Factors Affecting the Performance of Raingardens in Sydney LGA

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

1 URBANISM REPORT-2016

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).


2.2- 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.


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.


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.


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


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



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


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).


4. Stormwater

Quality

Improvement

Devises (SQID)

Sydney Water Total Suspended Solids: No

Total Nitrogen: No



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


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.


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


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).


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


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.


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.


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

http://www.google/


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


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.


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


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


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.


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.


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).


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.


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Documents

Factors Affecting the Performance of Raingardens in Sydney LGA