Australia; Water Sensitive Urban Design Technical Guidelines for Western Sydney

WA TER SEN SITIVE URBA N DESIGN TECH N ICA L GUIDELIN ES FOR

WESTERN SYDN EY

D R A F T F O R C O M M E N T

7 November 2003

Draft Prepared by: Supported by:

Document Management

Version Manager Date

Final Draft URS Australia 7 October 2003

Revised Final Draft UPRCT 7 November 2003

Contents

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1 Introduction ------------------------------------------------------------------------------------------------- 1-1

1.1 Background 1-1 1.2 What is WSUD 1-2 1.3 Proponent of the Technical Guidelines 1-3 1.4 Purpose and Use of the Technical Guidelines 1-3 1.5 Australian Runoff Quality 1-4 1.6 Document Structure 1-4

2 Key Physical Characteristics Affecting WSUD in Western Sydney ---------------------- 2-1

2.1 Climate 2-1 2.2 Geology and Soils 2-2 2.3 Groundwater/Salinity 2-3 2.4 Stormwater Management Objectives 2-5

2.4.1 Objectives for New Developments 2-5 2.4.2 Construction Phase 2-5 2.4.3 Post-Construction Phase 2-6

3 WSUD Measures and Application ------------------------------------------------------------------- 3-1

3.1 WSUD Measures 3-1 3.2 Relationship between WSUD Measures 3-2 3.3 Description of WSUD Measures and Implementation Issues 3-3

3.3.1 Vegetated Swales 3-3 3.3.2 Vegetated Filter Strips 3-5 3.3.3 Sand Filters 3-6 3.3.4 Bioretention Systems 3-7 3.3.5 Permeable Pavement 3-9 3.3.6 Infiltration Trenches 3-12 3.3.7 Infiltration Basins 3-15 3.3.8 Rainwater Tanks 3-18 3.3.9 Landscape Developments 3-20

4 WSUD Planning and Selection Guide -------------------------------------------------------------- 4-1

4.1 WSUD Planning Process 4-1 4.2 Applicability and Function of WSUD Measures 4-2 4.3 WSUD Selection and Treatment Train 4-4 4.4 Incorporation of WSUD Measures in Streetscapes 4-5

5 WSUD Design Specification --------------------------------------------------------------------------- 5-1

5.1 Introduction 5-1 5.2 Design Process 5-2 5.3 Design Specification DS1 – Vegetated Swales 5-3 5.4 Design Specification DS2 – Vegetated Filter Strips 5-10 5.5 Design Specification DS3 – Sand Filters 5-15 5.6 Design Specification DS4 – Bioretention Systems 5-20 5.7 Design Specification DS5 – Permeable Pavements 5-28 5.8 Design Specification DS6 – Infiltration Trenches 5-33 5.9 Design Specification DS7 – Infiltration Basins 5-38

Contents

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5.10 Design Specification DS8 – Rainwater Tanks 5-43 5.11 Design Specification DS9 - Landscape Developments 5-49

6 WSUD Operation and Maintenance ----------------------------------------------------------------- 6-1

6.1 Introduction 6-1 6.2 Vegetated Swales 6-2 6.3 Vegetated Filter Strips 6-4 6.4 Sand Filters 6-6 6.5 Bioretention Systems 6-8 6.6 Permeable Pavements 6-10 6.7 Infiltration Trenches 6-12 6.8 Infiltration Basins 6-14 6.9 Rainwater Tanks 6-16 6.10 Landscape Developments 6-18

7 Life Cycle Costs for WSUD Measures-------------------------------------------------------------- 7-1

7.1 Introduction 7-1 7.2 Vegetated Swales 7-1 7.3 Vegetated Filter Strips 7-2 7.4 Sand Filters 7-3 7.5 Bioretention Systems 7-3 7.6 Permeable Pavements 7-4 7.7 Infiltration Trenches 7-4 7.8 Infiltration Basins 7-5 7.9 Rainwater Tanks 7-5

8 References -------------------------------------------------------------------------------------------------- 8-1

List of Tables, Figures, Plates & Appendices

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Tables

Table 2.1 Average Monthly Rainfall in the Western Sydney Area Table 2.2 Average Evaporation Rates in Western Sydney Table 2.3 Rainfall Statistics – Mean Inter-Event Dry Period (hrs) for Sydney Table 2.4 Rainfall Statistics – Mean Storm Duration (hrs) for Sydney Table 2.6 Ranking of Objectives for New Development Table 2.7 Treatable Flow Rates and Runoff Depths (Blacktown City Council) Table 3.1 WSUD Measures Included in the Design Specifications Table 3.2 WSUD Treatment Measure Categories Table 3.3 Control Levels in the Urban Hydrological System Table 3.4 Minimum Clearances for Infiltration Systems Table 3.5 Site Assessment for Infiltration Systems Table 4.1 Scale of WSUD Application in Urban Catchments Table 4.2 Role and Function of WSUD Measures Table 4.3 Site Constraints for WSUD Elements Table DS1.1 Maximum Flow Velocities in Channels Table DS1.2 Selection of Flow Retardance Class Table DS2.1 Maximum Acceptable Flow Velocities in Channels Table DS2.2 Selection of Flow Retardance Class Table DS3.1 Sand Filter Particle Grading Specification Table DS4.1 Maximum Flow Velocities in Vegetated Channels Table DS6.1 Infiltration Rates for Homogeneous Soils Table DS6.2 Factors of Safety for Infiltration (Bettess, 1996) Table DS7.1 Infiltration Rates for Homogeneous Soils Table DS7.2 Factors of Safety (f) for Infiltration (Bettess, 1996) Table DS8.1 Design Roof Area and Number of Occupants for a given Lot Size Table DS8.2 Design Demand for Rainwater Tank (kL/year) Table DS8.3 Average Efficiency of Rainwater Tank for 250 m2 Lot Table DS8.4 Average Efficiency of Rainwater Tank for 350 m2 Lot Table DS8.5 Average Efficiency of Rainwater Tank for 450 m2 Lot Table DS8.6 Average Efficiency of Rainwater Tank for 1000 m2 Lot Table DS8.7 Average Available Detention Volume for 250 m2 Lot Table DS8.8 Average Available Detention Volume for 350 m2 Lot Table DS8.9 Average Available Detention Volume for 450 m2 Lot Table DS8.10 Average Available Detention Volume for 1000 m2 Lot Table 7.1 Estimated Unit Rate Construction Cost for Vegetated Swale Table 7.2 Estimated Swale Maintenance Costs – Model Farms High School Table 7.3 Estimated Unit Rate Construction Cost for Bioretention Trench

List of Tables, Figures, Plates & Appendices

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Figures

Figure 4.1 WSUD Planning Process Figure 4.2 Streetscape View Zone with Development Both Sides Figure 4.3 Streetscape View Zone with Development One Side Figure 4.4 Existing Road with Car Parking One Side Figure 4.5 Existing Road with Car Parking Two Sides Figure 4.6 2 m Wide Median Figure 4.7 4 m Wide Median Figure 4.8 6 m Wide Median Figure 4.9 2 m Median with Car Parking, Permeable pavement and Street Trees Figure 4.10 4 m Median with Car Parking, Permeable pavement and Street Trees Figure 4.11 6 m Median with Car Parking, Permeable pavement and Street Trees Figure 4.12 Open Space Edge Median with Car Parking, Permeable pavement and Street Trees

SECTION 1 Introduction

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1 Introduction

1.1 Background

Water Sensitive Urban Design (WSUD) is a relatively new approach to water management in urban areas. The objective of WSUD is to maintain or replicate the pre-development water cycle through the use of design techniques to create a functionally equivalent hydrological landscape. When urban development occurs, the natural water cycle is altered to the extent that stormwater runoff from individual properties and roads intensify, flows usually increase and potential contaminants from residential and commercial activity and associated vehicle use flow into the streams and watercourses. Traditionally stormwater generated from urban areas is conveyed efficiently to designed trunk drainage systems to reduce stormwater ponding and flooding risk. The effect of this type of water management approach on natural systems has in the past included:

• the intensification of flows in watercourses potentially resulting in stream bank erosion and sedimentation; and

• an increase in contamination of receiving aquatic environments resulting in generally adverse impacts on aquatic ecosystems.

• an increase in the use of water resources for domestic, commercial/industrial uses as well as outdoor irrigation of gardens and open space areas;

• an increased tendency for more severe flooding and increased areas of flooding;

Much of the Western Sydney area has recently been converted from a peri-urban and rural landuse to residential development. The implementation of WSUD in these areas can therefore be used to counteract disruptions to the natural water cycle.

The importance of increasing the use of WSUD has be recognised by local councils across Sydney with growing acceptances of the longer-term environmental benefits of the application of WSUD principles. However, implementation of WSUD has been limited in the past due to the lack of understanding of WSUD measures available and suitable to Western Sydney as well as a lack of established procedures, standards and approvals within councils. This creates a perception that there are unacceptable risks involved in approving alternative approaches to water management.

In order to encourage WSUD implementation in Western Sydney this document aims to provide WSUD best management practice design specifications for a diverse range of WSUD measures at the subdivision and allotment scales. The WSUD measures provided in this document are based upon innovative WSUD methods that have proven environmental, aesthetic and economic outcomes and are applicable to the local environment of Western Sydney. It should be noted however that the WSUD design specifications provided is not an exhaustive list of all possible WSUD measures that could be used in urban development. However, they do include those measures that are most likely to be used in the Western Sydney region.


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The appropriate WSUD measures, procedures and products are generally well known, even though ongoing research is providing additional information. The challenge is to gain broad-based acceptance and application of WSUD. This requires a greatly increased level of awareness and understanding of the techniques involved. This document is aimed at achieving broad application of WSUD measures by providing best practice design specifications for a number of those measures.

1.2 What is WSUD

WSUD is the integration of various Best Planning Practices (BPPs) and Best Management Practices (BMP) for the sustainable management of the urban water cycle. WSUD is concerned with the design of urban environments to be more “sustainable” by limiting the negative impacts of urban development on the total urban water cycle. Therefore WSUD is about:

• Trying to more closely match the pre-development stormwater runoff regime, in both quality and quantity;

• Reducing the amount of water transported between catchment, both in water supply import and wastewater export; and

• Optimising the use of rainwater that falls on the urban areas.

There is a wide range of approaches to WSUD, however the most commonly implemented is the improvement of stormwater management. Other approaches include management of water at the household scale such as the collection and use of rainwater from rooftops and water efficient landscape design. The key principles of WSUD as presented in the Urban Stormwater: Best Practice Environmental

Management Guidelines (Victorian Stormwater Committee, 1999) are:

• Protect natural systems (creeks, rivers and wetlands) within urban catchments.

• Protect water quality by improving the quality of stormwater runoff draining from urban developments.

• Integrate stormwater treatment into the landscape by using stormwater treatment systems in the landscape that incorporate multiple uses providing various benefits such as water quality treatment, wildlife habitat, public open space, recreational and visual amenity for the community.

• Reduce runoff peak flows from developments by on-site temporary storage measures (with potential for reuse) and minimise impervious areas.

• Add long-term value while minimising development costs.

• Reduce potable water demand by using stormwater as a resource through capture and reuse for non-potable purposes.


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WSUD applications can be sized to suit most individual sites from residential house blocks through to whole subdivisions. However, appropriate planning and design are required to ensure successful outcomes. The range of applications available may be applied in the following areas:

• new road/street in large or small development areas;

• existing streets and roadways;

• upgrade of drainage systems or pavements;

• publicly owned land

• new residential developments

• existing residential developments, redevelopments and infill areas;

• commercial or industrial developments; and

• carparks / driveways / access ways on public or private property.

It should be noted that WSUD measures cover a range of disciplines and therefore a multi-disciplinary team approach is required to promote urban design which integrates best practice water planning and management measures with attractive streetscapes and open spaces. This integration can create attractive and sustainable urban landscapes that can provide developers with a marketing advantage.

1.3 Proponent of the Technical Guidelines

This document has been prepared by URS Australia Pty Ltd (URS) for the Upper Parramatta River Catchment Trust (UPRCT). The UPRCT in conjunction with the Western Sydney Regional Organisation of Councils (WSROC), Sydney Coastal Councils Group (SCCG), Blacktown City Council (BCC) and Baulkham Hills Shire Council (BHSC) are working together to promote WSUD and sustainable water usage in the Sydney region. Other key organisations such as the Department of Environment and Conservation (DEC, formerly the NSW Environment Protection Authority), the Department of Infrastructure Planning and Natural Resources (DIPNR, formally the Department of Land and Water Conservation), Sydney Water Corporation (SWC) and Landcom are also enthusiastic to pursue WSUD projects and have provided valuable contributions to this project.

The NSW Government’s Stormwater Trust has principally funded the project, with financial support from Sydney Water Corporation and the Upper Parramatta River Catchment Trust.

1.4 Purpose and Use of the Technical Guidelines

The WSUD Technical Guidelines, incorporating the WSUD Design Specifications, are initially intended to provide information for those proposing development within the Blacktown City Council and Baulkham Hills Shire Council Local Government Areas (LGAs). The Guidelines explain how best to incorporate and design WSUD measures into urban developments. It provides guidance to councils, master planners, developers and builders through provision of best management practice design specifications for a number of WSUD measures suitable for application in the Western Sydney area. It


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does not discuss detailed site planning techniques or set water quality or quantity targets. Such targets will generally be set by the relevant Council or regulatory authority.

1.5 Australian Runoff Quality

This document is intended not to replace but be used in conjunction with Australian Runoff Quality, published by the Institution of Engineers’ National Committee on Water Engineering. This document is aimed at providing an overview of the current best practice in the management of urban stormwater quality in Australia and guidelines to the following:

• procedures for the estimation of a range of urban stormwater contaminants;

• design of stormwater quality improvement measures;

• procedure for the estimation on performance of stormwater quality improvement measures; and

• development of an integrated urban water cycle management strategy.

1.6 Document Structure

The technical guidelines are organised in the following sections:

Section 2 Physical Characteristics in Western Sydney

Section 3 WSUD Elements and Applications

Section 4 WSUD Planning and Selection Guide

Section 5 WSUD Design Specifications

Section 6 WSUD Operation and Maintenance

Section 7 Life Cycle Costs for WSUD Measures

This draft document has been prepared for review and comment by the WSUD Technical Specifications Project Steering Committee. Amendments will be made in response to comments received and a final document produced.

SECTION 2 Key Physical Characteristics Affecting WSUD in Western Sydney

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2 Key Physical Characteristics Affecting WSUD in Western Sydney

The key physical factors that affect the function, design and performance of WSUD measures in Western Sydney are:

• Climate;

• Geology and Soils;

• Groundwater/Salinity; and

• Catchment Water Quantity and Quality Objectives.

2.1 Climate

The climate in Western Sydney is typically characterised by warm, wet summers and cool, dry winters. Rainfall in summer is typically associated with thunderstorm activity. The major influences on Sydney's climate are the topography in and around the Sydney area, the sea-surface temperature of the coastal waters, and the orientation of the coastline. Western Sydney receives less rainfall than the coastal areas of Sydney and there is also significant variation in rainfall across the Western Sydney area. The average monthly distribution of rainfall across the year within Western Sydney is summarised in Table 2.1 below. Generally, the average rainfall is highest in the northeast and least in the southwest. Annual rainfall tends to vary from approximately 750 to 950 mm within the Western Sydney area

Table 2.1

Average Monthly Rainfall in the Western Sydney Area

Station Jan Feb Mar Apr May

Jun Jul Aug Sep Oct Nov Dec Total

Blacktown (1963 – 1993)

102 95 97 77 57 78 37 50 40 58 85 70 854

Prospect (1887 – 2003)

95 94 98 75 74 74 59 52 48 59 72 76 874

Windsor (1897 – 2003)

87 91 83 67 57 59 48 45 40 55 67 70 769

Parramatta (1832 – 1960)

89 96 99 91 80 82 80 55 51 63 63 72 921

Liverpool (1962 – 2001)

98 95 101 85 69 73 39 58 46 62 78 66 870

Bankstown Airport

(1968 – 2001) 94 107 118 94 67 81 43 52 42 64 82 73 917

Badgerys Creek

(1936 – 1996) 94 93 89 65 60 66 34 48 38 56 74 74 789


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Average monthly evaporation rates for the Western Sydney area are also summarised in Table 2.2 below.

Table 2.2

Average Evaporation Rates in Western Sydney

Station Jan Feb Mar Apr May

Jun Jul Aug Sep Oct Nov Dec Total

Prospect (1974 – 2003)

173 139 125 92 64 51 57 82 111 142 154 186 1373

A summary of the monthly rainfall statistics for the general Sydney area are summarised in Tables 2.3 and 2.4 (sourced from ARQ, 2003).

Table 2.3

Rainfall Statistics – Mean Inter-Event Dry Period (hrs) for Sydney

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual

70.3 64.7 66.6 69.3 70.2 73.4 91.5 98.5 97.8 77.9 68.9 76.3 75

Table 2.4

Rainfall Statistics – Mean Storm Duration (hrs) for Sydney

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual

11.0 11.4 12.2 13.8 13.5 16.2 13.4 13.0 11.0 11.3 10.7 11.5 12.4

2.2 Geology and Soils

The Hawkesbury Sandstone and Wianamatta Group Shales are the two main geologic groups in Western Sydney. Steep slopes and rolling plateaux are typical of a Hawkesbury Sandstone surface geology, which occurs generally in the northern and eastern regions. Gently undulating hills and flat plateaux are associated with Wianamatta Shales, which extend over most of Western Sydney.

Sandy soils are associated with Hawkesbury Sandstone parent material. These soils are generally shallow and highly permeable (Bannerman and Hazelton 1990). Clay soils are associated with the Wianamatta Shales. The clays characteristically have low permeability and poor drainage and in some areas high erosion hazard. The soils on Wianamatta Shales are also associated with areas of potential salinity hazard (refer Section 2.3).

Given the range of soil types and hazards, such as erosion and salinity, the application of WSUD measures, particularly those that involve infiltration, needs careful consideration to the soil types occurring at individual WSUD project sites.


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2.3 Groundwater/Salinity

Salinity is a well-recognised issue in Western Sydney. According to WSROC (2002) urban development in Western Sydney can contribute to salinity problems in the following ways:

By exposing sodic or saline sub-soils

When areas are developed using the process of cut and fill, particularly for slab on-ground construction, the upper layers of soils are removed or disturbed. If the lower soil profile has saline or sodic properties, this can result in the occurrence of salinity problems and erosion.

By increasing the level of regional groundwater and encouraging the development of perched water tables.

Traditional urban development can increase the amount of water entering the natural system. This is due to the irrigation of parks and gardens, leaking stormwater and sewer pipes and changed stormwater flows and concentrations.

By changing soil groundwater flow creating areas of impeded drainage or forced drainage.

This can result in sub-soil salinity being expressed on the surface at these points. For example where roads, house slabs, retaining walls or trenches intercept the soil water flow or create hydraulic pressure that raises the groundwater table.

By developing or disturbing areas sensitive to salinity.

Some areas exist in a delicate balance that once disturbed is difficult to restore and rapidly deteriorates. For example, removing established salt resistant vegetation in riparian corridors could increase erosion and downstream disturbances.

Due to the role of water in all salinity problems; the management of water and site drainage on sites in potential salinity hazard areas is essential. A potential salinity hazard therefore has direct implications for the selection and design of WSUD measures. The key principles of salinity management that need to be considered when implementing WSUD include:

• identify hazard areas and processes on the site;

• reduce water input and maintain natural water balance that limits groundwater rise and soil water flow-through;

• maintain good drainage and reduce water logging;

• retain or increase vegetation in strategic areas; and

• implement building controls and/or engineering responses where appropriate.


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Salinity Hazard Mapping for Western Sydney

To assist with the first step identified above, DLWC (now known as DIPNR) have produced a draft Salinity Hazard Map for Western Sydney. The most recent issue of this map is December 2000. The Salinity Hazard Map depicts potential salinity hazard zones and areas with known salinity problems. The map is at a scale of 1:100 000 and covers the area of the Penrith Soil Landscape map sheet (Bannerman and Hazelton 1990). It is noted in the draft guidelines which accompany the map that the salinity hazard zones should not be used at the property scale (i.e. individual lots) and that the maps should only be used to identify the general level of hazard in the locality of the site. This indicates the appropriate salinity assessment and management response for the site. To identify local variations in salinity hazard, it may be necessary to undertake on-site investigations, which the map cannot substitute for.

The Salinity Hazard Map identifies four classes of salinity hazard:

1. known areas of salinity;

2. areas of extensive salinity hazard;

3. areas of localised salinity hazard; and

4. no known hazard.

The draft salinity hazard map shows that salinity may occur throughout the Western Sydney LGA’s. Areas of extensive salinity hazard particularly include the lower slopes and streamlines, which have the potential to become waterlogged, or where the movement of water through the soil profile is low. Areas of localised salinity hazard cover the remainder of the map wherever Wianamatta Group shales and their derived soil materials are found. Areas of no known salinity hazard have been mapped on areas of Hawkesbury Sandstone and Narrabeen Group sedimentary rocks.

It should be noted that the Salinity Hazard Map is currently in draft (as at October 2003), and therefore users should ensure that the latest version is being sourced prior to use.

Salinity Hazard Assessment for Western Sydney

An assessment key accompanies the draft Salinity Hazard Map for Western Sydney to assist in the assessment of salinity hazard for specific sites in Western Sydney. It is intended for use at the planning scale and requires some field observation and reference to other maps and documents. It does not provide proof of the absence or presence of salinity but is simply a guide to planners and designers in deciding what type of salinity process model may be operating and whether further investigations may be necessary. The types, methodology and interpretation of salinity investigations suitable for urban developments is provided in WSROC (2002).

The WSUD design specifications provided in this document have generally taken into account salinity risk in the Western Sydney area. However, this does not preclude the need for appropriate site-specific investigations.


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2.4 Stormwater Management Objectives

2.4.1 Objectives for New Developments

Stormwater management objectives for new development aim to define those stormwater outcomes which councils and/or developers will seek to achieve in the development or redevelopment of land (EPA, 2000). The purpose of prescribing such objectives is to minimise the impact of new development (including redevelopment) upon receiving waterways and areas of natural heritage.

The objectives, recommended by the NSW Department of Environment and Conservation, aim to capture the greatest stormwater management opportunities that can be incorporated into new development, compared to those able to be cost-effectively retrofitted to existing urban areas. These objectives will provide guidance to both Council officers and development proponents in the management of stormwater from new developments.

For the construction and post-construction phases of development, the stormwater management objectives include both quantitative performance objectives for stormwater quality control measures, and qualitative management principles and objectives that should be adopted to mitigate any other known potential impacts on the environment. In addition, it is useful to consider stormwater management objectives for new development at the scales of individual lots, subdivisions and sub-catchments.

2.4.2 Construction Phase

The primary stormwater issue during the construction phase of new development is sediment eroded from exposed areas or flow paths, which leads to elevated levels of sediment and turbidity in stormwater discharges from the site. Secondary issues include chemicals (including fuels and oils) stored on site and litter generated by construction activities.

The construction phase objectives adopted by Council are listed in Table 2.5 below. As both Type C, Type D and Type F soils (as described in Managing Urban Stormwater: Soils and Construction document) are present within the local government area, objectives are expressed for runoff from these soil types. Type D (dispersible) soils are of particular concern because of the elevated levels of turbidity in stormwater runoff from exposed areas.

Table 2.5

Construction-phase Stormwater Management Objectives for New Development

Quantitative Objectives – applicable to subdivisions and all medium-large scale developments(a)

Pollutant / Issue Soil Type (b) Management Objective (b)

Type D (dispersible)

Type F (fine)

Suspended solids concentration not to exceed 50 mg/L for all 5-day rainfall totals up to the 75th

percentile rainfall event

Suspended solids and turbidity

Type C (coarse) Suspended solids concentration not to exceed 50 mg/L for all flow events up to 25% of the 1 Year ARI flow. (b)


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Qualitative Objectives – applicable to all new development, including individual building lots.

Pollutant / Issue Management Objective

Suspended solids (sediment)

Minimise soil erosion and the discharge of sediment by the appropriate design, construction and maintenance of erosion and sediment control measures.

Employ all practical measures to minimise soil erosion and the discharge of sediment in storm events exceeding the design storms specified in the Quantitative Objectives.

Motor Fuels, Oils and other Chemicals

All motor fuels, oils and other chemicals are stored and used on site in a manner which ensures no contamination of stormwater.

Litter No litter placed in a position where it may be blown or washed off-site.

Key (a) “medium-large scale developments” generally defined as those greater than 2,500 m2 total area

(b) Refer to Section 6.3, Managing Urban Stormwater: Soils and Construction document for further information.

(c) Assumes settling 0.02 mm particle in runoff from Type C soils achieves 50 mg/L suspended solids

2.4.3 Post-Construction Phase

Quantitative Objectives

Different types of land use typically generate specific stormwater pollutants in significant quantities. Consequently, the ‘key’ pollutants to be addressed from new development, and the control techniques employed, are a function of the type of development. Table 2.6 ranks the significance of pollutants likely to be generated by different land uses.

Table 2.6

Ranking of Objectives for New Development

Development Style Litter Coarse

sediment

Fine

particles

Total

phosphorus

Total

nitrogen

Hydrocarbons,

motor fuels, oils

and grease

Low Density Residential

Y N N Y Y N

High Density Residential

Y Y Y Y Y V

Commercial, Shopping & Retail

Outlets

Y Y Y N N N

Industrial Y Y Y V V Y


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Development Style Litter Coarse

sediment

Fine

particles

Total

phosphorus

Total

nitrogen

Hydrocarbons,

motor fuels, oils

and grease

Fast Food Outlets and Restaurants

Y N N N N V

Carparks, Service Stations and Wash

Bays

Y Y Y N V Y

Adapted from Upper Parramatta River Stormwater Management Plan, 1999

Key: Y – key pollutant – needs to be addressed V – variable – requires site-specific assessment N – not significant

Table 2.7 prescribes the degree of pollutant retention to be achieved in relation to the specific pollutants likely to be discharged from a range of new developments (denoted ‘Y’ above). For small developments (generally less or equal to 5 ha), these performance objectives shall be met for all flows up to 25% of the 1 year ARI peak flow from the development site.

For larger urban developments (greater than 5 ha in area for Blacktown City and Penrith City), or developments proposed within particularly sensitive catchments, proponents will be required to assess the magnitude of any change in stormwater pollutant loads caused by the development (with proposed stormwater controls) (‘Level 2’ or ‘Level 3’ modelling, Appendix F, Managing Urban Stormwater:

Council Handbook), and the likely impact of any increase in pollution levels.

Table 2.7

Quantitative Post-Construction Phase

Stormwater Management Objectives for New Development

Quantitative Objectives – applicable to subdivisions and all medium-large developments (a)(b)

Pollutant / Issue Retention Criteria

Coarse sediment 80% of average annual load for particles 0.5 mm or less

Fine particles 50% of average annual load for particles 0.1 mm or less

Total phosphorus 45% retention of average annual load

Total nitrogen 45% retention of average annual load

Litter 70% of average annual litter load greater than 5 mm

Hydrocarbons, motor fuels, oils and grease. 90% of average annual pollutant load

(a) “medium-large scale developments” generally defined as those greater than 1,000 m2 total area. (b) areas relate to total developments, rather than individual stages of a development.

Greater levels of pollutant retention than those listed in Table 2.7, including ‘no net increase’ in pollutant loads from the pre (existing) development situation, may be adopted in pursuit of sub-catchment


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objectives relating to the protection or restoration of catchment values. In such circumstances, enhanced source controls or ‘compensatory’ stormwater measures will be considered in order to achieve this greater degree of pollutant retention.

These stormwater management objectives will be adopted in relation to individual developments (particularly redevelopments) to the maximum extent practicable in respect of site constraints, opportunities and objectives. Urban salinity issues, for example, may constrain the scope of stormwater management options considered feasible in individual developments.

Qualitative Objectives

The retention of pollutants is only one part of stormwater management at new development sites. There are a number of stormwater management objectives that, although not quantifiable, are nonetheless critical to the pursuit of more sustainable stormwater management practices at new development sites. Councils will require developers to employ the following stormwater management principles, listed in Table 2.8, for managing stormwater from new developments.

Table 2.8

Qualitative Post-Construction Phase

Stormwater Management Objectives for New Development

Qualitative Objectives – applicable to all new development

Pollutant / Issue Management Objective

Impervious areas connected to the stormwater drainage system are minimised

Reuse of stormwater for non-potable purposes maximised

Use of vegetated flow paths maximised

Runoff volumes and flow rates(a)

Stormwater quality(a)

Use of stormwater infiltration ‘at source’ where appropriate

Riparian Vegetation and Aquatic Habitat

Protect and maintain natural wetlands, watercourses and riparian corridors.

All natural (or unmodified) drainage channels within the site which possess either:

a) base flow;

b) defined bed and/or banks; or

c) riparian vegetation

are to be protected and maintained.

“Natural” channel designs (b) should be adopted in lieu of floodways in areas where there is no natural (or unmodified) channel.

Flow Alterations to natural flow paths, discharge points and runoff volumes from the site are to be minimised.

The frequency of bank-full flows should not increase as a result of development. Generally, no increase in the 1.5 year and 100 year peak flows.

Amenity Multiple use of stormwater facilities to the degree compatible with other management objectives

Urban bushland Impact of stormwater discharges on urban bushland areas minimised

(a) Stormwater quality benefits of “Water Sensitive Urban Design” principles may contribute to the achievement of the above pollutant retention criteria.


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(b) “Natural channel designs” involves the creation of channels with attributes of natural channels, including a meandering plan, pool and riffle zones, use of natural materials and riparian/floodplain vegetation. (See Section C6.3, draft Managing Urban Stormwater: Council Handbook for further information and references).

Treatable Flow Rates

The approach required to demonstrate compliance with the above objectives and retention criteria is described Appendix F of Managing Urban Stormwater: Council Handbook – Draft (EPA, 1997). Three categories with associated modelling techniques are described in this appendix; however, a fourth category is to be included for catchments less than 5 ha. This category will specify a Treatable Flow Rate (TFR) for each hectare that contributes to the pollution control device. The TFR is defined as the minimum flow that a stormwater treatment measure must be capable of treating, without bypass, to achieve the desired Pollutant Retention Criteria for the particular development style and catchment size. The TFR will vary throughout the catchments of Western Sydney depending on site-specific rainfall and runoff relationships, and each Council would need to develop the appropriate TFR for use within its Local Government Area.

Blacktown City Council’s Stormwater Quality Control Policy (BCC, 2001) provides interim design Treatable Flow Rates and runoff depths for various pollutant types, and are summarised in Table 2.9 below.

Table 2.9

Treatable Flow Rates and Runoff Depths (Blacktown City Council)

Pollutant Type Treatable Flow Rate

(L/sec/ha) Runoff Depth (mm)

Gross pollutants / coarse sediments 60 30

Fine sediments / hydrocarbons 10 10

The Treatable Flow Rate for gross pollutants/coarse sediments are higher compared to rates for fine sediments, as they require greater energy and time for the mobilisation of these pollutants. Similarly, the concentration of gross pollutants/coarse sediments transport is dependent on flow rate and thus a higher treatable flow rate is required to achieve a similar mean annual load retention compared with pollutants associated with fine sediments.

SECTION 3 WSUD Measures and Application

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3 WSUD Measures and Application

3.1 WSUD Measures

This section provides general information on various WSUD measures in terms of their application, limitations, pollutant removal efficiency, construction and cost implications, to enable designers, engineers and planners to better understand the function of each WSUD measure and its role in the treatment train. The WSUD measures discussed in this section and for which design specifications have been prepared are presented in Table 3.1.

Table 3.1

WSUD Measures Included in the Design Specifications

Number Name

DS1 Vegetated Swales

DS2 Vegetated Filter Strips

DS3 Sand Filters

DS4 Bioretention Systems

DS5 Permeable Pavements

DS6 Infiltration Trenches

DS7 Infiltration Basins

DS8 Rainwater Tanks (single lot above ground)

DS9 Landscape Developments

These elements were chosen as they represent the key fundamental WSUD measures that are currently used in best management WSUD practices in both building and sub-division designs within Australia and overseas.

Primary treatment WSUD measures, such as litter traps and gross pollutant traps (GPTs) have not been included in this document, as there are numerous proprietary manufactured devices that are currently available that provide detailed technical design manuals and guidelines. If further information is required on the design and selection of appropriate traps, refer to EPA (1997) and WBM (2003).

Tertiary treatment WSUD measures, such as water quality ponds and constructed wetlands, have not been included in this document, as the design of these systems is very specialised, site specific and relatively complex and therefore (currently) outside the scope of the technical guidelines. If further information is required on the planning, design and layout of wetland ponds, refer to the Department of Land and Water Conservation: The Constructed Wetlands Manual (DLWC, 1998), the Cooperative Research Centre for Catchment Hydrology: Managing Urban Stormwater Using Constructed Wetlands (CRCCH, 1999), and Australian Runoff Quality (ARQ, 2003).


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3.2 Relationship between WSUD Measures

WSUD treatment measures can be grouped into three main categories: primary, secondary and tertiary. The definition of these categories are provided in the Victorian Stormwater Committee: Urban

Stormwater – Best Practice Environmental Management Guidelines, (Victorian Stormwater Committee, 1999) and typical WSUD measures appropriate to each category are summarised in Table 3.2 below.

Table 3.2

WSUD Treatment Measure Categories

Category Definition Typical Retained

Pollutant

Typical WSUD Measures

Primary Physical screening or rapid sedimentation techniques

Gross pollutants and litter, coarse sediments, free oil/grease

Gross pollutant traps (GPT’s), sediment traps, oil/grit separators

Secondary Finer particle sedimentation and filtration techniques

Fine particles and attached pollutants

Sand filters, permeable pavements, vegetated filter strips, vegetated swales, infiltration systems

Tertiary Enhanced sedimentation and filtration, biological uptake, absorption onto sediments

Nutrients and heavy metals Constructed wetlands, bioretention systems, natural stream systems

A fundamental feature of the WSUD philosophy is the restoration of natural features in the hydrological system. This is typically achieved by a series of hydrological design responses at four distinct treatment control levels or stages in the urban hydrological system. The description and the various WSUD measures that can be applied at each of the four levels are summarised in Table 3.3 below (sourced from Coombes, Donovan and Cameron, 1999).

Table 3.3

Control Levels in the Urban Hydrological System

Level Description / Location Typical WSUD Measures

Source Control At the individual building allotment

Rainwater tanks, infiltration trenches, vegetated filter strips, planting beds, permeable pavements

Conveyance Control Conveyance of stormwater to streets and channels

Vegetated filter strips and swales, on-line bioretention systems, natural channels, streetscapes

Discharge Control At the point where water leaves the lot, estate or catchment

Bioretention and infiltration basins, sand filters, constructed wetlands, detention ponds

Natural Systems Throughout the urban catchment

Natural water courses, creeks, floodplains, wetlands and vegetation


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3.3 Description of WSUD Measures and Implementation Issues

3.3.1 Vegetated Swales

Swales are formed, vegetated depressions that are used for the conveyance of stormwater runoff from impervious areas. They provide a number of functions including:

• Removing sediments by filtration through the vegetated surface;

• Reducing runoff volumes (by promoting some infiltration to the sub-soils); and

• Delaying runoff peaks by reducing flow velocities.

Swales are typically linear, shallow, wide, vegetation lined channels. They are often used as an alternative to kerb and gutter along roadways but can also be used to convey stormwater flows in recreation areas and car parks.

Application

• Most effective in removing coarse to medium sized sediments;

• Typically most practical and cost effective when serving catchment areas up to 2 ha and typically should not be used in catchments over 4 ha in area;

• Most often used as a pre-treatment for other stormwater treatment devices, such as bioretention and infiltration systems;

• Most applicable at the subdivision scale (i.e. along median strips, or through parks) but can be applied at allotment level, depending on catchment area;

• Best placed in central median strips rather than on edge of road where driveways and services are required, however driveways and services can be accommodated with swales as needed;

• Can substitute underground pipes, dish drains or kerb and gutters and are typically situated adjoining allotment boundaries or impervious surfaces such as roads.

Pollutant Trapping Efficiency

Typical pollutant removal efficiencies for swales are provided in the table below (source: WBM, 2003).

Gross

Pollutants*

Coarse

Sediment*

Medium

Sediment

Fine

Sediment

Free Oil and

Grease

Nutrients Metals

- 50 – 80% 30 – 50% 10 – 50% 10 – 50% 10 – 50% 10 – 50%

* Assumes gross pollutant pre-treatment provided.


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Limitations

1. Site limitations that may preclude the use of this WSUD measure include:

– Suitable only for gentle grades between 1% and 6%;

– Site requires adequate sunlight for vegetation growth.

2. Site limitations that may be overcome by modification to the design specifications include:

– Pre-treatment of gross pollutants required;

– Water ponding within relatively flat swale grades (<2%) to incorporate a subsoil drain to improve drainage.

3. Other issues which require consideration in design planning include:

– Wider road corridors may be required to incorporate swales and off street parking may be limited;

– Regular inspections and maintenance required during the establishment period;

– Potential for damage during construction of other developments;

– Establishment period for vegetation growth may be relatively long, particularly during winter;

– Residents need to be informed of swale function and benefits in order to prevent damage or misuse.

Vegetation Selection

The selection of vegetation for swales should not preclude the use of plants other than low grasses. It is often desirable for general landscape/aesthetics reasons, or to limit pedestrian or vehicular traffic to use more substantial plants and shrubs. Recommended plant species for swales is provided in Design Specification DS9 – Landscape Developments.

Construction Issues

The timing of swale construction must take account of the intended function of the area and device. If the swale is to be used during development construction, the swale should be constructed well in advance of development to provide enough time for the swale vegetation to establish. Depending on the site runoff sediment loads and flow rates, swales may need to be restored once development construction is complete.

If the swale is to be constructed for use after development completion, it should be protected from construction-site runoff and should be fenced during the construction period to prevent damage from heavy plant and vehicles.


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3.3.2 Vegetated Filter Strips

Vegetated filter strips (or buffers) are broad, sloped open vegetated areas that accept shallow runoff from impermeable areas as distributed or sheet flow. They provide a number of functions including:

• Removing sediments by filtration through the vegetation;

• Reducing runoff volumes (by promoting some infiltration to the sub-soils); and


Application

• Most effective in removing coarse to medium sized sediments and attached pollutants (such as nutrients, free oils/grease and metals);

• Typically used in conjunction with swales as an alternative to kerb and gutter and can form part of a multi-use corridor.

• Typically used as a pre-treatment for other stormwater treatment devices, such as bioretention and infiltration systems;

• Most applicable at the subdivision scale, with catchment areas less than 2 ha, however can be applied at allotment level (eg. buffering runoff from driveways, overflows from rainwater tanks etc) depending on catchment area


Typical pollutant removal efficiencies for vegetation filter strips are provided in the table below (source: WBM, 2003).

Gross

Pollutants*

Coarse

Sediment*

Medium

Sediment

Fine

Sediment

Free Oil and

Grease

Nutrients Metals

- 50 – 80% 30 – 50% 10 – 50% 10 – 50% 10 – 50% 10 – 50%


Limitations

1. Site limitations that may preclude the use of this WSUD element:

– Suitable only for relatively flat or gradually sloping areas; up to 5% grade;

– Require adequate sunlight for vegetation growth.

2. Other issues that require consideration in design planning:

– Regular inspections and maintenance required during the establishment period.


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Construction Issues

Filter strip construction must be timed to take account of the intended function of the area/device. If the filter strip is to be used during development construction, the filter strip should be constructed well in advance of development to allow adequate time for vegetation in the filter strip to establish. Depending on site runoff sediment loads and flow rates, filter strips may need to be restored once development construction is complete.

If the filter strip is to be constructed for use after development completion, it should be protected from construction-site runoff and should be fenced during the construction period to prevent damage from heavy plant and vehicles.

3.3.3 Sand Filters

Sand filters typically comprise of a bed of filter medium through which stormwater is passed to treat it prior to discharging to the downstream stormwater system. The filter media is usually sand, but can also contain sand/gravel and peat/organic mixtures. Sand filters provide a number of functions including:

• Removing fine to coarse sediments and attached pollutants by infiltration through a sand media layer; and

• Delaying runoff peaks by providing retention capacity and reducing flow velocities.

Sand filters can be constructed as either small or large scale devices. Small scale units are usually located in below ground concrete pits (at residential/lot level) comprising of a preliminary sediment trap chamber with a secondary filtration chamber. Larger scale units may comprise a preliminary sedimentation basin with a downstream sand filter basin-type arrangement.

Application

• Most effective in removing medium to fine sized sediments and attached pollutants (such as nutrients, free oils/grease and metals, specially when the sand is mixed with organic mulch);

• Best suited as near source treatment measures with small catchments (<0.4 ha) for residential, commercial and industrial developments with high percentages of impervious areas, such as parking lots, service stations, high density residential housing and roadways;

• Maximum catchment area should be less than 4 ha;

• Appropriate for retro-fitting, sites with space limitations and underground or under road pavement installations.


Typical pollutant removal efficiencies for sand filters are provided in the table below (source: WBM, 2003):


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Gross

Pollutants*

Coarse

Sediment*

Medium

Sediment

Fine

Sediment

Free Oil and

Grease

Nutrients Metals

- 50 – 80% 50 – 80% 30 – 50% 30 – 50% 30 – 50% 30 – 50%


Limitations


– High head loss due to vertical filtration;

– Restricted to relatively low flow rates through sand filter media;

– Requires frequent inspection and maintenance (ie. annual removal and replacement of surface filter sand).


– Upstream pre-treatment of litter and coarse sediments is essential to minimise filter clogging;

– Large land areas are required for large scale devices with limitations on future land use;

– To reduce infiltration to the groundwater system (ie. in known salinity hazard areas), provision of an underlying perforated pipe is to be provided to recover the filtered water; and

– Large sand filters without a vegetation cover can be unattractive in residential areas.

Construction Issues

The following issues are important to recognise during construction:

• The filter is not be used for sediment control during construction;

• The foundation area should be compacted to sustain the load placed on it by the filtration system; and

• The sand filter material and grading must meet the criteria specified on the technical specification for the works.

3.3.4 Bioretention Systems

Bioretention systems are essentially a surface and sub-surface water filtration system. They provide a number of functions including:

• Removing sediments and attached pollutants by filtering through surface vegetation, ground cover and through an underlying filter media layer; and


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Bioretention systems are similar in function to sand filters. Whereas sand filters rely on water quality treatment via passage of stormwater through a sand medium, bioretention systems incorporate both plants and an underlying filter media such as soil for the removal of contaminants. The vegetation enhances the filtration process as well as maintaining the porosity of the filter media. The filter media is usually the plant growing material, which may comprise soil, gravel, sand and peat mixtures.

Bioretention trenches can be constructed as either small or large scale devices. Small scale units are usually located in residential planter boxes (sometimes referred to as “rain gardens”), which pass collected stormwater and percolate it through the filer media to the outlet. Larger scale devices work on the same principle and can be located along the streetscapes and in conjunction with retarding basins over large open areas.

There are two main types of bioretention systems:

• Non-conveyance (off-line) systems – These use a freeboard for ponding above the bioretention surface to maximise the volume of runoff treated. Typically they contain the design inflow with higher flows discharged through overflow pits or bypass paths and are not required to convey flood flows. They are commonly installed in planting boxes or streetscapes as a landscape feature.

• Conveyance (on-line) systems – These treat the design inflow but are also able to convey minor storm events along longitudinal channels. These systems are commonly used in streetscape applications in combination with vegetated swales, which are used to convey street runoff to the designated bioretention system.

Application

• Most effective in removing medium to fine sized sediments and attached pollutants (such as nutrients, free oils/grease and metals), but have typically higher pollutant removal efficiencies for a wider range of contaminants due to enhanced filtration/biological processes associated with the surface vegetation;

• Best suited too small (<5 ha catchments) residential, commercial and industrial developments with high percentages of impervious areas, including parking lots, high density residential housing, roadways and bridges. “Planting box” type systems should restricted to catchment areas less than 0.1 ha (ARC, 2003).

• Commonly used in conjunction upstream vegetated filter strips or swales to provide effective water treatment chain and conveyance of stormwater runoff;

• May have aesthetic benefits due to the surface vegetation and therefore can be incorporated in streetscape and general landscape features; and

• Can be appropriate in areas where runoff is insufficient or unreliable, evaporation rates are high, or soils are too pervious to sustain the use of constructed wetlands.


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Typical pollutant removal efficiencies for bioretention systems are provided in the table below (source: WBM, 2003):

Gross

Pollutants*

Coarse

Sediment*

Medium

Sediment

Fine

Sediment

Free Oil and

Grease

Nutrients Metals

- 80 – 100% 50 - 80% 30 – 50% 30-50% 30-50% 30-50%


Limitations


– High head loss due to vertical filtration; and

– Require adequate sunlight for vegetation growth.


– Upstream pre-treatment of litter and coarse sediments is essential to minimise filter clogging;

– Regular inspections and maintenance required during the vegetation establishment period.

Construction Issues


• The filter system should not be used for sediment control during construction;

• The filter material and grading must meet the criteria specified on the technical specification for the works;

• Quality control relating to filter media placement is essential during construction.

3.3.5 Permeable Pavement

Permeable pavements, which are an alternative to typical impermeable pavements, allow runoff to percolate through hard surfaces to an underlying granular sub-base reservoir for temporary storage until the water either infiltrates into the ground or discharges to a stormwater outlet. They provide a number of functions including:

• Removing some sediments and attached pollutants by infiltration through an underlying sand/gravel media layer;

• Reducing runoff volumes (by infiltration to the sub-soils); and


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• Delaying runoff peaks by providing retention/detention storage capacity and reducing flow velocities.

Commercially available permeable pavements include pervious/open-graded asphalt, no fines concrete, modular concrete blocks and modular flexible block pavements.

There are two main functional types of permeable pavements:

• Infiltration (or retention) systems – temporarily holding surface water for a sufficient period to allow percolation into the underlying soils; and

• Detention systems - temporarily holding surface water for short periods to reduce peak flows and later releasing into the stormwater system.

Application

• Most effective in removing coarse to medium sediments and attached pollutants (such as nutrients, free oils/grease and metals),

• Most practical and cost effective when serving catchment areas between 0.1 and 0.4 ha;

• Best suited to catchment areas with low sediment loads and lightweight vehicle traffic such as small carparks, low traffic streets (eg. cul-de-sacs) and for paving within residential and commercial developments;

• Can be used to provide a more aesthetically pleasing surface compared to conventional asphalt/concrete pavements;

• Applicable for pavements grades of 1% or greater, with a maximum grade of 5%.


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Typical pollutant removal efficiencies for infiltration systems are provided in the table below (source: WBM, 2003):

Gross

Pollutants*

Coarse

Sediment*

Medium

Sediment

Fine

Sediment

Free Oil and

Grease

Nutrients** Metals

- 50 – 80% 50 – 80% 30 – 50% 30 – 50% 30 – 50% 30 – 50%

* Assumes gross pollutant pre-treatment provided. ** Bound to sediments and some dissolved nutrients.

Limitations


• Not suitable in areas of high traffic volumes or vehicle weights;

• Not suitable in areas where the catchment or high wind generates significant sediment loads;

• Pre-treatment of runoff from the pavement itself is not possible;

• Infiltration systems are generally not suitable in the following soil or terrain conditions:

– Loose sands or heavy clays;

– Exposed bedrock or shallow soils over rock or shale;

– Steep terrain (>5%);

– High water tables;

– Potential salinity hazard areas;

– Non-engineered fill or contaminated land.


• Inadequate maintenance frequencies can result in the underlying media becoming clogged.

Site Evaluation and Selection for Infiltration Systems

To reduce the likelihood of failure of infiltration systems in conjunction with permeable pavements, an initial site evaluation and assessment is required prior to undertaking planning and design. The methodology and procedure for this evaluation is outlined in Section 3.3.6.


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Construction Issues


• The pavement area should not be used for sediment control during construction;

• Pavements should not be laid until all catchment surface areas have been stabilised to prevent sedimentation and consequent premature clogging.

• For infiltration systems:

– Construction areas are to be fenced off to prevent heavy equipment compacting the underlying soils;

– The pavement subgrade is to be ripped/tyned before placement of the overlying aggregate of topsoil.

3.3.6 Infiltration Trenches

Infiltration trenches temporarily hold stormwater runoff within a sub-surface trench prior to infiltrating into the surrounding soils. Infiltration trenches provide the following main functions:

• Removing sediments and attached pollutants by infiltration through the sub-soils;


• Delaying runoff peaks by providing detention storage capacity and reducing flow velocities.

Infiltration trenches typically comprise of a shallow, excavated trench filled with reservoir storage aggregate. The aggregate is typically gravel or cobbles but can also comprise of modular plastic cells (similar to a milk crate). Runoff entering the system is stored in the void space of the aggregate material or modular cells prior to percolating into the surrounding soils. Overflow from the trench is usually to a downstream drainage system. Infiltration trenches are similar in concept to infiltration basins (refer Section 3.3.8), however trenches store runoff water below ground within a pit and tank system, whereas basins utilise above ground storage.

Application

• Primary function would be reducing runoff volumes (by infiltration to the sub-soils) and delaying runoff peaks by providing detention storage capacity. Secondary function would be removing fine sediments and attached pollutants (such as nutrients, free oils/grease and metals) by allowing infiltration through the sub-soils,

• Best suited to small (<2 ha catchments) residential, commercial and industrial developments with high percentages of impervious areas, including parking lots, high-density residential housing and roadways.


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• Allows surface area above the trench to be used for planting, gardens, temporary parking lots etc (i.e. advantage over infiltration basins);

• Commonly used in conjunction with overlying permeable pavements as an effective water treatment chain.


Typical pollutant removal efficiencies for infiltration trenches are provided in the table below (source: WBM, 2003):

Gross

Pollutants*

Coarse

Sediment*

Medium

Sediment

Fine

Sediment

Free Oil and

Grease

Nutrients** Metals

- 50 – 80% 50 - 80% 30 – 50% 30-50% 30-50% 30-50%

* Assumes gross pollutant pre-treatment provided. ** Bound to sediments and some dissolved nutrients.

Limitations


• Risk of groundwater contamination and low dissolved pollutant removal in coarse, high permeability subsoils;

• Generally not suitable in the following soil or terrain:

– heavy clays;


– Steep terrain;

– High water table;




• Upstream pre-treatment of litter and coarse sediments is essential to minimise clogging of the underlying infiltration surface;

• Inadequate maintenance frequencies can result in the underlying surface soils to become clogged.

Site Evaluation and Selection

To reduce the likelihood of failure of the infiltration trenches, an initial site evaluation and assessment is required prior to undertaking planning and design.


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Site Evaluation

The site is to be initially evaluated based on the following criteria (adapted from Horner, 1994):

• Catchments draining the infiltration system area to be less than 2 ha;

• Base of facility shall be at least 1.0 to 1.5 m above the seasonal high water table, bedrock, or a low permeability layer;

• Subsoil permeability shall be at least 0.8 to 1.3 mm/hr, unless an engineering analysis confirms the viability of less permeable subsoils;

• Subsoil shall not have more than 30% clay or 40% clay and silt content combined;

• The facility shall not be constructed on slopes greater then 15% unless an engineering analysis confirms the viability;

• The facility shall not be constructed on soils posing a potentially salinity hazard, nor fill or contaminated land;

• The facility shall not recharge a potential water supply aquifer; unless an engineering analysis confirms that no potential contamination of the aquifer will occur;

• The facility shall have sufficient clearance to property boundaries, buildings or other structures than indicated in the Table 3.4 below (source: Coombes, 2002a).

Table 3.4

Minimum Clearances for Infiltration Systems

Soil Type Hydraulic Conductivity

(mm/hr)

Minimum Clearance

(m)

Sand >180 1

Sandy Clay 180 – 36 2

Medium Clay 36 – 3.6 4

Heavy Clay 3.6 – 0.036 5

Construction Issues


• The infiltration device should not be used for sediment control during construction;

• The aggregate for filling of the infiltration trench should meet the criteria specified in the technical specification for the works;

• Base of trench to be ripped/tyned before placement of the overlying aggregate;

• Aggregate to be stored on-site to prevent contamination by fine sediments.


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3.3.7 Infiltration Basins

Infiltration basins can be situated in either natural or excavated open areas, designed to temporarily hold stormwater runoff prior to infiltrating through the basin floor. Infiltration basins provide the following main functions:

• Removing particulate and attached pollutants by infiltration through the sub-soils;

• Reducing runoff volumes runoff volumes (by infiltration the sub-soils); and


Infiltration basins can be constructed as either small or large scale devices. Small scale units (catchment <5 ha) are usually excavated pits or ponds, with larger scale units (catchments up to 50 ha) are typically located within natural surface depressions or gullies within the site occupying a large open area (i.e. playing field or parkland).

Application

• Most effective in removing coarse to fine sediments and attached pollutants (such as nutrients, free oils/grease and metals),

• Best suited to medium to large (5 to 50 ha catchment) residential, commercial and industrial developments with high percentages of impervious areas, including parking lots, high density residential housing and roadways.

• Best located within natural surface depressions or gullies within relatively large open areas (i.e. playing field or parkland).


Typical pollutant removal efficiencies for infiltration basins systems are provided in the table below (source: WBM, 2003):

Gross

Pollutants*

Coarse

Sediment*

Medium

Sediment

Fine

Sediment

Free Oil and

Grease

Nutrients** Metals

- 50 – 80% 50 – 80% 30 – 50% 30-50% 30-50% 30-50%

* Assumes pre-treatment provided. ** Bound to sediments and some dissolved nutrients.

Limitations


• Precludes development on the surface of the basin – area restricted to parkland or open space;


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• Risk of groundwater contamination and low dissolved pollutant removal in coarse, high permeability subsoils;

• Generally not suitable in the following soil or terrain:

– heavy clays;


– Steep terrain;

– High water tables;



2. Other issues that require consideration in design planing:

• Upstream pre-treatment of litter and coarse sediments is essential to minimise clogging of the underlying infiltration surface;

• Inadequate maintenance frequencies can result in the underlying sub-soils becoming clogged.

Site Evaluation and Selection

To reduce the likelihood of failure of the infiltration basins, an initial site evaluation and assessment is required prior to undertaking planning and design.

Site Evaluation

The site is to be initially evaluated based on the following criteria (adapted from Horner, 1994):

• Catchments draining the infiltration system area to be less than 2 ha;

• Base of facility shall be at least 1.0 to 1.5 m above the seasonal high water table, bedrock, or a low permeability layer;

• Subsoil permeability shall be at least 0.8 to 1.3 mm/hr, unless an engineering analysis confirms the viability of less permeable subsoils;

• If the facility recharges groundwater, the maximum subsoil infiltration rate shall be 60 mm/hr;

• Subsoil shall not have more than 30% clay or 40% clay and silt content combined;

• The facility shall not be constructed on slopes greater then 15% unless an engineering analysis confirms the viability;

• Base flows shall not enter the facility.

• The facility shall not be constructed on soils posing a potential salinity hazard, nor fill or contaminated land;


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• The facility shall not recharge a potential water supply aquifer; unless an engineering analysis confirms that no potential contamination of the aquifer will occur;

• The facility shall have sufficient clearance to property boundaries, buildings or other structures indicated in Table 3.4 above.

Site Assessment

A point score system provided in Table 3.5 below is to be used to assess each site on its suitability for use as an infiltration system (source: Camp, Dresser and McKee, 1993). A score of greater than 30 is considered excellent for stormwater infiltration. A site with a score of less than 20 is not suitable for infiltration, while a score between 20 and 30 is possible, depending on the outcome of more detailed site investigations.

Please note that a site evaluation (above) must be undertaken before the assessment and issues such as

potential salinity will automatically preclude the use of infiltration on the site.

Table 3.5

Site Assessment for Infiltration Systems

Item Condition Points

IA > 2 DCIA 20

DCIA < IA > 2DCIA 10 Ratio between the directly connected

impervious area (DCIA) and the infiltration area (IA) 0.5 DCIA < IA > DCIA 5

Coarse soil and low organic material 7

Normal humus soil 5 Nature of the surface soil layer

Fine grained soils and high organic matter 0

Gravel or sand 7

Silty sand or Loam 8 Underlying soils (if finer than surface

soils, else use surface soils classification) Fine silt or clay 0

S < 7% 5

7% < S < 20% 3 Slope of infiltration surface(s)

S > 20% 0

Healthy natural vegetation 5

Well established lawn 3

New lawn 0 Catchment vegetation cover

No vegetation (bare soil) -5

Negligible foot traffic 5

Average foot traffic (e.g. park lawn) 3 Degree of foot traffic on infiltration surface

Considerable foot traffic 0


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Construction Issues


• The infiltration basin should not be used for sediment control during construction;

• Construction area to be fenced off to prevent heavy equipment compacting the underlying soils;

• Base of basin to be ripped/tyned before placement of the overlying topsoil.

3.3.8 Rainwater Tanks

Rainwater tanks are sealed tanks designed to contain rainwater collected from roofs. Rainwater tanks provide the following main functions:

• Allow the reuse of collected rainwater as a substitute for mains water supply, for use for toilet flushing, laundry, or garden watering;

• When designed with additional storage capacity above the overflow, provide on-site detention, thus reducing peak flows and reducing downstream velocities; and

• It may be permissible to use rainwater tanks for internal hot water supply.

The water collected can be reused as a substitute for mains water supply either indoors (toilet flushing and laundry and possibly hot water supply) or outdoors (garden watering).

Rainwater tanks can be either above ground or underground. Above ground tanks can be placed on stands to prevent the need of installing a pump to distribute the water. Such systems are referred to as gravity systems. Pressure systems require a pump and can be either above or below ground tanks. Tanks can be constructed of various materials such as ColorbondTM, galvanised iron, polymer or concrete.

Application

• Can be used in residential, commercial and industrial developments;

• Best suited to new developments where the design of the tank can be incorporated into the house design and geometry; and

• Appropriate also for retro-fitting within existing developments, however the tank’s size may be limited to available space.

Limitations


• Rainwater cannot be collected from asbestos, copper, lead or tar based painted roofs;

2. Other issues that require consideration in design planing:


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• Due to large size/area requirements, difficulties may be experienced in retro-fitting in existing developments.

Construction Issues


• Tank must be installed by a licensed plumber;

• Rainwater tanks should be made of durable, watertight, non reflective, opaque materials with a clean, smooth interior such as ColorbondTM, galvanised iron, polymer or concrete. If a metal rain water tank is to be used, it shall comply with Australian Standard AS2179 "Rain Water Storage Tanks - Metal (Rain Water) Specifications";

• The tank is to be provided with suitable backflow prevention1 to the mains supply in accordance with Australian Standard AS3500.1.2 and the requirements of the relevant water authority.

• Tank to be fully enclosed to prevent mosquitoes breeding and access by insects, animals and birds;

• Tank system must be fitted with a first-flush device including a primary leaf/litter screen;

• Tank system must be fitted with a potable water trickle top-up and floats system;

• Tank should be located in a cool place;

• Sunlight should not penetrate the rainwater tank to prevent the growth of algae;

• The tank must be installed in compliance with the Building Code of Australia and must comply with the following standards:

– No tank shall be fixed to the wall of a building unless certified by a practising structural engineer;

– All tanks are to be placed on a structurally adequate base in accordance with the manufacturers or engineers details;

– All drainage connections are to be in accordance with the Drainage & Plumbing Code (1998), Australian Standard AS3500;

– No tank shall be permitted to have a cross connection with the potable water supply.

• An appropriate sign or plaque to be placed near the garden and/or tank outlet tap noting “Rainwater”;

• Pumps must be located and operated so as not to cause offensive noise (as defined under the Protection of the Environment Operations Act, 1997); and

• Gravity tanks should be constructed with sufficient head to achieve the required flow rates.

1 Sydney Water Corporation will supply backflow prevention devices at no cost for standard installations.


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3.3.9 Landscape Developments

The application of WSUD to landscape development involves the landscape design that aims to minimise irrigation water requirements and maximise the survival rate of plants during drought periods.

It involves the application of the following seven principles;

1. Appropriate landscape planning and design.

2. Limiting the extent of lawn

3. Ensuring irrigation efficiency

4. Improving soil for plant growth

5. Using surface mulches

6. Selecting low water demand plants

7. Carrying out effective landscape maintenance

The selection of low water demand plants gives preference to locally indigenous species that are adapted to the local soils and climate. However, the use of non-indigenous species may be appropriate in some situations to achieve a particular landscape outcome.

Application

• Can be applied to all scales of landscape development including residential, commercial, industrial and open space.

• Appropriate also for retrofitting existing landscape areas within existing residential, commercial and industrial development as well as parks and open space.

Limitations

• The use of low water demand plants in some situations may not allow the design to meet aesthetic or functional requirements.

• In situations where soil moisture remains relatively high, even during drought periods, the use of low water demand plants may not be appropriate.

• The use of extensive lawn areas requiring irrigation may be required in some situations to meet functional requirements (e.g. sports fields).

SECTION 4 WSUD Planning and Selection Guide

4-1

4 WSUD Planning and Selection Guide

4.1 WSUD Planning Process

Figure 4.1 below illustrates the recommended process to be followed in planning WSUD in western Sydney:

Step 1

Determine likely pollutant types and expected loads based on the development type and area (Refer Section 2.4)

Step 2

Determine required pollutant removal target levels given the development type and area (Refer Section 2.4)

Step 3

Determine short list of suitable WSUD measures or series of devices that will meet the pollutant removal target levels given the site’s physical constraints and to satisfy the overall treatment train process (Refer Sections 3.2 and 3.3)

Step 4

Determine the optimal WSUD measure given the applicability and function of the measure and its location and incorporation in the treatment train (Refer Section 4.2)

Step 5

Undertake detailed design of selected devices and complete checklist (Refer Section 5)

Step 6

Complete Operation and Maintenance Plan and Checklist (Refer Section 6)

Rev

iew

ste

ps 3

-6 t

o re

fine

desi

gn a

nd

chec

k fo

r

appr

opria

tene

ss a

nd a

ccur

acy

Figure 4.1 WSUD Planning Process


4-2

4.2 Applicability and Function of WSUD Measures

Prior to the selection of appropriate WSUD measures for incorporation in the treatment train it is important to recognise the appropriate scale of application (Table 4.1) and the primary role and function of each element (Table 4.2) so the key water management issues for individual sites in urban catchments can be effectively addressed (source: Lloyd et al, 2002).

Table 4.1

Scale of WSUD Application in Urban Catchments

Number WSUD Measure Allotment Scale

Subdivision Scale Open Space or Regional Scale

DS1 Vegetated Swales

DS2 Vegetated Filter Strips

DS3 Sand Filters

DS4 Bioretention Systems - Off-line (planting beds) - On-line (conveyance)

DS5 Permeable Pavements

DS6 Infiltration Trenches

DS7 Infiltration Basins

DS8 Rainwater Tanks


The results of Table 4.1 above indicate that nearly all of specified WSUD measures (with the exception of rainwater tanks) are adaptable to all spatial scales from allotment to open space or regional scale.

Table 4.2

Role and Function of WSUD Measures

Number WSUD Measure Water Quality

Treatment

Flow Attenuation* Reduction in

Runoff Volume*

DS1 Vegetated Swales H M L DS2 Vegetated Filter Strips H M L DS3 Sand Filters H M L DS4 Bioretention Systems H M L DS5 Permeable Pavements M H H DS6 Infiltration Trenches H H H DS7 Infiltration Basins H H H DS8 Rainwater Tanks L H H DS9 Landscape Developments M M L

Key: H – High level role; M – Medium level role; L – Low level role * Applies to frequent events.


4-3

Table 4.3 below presents a summary of the limitations of each WSUD measures. This table incorporates the key physical attributes of Western Sydney that can significantly influence the design of WSUD measures (limitations for each WSUD element are also discussed in Section 3).

Table 4.3

Site Constraints for WSUD Elements

No. WSUD Element

Ste

ep

Sit

e

Sh

all

ow

Be

dro

ck

Sa

lin

ity

Ha

zard

Lo

w P

erm

ea

bil

ity

So

ils

Hig

h P

erm

ea

bil

ity

So

ils

Hig

h W

ate

r T

ab

le

Hig

h S

ed

ime

nt

Inp

ut

La

nd

Av

ail

ab

ilit

y

Lim

ita

tio

n

Hy

dra

uli

c H

ea

d

Lo

ss

lim

ita

tio

n

DS1 Vegetated Swales C M M

M M C

DS2 Vegetated Filter Strips C M M

M M C

DS3 Sand Filters M M M

M C C M C

DS4 Bioretention Systems C M M

M C C C C

DS5 Permeable Pavements (Infiltration)

C C C C

C C C C

DS5 Permeable Pavements (Detention)

C M M

M C C C C

DS6 Infiltration Trenches C C C C

C C M C

DS7 Infiltration Basins C C C C

C C C

DS8 Rainwater Tanks

C


M M

M M C

Key: C – constraint may preclude the use of this WSUD M – constraint may be overcome with appropriate modifications to design

– generally not a constraint (i.e. design specifications apply)


4-4

4.3 WSUD Selection and Treatment Train

A fundamental feature of the WSUD philosophy is the restoration of natural features in the hydrological system by forming a hierarchy of allotment-level, subdivision-level and catchment-level solutions. Each WSUD measure should not be considered in isolation, but an element forming a continuous “Treatment Train” through the urban development catchment. The assembly of the Treatment Train is often based on achieving the desired outcomes within a system of stormwater management measures, for example, gross pollutant, coarse to medium sediment, fine sediment removal and soluble pollutant removal (as described in Table 3.2).

It is desirable to treat runoff and associated pollutants generated from impervious areas by WSUD measures located as close as possible to its source, thereby minimising the requirement for end-of-pipe or downstream catchment treatment measures. Pollutant removal mechanisms associated with these measures involve physical, biological and absorption processes. Treatment methods based on physical processors are often used first in the treatment train. Physical processes fundamentally involve initially trapping gross pollutants and coarse sediments (ie. primary treatment) followed by finer sediment particles and attached pollutants (ie. secondary treatment). Once these coarse pollutants are removed, other pollutant removal mechanisms involving biological and absorption processes can be effectively applied (ie. tertiary treatment). The optimal WSUD measure/s incorporated in the system is dependent on the following:

• The style of development and the type of pollutants likely generated (refer Table 2.6);

• Pollutant reduction objectives (refer Table 2.7);

• Location within the development catchment (ie. allotment, subdivision or catchment level);

• Role, function and effectiveness of the treatment measure (refer Section 3);

• Individual site assessment, physical constraints and design issues(such as soils, slopes, salinity, groundwater and space – refer Tables 4.1 to 4.3);

• Operation and maintenance issues (refer Section 6); and

• Life cycle cost considerations (refer Section 7).

Typical combination of WSUD treatment processes that are recommended for various development types (subject to site constraints) are provided below:

Residential Lots

• Rainwater tank (with first-flush device) for reuse in toilet flushing, outdoor use, laundry and possibly hot water supply, with overflow to detention/retention trench.

• Permeable pavement located along driveways and around building where possible, with overflow to the street drainage system.

• Stormwater runoff from impervious areas and lawns draining to landscape or garden bed areas.


4-5

• Excess runoff from impervious areas to detention/retention trench, with overflow to the street drainage system.

Roads and Commercial/Industrial Pavements

• Runoff from pavements draining to vegetated filter strips (replacing conventional kerb and gutter) with overland flow draining to bioretention or infiltration trenches, via vegetated swales.

• Alternately, runoff from pavements draining to in-pit GPTs, which drain to an underground sediment trap and sand filter system.

Carparks

• Permeable pavement over the carpark area with overflow to an oil/grit separator pit.

• Alternately, runoff from the carpark draining to vegetated filter strips with excess overland flow draining to bioretention or infiltration trenches, via vegetated swales.

4.4 Incorporation of WSUD Measures in Streetscapes

The streetscape aims to unify the various WSUD measures while creating the desired streetscape character intent as described in a development’s master plan. The combination of WSUD measures in the streetscape also aims to provide a visual public asset without interference, threat or compromise to public health and safety. Where locally endemic vegetation is used in the implementation of WSUD in the streetscape, this will provide habitat for wildlife and enhance green corridors.

The streetscape can be defined as those areas having a primary role in the composition and character of the public domain including public parks, open space areas, roads and car parking. In addition, the areas of commercial and house lots and roofs offer a secondary visual component to the character of the streetscape. When planning and designing the streetscape, consideration must be given to the visual composition of both the WSUD measures in the public spaces and adjoining private properties (refer Figures 4.2 and 4.3). The selection of landscape treatments in the public and private domain is equally important to the success of the visual character of the streetscape and this aspect is covered in Section 3.3.9 (Landscape Developments).

WSUD measures maximise passive stormwater treatment opportunities, reduce reliance upon traditional costly water treatment systems and reduce long-term maintenance costs. They can also contribute to an increase in public awareness and ownership through the use of interpretive signage and information.

The inclusion and use of WSUD measures does not restrict the design and configuration of the circulation system (street and paths). The simple road sections in Figures 4.2 – 4.12 demonstrate the combinations of road types and WSUD measures and their corresponding widths and possible landscape treatments. Typical of all these sections is the intention that stormwater treatment occurs as close to the source as possible. In many instances WSUD measures can be equally successful when applied to existing roads as they can be in new developments.


4-6

Using a typical car park and road as the source, Figures 4.2 to 4.12 show an appropriate combination of WSUD measures and their use. The primary filtration and infiltration process begins with permeable pavement that allows stormwater to infiltrate down to the porous bedding layer below, leaving sediments at the surface. Excess surface water is conveyed to the vegetated strips at the edge of the paved surface where, again, sediments are filtered out and water is detained to infiltrate into the natural soil. This infiltration of water replenishes the groundwater table and sustains the vegetation within the conveyance strip, reducing the need for irrigation as plant species are selected for their drought tolerance. Secondary treatment occurs when excess floodwater passes through a sediment trap and sand filter and then is piped to a bioretention system or bioretention basin, or is conveyed via another vegetated strip or swale. Again sediments and pollutants are filtered out. From the sediment trap and sand filter, water is conveyed via a pipe while surface floodwater is conveyed overland via a vegetated strip or swale to either an infiltration basin, constructed wetland or bioretention basin depending on site conditions, design intent and council approval. The detained stormwater is polished for the last time as it is conveyed to the receiving waters via vegetated filter strips, swales or reconstructed streams.

The extent and frequency of use of each of the WSUD measures is dependent the site conditions. WSUD measures can and should be considered as elements that can add value to a development while creating spaces of visual interest. The circulation systems (roads and pedestrian and cycle paths) bridging or interacting with conveyance and treatment elements, such as vegetated filter strips or reconstructed streams, can offer visual and educational opportunities. The open space areas can be defined and provided with visual interest by using WSUD measures. Active recreation areas can be defined and connected with bridges to passive recreation areas using vegetated filter strips, reconstructed streams and linear wetlands. Properly designed wetlands or bioretention basins can become a major visual asset particularly when used as an entry feature to a development. They are also useful islands for the enhancement of biodiversity.


4-7

Secondary View Zone

(Lot)

Primary View Zone

(Circulation System)

Secondary View Zone

(Lot)

Secondary View Zone

(Lot)

Primary View Zone

(Circulation System)

Secondary View Zone

(Open Space)

Secondary View

Primary Views Primary View Secondary View

Secondary View Primary Views

Figure 4.2: Streetscape View Zone With Development Both Sides

Figure 4.3: Streetscape View Zone With Development One Side

Secondary View

SECTION

SECTION


4-8

Verge and Footpath

Street Tree with Porous Pavement,

Slotted Kerb or Bollards

Draining Into Stormwater

Drainage

DS2 DS4 DS5

Verge and Footpath

Travel Lanes

Lot

Lot

SECTION

SECTION

Figure 4.4 Existing Road with Car Parking One Side

Figure 4.5 Existing Road with Car Parking

Two Sides

Verge and Footpath

Verge and Footpath

Travel Lanes

Lot

Lot




Drainage

DS2 DS4 DS5




Drainage

DS2 DS4 DS5


4-9

Verge and Footpath

Figure 4.6 2m Wide Median

Travel Lanes

2m Wide Median With



Drainage

DS1 DS2

Verge and Footpath

Travel Lanes

Lot

Lot

SECTION

PLAN


4-10

Verge and Footpath


Travel Lanes

4m Wide Median With


Draining Into Stormwater Drainage

DS1 DS2 DS4

Verge and Footpath

Travel Lanes

Lot

Lot

SECTION

PLAN


4-11

Verge and Footpath


Travel Lanes

6m Wide Median With



Drainage

DS1 DS2 DS4

Verge and Footpath

Travel Lanes

Lot

Lot

SECTION

PLAN


4-12

Verge andFootpath

Street Tree withPorous

Pavement,Slotted Kerb or

Bollards DrainingInto Stormwater

Drainage

DS2DS4DS5

Verge andFootpath

TravelLane

Lot Lot

SECTION

PLAN

Figure 4.9

2m Median With, Car ParkingPorous Pavement and Street Trees

TravelLane




Drainage

DS2DS4DS5

2m Median withSlotted Kerb or


Drainage

DS1DS2DS4


4-13

Verge andFootpath




Drainage

DS2DS4DS5

Verge andFootpath

TravelLane

Lot Lot

SECTION

PLAN

Figure 4.10 4m Median with Car Parking,Porous Pavement and Street Trees

TravelLane




Drainage

DS2DS4DS5



Drainage

DS1DS2DS4


4-14

Verge andFootpath




Drainage

DS2DS4DS5

Verge andFootpath

TravelLane

Lot Lot

SECTION

PLAN

Figure 4.11 6m Median with Car Parking,Porous Pavement and Street Trees

TravelLane




Drainage

DS2DS4DS5



Drainage

DS1DS2DS4


4-15

Verge andFootpath

Open Space Lot

SECTION

Figure 4.12 Open Space Edge Median withCar Parking, Porous Pavement

and Street Trees

Travel Lanes




Drainage

DS2DS4DS5

Median (widthvaries) with

Slotted Kerb orBollards DrainingInto Stormwater

Drainage

DS1DS2DS4

SECTION 5 WSUD Design Specification

5-1

5 WSUD Design Specification

5.1 Introduction

This section provides best practice specifications for a number of WSUD measures presented in Section 3 for the preparation of consistent designs that address all issues ranging from sizing of the device, implementation to construction requirements. These specifications provide a summary of the criteria that must be met as part of the preliminary planning and design process and provides a recommended methodology and step-by-step procedure for carrying out these designs. These design specifications should also be read in conjunction with Section 4 of this document, which provides guidance for the planning and implementation of the WSUD measures within the development area, and provides typical examples of their use and suitability in the overall treatment train process.

These design specifications have been prepared specifically for the use of developers, or engineering consultants working on behalf of developers, for preliminary design of WSUD measures during preparation of Stormwater Management Plans. They can also be used by Council Assessment Officers and Engineers assessing the suitability of these designs for development applications. The designs would then form the basis of more detailed design of these measures (if required) as part of the operational works applications.

These specifications are based on a number of published design guidelines and manuals currently available for the design of WSUD measures, which are referenced throughout the document. The reader is also encouraged to refer to these supplementary references if additional information is required. However, design criteria and guidelines specific to Western Sydney have been provided, where appropriate.

The sizing of the majority of the WSUD measures outlined in this document are based on hand calculations using empirical formulas that should provide an appropriate level of pollutant removal efficiency for a specific nominal detention time for each device. However, the performance of many of these measures (such as vegetated filter strips, swales and bioretention systems) can also be modelled using a continuous simulation export model, such as MUSIC (developed by the CRC for Catchment Hydrology). These types of models take into account the physical dimension of the system and the influence of climatic variability on pollutant export and removal efficiency. The effectiveness of retention systems for any runoff event is dependent on antecedent storage conditions and continuous simulation modelling of the hydrological behaviour of these systems is therefore the preferred method of design. However, such sophisticated modelling, in general, would only be undertaken for major development proposals with catchment areas typically greater than 5 ha based on Blacktown City Councils Stormwater Quality Control Policy (also refer Appendix F of the NSW EPA’s Managing Urban Stormwater: Council Handbook (1997b), for further information). If this type of modelling is undertaken, simple hand calculations (as provided in this document) should also be performed to provide an “order of magnitude” check of the results.


5-2

5.2 Design Process

The flow chart in Figure 5.1 below illustrates the recommended process to be followed in the design of WSUD measures in the western Sydney area:

Step 1

Determine the required Treatable Volume or Flow Rate or Detention Volume for the WSUD measure (Refer to Council’s Stormwater Management and Quality Control Guidelines)

Step 2

Determine optimal WSUD measure from the suite of WSUD measures and applications (Refer Section 3)

Undertake preliminary design of selected optimal WSUD measure (Refer Sections 4.2-4.4)

Check salinity hazard classification for the site area.

(Refer Salinity Hazard Maps)

Low permeability liner not required below WSUD measure

Step 4

Finalise detailed design

Limit soil infiltration. Include low permeability liner below WSUD measure.

Infiltration system not suitable. Choose alternate WSUD measure or modify design, where appropriate.

Potential Salinity Hazard

No Hazard

Complete design checklist (Refer Section 5)

YES

Infiltration system?

NO

Step 3

Step 5

Check maintenance suitability & life cycle cost of WSUD measure

(Refer Sections 6 & 7)

Suitable

Not suitable

Figure 5.1 WSUD Design Process


5-3

5.3 Design Specification DS1 – Vegetated Swales

(Refer Design Specification Drawing DSD1).

Function

Vegetated swales provide the following main functions:

• Removing particulates by filtration through the vegetation;

• Reducing runoff volumes runoff volumes (by promoting infiltration to the sub-soils); and


Design Approach

The design approach for vegetated swales is based on achieving the following objectives:

• Providing sufficiently low flow velocities through the swale to limit surface erosion and scouring; and

• Limiting the flow depth through the swale to maximise contact and filtration through the vegetation.

The design of vegetated swales will need demonstrate compliance of the above design objectives and criteria outlined below. References should be made to Council’s Drainage Design Guidelines for acceptable hydrological and hydraulic calculation methods. Where a modelling package is used that takes into account of the physical dimensions of the swale when estimating pollutant removal efficiencies (such as MUSIC2) then compliance with the above design objectives will not need to be demonstrated. However, design details such as swale geometry (i.e. length and cross-section), vegetation species and design flow velocities (to prevent scouring) are to be provided.

Design Criteria

Criteria for the design of vegetated swales are provided below (based on ARC, 2003):

• Swales shall be designed as trapezoidal or parabolic in shape with low sloping sides, preferably 6H:1V, maximum 4H:1V slope.

• Swales that traverse driveways or other pavements must match the crossover grade, generally 13H:1V on either side of the swale.

• Swales shall be blended or smoothed out to resemble the natural topography of the site and to prevent scalping when mowing occurs.

• Level spreaders or energy dissipators shall be provided at the inlet to swale channels from stormwater pipes or culverts (in accordance with NSW Department of Housing, 1998).

2 Model for Urban Stormwater Improvement Conceptualisation, developed by the CRC for Catchment Hydrology.


5-4

• Swale top width to depth ratio of 6:1 or greater.

• A maximum swale width of 2.5 m, unless structural measures are incorporated to ensure uniform spread of flow.

• A maximum longitudinal grade of 4% is appropriate. If check dams are included in the swale, up to 6% is permissible, but the toe of the upstream dam must be level with the spillway of the next downstream dam.

• All swales with longitudinal grades less than 2% require a subsoil drain below the swale invert to minimise surface ponding between rain events. In potential salinity hazard areas, a low permeability liner is required below the subsoil drain to minimise sub-surface infiltration;

• Longitudinal grades on the swale shall be uniform or gradual. The floor of the swale shall have no lateral grade.

• The swale aspect (ie length to width) ratio shall be within the range 3:1 to 10:1, or greater, to minimise short-circuiting.

• The flow depth in the swale during the design Treatable Flow Rate shall be equal to one-third to half the vegetation height, to a maximum depth of 75 mm.

• Velocities within the swale for all flows up to the design Treatable Flow Rate shall be <0.5 m/s (velocity where most grasses will be knocked over).

• Maximum acceptable flow rate velocities for conveyance of Peak Design Flows along the swale shall not exceed the recommended maximum scour velocities for various ground covers and soil erodibility presented in the Table DS1.1 below (NSW Department of Housing, 1998) and shall ideally be less than 1 m/s.


5-5

Table DS1.1

Maximum Flow Velocities in Channels

Maximum Acceptable Velocity (m/s) Soil Erodibility Ground Cover

Low Moderate High Mat or sword grasses with UV stabilised mesh 3.0 2.7 2.4

Kikuyu grass 2.5 2.2 1.9

Couch grass, carpet grass, Rhodes grass, sword forming grasses 2.0 1.8 1.4

Other improved perennials 1.6 1.3 0.9

Tussock grasses 1.3 0.9 0.5

Design Procedure

The following design steps are recommended when designing vegetated swales (based on ARC, 2003):

1. Determine the Treatable Flow Rate (Qd) based on the site specific characteristics such as catchment area, topography and impervious area to provide a level of treatment of pollutants (refer Council’s Stormwater Quality Control Policy Guidelines).

2. Determine the Peak Design Flow Rate (Qpeak) for the swale based on the site specific characteristics such as catchment area, topography and impervious area (refer Council’s Drainage Design

Guidelines).

3. Determine the average swale slope (s) based on-site conditions. Slopes to be between 1% and 4% or up to 6% if check dams are constructed.

4. Determine maximum swale top width (T) based on-site space/area restrictions and limitations (i.e. road carriageway widths).

5. Trial an initial swale flow depth (d) based on estimated maximum swale top width (T) and using sides slopes flatter than 4H:1V. Depths should be at least half the vegetation height or 75 mm maximum.

6. Select a swale base width (b). Adopt b minimum of 0.6 m and for maintenance requirements and maximum of 2.5 m, unless check dams are provided to ensure uniform spread of flow along swale.

7. Determine swale top width (T) using the following equation:

dZbT 2+=

where Z = 4 minimum when sides slopes are 4H:1V.

8. Check top width to depth ratio is 6:1 or greater. Modify b if required.

9. Calculate swale geometry for either a trapezoidal or parabolic swale shape:

2: ZdbdATrapezoid +=


5-6

dZbT 2+=

)12/( 22 +++= ZdbZdbdR

where A = swale flow area (m2);

b = swale base width (m);

d = swale flow depth (m)

R = hydraulic radius (m)

T = top width (m).

TdAParabola 3: =

dAT /5.1=

)45.1/( 222 dTdTR +=

8. Select the swale vegetation type and nominal vegetation height for the species. Recommended vegetation species for swales shall be in accordance with Design Guideline DG9 – Landscape Developments.

d

e

b

Z = e/d

T

T

d


5-7

9. Determine the Manning’s roughness coefficient (n) value based on the type of swale vegetation and average depth of flow. Select n values according to the following equations (from ARC, 2003):

For 150 mm grass and d < 0.06 m, n = 0.153 d–0.33 / (0.75 + 25s)

For 50 mm grass and d < 0.075 m, n = (0.54 - 228 d2.5) / (0.75 + 25s)

where: d = depth of flow (m) for water quality storm

s = longitudinal slope as a ratio of vertical rise/horizontal run (m/m)

10. Use Manning’s equation to determine flow rate in swale:

nSARQ /5.067.0=

where Q = swale flow rate (m3/s);

A = swale flow area (m2);

s = average swale slope (m/m) (from Step 2);

R = hydraulic radius (m) (from Step 7);

n = Manning’s roughness coefficient (from Step 9).

11. Compare Q with the Treatable Flow Rate (Qd) from Step 1. Perform iterations by changing flow depth d until Q = Qd (i.e. increase d if Q is to low).

12. Calculate the average flow velocity along the swale using equation:

AQV /=

where V = average swale flow velocity (m/s). Note that maximum V = 0.5 m/s.

13. Determine the Flow Retardance Class from the Table DS1.2 below based on grass height. The grass should be maintained at 100 to 150 mm minimum height.

Table DS1.2

Selection of Flow Retardance Class

Average Height of Grass (mm)

Flow Retardance Class

150 – 250 C

50 – 150 D

< 503 E

3 The grass should be maintained at 100 to 150 mm minimum height.


5-8

Figure DS1.1

14. Check Mannings roughness coefficient (n) with Figure DS1.1, depending on protocol below:

If Mannings n is not similar to that assumed in Step 9, perform iterations by using the n obtained from from Figure DS1.1 and changing d until the Treatable Flow Rate (Qd) is calculated and the n used corresponds to that in Figure DS1.1.

15. Check flow velocity for the design Treatable Flow Rate. If greater than 0.5 m/s, reduce the flow, increase the flow width or reduce the depth of flow.

16. Check flow velocity for the Peak Design Flow Rate (Qpeak) for the swale for conveyance with the recommended maximum velocities presented in Table DS1.1, or ideally less than 1 m/s.

17. Design swale check dams along portions of the swale with longitudinal slopes >4% where required (refer Figure DSD1).

18. Design swale subsoil drain along portions of the swale with longitudinal slopes <2% where required (refer Figure DSD1). Assess the swale subsoil drain capacity and size appropriately for the design Treatable Flow Rate.

19. Incorporate a low permeability liner along the swale length where required for potential salinity hazard areas.

20. Design inlet culverts and outlet pit size/grates for the Peak Design Flow Rate (Qpeak) along the swale.


5-9

21. Complete the Design Checklist for Vegetated Swale.

Vegetated Swales

Design Checklist

Design Feature Checked Satisfactory Unsatisfactory Comments

Side slope at least 4H:1V Y N

Longitudinal grade between 1% and 6%

Y N

Vegetation species selected

Y N

Velocity does not exceed recommended maximum for vegetation (0.5 m/s)

Y N

Check dams provided if grade >4%

Y N

Subsurface drainage and liner provided if grade < 2%

Y N

Soil salinity hazard assessment

Y N


5-10

5.4 Design Specification DS2 – Vegetated Filter Strips

(Refer Design Specification Drawing DSD2)

Function

Vegetated filter (or buffer) strips provide the following main functions:

• Removing sediments and attached pollutants by filtration through the vegetation;

• Reducing runoff volumes (by promoting infiltration to the sub-soils); and


Design Approach

The design approach for vegetated filter strips is similar to swales (refer Design Specification - DS1) and is based on achieving the following objectives:

• Providing sufficiently low flow velocities through the system to limit surface erosion and scouring; and

• Limiting the flow depth through the system to maximise contact and filtration through the vegetation.

The design of vegetated swales will need to demonstrate the compliance of the above design objectives and in accordance with the design criteria outlined below. References should be made to Council’s Drainage Design Guidelines for acceptable hydrological and hydraulic calculation methods. Where a modelling package is used that takes into account of the physical dimensions of the filter strip when estimating pollutant removal efficiencies (such as MUSIC) then compliance with the above design objectives will not need to be demonstrated. However, design details such as the filter strip geometry (i.e. length and area), vegetation species and design flow velocities (to prevent scouring) do need to be provided.

Design Criteria

Criteria for the design of vegetated filter strips are provided below (based on ARC, 2003):

• Filter strip shall have a longitudinal grade of between 1% and 5%.

• Concentration of flows through the filter strip to be avoided by providing level spreaders or energy dissipaters at the inlet/s from stormwater pipes or culverts (in accordance with NSW Department of Housing, 1998).

• The flow depth over the filter strip during the design Treatable Flow Rate shall be a maximum of 12 mm (Horner et al, 1994).

• The design flow velocity shall be <0.5 m/s (velocity where most grasses will be knocked over).


5-11

• Maximum flow rate velocities for conveyance of the Peak Design Flow shall not exceed the recommended maximum scour velocities for various ground covers and soil erodibilities presented in Table DS2.1 from (NSW Department of Housing, 1998), or ideally less than 1 m/s.

Table DS2.1

Maximum Acceptable Flow Velocities in Channels

Maximum Velocity (m/s)

Soil Erodibility Ground Cover

Low Moderate High

Mat or sword grasses with UV stabilised mesh 3.0 2.7 2.4


Couch grass, carpet grass, rhodes grass, sword forming grasses 2.0 1.8 1.4



Design Procedure

The following design steps are recommended when designing vegetated filter strips (based on ARC, 2003):

1. Determine the Treatable Flow Rate (Qd) based on the site specific characteristics such as catchment area, topography and impervious area to provide a level of treatment of pollutants (refer Council’s Stormwater Quality Control Policy Guidelines).

2. Determine the Peak Design Flow Rate (Qpeak) for the filter strip based on the site specific characteristics such as catchment area, topography and impervious area (refer Council’s Drainage

Design Guidelines).

3. Determine the average filter slope (s) based on-site conditions. Slopes to be between 1% and 5%.

4. Trial an initial filter strip flow depth (d). Depth shall be less than 12 mm.

5. Select a optimum filter width (b) based on-site area constraints.

6. Calculate filter strip flow area (A) and hydraulic radius (R):

A = bd

R = bd/(b+2d)


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where A = filter flow area (m2);

b = filter width (m);

d = filter strip flow depth (m)

R = hydraulic radius (m).

22. Determine the Manning’s roughness coefficient (n) value based on the type of filter strip vegetation and average depth of flow. Select n values according to the following equations (from ARC, 2003):

For 150 mm grass and d < 0.06 m, n = 0.153 d–0.33 / (0.75 + 25s)

For 50 mm grass and d < 0.075 m, n = (0.54 - 228 d2.5) / (0.75 + 25s)

where: d = depth of flow (m) for water quality storm

s = longitudinal slope as a ratio of vertical rise/horizontal run (m/m)

7. Use Manning’s equation to determine flow rate (Q) across the filter strip:

nSARQ /5.067.0= 8. Compare Q with the Treatable Flow Rate (Qd) from Step 1. Perform iterations by changing flow

depth d until Q = Qd (i.e. increase d if Q is to low).

9. Calculate the average flow velocity along the filter strip using equation:

AQV /=

where V = average filter flow velocity (m2/s);

10. Determine the Flow Retardance Class from the Table DS2.2 below based on grass height. The grass should be maintained at 75 to 100 mm minimum height.

Table DS2.2

Selection of Flow Retardance Class

Average Height of Grass (mm) Flow Retardance Class

150 – 250 C

50 – 150 D

< 504 E

4 The grass should be maintained at 75 to 100 mm minimum height.


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11. Check Manning’s roughness coefficient (n) with the Figure DS2.1 below, depending on protocol below:

Figure DS2.1

- If Mannings roughness coefficient (n) is not similar to that assumed in Step 7, perform iterations by using the n obtained from Figure DS2.1 and changing d until the Treatable Flow Rate (Qd) is calculated and the n used corresponds to that in Figure DS2.1.

12. Check flow velocity for the design Treatable Flow Rate. If greater than 0.5 m/s, reduce the flow, increase the flow width or reduce the depth of flow.

13. Check flow velocity for the Peak Design Flow Rate (Qpeak) for the filter strip for conveyance with the recommended maximum velocities presented in Table DS2.1, or ideally less than 1 m/s.

14. Design inlet culverts and outlet pit size/grate for the Peak Design Flow Rate (Qpeak) along the filter strip.

15. Complete the Design Checklist for Vegetated Filter Strip.


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Vegetated Filter Strips

Design Checklist


Grade between 1% and 5%

Y N

Sheet flow achievable

No lateral grade

Vegetation species selected

Y N

Velocity does not exceed recommended maximum

Y N

Soil salinity hazard assessment Y N


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5.5 Design Specification DS3 – Sand Filters


Function

Sand filters provide the following main functions:

• Removing fine to coarse particulates and attached pollutants by infiltration through a sand media layer; and


Design Approach

The design approach for sand filters is based on achieving the following objectives:

• Providing adequate sedimentation of gross pollutants and medium to course sediments within the stormwater runoff prior to entering the sand filter system;

• Providing an adequate hydraulic residence (filtration) time through the system to enable sediments and attached pollutants to be retained; and

• Selecting suitable filter media to provide the required hydraulic residence (filtration) time through the system.

The design of sand filters will need to demonstrate compliance of the above design objectives and criteria outlined below. References should be made to Council’s Drainage Design Guidelines for acceptable hydrological and hydraulic calculation methods.

Design Criteria

Criteria for the design of sand filters are provided below:

• Sand filters are to be located off-line with a high flow by-pass system.

• A sedimentation chamber/pond/basin shall be provided to remove litter and coarse sediments with the following key design criteria:

– inflow into the chamber shall not allow for re-suspension of previously deposited sediments;

– sediment chamber/basin designed to provide uniform sheet flow to the filtration chamber using a weir system;

– flow velocities through the sedimentation area shall be less than 0.25 m/s;

– recommended drawdown time of 24 hours for the settling basin; and

– settling of medium to coarse sediments down to the 0.125 mm size particle recommended to minimise clogging of the sand filter media.


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• Sand filter media shall be clean, washed aggregate, selected to provide the required retention time and minimise clogging and have a grading (or as close as possible) within the limits specified in Table DS3.1 (ARC, 2003):

Table DS3.1

Sand Filter Particle Grading Specification (Source: ARC, 2003)

Sieve Size (mm) Percentage Passing

(%)

9.5 100

6.3 95 – 100

3.17 80 – 100

1.5 50 – 85

0.8 25 – 60

0.5 10 – 30

0.25 2 – 10

• A sample (if available) of the proposed filter sand shall be tested in a NATA-registered laboratory to determine the average coefficient of permeability. The results shall be provided to Council, certifying compliance.

• Filter sand shall have a nominal permeability of between 1 and 5 m/day, but should be assessed to provide the required filtration time. ARC (2003) recommends a design sand hydraulic conductivity of approximately 1 m/day (0.04 m/hr), which is typically less than the typical conductivity of new sand media (ie. up to 8.6 m/day) and therefore allows for some clogging.

• Capacity of the facility shall be sufficient to provide adequate filtration time through the sand media. ARC (2003) adopts a filtration period for the mean storm of at least 30 to 50% of the mean inter-storm period. As the majority of the filtration occurs during the inter-event period, an approximate filtration period can be determined from an analysis of the site’s rainfall data history. A 2-day maximum filtration time is recommended (ARC, 2003). Drainage (filtration) of the design Treatable Volume through the filter media should be 30 to 50% of the mean inter-event dry period, or approximately 24 to 48 hours, for the Western Sydney area (refer Table 2.3).

• An underdrainage pipe collection system shall be provided below the sand filter media comprising a perforated lateral pipe/s system, sized to drain the design filter flow, with a minimum pipe size of 100 to 150 mm diameter. The underdrainage pipe/s shall be contained in fine to coarse gravel layer comprising clean gravel of generally uniform particle size and free from silt/clay fines or other deleterious matter. The pipe/s shall have 50 mm minimum cover of gravel material. The gravel material shall be selected to meet the following grading compatibility criteria with the filter sand:

8515 4 dD ∗≤


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where D15 = particle size (mm) in gravel material for which 15% by weight of particles are

smaller;

d85 = particle size (mm) in sand filter material for which 85% by weight of particles are smaller.

• Geofabric shall not be used as a filter/separation layer between the sand and gravel materials layers. A transition filter layer (200 mm min thick) may, however, be provided between the sand filter media and the gravel to satisfy the above grading compatibility criteria.

• Geofabric may be required along the side trench walls and base (only) to prevent the migration of surrounding fine soils into the system.

• A suitable backflushing system shall be incorporated in the design to enable flushing of the perforated pipe underdrainage system.

• A low permeability liner beneath the underdrainage system shall be provided in potential salinity hazard areas to minimise sub-soil infiltration.

Design Procedure

The following design steps are recommended when designing sand filters (based on ARC, 2003):

1. Determine the Treatable Volume (VT) based on the site specific characteristics such as catchment area, topography and impervious area to provide a level of treatment of pollutants (refer Council’s Stormwater Quality Control Policy Guidelines).

2. Determine the Treatable Flow Rate (Qd) and Peak Design Flow Rate (Qpeak) upstream of the sand filter system based on the site specific characteristics such as catchment area, topography and impervious area (refer Council’s Drainage Design Guidelines).

3 Determine the initial sediment chamber/basin size based on sediment settling velocity theory or retention curves (NSW Dept of Housing, 1998) and criteria outlined in this specification.

4. Determine maximum water depth in the filtration chamber (dmax) based on site/space/depth restrictions.

5. Calculate the surface area of the sand filter using the following equation:

))(/( tdhkdVA T ∗+∗∗=

where: A = minimum surface area of the filter (m2);

VT = Treatable Volume (m3) (from Step 1);

k = filter media hydraulic conductivity (m/day);

t = filtration time (days);

h = average depth of water above the sand (i.e. half dmax depth); and


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d = sand bed depth.

For initial sizing, use the following data:

t = 1 day minimum, 2 days maximum;

k = 1 m/day minimum for filter sand (assumes partially clogged);

d = 0.4 m minimum.

6. Optimise the filter area/size based site area and depth constraints and assessing alternate filter media permeability (k) properties.

7. Determine the underdrainage system pipe size/s based on the design flow through the system using the following equation:

dddkAQ /)( maxmax +∗∗=

where: Qmax = maximum outflow from the system (m3/s);

A = surface area of the system (m2);

k = filter media hydraulic conductivity (m/s) (from Step 5);

dmax = maximum depth of water above the soil (from Step 4);

d = filter soil depth (from Step 5).

8. Determine gravel material size and grading to satisfy grading compatibility criteria.

9. Design backflush system for the perforated underdrainage pipe/s.

10. Design high flow by-pass system for all flows above the Treatable Flow Rate (Qd) and up to the Peak Design Flow Rate (Qpeak).

11. Incorporate a low permeability liner beneath the underdrainage system where required for potential salinity hazard areas.

12. Complete the Design Checklist for Sand Filters.


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Sand Filters

Design Checklist

Design Feature Checked Satisfactory

Unsatisfactory Comments

Treatable Volume Y N

Off-line System? Y N

Sedimentation Chamber/ Basin

Y N

Filter Media Permeability

Y N

Detention time 1day> t <2days

Y N

Filter Media Depth >0.4 m

Y N

Underdrainage System Y N

Soil salinity hazard assessment

Y N

Low Permeability Liner Required

Y N

Underdrainage Pipe Backflush System

Y N

High Flow Bypass Y N


5-20

5.6 Design Specification DS4 – Bioretention Systems


Function

Bioretention systems provide the following main functions:

• Removing sediments attached pollutants by filtering through surface vegetation and ground cover and through an underlying filter media layer;

• Removing some dissolved pollutants through soil chemistry and vegetation nutrition;



Design Approach

The design approach for bioretention systems is based on achieving the following objectives:

• Providing an adequate hydraulic residence (filtration) time through the system to enable sediments and attached pollutants to be retained;

• Selection of suitable planting soil/filter media to provide required hydraulic residence (filtration) time through the system.

The design of bioretention systems will need to demonstrate compliance of the above design objectives and criteria outlined below. References should be made to Council’s Drainage Design Guidelines for acceptable hydrological and hydraulic calculation methods. Where a modelling package is used that takes into account of the physical dimensions of the bioretention system when estimating pollutant removal efficiencies (such as MUSIC) then compliance with the above design objectives will not need to be demonstrated. However, design details such as the bioretention system geometry (i.e. length and surface area), vegetation species, filter media type and properties and design flow velocities (to prevent scouring) are to be provided.

Design Criteria

Criteria for the design of bioretention systems are provided below:

• The primary filter media used within the bioretention system shall be permeable enough to allow runoff to filter through the media. The media shall meet the following general criteria (ARC, 2003):

– a loam/sand, or sand, or sand/gravel mix;

– clay content less than 25% (by mass);

– not susceptible to degradation or breakdown once incorporated in the works;


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– hydraulic permeability at least 0.3 m/day (i.e. silt/sand loam);

– free of stumps, roots, or other woody material over 25 mm in diameter;

– free of seeds or propagules from noxious plants.

A sample (if available) of the proposed filter media should be tested in the laboratory to determine its average coefficient of permeability.

• If a planting soil is used as the primary filter media within the system, the soil shall be permeable enough to allow runoff to filter through the media, while having characteristics suitable to promote and sustain a vegetation cover (ie. a sand/loam mix - 35 to 60% sand content by mass). A sample (if available) of the proposed soil should be tested in the laboratory to determine its average coefficient of permeability and chemical properties.

• If a planting soil is not used as the primary filter media within the system, a 100 mm minimum thick planting soil layer is to be provided above the primary filter media. If so, then the filter material grading shall be selected (preferably) to meet the following grading compatibility criteria to limit fines from the overlying planting soil layer entering and potentially clogging the filter media:

mmD 7.015 ≤

where D15 = particle size (mm) in filter material for which 15% by weight of particles are smaller;

Alternatively, an intermediate transition filter layer (200 mm min. thick) may be provided between the surface planting soil and the primary filter media. A geofabric layer shall not be used. The transition filter material shall a competent clean, sand material, with less than 5% fines, which is not susceptible to degradation or breakdown once incorporated in the works. The transition filter material shall be provided to meet the above grading compatibility criteria and have a hydraulic permeability equivalent or greater than the underlying or overlying primary filter media.

• Capacity of the system shall be sufficient to provide adequate filtration time through the primary filter media. ARC (2003) adopts a filtration period for the mean storm of at least 30 to 50% of the mean inter-storm period. As the majority of the filtration occurs during the inter-event period, an approximate filtration period can be determined from an analysis of the site’s rainfall data history. Drainage (filtration) of the design Treatable Volume through the filter media should be 30 to 50% of the mean inter-event dry period, or approximately 24 to 48 hours, for the Western Sydney area (refer Table 2.3).

• Recommended plant species for bioretention systems shall be in accordance with Specification DS9 – Landscape Development.

• An underdrainage collection system shall be provided below the primary filter media comprising a perforated lateral pipe/s system, sized to drain the design filter flow, with a minimum pipe size of


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100 to 150 mm diameter. The underdrainage pipe/s shall be contained in fine to coarse gravel layer comprising sound, clean stone or rock of generally uniform particle size (10 mm nominal) and free from silt/clay fines or other deleterious matter. The pipe/s shall have 50 mm minimum cover of gravel material. The gravel material shall be selected to meet the following grading compatibility criteria with the filter media:

8551 4 dD ∗≤

where D15 = particle size (mm) in gravel material for which 15% by weight of particles are smaller;

d85 = particle size (mm) in filter material for which 85% by weight of particles are smaller.

Geofabric shall not be used as a filter/separation layer between the sand and gravel materials layers. A transition filter layer (200 mm min thick) may, however, be provided between the primary filter media and the gravel to satisfy the above grading compatibility criteria.

• Geofabric shall be provided along the side trench walls and base (only) to prevent the migration of surrounding fine soils into the system.

• Inclusion of inspection well(s) to check the efficiency of the bioretention system.


• Incorporation of a low permeability liner is required where the site is within a potential salinity hazard area to minimise sub-soil infiltration.

• Maximum flow rate velocities for conveyance of the Peak Design Flow shall not exceed the recommended maximum scour velocities for various ground covers and soil erodibilities presented in Table DS4.1 below (NSW Department of Housing, 1998).


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Table DS4.1

Maximum Flow Velocities in Vegetated Channels

Maximum Velocity (m/s)

Soil Erodibility Ground Cover

Low Moderate High

Mat or sword grasses with UV stabilised mesh 3.0 2.7 2.4


Couch grass, carpet grass, rhodes grass, sword forming grasses 2.0 1.8 1.4



Design Procedure

Non-Conveyance Systems (Planting Beds)

The following steps are recommended when designing non-conveyance (off-line) bioretention trenches for small scale units, eg. planting beds or “rain gardens” (adopted from ARC, 2003):


2. Determine the Treatable Flow Rate (Qd) and Peak Design Flow Rate (Qpeak) upstream of the system based on the site specific characteristics such as catchment area, topography and impervious area (refer Council’s Stormwater Drainage Design Guidelines).

3. Determine maximum ponded surface water depth (dmax) – adopt 150 mm for planting beds.

4. Calculate the surface area of the bioretention system using the following equation:

))(/( tdhkdVA T +∗∗=

where: A = minimum surface area of the system (m2);

VT = Treatment Volume (m3) (from Step 1);

k = filter soil hydraulic conductivity (m/day);

t = filtration time (days);

h = average depth of water above the soil (i.e. half dmax depth); and

d = planting soil depth (m).


5-24


t = 1 day minimum, 2 days maximum;

k = 0.3 m/day minimum (assuming a sand/silt loam soil);

h = 0.075 m;

d = 1 m nominal.

5. Optimise the filter area/size based site area and depth constraints and assessing alternate soil media permeability properties and drainage time.

6. Determine the underdrainage system pipe size/s based on the design flow through the system using the following equation:

ddhkAQ /)( maxmax +∗∗=


A = surface area of the system (m2);

k = planting soil hydraulic conductivity (m/s) (from Step 4);

hmax = maximum depth of water above the soil (from Step 4);

d = filter soil depth (from Step 3).

7. Assess intermediate sand filter layer requirements by checking grading compatibility criteria.

8. Design and incorporate backflush system for the underdrainage pipes.

9. Design high flow by-pass system for all flows above the Treatable Flow Rate (Qd) and up to the Peak Design Flow Rate (Qpeak).

10. Incorporate a low permeability liner along the swale length if required for potential salinity hazard areas.

11. Complete the Design Checklist for Bioretention Systems.

Conveyance Systems (Road Design)

The following steps are recommended when designing conveyance (on-line) bioretention systems for medium to large-scale units, comprising vegetated channels for the treatment of road runoff in combination with grass swales (adopted from CRC, 2000):

1. Determine the Treatable Volume (VT) based on the site-specific characteristics such as catchment area, topography and impervious area to provide a level of treatment (refer Council’s Stormwater

Quality Control Policy Guidelines).


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2. Determine the Treatable Flow Rate (Qd) and Peak Design Flow Rate (Qpeak) upstream of the system based on the site-specific characteristics such as catchment area, topography and impervious area (refer Council’s Stormwater Management Guidelines).

3. Based on the site area/space/hydraulic constraints, determine the maximum surface water ponding depth available (hmax), average width of ponded channel cross-section (Wav), and depth of infiltration medium (d).

4. Calculate the effective length of the bioretention system using the following equation:

( )dWktWhIL avcav /// maxmax ∗+∗=

where: L = minimum effective length of the system (m);

Imax = Treatable Flow Rate (m3/s) (from Step 1);

k = filter soil hydraulic conductivity (m/s);

hmax = maximum ponding depth (m);

d = infiltration medium depth (m);

Wav = average width of ponded cross-section; and

tc = time of concentration for catchment (used in Step 2).


k = 10-5 m/s for a sand, 5 x 10-5 to 1 x 10-6 m/s for a sandy loam/clay soil;

hmax = 0.3 to 0.4 m;

d = 1 m nominal.

5. Determine the average outflow rate through the system using the following equation:

( ) ddhkWLQ aybaseav /+∗∗∗=

where: Qav = average outflow from the system (m3/s);

L = length of the system (m) (from Step 4);

Wbase = base width of the bioretention zone;

k = filter media hydraulic conductivity (m/s) (from Step 4);

hav = average depth of water above the soil (i.e. half hmax depth); and


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d = infiltration medium depth (from Step 4).

6. Determine the average Hydraulic Residence Time (THRT) in the bioretention zone under steady state conditions using the following equation:

( ) avavbaseHRT QhWdWLT /max∗+∗∗= φ

where: φ = porosity of the infiltration media;

Use a porosity of 0.2 for a graded sand or gravel filter media.

7. THRT should be 30 to 50% of the mean inter-event dry period, or approximately 24 to 48 hours, for the Western Sydney area (refer Table 2.3). Increase bioretention length, width or filtration media characteristics to increase THRT if required.

8 Determine the underdrainage system pipe size/s based on the maximum design outflow from the system using the following equation:

( ) ddhkkAQ /maxmax +∗∗=


9. Assess intermediate sand filter layer requirements by checking grading compatibility criteria.

7. Design backflush system for the underdrainage pipes.

8. Check flow velocity for the design Treatable Flow Rate through the bioretention system. If greater than 0.5 m/s, increase the flow width or reduce the depth of flow.

9. Check flow velocity for the Peak Design Flow Rate (Qpeak) through the bioretention system for conveyance with the recommended maximum velocities presented in Table DS4.1.

10. Design inlet culverts and outlet pit size/grate for the Peak Design Flow Rate(Qpeak) within the system.

11. Complete the Design Checklist for Bioretention Systems.


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Bioretention Systems

Design Checklist


Treatable Volume/Flow Rate

Y N

Off-line/On-line System

Y N

Pre-treatment System

Y N

Primary Filter Media Permeability

Y N

Detention time

1day> t <2days Y N

Primary Filter Media Depth

Y N

Underdrainage System

Y N

Surface Velocity (on-line systems)

Y N

Salinity hazard assessment

Y N

Low Permeability Liner Required

Y N

Perforated Pipe Backflush System

Y N


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5.7 Design Specification DS5 – Permeable Pavements

(Refer Design Specification Drawing DSD5)

Function

Permeable pavements provide the following main functions:

• Removing some sediments and attached pollutants by infiltration through an underlying sand/gravel media layer;


• Delaying runoff peaks by providing retention/detention storage capacity and reducing flow velocities.

Design Approach

Even though some filtration of sediments (and attached pollutants) will occur as runoff is drained through the underlying sand bedding and gravel reservoir layers, permeable pavements should only be designed as either infiltration systems (i.e. percolation to the underlying soils) or detention systems (i.e. holding of runoff for short periods to reduce peak flows).

The design approach for permeable pavements is based on achieving the following objectives:

• For infiltration systems, providing sufficient surface area and capacity of the reservoir (sub-base) storage to contain the treatment volume and allow infiltration to the subsoil between storm events;

• For detention systems, providing sufficient capacity of the reservoir (sub-base) storage to provide adequate detention during high runoff events to reduce peak outlet design discharges to specified pre-development conditions.

The design of permeable pavements will need to demonstrate compliance with the above design objectives and criteria outlined below. References should be made to Council’s Drainage Design

Guidelines for acceptable hydrological and hydraulic calculation methods.

Design Criteria

Criteria for the design of permeable pavements are provided below:

• Runoff directed to permeable pavements, where possible, to be pre-treated to remove coarse to medium sediments.

• Pavements shall be constructed on grades less than 1% where possible, and no steeper than 5%.

• Graded such that the area can drain to another downstream source control device or the street drainage system in an overflow event.


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• The pavement subgrade to comprise a sand or fine gravel filter/bedding course with an underlying reservoir storage gravel/stone sub-base layer.

• Bedding/filter material should comprise of clean, washed (no-fines) uniform size sand or fine gravel aggregate with a minimum thickness of 50 mm below the pavement, or as specified in the pavement manufactures technical manual.

• For infiltration systems, the surface area and capacity of the reservoir storage shall be sufficient to contain the treatment volume and allow infiltration to the subsoil between storm events. ARC (2003) adopts a filtration period for the mean storm of at least 30 to 50% of the mean inter-storm period. As the majority of the filtration occurs during the inter-event period, an approximate filtration period can be determined from an analysis of the site’s rainfall data history. Drainage (filtration) of the design Treatable Volume through the filter media should be 30 to 50% of the mean inter-event dry period, or approximately 24 to 48 hours, for the Western Sydney area (refer Table 2.3).

• For detention systems, the capacity of the reservoir storage shall be sufficient to provide adequate storage capacity to provide detention during high runoff events to reduce peak outlet design discharges to specified pre-development conditions (refer UPRCT’s On-site Detention Handbook).

• The reservoir storage sub-base layer material shall comprise coarse, sound, clean stone or rock of generally uniform particle size (typically 10 to 63 mm size) and free from silt/clay fines or other deleterious matter, or as specified in the pavement manufacturer’s technical manual.

• Geofabric may be required along the side walls and base to prevent the migration of surrounding fine soils into the system. A geofabric layer should not be installed above the gravel reservoir storage layer, unless otherwise specified or recommended in the pavement manufacturer’s technical manual. Where possible, suitable size/graded gravel or filter material shall be provided to meet the following grading compatibility criteria:

D15 ≤ 4 * d85



• For detention systems, an underdrainage collection system shall be provided, sized to limit outflow discharges. The underdrains shall comprise a perforated lateral pipe/s system, sized to drain the design filter flow, with a minimum pipe size of 100 to 150 mm diameter. The pipes shall be contained within the reservoir sub-base gravel layer with 50 mm minimum cover over the underdrain pipe/s. The pipes shall discharge to the downstream drainage system.


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• Incorporation of a low permeability liner required where the site is within a potential salinity hazard area to minimise sub-soil infiltration.

Design Procedure

Infiltration System

Please note that infiltration systems are unsuitable in areas of potential salinity hazard.

The following steps should be followed when designing permeable pavements for sub-surface infiltration (adopted from ARC, 2003, MDE, 2000):

1. Determine site suitability based on evaluation criteria provided in Section 3 of this document.

2. Determine optimum infiltration time for the site based on mean inter-event period (determined from an analysis of the site’s rainfall data history). An infiltration time of between 24 and 48 hours is recommended for the Western Sydney area.

3. Determine Design filtration Rate (I) for the subsoil based investigations undertaken at the site and applying the appropriate factor of safety (s). The classification of soil types on-site and the determination of infiltration rates shall be in accordance with AS 1547 – 1994. Refer Design Specification DS6 – Infiltration Trenches (Table D6.1) for guidance of typical infiltration rates for various soils, determination of the Design filtration Rate and appropriate factors of safety to be used (Table D6.2).


5. Determine the Peak Design Flow Rate (Qpeak) from the pavement area based on the site specific characteristics such as catchment area, topography and impervious area (refer Council’s Stormwater

Management Guidelines).

6. Calculate the maximum allowable reservoir layer depth (dmax) using the following equation:

dmax = I*t where: dmax = maximum allowable trench depth (m);

t = infiltration time (hr) (from Step 2);

I = Design Infiltration Rate of subsoil (m/hr) (from Step 3).


I = use minimum from Step 3;

t = 24hr minimum, 48hrs maximum.


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12. Select the optimum reservoir layer depth (d) based on the depth that is required above the seasonal high water table, bedrock, or a very low permeability layer, or a depth less than or equal to dmax, whichever is the smaller depth.

13. Calculate the permeable pavement surface area (A) using the following equation:

A = VT/(n*d)

where: A = required area of the permeable pavement (m2);


n = porosity of the aggregate filling material;

d = reservoir layer depth (m) (from Step 7).


n = 0.35 for uniform size stone/gravel, 0.25 for a graded gravel.

14. Design surface drainage system to downstream source control device or the street drainage system for all flows up to the Peak Design Flow Rate (Qpeak).

Detention with Underdrainage System

The following steps should be followed when designing permeable pavements for stormwater detention only (no sub-surface infiltration):

1. Determine the Site Storage Requirement (SSR) and Permissible Site Discharge (PSD) and Peak Design Flow Rate (Qpeak) from the pavement area based on the site specific characteristics such as catchment area, topography and impervious area (refer Council’s Stormwater Management Guidelines and UPRCT’s On-site Detention Handbook).

2. Select the optimum trench depth (d) based on-site area/depth/hydraulic structural constraints,

3. Calculate the permeable pavement surface area (A) using the following equation:

A = SSR/(n*d)

where: A = minimum required area of the permeable pavement (m2);

SSR = Site Storage Requirement (m3) (from Step 1);

d = reservoir layer depth (m) (from Step 2).

n = porosity of the reservoir aggregate material;



4. Design surface drainage system to downstream source control device or the street drainage system in an overflow event.


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5. Determine the underdrainage system pipe size/s based on the maximum design outflow from the system using the following equation:

Q = A*k

where: Q = discharge outlet rate from the system (m3/s);

k = reservoir storage aggregate material hydraulic conductivity (m/sec);

use k = 1 x 10-3 m/s for uniform size stone/gravel.

6. Check if Q is less than the Permissible Site Discharge (PSD) (from step 1). If greater, reduce reservoir storage aggregate material hydraulic conductivity proportionally to limit Q to PSD.

7. Design backflush system for the underdrainage pipes.

8. Design high flow overflow system for all flows up to the Peak Design Flow Rate (Qpeak).

9. Incorporate a low permeability liner below the underdrainage system where required for potential salinity hazard areas.

10. Complete the Design Checklist for Permeable Pavements.

Permeable Pavement

Design Checklist


Site Evaluation Y N

Infiltration Time Y N

Subsoil Infiltration Rate Y N

Treatment Volume Y N

Required Reservoir Layer Depth

Y N

Pavement Surface Area Y N

Overflow System Y N

Perforated Pipe Underdrainage System

Y N

Salinity hazard assessment

Y N

Low Permeability Liner Y N


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5.8 Design Specification DS6 – Infiltration Trenches

(Refer Design Specification Drawing DS6).

Function

Infiltration trenches provide the following main functions:




Design Approach

Even though some filtration of sediments (and attached pollutants) will occur as runoff is drained through the underlying gravel reservoir layers, infiltration trenches should only be designed as infiltration systems where runoff is stored within the trench until it has percolated into the underlying soils.

The design approach for infiltration trenches is based on achieving the following objectives:

• Ensuring that the in-situ soil permeability properties (i.e. porosity) and other physical constraints (such as geology, soil salinity, terrain and groundwater table) within the site are appropriate for infiltration to occur between rain events;

• Providing sufficient trench depth, surface area and capacity storage to contain the treatment volume and allow infiltration to the subsoil between rainfall events (i.e. drainage time).

The design of infiltration trenches will need to demonstrate compliance of the above design objectives and criteria outlined below. References should be made to Council’s Drainage Design Guidelines for acceptable hydrological and hydraulic calculation methods. Design of infiltration systems will be required to be based on in-situ soil properties (i.e. infiltration rates) determined by site-specific investigations (refer to AS/NZS 1547:2000 for methods to determine infiltration rates).

Design Criteria


Criteria for the design of infiltration trenches are provided below:

• Stormwater entering the trench must be pretreated (eg. GPT, sediment trap or basin, vegetation filter/buffer strips, grass swale or combinations) to remove gross pollutants, coarse to medium sediments and organic matter.

• Capacity of the facility must be sufficient to provide adequate infiltration time into the subsoil. The optimum infiltration time is related to the mean inter-event period, which can be determined from an analysis of the site’s rainfall data history. An infiltration drainage time of between 24 and 72 hours is recommended (ARC, 2003).


5-34

• Field measured infiltration rates using borehole or percolation tests are required for each site. The classification of soil types and the determination of infiltration rates shall be in accordance with AS/NZS 1547:2000. The typical range of infiltration rates for homogeneous soils are provided in Table DS6.1 below (source: ARQ,2003):

Table DS6.1

Infiltration Rates for Homogeneous Soils

Soil Texture Infiltration Rate (m/sec)

Infiltration Rate (mm/hr)

Infiltration Rate (m/day)

Deep Sand (Confined/Unconfined)

5 x 10-5 Minimum 180 Minimum 4.3 Minimum

Sandy Clays 1 x 10-5 to 5 x 10-5 36 to 180 0.86 to 4.3

Medium Clays 1 x 10-6 to 1 x 10-5 3.6 to 36 0.086 to 0.86

Heavy Clays 1 x 10-8 to 1 x 10-6 0.036 to 3.6 0.00086 to 0.086

• A factor of safety (s) shall be applied to the measured infiltration rate to ensure the system will function as designed, which follows:

I = f/s

where I = Design Infiltration Rate (m/hr);

f = measured or estimated infiltration rate (m/hr); and

s = factor of safety based on Table DS6.2 below.

Table DS6.2

Factors of Safety for Infiltration (Bettess, 1996)

Consequence of failure Size of area to be drained

No damage or inconvenience

Minor inconvenience (e.g. surface water on carpark)

Damage to buildings or structures, flooding of

major roads, etc

< 100 m2 1.5 2 10

100 m2 to 1,000 m2 1.5 3 10

> 1,000 m2 1.5 5 10

• If runoff inflow to the trench sub-base is to occur from the surface, an intermediate transition filter layer (200 mm min thick) is to be provided between the surface planting soil and the primary filter media. A geofabric layer shall not be used. The transition filter material shall a competent clean, sand material, with less than 5% fines, which is not susceptible to degradation or breakdown once incorporated in the works. The transition filter material shall be provided to meet the above grading


5-35

compatibility criteria and have a hydraulic permeability equivalent or greater than the underlying or overlying primary filter media.

D15 ≤ 0.7 mm

where D15 = particle size (mm) in filter material for which 15% by weight of particles are smaller;

• The aggregate trench material shall be clean, washed stone/gravel of 25 to 75 mm diameter with the highest available surface area (i.e. granite preferred over basalt). If runoff inflow to the trench sub-base is to occur from the surface, the gravel material shall be selected to meet the following grading compatibility criteria with the overlying transition filter layer:

D15 ≤ 4 * d85



Geofabric shall not be used as a filter/separation layer,. However, if there is no surface inflow, a geofabric layer can be used as separation layer between the aggregate trench material and the surface soils.

• Geofabric shall be provided along the side trench walls and base to prevent the migration of surrounding fine soils into the system.

• An optional perforated inlet pipe can extend through the upper portion of the trench to maximise the diffusion of water throughout the trench.

• A high level overflow pit/pipe is required, connected to the stormwater system or a safe overland flow path. The overflow pipe shall be separate from the inlet diffuser pipe to prevent opportunities for short-circuiting through the trench.

• An observation well is to be installed to the base of the infiltration trench to allow performance monitoring of the system. The well is to comprise a 100 to 200 mm diameter, perforated pipe with a suitable footplate and lockable end cap.

• The invert of the trench shall be at least 1 m above any seasonable high water table or impermeable soil layer at depth.

Design Procedure

The following steps are recommended followed when designing a standard sub-surface infiltration trench (adopted from ARC, 2003, MDE, 2000):



5-36

2. Determine the optimum infiltration time (t) for the facility based on the mean inter-event period (determined from an analysis of the site’s rainfall data history). An infiltration time of 30 to 50% of the mean inter-event dry period, or approximately 24 to 48 hours, for the Western Sydney area (refer Table 2.3).

3. Determine the Design Filtration Rate (I) for the subsoil based investigations undertaken at the site and applying the appropriate factor of safety (s) (Table DS6.2). The classification of soil types on-site and the determination of in-situ infiltration rates shall be in accordance with AS 1547 – 1994.




6. Calculate the maximum allowable trench depth (dmax) using the following equation:

dmax = I*t/n

where: dmax = maximum allowable trench depth (m);

t = infiltration (drainage) time (hr) (from Step 2);

I = Design Infiltration Rate of subsoil (m/hr) (from Step 3);

n = porosity of the aggregate trench material.


I = use minimum determined from Step 3;

t = 24 hr minimum, 48 hours maximum;


7. Select the optimum trench depth (d) based on the depth that is required above the seasonal high water table, bedrock, or a very low permeability layer, or a depth less than or equal to dmax, whichever is the smaller depth.

8. Calculate the infiltration trench surface area (A) using the following equation:

A = VT/(n*d - P + IT)

where: A = area of the infiltration trench (m2);



5-37

d = optimum trench depth (m) (from Step 7);

n = porosity of the aggregate trench material;


P = Rainfall from design event (m);

T = Filling time of the trench (hrs).



Please note that the storage infiltration volume loss during the design event and the additional design water volume that enters the basin during the event are assumed to be negligible with the design volume (VT), and have been ignored for these calculations. However, these inputs/losses should be included in the calculations if deemed considerable.

9. Determine minimum length of trench required but adopting an optimal trench width (minimum 450 mm) based on site space/area constraints and limitations.

10. Design inlet culverts and outlet pit size/grate for the Peak Design Flow Rate(Qpeak) within the system.

11. Complete the Design Checklist for Infiltration Trenches.

Infiltration Trenches

Design Checklist


Site Evaluation Y N


Subsoil Infiltration Rate

Y N



Y N

Aggregate Fill Permeability

Y N

Trench Depth Y N

Trench Surface Area

Y N

Overflow System Y N


5-38

5.9 Design Specification DS7 – Infiltration Basins

(Refer Design Specification Drawing DS7).

Function

Infiltration basins provide the following main functions:




Design Approach

The design approach for infiltration basins is based on achieving the following objectives:

• Ensuring that the in-situ soil permeability properties (i.e. porosity) and other physical constraints (such as geology, soil salinity, terrain and groundwater table) within the site are adequate for infiltration to occur between rain events;

• Providing sufficient surface area and capacity of the reservoir (sub-base) storage to contain the treatment volume and allow infiltration to the subsoil between rain events.

The design of infiltration basins will need demonstrate compliance of the above design objectives and criteria outlined below. References should be made to Council’s Drainage Design Guidelines for acceptable hydrological and hydraulic calculation methods. Design of infiltration systems will be required to be based on in-situ soil properties (i.e. infiltration rates) based on site-specific investigations (refer to AS/NZS 1547:2000 for methods for determining infiltration rates).

Design Criteria


Criteria for the design of infiltration basins are provided below:

• Stormwater entering the trench must be pretreated (eg. GPT, sediment trap or basin, vegetation filter/buffer strips, grass swale or combinations) to remove gross pollutants, coarse to medium sediments and organic matter.

• Capacity of the facility must be sufficient to provide adequate infiltration time into the subsoil. The optimum infiltration time is related to the mean inter-event period, which can be determined from an analysis of the site’s rainfall data history. An infiltration time of between 24 and 72 hours is recommended (ARC, 2003).


5-39

• Field measured infiltration rates using borehole or percolation tests are required for each site. The classification of soil types and the determination of infiltration rates shall be in accordance with AS/NZS 1547:2000. The typical range of infiltration rates for homogeneous soils are provided in Table DS7.1 below (source: ARQ, 2003):

Table DS7.1

Infiltration Rates for Homogeneous Soils

Soil Texture Infiltration Rate (m/sec)

Infiltration Rate (mm/hr)

Infiltration Rate (m/day)

Deep Sand (Confined/Unconfined)

5 x 10-5 Minimum 180 Minimum 4.3 Minimum

Sandy Clays 1 x 10-5 to 5 x 10-5 36 to 180 0.86 to 4.3

Medium Clays 1 x 10-6 to 1 x 10-5 3.6 to 36 0.086 to 0.86

Heavy Clays 1 x 10-8 to 1 x 10-6 0.036 to 3.6 0.00086 to 0.086

• A factor of safety (s) shall be applied to the measured infiltration rate to ensure the system will function as designed, which follows:

I = f/s

where I = Design Infiltration Rate (m/hr);

f = measured or estimated infiltration rate (m/hr); and

s = factor of safety based on Table DS7.2 below.

Table DS7.2

Factors of Safety (f) for Infiltration (Bettess, 1996)

Consequence of failure

Size of area to be drained

No damage or inconvenience

Minor inconvenience (e.g. surface water on carpark)

Damage to buildings or structures, flooding of

major roads, etc

< 100 m2 1.5 2 10

100 m2 to 1,000 m2 1.5 3 10

> 1,000 m2 1.5 5 10

• Energy dissipators to be provided at the inlet to minimise erosion and distribute flows across the basin floor;

• Basin floor and embankment walls to be vegetated to minimise erosion and potential clogging of the basin floor;


5-40

• Basin floor to be generally flat with side slopes to be greater than 4H:1V to meet maintenance and safety requirements;

• Provision of maintenance vehicle access to the basin floor.

• A high by-pass spillway channel is required, connected to the stormwater system or a safe overland flow path.

Design Procedure

The following steps are recommended when designing a standard trapezoidal shaped excavated infiltration basin (adopted from ARC, 2003, MDE, 2000). This design procedure could also be used to approximate a parabolic shaped basin.


2. Determine optimum infiltration time (t) for the facility based on mean inter-event period (determined from an analysis of the site’s rainfall data history). An infiltration time of between 24 and 72 hours is recommended.

3. Determine the Design Filtration Rate (I) for the subsoil based investigations undertaken at the site and applying the appropriate factor of safety (s). The classification of soil types on-site and the determination of in-situ infiltration rates shall be in accordance with AS 1547 – 1994.




6. Calculate the maximum allowable basin depth (dmax) using the following equation:

dmax = I*t

where: dmax = maximum allowable basin depth (m);

t = infiltration drainage time (hr) (from Step 2);



I = use minimum determined from Step 3;

t = 24 hr minimum.


5-41

7. Select the optimum average basin depth (d) based on the depth that is required above the seasonal high water table, bedrock, or a very low permeability layer, or a depth less than or equal to dmax, whichever is the smaller depth.

8. Determine the allowable top surface area (At) based onsite area/space constraints;

9. Calculate the infiltration basin bottom area (Ab) using the following equation (assuming trapezoidal shaped basin:

Ab = (2*VT – At*d)/(d –2P +2IT)

where: Ab = infiltration basin bottom area (m2);


At = infiltration basin top area (m2) (from Step 8);

d = average basin depth (m);


P = Rainfall from design event (m);

T = Filling time of the trench (hrs).

Please note that the storage infiltration volume loss during the design event and the additional design water volume that enters the basin during the event are assumed small compared with the design volume (VT), and have been ignored for these calculations. However, these inputs/losses should be included in the calculations if deemed considerable.

10. Design inlet culverts and outlet spillway for the Peak Design Flow Rate (Qpeak) within the system.

12. Complete the Design Checklist for Infiltration Basins.


5-42

Infiltration Basin

Design Checklist


Site Evaluation Y N


Subsoil Infiltration Rate

Y N



Y N

Required Trench Depth

Y N

Trench/Basin Surface Area

Y N

Overflow Spillway Y N


5-43

5.10 Design Specification DS8 – Rainwater Tanks


Function

Rainwater tanks provide the following main functions:

• Allow the reuse of collected rainwater as a substitute for mains water supply, for use for toilet flushing, laundry, garden watering or possibly hot water supply; and

• When designed with additional storage capacity above the overflow, provide on-site detention, thus reducing peak flows and reducing downstream velocities.

Design Approach

Rainwater tank capacity is to be designed based on one or both of the following system functions:

• Water reuse system – only sufficient capacity to supply (on average) a specified proportion of the total water demand supply for the development; and

• Water reuse and detention system – provides additional storage capacity above the design capacity for reuse to temporarily detain a specified volume prior to releasing downstream stormwater system.

The proportion of water reuse from the tank depends on several variables including effective roof catchment area, water use rate, tank capacity and long-term rainfall characteristics of the area.

The required detention volume depends on several variables including the roof and other impervious areas within the site, tank capacity, outlet orifice size and storm characteristics. This can be matched by the available detention volume, which is dependent on the tank size, roof size and consumption rate.

The tank efficiency and the average available detention volume have been estimated using the Probabilistic Urban Rainwater and wastewater Reuse Simulator (PURRS, 2002), using 10 years of continuous rainfall data from Prospect Reservoir gauging station. The design capacity requirements for both rainwater reuse and reuse/detention tank systems are addressed in this document.

Design Criteria

The adopted design assumptions and criteria for design of rainwater tanks are provided below:

• For detention systems, at least 80% of the roof area of the development must be diverted to the tank with the remainder being diverted to other WSUD measures, or to the stormwater system.

• All tanks must be fitted with a first flush device, which diverts the first 1 mm of roof water. The device is to include a primary litter/leaf mesh screen and a first flush containment storage with a small orifice (5 mm nom. diameter) to empty the storage between rain events. The first flush water is


5-44

to be directed to another WSUD measure (such as a grassed filter strip/buffer) before discharging to the stormwater system.

• The tank is topped up with potable mains water to a level when the tank levels falls below a minimum specific level. A float valve system is to be incorporated to close, or partially close, the top up pipe in response to water levels.

• The Design Roof Area and Design Number of Occupants have been estimated for four lot sizes, as shown in Table DS8.1. The Design Roof Area has been used in PURRS to calculate the rainfall that will be available for use by the rain tank, while the Design Number of Occupants determines the demand for toilet and washing requirements.

Table DS8.1

Design Roof Area and Number of Occupants for a given Lot Size

Design Lot Size (m2) Design Roof Area (m2)

Design Number of Occupants

250 150 2

350 210 3

450 270 4

1000 450 5

• The Design Demand has been determined based on Design Number of Occupants in the household and the Demand Use of the rainwater tank, and is shown in Table DS8.2 below.

Table DS8.2

Design Demand for Rainwater Tank (kL/year)

Design Lot Size (m2) Demand Use

250 350 450 1000

Gardening* only 303 426 548 1277

Toilet** 271 407 542 678

Washing Machine*** 248 372 496 619

* Seasonally based - 50% of annual demand occurs in summer, 20% in autumn and spring and 10% in winter. ** Based on typical toilet consumption of 35 L/person/day (ARQ, 2003). ** Cold water usage for washing machines only.

• The minimum required Rainwater Tank Capacity for reuse is determined based on the Design Demand and the Minimum Rainwater Tank Efficiency for the appropriate effective Roof Catchment Area, using PURRS. The results of this modelling have been provided in Tables DS8.3, DS8.4, DS8.5 and DS8.6.

• Overflows from the rainwater tank to be connected to the downstream stormwater system.


5-45

Table DS8.3

Average Efficiency of Rainwater Tank for 250 m 2 Lot

Demand Use Rainwater Tank Capacity for Reuse (kL)

1 2 5 7.5 10 15

Gardening only 53 73 93 96 98 99

Gardening + Toilet 37 57 80 88 92 95

Gardening + Toilet + Washing Machine

27 44 66 74 78 85

Table DS8.4


Demand Use

Rainwater Tank Capacity for Reuse (kL)

1 2 5 7.5 10 15




21 35 58 67 73 80

Table DS8.5


Demand Use


1 2 5 7.5 10 15



Gardening + Toilet + Washing Machine***

16 27 50 59 65 74

Table DS8.6


Demand Use


1 2 5 7.5 10 15




11 19 39 49 56 66

• The Average Available Detention Volume is determined by subtracting the average volume of water in the tank from the total tank volume. The average volume for each tank size has been calculated


5-46

by PURRS, based on the Roof Area, Lot Size and Demand Use, and is provided in Tables DS8.7, DS8.8, DS8.9 and DS8.10.

Table DS8.7

Average Available Detention Volume for 250 m 2 Lot

Demand Use

Average Available Detention Volume (kL)

1 2 5 7.5 10 15

Gardening only 0.18 0.37 0.79 0.90 1.07 1.39

Gardening + Toilet 0.26 0.63 1.68 2.27 2.78 4.19


0.29 0.73 2.22 3.33 4.41 6.87

Table DS8.8


Demand Use


1 2 5 7.5 10 15

Gardening only 0.13 0.31 0.72 0.93 1.01 1.28



0.23 0.59 2.09 3.21 4.23 6.58

Table DS8.9


Demand Use


1 2 5 7.5 10 15

Gardening only 0.19 0.40 0.97 1.3 1.52 1.82



0.30 0.66 2.18 3.47 4.62 7.19


5-47

Table DS8.10


Demand Use


1 2 5 7.5 10 15

Gardening only 0.18 0.40 1.18 1.77 2.36 3.37



0.26 0.55 1.75 2.89 4.11 6.59

Design Procedure

The following steps should be followed when designing the capacity of a rainwater tank:

Water Reuse System

1. Determine the Minimum Rainwater Tank Efficiency for the development from Council’s Planning Policy Guidelines.

2. Determine the Design Lot Size for the development using Table DS8.1.

3. Determine the Demand Use for the rainwater tank to supply the household using Table DS8.2.

4. Determine the minimum required Rainwater Tank Capacity for Reuse using Tables DS8.3, DS8.4, DS8.5 and DS8.6, as appropriate based on the Lot Size (from Step 2), the Design Use (from Step 3) and the Minimum Rainwater Tank Efficiency (from Step 1). Interpolate between alternate Lot Sizes if different than those specified for the tables.

5. Determine pre-treatment requirements (i.e. leaf diverters, first-flush system etc.).

6. Determine outlet pipe requirements for the Peak Design Flow Rate (Qpeak) from the effective Roof Catchment Area (refer Council’s Stormwater Management Guidelines).

Water Reuse and Detention System

1. Determine the required rainwater tank capacity for reuse (Steps 1-6 from Water Re-use System design procedure).

2. Determine the average available detention storage using Tables DS8.7, DS8.8, DS8.9 and DS8.10 for the specified Tank Size, Lot Size and Demand Use.

3. Determine the Total Impervious Area requiring detention, which is the effective Roof Catchment Area that is draining to the rainwater tank plus any other impervious areas on-site that are not connected to the tank, such as driveways, carparks (up to a maximum of 120 m2 for non-roof impervious areas).


5-48

4. Determine the Site Storage Requirement (SSR) and Permissible Site Discharge (PSD) based on the Total Impervious Area within the site requiring detention (refer Council’s Stormwater Management Guidelines and UPRCT’s On-site Detention Handbook).

5. Combine the required Rainwater Tank Capacity for Reuse (from Step 2) and the required SSR volume (from Step 2) to obtain the total effective rainwater tank volume.

6. Determine the required orifice and outlet requirements (refer UPRCT’s On-site Detention Handbook).

7. Determine pre-treatment requirements (i.e. leaf/litter screen, first-flush system etc.).

8. Determine outlet pipe requirements for the Peak Design Flow Rate (Qpeak) from the effective Roof Catchment Area (refer Council’s Stormwater Management Guidelines).

Rainwater Tank

Design Checklist


Minimum Rainwater Tank Efficiency

Y N

Design Number of Occupants Y N

Demand Use Y N

Design Demand Y N

Roof Catchment Area Y N

Rainwater Tank Capacity For Reuse

Y N

Site Total Impervious Area Y N

Site Storage Requirement (SSR) Y N

Total Rainwater Tank Volume

Overflow system

First Flush Device Y N


5-49

5.11 Design Specification DS9 - Landscape Developments

Function

Application of WSUD principles to landscape design aims to perform the following functions:

• Maximising the survival of plants during periods of low rainfall.

• Conserving an effective vegetation cover for WSUD measures that incorporate vegetation such as drainage needs and filter strips.

• Enhancing biodiversity and habitat values by giving preference to locally indigenous plant species.

Appropriate Landscape Planning and Design

• Landscape design shall be prepared by a suitably qualified person (eg landscape architect with ecological knowledge) based on the six principles listed in section 1.2

• Determine general soil category (clay, alluvial, sandstone derived).

• Identify the regional climatic factors that will influence the selection of suitable low-water demand plants.

• Carry out a site analysis to identify microclimate relevant factors including shading, sun and wind exposure, low-lying areas subject to frost.

• Determine landscape situation (eg residential development, local street, riparian zone).

• Assess soil conditions (fertility, physical structure, permeability) to identify factors that influence plant growth as well as any requirements to modify these conditions to assist plant growth.

• Irrigation water requirements are to be minimised by grouping species with similar water requirements and selecting plants that are adapted to local climatic and soil conditions.

• The extent of lawn should be limited in order to minimise requirements for irrigation water. Xerophytic vegetation is preferred.

• Ground cover plants and native grasses are to be used as an alternative to lawn as much as possible.

• Inorganic material such as gravel (not sourced from rivers and creeks) should be used as an alternative surface cover to lawn where appropriate.

• Consumption of water for irrigation shall be eliminated where possible through the selection of suitable species. Where irrigation is necessary the volume of water consumed shall be minimised by ensuring that the most efficient irrigation system is installed and properly maintained.

• Drip irrigation systems shall be used wherever possible in preference to spray irrigation.


5-50

• Automatic irrigation systems are to incorporate soil moisture sensors that avoid irrigation during periods of rainfall.

Implementing soil improvements

• Site soil should be tested to identify conditions that may inhibit plant growth such as compaction, soil salinity and deficiencies in soil nutrient and organic matter content.

• Improvements to the physical and nutrient condition of the soil should be carried out to ensure rainfall infiltration and soil moisture retention are achieved in order to assist plant growth.

Using surface mulches

• Evaporation from the soil surface can be minimised through the application of a 75 mm min. thick layer of surface mulch, such as weed-free woodchip mulch, where appropriate in planted landscape areas.

• Surface water run-off may be minimised through the application of suitable mulch material.

Selecting low water demand plants

• Low water demand plants shall be selected from the tables presented for the appropriate soil category and landscape situation.

Implementing appropriate landscape maintenance

• A program of landscape to assist health and vigour of plants that includes

• regular weed control measures

• maintenance of installed vegetation

• repair of any soils and mulch erosion that occurs

• conserving soil on site

• Plant species shall be selected from the following schedules for the appropriate landscape situation and soil type.

• Selecting locally endemic plant species can reduce landscape maintenance requirements due to their adaptations to the local climate.


5-51

Table DS9.1 Suggested Landscaping Plants for Clay Soils

Landscape Situation

Res

iden

tial

D

evel

op

men

t

Lo

cal S

tree

ts

Urb

an P

arkl

and

s

Pu

blic

Op

en S

pac

e

Rip

aria

n Z

on

es

infi

ltra

tio

n/D

eten

tio

n

Bas

ins

Sw

ales

& F

ilter

S

trip

s

Bio

rem

edia

tio

n

Sys

tem

s/W

etla

nd

s

Trees

Acacia decurrens x x x x

Acacia elata x x x x

Acacia melanoxylon x

x x x x

Acacia parramattensis

Angophora bakeri x x

Angophora floribunda x x x

Angophora subvelutina

Brachychiton populneum x x

Casuarina glauca x x

Eucalyptus acmenoides

Eucalyptus amplifolia

Eucalyptus bauerana

Eucalyptus benthamii

Eucalyptus crebra x x

Eucalyptus eugenoides

Eucalyptus fibrosa x x

Eucalyptus molluccana x x x

Eucalyptus paniculata

Eucalyptus tereticornis x x x x x x

Melaleuca decora x

x x x

Melaleuca linariifolia x

x x x x

Melaleuca stypheloides x x x x

Melaleuca bracteata x x x x


5-52

Landscape Situation

Res

iden

tial

D

evel

op

men

t

Lo

cal S

tree

ts

Urb

an P

arkl

and

s

Pu

blic

Op

en S

pac

e

Rip

aria

n Z

on

es

infi

ltra

tio

n/D

eten

tio

n

Bas

ins

Sw

ales

& F

ilter

S

trip

s

Bio

rem

edia

tio

n

Sys

tem

s/W

etla

nd

s

Platanus x hybrida x x

Shrubs

Acacia glaucescens x

Callistemon salignus x x x x

Callistemon viminalis x x x x

Callistemon citrinus x x x x

Clerodendrum tomentosum

Dillwynia retorta

Kunzea ambigua x

x x x

Leptospermum lanigerum X

X x

Ground Cover

Myoporum debile x x x x

Hardenbergia violacea x x x x x

Grasses

Themeda australis x x x x x

x

Danthonia tenuior x x x x x

x

Imperata cylindrica x x x x

x

Lomandra longifolia x x x x x x x

Microlaena stipoides x x x x x x x

Poa labillardieri x x x x x

x

Macrophytes

Baumea articulata x

Baumea rubiginosa x

Bolboschoenus caldwellii x

Bolboschoenus fluvitalis x

Carex appressa x


5-53

Landscape Situation

Res

iden

tial

D

evel

op

men

t

Lo

cal S

tree

ts

Urb

an P

arkl

and

s

Pu

blic

Op

en S

pac

e

Rip

aria

n Z

on

es

infi

ltra

tio

n/D

eten

tio

n

Bas

ins

Sw

ales

& F

ilter

S

trip

s

Bio

rem

edia

tio

n

Sys

tem

s/W

etla

nd

s

Cyperus exaltatus

x

Eleocharis acuta x

Eleocharis sphacelata x

Juncus usitatus

x

Isolepsis inundata x

Phragmites australis x

Schoenoplectus mucronatus x

Schoenoplectus validis x


5-54

Table DS9.2 Suggested Landscaping Plants for Alluvial Soils

Landscape Situation

Res

iden

tial

D

evel

op

men

t

Lo

cal S

tree

ts

Urb

an P

arkl

and

s

Pu

blic

Op

en S

pac

e

Rip

aria

n Z

on

es

infi

ltra

tio

n/D

eten

tio

n

Bas

ins

Sw

ales

& F

ilter

Str

ips

Bio

rem

edia

tio

n

Sys

tem

s/W

etla

nd

s

Trees

Acmena smithii x x x x x

Angophora floribunda x

x x x

Backhousia myrtifolia x x x x x

*Acer buergerianum x x x

Brachychiton acerifolium x x

Brachychiton populneum x x

Callistemon citrinus x x x x


Callistemon viminalis x x x x

Casuarina cunninghamiana x x x x x

Corymbai citriodora x x x

Corymbia maculata x x x

Corymbia gummifera x x x x

Elaeocarpus reticulatus x x x x x

Eucalyptus amplifolia x x x

Eucalyptus deanei x x x

Eucalyptus elata x x x

Eucalyptus parramattensis

Eucalyptus pilularis x x x

Eucalyptus sideroxylon x x x x

Eucalyptus robusta x x

Eucalyptus saligna x x x

Eucalyptus sclerophylla


5-55

Landscape Situation

Res

iden

tial

D

evel

op

men

t

Lo

cal S

tree

ts

Urb

an P

arkl

and

s

Pu

blic

Op

en S

pac

e

Rip

aria

n Z

on

es

infi

ltra

tio

n/D

eten

tio

n

Bas

ins

Sw

ales

& F

ilter

Str

ips

Bio

rem

edia

tio

n

Sys

tem

s/W

etla

nd

s

Eucalyptus viminalis

Lophostemon confertus x x x x x

Melaleuca quinquenervia x x x x x

Syncarpia glomulifera x x x

Grevillea robusta x x x

Livistona australis

Melaleuca deocra x x x x x x

Melaleuca eliptica x x x

Melaleuca bracteata x x x

Melaleuca stypheloides x x x x x

*Populus deltoides x

Tristania conferta x x x x x

Shrubs

Acacia elata x

x x x

Acacia pendula x

x x x

Acacia promine x

x x x

Acacia spectabilis x

x x x

Acacia floribunda x

x x x

Banksia intergifolia x

x x

Banksia robur x

x x

Leptospermum petersonii x

x x

Leptospermum laevigatum x x x x

Olearia pimeloides x

x x

Westringia fruticosa x

x x

Westringia grandifolia x

x x


5-56

Landscape Situation

Res

iden

tial

D

evel

op

men

t

Lo

cal S

tree

ts

Urb

an P

arkl

and

s

Pu

blic

Op

en S

pac

e

Rip

aria

n Z

on

es

infi

ltra

tio

n/D

eten

tio

n

Bas

ins

Sw

ales

& F

ilter

Str

ips

Bio

rem

edia

tio

n

Sys

tem

s/W

etla

nd

s

Ground Cover

Hibbertia scandens x

x x x

x

*Clivea mineata x x x

Myoporum debile x x x x x

x

Wahlenbergia gracillis x x x x

Dianella caerulea x x x x x

x

*Cerastium tomentosum x x x

*Hemerocallis hybrids x

x x

Kennedia rubicunda x x x x x

Hardenbergia violacea x x x x x

x

Grasses

Dichelachne crinata x x x x x

Danthonia caespitosa x x x x x


x

Lomandra longifolia x x x x x x x

Microlaena stipoides x x x x x x x

Poa labillardieri x x x x x

x

Stipa spp. x x x x x

x

Themeda australis x x x x x

x

Macrophytes

Baumea articulata x

Baumea rubiginosa x


Bolboschoenus fluviatilis x

Carex appressa x

Cyperus exaltatus

x


5-57

Landscape Situation

Res

iden

tial

D

evel

op

men

t

Lo

cal S

tree

ts

Urb

an P

arkl

and

s

Pu

blic

Op

en S

pac

e

Rip

aria

n Z

on

es

infi

ltra

tio

n/D

eten

tio

n

Bas

ins

Sw

ales

& F

ilter

Str

ips

Bio

rem

edia

tio

n

Sys

tem

s/W

etla

nd

s

Eleocharis acuta x


Juncus usitatus

x






5-58

Table DS9.3 Suggested Landscaping Plants for Sandstone Derived Soils

Landscape Situation

Res

iden

tial

D

evel

op

men

t

Lo

cal S

tree

ts

Urb

an P

arkl

and

s

Pu

blic

Op

en S

pac

e

Rip

aria

n Z

on

es

infi

ltra

tio

n/D

eten

tio

n

Bas

ins

Sw

ales

& F

ilter

S

trip

s

Bio

rem

edia

tio

n

Sys

tem

s/W

etla

nd

s

Trees

Allocasuarina portuensis

Angophora costata x x x

Angophora bakeri x x x

Angophora hispida

Banksia ericifolia

Banksia marginata x x x x

Banksia serrata x x x x

Callitris rhomboidea x x


Casuarina glauca

Casuarina littoralis x x x x x x

Casuarina torulosa x x x x x x

Corymbia gummifera

Eucalyptus eximia x x

Eucalyptus gummifera x x

Eucalyptus haemastoma x x x x x

Eucalyptus piperita

Eucalyptus punctata x x x

Eucalyptus racemosa x x

Eucalyptus resinifera x x

Eleaocarpus reticulata x

x x x

Ficus rubiginosa x x

Glochidion ferdinandi


5-59

Landscape Situation

Res

iden

tial

D

evel

op

men

t

Lo

cal S

tree

ts

Urb

an P

arkl

and

s

Pu

blic

Op

en S

pac

e

Rip

aria

n Z

on

es

infi

ltra

tio

n/D

eten

tio

n

Bas

ins

Sw

ales

& F

ilter

S

trip

s

Bio

rem

edia

tio

n

Sys

tem

s/W

etla

nd

s

Syncarpia glomulifera x x x

Tristaniopsis laurina

Trochocarpa laurina x x x

Shrubs

Acacia buxifolia x

x x x

Acacia prominens x

x x x

Acacia calamifloia x

x x x

Acacia longifolia x

x x x

Acacia conferta x

x x x

Acacia fimbriata x

x x x

Acacia floribunda x

x x x

x

Acacia glaucescens x

x x x

Acacia longifolia var longifolia x

x x x

x

Callicoma serratifolia x

x x x

Callistemon citrinus

Ceratopetalum gummiferum x

x x x

Clerodendrum tomentosum

Dillwynia retorta x

x x x

Dodonaea micozyga x x x

Dodonaea viscose x x x

Doryanthes excelsa x x x x x

x

Eremophila bowmanii x

x x x

Eremophila divaricata x

x x x

Grevillea juncifolia x

x x x

Hakea dactyloides x

x x x

Hakea sericea x x x


5-60

Landscape Situation

Res

iden

tial

D

evel

op

men

t

Lo

cal S

tree

ts

Urb

an P

arkl

and

s

Pu

blic

Op

en S

pac

e

Rip

aria

n Z

on

es

infi

ltra

tio

n/D

eten

tio

n

Bas

ins

Sw

ales

& F

ilter

S

trip

s

Bio

rem

edia

tio

n

Sys

tem

s/W

etla

nd

s

Isopogon anethifolius x

x x x

Kunzea ambigua x

x x x

Leptospermum polygalifolium x

x x x

Leptospermum petersonii x

x x x

Melaleuca nodosa x

x x x

Myoporum insulare x

x x x

Olearia microphylla

Pittosporum revolutum

Westrigia fruticosa x

x x x

Westringia ‘Wynyabbie Gem’ x

x x x

Ground Cover

Carpobrotus modestu x

x x

Cissus antartica x

x x

Clematis aristata x

x x x

Darwinia grandiflora

Dianella caerulea

Dianella revoluta

Echinopogon caespitosus

Einadia nutans x

Hardenbergia violacea

Hibbertia scadens x x x x x

x

Hibbertia pendunculata x

x x x

x

Myoporum parvifolium x x x x x

x

Pandorea pandorana x

x x

Grasses

Blandfordia nobilis


5-61

Landscape Situation

Res

iden

tial

D

evel

op

men

t

Lo

cal S

tree

ts

Urb

an P

arkl

and

s

Pu

blic

Op

en S

pac

e

Rip

aria

n Z

on

es

infi

ltra

tio

n/D

eten

tio

n

Bas

ins

Sw

ales

& F

ilter

S

trip

s

Bio

rem

edia

tio

n

Sys

tem

s/W

etla

nd

s

Dichelachne crinata x x x

x

Danthonia tenuior x x x

x


x

Lomandra longifolia x x x x

x

Microlaena stipoides x x x x

x

Poa labillardieri x x x x

x

Stipa ramossissima x x x x

x

Themeda australis x x x x

x

Macrophytes

Baumea articulata x

Baumea rubiginosa x


Bolboschoenus fluviatalis x

Carex appressa x

Cyperus exaltatus

x

Eleocharis acuta x


Juncus usitatus

x





SECTION 6 WSUD Operation and Maintenance

6-1

6 WSUD Operation and Maintenance

6.1 Introduction

Detailed operation and maintenance programs for structural WSUD measures are crucial to optimise their long-term performance. Industry experience has shown that many WSUD measures are inadequately protected during construction or poorly maintained, leading to the reduction in design performance and, in some instances, failure. Inadequate maintenance may arise from a number of issues (CRCCH, 2003) and include:

• A lack of consideration of design attributes during the planning phase of the WSUD measure that facilitate maintenance programs being adequately conducted on site (ie. access ramps, dewatering facilities etc.);

• Maintenance personnel unsure of what maintenance procedures to follow during routine site inspections and clean out programs; and

• Inadequate budgetary resources set aside by organisations responsible to maintain the WSUD measure at a frequency necessary to ensure a system operates as it was designed to.

The following section provides:

• Guidance on the critical items should be monitored during inspections for each WSUD measure;

• Guidance on the critical maintenance requirements and activities for each WSUD measure; and

• Preliminary Maintenance and Inspection Checklists for each WSUD measure, providing critical items to be inspected, specifying maintenance requirements and inspection frequencies, that can be used for maintenance personnel (based on ARC, 2003).


6-2

6.2 Vegetated Swales

Inspection and Monitoring

Following construction, swales should be inspected every 1 to 3 months (or after each major rainfall event) during the initial establishment period to determine whether or not the swale surface and vegetation requires immediate maintenance. The following critical items should be monitored:

• Channelisation and erosion;

• Density of vegetation;

• Weed infestation;

• Integrity of inlet and outlet areas and check dams;

• Inappropriate access and wear;

• Nuisance problems such as mosquitoes and boggy areas

After the initial establishment period (typically 1 to 2 years), inspections may be extended to the frequencies shown in the Maintenance and Inspection Checklist for Vegetated Swales.

Maintenance

Vegetated swales require on-going maintenance such as mowing, watering (depending on the plant species selected), weeding, fertilising (depending on the plant species selected), sediment and litter removal and scour and erosion repair. Vegetation should maintained preferably above 100 to 150 mm in height.

Residents need to be informed of the function of any swale(s) within or bordering their properties and its benefits to prevent damage and or misuse. The erection of signage near swales on publicly accessible land may be useful in informing the public of their function and use.

For grass-lined channels, which are not recommended in comparison to vegetated swales, mowing of the grass shall be at least to 25 mm height. Mowing shall be outside predicted rain periods.


6-3

Vegetated Swales

Maintenance and Inspection Checklist

Checked Maintenance Needed

Inspection Frequency

Items Inspected

DEBRIS CLEANOUT

6M

Swale and contributing areas clean of debris

Observed dumped domestic litter/debris in swale channel

SWALE SURFACE 6M

Evidence of erosion

Vegetation condition

SWALE VEGETATION

6M

Vegetation trimming/mowing

Fertilisation required where grass is used (slow release organic fertilisers recommended)

Weed infestation

PONDING

6M

Evidence of ponding water

Inspection Frequency Key:

A = Annual M = Monthly 3M = Three Monthly 6M = Six Monthly 3-6M = Three to Six Monthly 1-3M = One to Three Monthly


6-4

6.3 Vegetated Filter Strips


Following construction, filter strips should be inspected every 1 to 3 months (or after each major rainfall event) for the initial establishment period to determine whether or not the filter strip surface and vegetation requires immediate maintenance. The following critical items should be monitored:

• Channelisation and erosion;

• Density of vegetation;


• Inappropriate access and wear;

• Nuisance problems such as boggy areas.

After the initial establishment period (typically 1 to 2 years), inspections may be extended to the frequencies shown in Maintenance and Inspection Checklist for Vegetated Filter Strips.

Maintenance

Vegetated strip or buffers require regular maintenance such as mowing, watering (depending on the plant species selected), weeding, fertilising (depending on the plant species selected), sediment and litter removal and scour and erosion repair. Vegetation should be maintained preferably above 75 mm in height.

Residents need to be informed of the function of any filter strip(s) within or bordering their properties and its benefits to prevent damage and or misuse. The erection of signage near filter strips on publicly accessible land may be useful in informing the public of their function and use.


6-5

Vegetated Filter Strip




Items Inspected

DEBRIS CLEANOUT

6M

Filter and contributing areas clean of debris

Observed dumped domestic debris/litter within area

SWALE SURFACE 6M

Evidence of erosion


Erosion repair required

FILTER STRIP VEGETATION

6M

Vegetation trimming/mowing required

Fertilisation required where grass is used (slow release organic fertilisers recommended)

Weed infestation

PONDING

6M

Evidence of ponding water




6-6

6.4 Sand Filters


Following construction, sand filters should be inspected every 1 to 3 months (or after each major rainfall event) for the initial 6 months of operation to determine whether or not the filter requires maintenance or the media requires replacement. The following items should be monitored:

• Ponding, clogging and blockage of the filter media;

• Depth of sediment in the sediment chamber; and

• Blockage of the outlet from the sediment chamber.

After the initial 6 months, inspections may be extended to the frequencies shown in Maintenance and Inspection Checklist for Sand Filters.

Maintenance

If the filter is not cleaned frequently, the entire filter media may need to be replaced due to clogging of the media material with fine particles. This means frequent maintenance will be more cost effective in the long term. The following maintenance activities will be required with inspection frequencies shown in the Maintenance and Inspection Checklist:

• Sediment and litter to be removed from the sediment chamber if the sediment storage capacity is approaching half full, with drying of the sediment possibly required before disposal.

• Litter and debris over the filter surface to be cleaned out and the surface raked to break up any crusts (to improve infiltration).

• The top layer (50 to 100 mm) of the filter media to be periodically removed and replaced depending on loading rates.

• Repair of structure components including side walls, pits and grates;

• Repair outlet and overflow structures and the removal of accumulated debris.


6-7

Sand Filters




Items Inspected

M/A

DEBRIS CLEANOUT

6M

Contributing areas clean of debris

Filtration surface clean of debris

Inlets and outlets clear of debris

FILTER SURFACE REPAIRS 6M

Surface of filter clean

Top layer require raking

Top 100 mm layer requires replacement

Entire filter media requires replacement

OIL AND GREASE 6M

Evidence of filter surface clogging

Activities in drainage area minimise oil and grease entry

VEGETATION (UPSLOPE) 6M

Contributing drainage area stabilised

Evidence of erosion

SEDIMENT TRAPS, FOREBAY, OR BASIN

6M

Sediment chamber at normal pool depth?

Sediment chamber not more than 50% full of sediment

STRUCTURAL COMPONENTS

A

Evidence of structural deterioration

Grate/pit condition

OUTLETS/OVERFLOW SPILLWAY

A

Good condition, no need for repair

No evidence of erosion (if draining into a natural channel)




6-8

6.5 Bioretention Systems


Following construction, bioretention systems should be inspected every 1 to 3 months (or after each major rainfall event) for the initial vegetation establishment period to determine whether or not the bioretention zone requires maintenance or the media requires replacement. The following critical items should be monitored:

• Ponding, clogging and blockage of the filter media;

• Establishment of desired vegetation/plants and density;

• Blockage of the outlet from the bioretention system.

After the initial establishment period (typically 1 to 2 years), inspections may be extended to the frequencies shown in Maintenance and Inspection Checklist for Bioretention Systems.

Maintenance

If the bioretention system is not maintained frequently, the entire filter media may need to be replaced due to clogging of the media material with fine particles. This means frequent maintenance ismore cost effective in the long term. The following maintenance activities will be required with inspection frequencies shown in the Maintenance and Inspection Checklist:

• Maintenance of flow to and through the system;

• Maintaining the surface vegetation;

• Preventing undesired overgrowth vegetation/weeds from taking over the area;

• Removal of accumulated sediments; and

• Debris and litter removal.


6-9

Bioretention System




Items Inspected

DEBRIS CLEANOUT

6M

Surface clear of debris & litter

Inlet area clear of debris & litter

Overflow clear of debris & litter

TRENCH SURFACE VEGETATION

6M


Vegetation trimming/maintenance

Weed infestation

Evidence of erosion

DEWATERING

6M

Trench surface dewatering between storms

Top soil layer require replacing?

Entire planting media require replacing?

OUTLET/OVERFLOW CHANNEL OR PIT

A

Pit/grate condition

Evidence or cracking or spalling of concrete structures

Evidence of erosion in downstream channel




6-10

6.6 Permeable Pavements


Following construction, infiltration system should be inspected every month (or after each major rainfall event) for the initial 6 months of operation to determine whether or not the infiltration zone requires immediate maintenance. The following critical items should be monitored:

• Surface ponding (which would indicate clogging or blockage of the underlying aggregate);

• Sediment build-up;

• Blockage of the outlet pipe (if applicable).

After the initial 6 months, inspections may be extended to the frequencies shown in Maintenance and Inspection Checklist for Permeable Pavements.

Maintenance

For efficient operation of permeable pavements it is essential that the gaps between the paver and the underlying bedding layer do not become clogged by fine sediment. To prevent this from occurring, permeable pavements require the following maintenance activities:

• High pressure hosing, sweeping or vacuuming depending on the manufacturer’s specifications to remove sediments and restore porosity;

• Repair of potholes and cracks;

• Replacement of clogged/water logged areas;

• Rectification of any differences in pavement levels.


6-11

Permeable Pavement




Items Inspected

DEBRIS CLEANOUT

3M

Pavement surface clear of debris

PAVEMENT SURFACE 3M

Sediment build-up

Potholes

Cracking of pavement

Significant pavement deflection

DEWATERING

3M

Pavement surface dewatering between storms?

Replacement required of clogged pavement

OUTLET/OVERFLOW

A

Outlet condition

Evidence of erosion downstream




6-12

6.7 Infiltration Trenches


Following construction, the infiltration system should be inspected every 1 to 3 months (or after each major rainfall event) for the initial 6 months of operation to determine whether or not the infiltration zone requires immediate maintenance. The following critical items should be monitored:

• Surface ponding (which would indicate clogging or blockage of the underlying aggregate);

• Water remaining above the trench base after the design infiltration period (which would indicate clogging of the underlying aggregate or the base of the trench);

• Sediment in the upper layer of aggregate;

• Pre-treatment sediment traps, forebays or swales are working in accordance with the design; and

• Blockage of the outlet pipe from the system.

After the initial 6 months, inspections may be extended to the frequencies shown in the Maintenance and Inspection Checklist for Infiltration Trenches.

Maintenance

If the infiltration trench is not maintained frequently, the entire trench aggregate may need to be replaced due to clogging of the media material with fine particles. This means frequent maintenance is more cost effective in the long term. The following maintenance activities will be required with inspection frequencies shown in the Maintenance and Inspection Checklist:

• Removal of the top sand/aggregate layer and the geofabric layer if excessively clogged;

• Maintaining the surface vegetation (if present);

• Debris and litter removal.


6-13

Infiltration Trench




Items Inspected

DEBRIS CLEANOUT

6M

Trench surface clear of debris & litter


Overflow clear of debris & litter

SEDIMENT TRAPS, FOREBAYS, OR PRE-TREATMENT SWALES

6M

Trapping sediment effectively

Facility not more than 50% full of sediment

TRENCH SURFACE 6M

Evidence of surface erosion/scouring

TRENCH SURFACE VEGETATION (if applicable)

6M



Weed infestation

DEWATERING

3-6M

Trench surface dewatering between storms

Trench base dewatering between storms

Top aggregate layer/geofabric need replacing?

Entire aggregate requires replacing?

OUTLET/OVERFLOW SPILLWAY

A

Outlet/overflow condition





6-14

6.8 Infiltration Basins


Following construction, infiltration basins should be inspected every 1 to 3 month (or after each major rainfall event) for the initial 6 months of operation to determine whether or not the infiltration zone and inlet and outlet structures require immediate maintenance. The following critical items should be monitored:

• Monitoring duration of ponding compared to the design infiltration period;

• Inspection of basin floor for erosion, sediment deposition and grass growth;

After the initial 6 months, inspections may be extended to the frequencies shown in the Maintenance and Inspection Checklist for Infiltration Basins. A long-term monitoring program of infiltration rates should be initiated to compare/confirm actual rates against design rates over time.

Maintenance

The following critical maintenance activities will be required with inspection frequencies shown in the Maintenance and Inspection Checklist.

• Removal of deposited sediment, debris and litter from the basin floor and inlet/outlet areas;

• Surface ripping to enhance infiltration rates (if dropped to unacceptable rates); and

• Vegetation trimming and maintenance.


6-15

Infiltration Basin




Items Inspected

DEBRIS CLEANOUT

6M

Storage clear of debris & litter


SEDIMENT TRAPS, FOREBAYS, OR PRE-TREATMENT SWALES

6M

Trapping sediment effectively

Facility not more than 50% full of sediment

BASIN SURFACE 6M

Evidence of surface erosion/scouring

Good vegetation layer exists

BASIN VEGETATION

6M



Weed infestation

DEWATERING

3-6M

Basin dewatering between storms?

OUTLET/OVERFLOW SPILLWAY

A

Outlet/overflow condition





6-16

6.9 Rainwater Tanks


For rainwater tanks the following items should be inspected:

• Clogging and blockage of the first flush device;

• Clogging and blockage of the tank inlet leaf/litter screen; and

• Depth of sediment within the tank.

Inspections should be undertaken at the frequencies shown in the Maintenance and Inspection Checklist for Rainwater Tanks.

Maintenance

The following maintenance activities will be required with inspection frequencies shown in the Maintenance and Inspection Checklist:

• First flush device to be cleaned out;

• Leaves and debris to be removed from the inlet leaf/litter screen;

• Removing leaves and debris from roof gutters; and

• Sediment and debris to be removed from rainwater tank floor.

Adequate first flush systems and mesh screens on tanks inlets will reduce the amount of sediment and debris entering the tank rendering cleaning only necessary approximately 10 years or so.


6-17

Rainwater Tank




Items Inspected

DEBRIS CLEANOUT

6M

Basin surface clear of debris

Inlet area clear of debris

Overflow pipe clear of debris

FIRST FLUSH DEVICE

2M

Leaves and debris in sump/vessel

INLET SCREEN

6M

Leaves and debris on surface

ROOF GUTTERS 6M

Leaves and debris in gutters

SEDIMENT LEVEL IN TANK

2A

Sediment level

TANK STRUCTURE

2A

Check for corrosion

Check footings

OUTLET PIPE

A

Pipe condition

Evidence of blockage


2A = 2 Years A = Annual M = Monthly 3M = Three Monthly 6M = Six Monthly 3-6M = Three to Six Monthly 1-3M = One to Three Monthly


6-18

6.10 Landscape Developments


For landscape areas the following items should be inspected:

• Signs of plant moisture stress;

• Dead or damaged vegetation;


• Signs of surface erosion and scouring.

Inspections should be undertaken at the frequencies shown in the Maintenance and Inspection Checklist for Landscape Development.

During the first twelve months of the landscaping – the establishment period – inspections should be carried out more frequently (e.g. fortnightly during summer and monthly otherwise) to ensure that watering systems are operating and plants are attended to. Based on the maintenance activities and regimes experienced during the establishment period, the Maintenance and Inspection Checklist may need to be amended.

Maintenance

The following maintenance activities will be required with inspection frequencies shown in the Maintenance and Inspection Checklist:

• Repair/replace any damaged vegetation;

• Reapply or apply mulch layer

• Watering;

• Repair surface erosion and scouring.


6-19

Landscape Development




Items Inspected

PLANT SURVIVAL

3M

Dead plants identified and replaced

Alternative species used if soil moisture unsuitable

IRRIGATION SYSTEM CHECK 3M

Plants show no evidence of moisture stress

Repair/replace any damaged components

Adjust irrigation program if necessary

DRAINAGE PATTERN

3M

Subsurface drainage required to prevent waterlogging

Modification of surface drainage required to direct stormwater to planted areas



7-1

7 Life Cycle Costs for WSUD Measures

7.1 Introduction

This section provides capital and on-going maintenance associated with the specified WSUD measures in this document. The costs provided in this section have been obtained from a number of sources, as follows:

• Collation of indicative construction and maintenance costs from the latest manuals and reports, as referenced in this section;

• Using unit rates of the supply and construction of individual items or activities associated with each WSUD measure, using Rawlinsons - Australian Construction Handbook (2002) for the Sydney area; and

• Where possible, actual construction and maintenance costs from recently completed projects in Sydney and Victoria, as referenced in this section.

All costs provided in this document are based on 2003 unit rates and values.

7.2 Vegetated Swales

The construction cost for vegetated swales would depend on the surface area/width, type of vegetation and the steepness of the area (ie. requiring intermittent check dams). The estimated unit rate construction costs for a nominal 3 m wide swale in the Sydney area (as shown on Drawing DSD1) is summarised in Table 7.1 below:

Table 7.1

Estimated Unit Rate Construction Cost for Vegetated Swale

Works Description Quantity Unit Rate Cost

($/m)

Excavate and profiling swale channel 3.0 m2/m 2.0 6

Supply and place topsoil layer (100 Nom thick) 3.0 m3/m 7.0 21

Supply and apply grass seed, fertiliser and watering 3.0 m2/m 1.0 3

TOTAL

30

Based on the above, the unit cost is approximately $30/m length of swale or approximately $10/m2 of swale. For swales with an underlying subsoil drain (ie. for grades less than 2%) include an additional $30/m for the construction of the subsoil drain, including excavation, perforated pipe, gravel and sand backfill and geofabric surround. If rolled turf is used instead of seed, the estimated unit cost of the swale would increase to approximately $18/m2 (excluding subsoil drain).

7-2

Construction of a swale covering 2700 m2 at Kinfauns Estate, Hasting, Victoria cost $41,750, which equates to approximately $15.50/m2 (Lloyd et al, 2002).

Estimated swale maintenance costs for the Model Farms High School at Baulkham hills are provided in the Table 7.2 (Beecham, 2002). The overall maintenance cost obtained is $3.13/m2 of swale. CRCCH (2003) indicates that maintenance of grass swales for the removal of litter and mowing is about $2.5/m2

(based on figures provided by Vicroads).

Table 7.2

Estimated Swale Maintenance Costs – Model Farms High School

Component Estimated

Unit Cost Swale Size 1 Swale Size 2 Comments

($)

0.5m deep 0.3m bottom

width 3m top width

($)

1m deep 1m bottom

width 7m top width

($)

Mowing 1.62/100m2 264.6 440.1 Mow 2/3 times per year

General Grass Care 16.2/100m2 297 499.5 Grass Maintenance

area is (top width + 3m) x length

Debris/Litter Removal 0.95/m2 170.1 170.1

Reseeding/ Fertilisation

0.65/m2 10.8 18.9 Area revegetated is 1%

of maintenance area per year

Inspection and General Administration

1.35/m2 421 421 Inspection once per

year

TOTAL 3.13/m2 1164 1550

7.3 Vegetated Filter Strips

The construction cost for vegetated filter strips would depend on the surface area and type of vegetation used. The construction cost for a filter strip (as shown on Drawing DSD2) comprising surface preparation (grading, tyning), topsoiling, and seeding (with grasses) would be in the order of $10 to $15/m2 in the Sydney area. The cost would increase to around $20 to $50/m2 if the area was planted with a ground cover of established native grasses and shrubs.

CRCCH (2003) indicates that maintenance of buffer strips for the removal of litter and mowing is about $2.5/m2 (based on figures provided by Vicroads).

7-3

7.4 Sand Filters

The capital cost will vary depending on the scale of the system. The estimated costs for supply and installation will therefore likely range from $5,000 to $50,000 (WBM, 2003).

Maintenance costs will also likely range from $1,000 to $5,000 per year depending on the scale of the device (WBM, 2003). For small filters, this would involve the use of a shovel, a wheelbarrow and a small utility to transport the clean sand and the removal of the contaminated sand to an approved solid waste disposal site. Larger sand filters may require larger equipment (eg. a grader or a front-end loader).

7.5 Bioretention Systems

The construction cost for bioretention systems would depend on the surface area/width, depth, type of surface vegetation and the inlet/outlet structures. The estimated unit rate construction costs for a 3 m wide x 1 m nominal deep, on-line bioretention trench (as shown on Drawing DSD4b) is summarised in Table 7.3 below.

Table 7.3

Estimated Unit Rate Construction Cost for Bioretention Trench

Works Description Quantity Unit Rate Cost ($/m)

Excavate trench (3m x 1.5m) and stockpile 4.8 m3/m 20 96

Supply and install geofabric liner 6.2 m2/m 5 31

Supply and place underdrainage pipe (100 diameter) 1.0 m/m 13 13

Supply and place gravel drainage layer 0.7 m3/m 45 31.5

Supply and place filter media (sand/gravel soil) 3.0 m3/m 55 165

Supply and place graded filter sand layer (150 Nom thick) 0.5 m3/m 45 22.5


Supply established vegetation ground cover including planting, fertiliser and watering

3.0 m2/m 10 30

TOTAL

410

Based on the above, the unit cost 3 m wide x 1 m nominal deep bioretention trench is approximately $410/m by length, or approximately $137/m2 of trench surface area. Costs, however, will tend to differ with the type of surface landscaping, and the sand and gravel type and source location.

Long-term maintenance cost for are bioretention systems largely unknown but likely to be dominated by activities similar to those of swales, ie. $1.5 to $2.5/m2 for landscaped systems (CRCCH, 2003).

7-4

7.6 Permeable Pavements

The construction cost for permeable pavements would depend largely on the type of permeable pavement selected (ie. no fines asphalt/concrete or block) and the depth of the underlying gravel reservoir layer. The supply cost for permeable pavement blocks typically varies from $30 to $50/m2. Installation costs of permeable pavements is typically greater than conventional pavements.

The estimated unit rate construction costs for a typical permeable pavement area (using block pavers with a 400 mm thick sub-base sand/gravel layer, as shown on Drawing DSD5) is summarised in Table 7.4.

Table 7.4

Estimated Unit Rate Construction Cost of Permeable Block Pavement

Works Description Quantity Unit Rate) Cost ($/m2)

Excavate and profiling subgrade surface 1.0 m2 2 2

Supply permeable pavement blocks 1.0 m2 40 40

Install pavement blocks 1.0 m2 25 25

Supply and install geofabric liners 2.0 m2 5 10

Supply and place gravel reservoir layer (350 thick) 0.35 m3/m2 55 19.2

Supply and place bedding sand layer (50 thick) 0.05 m3/m2 45 2.2

TOTAL

98.4

Based on the above, the unit cost for permeable block pavements is approx. $100/m2 of pavement surface.

Maintenance costs are generally higher than that of traditional pavements due to the extra cleaning costs required to ensure pavements do not become clogged with sediment. Accurate maintenance cost estimates were unavailable at the time of writing.

7.7 Infiltration Trenches

The construction cost for an infiltration trench would depend largely on the surface area/width and depth of the trench. The estimated unit rate construction costs for a typical 1 m wide x 1 m nominal infiltration trench in the Sydney area (as shown on Drawing DSD6) is summarised in Table 7.5 below.

7-5

Table 7.5

Estimated Unit Rate Construction Cost of Infiltration Trench

Works Description Quantity Unit Rate Cost ($/m)

Excavate trench (1m x 1.25m) and stockpile 1.25 m3/m 20 25

Supply and install geofabric liner 4.0 m2/m 5 20

Supply and place perforated pipe (100 dia) 1.0 m/m 13 13

Supply and place gravel storage layer 1.0 m3/m 65 65

Supply and place filter layer (100 Nom thick) 0.15 m3/m 45 7


Supply and apply grass seed, fertiliser and watering 1.0 m2/m 1.0 1

TOTAL

138

Based on the above, the unit cost is approximately $140/m of infiltration trench (or $130/m2 of trench surface).

Maintenance costs will differ depending on the scale of the device. Maintenance costs for infiltration trenches are unknown at present.

7.8 Infiltration Basins

The construction cost for an infiltration basin would depend largely on the excavation volume required to construct the basin (i.e. surface area/width and depth). Excavation would typically be less in natural depressions or gullies on sites. Excavation costs would also depend on subsurface ground conditions, with rates varying from $20/m3 in light soils, to over $50/m3 in soft rock that can be easily ripped.

Maintenance costs will differ depending on the scale of the basin. Maintenance labour/plant requirements will range from hand and shovels for small basins, to front-end loaders and trucks for larger basins. Maintenance costs for infiltration basins are unknown at present.

7.9 Rainwater Tanks

The cost for the supply of rainwater tanks will largely depend on the tank’s fabrication material. Supply costs for a range of tank types are summarised in Table 7.6 below (Coombes, 2002b, Practice Note 4).

7-6

Table 7.6

Typical Rainwater Tank Supply Costs (2002)

Supply Cost for various Tank Size ($) Tank Type

4.5 kL 9 kL

AquaplateTM 540 860

Galvanised Iron 440 640

Polymer 670 1150

Concrete 1,300 1,800

The cost of installing a rainwater tank can vary considerably depending on-site constraints. Installation, fitting and plumbing costs for a typical 5 to 10 kL size tank are also summarised in Table 7.7 below.

Table 7.7

Typical Rainwater Tank Installation and Fittings Costs (2002)

Tank Item Cost ($)

Pump and Pressure Controller 350

Tank stand/base 300

Fittings including float system 500

Installation 450

TOTAL

$1600

The additional costs for installation of a below ground rainwater tank would be around $2,000 (Coombes, 2002b, Practice Note 4).

The on-going operating costs for the pump motor would be approximately $150/year (based on $13.53 cents/kWh for a 0.75 kW pump for an average 4 hr operating day).

A conservative estimate of annual maintenance costs incurred for a rainwater tank is about $70/year.

SECTION 8 References

8-1

8 References

(ARC, 2003), Stormwater Treatment Devices: Design Guidelines Manual, Technical Publication 10. Auckland Regional Council, July 2003.

(ARQ, 2003) Australian Runoff Quality, Proceedings of Launch of Guidelines, June 2003, The Institute of Engineers Australia, National Committee on Water Engineering.

(BCC, 2001) Stormwater Quality Control Policy - Background Information and Guidelines for

Application, Blacktown City Council.

(Bannerman and Hazelton, 1990) Soil landscapes of the Penrith 1:100,000 Sheet. Soil Conservation Service of NSW, Sydney.

(Beecham, 2002) Development of Water Sensitive Urban Design (WSUD) Concepts for Model Farms High School, prepared by Dr Simon Beecham.

(Camp, Dresser and McKee, 1993), California Storm Best Water Management Practice Handbooks: Municipal, prepared for California Stormwater Quality Task Force.

(Coombes, 2002a), Practice Note No.5 – Infiltration Devices, LHCCREMS, NSW.

(Coombes, 2002b), Practice Note No.4 – Rainwater Tanks, LHCCREMS, NSW.

(Coombes, Donovan and Cameron, 1999) Water Sensitive Urban Development: Implementation Issues

for the Lower Hunter and Central Coasts, Lake Macquarie City Council, Speers Point, on behalf of the Lower Hunter and Central Coast Environmental Management Strategy.

(CRCCH, 1999) Managing Urban Stormwater Using Constructed Wetlands, Cooperative Research Centre for Catchment Hydrology, Melbourne.

(CRCCH, 2000) Water Sensitive Road Design – Design Options for Improving Stormwater Quality of

Road Runoff. Technical Report 00/1, Cooperative Research Centre for Catchment Hydrology ,August, 2000.

(CRCCH, 2003) Stormwater Flow and Quality and The Effectiveness of Non-Proprietary Stormwater

Treatment Measures (Draft), prepared by the Cooperative Research Centre for Catchment Hydrology, Melbourne for the NSW EPA.

(DLWC, 1998) The Constructed Wetlands Manual. Volumes 1 and 2, Department of Land and Water Conservation, New South Wales.

(DoH, 1998), Managing Urban Stormwater – Soils and Construction, NSW Department of Housing, NSW August 1998.

(EPA, 1997a), Managing Urban Stormwater: Treatment Techniques, Draft, Environment Protection Authority, NSW, November 1997.

SECTION 8 References

8-2

(EPA, 1997b) Managing Urban Stormwater: Council Handbook – Draft, Environment Protection Authority.

(EPA, 2000) Example Stormwater Management Plan – Draft, Environment Protection Authority.

(EPA, 2002c) Environmental Management Plan for Landscaping Works, NSW EPA and The Stormwater Trust, August 2002.

(EPA, 2002d) Landscape Industry Fact Sheets: environmental information. Sheet 7 – Using Water Wisely. Sheet 8 – Controlling Erosion and Sediment. Home Landscaping.

(Horner, Skupien, Livingston and Shaver, 1994), Fundamentals of Urban Runoff Management, Terrene Institute, Washington, DC.

(KNOX, 2003) Water Sensitive Urban Design Guidelines for City of Knox, prepared for Knox City Council by MDG Landscape Architects and KLM Development Consultants.

(MDE, 2000), Maryland Stormwater Design Manual, Volumes 1 and 2, Maryland Department of the Environment and the Center Watershed Protection, Maryland, 2000.

(RTA, 2003), Procedure for Selecting Treatment Strategies for Road Runoff. Road and Traffic Authority

(S Lloyd, T Wong, and C.Chesterfield, 2002), Water Sensitive Urban Design – A stormwater

Management Perspective, prepared for Cooperative Research Centre for Catchment Hydrology and Melbourne water Corporation.

(SCS, 1990), Soil Landscapes of the Penrith 1:100 000Sheet. Bannerman, S.M. and Hazelton, P.A. Soil Conservation Service of NSW.

(UPRCT, 1999) On-site Detention Handbook, Upper Parramatta River Catchment Trust.

(UPRCT, 2002), Upper Parramatta River Stormwater Management Plan, Upper Parramatta River Catchment Trust.

(Victorian Stormwater Committee, 1999), Urban Stormwater – Best Practice Environmental Management Guidelines, Victorian Stormwater Committee, CSIRO Publishing, Victoria.

(Walsh, 1993) Water-Saving Gardening in Australia, Kevin Walsh, Reed Books Australia

(WBM, 2003), Stormwater Treatment Framework and Stormwater Quality Improvement Device

Guidelines – Exhibition Draft – Version 3, WBM Oceanics Australia.

(WSROC, 2002), Final Draft Salinity Code of Practice, Western Sydney Regional Organisation of Councils, July 2002.

Australia; Water Sensitive Urban Design Technical Guidelines for Western Sydney

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