WATER SENSITIVE Strataflow

TM

WATER SENSITIVE URBAN DESIGN GUIDE FOR

Strataflow™

ADVANCED TREEPIT SYSTEMS

1 IntroductionThe Strataflow WSUD Design Guide is a technical guideline developed to assist designers and specifiers integrate Citygreen’s structural modules into WSUD tree pit and raingarden projects. This document includes a suite of design applications and standard drawings, as well as a discussion of design considerations that are critical to successful implementation of WSUD tree pit projects.

This guideline should be read in conjunction with Citygreen’s Specifier Reference Manual, Stratacell and Stratavault and Stratavault Standard Drawings and Specifications.

1.1 Document OutlineThe design guide is organised into the following sections.

Introduction This section provides a brief background to WSUD principles and context with respect to street trees. It also describes the typical benefits and stormwater management issues involved in implementing street trees.

Integrating WSUD and Stratacell This section explains the application of Stratacells as WSUD elements, providing a general description of the cells, benefits to WSUD and an overview of considerations with respect to tree pits.

Design Methodology The design method is described in step by step detail in this section. A design procedure followed by potential Strataflow configurations relevant to different project objectives is presented. Then a detailed look at vital technical information covering system sizing, hydraulics, and tree and soil specification.

Water Quality Modelling and Toolkit

Provides a brief guide to setting up a MUSIC model and inputting appropriate parameters to enable water quality assessment of the WSUD Stratacell.

Water Balance Toolkit An overview of the water balance and step by step procedure is provided in this section.

Appendices Contains Decision Tree, Engineering Drawings, Raingarden Material Specification and Water Balance Toolkit User Guide

1.2 WSUD principlesWater Sensitive Urban Design (WSUD) is a design approach that considers and attempts to mitigate the impacts of urban stormwater runoff in a manner which is sympathetic to the receiving environments where water flows. In this sense, key WSUD outcomes are to improve water quality and to detain and retain flows so that the water reaching a watercourse downstream is treated and has been attenuated such that the receiving water ecosystems will not suffer.

Other outcomes that WSUD embraces include

• Utilisation of stormwater as resource,

• Reduction in potable water use,

• Healthy and resilient green spaces,

• Protection of groundwater,

• Improved air quality

• Improved urban amenity,

• Improved micro-climate and shading

These outcomes will be achieved through implementation of WSUD. This document will assist designers to utilise Strataflow in the design solution.

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1.3 WSUD in context of Street TreesStreet trees provide a multiplicity of benefits and are precious natural assets in urban spaces where green spaces are limited.

The common scenario for street trees is that they are planted in confined spaces within the urban streetscape and provided limited resources needed to remain healthy and thrive. The soils around street trees are often compacted during the construction of surrounding paved surfaces.

Street trees need access to water, adequate soil volume and a good soil mixture.

To provide the necessary watering requirements for a healthy street tree, active watering or irrigation is usually required where the tree can’t access adequate volumes of water from its surroundings.

Enhanced design, ensuring street trees are configured for passive watering via stormwater runoff provides many benefits:

• Reduced requirements for active watering, ensuring tree longevity and resilience subject to drought periods and water restrictions;

• Mitigates many of the impacts of urban stormwater runoff. Research has shown that WSUD street trees can substantially reduce nitrogen and other pollutant loads in stormwater; and

• Reduced stormwater runoff volumes.

WSUD street trees can be configured as stand alone bioretention street trees or planted within larger raingardens and function as biofilters for stormwater.

A WSUD street tree incorporating an adequate soil volume is ultimately a healthier and more resilient tree, with greater access to soil, nutrients, and water from the surrounding environment.

The support provided by Stratacells can therefore play a crucial role in maintaining a sustainable urban forest. To obtain an adequate soil volume in a confined streetscape, Citygreen’s Strataflow systemcan be used under pavements as a structural support whilst creating a sub-surface space for uncompacted soils required for trees root expansion. An adequate soil volume within a Strataflow matrix can provide tree roots the space they need to grow to full size.

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1.4 Stormwater Treatment (Bioretention) in WSUD Street TreesWSUD street trees are a combination of a bioretention (biofiltration) system and a street tree.

Bioretention systems are shallow depressed vegetated filter beds which are designed to collect and treat stormwater runoff. The Figure 1 depicts a typical bioretention profile

Figure 1 – Typical Bioretention Profile

Stormwater runoff intercepted by a bioretention system is treated via a number of natural processes including sedimentation, physical filtration by soil and plants, chemical adsorption, biological processing by plants and bacteria within the soil. Depending on the configuration, treated water may drain into the surrounding soils or via perforated pipes that connect into the downstream drainage system.

As well as improving the quality of stormwater, bioretention systems can improve urban hydrology by storing and detaining water, and reducing the volume of runoff through evapotranspiration and in some cases infiltration into the surrounding soil.

In a Strataflow tree pit, filter media is used as a replacement for typical growing media at the pavement opening, and stormwater runoff from surrounding hardstand areas is directed towards the tree where the stormwater is cleaned as it drains through the bioretention media. Tree pit openings are usually small systems, accommodating a single tree and are usually a few square metres in size.


In a raingarden configuration, trees can be planted amongst shrubs and grasses within a filter bed. These are larger garden beds usually constructed in less constrained spaces where there is sufficient space for more landscaping.

Examples of these concepts are shown in Figure 2 which provides a preliminary look on how this can translate to the Strataflow system showing typical details.

a) Strataflow Tree Pit b) Raingarden and Strataflow Treepit

Figure 2 – Typical Strataflow Bioretention ProfileA detailed description of the Stratacell and Stratavault is provided in the next section.


2 Integrating WSUD and citygreen modular cells2.1 Stratacell™ and Stratavault™ ModulesCitygreen’s Stratacell and Stratavault modules are extremely high strength modules for support of pavements and traffic loads, and provides large volumes of uncompacted soil for healthy tree root systems and water harvesting. Figure 2 illustrates the configuration of a Stratavault treepit.


Figure 3 – Structural Soil Cell Configuration Refer to Citygreen’s Technical Sheets for more detailed information.

2.2 WSUD Strataflow Tree PitA WSUD Strataflow tree pit is a configuration that incorporates the following elements to provide the necessary resources for healthy tree development:

• Citygreen’s Stratacell and Stratavault modules provide a suitable uncompacted soil volume,

• stormwater conveyance measures to facilitate passive watering,

• a storage component for detention or retention of stormwater, and

• a biofiltration soil medium (in some applications – shown in Figure 2) to provide a stormwater filtration function.

Stratacell™ Stratavault™


Step 1 Preliminary Analysis/Feasibility

• Preliminary Layout

• Identify Catchments

• Initial modeling accordint to objectives

• Water Quality/Detention/Infiltration Tree health

• Services and constraings assessment

• Assess site conditions

Step 3 Prepare Functional Layout Plans

• Develop a functional layout based on the selected standard configuration

• The results of the preliminary analysis should inform the functional design

• Create a project specific functional design drawing using WSUD standard drawings and specifications

Step 2 Determine Configuration

• Refer to:

1. List of Strataflow WSUD configurations (section 3.2)

2. Decision tree (Appendix A)

3. Standard drawings (Appendix B), to determine the most ideal configurations for the site

Step 4Civil and Landscape Design

• Design appropriate edge treatments and safety measures

• Design stormwater inlet and outlet arrangement

• Design pre-treatment and necessary filtration elements

• Final selection of appropriate tree and plant species

• Specification of appropriate soil / media fill

* The objectives will determine the sizing method and will be influenced by the configuration adopted. A number of Tools are provided to size for detention, infiltration and plant health. The water balance will also aid with tree selection. MUSIC is typically used to size for water quality.

3 Design GuideWSUD Strataflow tree pit systems can be configured in a number of ways depending on specific site opportunities and constraints. The fundamental elements of a WSUD tree pit system include the following;

• Sufficient structural soil volume to facilitate tree growth and root expansion

• Available impervious catchment for passive watering

• Inlet for stormwater supply to tree

• Suitable tree species

• Treatment element for management of solids and pollutants in stormwater

• Drainage layer and stormwater outlet connection (as required)

A design procedure has been prepared to assist specifiers and designers with the integration of WSUD features into Strataflow Tree pit systems. The procedure is presented as a flowchart which describes the general steps involved in the design process.

3.1 Design Procedure for Strataflow™ systems


WSUD Strataflow Tree Pit• Street tree pit modified to incorporate a bioretention function. This system provides stormwater treatment as

well as passive watering for the tree.

StrataflowRaingarden• A vegetated bioretention system or ‘raingarden’ incorporating a tree in addition to smaller shrubs and

groundcovers.• This system provides a greater level of stormwater treatment due to it’s footprint, and provides passive

watering for the tree and vegetation within the garden bed.

StrataflowTree Pit with Permeable Pavement Surround• Street tree pit incorporating a permeable pavement surround.• Permeable pavement encourages infiltration of stormwater and can facilitate passive watering for the tree.

StrataflowTree Pit configured for passive watering• Street tree pit incorporating an inlet for stormwater and detention storage.• This system provides passive watering for the tree and may enable capture of stormwater pollution.

StrataflowTank for Detention or Retention• Underground modular and trafficable storage system that can be configured for detention, retention and/or

infiltration of stormwater.• Tanks can be linked to tree pit systems.

3.2 Strataflow ConfigurationsA number of standard configurations have been detailed to aid in implantation of the design solution. These are described below. A decision tree, provided in Appendix A, assists the designer to choose the right configuration for the appropriate application (dependent on project objectives). The drawings located in Appendix B illustrate the standard configurations.

3.3 Design ConsiderationsThe following design considerations should be accounted for prior to undertaking the functional design of a WSUD Tree Pit system.

3.3.2 Siting the System

A typical street tree is usually confined by impervious pavements and relies on a catchment of a few square metres for passive watering. In this typical scenario a street tree will seldom receive a sufficient supply of water from stormwater runoff and would be reliant on active watering to facilitate growth and maintain tree health.

Understanding the characteristics of the stormwater catchment draining to a street tree or raingarden is important for determining the feasibility and opportunities for WSUD. During the initial feasibility assessment, catchments should be assessed and the extents of directly connected impervious area draining to the tree system delineated.

An understanding of stormwater catchments is critical for determining the following;

• Feasibility of passively watered tree systems;

• The amount of water that is generated by the site and that can be delivered to the tree for passive watering and stormwater treatment;

• The optimal tree spacing, and

• The pollution retention potential of WSUD systems.


3.3.2 System Sizing

WSUD devices should ideally be sized to optimise treatment efficiency and to achieve a set of stormwater management objectives which may vary across different regions.

Sizing of WSUD systems should be undertaken during the concept design phase. In some cases performance curves, and deem to comply tools can be used to approximate the size of WSUD systems to achieve stormwater management objectives.

For most projects a water quality model should be developed for the project using eWater’s MUSIC software. MUSIC can assist with optimising the size of WSUD elements and to determine its efficiency in meeting stormwater management objectives for the project. Refer to Section 4.0 for more detail on modelling WSUD systems in MUSIC.

The additional tools available to be used in conjunction with MUSIC are:

• Strataflow Infiltration Sizing Calculator – to be used to help determine the filter area to enter into MUSIC based on the soil selection and the desired storm event to be treated

• Stormwater Quantity Calculator – can be used to understand the total number of Stratacells required to treat the desired storm event particularly if detention is also a project objective.

Figure 4 depicts a graphical representation of these tools with key inputs being catchment area, runoff depth or storm intensity and soil properties.

3.3.3 Existing Services

Asset location and/or service proving should be undertaken to identify any services that might constrain the site and impact on the feasibility of the proposed tree pit system.

As a minimum a Dial-Before-You-Dig (DBYD) enquiry should be made and service information requested. A services plan should be developed to map out service conduits in the vicinity of the proposed works.

Some services can be accommodated with ease within a Strataflow tree pit, others may require costly protection works to accommodate within a tree pit system. Where services may impact on a tree pit design a service proving contractor should be engaged to determine the alignment and depth to conduit, and the asset owner should be contacted to ensure that their requirements are known prior to finalising the design.

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3.3.4 Soil Media Fill

Selecting the appropriate soil media is critical for ensuring a healthy tree, and with respect to integrated stormwater applications that stormwater treatment objectives can be met.

Typically a growing media with a suitable organic content is specified for standard street tree installations, and they are formulated to sustain trees both nutritionally and physically (provide adequate nutrients and water holding capacity).

For WSUD tree systems the key objective is to ensure that the soil medium can sustain a healthy tree as well as provide a suitable well-draining medium for stormwater filtration and pollutant retention.

For a WSUD tree pit system incorporating bioretention filter media, the soil profile will usually consist of three layers.

1. Filter media (minimum 750mm deep or as specified in the engineering design)

2. Transition Layer (100mm deep coarse sand)

3. Drainage Layer (150mm deep fine washed gravel)

The soil media fill specified for a WSUD tree system will vary depending on the project objectives and the site context. Designers should consider the application, context, and project objectives prior to selecting a suitable soil media specification. The figure below provides a basic decision tree to assist designers in selecting the appropriate soil media a fill.

3.3.5 Tree and Plant Selection

Ideal tree species to be planted within a WSUD or Bioretention Tree Pit are those which have the following traits;

• Effective for removal for nutrients in stormwater

• Can grow and develop in freely draining filter media

• Can tolerate variable periods of inundation.

• Ideally non-deciduous

• Preferably native and occur naturally in the local area

• Where trees are planted in a raingarden they need to have relatively sparse canopies to allow light penetration to support dense groundcover vegetations.

A basic list of recommended tree species for WSUD tree pit systems has been compiled as shown in Table 1. This is by no means an exhaustive list of tree species suited to WSUD tree systems. The tree species listed display some or all of the desirable traits for tree species planted within bioretention systems. The tree species listed occur in various regions across Eastern Australia and should be specified if they are suited to the local environment and site conditions. Ideally a qualified professional should tailor a site specific planting plan incorporating appropriate tree species.


Determine Primary Project Objectives

Horticultural and Water Quality

Objectives

Select mix of fills (growing media and filter media)

Filter media to be specified around the root ball and

where necessary to intercept stormwater inflows. A low fines growing media to be

specified around the extents of the Strataflow pit.

Refer to WSUD Strataflow Tree Pit

standard drawings and soil media specifications

Horticultural Objectives

Select growing media fill

Similar to typical street tree configuration.

Suitable for passive watering applications only.

Refer to Citygreen’s standard Growing

Media Specifications


Table 1 – Recommended Tree Species for WSUD Tree Pit Systems

Species Name Common Name Type

Banksia Robur Swamp Banksia Small Tree

Banksia Species - Shrub to Tree

Callistemon Salignus Willow Bottlebrush Small Tree

Callistemon Viminalis Red Bottlebrush Small Tree

Kunzea Ericoides Burgan Small Tree

Leptospermum Continentale Prickly Tea-tree Small Tree

Leptospermum Lanigerum Woolly Tea-tree Small Tree

Melaleuca Ericifolia Swamp Paperbark Small Tree

Melaleuca Linariifolia Flax-leaved Paperbark Small Tree

Melaleuca Nodosa Prickly-leafed Paperbark Small Tree

Melaleuca Sieberi Small-leaved Paperbark Small Tree

Melaleuca Squarrosa Scented Paperbark Small Tree

Melaleuca Viridiflora Broad leaved Tea-tree Small Tree

Elaeocarpus Obovatus Hard Quandong Tree

Brachychiton Acerifolius Illawarra Flame Tree Tree

Callistemon Sieberi River Bottlebrush Tree

Casaurina Glauca Swamp Oak Tree

Casuarina Cunninghamiana River Sheok Tree

Eucalyptus Camaldulensis River Red Gum Tree

Eucalyptus Ovata Swamp Gum Tree

Laphostemon Confertus Brush Box Tree

Lophostemon Confertus Brush Box Tree

Lophostemon Suaveolens Swamp Mahogany Tree

Maclura Pomifera ‘Wichita’ Wichita Osage Orange Tree

Melaleuca Bracteata Black Tea-tree Tree

Melaleuca Quinquenervia Broad-leaved Paperbark Tree

Pyrus Calleryana Callery Pear Tree

Further guidance and detailed plant and tree species lists for WSUD systems are provided in the publications listed in the Table 2. Guidance on the selection of suitable plant and tree species for bioretention systems can also be in the FAWB Adoption Guidelines1.

Region Publication

Victoria Appendix A, WSUD Engineering Procedures: Stormwater1

South East Queensland, QLD Section 3.6, Bioretention Technical Design Guidelines2

Western Sydney, NSW Section 5, UPRCT WSUD Technical Guidelines for Western Sydney3

1. FAWB, 2009. Adoption Guidelines for Stormwater Biofiltration Systems, Facility for Advancing Biofiltration, Monash University, June 2009.

2. Pilgrim, DH (ed), 1998. Australian Rainfall and Runoff – A Guide to Flood Estimation, Institute of Engineers, Australia, ACT.


3. Melbourne Water, 2005. WSUD Engineering Procedures: Stormwater, CSIRO Publishing, VIC

4. Water by Design, 2012. Bioretention Technical Design Guidelines. Healthy Waterways Brisbane, QLD.


3.3.6 Tree Health Considerations

Following tree selection, and subject to preliminary design including soil media selection, the hydrology of the system can be assessed. The aim of this stage of the design is to determine if the selected tree can maintain healthy growth within the constraints of the design. The water balance toolkit, outlined in Section 4.2, examines the water consumption of the tree against the stormwater inflow and field capacity of selected soils to determine the likely health of the tree. This looks at the wilting threshold of the soil to indicate whether the tree will require additional water, which may be in the form of irrigation (Section 3.3.12) or re-design of the concept to produce more favourable conditions. Refer to the Water Balance User Guide (Appendix D) to appreciate some of the options available to improve tree conditions.

3.3.7 Hydraulic Design Considerations

Following catchment delineation, a suitably qualified designer should confirm that stormwater runoff can be delivered to the proposed WSUD Tree Pit system under gravity (i.e. via an overland kerb opening, pipe connection, or a surcharge system, etc…).

The designer should also confirm that stormwater flows can discharge from the system under gravity. If surrounding soils are suitable exfiltration can be relied upon to drain the system otherwise underdrainage is required. A sufficient profile depth should be allowed for if underdrainage is required. Refer to Underdrainage Requirements in this section, for more information.

Stormwater inlets, outlets, overflow weirs, etc. should be designed by a suitably qualified professional and in accordance with appropriate engineering practice and industry design guidelines.

WSUD systems are usually designed to convey and treat stormwater runoff from an urban catchment based on a ‘treatable flow rate’ which is typically set as the 3 month and 1 year ARI peak flow. Design flows through systems can be calculated using the rational method or an appropriate hydrological computer model.

The Urban Rational Method is discussed in detail in the Australian Rainfall and Runoff (AR&R) Book VIII, Section 1.5.5.

A detailed procedure of hydraulic design calculations for WSUD systems are provided in sections 5 and 6 of WSUD Engineering Procedures: Stormwater and sections 3.4 and 3.5 of Bioretention Technical Design Guidelines

3.3.8 Underdrainage requirements

Frequently inundating a tree pit with stormwater creates the need for suitable underdrainage to prevent a tree pit soils from becoming waterlogged.

Depending on the configuration, treated water may drain into the surrounding soils or via perforated pipes that connect into the downstream drainage system.

Figure 5 has been adapted from the Bioretention Technical Design Guidelines (Water by Design, 2012) and provides a basic decision tree to assist designers in selecting the appropriate drainage profile within a WSUD tree pit system.

Drainage profile options include the following;

• Submerged Zone - Partially lined system with saturated drainage layer including slotted pipe underdrainage

• Fully Lined system - with slotted pipe underdrainage

• Unlined system with slotted pipe underdrainage

• Unlined system without underdrainage

Detailed guidelines for designing the drainage profile for WSUD systems is provided in Section 3.2 of Bioretention Technical Design Guidelines


Saturated Zone required for plant health

and/or stormwater treatment

Submerged Zone drainage profile

Fully lined system with slotted pipe underdrainage

Unlined system with sloteed pipe

Unlined system without slotted pipe

underdrainage

Treated Water will be harvested

Dispersive, sodic, or acid sulphate soils present

Elevated groundwater table present

Restrictions to infiltration of flow present (i.e. infrastructure damage)

and building foudations within 3m

Surrounding soil hydraulic conductivity <0.25mm/hr

Surrounding soil hydraulic conductivity

< twice that of the filter media

NO

YES

YES

YES

YES

YES

YES

YES

NO

NO

NO

NO

NO

NO

Figure 5 – Drainage Profile Decision Tree for WSUD Tree Pit

3.3.9 Liners and Geotextiles

Impermeable and permeable liners are often required in WSUD tree pit systems. Impermeable liners may be required to partially or fully line the base and sides of WSUD tree pit system if water cannot be exchanged between the tree pit and the surrounding soil profile.

Non-woven geotextile liners should be applied to the sides of a WSUD tree pit system where the surrounding soils can migrate into more porous fill layers like the filter, transition, or drainage layers.

Reinforced filter fabrics are heavy grade non-woven geotextile liners which incorporate a reinforced mesh. The liners are applied to the top and sides of a tree pit system to spread vertical and lateral loads as well as prevent the migration of fines into the tree pit soil fill profile.

A detailed discussion regarding the application of liners in WSUD systems is provided in Section 3.2.4 of Bioretention Technical Design Guidelines6



Figure 6 – Example Pre-treatment for Street Tree

3.3.10 Trafficable Pavements

In applications where Strataflow Tree pit systems are installed beneath trafficable pavements, a geotechnical analysis of the site subgrade should be undertaken to determine the appropriate specification and composition of the tree pit drainage layer and substructure.

The tree pit’s foundation is the angular crushed rock material placed between the subgrade soils and the Strataflow base.

The site sub-grade bearing capacity and cover depth above the Strataflow system will determine the depth of crushed angular rock required for the Strataflow Tree Pit foundation.

A geotechnical engineer may be required to review the installation of a Strataflow Tree Pit system where weak subgrade material is present.

3.3.11 Management of Sediment and Gross Pollutants

Urban stormwater runoff typically carries with it pollutants ranging from large debris and rubbish through to fine particulate matter and dissolved chemicals. Stormwater runoff should be ‘pre-treated’ as required prior to discharge into the tree pit system and distribution into the Strataflow matrix.

Stormwater pollutants that settle out and accumulate within a Strataflow Matrix can clog the system over time and may be difficult to remove in some cases without significant intervention.

Fine particulate matter should be isolated up front and filtered out prior to distribution throughout the system. By capturing some of the pollution prior to the entry into the system, the service life of the system can be extended and the ongoing maintenance burden can be reduced.

Pre-treatment systems vary greatly in terms of cost, complexity, and size and should be designed with consideration to the project site and context. An appropriate pre-treatment device is one that excludes fine particulate matter from the Strataflow system.

A few pre-treatment systems which are considered appropriate for the typical urban context include the following;

• Sediment forebay and bioretention filter media;

• Permeable pavement;

• An inlet pit incorporating a filter screen/pit insert filter and a sump (example shown in Figure 6);

• A Gross Pollutant Trap (GPT)

3.3.12 Irrigation

In most situations the supply of stormwater runoff for passive watering may need to be supplemented by active watering (irrigation). Citygreen’s RootRain Arborvent is a dual inlet for deep watering and aeration and can be used to provide an active watering conduit to the confined root zone within a tree pit system.

The active watering manifold should be installed higher in the profile than the drainage layer to prevent short circuiting of irrigation water via slotted pipe underdrainage to the stormwater system. A non-woven geotextile sock can be fitted to the active watering manifold to prevent the ingress of fines from the surrounding soil media.


3.3.12 Inspection and Maintenance Considerations

Fundamental to the design of any WSUD system is consideration to inspection and maintenance. Asset owners are very interested in the provision of easy access, safety and minimal maintenance requirements. Additionally, the longevity, resilience and effectiveness of the system is largely dependent on appropriate maintenance, which therefore requires careful consideration in design. The following may be used as a guide.

An Inspection and Maintenance plan should be undertaken after design of Strataflow tree pit sets which as a minimum would identify the requirements and frequency of maintenance. Maintenance staff should be periodically inspecting the system for: damage (cracking, subsidence), blockage and erosion.

Issue Description Guidance

Access Easy access to inspect the system is fun-damental to ensuring maintenance can be carried out efficiently.

Consider lifting weights of the grate

Consider lifting procedure (specialised open-ings requiring specific equipment to prevent public access)

Blockage Inlets and outlets must remain free of sedi-ment, gross pollutants and debris to ensure that stormwater is not hindered in entering and exiting the tree pit – maximising perfor-mance.

The system is designed with an inspection port (vertical pipe) connected to the underd-rain system. The inspection pipe comprises a screw-cap above the surface of the tree pit which may be opened to check the underd-rain and use a water jet or pipe rod to flush if necessary.

Damage Damage to tree pit infrastructure can cause stormwater to bypass the tree pit affecting treatment performance. Tree pits located ad-jacent to roads and car parks are particularly susceptible to damage from traffic.

Consider traffic management options such as bollards.

Maximise structural integrity of tree pit infra-structure. Include

Erosion Erosion of mulch or the filter media may occur, particularly during large events, if not designed appropriately

Inlet design should consider minimising ve-locity and sharp hydraulic drops. Rockwork may be required.

Tree, Vegetation and Mulch

The tree will inevitably require maintenance in the form of pruning and weeding. Design can nonetheless minimise the burden of maintenance in some aspects

Tree supports are often required during the establishment phase to stabilise plants.

Consider using mulch that isn’t displaced easily

Health and Safety All maintenance activities must be under-taken in such a way to minimise the occupa-tional health and safety risk to maintenance personnel and general health and safety of the public.

Consider siting

Consider lifting weights


A more detailed step by step procedure is outlined in Appendix C.

4 Modelling WSUD Tree Pits

4.1 Water Quality Modelling

4.1.1 MUSIC by eWater

MUSIC (the Model for Urban Stormwater Improvement Conceptualisation) is a modelling tool used to provide water quality analysis of proposed stormwater treatment systems. This model was initially developed by the Cooperative Research Centre for Catchment Hydrology (CRCCH) and is a standard industry model for this purpose. MUSIC is suitable for simulating catchment areas of up to 100 km2 and utilises a continuous simulation approach to model water quality.

By simulating the performance of a range of stormwater treatment measures, MUSIC can be used to determine the ideal treatment solution and optimise the treatment size to achieve stormwater management objectives. The primary water quality constituents modelled in MUSIC include Total Suspended Solids (TSS), Total Phosphorus (TP) and Total Nitrogen (TN) and Gross Pollution (GP).

4.1.2 Modelling Stratacell/Stratavault in MUSIC

MUSIC can be used to model the storage benefits of an empty Strataflow Tank System and/or the water quality benefits of WSUD Strataflow tree pit with a soil media fill.

A procedure has been developed to assist in sizing and specifying a tree pit or tank system to meet water quality or water quantity objectives, within MUSIC.

The aim of the following procedure is to provide a quick guide of how to setup a MUSIC model and input appropriate parameters to ensure the model produces the most accurate and representative results.

An overview of the modelling procedure is provided in the chart below.

Step 1 - Model Set Up• Set up the meterological template for the project site

• Create a catchment node

• Create a bioretention node to simulate a WSUD Strataflow tree pit, OR

• Create a Tank Node to simulate a StrataflowTank Configuration, OR

• Create an Infiltration Node to simulate a leaky tank or infiltration system

• Identify Catchments

• Initial modeling accordint to objectives

• Water Quality/Detention/Infiltration Tree health

• Services and constraings assessment

• Assess site conditions

Step 2 - Catchment Node• Edit the catchment parameters to specify the

catchment area and % imperviousness

• Edit the pollution generation parameters within the catchment node to match the charcteristics and landuse of the modelled catchment. If the site specific pollutant generation parameters are not available, leave the default values.

Step 3 - Treatment Node• Edit the tree-pit (bioretention) node to specify its area,

ponding depth, filter depth, infiltration rate, etc... OR

• Edit the Tank node to specify it’s area, tank height, outlet size, etc... OR

• Edit the Infiltration node to specify it’s area, storage depth, infiltration rate, etc...

Step 5 - Model Optimisation• Adjust the design/tree pit size until stormwater targets are met (or best possible outcome is achieved).

Step 4 - Run Model and Output Results• Run the model

• Generate a model output to determine if the design meets stormwater targets


4.2 Water Balance Tool

4.2.1 Model Objectives

The overall objective of the water balance is to test the performance of the designed Strataflow system with respect to the likely water requirements and behaviour of the proposed tree. Specifically, the water balance aims to:

• Determine the water storage behaviour of the tree pit

• Assess the water availability of the designed configuration, comparing a WSUD tree with a traditional tree pit

• Predict the health of the tree using a wilting point as the threshold at which the tree is likely to fail

• Evaluate the sensitivity of the proposed configuration to varied parameters (eg size of diverted catchment, size of the tree, high water consumer vs low water consumer, soil properties, irrigation input)

• Inform design. For example, will a larger tree spacing be required, such that more stormwater is diverted into the tree pit to enable survival? Will a smaller tree be more suitable? Will a native tree be more suitable?

4.2.2 Performance Indicators

If there is not enough water, the tree will suffer and wilt. As such the system should be assessed based on whether the defined wilting point threshold is reached. If the soil moisture (represented as a percentage of field capacity) drops below this threshold. Parameters should be changed under the ‘optimisation’ phase of the modelling (refer Appendix D), to bring the soil moisture above this threshold.

On the flip side, if there is too much water, the tree can become water logged and suffer.

4.2.3 Model Variables

The interface, as shown on Figure 6, depicts the variables that can be input and adjusted to simulate site specific conditions. The subsequent chart summarises the general procedure to adjust these inputs and undertake the modelling.

A detailed step by step guide is provided in Appendix D.

Figure 7 – Tree Water Balance User Interface


Step 1 - Site DescriptionThis step characterises the site specific climate conditions

• Enter the project name• Use the drop down menu to indicate your location. Each Australian State City is available,

otherwise, select ‘User Defined’

• Identify the type of evaporation data to be input: daily or monthly. If a city was selected in the previous step, then this should be left as daily.

Step 1a - If Location = ‘User Defined’If a user defined location has been selected, rainfall and evaporation data must be entered.

• Click on ‘Add Climate Data’ • Copy 1 year of daily precipitation and pan evaporation into the white blank cells (if no white cells are

visible, go back to the user interface and select ‘User Defined’ under the Location drop down.

• If only monthly evaporation data is available, make sure to go back to the User Interface and select ‘Monthly’ evaporation. Enter monthly evaporation in the white blank cells.

Step 2 - Runoff Characteristics

• After delineating the upstream catchments, enter each of the areas in m2 in the appropriate cells.

• If initial losses are preferred to be entered, select “User Defined”, otherwise keep selected as default. The default initial losses are:

• Tree = 5 mm

• Road = 2 mm

• Footpath = 2 mm

• Other = 1 mm

Step 3 - Soil Characteristics• Select the soil type. This will most likely be a sandy

loam for typical tree pits. This relates to the field capacity and wilting point of the system. These can be adjusted by the user by selecting ‘Yes’ under the ‘Soil Properties’ box coloured in brown.

• Enter the soil depth. Typically between 300 - 750mm.

• Enter the Type 1 soil cross-section. This is typically 1 m, but may be zero, depending on the design

Step 6 - Optimise Model• Analyse the results graphs.

• The ideal condition is that the soil moisture is kept above the Wilting Point to avoid tree failure.

• If this drops below:

- Increase the catchment (this will affect tree spacing)

- Adjust the water use category to a more efficient tree (low or very low).

- Adjust the canopy diameter to reflect a smaller tree

Step 4 - Tree Characteristics• Enter the water use category of the desired tree type. This relates

to how much water the tree consumes, and affects the default crop factors (which may be changed manually).

• Define whether default crop factors or user defined values will be used. The default values relate to the water use category.

• Define the Crop Factor Behaviour. This shows the relationship between crop factor varying through depth. If unknown, leave as default.

• Define the tree type as ‘native’ or ‘exotic’.

• Adjust the canopy diameter. This affects the surface from which evapotranspiration can occur. A range from small to large trees is given as 5m - 16m. Some trees, can be as large as 30m, such as the Sydney Blue Gum.

• Identify if the root growth will be constrained. For the Strataflowsystem this should be kept as Y.

• Identify the typical root diameter.

• Identify the root depth (this is likely to be equivalent to soil depth)

Step 5 - Watering Requirements• Identify if irrigation will be required. The answer will be

‘Yes’ if the results graph shows the WSUD tree moisture dropping below the orange Wilting Point line. If the the desired answer is No, but the Wilting Point line is crossed, go to ‘Step 6’

• If Yes above, adjust the irrigation application rate, to bring the soil moisture above the Wilting Point threshold.

• Adjust the canopy diameter. This affects the surface from which evapotranspiration can occur. A range from small to large trees is given as 5m - 16m. Some trees, can be as large as 30m, such as the Sydney Blue Gum.

• Identify if the root growth will be constrained. For the Strataflowsystem this should be kept as Y.

• Identify the typical root diameter.

• Identify the root depth (this is likely to be equivalent to soil depth)

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WATER SENSITIVE Strataflow

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