Ingleside Precinct Water Cycle Management and Flooding Assessment

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Final Draft Report

Ingleside Precinct Water Cycle Management and Flooding Assessment

59914096

Prepared for

NSW Department of Planning and Environment

31 May 2016

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Contact Information

Cardno (NSW/ACT) Pty Ltd ABN 95 001 145 035

Level 9, The Forum 203 Pacific Highway St Leonards NSW 2065 Australia

Telephone: +61 2 9496 7700 Facsimile: +61 2 9439 5170 International: +61 2 9496 7700 [email protected] www.cardno.com

Author(s):

Shefali Chakrabarty Water Engineer

Approved By:

Emma Maratea

Senior Environmental Engineer

Document Information

Prepared for NSW Department of Planning and Environment Project Name Ingleside Precinct Water

Cycle Management and Flooding Assessment File Reference 59914096 R003 Rev 3

Ingleside Precinct WCM Final Draft Report.docx Job Reference 59914096

Date 31 May 2016

Version Number Rev 3

Effective Date 31/05/2016

Date Approved: 31/05/2016

Document History

Version Effective Date

Description of Revision Prepared by: Reviewed by:

A Preliminary Draft MG NE

B June 2014 Stage 1 Report NE LCC

0 Nov 2015 Draft Report SC/TF EM

1 Apr 2016 Final Draft Report SC LG (DP&E) MS (Pittwater Council)

2 May 2016 Final Draft Report SC LG (DP&E)

3 May 2016 Final Draft Report SC MC (DP&E)

© Cardno 2016. Copyright in the whole and every part of this document belongs to Cardno and may not be used, sold, transferred, copied or reproduced in whole or in part in any manner or form or in or on any media to any person other than by agreement with Cardno.

This document is produced by Cardno solely for the benefit and use by the client in accordance with the terms of the engagement.

Cardno does not and shall not assume any responsibility or liability whatsoever to any third party arising out of any use or reliance by any third party on the content of this document.

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Executive Summary

Introduction

NSW Department of Planning & Environment (DP&E) is proposing to re-zone the Ingleside Release Area (Ingleside Precinct) for residential purposes. The area identified for rezoning is approximately 700 hectares and currently has a non-urban zoning. Cardno has been commissioned by DP&E to prepare a Water Cycle Management and Flooding Assessment Strategy for this Precinct. The Strategy will form part of the Precinct Planning Process to confirm development potential and to establish planning controls to enable development consistent with that potential.

Objective

The objective of this study is to prepare a strategic level Water Cycle Management Strategy for incorporation into the Ingleside Draft Plan through documentation of the following:

 Identification of water management targets (water quality, water quantity and social/ecological requirements) for the future urban development in the precinct.

 Ensuring no adverse impact to flows and flood behaviour in downstream areas.

 Preparation of a water cycle assessment/water balance modelling.

 Consideration of ecological impacts including sustainable environmental flows to Warriewood Wetlands.

 Preparation of a water quality monitoring plan as a determinant of pre and post development impacts.

 Assessment of site constraints and opportunities including:

o Potentially feasible water management strategies;

o Management of environmental flows in creeks;

o Stormwater re-use options;

o Source control measures; and o WSUD options.

 Consolidation of stormwater quality and quantity controls in order to control construction costs and reduce allocation of valuable land for water management purposes.

 Development of feasible options through consideration of:

o Compliance with management objectives;

o Reliability;

o Operation and Maintenance;

o Land Take; and

o Stakeholder Acceptance.

The water management targets set for the Ingleside Precinct in consultation with Council and DP&E are provided below. These targets have been established with the aim to reduce impacts from the Ingleside Precinct development on the surrounding environment and neighbouring properties.

ELEMENT TARGET REFERENCE

Potable Water Household use – 192 L/day/dwelling (2.5 Pax) BASIX (40% reduction target of 320L/dwelling)

Non-potable Water

Irrigation – 125 L/day/dwelling

Supply with non-potable water supply from rainwater/wastewater re-use.

EDAW 2008

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Water Quantity (Design Storm Hydrograph)

For the 2 and 100 year ARI events and the 2hr durations:

a) Peak flow is +/-5% of predevelopment condition.

b) Pre and post development hydrographs are to be shown on one graph with tail cut at given storm duration.

c) The developed hydrograph is to be no more than +/-10% of pre-development at any location on rising/falling limbs.

Warriewood Water Management Specification

Water Quality

90% capture of gross pollutants 85% reduction of TSS

65% reduction of TP 45% reduction of TN

Limit impacts on water quality during

construction using soil and water management plans and water quality monitoring.

Sydney Catchment

Management Authority (now Local Land Services)

Pittwater DCP

Environmental Flows

Flow volume of the post development conditions is to be within +/-5% of pre-development based on a daily water balance (MUSIC) with 31yr simulation period.

Groundwater

Maintain baseflows so that there are no more than +/-10% of pre-development daily volumes represented in a daily water balance model (MUSIC) with 31yr simulation period.

Groundwater Dependent Ecosystems (Ecological 2014)

Methodology Flooding Assessment

A computer-based RAFTS model has been used to determine the existing, pre-development stormwater discharges for the site and for the proposed development. In this way, it is possible to assess the potential impacts of the proposed development on the flows. As expected, the modelling showed that the proposed development generally increased the intensity of stormwater flows within and from the site. This is due to the changes in land use, with the transition from green space and bushland that slowly absorb stormwater to a higher proportion of hard surfaces.

Flood detention basins have been proposed for incorporation into the Draft Plan to attenuate the peak stormwater flows to existing levels in the Precinct. Both on-line (i.e. on the existing watercourse) and off-line (located away from watercourses) basins are proposed to provide peak stormwater flow control and ensure there are no adverse impacts on stormwater flows and flood behaviour within and downstream of the developed Precinct.

Various possible locations were identified and evaluated for the basins. On-line basins are more efficient in terms of land-take and consolidate maintenance within the natural drainage corridor. The off-line basins were located based on site topography, location of conservation significant vegetation and modelled design flood extents.

A SOBEK model has been established to assess the impact of urban development options to existing flood behaviour. Flood mapping for existing conditions and proposed development have been undertaken to demonstrate that the water management targets for flooding are achieved.

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Water Cycle Management

The computer-based Model for Urban Stormwater Improvement Conceptualization (MUSIC) was used for the analysis of the stormwater management requirements for the Precinct. A stormwater ‘treatment train’

approach incorporating different types of Water Sensitive Urban Design systems was evaluated. Based on the outcomes of this analysis, the following treatment train approach has been proposed to achieve the water quality and water quantity targets:

 Rainwater harvesting and re-use of residential, mixed use, community centre and school roof runoff by utilising rainwater tanks;

 Gross Pollutant Traps (GPT) to pre-treat runoff prior to discharge into basins;

 Bioretention basins which will receive flows from the GPTs;

 Detention basins as water retention ponds; and

 Stormwater harvesting for re-use in irrigation of sports field.

Conclusion

This Water Cycle Management Strategy has been prepared to inform the Precinct Planning process and support the rezoning process for the Ingleside Precinct. It presents guiding principles for WCM across the precinct and preliminary management measures. This includes conceptual sizes and locations for elements of the stormwater management network, including detention and water quality treatment infrastructure, and maintenance requirements in determining the best water cycle management option. Indicative layouts of detention basins and bioretention systems have been provided. This will be subject to more detailed assessment during the design phase based on detailed site survey, detailed geotechnical and soil investigations, and also when the final development plan for the sub-catchments is finalised.

In May 2016 Pittwater Council was merged into a new body, the Northern Beaches Council. As this report was prepared prior to these changes, it makes reference to the former council. The plans and strategies of the former council continue to apply to the former local government area until the new council prepares its own plans and strategies.

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List of Abbreviations

AEP Annual Exceedance Probability ALS Aerial Laser Survey

ANZECC Australia and New Zealand Environment and Conservation Council ARI Average Recurrence Interval

AR&R Australian Rainfall and Runff BOM Bureau of Meteorology DTM Digital Terrain Model

LGA Local Government

MHL Manly Hydraulic Laboratory OSD On-site Detention

PMF Probably Maximum Flood WSUD Water Sensitive Urban Design

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Appendices

Appendix A Hydrology Appendix B Hydraulics Appendix C Water Quality Appendix D Aquatic Monitoring

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Tables

Table 3-1 Water Management Targets 13

Table 4-1 Proposed Off-line Basins 20

Table 4-2 Peak Flows at Downstream Boundary of the Off-line Basins 21

Table 4-3 Proposed On-line Basins Adopted in SOBEK model 23

Table 4-4 Peak Flows at Downstream of the Online Basins 24

Table 4-5 Peak Flows at Key Downstream Locations 24

Table 4-6 Culverts included in the SOBEK model 25

Table 4-7 Roughness Values for 2D Domain 26

Table 5-1 Evacuation Route Crossing Summary 31

Table 6-1 Typical WSUD devices 34

Table 6-2 Water Cycle Management Measures for Ingleside Precinct 35

Table 6-3 MUSIC Pollutant Reduction Targets 37

Table 6-4 Environmental Flow and Groundwater Flow Targets 40

Table 6-5 MUSIC Model Water Quality Results 45

Table 6-6 MUSIC Model Water Quantity Results 46

Table 6-7 WSUD maintenance schedule 50

Table 8-1 Monitoring Frequency 59

Table 8-2 Monitoring Parameters and Trigger Levels 60

Table 8-3 QA/QC Requirements 64

Table 8-4 Reporting Requirements 65

Figures

Figure 2-1: Ingleside Precinct Study Area 4

Figure 2-2: Ingleside Precinct Waterways 5

Ingleside Draft Plan 15

XP RAFTS Existing Condition Subcatchment Layout 17

XP RAFTS Developed Condition Subcatchment Layout 19

Detention Basin Locations 22

Flood Evacuation Routes for Ingleside Precinct 30

Total Water Cycle (Source http://www.physicalgeography.net) 33

Waterway catchments 38

MUSIC catchment delineation 39

MUSIC model – Existing Scenario 41

MUSIC model – Developed Scenario 42

MUSIC Model – Mitigated Scenario 44

Typical Bioretention Layout – Flat Terrain 47

Typical Bioretention Detail – Flat Terrain 47

Typical Bioretention Layout – Steep Terrain 48

Typical Bioretention Detail – Steep Terrain 48

Strahler Stream Order and Corresponding Riparian Corridors (Eco Logical, 2016) 53

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Proposed Indicative Water Quality and Sediment Monitoring Locations 58

Corrective Action Process 63

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1 Introduction

Cardno has been commissioned by the NSW Department of Planning & Environment (DP&E) to prepare a Water Cycle Management and Flooding Assessment (WCM) for the Ingleside Precinct. The WCM will form part of the Precinct Planning Process to confirm development potential and to establish planning controls to enable development consistent with that potential.

This Report summarises the following:

> Section 2 – Background: Provides background on the Study Area, the previous water related studies

conducted in the area, and the various development controls and policies that are relevant to the study area;

> Section 3 – Objectives: Based on the development controls and policies relevant to the study area, sets

specific flooding and water quality and quantity design objectives for the Precinct WCM Strategy that satisfy all relevant controls and take into account the water cycle management issues relevant to the study area;

> Section 4 – Flooding Assessment: Summarises the modelling methodology and demonstrates how the

flooding objectives for the Precinct have been met;

> Section 5 – Flood Emergency Response: Assesses the flood emergency response implications of

development of the Ingleside Precinct;

> Section 6 – Water Cycle Management Strategy: Summarises the modelling methodology and identifies

the management approaches required to meet the water quality and quantity objectives for the Precinct;

> Section 7 – Riparian Corridor Assessment: Based on the assessment of the riparian lands within the

Ingleside Precinct that has been undertaken by Eco Logical Australia, provides concept design for basins (detention and bioretention) to be located within the riparian corridors; and

> Section 8 – Water Quality Monitoring Program: Establishes the general framework for water quality

monitoring within and downstream of the Ingleside Precinct for the purpose of managing any impacts associated with the proposed land development.

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2 Background

2.1 Study Area

2.1.1 Location

Ingleside is a suburb of Sydney’s northern beaches area, approximately 30km north of the CBD, and is located along the ridge line 2km to the west of North Narrabeen and Warriewood Beaches. The precinct area is approximately 700 hectares as shown in Figure 2-1. The Precinct is delineated by major roads, conservation areas and crown lands. Mona Vale Road bisects the Precinct and also forms part of its south-western boundary. Ku-ring-gai Chase National Park is located to the north of the precinct, Garigal National Park to the south, Katandra Bushland Sanctuary and Warriewood Wetlands to the east.

2.1.2 Climate

The Ingleside climate is related to the recorded information for Sydney where average temperatures range from 13.8 to 21.7 ⁰C and an average annual rainfall of 1,213mm is recorded (www.bom.gov.au). Summer months generally experience the highest quantity of rainfall and evaporation. In 2013 temperatures were recorded approximately 2⁰C higher than the average maximum and a considerably higher quantity of rainfall occurring in autumn and winter months. Conversely the years of 2010, 2011 and 2012 all featured lower than average temperatures, particularly in the first half of the years with higher than normal amounts of rainfall. This is generally attributed to a La Nina pattern. Current predictions indicate that an El Nino pattern would be experienced in 2014, generally involving drier weather and warmer temperatures for the latter part of 2014.

2.1.3 Topography

Ingleside includes a range of topography due to its location on the Warriewood Escarpment. Above the escarpment the land gently undulates from the ridge line of Mona Vale Road into a number of waterways.

These elevated areas then begin to increase in slope before reaching the escarpment. In general the escarpment delineates the boundary of the precinct; conservation areas and urban development exist thereafter. The urban settlements of Warriewood, Elanora and North Narrabeen are located to the east of the precinct over a steep transition of the escarpment to the foothills before continuing at a lower grade to Warriewood Wetlands and Narrabeen Lagoon. To the north, the urban areas of Church Point and McCarrs Creek are located along the transition from the escarpment to the foreshore. To the west the land slopes down to Wirreandra Creek, then winds its way to the north meeting McCarrs Creeks and ultimately Pittwater. To the south, the escarpment is located beyond the precinct boundary within Garrigal National Park and slopes away to Elanora Heights and eventually to Narrabeen Lagoon.

2.1.4 Land Use

Historically Ingleside has been used as a rural residential area with large homes accommodating large lots. It is not uncommon to encounter grazing and equine uses on a small scale in Ingleside. In addition, light industrial uses are evident along with market gardens and nurseries.

The land use immediately surrounding the precinct boundary is mostly National Parks and Conservation Lands, with the exception of urban areas of Bayview and Monash Country Club and Elanora country club golf courses.

2.1.5 Waterways

The Ingleside Precinct waterways are shown in Figure 2-2. The northern and western portions of the Precinct flow into McCarrs Creek, which discharges into Pittwater. McCarrs Creek is a natural waterway and has a catchment dominated by National Park and recreational grounds. Tributaries to McCarrs Creek located within the Precinct include Crystal Creek, which flows in a westerly direction by the northern boundary before joining Wirreandra Creek, and Cicada Glen Creek flowing through the centre of the Precinct in a northerly direction until it discharges into McCarrs Creek. Wirreandra Creek located on the western part of the Precinct flows north through Ku-ring-gai Chase National Park and further downstream into McCarrs Creek.

A number of tributaries of Mullet and Narrabeen Creeks are located on the eastern side of the Precinct. The

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environmentally sensitive and regionally significant Warriewood Wetlands, and ultimately into Narrabeen Lagoon.

2.1.6 Soils

As per the Preliminary Land Capability, Salinity and Contamination Assessment report (SMEC, 2014), the Precinct is mapped by a variety of soil landscapes including Gymea, Oxford Falls, Hawkesbury, Somersby and Lambert. The site is entirely underlain by the Hawkesbury Sandstone formation of the Wianamatta Group from Triassic Period. The Hawkesbury Sandstone formation typically comprises medium to coarse-grained quartz sandstone with very minor shale and laminate lenses.

The Precinct is considered to have a higher susceptibility to erosion due to the characteristics of a colluvial and erosional soil landscape combined with high rainfall intensity resulting in high soil loss conditions.

As per advice from SMEC, the hydraulic conductivity of the soil could vary from 60mm/hr to 120mm/hr due to the variation in soil textures. Soil depths are generally less than 0.5m before encountering bedrock. Exposed bedrock is present on site and gullies could have 2.0m soil over bedrock.

2.1.7 Groundwater

According to the SMEC 2014 report, local groundwater occurs at depths ranging from 10 to 20 metres below ground level (mbgl) and regional groundwater are likely to be deeper at 100 to 200 mbgl (SMEC, 2013). As per the report, groundwater is of reasonable quality with non-saline characteristics.

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Figure 2-1: Ingleside Precinct Study Area

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Figure 2-2: Ingleside Precinct Waterways

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2.2 Previous studies

2.2.1 Effects of urbanisation on water quality in creeks draining Hawkesbury Sandstone (Laxton, 2001)

Water quality data and macroinvertebrate counts were collected and compared for the Cowan and McCarrs Creek catchments containing the Hawkesbury soil landscape. The locations of sampling focused on a subcatchment and receiving environment scale to formulate an analysis of urban development impact. The land use in the sub-catchments ranges between natural bushland, partially urbanised catchments and rural residential. A number of the McCarrs Creek sub-catchments included in the study are part of the Ingleside precinct. It was found that urban development, even for a small portion of a sub-catchment, can modify the water quality in the receiving environment. Within McCarrs Creek it was shown that pH, TSS and TN parameters were modified when comparing the natural bushland sub-catchments with those containing urban/rural uses. Typically pH increased from approximately 5 in natural areas up to around 7 in the urban areas. This indicates that the naturally acidic water quality, originating from dispersive sandstone runoff, becomes neutralised once land is cleared and materials such as concrete are introduced. Furthermore TSS increases from around 1mg/l in natural areas to around 6mg/l in the urban areas.

The findings indicate that the modification in land use directly affects water quality in receiving waters of McCarrs Creek. The impacts on aquatic ecology also demonstrate how the urban development modifies its receiving environment. In natural catchments species such as Mayfly Larvae, Stonefly Larvae, Shrimps and Crayfish were recorded. In urban catchments the above species were less prominent and the Gastropod species were predominant. This can be indicative of water quality that is more turbid and less acidic where the conditions suffocate the species of natural catchments and allow proliferation of Gastropods which thrive on turbid conditions and are reported to feed on algae. These findings were found in McCarrs Creek, where little urban development exists, and supported with water quality analysis from Cowan Creek where more dense urban development can be found.

2.2.2 Warriewood Valley Water Management Specification (Lawson & Treloar, 2001) 2.2.2.1 Summary

Warriewood Valley had urban development planned for rural land areas surrounding the sensitive Warriewood Wetlands. Pittwater Council moved to develop an Integrated Water Cycle Management (IWCM) strategy in 1995 that set out management objectives and treatment targets to mitigate the impacts of the planned development. The Warriewood Valley Water Management Specification (WMS) was prepared to supplement the IWCM strategy and provide development controls to protect existing water quality and aimed to prevent degradation to existing ecosystem conditions. The sensitivity of the receiving environment led to the planning controls requiring nil impact on water quality and quantity for urban development. A staged approach to the consideration of the water cycle assessments was presented relative to common steps in the planning process (rezoning, development application, construction certificate, construction and hand-over).

The steps of the planning process prior to construction certificate rely on preparation of a Water Management Report at each stage of the process. Thereafter, an Environmental Management Plan & Erosion and Sediment Control Plan would outline requirements for construction followed by quarterly water quality reports during the maintenance liability period.

The various aspects of the water cycle that require assessment and reporting on include:

1. Water cycle assessment – overview of the total water cycle at the site and a daily water balance model that addresses overland flow, baseflow and changes in sub-surface water levels on an annual basis.

A comparison of the existing and developed case scenarios is to be made demonstrating how nominated management measures provide no adverse impact to the existing scenario.

2. Water quality assessment – A water quality monitoring plan is to be developed both with baseline data and additional sampling for water quality in the nearest riparian watercourse. Sampling is to be undertaken upstream and downstream of the development input to the water course along with sampling from the development itself. Reporting of the testing results is to be included throughout all stages of the planning process.

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3. Water quality management – Pre and post development condition pollutant estimations are to be made using a proven method using established pollutant load concentrations provided in the specification.

The objective is set for no worsening of pre development runoff quality (expressed in terms of pollutant loads) in addition to seeking to meet ANZECC ecosystem protection criteria for in-stream measured water quality (ANZECC, 2000). It is suggested that the daily flow output from the water balance model could be coupled with the pollutant concentrations to establish export load values for Total Suspended Solids, Total Phosphorous and Total Nitrogen.

4. Watercourse and Creekline Corridor preservation/restoration – Riparian corridors are to be established/retained along creek lines to observe WSUD principles. A number of technical requirements are outlined for the estimation of environmental flows, riparian corridor width, channel characteristics and buffer widths. A number of design requirements are outlined to guide the preparation of channel/riparian corridor design. An erosion and sedimentation control plan is required for construction management.

5. Flood protection – Planning controls for flood planning levels and requirements of the flood modelling are outlined for inclusion in the Water Management Report at each stage of the development process.

Aspects of the flood protection section require information on flood modelling methodology, plans showing flood levels, interim flood protection works and a flood evacuation plan. Consideration of design storm events include the 50%, 20%, 5%, 1% AEPs together with the PMF.

6. Stormwater quantity management – On-site detention parameters are outlined for the various sectors of development in the valley in order for flows from development sites to be retarded so they do not exceed pre development conditions for the full range of durations and frequencies up to the 1% AEP.

Replication of the base case hydrograph is required. This is to be achieved through both detention and retention of stormwater and a number of options to achieve this are identified (basins, ponds, OSD systems, seepage and re-use). Specific requirements for the hydrograph replication are noted as per below:

a. Peak flow is +/-5% of pre-development condition;

b. Pre and post development hydrographs are to be shown on one graph with tail cut at given storm duration; and

c. The developed hydrograph is to be no more than +/-10% of pre-development at any location on rising/falling limbs.

7. Stormwater drainage concept plan – Design of the water management measures and findings of the various assessments are to be documented on a concept plan in support of the Water Management Report.

8. Wastewater Infrastructure Considerations – Generally refers to the requirements of Sydney Water.

Collection of field data for parameters such as stream flow, rainfall, infiltration, soil type and water quality is required to inform the various assessments listed above. It is noted that whilst this information may not easily be obtained for some of the locations within the land release area there is common data collection locations located within the vicinity such as the flow gauges on Fern and Mullet Creeks operated by Manly Hydraulics Laboratory on behalf of Council and partially grant funded by the Office of Environment and Heritage.

2.2.2.2 Application to Ingleside

It is noted that the Warriewood Valley Water Management Specification (2001) outlines stringent objectives aiming to limit the impact of urban development across all aspects of the water cycle and sought to implement a zero net change approach to impact (a pre-cursor to the Neutral or Beneficial Effect concept used for water quality controls on development in the Sydney Catchment Authority catchment area). The Ingleside Precinct is the neighbouring land release area to Warriewood Valley and a portion of the precinct drains to the same creeks as Warriewood Valley (and ultimately Narrabeen Lagoon). The majority of the area to the east of Mona Vale Road has similar land uses and physical characteristics to Warriewood Valley, whilst the area located to the western side of Mona Vale Road has a lower density of urban land use and drains to McCarrs Creek (and ultimately the estuary of Pittwater).

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It can be expected that similar overall water management objectives could apply to the Ingleside precinct considering that the receiving environments on both sides contain valuable ecosystems. However, it should be noted that the ecosystems in and around McCarrs Creek and Pittwater estuary are different to those of Narrabeen Lagoon and its tributaries and both have been impacted by existing development to differing levels.

Common overall water management objectives are considered applicable to the precinct because of shared geography and expectations of stakeholders. Therefore it is prudent to consider the foundations of the Warriewood Valley Water Management Specification (2001) and how it may be modified to maintain the water management objectives and improve upon the experiences of recent urban development in the valley.

2.2.2.3 Lessons Learned

Ongoing urban development in Warriewood Valley has been undertaken with reference to the Water Management Specification (2001).

The WMS (2001) was applied to all rezoning and development applications received for the various sectors in the Warriewood Valley. Key learnings from the review of applications by Cardno over the period from 1999 to 2009 were:

 Only the absolute minimum water quality data required was collected and mostly consent had to be withheld until such time as the data was collected and submitted to Council. Water quality data collected during construction and post construction phases were often supplied to Council months after an impact was shown and no action was taken at the time of the incident, nor was it able to be taken long after the incident had occurred. An improved system of construction and post-construction phase monitoring that ensures that action is taken or penalties are applied would be appropriate to achieve the environmental outcomes necessary for the receiving systems.

 A review of potential water quality issues for the locality was better conducted with consideration of Phase 1 and (where available) Phase 2 contaminated land investigations.

 It was difficult to demonstrate compliance with flow and flood requirements of the WMS (2001) without considered incorporation of these concepts in the initial rezoning application. Council eventually set some limits on acceptable flood impacts where zero impact could not be reasonably demonstrated using flood modelling, especially for sector developments in large complex systems. The use of regional flood models established by Council ended up being a more effective means of assessing regional flood impacts of a development, rather than requiring individual developments establish their own flood models for each locality.

 Having set local (sector-specific) requirements for on-site detention (site storage requirements and permissible site discharges) meant that applications could be more easily assessed against these pre- set requirements.

 Requirements for zero net change in pollutant loads were challenging but could be addressed with innovative solutions and ensuring that sufficient space for these solutions was set aside early (such as in the rezoning application).

 The use of infiltration as a mechanism for achieving a water balance was not always possible with local geology (rock close to the surface) and proper testing using double ring infiltrometer testing at rezoning stage allowed for early identification of these constraints. Alternatives, such as larger rainwater tanks or more extensive irrigation or in-house/on-lot reuse (e.g. for laundry as well as toilet flushing and garden irrigation) could be flagged earlier, which contributed to lot sizing requirements.

 The then Part 3A process (repealed in 2011) for some developments within the land release area largely circumvented some of the detailed requirements laid out in the WMS (2001) and often these requirements were relaxed or reduced and did not allow for proper integration of the overall regional strategy.

 Legacy issues for former agricultural sites were present and not always able to be addressed effectively with respect to the protection of receiving environments. For example, groundwater in some areas showed very high nutrient concentrations and controlling the mobilisation of these nutrients associated with large scale earthworks and stormwater infiltration systems incorporated into

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 Where a creekline corridor was shared and the creek was to be rehabilitated, constructing one half of the creekline as part of the development on that side of the creek was achievable but presented challenges in the interim period prior to the other half being constructed. Flood impact assessments also had to demonstrate that a half-creek construction did not result in short term flood impacts upstream or downstream from a site.

 Integration of wastewater infrastructure in the creekline corridor designs (often outside of the scope of an individual development and managed by Sydney Water) would be more effective at an early planning stage rather than after a creekline corridor was constructed.

 In addition to on-lot controls for dwellings across all developed areas (e.g. on-lot rainwater tanks and associated in-dwelling or irrigation re-use, on-lot on-site detention facilities, on-lot infiltration facilities),a number of gross pollutant traps, proprietary stormwater treatment systems, ponds, detention basins/systems, infiltration facilities, swales, bioretention systems and constructed wetlands exist within the public domain space (i.e. in the private buffer areas of the riparian corridors and beyond) managing flows from cluster developments. The water management controls in public domain areas have largely been handed over to Council, but some have been retained in private ownership (e.g. those in the Shearwater Estate, also known as Sector 12) with the inherent maintenance responsibility. Often the maintenance requirements have not been fully implemented for those facilities in private ownership by the residents and the water quality treatment performance is compromised as a result. Some facilities, such as dry detention basins can perform their water quantity management function with a limited amount of maintenance (such as lawn mowing of batters).

2.2.3 Mullet Creek Rehabilitation Plan (Hyder, 2008)

The rehabilitation plan aimed to conserve Mullet Creek and its receiving environment through providing a strategic framework for rehabilitation. The plan identified a number of social and ecological values of the waterway that are of high value and outlined actions for conservation. Objectives were listed that aimed to improve the understanding of the Mullet Creek hydrology, geomorphology, water quality and aquatic ecology in order to inform a set of creek management recommendations. Investigations into the key indicators for creek health were undertaken through site inspection, community consultation and review of previous studies. The key indicators were then listed in a matrix with corresponding condition, issues and causes. In general it was found that the creek was degraded as a result of land clearing, rural residential uses, increased nutrient loading, water extraction and modified hydrology.

Identification of issues for various reaches of the creek were tabulated with corresponding management actions. The actions were then prioritised based on a qualitative assessment of a range of criteria including cost, stakeholder acceptance, severity of the issues and effectiveness of the management measure. The measures were both preventative and responsive. The consultant undertaking the study and Council then rationalised the management actions to a refined list before going to public exhibition. Following the collation of comments the list was finalised and funding was to be sought for further action. The preparation of the Ingleside Water Cycle Management (WCM) Strategy was identified as management action number 3. The WCM should integrate the various riparian, geomorphology, flooding and water quality principles to sustain the creek health during and following urban development. Management actions were also identified that lead to the development of the studies have been reviewed in Sections 2.2.4 and 2.2.5.

2.2.4 Mullet Creek Environment Flow Assessment (SKM, 2010)

Pittwater Council implemented a Rehabilitation Plan for Mullet Creek in 2008, as discussed in Section 2.2.3.

As an outcome of the rehabilitation plan, an environmental flow assessment was prepared to determine the impact of urban development to flow regimes in Mullet Creek and the projected impacts of further irrigation were also investigated. The assessment identified the time series flows in Mullet Creek through WaterCAST modelling of hydrology. Since the catchment involves a range of land uses particular attention was made to the significant water users such as the Monash Country Club and Elanora Golf Course. Urban development occupies approximately 10% of the catchment and rural residential uses occupy approximately 40%. The remainder of the catchment is bushland.

The study uses rainfall data from a MHL operated station in Narrabeen Creek, evaporation data from the BOM station at Sydney Airport and water level data from the MHL gauge at Garden Street. The model was built to represent 11 sub-catchments using a DTM created from Council’s ALS. Validation of the model was

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undertaken by comparing flows of the catchment model to those of a rating table developed for the water level recorded by the MHL gauge in the Garden Street culvert. It was found that the rating table estimations of flow volume were abnormally high and disregarded. Alternatively, a volumetric runoff coefficient of 0.3 was used to adjust the catchment parameters to suit. A number of dams and irrigation demands of those were also included.

The study found that the natural hydrology of Mullet Creek, prior to European settlement, had a similar regime to that of existing conditions with the irrigation demands included. Analysis of the existing condition without the irrigation proved that there were increases in the amount of flow at the Garden Street culvert. It was concluded that the impact of the relatively low urbanisation of the catchment decreased the low flows received by Warriewood Wetland and increased the high flows. This was a more significant impact than the harvesting of flows for irrigation of the golf courses. It is noted that the golf courses were not using their full water license allowances and if they were to increase, then impact on the high and medium flows in Mullet Creek could be experienced.

2.2.5 Mullet Creek Water Quality Monitoring Program and Design (Bio-Analysis, 2010)

The monitoring program report was commissioned to investigate the aquatic ecology of Mullet Creek and to outline methods for testing response of the creek to planned development. It was anticipated by Council that the Ingleside precinct would be rezoned for urban development and this program is one of the management actions coming out of the Mullet Creek Rehabilitation Plan. The report notes that creek rehabilitation works are likely to improve water quality in the short term, however, there remains concerns over the impact of future development. The program is informed by previous water quality assessments undertaken by Council and its consultants.

Aquatic habitat was inspected visually and reported to inform the design of the program. A review of available information regarding water quality is summarised below:

 Low dissolved oxygen levels;

 Elevated nutrients;

 Blooms of phytoplankton;

 Faecal contamination;

 Nuisance macro algae and aquatic plant growth;

 Obstruction of flows by dams and culverts;

 Elevated levels of suspended solids;

 Sedimentation; and

 Concentration of heavy metal was below upper limit of ANZECC guideline.

A short description of the aquatic habitat was reported to contain three distinct reaches being:

1. A wetland upstream of Jackson’s Road – wetland similar to those found in Warriewood Wetland.

2. From the wetland limit at around Garden Street to the first waterfall in Epsom Park – Shallow sandy channel with shallow pools having dense riparian vegetation and many weeds. Water quality appeared poor due to turbidity.

3. Upstream of the confluence of the two arms of the Creek that drain either side of Powderworks Road – The south-western arm is relatively undisturbed with several land developments, Monash golf course and dams located adjacent to the creek. The north-western arms is disturbed as a result of land clearing, rural development and road crossings.

An assessment of water quality and related studies identifies that the aquatic habitat is under stress as a result of high nutrient and sediment levels. In addition aquatic biota is predicted to suffer as a result of urban development adjustments to water quality parameters such as conductivity, dissolved oxygen, pH and temperature. Traditionally water quality sampling concentrates on nutrients and suspended solids when monitoring the impact of urban development. It is recommended that sampling of macro-invertebrates is coupled with the water quality testing. Macro-invertebrates are a key indicator of the aquatic biota present in a

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It is proposed in the report that the program should test water quality at a number of locations along the waterway over a given time period to accurately monitor water quality changes. If the water quality and macroinvertebrate levels increase above the average baseline data then it would be determined that impacts have become incurred. Reference to ongoing monitoring in related catchments in the Hornsby and Warringah LGAs could be used for analysis to outline common response of undeveloped catchments to climatic conditions. This would be supportive data to allow a clear identification of urban development impact independent of other variables. Further discussion of the program and how it would be applied for the Ingleside precinct is included in Section 7.

2.2.6 Ingleside Water Management Option (EDAW, 2008)

The Ingleside Water Management Option report was commissioned by Landcom (now UrbanGrowth NSW) to investigate opportunities for water management in the Ingleside land release area. Potable water, wastewater and stormwater management infrastructure options were investigated. It is noted that potable water is most likely to be supplied by a new centralised piped network considering the lack of existing infrastructure.

Recommendations are made to reduce potable water demands through rainwater tanks to supply hot water demands and recycled wastewater for non-potable uses.

Wastewater services have been investigated by Worley Parsons and would involve expansion of the Warriewood STP reticulation network. This would be cost effective in servicing locations in the precinct on the eastern side of Mona Vale Road.

Stormwater management is generally recommended to include WSUD, retention of post development flows for events up to the 1.5 year ARI and retard stormwater flows to mimic pre development hydrology. It is noted that there are a wide range of options to meet these stormwater management objectives and could be either located in public domain or within private property. In general land take requirements for stormwater management are reported to be between 1-3% of the development area. Considering the sensitivity of the receiving environments discussion is focussed on the capture, treatment and harvesting of stormwater to reduce the predicted modification of hydrology in Ingleside.

2.2.7 Narrabeen Lagoon Flood Study (BMT-WBM, 2013)

This study was commissioned by Council with the support of the Office of Environment and Heritage and describes the flood behaviour in the Narrabeen Lagoon catchment. Mullet and Fern Creeks are tributaries to the Lagoon. Further discussion of this study in included in Appendix A.

2.2.8 Pittwater Overland Flow Flood Study (Cardno, 2013)

This study was commissioned by Council with the support of the Office of Environment and Heritage and aims to increase awareness of Overland Flow in the Pittwater LGA. Various models were prepared that include the entire Ingleside Precinct. Further discussion of this study in included in Appendix A.

2.2.9 Mona Vale – Bayview Flood Study (DHI, 2002)

A small part of the Ingleside precinct drains to the north through the Mona Vale – Bayview catchment where flood behaviour was estimated by this Flood Study using a Mike 11 model. Further discussion of this study in included in Appendix A.

2.2.10 Warriewood Valley Flood Study (Cardno Lawson Treloar, 2005)

This study was commissioned by Council to investigate the flood behaviour of Warriewood Valley where ongoing urban development was in progress. It has now been superseded by the Narrabeen Lagoon Flood Study (BMT WBM, 2013) and the Pittwater Overland Flow Flood Study (Cardno, 2013). Further discussion of this study in included in Appendix A.

2.3 Relevant Development Controls and Policies

2.3.1 Pittwater Local Environmental Plan (LEP)

The Pittwater LEP was gazetted in May 2014 and came into effect in June 2014. It defines the Flood Planning Level in Section 7.3 Flood Planning, under Item (5):

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Flood planning level means the level of a 1:100 ARI (Average Recurrence Interval) flood event plus 0.5m freeboard, or other freeboard determined by an adopted floodplain risk management plan.

Section 7.4 Floodplain Risk Management of LEP 2014 outlines safe occupation and evacuation requirements and applies to land as defined under Item (2):

”This clause applies to land between the flood planning level and the level of the probably maximum flood, but does not apply to land subject to the discharge of a 1:100 ARI (average recurrent interval) flood event plus 0.5 metre freeboard, or other freeboard determined by an adopted floodplain risk management plan. “ 2.3.2 Pittwater Development Control Plan (DCP)

The Pittwater 21 DCP was first adopted in 2003 and has since been amended seventeen (17) times and most recently came into force on 14^th November 2015. It currently applies planning controls to land uses mapped in the Pittwater LEP 2014 with specific requirements for land release areas such as Warriewood Valley. In regard to water cycle management the DCP includes specific hazard controls for flooding that relate to associated flood hazard maps. The controls recommend a range of flood risk management considerations in the planning and design of urban development. The flooding controls are similar to what has been documented throughout NSW under the Floodplain Risk Management process as defined by the NSW Floodplain Development Manual (NSW Government, 2005). It is noted that specific controls are included for minor and major overland flow paths that are particularly relevant to flood behaviour in Ingleside.

Section C6.1 outlines the controls for integrated water cycle management within the Warriewood Valley locality and a summary of this is included below:

1. Water Management Report – This report is to be prepared by a qualified professional and is to be in accordance with Council’s Warriewood Valley Urban Land Release Water Management Specification (2001) and relevant legislation taking into account the Narrabeen Lagoon Flood Study (2013) and the Pittwater Overland Flow Flood Study (2013).

2. Flooding – Flood levels are to be determined as part of the Water Management Report along with assessment of the likely flood impacts from the development.

3. Creekline Corridor – Any creek that passes through/aligns/abuts a sector, buffer area or development site, is required to comprise a total width of 100m. This comprises of a 50m wide Inner Creekline Corridor which would be under Council ownership and contain the 1% AEP flow plus climate change;

and an Outer Creekline Corridor 25m wide on each side of the Inner Creekline Corridor. This would be in private ownership and perform the function of part water quality and park fauna/flora corridor.

4. Stormwater Drainage Management – Design of piped stormwater drainage system network with 5%

AEP capacity including climate change impacts is required. All development stages are to meet or exceed the water quality criteria within the Warriewood Valley Urban Land Release Water Management Specification (2001).

5. Groundwater – If groundwater is required to be manages as a result of excavation/basement/stormwater or flood mitigation measures then groundwater management measures are to be assessed.

6. Greywater Reuse – if greywater reuse is proposes then on-site treatment, disposal and/or reuse must demonstrate feasibility, compliance with relevant State and Federal regulatory requirements, and achieve current NSW Heath Accreditation.

Section B.25 of the DCP outlines the flood emergency response planning control for areas impacted by flash flooding or overland flow or lagoon flooding or a combination of flooding to ensure that development is undertaken in a way that is reflective of the flood risk.

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3 Objectives

The objectives of the WCM strategy are to prepare a strategic level WCM strategy for incorporation into the Ingleside Draft Plan through documentation of the following:

 Identification of water management targets (water quality, water quantity and social/ecological requirements) for the future urban development in the precinct.

 Ensuring no adverse impact to flows and flood behaviour in downstream areas.

 Preparation of a water cycle assessment/water balance modelling.

 Consideration of ecological impacts including sustainable environmental flows to Warriewood Wetlands.

 Preparation of a water quality monitoring plan as a determinant of pre and post development impacts.

 Assessment of site constraints and opportunities including:

o Potentially feasible water management strategies;

o Management of environmental flows in creeks;

o Stormwater re-use options;

o Source control measures; and o WSUD options.

 Consolidation of stormwater quality and quantity controls in order to control construction costs and reduce allocation of valuable land for water management purposes.

 Development of feasible options through consideration of:

o Compliance with management objectives;

o Reliability;

o Operation and Maintenance;

o Land Take; and

o Stakeholder Acceptance.

3.1 Water Management Targets

The water management targets set for the Ingleside precinct in consultation with Council and DP&E are provided in Table 3-1. These targets have been established with the aim to reduce impacts from the Ingleside Precinct development on the surrounding environment and neighbouring properties.

Table 3-1 Water Management Targets

Potable Water Household use – 192 L/day/dwelling (2.5 Pax) BASIX (40% reduction target of 320L/dwelling)

Non-potable Water

Irrigation – 125 L/day/dwelling

Supply with non-potable water supply from rainwater/wastewater re-use

EDAW 2008

Water Quantity (Design Storm Hydrograph)

For the 2 and 100 year ARI events and the 2hr durations:

a. Peak flow is +/-5% of predevelopment condition

b. Pre and post development hydrographs are to be shown on one graph with tail cut at given storm duration

c. The developed hydrograph is to be no more than +/-10% of pre-development at any location on rising/falling limbs

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Water Quality

90% capture of gross pollutants 85% reduction of TSS

65% reduction of TP 45% reduction of TN

Limit impacts on water quality during

construction using soil and water management plans and water quality monitoring

Sydney Catchment

Management Authority (now Local Land Services)

Pittwater DCP

Environmental Flows

Flow volume of the post development conditions is to be within +/-5% of pre-development based on a daily water balance (MUSIC) with 31yr simulation period

Groundwater

Maintain baseflows so that there are no more than +/-10% of pre-development daily volumes represented in a daily water balance model (MUSIC) with 31yr simulation period

Groundwater Dependent Ecosystems (Ecological 2014)

The following sections will provide further discussion on how the water cycle management and flooding objectives and water management targets will be achieved.

The Ingleside Draft Plan is provided in Figure 3-1.

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Ingleside Draft Plan

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4 Flooding Assessment

¹

4.1 Hydrology

This study adopted a traditional hydrological XP_RAFTS model for the entire Ingleside precinct catchment to generate the input hydrographs to a hydraulic SOBEK model which covers the Ingleside Precinct.

An XP_RAFTS model is the most widely used hydrological modelling tool to predict the storm discharge for the pre and post development conditions and to estimate the requirements for stormwater detention. The model allows the user to rapidly update parameters such as impervious percentage, rainfall losses and roughness to assess greenfield development.

The aims of the hydrological analyses were to:

 Assemble a rainfall/runoff model of the existing catchment and the post development catchment;

 Estimate catchment runoff under existing catchment conditions for the 2, 20, 100, 200 and 500 year ARI and PMF events;

 Estimate catchment runoff under post development conditions to ascertain the impacts of the proposed development for the 2 year ARI and 100 year ARI events;

 Assess the impact of climate change by estimating 10%, 20% and 30% increases in 100 year ARI rainfall under post development conditions;

 Size detention basins to reduce the 2 and 100 year ARI peak flows as specified in the water management targets (Table 3-1):

o Peak flow is +/-5% of predevelopment condition;

o Pre and post development hydrographs are to be shown on one graph with tail cut at given storm duration; and

o The developed hydrograph is to be no more than +/-10% of pre-development at any location on rising/falling limbs.

 Assess the ramifications of climate change on the volumetric requirement for structural flood risk management measures.

The catchment model and parameters are outlined in Appendix A.

4.1.1 Existing Conditions

An XP_RAFTS model was developed under the catchment existing conditions to generate hydrographs for inputs to a SOBEK model. The catchment was divided into 64 subcatchments based on topographic features, the likely overland flowpaths and the input requirements of the hydraulic model.

The XP_RAFTS subcatchment layout for the existing scenario is shown in 0.

A full range of design events was simulated for the existing scenario, including the 2, 20, 100, 200 and 500 year ARI and PMF events. The estimated peak flows for each subcatchment for these design events are summarised in Appendix A.

1 Subject to further amendments to the draft Plan, the flood assessment will be updated post public

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XP RAFTS Existing Condition Subcatchment Layout

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4.1.2 Results comparison

Since calibration data is not available in the study area, the XP_RAFTS model was validated by comparing the peak flows for 100 year ARI at a common node on Mullet Creek with previous available studies. An assessment of peak flow from the XP_RAFTS models available at the time of reporting found the following 100 year ARI, 2hr peak flows at a common node on Mullet Creek.

 Narrabeen Lagoon – 97.2 m³/s

 Warriewood FS – 40.4 m³/s

 Ingleside Precinct WCM – 100.7 m³/s

The Ingleside peak flow of the 100 year ARI is similar to that of the Narrabeen Lagoon Flood Study (BMT- WBM, 2013). This is not surprising considering that the same hydrological model parameters have been adopted. The reason why the Ingleside flows are slightly higher than those identified in the Narrabeen Lagoon Flood Study is because the catchment slope has generally been estimated higher in the current study. The flows estimated for the Warriewood Valley Flood Study (Lawson & Treloar, 2015) involved a detailed investigation of losses and much higher Bx values that would reduce the discharge. It is interpreted from the Warriewood study that the higher losses/Bx were used to calibrate the model to local stream gauge data. It is evident that in the Narrabeen Lagoon Flood Study higher losses were also estimated in order to calibrate models. It is noted that the loss values in these previous studies were averaged over a large catchment and may not provide adequately conservative values for the Precinct. As a result, industry standard valued recommended by AR&R were adopted and is consistent with the Ingleside model approach.

4.1.3 Developed Conditions

The existing XP_RAFTS model was modified for the development conditions to represent the land uses proposed in the revised Ingleside Concept Plan in August 2014. The key modifications include:

 Configuration of subcatchment layout; and

 Impervious percentage for different land uses.

The catchment was divided into 72 subcatchments by considering the proposed design layout, land uses and the existing subcatchment layout. The XP_RAFTS subcatchment layout for the development scenario is shown in Figure 4-2.

The design events of 100 year ARI and 2 year ARI were simulated for the developed conditions. The modelled peak flows for each subcatchment for these design events are summarised in Appendix A.

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XP RAFTS Developed Condition Subcatchment Layout

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4.1.4 Basin assessment

A hydrological assessment of possible detention basin options was undertaken. The aim of the assessment was to meet the following water management targets:

 For the 2 year and 100 year ARI events and the 2 hour durations:

o Peak flow is +/-5% of predevelopment condition;

o Pre and post development hydrographs are to be shown on one graph with tail cut at given storm duration; and

o The developed hydrograph is to be no more than +/-10% of pre-development at any location on rising/falling limbs.

The potential detention basin locations are shown in Figure 4-3, including seven off-line basins and three on- line basins.

The on-line basins are located at the three locations along Mullet Creek and Cicada Glen Creek, to capture flows from all of upstream catchments. These creeks are 1^st and 2^nd order creeks and as per NOW Controlled Activity Riparian Corridor Guidelines, online basins are allowed on these creeks. The on-line basins will play a key role to meet the specified water management targets for the downstream flows along the creeks under the developed conditions.

The off-line basins would be situated adjacent to the creek within the outer 50% of the Vegetated Riparian Zone. They will capture flows from its local catchment and include a biofiltration area.

The design of the basin size and outlet structures is crucial to control the peak flows downstream and to achieve the optimal efficiency of the detention systems. This study adopted two approaches in sizing off-line basins and on-line basins.

4.1.4.1 Off-line Basins

Off-line basins were estimated using XP_RAFTS model under the developed conditions. Off-line basins generally considered the flows from its location subcatchment.

The basin size and outlet structure for each off-line basin were determined by adjusting the basin design parameters in XP_RAFTS to achieve the targeted downstream peak flows mentioned above. The detailed information regarding these off-line basins are summarised in Table 4-1.

Table 4-1 Proposed Off-line Basins

Offline Basin ID

Subcatchment Area (ha)

Peak Depths (m) Indicative Storage Volume (m³)

100 year ARI Spillways

2 year ARI Outlet 100 year (m)

ARI

2 year ARI

100 year ARI

2 year ARI

Width (m)

Spillway Height

(m)

B_M1 16.16 2.18 1.23 5,500 3,100 5 1.8 1.4×1.0

B_M13 24.94 2.05 1.11 8,100 4,400 8 1.8 1.6×1.0

B_M11b 9.63 2.14 1.13 2,800 1,500 4 1.8 1.0×0.8

B_N3 44.25 2.40 1.23 12,000 6,200 8 1.8 2.9×0.9

B_U1 19.75 2.14 1.22 5,400 3,100 8 1.8 1.8×1.0

B_U2 18.04 1.90 1.20 4,200 2,700 8 1.8 1.8×1.0

B_C10b 5.988 1.73 1.18 740 510 4.5 1.5 1.0×0.7

B_N3 was designed to capture flows from all of the upstream subcatcments, including N3b, N3, N12a and N12, which the total subcatchment area is 44.25 hectares.

For B_M13 and B_C10b, there is no identical corresponding subcatchment for the existing RAFTS model due to the subcatchment split under the developed conditions. The hydrographs under the existing conditions were

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obtained by simulating the same subcatchment area as the developed conditions and the impervious percentage under the existing conditions.

The peak flows at downstream end of off-line basins are provided in Table 4-2.

Table 4-2 Peak Flows at Downstream Boundary of the Off-line Basins Off-line

Basin

100 year ARI Peak Flow (m³/s)

100 year ARI Flow Percentage

(%)

2 year ARI Peak Flow (m³/s) 2 year ARI Flow Percentage Existing Developed Existing Developed (%)

B_M1 7.22 7.12 -1.4 2.30 2.40 4.3

B_M13 7.17 7.19 0.4 2.08 2.07 -0.2

B_M11b 4.43 4.37 -1.4 1.52 1.56 3.1

B_N3 17.29 17.23 -0.2 5.64 5.39 -4.8

B_U1 9.52 9.13 -4.2 3.09 3.05 -1.4

B_U2 8.00 8.16 1.9 2.65 2.71 2.3

B_C10b 3.39 3.23 -4.9 1.23 1.18 -4.1

The results indicate that the off-line basins are capable of managing the peak flows within +/-5% of predevelopment condition.

4.1.4.2 On-line Basins

This study proposed three on-line basins, which the locations are shown in Figure 4-3. OSD_C3 and OSD_C6 are located along Cicada Glen Creek, whilst OSD_M5 is located along Mullet Creek. The main purpose of these on-line basins is to manage the flows downstream of the study area along these two major creeks in a range of ± 5% of the predevelopment conditions. These downstream flow control locations are shown in Figure 4-3 as flow measurement lines.

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Detention Basin Locations

Ingleside Precinct Water Cycle Management and Flooding Assessment

Final Draft Report