Basin. Many groundwater management subareas of the Gnangara groundwater system are currently over-allocated and have shown water level decline over the last ~40 years due to groundwater abstraction, reduction in rainfall and pine plantations which limit recharge to the aquifer.

4.2 Aquifers The aquifers underlying the site include the Perth-Superficial Swan aquifer, Leederville partially confined aquifer and the Perth Yarragadee North confined aquifer. The Perth-Superficial Swan aquifer is in the Superficial Formation and is mainly fresh. The thickness of the superficial aquifer ranges from approximately 50 to 60 m.

4.3 Groundwater levels A search of the DWER’s Water Information Reporting Database identified that there are 60 bores across the site that are screened in the Superficial aquifer that have suitable long-term temporal datasets to determine maximum groundwater levels (MGLs) and average annual maximum groundwater levels (AAMGLs) within the site.

Historic MGL and AAMGL contours were produced from monitoring data using the kriging method in a computer program. MGL contours were calculated to range from 55 m AHD in the north east to 36 m AHD in the west, with the gradient increasing to the west (Figure 9). Contours created for the AAMGL determined levels to range from 52 m AHD in the north-east to 36 m AHD in the west of the site (Figure 10).

In more recent years water levels in the Superficial Aquifer have risen as a result of higher rainfall since 2010, land use changes and reduced public water supply abstraction. Groundwater levels (known as ‘potentiometric head’) in both the Leederville Aquifer and the Yarragadee Aquifer have declined at a rate of 1m and 2 to 6m per year respectively.

4.4 Groundwater flow Hydrographs from DWER observational bores indicate that groundwater typically flows in a west/south direction.

RPS (2009) previously reported that the hydraulic gradient of the Superficial aquifer in the area is approximately 0.0025, with a steeper gradient directly east of Lake Joondalup (RPS 2009). The hydraulic conductivity of Bassendean Sands has been estimated to be on average 15 m/day (Davidson and Wu 2006), while the Spearwood Sand has an estimated hydraulic conductivity of 7 to 15 m/day.

4.5 Groundwater quality Overall, since 1995 the salinity of the groundwater of the Gnangara groundwater area has remained relatively constant, with groundwater salinity typically increasing closer to the coast (DoW 2009b). Salinity in the Superficial Aquifer ranges from fresh (<250 mg/L TDS) to marginal (1000 mg/L TDS) (DoW 2018a). Elevated nutrient concentrations are common due to horticultural practices in the area, with nitrate concentrations frequently exceeding 20 mg/L and phosphorus concentrations frequently above 0.5 mg/L (DoW 2009b).

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Figure 9: Historic MGL contours

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Figure 10: Historic AAMGL contours

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5 DRAINAGE MANAGEMENT PRINCIPLES

5.1 Drainage principles The pre-development surface water hydrology of the site is dominated by groundwater recharge through predominantly sandy soils and evapotranspiration. The stormwater drainage strategy will be consistent with the overarching regional water management strategy (Essential Environmental 2016a) which encompasses the site.

This IWMF was prepared with reference to several guidance documents including Decision process for stormwater management in Western Australia (DWER 2017a) which is a part of the Stormwater Management Manual (DoW 2004-2007), and the Gnangara Sustainability Strategy (DoW et al. 2009) which identified that civil engineering solutions should direct all stormwater runoff from hard surfaces away from P1 and P2 areas or wellhead protection zones (WHPZs) if the area is rezoned to P3*.

Integrated urban water management recognises that the urban water cycle should be managed as a single system and water supply, stormwater, wastewater, flooding, water quality and wetlands are interconnected (WAPC 2006). Water sensitive urban design principles to be adopted across the site include:

• Protection of natural systems

• Integration of stormwater treatment into the landscape to maximise the visual and recreational amenity of the development

• Protection of water quality

• Maintaining peak flows to pre-development rates if discharging off site

• Adding value to the development.

The site will effectively manage stormwater quantity and quality generated from minor and major events, incorporating best practice WSUD principles. The stormwater management criteria outlined in the following sections will inform the detailed drainage design for the EWDSP throughout the various stages of development.

5.2 Drainage strategy overview All stormwater will be treated and managed to ensure that there are no detrimental impacts to wetlands and public drinking water sources. Best management practice WSUD is to be implemented across the site to ensure groundwater resources including public drinking water sources and the wetland receiving environments will be protected. Stormwater management will also ensure adequate flood protection is provided for the development.

5.2.1 Minor drainage system including treatment of the first 15 mm Stormwater runoff from constructed impervious surfaces generated by the first 15 mm of rainfall will be managed, retained/detained and treated (if required) — at-source as much as practical. At-source means that lot runoff is managed within lots and road runoff is managed within road reserves and the stormwater has not entered a piped or lined channel conveyance system.

Where site conditions do not allow for the full runoff to be managed at-source, the runoff will be managed as much as practical at-source, subject to the pre-development hydrology. The remaining runoff will be conveyed from the lot or road reserve via overland flow wherever practical. Impervious areas which cannot be treated at-source will require treatment systems such as bioretention basins, raingardens or equivalent treatment systems. The standard requirement is that the total bioretention area is sized to a minimum 2% of the total connected impervious catchment area. Bioretention areas are to also be required to be vegetated and designed to accommodate the 1 EY (15 mm) rainfall event.

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5.2.1.1 Lot drainage The general principle for managing lot generated runoff from the first 15 mm of rainfall is for this stormwater to be retained and infiltrated within the lot boundary using soakwells. Due to the predominantly sandy soils present in-situ and as any fill brought onto the site will also be sand, this will enable the use of soakwells. As per CoW requirements, soakwell sizing can be based on 1cum per 60m² of constructed impervious surface runoff area.

For lots with low clearance to groundwater, subsoil drainage may also be used to ensure sufficient clearance to groundwater is maintained.

5.2.1.2 Road reserve The road drainage network will ensure that roads will remain passable in the 20% AEP event. This is proposed to be achieved through pipeless systems. The pipeless systems can include rain gardens, tree pits as well as roadside and median swales. Utilisation of the pipeless systems will allow for less POS to be restricted by drainage in the minor events. If there are situations where a pipeless system cannot be utilised and the runoff enters a closed pipe network, stormwater at the outlet will be required to be treated through biofiltration areas.

Biofiltration areas will be sized to at least 2% of the equivalent connected impervious catchment area. Erosion and scour protection mechanisms including rock pitching will be put in place at outlet points to biofiltration areas. Where possible, biofiltration areas will be located “offline” so that they are not inundated during flooding. Biofiltration areas, along with other formal drainage infrastructure, will be required to be located outside of wetlands and their associated buffers.

Biofiltration areas will be planted with reeds, sedges and other plant species that will be selected based on their ability to uptake nutrients. The plant species will be selected from Vegetation guidelines for stormwater biofilters in the south-west of Western Australia (Monash University 2014). The biofiltration areas will be designed in compliance with the Adoption Guidelines for Stormwater Biofiltration Systems (Payne et al. 2015). The biofiltration areas will be underlain by a series of layers including filter media layer, a transition layer and drainage layer, which allows stormwater runoff to be retained in the media, allowing sufficient time for treatment and uptake/retention of nutrients to occur.

5.2.2 Major drainage system For all rainfall events greater than the 20% AEP event up to and including the 1% AEP event, any runoff that does not infiltrate into the sandy soils will be conveyed via overland flow paths either via the road network or via drainage swales and living streams. This stormwater will be directed to either wetlands that have been pre-determined that they can receive flood waters or basins. Road centrelines should be designed to be considerably lower than the surrounding lots such that stormwater moving along the road can overtop without risk of flood damage to properties.

Events that exceed the capacity of bio-filtration areas (i.e. greater than the first 15 mm of runoff) will overtop to flood detention areas. Where biofiltration areas are located adjacent to wetland buffers, it is proposed that stormwater will overflow as sheet flow over a vegetated surface towards the wetland buffer and ultimately the wetland. Further hydrological modelling work and consultation with the Department of Biodiversity, Conservation and Attractions (DBCA) will be required prior to designing a stormwater system that will utilise a wetland for flood storage.

5.3 Drainage characteristics

5.3.1 Post-development catchments A district-level hydrological assessment has been conducted on the EWDSP area. This assessment determined that following future earthworks design: the structure plan will be split into the following three catchment categories, each of which will have distinct stormwater discharge points and storage types:

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• Catchments draining to CCWs and REWs: runoff generated in these catchments discharges into the wetlands which provide flood storage. If a wetland is deemed to have insufficient capacity to store runoff in the 1% AEP event, then additional flood storage areas will need to be provided.

• Catchments draining internally to non CCW/REW areas: these will require an infiltration basin sized to take runoff up to the 1% AEP storm event.

• Catchments draining off site: upon development, these will require detention basins to ensure post development discharge is not greater than predevelopment flow rates.

The above catchments are shown in Figure 11.

5.3.2 Flood storage areas in catchments with basins The flood storage volumes for catchments draining to internal basins should be sufficient to store the critical 1% AEP event while taking account the infiltration capacity of the basin. If not inundated with groundwater, flood storage areas can be vegetated and integrated into useable POS. Final flood storage areas required will depend on factors such as final catchment areas, number and locations of drainage areas, achievable infiltration capacity and density of development.

5.3.3 Flood storage in catchments with REWs or CCWs An initial assessment of the capacity of the REWs and CCWs to receive stormwater runoff during the 1% AEP event was conducted to support the IWMF. The volume of runoff and the corresponding rise in water levels within the wetlands and their buffers was taken into consideration. The overall rainfall storm event increase in wetland water levels was found to be less than 0.5 m. This is largely due to the land use types and total area of each catchment relative to the size of the wetland and its buffer. Where the maximum water depth within a wetland exceeds 1 m, additional flood storage within these catchments would need to be provided in the form of storage basins. At least 0.5 m of clearance from the 1% AEP top water level to lot level will be required to minimise risk of flooding given that there is no discharge point from the wetlands.

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Figure 11: Probable post-development catchment types

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5.4 Future Local Structure Plan Areas and Future Water Planning The East Wanneroo DSP aims to remove the risk of inconsistent and incoherent drainage planning and associated arterial flood management at the DSP stage. The East Wanneroo DSP area does not have an arterial drainage system and its dominant hydrological process is infiltration of rainfall events within many isolated drainage catchments. The DSP area contains 94 pre-development hydrologic catchments where rainfall run-off drains internally, mostly grading to wetlands in the low points of each catchment. A hydrological assessment of likely post-development drainage design outcome rationalises the 94 pre-development catchments to a smaller and more manageable number of 66 post-development catchments. Future Local Structure Plan (LSP) areas have been determined considering these post-development catchments.

To remove the risk of inconsistent drainage management, the DSP proposes to implement the principle that all surface and groundwater encountered within individual local structure plan (LSP) areas (referred to in the DSP as ‘precincts’) must be managed within that LSP area. The hydrological assessment indicates that principle can be made to work in practice as the post-development hydrological catchments will be contained with these pre-determined precincts or LSP areas.

The DSP area is to be divided into 27 precincts for which individual local structure plans and LWMSs will need to be prepared and approved before subdivision can proceed. The only exception to this rule is for Precincts 17, 26 and 27 for which the DSP proposes no change and therefore no local structure plan will be necessary. Figure 12 shows the precinct boundaries proposed for the DSP. Figure 13 shows the post development catchments identified during the hydrological assessment and are overlain onto the proposed precincts and indicates the direction of overland flow and approximate infiltration basin locations where needed. The figures were created using the guiding principles set out below: • All surface and groundwater arising within a precinct must be managed within that precinct. Run-off

from one precinct will not be allowed to drain through to an adjacent precinct. An exception to this may be permitted in locations where there are designated green areas which can be used to convey flows from a precinct boundary to a Conservation Category Wetland (CCW) or Resource Enhancement Wetland (REW).

• All existing CCWs and REWs will be maintained and runoff from an LSP area will be discharged to these where they are located entirely or partially within an LSP area, or on the boundary of an adjacent LSP area.

• There are at least 15 catchments without CCWs or REWs which drain internally through infiltration. LSPs will cater for these using infiltration basins which mimic predevelopment conditions.

Initial drainage considerations for each of the proposed precincts (LSP areas) is provided in Table 1 below. The DWMS will provide further details on the drainage considerations for each precinct, which will have to be given effect to though the LSP (and corresponding LWMS) prepared for each area. It will also be recommended that more detailed hydrological assessment and water modelling will be required at the LSP stage through the LWMS when more detailed design inputs needed will be available. By utilising the principles associated with the “self-management” of drainage for individual precinct areas, the risk of inconsistent or incoherent drainage design and associated arterial flood management as subdivision and development proceeds across the DSP area is removed.

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Figure 12 Post-development precinct and catchment boundaries

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Figure 13 Post-development hydrological catchment and broad drainage management areas

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Table 1: Water Management and Drainage Proposed for East Wanneroo DSP area

Water management and drainage proposed for East Wanneroo DSP area Precinct Land use context Drainage considerations P1 Residential area with access to

regional reserves and wetland system

Contains three catchments. Runoff from this precinct is not conveyed through other precincts. The majority of runoff generated within this precinct discharges into a CCW located in the east of the precinct. The remaining precinct area drains into a detention basin in the west of the precinct prior to discharge offsite or into an infiltration basin located in the south of the precinct.

P2 Low Density Residential area with access to regional reserves and wetland system

Contains two catchments. Runoff from this precinct is not conveyed through other precincts. Large lot size proposed (minimum 0.25 ha) means all stormwater will be infiltrated on site. Much of this precinct discharges into a CCW located partially within the precinct in the east. A very small portion of the precinct drains into a detention basin located in the south west of the precinct prior to discharge offsite.

P3 Residential area. Access is available to regional reserves; however wetland system is some distance to the precinct boundary.

Contains two catchments with infiltration basins. Potentially these basins will overtop into adjacent regional open space areas in large storm events.

P4 Residential area Contains two catchments. Runoff from this precinct is not conveyed through other precincts. The majority of the precinct discharges to an infiltration basin located within the precinct. A portion of the precinct in the west drains into a detention basin located within the precinct prior to discharge offsite.

P5 Residential area bisected by road and rail corridor. Eastern section has access to regional reserves and wetland systems. Includes area of public purpose reserve (water supply tanks).

Contains five catchments. Runoff from this precinct is not conveyed through other precincts. The majority of runoff generated within this precinct drains into infiltration basins or discharges into a CCW located partially within the precinct in the east. The precinct also requires a detention basin in the west prior to discharge offsite.

P6 Residential area including large park and recreation area in the centre. Access to Lake Mariginiup is available through reserve 46711.

Contains three catchments. The majority of the precinct drains to the north and through Precinct 7 within a regional open space corridor prior to discharging into a CCW located in Precinct 7. The remaining precinct area drains to the east into a CCW located partially within the precinct.

P7 Residential area bisected by road and rail corridor. Access to regional reserves and wetland systems is available both sides of the corridor.

Contains two catchments. Runoff from this precinct is not conveyed through other precincts. The majority of the precinct drains into a central CCW located partially within the precinct. The remaining precinct area in the east drains into a CCW located paritially within the precinct. The precinct also conveys some runoff from Precinct 6 which is drained via the regional open space corridor strip into the CCW.

P8 Residential area with access to regional reserves and wetland system

Contains seven catchments. Runoff from this precinct is not conveyed through other precincts. The majority of the precinct drains into MUWs and CCWs which are all located partially or completely within the precinct or on the precinct boundary. A relatively small portion of the precinct drains into a detention basin located in the west of the precinct prior to discharge offsite into a MUW. Similarly, a relatively small portion of the precinct drains northward directly to Lake Adams.

P9 Low density residential area with access to regional reserves and wetland system

Contains two catchments. Runoff from this precinct is not conveyed through other precincts. The majority of the precinct drains into a central CCW located within the precinct. A relatively small portion of the precinct drains into a detention basin in located in the west of the precinct prior to discharge offsite.

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Water management and drainage proposed for East Wanneroo DSP area Precinct Land use context Drainage considerations P10 Residential area affected by mining

tenements. Topography may be subject to change. Precinct bisected by road and rail reserve.

Contains nine catchments. Runoff from this precinct is not conveyed through other precincts. The precinct drains into infiltration or detention basins located within the precinct; or MUWs and CCWs which are all located partially or completely within the precinct or on the precinct boundary.

P11 Residential area with access to regional reserves and wetland systems

Contains five catchments. Runoff from this precinct is not conveyed through other precincts. The precinct drains into infiltration basins located within the precinct; or MUWs and CCWs which are all located partially or completely within the precinct or on the precinct boundary.

P12 District Activity Centre including a City park. Higher density development expected.

Contains seven catchments. Runoff from this precinct is not conveyed through other precincts. Runoff drains into infiltration or detention basins, a MUW or CCWs. The infiltration and detention basin, MUW and one of the CCWs are located within the precinct. The remaining two CCWs are located on the precinct boundary.

P13 Residential area with access to regional reserves and wetland systems.

Contains eight catchments. Runoff from this precinct is not conveyed through other precincts. The majority of the precinct drains to a CCW located partially in the precinct in the north. The remaining precinct area drains into infiltration basins or a MUW located within the precinct.

P14 Lake Jandabup and associated regional reserves.

Contains one catchment which drains internally into a CCW which is located partially within the precinct. Runoff from this precinct is not conveyed through other precincts.

P15 Neighbourhood Centre precinct. Higher residential density expected. 50ha regional sporting facility proposed in eastern part of precinct. Precinct bisected by road and rail reserve.

Contains five catchments. Runoff from this precinct is not conveyed through other precincts. The precinct drains into MUWs and CCWs which are all located partially or completely within the precinct or on the precinct boundary.

P16 Residential area. Precinct bisected by road and rail reserve.

Contains five catchments. Runoff from this precinct is not conveyed through other precincts. The eastern portion of the precinct drains into infiltration basins located within the precinct. The western portion of the precinct drains into three CCWs which are located within the precinct and on the precinct boundary.

P17 Existing Industrial and Bush Forever area subject to approved Local Structure Plan

NA

P18 Large Format Service Commercial Area with access to regional reserves and wetland systems.

Contains two catchments. Runoff from this precinct is not conveyed through other precincts. The eastern portion of the precinct drains into a CCW on the precinct boundary. Runoff in the south western portion of the precinct drains into an infiltration basin located within the precinct.

P19 Lake Gnangara and associated reserves. No changes proposed to Rural zoned lots in south east corner.

Contains three catchments. Runoff from this precinct is not conveyed through other precincts. The majority of the precinct drains into a large CCW located within the precinct. If the land remains undeveloped no detention or infiltration basins will be required.

P20 Residential area with access to regional reserves and wetland systems.

Contains five catchments. The majority of the precinct drains into infiltration basins or a MUW located within the precinct. A relatively small portion of the precinct drains into rural open space located within Precinct 19 prior to discharging into a CCW.

P21 Rural area and State Forest. Subject to Bush Forever. No land use change.

NA

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Water management and drainage proposed for East Wanneroo DSP area Precinct Land use context Drainage considerations P22 Residential area with access to

regional reserves and wetland system.

Contains three catchments. Runoff from this precinct is not conveyed through other precincts. The majority of the precint discharges into CCWs and a MUW which are located partially witihn the precinct. A portion of the precinct in the south drains into an infiltration basins located within the precinct.

P23 Long term strategic industrial area affected by mining tenement. Topography may be subject to change.

Contains nine catchments. Runoff from this precinct is not conveyed through other precincts. The precinct drains into infiltration basins and a MUW located within the precinct, and also into a CCWs which located partially within the precinct.

P24 Long term strategic industrial area affected by mining tenement. Topography may be subject to change.

NA

P25 Long term planning investigation area.

Contains three catchments. Runoff from this precinct is not conveyed through other precincts. Most of the precinct drains south and if developed may require a detention basin prior to discharge offsite. A relatively small portion of the precinct drains into a MUW located within the precinct.

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6 GROUNDWATER MANAGEMENT As identified in Section 4, both historic mapped groundwater contours and PRAMS future groundwater levels have identified areas that have a shallow clearance to groundwater. However, it should also be noted that groundwater resources in the Gnangara groundwater area have been in decline over the past thirty years. This decline in groundwater levels is a potential threat to groundwater dependent ecosystems.

6.1 Predicted future groundwater level rise Changing the land use within the site will also change the rate of recharge to groundwater. Sharma et al. (1991) and Silberstein et al. (2004) estimated that approximately 40% of rainfall reaches the Superficial aquifer under market garden land use which is currently the predominant land use within the Site. While recharge in urban areas has been reported to range from 48-60% of rainfall (Silberstein et al. 2004) to 27% (Appleyard 1995).

Recharge for pine plantations has been reported as -40% (Silberstein et al. 2004), 15% (Farrington and Bartle 1991) and no recharge (Sharma and Pionke 1984). A significant portion of the pines within the state forest have already been removed and there is a State agreement which stipulate that the remaining forests will be harvested over the next 20 years. This reduction in pines, abstraction for public drinking water supply as well as licenced private abstractions is likely to cause increase groundwater recharge rates and groundwater levels.

With the expected increase in recharge from the removal of the pines, lower evapotranspiration due to urbanisation and reduced abstraction from licenced groundwater users, groundwater levels in the Superficial aquifer are predicted to rise.

The PRAMS model undertaken by DWER to predict future groundwater levels by 2030 uses a future medium climate scenario (DoW 2015), and takes into consideration future urban and industrial development, pine management and a reduction to licenced abstractions (particularly private licenced abstraction and garden bore abstraction) scheduled to occur to 2030. The PRAMS modelling results will be used by DWER to inform the next Gnangara groundwater allocation plan.

The PRAMS model shows groundwater levels in the Superficial aquifer are estimated to increase by 3 to 4 m over most of the site (Figure 14), with a level of 35 to 60 m AHD by 2030 (Figure 15). This will result in a significant proportion of the existing site being inundated with groundwater. The majority of inundated areas will occur where there are geomorphic wetlands. As the PRAMS model is a relatively coarse regional scale model with a 500 m x 500 m grid size it should only be used as a guide. The PRAMS modelling also assumes full build-out of the development by 2030. However, it is likely that full build-out of the DSP will occur over a much longer timeframe, the working assumption being that the DSP will develop generally from west to east over the next 30 to 40 years. Therefore, groundwater level rise predicted by the PRAMS modelling will be more gradual.

Groundwater levels will need to be assessed in more detail for future development and this is discussed more in Section 6.3 and 6.4.

6.2 Implications on fill and infrastructure due to predicted groundwater level rise

6.2.1 Impact on infrastructure The need for separation to groundwater is primarily driven by the design requirements of pavements and drainage infrastructure. Pavements such as roads, car parks and driveways need sufficient separation from groundwater in order to prevent failure and to be able to function as stormwater conveyance systems. Similarly, drainage services which are typically located underneath pavements require sufficient spacing from both the pavement above, and the groundwater underneath. The coupling of these two requirements generally results in the need for a minimum of 0.7m separation between the road pavement and the peak groundwater levels.

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Due to the potential impacts of future groundwater levels on infrastructure, further groundwater level modelling, earthworks and subsoil design will need to be undertaken at the LWMS and UWMP stages in order to ensure sufficient clearance from predicted groundwater levels is achieved.

6.2.2 Fill and subsoil drainage If groundwater levels are not controlled a substantial amount of fill will be required across the site to create sufficient clearance from currently predicted future groundwater levels (based on PRAMS). Due to increasing scarcity and cost of clean sand fill from the Swan Coastal Plain, the minimisation of fill through optimising earthwork design coupled with a strategic groundwater control mechanism to control rising groundwater levels will be required. This mechanism needs to be assessed at a district level and implemented uniformly across the low-lying areas of the DSP.

6.3 Controlling groundwater level rise The PRAMS model estimates groundwater levels in the Superficial aquifer will increase by 3 to 4 m over most of the site. This modelling indicates that without controlling future predicted groundwater level rise significant proportions of the central and eastern DSP area will be inundated or have low clearance from groundwater.

Based on current development and construction practices and standards, lot levels are required to have at least 1.2 m to 1.5 m clearance above groundwater level. Given this context, a large portion of the site will require significant amounts of imported sand fill if future groundwater levels are uncontrolled and expected to be similar to PRAMS groundwater levels.

Controlling groundwater level rise post-development will significantly reduce development costs and lead to more affordable housing, as well as leading to a more environmentally sustainable outcome as the unnecessary use of basic raw materials could be reduced.

A critical design criterion that will need to be determined and presented in a future District Water Management Strategy (DWMS) will be a controlled groundwater level across the DSP area. Future groundwater modelling and assessment and the DWMS will need to assess historic, current and future predicted groundwater levels and provide guidance on a future controlled groundwater level across the DSP area. This future controlled groundwater level will need to be set based on Water resource considerations when controlling groundwater levels in urban development (DWER 2013) and giving considerations to the water dependent ecosystems and wetland and lakes with Ministerial water level criteria.

6.4 Additional groundwater modelling The current estimates of groundwater level rise post-development are based on DWER’s current PRAMS groundwater modelling (PRAMS). However, the PRAMS model is a coarse model used to determine groundwater allocation and groundwater resource management across a large scale (i.e. most of the Swan Coastal Plain). It is not suitable to be used to make engineering design decision that requires more site specific and granular assessment. It is acknowledged that additional groundwater assessments and the determination of a proposed controlled groundwater level criteria needs to be determined at a DSP scale to provide certainty to the local structure plans.

DPLH is, therefore, intending to conduct a more detailed groundwater modelling exercise with the aim of establishing an environmentally acceptable maximum groundwater level across the DSP area in consultation with the key stakeholders. The proposed district scale groundwater model would use the input parameters and future predicted climate change scenarios used by DWER to conduct the PRAMS modelling. The proposed district scale groundwater model will be more detailed (reduced cell sizes) than the PRAMS and provide specific control groundwater levels for specific LSP areas. The model will also better quantify the change in water balance due to the change in land use and assess the options to control groundwater levels post-development. The intention is to conduct this groundwater modelling exercise and present the results in the finalised DWMS.

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6.5 Groundwater management principles

6.5.1 Management principles - controlled groundwater levels Groundwater levels will likely be required to be controlled through the installation of subsoil drains in areas where there is currently a low clearance to groundwater or where there is predicted to be a low clearance to groundwater based on PRAMS modelling undertaken by DWER. The subsoil drainage system will need to be designed so that it does not cause any drawdown and reduction in water levels of CCWs or REWs.

At the LWMS stage of development appropriate controlled groundwater levels will be required to be defined with considerations of groundwater dependent ecosystems. The controlled groundwater level and subsoil drainage will need to be designed in accordance with the IPWEA (2016) Draft Specification Separation Distances for Groundwater Controlled Urban Development as well as the DoW (2013) Water resource considerations when controlling groundwater levels in urban developments.

Modelling undertaken should then be further refined at the UWMP stage of development to incorporate the designed urban form, site specific geotechnical conditions and the designed drainage system (IPWEA 2016).

6.5.2 Management principles - groundwater quality Groundwater quality will be managed through the use of WSUD systems which will treat the first 15mm runoff from impervious areas prior to infiltration. The infiltration of the treated stormwater will further remove pollutant loads through mechanical filtration as well as soil adsorption (soils have a natural phosphorus retention capacity). The use of amended soils beneath bioretention basins and raingardens may also be implemented to removed pollutants prior to entering groundwater.

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Figure 14: Change in groundwater levels (m) of the Superficial Aquifer by 2030, modelled using PRAMS

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Figure 15: Groundwater levels (m AHD) of the Superficial Aquifer by 2030, modelled using PRAMS

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7 WATER DEPENDENT ECOSYSTEMS MANAGEMENT PRINCIPLES

The water dependant ecosystem (WDE) management of the site as part of future development will involve the protection and enhancement of onsite ecological systems and the treatment of water in order to protect downstream ecological systems. There are various impacts that new developments may have on WDEs. These include the encroachment, discharge of pollutants, erosion, scouring due to high flow rates and groundwater subsidence.

The protection of WDEs will involve the implementation of buffers around existing onsite ecological systems (e.g. wetlands), controlling groundwater levels and implementing WSUD systems to treat stormwater runoff.

7.1 Protection of on-site wetlands As previously identified in Section 3.1, the site contains a significant number of CCWs and REWs, with the three largest ones being Lake Mariginiup, Jandabup Lake and Gnangara Lake. In order to protect the wetlands from encroachment and to protect wetland function, appropriate buffers or separation distances will be implemented. This is typically 50 m buffers for CCWs and 30 m buffers for REWs.

Due to the large number of CCWs and REWs which will be retained, all land development will be required to ensure these wetlands are protected with regards to quality and quantity. Development will need to consider wetland water levels, groundwater levels, surface water discharge and stormwater water treatment.

Developments surrounding significant wetlands will require the preparation of wetland management plans to be undertaken at the LSP and subdivision stage.

7.2 Water sensitive urban design WSUD will play a major role in reducing the pollutant loads to both internal and external WDEs. WSUD may be implemented through various means, the most common being the use of bioretention basins, raingardens, tree pits and swales. The WSUD systems are typically designed to treat the first 15 mm of rainfall and are incorporated into the drainage system in a series formation, known as a treatment train.

Each WSUD system within a treatment train plays an important role in removing pollutants. The treatment train typically begins with systems that remove gross pollutants such as drainage pit grates and litter traps. Alternatively, roads may drain directly to vegetated swales or raingardens which can remove gross pollutants, suspended solids and nutrients.

At the final stages of the treatment train, bioretention systems such as bioretention basins or constructed wetlands are typically utilised to provide nutrient removal trough plant absorption, the settling of suspended solids and filtration through the use of filter media at the base of the basins. Amended soil with a phosphorus retention index (PRI) of at least 10 will be used in any biofiltration areas to assist in phosphorus retention in the soil profile. At the end of the treatment train, the treated water will then be discharged to the external drainage systems, wetlands or groundwater, depending on the catchment.

The following list includes typically used WSUD systems within Western Australia which may be incorporated at the site:

Primary treatment:

• Litter and sediment traps

• Pervious paving

• Buffer strips.

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Secondary treatment:

• Swales

• Living streams

• Raingardens.

Tertiary treatment (end of line):

• Bioretention basins

• Infiltration basins

• Constructed wetlands.

(Source: DWER 2019)

Figure 16: Typical vegetated swale (left) and bioretention basin (right)

In areas where subsoil drainage is required to provide a controlled groundwater level to ensure enough clearance from the finished lot level, the subsoils will outlet to a biofiltration treatment area to provide treatment. These treatment systems will be installed “off-line”.

Where stormwater is discharged offsite, peak flow rates will also be managed through the use of attenuation measures (e.g. basins) to ensure post-development flow is not greater than predevelopment flow. This will ensure that ecologically sensitive areas are not impacted by additional flows or inundation. This will be the case for both minor and major rainfall events.

UNDO modelling may need to be undertaken at the LWMS and UWMP stages of the development in order to ensure that nutrient loading (from surface runoff) to downstream environments is acceptable.

7.2.1 WDE protection through vegetation Vegetation will play a major role in the water quality treatment train. As previously mentioned in Section 5.2.1, bio-filtration areas created to treat runoff from the 15 mm rainfall event will be planted with reeds, sedges and other plant species that will be selected based on their ability to uptake nutrients and will be selected from Vegetated guidelines for stormwater biofilters in the south-west of Western Australia (Monash University 2014).

Vegetation will be used throughout the development for water quality treatment including for nutrient uptake and reducing stormwater velocities, reducing erosion and sediment transport. This will be achieved through planting in biofiltration areas, living streams, swales and tree pits.

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7.2.1 Other water quality control measures Other water quality treatment measures will include (but not limited to):

• Rock pitching for scour protection at outlet points in the drainage network

• Using water for dust suppression during construction activities to minimise erosion

• Street sweeping to remove sediments and other pollutants from the road before they enter the drainage network.

7.3 Mosquito management Mosquitoes and chironomid midges will be managed by ensuring that the drainage systems do not hold standing water over the spring and summer months, with all bioretention systems being designed to be emptied through outflows or infiltration within 96 hours, in accordance with standard practice. Mosquito and midge prevention measures will be outlined in more detail as part of the detailed design of any proposed drainage infrastructure.

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8 IMPLEMENTATION AND GOVERNANCE PLAN The water management principles and strategies outlined in this report will be implemented through a range of governance frameworks which will require the production of a range of studies and reports throughout the various stages of the development process.

DWER with assistance from the DPLH, and in consultation with relevant stakeholders will guide the requirements for the water management framework as the planning of the development progresses.

The following sections provide guidance to the various requirements each stakeholder may need to accomplish as part of the staged development process. The list of requirements is provided to give direction and is not necessarily exhaustive.

8.1 BUWM framework The Better Urban Water Management (BUWM) framework (WAPC 2008) establishes a requirement for a DWMS to be prepared in support of a region scheme amendment or DSP. The objective of the DWMS is to demonstrate that the area can support future development in terms of water supply planning, flood mitigation, drainage management and water quality protection.

An LWMS is typically undertaken at the Local Structure Plan (LSP) stage to support and facilitate approval of the LSP. The LWMS details the integrated water management strategies that will be implemented and demonstrates that the land can facilitate development whilst achieving sustainable and environmental outcomes.

An Urban Water Management Plan (UWMP) is typically required at subdivision stage, its purpose being to support subdivision approval. A UWMP provides the detail to the design proposed in the LWMS.

8.2 Individual developer requirements • Complete LWMS and UWMP for the relevant development area

• Undertake a detailed drainage assessment as part of the above plans and in accordance with local overarching strategies and policies stipulated by relevant local authorities

• Determine the localised groundwater levels and quality through a monitoring program

• Refine the DSP scale model and assess the groundwater regime, taking into account climate change scenario and land use change.

• Model and assess flooding scenarios to inform and assess drainage easement widths, flood storage requirements and critical flood levels across each LSP area.

• Determine the localised surface water quality through a monitoring program

• Geotechnical assessment and acid sulfate soil investigation

• Produce and implement construction and sedimentation control reports

• Implement all servicing and drainage infrastructure in accordance with the overarching strategies set out in the LWMS and UWMP

• Apply appropriate fill and groundwater control structures where required

• Implement appropriate street-side and public open space landscaping, and planting of vegetation within basins, swales and foreshore areas in accordance with local council requirements

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• Provide lot owners with information regarding waterwise practices relevant to the development landscape and servicing strategy

• Undertake post-development monitoring as required by local authorities or as stipulated in the LWMS or UWMP.

8.3 DWER requirements • Assist with direction for the provision of water services.

• Review and approve the DWMS and LWMSs, and provide advice on corresponding UWMPs.

8.4 DPLH requirements • Provide necessary layouts and guidance on land use planning for the subject land.

• Finalise the DWMS including completing DSP scale groundwater modelling to determine future proposed controlled groundwater levels

• Provide assistance where possible with developing suitable water servicing options.

8.5 City of Wanneroo requirements • Maintain the internal stormwater system, after a mutually agreed time period after construction or as

outlined in the governance framework.

• Encourage the use of Waterwise and nutrient wise practices for lot owners.

• Maintain water management strategies within the POS and MUC area after handover.

• Undertake compliance control at lot development stage.

• Develop and implement a development contribution plan.

8.6 Service provider requirements • Develop workable service strategies that covers one or more water related service (e.g. potable, non-

potable, wastewater, etc.).

• Have the Servicing Strategies approved through the Economic Regulatory Authority in line with the Water Services Act.

• Implement and run the required services as required by legislation.

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9 ACID SULFATE SOILS The WAPC and former Department of Environment Regulation (DER) produced regional ASS mapping, which is based on regional surface geology mapping and provides a broad scale indication of the risk of occurrence of ASS. ASS risk mapping (Figure 17) has identified that the majority of the eastern third of the site has a moderate to low risk of ASS occurring within 3 m of the natural soil surface, with sections of the site, typically associated with wetlands and peaty clay soils, classified as having a high to moderate risk of ASS occurring within 3 m of the natural soil surface. The western portion of the site has no known risk of ASS occurring within 3 m of the natural soil surface.

9.1 Acid sulfate soils process Acid Sulfate Soils (ASS) are formed naturally under waterlogged, iron and sulfate-rich conditions. These soils contain iron sulfide minerals (most commonly pyrite) and other sulfidic minerals. These soils remain stable under anaerobic conditions, but exposure to air can lead to their oxidation, with the result being the formation of sulfuric acid and the release of iron, aluminium and other heavy metals and nutrients from soils into groundwater and surface water bodies.

Development of land containing or underlain by ASS thereby introduces a potential risk to cause environmental harm including contamination of local wetlands. Traditionally, earthmoving, dewatering and drainage works can result in exposure of these soils to oxidation either directly; or indirectly through the lowering of water tables.

9.2 ASS risk due to groundwater level rise The geology of the DSP area is dominated by Bassendean Sands intermingled with wetland sediments. Within the Bassendean Sands soil profile can be layers of iron cemented organic rich sands (known as coffee rock). Dewatering and or disturbance of these Bassendean Sands and coffee rock, if not managed correctly, can result in the mobilisation of acidity and acidification products, e.g. metals (predominantly iron and aluminium), which can further lead to acidification of the shallow groundwater aquifer. The Bassendean Sand Dune systems across the Swan Coastal Plain are especially susceptible to acidification due to their insufficient capacity in the sands to neutralise/buffer the acidity.

The reduction in groundwater levels since the 1970s across the Gnangara Mound is well documented. The general lowering of groundwater levels cross the DSP area is also well noted and observed in long-term datasets. This reduction in groundwater levels may have resulted in the oxidation of soils that were previously saturated below the water table, which can result in the acidification of the water table and release of metals as described above.

The predicted groundwater level rise in a post-development scenario will likely re-saturate this potentially oxidised material and cause mobilisation of low pH water and elevated levels of acidity and metals in the shallow groundwater table. It could result in very acid groundwater as was the experience during the development of Ellenbrook. Given this site is largely within a trapped hydrogeological catchment and most local groundwater moves towards the wetlands which are the low point or groundwater sinks – there is a potential risk that these environmentally sensitive areas could be the receptors of acidified groundwater.

9.3 The role of controlling groundwater to reduce ASS risk There is no broad scale remediation technology that could be applied to reduce the mobilisation of acidity across an area this large. The most reasonable and practical approach would be to halt the groundwater level rise and reduce the potential for the re-saturation of potentially oxidised ASS and subsequent release of acidity and associated acidification products. Therefore, it is important that post-development groundwater level rise is controlled. The infrastructure used to control groundwater levels will also need to have the ability to treat water for low pH as a contingency in the event that groundwater acidity becomes an issue.

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Figure 17: ASS risk mapping

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10 REFERENCES Appleyard S. 1995. The impact of urban development on recharge and groundwater quality in a coastal

aquifer near Perth, Western Australia. Hydrogeology Journal 3, no. 2, 65-75.

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City of Wanneroo. 2015. Wanneroo Development Design Specification WD5, Stormwater Drainage Design.

Davidson W.A. 1995. Hydrogeology and groundwater resources of the Perth region, Western Australia. Western Australian Department of Mines, Perth.

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Department of Water (DoW) 2015. Selection of future climate projections for Western Australia, Water Science Technical Series, report no 72. Government of Western Australia, Perth, Western Australia.

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Department of Water (DoW). 2017b. Northern Perth Basin: Geology, hydrogeology and groundwater resources, Hydrogeological bulletin series, report no. HB1. Department of Water, Government of Western Australia, Perth.

Department of Water and Environmental Regulation (DWER). 2017a. Decision Process for Stormwater Management in Western Australia. Government of Western Australia.

Department of Water and Environmental Regulation (DWER). 2009. Gnangara Groundwater Areas Allocation Plan. Government of Western Australia.

Department of Water and Environmental Regulation (DWER). 2017b. Water Information Reporting. Accessed October 2017. Available from: http://wir.water.wa.gov.au/Pages/Water-Information-Reporting.aspx

Department of Water and Environmental Regulation (DWER). 2017c. Contaminated Sites Database. Accessed: December 2017. Available from: https://dow.maps.arcgis.com/apps/webappviewer/ index.html?id=c2ecb74291ae4da2ac32c441819c6d47

Department of Water and Environmental Regulation (DWER). 2018. Protecting water quality in P3* areas, Water Quality Protection Note No. 38 (Draft). Department of Water, Government of Western Australia, Perth.

Department of Water and Regulation (DoW). 2018a. Perth Groundwater Map. Accessed: May 2018. Available from: https://maps.water.wa.gov.au/#/webmap/gwm

Emerge. 2018. Draft Environmental Assessment Study, East Wanneroo District Structure Plan. Prepared for the Department of Planning, Lands and Heritage.

Essential Environmental 2016a. North West Metropolitan Subregion, Regional Water Management Strategy. Prepared for Department of Planning.

Essential Environmental. 2016b. Pinjar South Development Area – District Water Management Strategy. Prepared for Landcorp and Housing Authority.

Farrington P. and Bartle G.A. 1991. Recharge beneath a banksia woodland and a pinus pinaster plantation on Coastal Deep Sands in South-Western Australia. Forestry Ecology and Management, v. 40, pp. 101-118.

Heddle E.M., Lonergan O.W. and Havel J.J.1980. Atlas of Natural Resources Darling System, Western Australia. Department of Conservation and Environment.

Heritage Council, State Heritage Office. 2018. Inherit database. Government of Western Australia. Accessed January 2018. Available from: http://inherit.stateheritage.wa.gov.au/Public/Search/Results?new Search=True&placeNameContains=&streetNameContains=&suburbOrTownContains=&lgaContains=wanneroo&isCurrentlyStateRegistered=false#

Institute of Public Works Engineering Australasia (IPWEA). 2016. Specification Separation distances for groundwater controlled urban development.

Monash University. 2014. Vegetation guidelines for stormwater biofilters in the south-west of Western Australia. Monash University, Victoria.

Payne E., Hatt B., Deletic A., Dobbie M., McCarthy D. and Chandrasena G. 2015. Adoption Guidelines for Stormwater Biofiltration Systems. Cooperative Research Centre for Water Sensitive Cities. Melbourne, Victoria.

RPS. 2009. Local area model of groundwater flows and lake interactions, Lakes Mariginiup and Jandabup. Prepared for the Department of Water and Cedar Woods Properties Ltd.

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Sharma M.L. and Pionke H.B. 1984. Estimating groundwater recharge from measurements of environmental tracers in the vadose zone, pp. 799-819. In NWWWA/US EPA Conference on Characterisation and Monitoring of the vadose zone. Edited by Nielsen D.M. Las Vegas, Nevada, USA.

Sharma M.L., Byrne J.D., Herne D.E. and Kin P.G. 1991b. Impact of horticulture on Water and Nutrient fluxes to a sandy aquifer. CSIRO Report 91/33.

Silberstein R., Barr A., Hodgson G., Pollock D., Salama R. and Hatton T. 2004. A vertical flux model for the Perth Groundwater Region. CSIRO.

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Western Australian Planning Commission (WAPC). 2008. Better Urban Water Management. Department of Planning, Perth, Western Australia.

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