Australia: Knox: Rain garden design

1. 1
Rain garden: design, construction and maintenance recommendations
based on a review of existing systems
N. Somes 1, M. Potter2, Joe Crosby1, M Pfitzner3
1
Ecodynamics, nick.somes@ecodynamics.com.au
2
Melbourne Water Corporation, matthew.potter@melbournewater.com.au
3
Kingston City Council,
In order to better understand factors that contribute to the successful implementation of street scale Water
Sensitive Urban Design (WSUD) assessments were undertaken at 22 sites across Melbourne. The sites use a
variety of treatment methods with most using Raingardens. The review was undertaken in response to concerns
regarding poor plant growth at a number of sites.
The inspections found civil works and maintenance (weed and litter control) were generally undertaken well.
Some issues identified with civil works include the use of inappropriate mulches and loss of extended detention
storage due to overfilling of filters. Infiltration rates of filters were quantified via insitu and laboratory methods.
Most sites tested had infiltration rates of less than 80 mm/h. Infiltration rates below design specifications results
in reduced pollutant removal as systems bypass more frequently. Low hydraulic conductivity of the systems
resulted from use of filter material or mulches containing fines which impeded flow. Surface clogging was not
evident. Plant growth was variable with poor growth often linked to water logged soils due to soils with very
low hydraulic conductivity. In response to these findings a revised specification for the design and
implementation of rain garden filters was developed.
Introduction
Since the early 1990s a number of activities have been undertaken to reduce the impact of
urban stormwater on the environment in greater Melbourne. The water sensitive urban design
(WSUD) measures implemented were at the regional scale and included wetlands and
sedimentation basins. These treatments are often referred to as “end of pipe” treatments and
typically treat large catchments (> 60 ha). By the late 1990s attention was also being focused
on the use of treatment measures at the street scale. This approach avoided the need to
construct regional scale systems by treating runoff at or near source. To date a large number
of street scale systems have been constructed in new areas of development or retrofitted to
existing urban areas.
Common street scale treatment measures include swales, bioretention systems, infiltration
systems and porous paving. Street scale systems are characterised by treating runoff by
filtration through vegetation and infiltration through soil profiles. The most common form of
street scale WSUD are bioretention systems, commonly referred to as raingardens. Street
scale treatments face a number of design pressures not present in regional scale measures.
Retrofitted systems have further limitations as they must be accommodated into existing
streetscapes and deal with issues such as existing services and limited space.
Figure 1 is a typical section of a raingarden showing the key elements and dominant flow
paths. Raingardens treat runoff by storing catchment flows in the extended detention storage
and allowing it to infiltrate through a series of layers. To ensure drainage function is
maintained most systems have an overflow which operates when the storage capacity of the
extended detention storage is exceeded.
Background
In 2006, Kingston City Council and the Better Bays and Waterways Project embarked on a
joint venture to investigate the condition of 22 street scale WSUD devices across Melbourne
and review design, construction, landscaping and maintenance practices. The sites had been

2. 2
Ca Extended
tc
Ru hme Detention Zone
no nt
ff
Overflow
Batter
Kerb
Overflow
Infiltration
Infiltration
Mulch
Filter
Transition
Drainage
Figure 1 – Raingarden section
designed and constructed in the past five years and most had been retrofitted into existing
streetscapes. A variety of systems had been built, the majority being raingardens. Infiltration
systems, swales and wetlands had also been constructed. The review was initiated due to
concerns regarding plant growth in a number of systems. The study also provided an
opportunity to inform local government of lessons learnt in implementing raingardens, in
particular for retrofitted systems.
The study was undertaken during winter of 2006. Site assessments included a review of
available documentation, discussions with Council staff associated with individual projects
and site inspections. The objective of the review was to define the current condition of each
site compared to the design and the maintenance regime.
Design
Raingarden design is an area of ongoing development. The earliest projects reviewed were
designed in 2002. At this point in time, design tools were in their infancy and limited to
overseas guides and based on results of previous projects (Knox City Council, 2002).
Melbourne Water produced the WSUD Engineering Procedures: Stormwater (Melbourne
Water, 2005) and represented a significant step forward in the design resources available.
The improvement in design resources was reflected in design practice.
The sites were designed over a 5 year period and reflected the developments in design
practice that occurred over that period. Many of the changes to design practice related to the
aesthetics, maintainability and integration into the streetscape. In early systems, some designs
did not differentiate the raingarden from the streetscape, with edges typically battered and
planting areas adjoining grassed areas. These design decisions have resulted in ongoing
maintenance issues, e.g. weed control is required around the raingardens to prevent grass
entering the garden bed, in undertaking these works overspray with herbicide has killed both
grass and the raingarden plants. To overcome this, recent designs have included provisions of
features such as concrete edge strips to delineate gardens and assist maintenance. Other
developments are the use of steps and retaining walls rather than batters to reduce the area
required to take up changes in grade.
Further improvements can be made in design practice as new design models are pursued to
incorporate raingardens into items such as street furniture, traffic calming devices and street
trees. The design process should also include streetscape design and community engagement
to increase the rate of stakeholder “buy in”. Stakeholder involvement is discussed later in this
paper in relation to its ability to reduce maintenance costs.

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Project Implementation
The findings of the review of sites are summarised in Figure 2, which outlines the number of
systems which were rated good, moderate or poor across five functional characteristics
(Kingston City council et al, 2007). The selected characters represent the key elements of
street scale WSUD elements. It should be noted that the infiltration rate data set was based on
results from 12 sites, while other categories were based on results from all 22 sites. The
results are discussed in detail in the following sections.
100%
90% Good
80%
Moderate
70%
Proportion of Sites (%)
Poor
60%
50%
40%
30%
20%
10%
0%
Civil Works Plant Condition Infiltration Rate Weeds Litter
Figure 2 – Summary of site investigation findings
Civil Works
Civil works are the hard works required to construct raingardens, e.g. concrete paving and
drainage works. The results indicate that civil works were generally well constructed. The
cases of moderate civil construction were related to surface grading of the raingarden not
being in accordance with the design and reducing the extended detention depth. Another
issue identified with civil construction was the selection of gravel mulches that contained
fines, which formed a compacted and relatively impermeable crust over the surface of the
raingarden. Other issues included bypass inlets set at wrong heights and inlet structures
which restricted flow or were prone to blockage.
The good rating for civil works is not surprising, as all of the works were constructed by civil
construction firms with experience in the construction of hard works. Errors were often
associated with departures from standard engineering practice. For example, it is common
practice to fill around pits and back up kerbs. This had occurred in several raingardens and
resulted in the loss of the extended detention storage area, which will significantly reduce the
effectiveness of the systems. From discussions with Council staff it was concluded that these
sorts of errors reflected a lack of understanding on behalf of the contractor and Council
supervisors of the intended function of the systems.
Appropriate supervision and knowledge transfer is essential to ensure construction projects
are built to reflect the design. In most of the projects reviewed, implementation of the works
was supervised by Council Officers, whose background was in the supervision of civil works.
Typically officers had not been provided with additional training regarding the design and
implementation of WSUD works. Their experience is reflected in the works, in nearly all
cases the civil works are constructed to a high standard. However, the landscape works, in
particular filters, have been implemented poorly, with few of the raingardens installed as per
the design. In most cases the poor implementation has been a result of the specification being
partially adopted or incorrectly adopted. It is considered that appropriate training is needed to

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provide supervisors with the knowledge to ensure works are constructed to design
specifications.
At a minimum the following areas of the works should be checked or confirmed as works
proceed:
• Connections to drainage system, including sub-surface drains, inlets and overflows;
• Levels of inlet, overflow pit, and extended detention area base (top of mulch layer);
• All materials used in the construction comply with the specification, in particular
hydraulic conductivities and grading;
• Placement of the filter to design levels; and
• That any amelioration to filter material has been undertaken in accordance with the
recommendations;
Plant Selection and Health
A wide variety of plant species had been used in projects reviewed. Plants used included
trees, shrubs, tussocks and ground covers. Where filters had been implemented correctly, all
species of plants grew well. Poor plant growth was typically associated with poor filter
function or water logged soils. In this regard terrestrial plants, e.g. Dianella species, were
found to offer an advantage as they demonstrated poor growth if the filter had low infiltration
rates (refer to Figure 3). The poor growth prompted managers to investigate reasons for the
poor growth, which subsequently led to filter function being identified as being of concern.
Where semi-aquatic plants (e.g. Ficinea nodosa or Juncus species) were used in filters with
low infiltration rates, the plants grew well and the poor filter function was not identified
through plant health. It is considered that the use of some plants which prefer well drained
conditions is preferable, as they will indicate poor filter function.
It is considered that a wider variety of plants than has been used to date are suitable for
raingardens. It is recommended that designers expand their plant palettes to include plants
that are not normally associated with raingardens. Good plant cover was achieved best where
groundcovers were included in the plant palette.
Filter Specification and Implementation
To understand filter function better, samples from filters exhibiting poor plant growth and
good plant growth were analysed at the outset of the study to determine horticultural and
drainage properties and compare these to design values. Soil fertility was found to be
adequate at both sites, with plant growth not limited by it. Drainage rates were quantified via
hydraulic conductivity tests (AS4419, 2003) and were found to be low compared to design
values. The designs specified hydraulic conductivities in the range of 80 – 180 mm/h.
Samples from the sites were found top have hydraulic conductivities of less than 5 mm/h.. At
these sites many of the plants were showing signs of being too wet and soils were water
logged.
To better understand filter function across a wide range of sites, 44 in situ infiltration tests
were undertaken at 12 sites. Beds were selected to provide a cross section of plant growth,
i.e. poor to good, and in some cases multiple infiltration tests were conducted in the same
raingarden. As a result of the sites not being randomly selected and multiple tests in a number
of beds the results should not be considered definitive. However, they provide a useful
insight into filter function across the sites and are considered representative of all the sites
investigated.

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Figure 3 – Example of good and poor growth at adjacent raingardens planted with
Dianella tasmanica. The hydraulic conductivity of the right hand site was two orders of
magnitude lower than the left hand site.
The in-situ test method used a steel collar (Figure 4) that was driven into the filter profile to
the drainage layer (Figure 5). A constant head was maintained within the steel tube and
topped up on a regular basis, with the timing and volumes of additions recorded. The tests
were conducted by Land and Water Constructions and the Facility for Advancing Water
Biofiltration (FAWB) with results pooled. Please note, the results are described as infiltration
rates not saturated hydraulic conductivities. This description has been adopted as test
conditions could not be controlled as they are in the laboratory tests which comply with
AS4419.
Figure 4 Infiltration tube Figure 5 Infiltration tube installed
in a raingarden
The results of the infiltration tests are summarised in Figure 6 and show that more than half
the tests indicated infiltration rates below 40 mm/h. These results indicate that many
raingardens are not functioning appropriately and will therefore be bypassing some design
flows. An assessment of the impact of reducing infiltration rates was made using MUSIC V3.
A simple model of a raingarden was developed and run with a range of infiltration rates, with
the results summarised in Figure 7. Figure 7 shows that optimal treatment is achieved at
infiltration rates between 150 and 250 mm/h. Large reductions in removal efficiency occur
when the infiltration rate drops below 100 mm/h, due to the increased rate of bypass. The
results suggest that many of the raingardens reviewed would have the design pollutant
removal effectiveness reduced by 10 to 15% due to their low infiltration rates.

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25 50
Annual Nitrogen Load Reduction (%
20 45
Infiltration tests (number)
15 40
10 35
5 30
0 25
<40 40 - 80 80 - 180 >180 0 100 200 300 400 500
Infiltration rate (mm/h) Infiltration Rate (mm/h)
Figure 6 Summary of in situ Figure 7 Effect of altering
infiltration rates measured hydraulic conductivity on raingarden
performance
Filters were frequently specified in the following manner:
• A material description e.g. sandy loam;
• A desired hydraulic conductivity, typically in the range of 80 to 180 mm/h; and
• In some cases, a grading was provided.
In many cases the specifications were in conflict or were contradictory. The description of
soils, e.g. sandy loam, includes a relatively wide grading range and can contain up to 20% of
clay material (AS 4419-2003 Table I1). This proportion of clay material is considered to be
too high to maintain the required hydraulic conductivity. The term sandy loam is also widely
used within the landscape supply industry to indicate a soil with sandy characteristics. Sandy
loams have highly variable gradings and properties reflecting the variety of sources and
minimal processing that the soils undergo. Many of the filters reviewed had been constructed
with topsoil and had low infiltration rates. The widespread use of topsoil within filters is
thought to be due to ambiguous specification and a lack of experience on behalf of
supervisory and construction contractors.
In light of these findings a new specification was developed for raingarden filters. The
specification is based on “Recommendations for the Establishment of Putting Greens”
(USGA, 2004) and the experience of the authors in the construction of landscape systems.
The objective is the provision of a specification that is robust and can be readily implemented.
It is acknowledged that it may appear overly prescriptive, but the experience of the review of
existing systems has demonstrated that a greater degree of prescription is required to achieve
appropriate outcomes. The specification is contained in the project report available on line
from Melbourne Waters website.
(http://wsud.melbournewater.com.au/content/technical_reports/technical_reports.asp)
The raingarden specification provides guidance on the following layers:
• Mulch – to suppress weeds and retain moisture within the underlying filter media;
• Filter – soil layer which acts as a pollutant filter and supports plant growth;
• Transition Layer – layer to separate filter layer from the drainage layer to avoid
clogging of drainage pipe; and
• Drainage Layer – relatively free draining layer containing perforated drainage pipe.
Departures from current specifications (Melbourne Water, 2005, FAWB, 2006, Fletcher et al,
2006) include the recommendation of mulch in all systems and the use of washed sand as the
preferred filter material. Stone mulch is recommended to improve plant growth by

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suppressing weed growth and maintaining soil moisture. Timber mulches or jute mat are not
recommended as they are not long lasting and not practical for regularly inundated systems.
Suitable stone mulches are sized 4 to 13 mm, screened and applied in a 40 - 75 mm layer.
Selection of mulch materials is based largely on aesthetic requirements, with a wide variety of
stone sources available.
The filter layer is the critical layer in determining the operation of the system and therefore
the greatest care must be taken in sourcing the appropriate materials. In recognition that the
materials need to be controlled, the approach outlined in the specification recommends the use
of a washed sand product. This approach has been adopted to limit the amount of fines that
will be present. The major determinant of suitability is the hydraulic conductivity of the
material which should be in the range of 100 – 300 mm/h.
As the filter is a washed product it will not support plant growth and will require amelioration
with a range of organic and inorganic fertilizers and trace elements. These materials are
added as a “one off” addition during construction. The total nutrient load of this addition is
equivalent to the annual nutrient load (N and P) captured within the filter. Subsequent
nutrient additions are not usually required and are provided by stormwater runoff.
The transition layer and drainage layer have similar specifications to existing guidelines and
are not discussed here in detail.
Experience with the supply of materials to the projects reviewed and subsequent projects
indicates that supply of materials is not a simple task. When sourcing materials, a quality
control program must be implemented that test materials prior to and following delivery. For
large scale projects this requires prospective materials to be stockpiled at their source and
tested prior to delivery. Follow up tests of materials delivered to site should also be
undertaken to ensure material properties do not vary throughout the delivery process.
Maintenance
Maintenance is considered to include management of the following:
• Aesthetics – management of litter and sediment;
• Horticultural – weed control, replanting and re-mulching;
• Damage – repair of accidental damage and vandalism; and
• Inspections – regular inspections of systems to ensure it is functioning.
The review of existing systems found that aesthetic and horticultural maintenance was
generally done well. Weeds were only a problem where maintenance responsibility at council
had not been identified. Litter load generally reflected the surrounding areas, with inner city
areas typically having higher litter loads.
As part of the project an attempt to quantify the maintenance cost of raingardens was made.
The results varied markedly with many Councils not being albe to identify a maintenance
cost. Where costs were identified they ranged between $3.80 and $20 per square metre of
raingarden per annum for planted systems. The costs were considered to reflect the
maintenance cost of any landscape at that location, not the specific cost of maintaining a
raingarden. This finding is reflective of most of the maintenance cost being associated with
site visits for aesthetic purposes. For example a high profile park location will require a visit
each week to ensure litter is managed.
Cost effective maintenance is aided by effectively constructed systems with good plant cover
and edge delineation. Regular maintenance activity coordinated with maintenance of other
public landscapes also reduces costs.

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Conclusions
The review of 22 street scale WSUD sites found that whilst design practice had improved
significantly over the past 5 years and implementation was generally well done, opportunities
for improvement still existed.
The key areas for improvement are in the specification and implementation of filters. To date
many of the filters constructed have been constructed with topsoil as the filter media. These
materials contain too high a proportion of fines therefore impeding infiltration. The reduced
hydraulic conductivity significantly reduces the pollutant removal effectiveness. To
overcome these issues it is recommended that a revised specification be adopted and that
greater attention be given to supervision of the works.
Other key findings were:
• Civil works are generally well constructed;
• A wide range of vegetation types and species are suitable; and
• Maintenance costs for raingardens should be considered to be of similar magnitude to
adjacent landscapes.
Acknowledgements
The project used as a basis for this paper was conceived and funded by the City of Kingston
and the Better Bays and Waterways - Institutionalising Water Sensitive Urban Design and
Best Management Practice in Greater Melbourne Project which is supported with funding
through the Australian Government Coastal Catchment Initiative. Fieldwork to quantify
hydraulic conductivity was undertaken in partnership with the Facility for Advancing Water
Biofiltration.
References
FAWB (Facility for Advancing Water Biofiltration). 2006. Guideline Specifications for Soil
Media in Bioretention Systems.
Fletcher, T., Wong, T. and Breen, P. 2006. Buffer Strips, Vegetated Swales and Bioretention
Systems – Chapter 10 in Australian Runoff Quality – A Guide to Water Sensitive Urban
Design. Engineers Australia.
Kingston City Council and Better Bays and Waterways, 2007, Review of Street Scale WSUD
in Melbourne, Technical Report Prepared by Land and Water Constructions, available at
(http://wsud.melbournewater.com.au/content/technical_reports/technical_reports.asp)
Knox City Council, 2002, Water Sensitive Urban Design (WSUD) Implementation
Guidelines, Technical Guidelines published by Knox City Council, available at
www.knox.vic.gov.au, April 2002.
Melbourne Water, 2005. WSUD Engineering Procedures: Stormwater. CSIRO Publishing.
Melbourne.
Standards Australia, (2003), AS4419 Soils for landscaping and garden uses, Standards
Australia International, Sydney.
USGA, 2004, USGA recommendations for a method of putting green construction, United
States Golf Association Green Section Staff, 11 pages, sourced from www.usga.org