URBANISM REPORT
Factors Affecting the Performance of
Raingardens in City of Sydney LGA
5/13/2016
Source: Raingardens across Marylands (www.mdcoastalbays.org/files/pdfs_pdf/rain_gardens.pdf)
Factors affecting the performance of raingardens
In City of Sydney LGA
ABSTRACT
Local government of City of Sydney is committed to meet stormwater pollution reduction
targets as set in Decentralized Water Management Plan (DWMP). A number of policy
initiatives are being taken at all government levels under the broad spectrum of Water
Sensitive Urban Design (WSUD) to meet pollution reduction targets.
Raingarden is one of WSUD initiative to intercepting stormwater runoff, which then
infiltrates vertically through a filtration media where treatment is achieved by process of
filtration through engineered soil layers (Heish,C., Davis, A.P., 2005). A number of
raingardens are developed across the City of Sydney since 2008, incorporating different
techniques to improve their efficiency and performance.
Research report seeks a detailed visual inspection of raingardens to investigate factors
which can potentially limit the efficiency of raingardens. Visual inspection is carried out
in accordance with criteria provided by City of Sydney Council to analyze the
deficiencies in the current system. Guidelines by Faculty of Advanced Water
Bioretention (FAWB) are considered as benchmark to develop the criteria of inspection.
CONTENTS
1.0- Introduction 1
2.0- WSUD & Policy Initiatives 1
2.1- Initiatives at Commonwealth Level 1
2.2- Initiatives at State Level 2
2.3- Initiatives at Local Government Level 2
3.0- Stormwater Pollution and Sydney 2
4.0- Implementation of WSUD in City of Sydney 3
4.1- Raingardens in City of Sydney LGA 3
5.0- Role of Raingardens 5
5.1- Stormwater Treatment Train 6
5.2- The Hydrological Spectrum of Total Rainfall 8
6.0- Evolution of Raingardens in City of Sydney 8
7.0- Methodology and Criteria for Evaluation 11
7.1- Comparative Analysis 12
7.2- Schematic Design 12
7.3- Observations 13
7.4- Discussion on Observations 14
8.0- Factors Affecting the Performance of Raingardens 16
8.1- Large Catchment Areas 16
8.2- High Street Gradients 16
8.3- Stormwater Inlet 17
8.4- Overflow Pit/Stormwater Outlet 18
8.5- Narrow and Small Size 19
8.6- Soil Erosion 19
8.7- Variation in Ponding Depth 20
8.8- Impact of Existing Trees 20
8.9- Deposited Sediments 20
8.10- Lack of Maintenance 20
8.11- Vegetation Cover 21
8.12- Signage 21
8.13- Rainfall Erosivity 21
9.0- Conclusion 22
10.0- References 23
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1.0- INTRODUCTION:
Urban developments have resulted in an enormous increase in impervious surfaces
which has disturbed the water cycle worldwide. Pervious areas provide an opportunity
to treat stormwater by filtration, oxidation and solar irradiation before entering into our
waterways (Brabec, E., Schutle, S., P., 2002, pg.499-514). Significant increase in
impervious surfaces lead to high stormwater runoff which washes off significant amount
of pollutants including litter, suspended solids, nitrogen and phosphorous (Leopold,
1968). Unlike wastewater, stormwater isn't treated before entering directly into creeks,
rivers and oceans and it can potentially carry all pollutants to waterways and damages
environmental ecology and marine life.
A range of systems and strategies have been developed to control pollution to protect
waterways as a part of Water Sensitive Urban Design (WSUD). These systems
primarily aim to control contamination and velocity of urban runoff by incorporating
bioretention facilities, flow buffers and introduce engineered terrestrial phase to provide
natural and physical stabilization of stormwater (Singh et al. 2009, p.144-150).
Prominent initiatives by WSUD to treat urban runoff include porous pavers, bioretention
basins (raingardens), infiltration trenches, wetlands and sand filters.
2.0- WSUD AND POLICY INITIATIVES:
‘Water sensitive urban design is an emerging discipline that employs planning and
design procedures that better integrate urban development with the elements of the
local and surrounding natural environment, in order to secure more sustainable
environmental, social and economic outcomes’ (Sydney Metropolitan Catchment
Management Authority,SMCMA, 2007).
WSUD strategies and policies are encouraged to be incorporated at all government
levels from commonwealth to local government level.
2.1- Initiatives at Commonwealth Level:
What is WSUD?
WSUD is backed by numerous statutory and policy initiatives. At commonwealth level,
National Water Initiative (NWI) was adopted in 2004 and it started a water reform
journey that provides a clear national strategic action plan for the sustainable
management of Australia’s water resources. It is further endorsed at commonwealth
level by Australian Guidelines for Urban Stormwater Management (ARMCANZ and
ANZECC 2000). It aims to provide a nationally consistent approach for the management
of urban stormwater and recommends inclusion of WSUD in greenfield and infill
developments (Water Sensitive Urban Design Program, 2014).
2 URBANISM REPORT-2016
2.2- Initiatives at State Level:
At the State level, planning and environmental legislations promote the principles of
Ecologically Sustainable Development (ESD) with state planning and environmental
policy frameworks. It provides guidelines for implementation of WSUD with primary
focus on stormwater quality (Water Sensitive Urban Design Program, 2014).
2.3- Initiatives at Local Government Level:
The Local Government Act, 1993 requires councils across NSW to protect and enhance
waterways by managing stormwater properly in accordance with the principles of
ESD. It places a strong onus on small scale interventions by local government
authorities to ensure urban ecology and harbor water quality (Water Sensitive Urban
Design Program, 2014).
3.0- STORMWATER POLLUTION AND SYDNEY:
Sydney waterways are much debated for its pollution levels over the last decade.
According to Gavin Birch (2011), associate professor at the School of Geosciences at
University of Sydney, expressed in an interview that greatest threat to harbor health is
pollution that is flowing into harbor very frequently in form of stormwater. Stormwater is
more than the rain and can contain heavy metals produced by fossil fuels and vehicular
activity combines to form a potentially hazardous mix in waterways.
The highly urbanized catchment of City of Sydney LGA is spread on an area of 2,699
Hectares with less than 20% pervious area and considered as one of the potential
sources of stormwater pollution (DWMP, 2012).Concerns about the increasing level of
pollution in Sydney waterways were translated in objectives of Sustainable Sydney
2030 as follows;
OBJECTIVE 2.3: Reduce stormwater gross pollutant loads to the catchment within the
local government area. (Sustainable Sydney 2030, pg.33)
ACTION 2.3.3: Continuously improve Water Sensitive Urban Design standards to reduce
pollutant loads in city waterways. (Sustainable Sydney 2030, pg.33)
Sustainable Sydney 2030 was adopted by the City of Sydney in 2008 and it was
proposed to include WSUD as a part of the new developments and urban renewal
works since 2008 in accordance with the directive.
3 URBANISM REPORT-2016
Implementation of WSUD was further endorsed by Decentralized Water Master Plan
(DWMP), prepared by City of Sydney in 2012. It is committed to reduce the pollution
entering into waters to 50 % by 2030 (DWMP, 2012, pg.42). Here is a brief of targets of
pollution reduction as set in DWMP (2012):
Water quality Perimeters Current pollutant
quantities entering in
Sydney waterways
Pollutant removal
targets by DWMP by
2030
1. Total Suspended Solids 3192 tones 85%
2. Total Nitrogen 56 tones 45%
3. Total Phosphorous 7 tones 65%
4. Gross Pollutants 575 tones 90% (>5mm)
4.0- IMPLEMENTATION OF WSUD IN CITY OF SYDNEY:
Several methods are suggested in DWMP including construction of bioretention
systems e.g. rain gardens, swales, infiltration trenches within streetscapes and open
spaces (DWMP, 2012, pg.39). According to City of Sydney Council officials (personal
communication, 28 April 2016) major challenge to implement WSUD lies in the highly
urbanized setting of the LGA and City of Sydney largely relies on the retrofitting WSUD
elements in existing urban areas. Raingarden is considered as the most suitable
bioretention system because it needs less space and can be easily incorporated in
urban infill and renewal works as compared to swales and infiltration trenches. In 2011
City of Sydney adopted ‘Raingarden Policy’ in line with the objectives of Sustainable
Sydney 2030. It aims to reserve areas for raingardens in foot path works in new and
renewal projects.
4.1- Raingardens in City of Sydney LGA:
What is a raingarden?
According to the website of ‘Melbourne Water’ raingarden is an excavated, shallow
garden bed composed of several engineered layers of composite soil. Raingarden
receives stormwater runoff from the surrounding catchment area through inlet point and
allowing it to soak through the surface of raingarden bed. Soil and plant roots work
together to remove the pollution contents. After filtration clean stormwater is released in
stormwater system or allowed to soak in soil to recharge ground water. An overflow or
surcharge pit is installed to bypass excessive stormwater entering in raingarden during
an event of heavy rainfall. A schematic diagram of a typical rain garden is shown in
figure 1.
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Figure 1. Main components of raingarden system. Source: Drawn by Author in accordance with FAWB
(Storm Water Biofiltration System Adaptation Guidelines, 2009)
City of Sydney Council officials (personal communication, 28 April 2016) were able to
confirm that in accordance with ‘Raingarden Policy’ a series of raingardens are installed
across the LGA to meet the pollution reduction targets since 2008. Currently more than
140 raingardens are installed in LGA with an approximate land cover of 2800m2.
Installing a rain garden in highly urban setting is challenging and almost all the
raingardens in LGA are retrofitted and installed in footpath kerb extensions.
FAWB (2009) guidelines are adopted as best management practices (BMPs) for the
design of raingardens. Two types of raingardens are generally employed which are
lined and unlined systems. Unlined systems are the preferred type since they infiltrate
water and recharge the ground water. These are predominately installed in the southern
part of LGA where there are sandy soils and houses are offset from the boundary. In the
north and west, the soils are less suitable for infiltration and buildings are built on or
close to the boundary. In these areas lined systems were more commonly used. Lined
systems are enclosed by a concrete layer as shown in figure 2. Location of lined and
unlined raingardens across the City of Sydney LGA is translated in figure 3.
5 URBANISM REPORT-2016
Figure 2. Lined raingarden system (left) Unlined raingarden system (right). Source: FAWB, 2009.
Figure 3.Red dots representing the locations of raingardens in City of Sydney LGA.
Source: Google maps and Author.
5.0 ROLE OF RAINGARDEN:
In ideal circumstances raingardens are expected to meet the pollution removal targets
as set by DWMP (2012). An efficient raingarden should be able to successfully infiltrate
the first flush of rain which collects most of the pollutants from the roads and walkways
(Marsalek at el. 2002, p. 1-17). According to scientific literature these systems are
6 URBANISM REPORT-2016
recorded to reduce total suspended solids by 90% and nutrients up to 60-80% under the
ideal conditions (Davis at el. 2001, pg.5-14).
Apart from providing treatment for urban stormwater raingarden can also add to the
streetscape character as it is installed along the walkways and footpaths; which makes
it highly visible in urban setting. Landscape feature of raingarden largely depend on the
green cover and plantation. Dense plantation with 75-100% land cover is inevitable to
enhance the character of streetscape (Chris at el, 2013, pg. 31).
Raingardens cannot be seen in isolation to achieve the pollution removal targets rather
it works as a part of stormwater treatment train, which is comprised of gross pollutant
traps (GPT), stormwater quality improvement devises (SQID), wetlands and bioretention
facilities along with street sweeping.
5.1- Stormwater Treatment Train:
Raingardens work as a part of stormwater treatment train which is currently comprised
of four different initiatives taken by City of Sydney and Sydney Water. Raingarden is
considered as most important in this system because of its capacity to remove all
pollutants as mentioned in DWMP (2012) ranging from suspended solids to gross
pollutants. Stormwater Quality Improvement Devices (SQID) is the only initiative which
lies outside the custodianship or monitoring jurisdiction of the City of Sydney. SQIDs are
generally installed along creeks and streams which receive stormwater from different
local government areas (SMCMA, 2007).
Stormwater Treatment Train
Initiative Managing
Authority
Impact Remarks
1. Street
Sweeping
Local Government
(City of Sydney
Council)
Total Suspended Solids: No
Total Nitrogen: No
Total Phosphorous: No
Gross pollutants: Yes
It is a precautionary measure to
minimize the litter and debris on roads
and walkways which is carried by
stormwater runoff in any event of
rainfall.
2. Gross Pollutant
Traps
Local Government
(City of Sydney
Council)
Total Suspended Solids: No
Total Nitrogen: No
Total Phosphorous: No
Gross pollutants: Yes
Gross Pollutant Traps are installed
along the roads across the LGA. It is a
filtering device and effective to remove
debris and litter from stormwater.
Cross section of GPT is shown in
figure 5.
3. Biofiltration
Systems/
Raingardens
Local Government
(City of Sydney
Council)
Total Suspended Solids: Yes
Total Nitrogen: Yes
Total Phosphorous: Yes
Gross pollutants: Yes
Raingarden distinguishes itself from
rest of the components of stormwater
treatment train for its effectiveness to
remove all type of pollutants as
mentioned in DWMP (2012).
7 URBANISM REPORT-2016
4. Stormwater
Quality
Improvement
Devises (SQID)
Sydney Water Total Suspended Solids: No
Total Nitrogen: No
Total Phosphorous: No
Gross pollutants: Yes
It works on the same principle as gross
pollutant trap (GPT). Sydney water has
deployed about 70 SQIDs; which
helped to remove over 35,000 cubic
meters of litter and organic waster as
well as 39,000 tones of sediments from
stormwater before it reaches Sydney’s
natural waterways. (Sydney Water,
Stormwater)
Figure 4.Gross Pollutant Trap Cross section. Source: Urbanwater Melbourne
Figure 5. Stormwater Quality Improvement Device (SQID). Source: Sydney Water, Stormwater
8 URBANISM REPORT-2016
5.2-The Hydrological Spectrum of Total Rainfall:
The hydrological spectrum of total rainfall is of great importance in understanding that
how far raingarden system itself can go as stand-alone unit to achieve pollution removal
targets. Rainfall events can be distinguished into three categories small, large and
extreme storms. Raingarden is primarily designed to manage runoff caused by small to
large storm which makes 99% of rainfall. In case of heavy storm it is expected to
withstand the strong water current and bypass the excessive stormwater through
overflow pit (SMCMA, 2007).
Hydrological Spectrum Of Total Rainfall Events (SMCMA, 2007)
Rainfall Event Small/Medium
Storms
Large Storms Extreme Storms
Frequency of
Occurrence
~95% ~4% ~1%
Expectation form
Raingardens as
stand-alone unit
Stormwater can efficiently
infiltrate through the
filtration media with
removal of pollutants in
accordance with DWMP
targets.
First flush of stormwater
can be infiltrated efficiently
with optimum removal of
pollutants and only minor
quantities of runoff are
diverted to the overflow
pit. Any debris or clogging
in inlet system can
potentially affect the
performance of raingarden
system.
A carefully maintained
raingarden can infiltrate
first flush with optimum
removal of pollutants and
excessive stormwater is
efficiently bypassed
through overflow pit
without any loss to green
cover and mulch layer.
6.0 EVOLUTION OF RAINGARDENS IN CITY OF SYDNEY:
A series of raingardens were installed across the LGA after adopting the ‘Raingarden
Policy’ by City of Sydney since 2008. Early systems were installed on experimental
basis as there was no precedent. The very first raingardens, based on lined system,
were established in Surry Hills and Redfern. Stormwater inlets in these systems were
provided by creating an opening in kerb stone as shown in figure 7. These systems
were predominantly linear with shorter dimension of about 1.0 meter. Raingardens
along Crown and Reservoir Street in Surry Hills are prominent examples of early
systems.
9 URBANISM REPORT-2016
Figure 6. Raingarden along Crown St, Surry Hills, an example of early systems installed in LGA.
Source: Google maps
Early systems were found to be inefficient primarily because of their narrow size and
minimum dimension of 1.5m-2.0m was adopted in later developments as shown in
figure 8. Raingarden at cross section of Walker and Cooper Street in Redfern is an
example of this system. These raingardens were better able to retain their green cover
but opening in kerb stone remained a cause for deposition of debris and leaf litter on the
raingarden bed.
Figure 7. Raingarden at cross section of Walker & Cooper St, shorter dimension of the system was
increased to 1.5m-2m. Source: Google maps
In new developments entry pit was introduced rather creating an opening in kerb stone
to control the debris and leaf litter entering in the raingarden as shown in figure 8. This
intervention improved the landscape character of raingardens. Raingardens in
10 URBANISM REPORT-2016
Rosebery are examples of such systems as shown in figure 9. Entry pit also helped to
reduce the velocity of runoff; which caused significant soil erosions in earlier systems.
Furthermore an arrangement of small rocks of approximately 500mmX500mm was
placed at stormwater inlet points to stabilize the raingarden bed to withstand the fast
runoff in an event of heavy rain.
Figure 8. Schematic design if entry pit. Source: Author
Figure 9.Installed entry pit (Left) & arrangement of rocks to control soil erosion (Right). Source: Author
The disadvantage associated with installation of entry pit is its limited capacity to convey
water to raingarden bed as it can be affected by clogging and cause flooding. In order to
counter flooding issues overflow points were provided along the raingarden kerb as
shown in figure 9. All these interventions have improved the capacity and outlook of
raingardens with a varying degree of success (personal communication with City of
Sydney Officials, 31 May 2016).
11 URBANISM REPORT-2016
Figure 10.Provision of overflow point by cutting kerb stone to avoid flooding. Source: Author
7.0-MATHODOLOGY & CRITERIA FOR EVALUATION:
The City of Sydney has developed a criteria in accordance with FAWB guidelines
(Storm Water Biofiltration System Adaptation Guidelines, 2009) for physical inspection
of raingardens to evaluate their efficiency and to investigate the factors which may limit
the performance of rain gardens.
The Criteria is based on:
Size and Location:
Topography and catchment area:
Vegetation (Plant health and cover):
Presence of Mulch:
Pollutants and debris:
Design scheme (Inlet/Outlet):
Erosion or blockage at entry point:
Ponding depth: (Vertical distance between raingarden bed and overflow pit)
Soil Erosion:
Landscape character
Signage
12 URBANISM REPORT-2016
All the raingardens across City of Sydney LGA are assessed on the basis of above
mentioned criteria by author. The most common problems are erosion of top layer,
ineffective ponding depth, leaf litter and debris which ultimately adversely affect the
vegetation cover and performance of raingarden.
7.1-Comparative Analysis:
The report seeks to focus the discussion on two raingardens in LGA. Raingardens along
Rosebery Avenue at cross section of Hayes Road and Morley Avenue are selected for
comparative analysis as shown in figure 11. These raingardens are installed in 2014
and based on current design scheme adopted by council as shown in figure 12. These
systems have similar configuration, design, size, urban context and catchment
characteristics and allow to extend investigation beyond the physical parameters.
Figure 11. Location of selected raingardens. Source: Google maps & Author.
7.2-Schematic Design:
These raingardens are developed at the cross section of two roads. One landscape
area (not raingarden) and two interconnected raingarden beds are incorporated in
design. Both raingarden beds are connected by underground pipe which enables these
to perform with single inlet (entry pit) and outlet (overflow pit) system. Schematic design
is illustrated in figure 12.
13 URBANISM REPORT-2016
Figure 12. Schematic design of selected raingardens. Source: Author & City of Sydney, Sydney street
technical specifications.
7.3-Observations:
Criteria Morley Avenue Hayes Road
1 Size and Location Overall area is approximately 150sqm
and it is located at cross-section of
Morley Avenue and Rosebery Ave.
Overall area is approximately 150sqm and it
is located at cross-section of Hayes Road
and Rosebery Ave.
2 Topography and
Catchment Area
Its catchment area lies between Morley
Ave-Dalmaney Ave cross section and
Rosebery Ave-Crewe Place cross
section with medium gradients as shown
in figure 14.
Its catchment area is extended to Morley
Ave-Rosebery Ave cross section to Trevilyan
Ave-Bannerman Cres cross section with high
gradients as shown in figure 13.
14 URBANISM REPORT-2016
3 Vegetation Cover Medium plant cover (approximately 50-
70%). (Chris at el, 2013)
Sparse plant cover (approximately 0-25%).
(Chris at el, 2013)
4 Mulch Layer Mulch layer is present with some
symptoms of erosion.
Mulch layer is significantly eroded.
5 Soil erosion Symptoms of soil erosion. Soil erosion up to 150mm is recorded.
6 Ponding depth Ponding depth varies from 150mm to
200mm.
More than 300mm
7 Debris Traces of debris and leaf litter found. Traces of debris and leaf litter found.
8 Inlet Inlet pit is installed to control leaf litter
entering in raingarden.
Inlet pit is installed to control leaf litter
entering in raingarden.
9 Outlet 500mmX500mm outlet pit is provided to
manage overflow.
500mmX500mm outlet pit is provided to cater
overflow.
10 Contribution in
streetscape
It contributes to streetscape with varying
degree of success as green cover is
visible and covers most of the raingarden
bed as shown in figure 16.
It does not contribute to streetscape.
Raingarden surface is clearly visible owing to
absence of green cover as shown in figure
15.
7.4-Discussion on Observations:
The primary reason for the difference in performance of these two raingardens lies in
their catchment areas. Hayes Street raingarden is exposed to large catchment area and
draws a significantly large amount of water than that of its capacity; leading to limited
performance. Streets approaching to Hayes Street raingarden have higher gradients.
These factors primarily led to soil erosion, loss of ground cover and inadequate ponding
depth.
Figure 13. Catchment area of raingarden at cross section of Hayes Rd and Rosebery Avenue. Source: Author & www.google maps.com
15 URBANISM REPORT-2016
Figure 14.Catchment area of raingarden at cross section of Morley Avenue and Rosebery Avenue. Source: Author & www.google maps.com
Figure 15. Raingarden at cross section of Rosebery Ave and Hayes Rd. Source: Author
Figure 16. Raingarden at cross section of Rosebery Ave and Morley Ave. Source: Author
16 URBANISM REPORT-2016
8.0- FACTORS AFFECTING THE EFFICIENCY OF RAIN GARDENS:
8.1- Large Catchment Areas:
In ideal circumstances size of raingarden should be 2% of the total impervious area of
the catchment (FAWB, 2009). Raingardens designed in City of Sydney LGA are
retrofitted and exposed to larger catchment areas which can draw runoff more than the
capacity of raingardens. It leads to a significant quantity of stormwater bypassing the
filter media and running through the overflow pit which potentially limits the efficiency to
filter pollutants (Davis at el. 2009, p. 109-117). Furthermore recent studies have
revealed that doubling the catchment impervious area causes 10 times increase in
frequency of runoff. (Fletcher at el. 2007, p.265-272)
Raingardens at the intersection of Hayes Road and Rosebery Ave receive a significant
quantity of first flush from neighboring streets as these are located at relatively lower
level than Travilyan Ave and Dalmeny Ave. In an event of heavy rain stormwater cannot
be filtered through the system and pollutants might escape in storm water reticulation by
overflow pit. Catchment area of subject rain garden is reflected in figure 17.
Figure 17.Catchment area of raingarden at intersection of Hayes Rd and Rosebery Ave.
Source: Google maps and Author.
8.2- High Street Gradients:
Velocity of stormwater runoff is largely associated with the gradient of the impervious
surface. Top soil of raingarden is composed of loosely compacted layer of mulch or
small pebbles which cannot withstand the strong current of water resulting in soil
erosion and loss of vegetation cover. (FAWB, 2009)
Raingarden along the Hayes Road shows the symptoms of significant soil erosion as
top layer is flooded by the high velocity of runoff as shown in figure 18.
17 URBANISM REPORT-2016
Figure 18. Raingarden affected by fast stormwater runoff. Source: Author.
8.3- Stormwater Inlet:
Design of stormwater inlet is one of the most critical features of raingarden and it is not
only expected to slow down the runoff velocity but also to remove the litter and debris
from stormwater prior filtration through the raingarden bed. Conventionally inlet to
raingarden was provided by creating an opening in kerb stone. This technique is proved
inefficient as it allows all the litter and debris to enter in the system which can deposit on
raingarden bed. Most of the raingardens across LGA still have this type of inlets. Leaf
litter in a raingarden along Baptist Street is shown in figure19.
Figure 19. Debris and litter in a raingarden along Baptist Street.
Source: Google maps
18 URBANISM REPORT-2016
An inlet pit is installed in recently established raingardens which can trap litter and
debris from stormwater prior to filtration. Installation of entry pit has improved the
performance of raingardens with a varying degree of success and relatively less litter is
noticed in these systems as shown in figure 20.
Figure 20. Reduced debris and litter due to installation of entry pit in a raingarden along Hayes Road.
Source: Author.
8.4-Overflow Pit/ Stormwater Outlet:
An outlet pit is installed in raingardens to drain excessive stormwater; more than the
ponding depth. A 500mmX 500mm overflow pit is noticed in most of raingardens. Large
outlet pits can potentially limit the performance of raingarden as it occupies a significant
space in the system as shown in figure 21.
Figure 21. Overflow pit in a rain garden along Buckland Street.
Source: Google maps
19 URBANISM REPORT-2016
8.5-Narrow and Small Size:
Small size raingardens are more likely to get clogged by debris and leaf litter which
affect the performance of filter media. Limited capacity to hold stormwater leads to quick
overflow and reduce the volume of stormwater passing through the raingarden bed.
Visually they have poor outcome with plants struggling to survive due to being
smothered by silt and debris. Most of raingardens with 1m width are found in poor
conditions as shown in the figure 22.
Figure 22.A narrow raingarden along the Marriott Street.
Source: Google maps
8.6-Soil Erosion:
Soil erosion is noticed in most of the raingardens across City of Sydney LGA. Major
reason for soil erosion is fast runoff and inadequate configuration of stormwater inlet to
slow down water entering in the system. In some raingardens an arrangements of small
rocks of approximate 500mmX500mm is placed at inlet to avoid erosion. The
arrangement of rocks offers a limited performance in case of high speed run off. Soil
erosion adversely affects the vegetation cover. A raingarden along Hayes Street is
suffered by massive soil erosion as shown in figure 23.
Figure 23. Significant soil erosion in a rain garden located at intersection of Hayes St and Rosebery Ave.
Source: Author.
20 URBANISM REPORT-2016
8.7-Variation in Ponding Depth:
Variation in ponding depth is generally caused by soil erosion and deposited sediments.
It can potentially limit the capacity of raingarden to hold and infiltrate stormwater.
According to guidelines of FAWB standards ponding depth of an efficient raingarden
facility should be from 150mm to 200mm. It enables system to perform efficiently with
debris and litter and minimize the maintenance. Some raingardens are noticed with less
than 100mm ponding depth. A raingarden at intersection of Myrtle Street and Buckland
Street has almost no ponding depth as shown in figure 24.
Figure 24. Deposited sediments in a rain garden located at intersection of Buckland St and Myrtle St.
Source: Google maps
8.8-Impact of Existing Trees:
Raingardens located under dense trees suffer by the leaf litter which affects the
performance of filter media and need more maintenance. Dense trees may also block
the sunlight which is necessary for the plant health. City of Sydney is experimenting
different indigenous plants which can survive with less direct sunlight.
8.9-Deposited Sediments:
Sediments are noticed in some of raingardens which not only cause to reduce the
ponding depth of rain garden but also affects the visual outlook of raingarden.
8.10-Lack of Adequate Maintenance:
Inlet and overflow pits require maintenance as are prone to scour and litter build up. In
general raingarden in City of Sydney are not properly maintained and debris and
rubbish is noticed in most of raingardens as shown in figure 25.
21 URBANISM REPORT-2016
Figure 25. Debris in a raingarden located at intersection of Elizabeth St and Holt St Surry Hills. Source:
Google maps.
8.11-Vegetation Cover:
Vegetation not only enhances the visual character of a raingarden system but also work
with filter media to remove pollutants. It leads the complex decomposition process for
removal of nutrients and provides reinforcement to garden bed and helps to resist soil
erosion. Vegetation should be 75-100% of raingarden bed for optimum performance
(Chris at el, 2013). Loss of vegetation is noticed in most of raingardens which is majorly
caused by inadequate maintenance and massive soil erosion.
8.12-Signage:
Signage is noticed in just two raingardens across the LGA. Raingardens at cross
section of Boronia and Baptist Street in Redfern and at cross section of Iredale and
Union Street in Newtown have signage with information about the importance and
function of raingardens.
Signage can be employed to educate people about the function and importance of
raingarden and making it a responsibility of community to protect their green assets.
8.13-Rainfall Erosivity:
Rainfall erosivity is a measure of the rainfall intensity and duration at a given location. It
indicates the amount of water, the force of rain drop impact and the force with it flows
over the soil surface. Rain erosivity values are higher for Sydney as it is a located along
the coast. Rain erosivity can cause severe soil erosion in absence of vegetation cover
(SMCMA, 2007).
22 URBANISM REPORT-2016
9.0- CONCLUSION
City of Sydney has installed raingardens on experimental basis as it is a relatively new
discipline and there is no ideal design available (Wong at el. 2006, p.58-70). MUSIC
modeling is being used for design analysis of raingardens which is still inadequate to
predict the outcome of a raingarden in urban fabric. Consequently most of the
raingardens in City of Sydney are not performing at optimum levels; leading to failure to
meet pollution removal targets. Factors affecting the performance of raingardens are
highly interconnected. The major factors are large catchment areas, than the FAWB
(2009) guidelines, and higher gradients causing faster runoff; which not only limit the
performance and efficiency of the system but also cause soil erosion, loss of vegetation
and inadequate ponding depth.
There are two dimensions to address this problem. The straight forward solution can be
to incorporate more raingardens across the LGA with relatively larger areas to mitigate
the FAWB (2009) guidelines. It is highly challenging to execute because of loss of
parking space along the roads which is highly desirable. New mechanisms can be
incorporated to enable raingardens to withstand the high velocity runoff such as
Introduction of entry pit; which is very successful for raingardens with medium gradients.
Apart from inclusion of new interventions high rain erosivity and extreme storms are
likely to pose some damages to raingardens which should be countered by regular
maintenance. Most of the raingardens across the LGA are in adverse condition due to
lack of proper maintenance.
Second dimension to resolve the problem is to reduce the stormwater runoff to
decrease load on the raingarden system. Raingardens is an initiative endorsed by
DWMP (2012) and it should be implemented with the greater perspective of stormwater
management strategies which also includes stormwater harvesting systems such as
wetlands. Stormwater harvesting systems can efficiently contribute in controlling the
urban runoff and enhance the capacity of raingardens to meet the pollutants removal
targets.
The solution to manage urban stormwater efficiently lies in mitigation between the both
perspectives. Green areas across the LGA should be utilized to maximum extent to
reduce runoff reaching raingardens along with the installation of new raingardens.
Noticeable stormwater harvesting facility is implemented in Sydney Park and can be a
precedent for future installations.
23 URBANISM REPORT-2016
10.0-REFRENCES
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24 URBANISM REPORT-2016
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