Assessing_the_effectiveness_of_WSUD_in_SEQ

ASSESSING THE
EFFECTIVENESS OF WATER SENSITIVE
URBAN DESIGN IN SOUTHEAST QUEENSLAND
Nathaniel Parker
B.Sc. (Crops and Rangelands)
A THESIS SUBMITTED
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS OF THE
DEGREE OF MASTER OF ENGINEERING
FACULTY OF BUILT ENVIRONMENT AND ENGINEERING
QUEENSLAND UNIVERSITY OF TECHNOLOGY
2010

i
KEYWORDS
Water Sensitive Urban Design (WSUD), Low Impact Design (LID), urban water
quality, stormwater quality treatment, bioretention basins, bioretention swales,
constructed wetlands.

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ABSTRACT
Water Sensitive Urban Design (WSUD) systems have the potential mitigate the
hydrologic disturbance and water quality concerns associated with stormwater runoff
from urban development. In the last few years WSUD has been strongly promoted in
South East Queensland (SEQ) and new developments are now required to use
WSUD systems to manage stormwater runoff. However, there has been limited field
evaluation of WSUD systems in SEQ and consequently knowledge of their
effectiveness in the field, under storm events, is limited.
The objective of this research project was to assess the effectiveness of WSUD
systems installed in a residential development, under real storm events. To achieve
this objective, a constructed wetland, bioretention swale and a bioretention basin
were evaluated for their ability to improve the hydrologic and water quality
characteristics of stormwater runoff from urban development. The monitoring
focused on storm events, with sophisticated event monitoring stations measuring the
inflow and outflow from WSUD systems.
Data analysis undertaken confirmed that the constructed wetland, bioretention basin
and bioretention swale improved the hydrologic characteristics by reducing peak
flow. The bioretention systems, particularly the bioretention basin also reduced the
runoff volume and frequency of flow, meeting key objectives of current urban
stormwater management.
The pollutant loads were reduced by the WSUD systems to above or just below the
regional guidelines, showing significant reductions to TSS (70-85%), TN (40-50%)
and TP (50%). The load reduction of NOx and PO4
3-
by the bioretention basin was
poor (<20%), whilst the constructed wetland effectively reduced the load of these
pollutants in the outflow by approximately 90%.
The primary reason for the load reduction in the wetland was due to a reduction in
concentration in the outflow, showing efficient treatment of stormwater by the
system. In contrast, the concentration of key pollutants exiting the bioretention basin

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were higher than the inflow. However, as the volume of stormwater exiting the
bioretention basin was significantly lower than the inflow, a load reduction was still
achieved.
Calibrated MUSIC modelling showed that the bioretention basin, and in particular,
the constructed wetland were undersized, with 34% and 62% of stormwater
bypassing the treatment zones in the devices. Over the long term, a large proportion
of runoff would not receive treatment, considerably reducing the effectiveness of the
WSUD systems.

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TABLE OF CONTENTS
Chapter 1 - Introduction 1
1.1 Overview of Water Sensitive Design in Australia 1
1.2 Justification of Research 2
1.3 Aims and Objectives 3
1.4 Project Scope 4
1.5 Thesis Outline 4
Chapter 2 - Urban Stormwater Management 6
2.1 Background 6
2.2 Urban Runoff Behaviour 6
2.3 Construction of Impervious Surfaces 7
2.3.1 Directly Connected Impervious Surfaces 9
2.4 Retention Capacity 9
2.5 Groundwater Influences 10
2.6 Civil Works 11
2.7 Primary Pollutants in Urban Stormwater Runoff 12
2.7.1 Total Suspended Solids (TSS) 12
2.7.2 Nutrients 13
2.7.3 Phosphorus 14
2.7.4 Nitrogen 15
2.7.5 Heavy Metals 16
2.7.6 Hydrocarbons 16
2.8 Pollutant Build-up 17
2.9 Pollutant Wash-off 18
2.10 Water Quality Objectives 18

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2.11 WSUD Philosophy 19
2.12 Pollutant Removal Processes by WSUD Systems 22
2.12.1 Settling 23
2.12.2 Filtration 23
2.12.3 Volatilisation and Photolysis 23
2.12.4 Adsorption 23
2.12.5 Flocculation 24
2.12.6 Precipitation 24
2.12.7 Plant and Microbial Removal 25
2.13 Overview of WSUD technologies 25
2.13.1 WSUD Treatment Measures 26
2.14 Rainwater Tanks 28
2.15 Swales (Vegetated filter strips) 29
2.15.1 Swale Design 29
2.15.2 Swale Effectiveness 30
2.16 Bioretention Systems 31
2.16.1 Bioretention Treatment 31
2.16.2 Bioretention Filter Media 33
2.16.3 The Rhizosphere 34
2.16.4 Bioretention Size 35
2.16.5 Bioretention Effectiveness 35
2.16.6 Clogging 38
2.17 Constructed Wetlands 40
2.17.1 Wetland Treatment 40
2.17.2 Wetland Design 42
2.17.3 Wetland Effectiveness 43
2.18 Porous Paving 45
2.18.1 Porous Paving Treatment 45
2.18.2 Porous Paving Effectiveness 46

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2.19 Conclusions 47
Chapter 3 - Study Area 51
3.1 Background 51
3.2 Site Selection 51
3.3 Study Area 52
3.4 Summary 56
Chapter 4 - Materials and Methods 57
4.2 Flow Measurement 57
4.3 Weir Design and Installation 59
4.4 Monitoring Stations 60
4.5 Weir Calibration 65
4.5.1 Laboratory Weir Calibration 66
4.5.2 Field Weir Calibration 68
4.6 Water Sample Collection 69
4.6.1 Laboratory Water Analysis 71
4.6.3 Nitrogen and Phosphorus 72
4.7 Monitoring Challenges at Coomera Waters 73
4.8 Details of Monitoring Sites 76
4.8.1 Bioretention Swales (Site A) 76
4.8.2 Constructed Wetland (Site C) 80
4.8.3 Bioretention Basin (Site E) 86
4.9 Conclusions 90
Chapter 5 - Hydraulic Performance of WSUD systems 92
5.1 Background 92
5.2 Rainfall Characteristics 93
5.2.1 Defining Rainfall Events 94

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5.3 Hydraulic Performance of the Bioretention Basin 95
5.3.1 Overall Hydrological Performance of the Bioretention Basin 95
5.3.2 Bioretention Basin Peak Flow Reductions 96
5.3.3 Bioretention Basin Volume Reductions 98
5.3.4 Discussion on Bioretention Basin Volume Reduction 102
5.4 Hydraulic Performance of the Bioretention Swales 104
5.4.1 Swale Peak Flow Reductions 105
5.4.2 Swale Volume Reductions 106
5.5 Hydrological Performance of the Constructed Wetland 107
5.5.1 Overall Hydrological Performance of the Wetland 107
5.5.2 Wetland Peak Flow Reductions 108
5.5.3 Wetland Volume Reductions 110
5.5.4 Wetland Bypass 112
5.6 Treatment Train Effectiveness 113
5.6.1 Rainfall - Runoff Response of Bioretention Basin Catchment 114
5.6.2 Rainfall - Runoff Response of the Wetland Catchment 115
5.7 Modelling Bypass and Volume Retention 117
5.8 Conclusions 119
Chapter 6 - Water Quality Performance of WSUD systems 122
6.2 Data Preprocessing 122
6.3 Pollutant Characterisation and Comparison 125
6.3.2 Nutrients 128
6.3.4 Measured EMCs vs. MUSIC model input EMC parameters 135
6.3.5 Comparison with ANZECC guideline values 138
6.4 Bioretention Basin Water Quality Performance 141
6.4.1 Load Reductions 141
6.4.2 Analysis of Concentration 142

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6.4.3 Discussion of Results 142
6.5 Swale Water Quality Performance 144
6.6 Constructed Wetland Water Quality Treatment 146
6.6.3 Discussion of Results 147
6.7 Water Quality Treatment Train Benefits 149
6.8 Conclusions 150
Chapter 7 - Field observation of WSUD systems in SEQ 153
7.2 Evidence of Poor Maintenance and WSUD Failures 153
7.3 WSUD sizing 155
7.4 Conclusions 157
Chapter 8 - Conclusions and Recommendations 158
8.1 Introduction 158
8.2 Conclusions 158
8.2.1 Investigation Methodology 158
8.2.2 Hydraulic Performance of the WSUD Systems 159
8.2.3 Pollutant Inflow Characterisation 160
8.2.4 Water Quality Performance of the WSUD Systems 162
8.3 Recommendations for Future Research 163
References 165

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List of Appendices
Appendix 1: Housing design for autosamplers 183
Appendix 2: Configuration of telemetry setup at Coomera Waters 183
Appendix 3: Weir calibration results 185
Appendix 4: Sample collection protocol and sticker sheet for bottle
collection 185
Appendix 5: Maintenance schedule 188
Appendix 6: Details of the events used for hydraulic analysis for
the bioretention basin at Coomera Waters 189
Appendix 7: Details of the events used for hydraulic analysis for
the swale at Coomera Waters 191
Appendix 8: Details of the events used for the hydraulic analysis
for the constructed wetland at Coomera Waters 192
Appendix 9: MUSIC model calibration values and input
parameters for the bioretention basin 194
Appendix 10: MUSIC model calibration values and input
parameters for the constructed wetland 195
Appendix 11: Water quality results for the Coomera Waters
bioretention basin 196
bioretention swale 200
constructed wetland 204

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LIST OF TABLES
Table 2.1 - TSS concentrations in stormwater from urban surfaces 13
Table 2.2 - Metals contained in urban stormwater runoff 16
Table 2.3 - Devices within the WSUD treatment levels 26
Table 2.4 - Recommended soil types for bioretention filter media 33
Table 4.1 - Details of the seven event monitoring stations at Coomera Waters 64
Table 4.2 - Challenges in instrumentation 73
Table 4.3 - Challenges in logistics 74
Table 4.4 - Challenges in programming 74
Table 5.1 - Default and calibrated rainfall-runoff parameters for Coomera
Waters 118
Table 5.2 - Modelled bypass and volume of stormwater retained for the
bioretention basin and constructed wetland 118
Table 6.1 - Proportion of roads, roofs and landscape surfaces in catchments
at Coomera Waters where inflow was measured 126
Table 6.2 - Proportions of nitrogen at Coomera Waters compared to
similar studies 129
Table 6.3 - Comparison of the GCCC (2006) EMCs values for MUSIC,
the measured SMC values at Coomera Waters and the MUSIC
default EMC values 137
Table 6.4 - The SMC’s of pollutants in the inflow and outflow for the
bioretention basin, the constructed wetland and the swale 140

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LIST OF FIGURES
Figure 2.1 - Comparison of hydrographs before and after urbanisation 6
Figure 2.2 - Hydrological cycle before and after urbanisation 7
Figure 2.3 - Relationship between stream health and total impervious area 8
Figure 2.4 - Effect of erosion and sediment control measures on TSS
concentrations in stormwater runoff 11
Figure 2.5 - Simplified phosphorus cycle in the urban landscape 14
Figure 2.6 - Simplified nitrogen cycle in the urban landscape 15
Figure 2.7 - Fundamental unit process in relation to WSUD characteristics
and pollutant removal behaviour 22
Figure 2.8 - Three levels of WSUD treatment 26
Figure 2.9 - WSUD treatment measures and acceptable hydraulic loading rates 27
Figure 2.10 - Swales in residential developments in SEQ 29
Figure 2.11- Bioretention systems 31
Figure 2.12 - Bioretention swale at Coomera Waters 32
Figure 2.13 - Typical bioretention basin design 34
Figure 2.14 - Constructed Wetland cross section 40
Figure 2.15 - Identification of the cyclic variables in a treatment wetland 41
Figure 2.16 - Hydraulic efficiency of various wetland geometries 42
Figure 2.17 - Examples of porous pavement 46
Figure 3.1 - Location of Coomera Waters in Queensland Australia 53
Figure 3.2 - Study site at the Coomera Waters development 54
Figure 3.3 - Location of monitoring stations 55
Figure 3.4 - Sub catchment monitoring areas defined 56
Figure 4.1 - The constructed wetland sedimentation basin, showing
inundation/ponding of the wetland inlet 58
Figure 4.2 - Weir design and rocks at the front of the weir to minimize soil
erosion 60
Figure 4.3 - Examples of the assess chambers, down which equipment was
installed to monitor stormwater runoff 61
Figure 4.4 - Design of the monitoring system at Coomera Waters 61

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Figure 4.5 - Instrumentation used for event monitoring of stormwater runoff 62
Figure 4.6 - Community education sign 63
Figure 4.7 - Wiring panel, with CR1000 logger, RF411 radio transmitter,
modem & solar regulator 63
Figure 4.8 - Housing, solar panel, RF radio antenna and rain gauge 65
Figure 4.9 - Calibration setup for underground weirs 67
Figure 4.10 - Example of rating curve obtained for the constructed wetland
outlet weir from the laboratory calibrations 67
Figure 4.11 - Calibration method for stormwater discharges through the weirs 68
Figure 4.12 - Example of rating curve obtained for a weir from the field
calibrations 69
Figure 4.13 - Collecting stormwater samples from Coomera Waters 71
Figure 4.14 - Typical sampling program for a storm event 75
Figure 4.15 - Sampling program required to capture samples during sequential
rainfall events 75
Figure 4.16 - Bioretention swale at Coomera Waters 76
Figure 4.17 - Monitoring location for measuring outflow from the bioretention
swale 77
Figure 4.18 - Monitoring station and weir installation for monitoring outflows
from the bioretention swales 78
Figure 4.19 - Weir face plate for bioretention swale outlet 79
Figure 4.20 - The constructed wetland and insturmentation 80
Figure 4.21 - Monitoring locations for measuring outflow from the wetland 81
Figure 4.22 - Weir design for the wetland inlet (big) 82
Figure 4.23 - Monitoring station and weir installation for monitoring inflows
into the constructed wetland from the small stormwater pipe 83
Figure 4.24 - Weir design for the wetland inlet (small) 84
Figure 4.25 - The outflow control riser and weir measuring flow from the
constructed wetland outlet 84
Figure 4.26 - Weir design for the wetland outlet 85
Figure 4.27 - Constructed wetland bypass, and monitoring station 85
Figure 4.28 - Bioretention basin and bioretention basin inflow monitoring site 86
Figure 4.29 - Bioretention basin showing weir locations to measure the volume
of incoming and outgoing stormwater 87

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Figure 4.30 - Monitoring locations for measuring outflow from the
Figure 4.31 - 450
V notch Weir installed into the small access pit to measure
the outflow from the bioretention basin 88
Figure 4.32 - Weir Design for bioretention basin inlet 89
Figure 4.33 - Weir Design for bioretention basin outlet 90
Figure 5.1 - ARI for 1 year, 6 month, 3 month and 1 month rain events 94
Figure 5.2 - Cumulative rainfall and corresponding inflow and outflow
hydrographs from the bioretention basin 96
Figure 5.3 - Comparison of peak outflow to peak inflow when no bypass of the
bioretention basin occurred 97
Figure 5.4 - The relationship between inflow and outflow volumes from the
Figure 5.5 - The effect of antecedent dry hours on retention capacity 99
Figure 5.6 - Typical hydrograph of bioretention outflow showing when bypass
occurred 100
Figure 5.7 - Rainfall events and volume of stormwater that bypassed outflow
from the bioretention basin 101
Figure 5.8 - Cumulative rainfall and outflow hydrological response of the
catchment treated by the swale 104
Figure 5.9 - Reduction in peak flows for the bioretention swale 105
Figure 5.10 - Rainfall events and the inflow and outflow volumes generated at
the swale 106
Figure 5.11- Cumulative rainfall and corresponding inflow and outflow
hydrographs from the constructed wetland 108
Figure 5.12 - Peak inflow and peak outflow from the constructed wetland, the
equation was calculated for events when bypass occurred 109
Figure 5.13 - Peak inflow and peak outflow from the constructed PVC riser
for 8 events with no bypass 110
constructed wetland for all 17 events 111
constructed wetland for 8 events with no bypass 112

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Figure 5.16 - Rainfall events and volume of outflow from the constructed
wetland 113
Figure 5.17 - Rainfall-runoff relationship for the bioretention catchment at
Coomera Waters. The dotted line shows the 1:1 line. 115
Figure 5.18 - Rainfall-runoff relationship for the constructed wetland
catchment at Coomera Waters 116
Figure 6.1 - The TSS SMCs measured at WSUD system inlets 127
Figure 6.2 - TN (NOx, NH4 and organic N) SMCs at WSUD system inlets 129
Figure 6.3 - Variation of nitrogen species concentrations with TSS
concentration 130
Figure 6.4 - The TP (PO4
3-
and organic P) SMCs of stormwater runoff
measured at WSUD system inlets 132
Figure 6.5 - Variation of phosphorous species concentrations with TSS
concentration 133
Figure 6.6 - Comparison of site mean concentrations of heavy metals from
the ‘bioretention basin inflow’, the ‘wetland big inflow’ and
the ‘wetland small inflow’ 134
Figure 6.7 - Comparison of TSS, TN, and TP concentrations between the
default MUSIC model values and the values measured at
Coomera Waters 136
Figure 6.8 - Reduction in the loads of total suspended solids, nitrogen and
phosphorus after treatment by the bioretention basin 141
Figure 6.9 - Loads of total suspended solids, nitrogen and phosphorus after
treatment by the swale 145
Figure 6.10 - Reduction in the loads of total suspended solids, nitrogen and
phosphorus after treatment by the constructed wetland 146
Figure 7.1 - Event monitoring stations and weir filled with leaking filter
media from a bioretention basin in SEQ 154
Figure 7.2 - A bioretention system at the end of a subdivision 156

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LIST OF ABBREVIATIONS
Abbreviation Description
Al Aluminium
ANZECC Australian and New Zealand Environmental and
Conservation Council
AR&R Australian Rainfall and Runoff
ARI Average Recurrence Interval
BCC Brisbane City Council
BMP Best Management Practice
BOM Bureau of Meteorology
Cu Copper
DCIA Directly Connected Impervious Area
EMC Event Mean Concentration
GCCC Gold Coast City Council
GPT Gross Pollutant Trap
LID Low Impact Development
MUSIC Model for Urban Stormwater Improvement Conceptualisation
NH4 Ammonium
NOx Oxidised Nitrogen
NO2
-
Nitrate
NO2
-
Nitrite
NTU Nephelometric Turbidity Units
Pb Lead
PCWMP Pimpama Coomera Waterfuture Master Plan
PHA Polycyclic aromatic hydrocarbons
PO4
3-
Phosphate
SD Standard Deviation
SEQ South East Queensland
SMC Site Mean Concentration
SUDS Sustainable Urban Drainage Systems
TIA Total Impervious Area
TKN Total Kjeldahl Nitrogen
TKP Total Kjeldahl Phosphorus

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TN Total Nitrogen
TP Total Phosphorus
TSS Total Suspended Solids
USEPA United States Environmental Protection Agency
WSUD Water Sensitive Urban Design
Zn Zinc

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STATEMENT OF ORIGINAL AUTHORSHIP
This thesis contains no material accepted for the award of any other degree or diploma in any
university and, to the best of my knowledge and belief, contains no material previously
written or published by another person except where due reference is made in the text.
Nathaniel Parker
………………………………..
Date: / /

xxiii
ACKNOWLEDGEMENTS
There were many people who guided, supported and encouraged me during this
exciting and challenging research project.
I would like to thank my supervisory team; Professor Ashantha Goonetilleke who
provided me with guidance, support and expert academic supervision. Adjunct Prof.
Ted Gardner who professionally guided me as academic mentor and team manager.
Dr. Prasanna Egodawatta for his technical insights and support during this research.
I would also like to acknowledge the Department of Environment and Resource
Management (DERM), for providing me with study time for my coursework subjects
and funding this research project.
Thanks to my colleagues at DERM who provided me with help, valuable advice and
support. In particular, I would like to thank Daniel Giglio who helped me purchase,
install and maintain the instrumentation at Coomera Waters. Richard Gardiner, who
helped me collect copious numbers of water samples, Marianna Joo for her load
calculation template, Joe Lane for his excel skills, Barry Hood for his helpful
suggestions, and Alison Vieritz for coding the Natifier.
This research would not have been possible without the support of a number of
external organisations. Thanks go to Evan Thomas and his team from the Gold Coast
City Council for technical assistance during the project. I would like to express my
appreciation to Coomera Waters Management and residents for their support and
collaboration. My appreciation is further extended to Shaun Leinster for providing
technical advice and input into this project.
To my parents, family and friends thanks for your encouragement, and the times
shared over the last few years that helped break the monotony of study life. Finally,
Kylie thanks for your amazing support over the last 3 years, even when we threw a
third baby into the mix. You have been a great inspiration and source of
encouragement.

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DEDICATION
For Rohan, Taylah and Luke, thanks for your earnest 5pm greetings, helping me
grow up a bit more and see the important things in life.
‘Be wise at a young age’

1 Chapter 1 - Introduction
Chapter 1 - Introduction
1.1 Overview of Water Sensitive Design (WSUD) in Australia
Urban development creates large areas of impervious surfaces, preventing much of
the rainfall from infiltrating into soil, necessitating the construction of large
stormwater conveyance systems (Arnold and Gibbons 1996). The urban landscape
results in a profoundly altered hydraulic and hydrologic regime and significantly
reduced receiving water quality. These changes have seen the degradation of streams
and impacted estuaries and bays throughout the world (Booth and Jackson 1997;
Walsh et al., 2005).
In Australia, recurring droughts have threatened water resources. This has paralleled
the realisation that many environmental waters have declining water quality. As
society has become more environmentally conscious, the demand to protect the
environment in urban areas has been heard by regulatory authorities. This impetus
has seen more stringent regulations being imposed on water resource managers and
means of water quality improvement which are ecologically sound are being
explored.
Through the 1990s a new philosophy for urban design emerged called Water
Sensitive Urban Design (WSUD). This was not unique to Australia and similar
principles have been developed in the USA called (LID) Low Impact Development
(USEPA 2000; Barrett et al.,1998; Davis et al., 2001; Hunt et al., 2006). In other
countries WSUD systems gained popularity and in the United Kingdom are called
Sustainable Drainage Systems (SUDs) (Eriksson et al., 2007).
The new philosophy promotes ‘source control’ whereby small distributed WSUD
systems are built throughout the subdivision to mitigate the effects of land use
changes, and protect downstream water quality. This is achieved by using vegetation
and storage systems and placing them close to the source area (roof or paved areas)
and along the transport pathways (pipes and roads) of stormwater. This reduces the
direct connection of impervious surfaces and can reduce peak flow, runoff frequency

and runoff volume and increase the retention of pollutants (Argue 2004; Hogan and
Walbridge 2007; Hunt et al., 2006; Walsh et al., 2009).
Among the structural WSUD systems, bioretention systems, swales and constructed
wetlands are commonly used treatment measures for stormwater management.
These WSUD systems facilitate the desired changes in the stormwater inflow
characteristics through the interception of stormwater by vegetation, temporarily
detaining stormwater and/or infiltrating stormwater into the subsurface through a
filter media.
1.2 Aims and Objectives
Key aims of the study were:
Developing innovative methods to collect high quality data for the evaluation of
the performance of WSUD systems under real storm events.
Comparing and characterising the hydraulic performance of the WSUD systems
in reducing peak flow, volume and frequency of runoff from urban development.
Comparing the inflow pollutant concentration values from urban areas to findings
in literature.
Comparing the outflow pollutant load from WSUD systems with the inflow
pollutant load.
Objectives:
The primary objective of this research was to assess the treatment effectiveness of a
constructed wetland, bioretention basin and bioretention swale in improving urban
stormwater runoff and the ability to meet key stormwater management objectives,
including:
• reducing peak flow
• reducing runoff volume
• reducing the frequency of flow
• improving water quality

1.3 Justification for the Research
Southeast Queensland (SEQ) is expected to grow at a rate of 2% per annum from 4.0
million people in 2006 to 5.6 million people by 2026, and to reach 7.1 million by
2050 (Queensland Government 2006). The South East Queensland Regional Plan,
covering the area from the NSW border to Noosa, and west to Toowoomba,
estimates that by 2026 a further 800,000 new dwellings will be needed,
accommodating approximately 1.6 million extra residents (PIFU 2007). Many of
these new dwellings will be in master planned communities – large, greenfield sub-
divisions, requiring the conversion of previously rural land into housing
development. The unprecedented growth in this region could lead to significant
degradation of water resources.
Lot based WSUD measures or ‘source control’ (see Section 2.12) is being
encouraged as the best way forward for urban stormwater management (Argue 2004;
IEAust 2006). However, one of the impediments to realising ‘source control’ is that
many small lot based WSUD treatment systems will be required to be maintained
(Morzaria-Luna 2004). Without the demonstration of the ability to meet key
performance criteria, it is unlikely that source control strategies will be implemented,
as they are perceived to require more effort on the part of the developer and local
authorities (BCC 2000). It is important that WSUD systems are proven to be
effective, in order to justify the added costs.
Despite comprehensive WSUD design guidelines (Moreton Bay Waterways and
Catchments Partnership 2006; BCC 2005), there is inadequate understanding of
WSUD system operation and pollutant processes. Consequently, WSUD systems are
often under-designed and/or poorly built. Furthermore, once WSUD systems are
constructed they are poorly maintained (See Section 7.1). Contractors and
maintenance staff do not realise that the systems need to be built to design
specifications and require regular maintenance to be effective.
The WSUD approach has had limited field validation in Australia. Lynbrook Estate
is a notable example of a WSUD development in Melbourne where the systems were
assessed under natural rain events (Lloyd 2004) (see Section 2.8.5). In the

subtropical environment of SEQ with frequent intense storms, WSUD remains
largely untested, yet now underpins water quantity and quality management for the
protection of aquatic environments.
1.4 Project scope
The physical impacts resulting from urbanisation can be grouped into four major
categories: 1) hydrologic impacts, 2) water quality impacts, 3) impacts on
geomorphology and channel stability, and 4) thermal impacts. This thesis focuses on
the effectiveness of WSUD in reducing hydrologic and water quality impacts from
urban lands. The other factors mentioned are recognised as important stressors to
aquatic habitats, but were not explored in this study.
This research study was confined to a small residential catchment in the Gold Coast.
Though the knowledge built from the study has broad application, the specific
outcomes of the research are limited to the regional climatic conditions and the
WSUD features observed. Furthermore, the water quality condition and the
ecological health of the receiving waters were not evaluated as this was beyond the
scope of the project.
1.5 Thesis Outline
The principal objective of this study was to assess the effectiveness of WSUD
systems in mitigating the hydrologic and water quality impacts of stormwater runoff
on aquatic ecosystems. The structure of the thesis is outlined below.
A literature review provides background to the thesis in Chapter 2. It provides an
overview of the hydrological impacts and water quality concerns resulting from
urban runoff and reviews the reported effectiveness of key WSUD systems. Chapter
3 discusses the criteria adopted in selecting the study site. The study site is defined
and the location of the event monitoring equipment given. Chapter 4 details the
monitoring methods used to assess performance of the WSUD devices and describes
the design and installation of the event monitoring stations, the laboratory methods

used to analyse water quality parameters and the challenges associated with
monitoring storm events. Chapter 5 details the hydrological performance of the
WSUD systems. The effectiveness of the WSUD systems in reducing peak flow,
flow frequency and volume is assessed. MUSIC modelling quantifies the stormwater
bypass of the constructed wetland and bioretention basin. Chapter 6 details the water
quality performance of the WSUD systems at the study. The inflow concentrations
were evaluated and compared to literature. The water quality improvement provided
by the constructed wetland, bioretention basin and bioretention swale was assessed.
Chapter 7 provides an addendum to this research by discussing the wider application
of WSUD in SEQ. Chapter 8 summaries the main conclusions of the thesis and
recommendations for further research are also provided.

6 Chapter 2 - Urban Stormwater Management
Chapter 2 - Urban Stormwater Management
2.1 Background
This chapter focuses on indentifying the changes that accompany urban
development, leading to deleterious impact on receiving waterways. The chapter
provides an overview of the hydrologic impacts and water quality concerns resulting
from urban runoff. WSUD stormwater management philosophy is discussed and
various WSUD technologies described. The pollutant removal mechanisms in
WSUD systems are explained and the effectiveness of key WSUD systems are
reviewed.
2.2 Urban Runoff Behaviour
An undisturbed natural catchment is pervious and contains a naturally developed
arrangement of drainage paths. Usually trees, shrubs and grass intercept flow, and
less than 20% of annual rainfall becomes surface runoff (Argue 2004). For a typical
series of rainfall events, there is generally little erosion or steam disturbance and
damage to aquatic ecosystems is minimal. In the urban environment, lawns and road
verges are examples of pervious areas that allow rainfall to infiltrate into soils. Due
to reduced infiltration capacity, the pervious areas have a rainfall - runoff response
that is markedly different to pervious areas (Figure 2.1).
Figure 2.1 - Comparison of hydrographs before and after urbanisation
(Adapted from County of Santa Cruz Redevelopment Agency)

2.3 Construction of Impervious Surfaces
Urbanisation brings with it a high percentage of impervious surfaces (40-70% of the
catchment) and traditionally pipes replace natural drainage paths to the local creek
(Argue 2004). Impervious surfaces common in urban surroundings belong either to
the transportation system (roads, sidewalks, parking lots), or rooftops (Sleavin et al.,
2000). Urbanisation not only changes runoff behaviour during storm events, but has
long term consequences on the hydrologic cycle within the catchment (Figure 2.1
and 2.2). Changes to runoff behaviour due to urbanisation include:
• Increase in the magnitude and frequency of runoff events
• Reduced evapotranspiration
• Reduced infiltration of rainfall into the soil and groundwater
• Reduced catchment storage capacity
Figure 2.2 - Hydrological cycle before and after urbanisation
Pre-development
Post-development

The hydrologic consequence of introducing impervious surfaces is that runoff events
from all storms are greatly increased. However, the greatest change in runoff is from
the small storms (Holman-Dodds et al., 2003). With urbanisation, the catchment
experiences ‘flashy floods’ which cause stream incision and bank erosion, can
destroy habitats for aquatic fauna and carry high concentrations of nutrients and
sediments (Ladson et al., 2004; Walsh et al., 2005). Furthermore, overall stream
geomorphology and health is influenced by small but more frequent sediment
transporting flows (Holman-Dodds et al., 2003; Walsh et al., 2005). Flows that result
from small rainfall events are unlikely to cause hydraulic stress to streams. However,
these flows can still result in stream degradation through pollutant and thermal
contamination (Walsh et al., 2005).
Hollis (1975) found that urbanised catchments with 20-30% total impervious area
(TIA) have small flood events 15 times more frequently for a 1- year storm than a
pervious catchment. Schiff and Benoit (2007) found that stream health to be
significantly impaired at a TIA of 10%, caused by the increase in the magnitude and
frequency of runoff events. This is consistent with numerous other studies (for
example, Booth and Jackson 1997; Sonneman et al., 2001; Walsh et al., 2004; Walsh
et al., 2005). This is demonstrated in Figure 2.3, which shows that with increasing
impervious area, there is a corresponding decrease in stream ecological health.
Figure 2.3 - Relationship between stream health and total impervious area.
(Adapted from Walsh et al., 2004).

2.3.1 Directly Connected Impervious Surfaces
The direct connection of impervious surfaces to streams, called the effective
impervious area (EIA) is understood to be an important explanatory variable for
stream degradation (Booth and Jackson 1997; Walsh et al., 2005). Walsh et al.,
(2005) showed that many ecological indicators, including concentrations of
contaminants, algal biomass, algal assemblage composition and macroinvertebrate
assemblage composition, had a negative response to EIA. Significant ecological
damage to streams can occur at very low extents of EIA <5% (Schiff and Benoit
2007) and Walsh et al., 2005). Sonneman et al., (2001) compared rural sites with
catchments of similar total impervious area but with varying degrees of direct
connection of drainages to streams. They found that despite the same TIA in the
catchments, the urban catchments with direct connection of impervious surfaces to
the stream were more ecologically degraded.
2.4 Retention Capacity
It has been proposed that to properly manage stream health, the most appropriate
catchment-scale objective for stream protection is 0% EIA, as any directly connected
impervious area results in declining stream health (Walsh et al., 2009). Similarly
Taylor et al., (2005) has suggested that the key management strategy for maintaining
stream water quality would be to reduce the direct connection of the drainage system
so that flow to streams does not occur for frequent, small rain events.
Walsh et al., (2009) proposed an index called the ‘Retention Capacity’ (RC) of a
catchment, where the increased runoff frequency from impervious areas is reduced to
post-development runoff frequency by the retention of small rain events. A
maximum RC value of 1 will be achieved under the condition that frequent flows
from the urban catchment can mimic the pre-urban state. However, when
impervious areas are directly connected to the receiving water with no retention of
storm events such as in a typical urban design, a minimum RC value of 0 is given
(see Equation 2.1).

0,max1 





−
−
−=
nu
nt
RR
RR
RC Equation 2.1
Where tR = frequency of runoff per year from the surface after WSUD treatment
Where nR = frequency of runoff per year in the pre urban state
Where uR = frequency of runoff per year if directly connected to the urban stream
Walsh et al., (2009) advocated the use of WSUD treatment measures such as
rainwater tanks and bioretention systems which can significantly increase stormwater
volume retention capacity. This in effect will reduce frequent flow events, and offer
the best opportunity for urban stream restoration.
It should be understood that there can be more than one receiving environment for
urban stormwater. Accordingly there are different environmental stressors which
need to be managed depending on the receiving ecosystem. WSUD objectives have
been based around pollutant load reduction targets. It is common that coastal waters
(wetlands, estuaries and bays) are allocated nitrogen and phosphorus load reduction
targets to mitigate environmental stress. However, to protect the ecological health of
local streams, the magnitude and frequency of post-development runoff needs to
mimic pre-development runoff (Taylor et al., 2005; Walsh et al. 2005; Walsh et al.,
2009). Objectives for WSUD need to include both pollutant load reduction targets
and reduction targets for hydrologic parameters such as frequent flows (Walsh et al.,
2009).
2.5 Groundwater Influences
A consequence of converting pervious to impervious surfaces is the reduction in the
amount of rainwater that infiltrates into the ground for recharging of groundwater
and providing base flows to streams (Schueler 1994; Argue 2004). Conversely,
rising water tables in cities are reported due to the leakage of imported water from
centralised supply or urban sewerage infrastructure (Lerner 2002; Ragab et al., 2003;
Walsh et al., 2005).

2.6 Civil Works
The civil works associated with urban development can have a major impact on the
quantity and quality of water leaving a construction site and provides significant
pollutant risks to local streams. During construction, heavy vehicles clear vegetation,
and compact and/or loosen soils, leading to an increase in erosive potential from
runoff and sediment mobilisation. The potential for inputs of suspended solids loads
to streams can be increased by a factor of 100 times or more (Pisano 1976; Wolman
and Schick 1967). For example, 300 tonnes of sediment loss per hectare per annum
have been estimated in runoff from catchments in SEQ’s Sunshine Coast (Pers.
Comm. Maurice Mathews - Sunshine Coast Regional Council).
Daniel et al., (1979) reported concentrations of TSS up to 60,000mg/L in runoff
originating from construction sites for large events. After development, TSS
concentrations are typically in the range of 50 mg/L to 500 mg/L (Duncan 1999).
This means that the TSS concentration expected from developed urban catchments
can be 100 to 1000 times lower. Schueler and Lugbill (1990) found that the average
concentration of TSS was 4,145 mg/L in stormwater runoff from uncontrolled
construction compared with 50 mg/L for post-construction. The study also found that
if erosion and sediment control measures were effectively installed, a significant TSS
reduction could be achieved (Figure 2.4).
Figure 2.4 - Effect of erosion and sediment control measures on TSS
concentrations in stormwater runoff
(Adapted from Schueler and Lugbill 1990)

The comparative TSS load which is washed from an urbanised catchment can be
very small compared to one under construction. Although WSUD systems may
provide protection to receiving waterways after civil works and housing construction
is completed, the development process itself may provide significant pollutant risks
to local streams. This could potentially outweigh the future benefits of WSUD.
2.7 Primary Pollutants in Urban Stormwater Runoff
Urban stormwater transports high levels pollutants from urban surfaces to receiving
waters (Bannerman et al., 1993; Dodson 2005; Duncan 1999; Mitchell 2006).
During storm events, rainwater first washes out atmospheric pollutants then, on
surface impact, picks up deposits on surfaces and transports into receiving water
bodies.
2.7.1 Total Suspended Solids (TSS)
Suspended solids are important not only because of the potential for direct physical
impacts on ecosystems (such as siltation), but also because high concentrations of
other pollutants are often associated with the suspended material. Suspended
sediment increases the turbidity of water, reducing light penetration and
photosynthesis. In case a large proportion of the TSS in the water is organic particles,
biological oxygen demand may increase as the microbe population increases due to
the abundant food source. The microbes use oxygen in the water as they respire and
which can lead to a lack of oxygen in the water for other organisms.
Pitt (1979) reported that TSS is sourced from dry atmospheric deposition, wear of
road surfaces and from vehicles, soil disturbance due to construction activities and
erosion of pervious areas by wind and water. Duncan (1999) conducted a wide
ranging review of published literature and found the event mean concentration
(EMC) of TSS in runoff from residential areas was 141 mg/L. Similarly, BCC (2004)
reported a mean TSS concentration of 151 mg/L from residential catchments in
Brisbane. TSS concentration in stormwater runoff from roofs is consistently reported
to be lower than for roads as seen in Table 2.4. Studies reviewed by Brodie (2007)

and Pitt and Voorhees (2000) reported that stormwater runoff from roofs had a mean
TSS concentration of 24 and 22 mg/L respectively.
Table 2.1 - TSS concentrations (mg/L) in stormwater from urban surfaces
(Adapted from Brodie 2007)
Study Roofs Roads Landscaped
Statistical mean of EMCs from the review of
research literature by Duncan (1999)
36 257
1
, 69
2
-
Residential area, Canada from Pitt & McLean
(1986)
13 242 100
Central Paris, France from Gromaire-Mertz et
al. (1999) – EMCs
29 93 74
3
Monroe, Wisconsin, USA from Wachbusch et
al (1999) – EMCs
18 60
4
, 64
5
75
6
Notes:
1. Classed as ‘High’ urban – greater than 67% residential development
2. Classed as ‘Low’ urban – less than 67% residential development
3. Landscaped areas included grassed and paved yards
4. Classed as ‘Feeder’ street – pavement runoff excluding kerb flow
5. Classed as ‘Arterial’ street – pavement runoff excluding kerb flow
6. Landscaped areas include lawns only
2.7.2 Nutrients
Nutrients are chemical substances vital to the development of plant and animal life.
In creeks and rivers, nutrients are needed for the growth of autotrophic organisms
(plants and algae) that provide energy and sustenance to aquatic ecosystems (Dodson
2005). However, when receiving waters become nutrient rich they cause
environmental harm such as excessive growth of plants, algae, and periphyton which
is called eutrophication. The adverse effects of eutrophication on ecosystems
include:
1. Loss of species diversity and a change in the dominant biota (Mason 2002).
2. An increase in turbidity, reducing light to aquatic biota leading to a change in
species composition (Mason 2002).
3. Anoxic conditions, which result from increased biological production, and
organic matter decomposition by bacteria, which remove dissolved oxygen
(DO) from water. A decrease in (DO) will severely limit the ability of
organisms to survive in aquatic environments (Dodson 2005).
4. The water environment looks aesthetically poor and can smell and taste bad.

5. Plant and animal biomass increases and algal booms occur (Mason 2002).
The peak of an algal bloom is often followed by a rapid decline in the algal
population, greatly reducing the oxygen content in the water body. Depending on the
algae present, the toxins released may be harmful to aquatic life and terrestrial life
which drink from these contaminated waters. These anoxic conditions also cause
pollutants, such as phosphorus and heavy metals, to be released from sediments to
which they were bound, causing water quality to decline further. The most common
nutrient pollutants entering urban waterways which cause eutrophication and algal
blooms are phosphorus and nitrogen (Taylor et al., 2005). Phosphorus is usually the
main limiting nutrient in fresh water while nitrogen is in estuaries (Kadlec and
Knight 1996; Dodson 2005; Mason 2002).
2.7.3 Phosphorus
Phosphorus principally originates from calcium phosphate minerals which weather
and release biologically available (reactive) phosphorous to the environment (Figure
2.5). Phosphorus becomes more mobile under neutral conditions, but is strongly
bound to cations such as Fe and Al in acidic conditions (Sonoda and Yeakley 2007).
In stormwater, phosphorous is present in inorganic (orthophosphate) and organic
forms.
Figure 2.5 - Simplified phosphorus cycle in the urban landscape

Orthophosphate (PO4
3-
) is readily available to aquatic organisms while organic forms
of phosphorus need to be decomposed by oxidising agents in the environment before
it becomes available for metabolic process (Dodson 2005).
2.7.4 Nitrogen
In stormwater runoff, nitrogen is composed of organic and inorganic nitrogen (Figure
2.6). Organic nitrogen consists of amino acids and nucleotides which are found in
living tissues and excretory products from animals (Dodson 2005). Ammonia is
formed from the excretory products of animals and from the bacterial breakdown of
organic material. Oxidation in the environment forms nitrite ions (NO2
-
) and
eventually nitrate ions (NO3
-
). The sum of nitrite and nitrate concentrations is known
as oxidized nitrogen. Total nitrogen include; ammonia, oxidized nitrogen and
organically bound nitrogen or kjeldahl nitrogen.
In urban runoff most of the particulate nitrogen is transported as organic nitrogen
(Harris et al., 1996). High loads of particulate organic nitrogen can be harmful to
aquatic ecosystems, principally by increasing populations of decomposing bacteria,
leading to low oxygen levels in the water column (Dodson 2005; Boulton and Brock
1999). As particulate organic nitrogen is decomposed it becomes available to
macrophytes and phytoplankton, resulting in eutrophication and algal blooms.
Figure 2.6 - Simplified nitrogen cycle in the urban landscape

2.7.5 Heavy Metals
Industrialisation in the past 200 years has seen the extraction and distribution of
heavy metals from their natural deposits. These natural deposits undergo chemical
changes and originate as fine atmospheric dust particles from industrial emissions
and from urban sources such as tyres and brake pads, pipes and roofs (Davis et al.,
2001; Duncan 1999; Gobel et al., 2007). Studies have reported that urban stormwater
runoff can contain significant quantities of a variety of heavy metals from both point
and non-point sources (Dodson 2005 Duncan 1999). Small concentrations of some
heavy metals are essential for aquatic ecosystem function. However, many are highly
toxic, particularly when they occur in high concentrations (Table 2.2).
Table 2.2 - Metals contained in urban stormwater runoff (from Dodson 2005)
2.7.6 Hydrocarbons
Polycyclic aromatic hydrocarbons (PAHs) are a major hydrocarbon species in urban
stormwater runoff known to cause a wide range of human heath problems and
diseases in aquatic organisms (Dodson 2005). Brown et al., (1985) showed that
significant hydrocarbon concentrations were associated with urban stormwater runoff
and are primarily attached to bed sediment in streams. Similarly Datry et al., (2003)
found high concentrations of hydrocarbons in the bed sediment of detention basins
treating stormwater runoff. Sources of PAHs in urban runoff include asphalt

leaching, particles from tyre abrasion, automobile exhausts and other combustion
processes and lubricating oils (Ngabe et al., 2000). PAHs are often found bound to
organic material and are attached to particulate matter rather than being in the
dissolved phase (Datry et al., 2003).
2.8 Pollutant Build-up
In urban areas, pollutant accumulation on urban surfaces is primarily from vehicular
traffic, industrial processes, construction, deterioration of surfaces, animal wastes
and spills particularly in industrial areas (Duncan 1999). These pollutants build-up
on impervious surfaces and are displaced to the local stream by the wash-off process
during a storm event (Egodawatta et al., 2006; Gobel et al., 2007). The pollutant load
being transported from urban surfaces by stormwater can be higher than in secondary
treated sewage effluent (House et al., 1993; Duncan 1999).
Build-up of pollutants on urban surfaces has been found to be strongly correlated to
the antecedent dry period and re-distribution processes (Egodawtta et al., 2006).
Sediment accumulation on urban surfaces usually reaches a threshold at which point
no significant build-up occurs due to redistribution processes, caused by wind and
vehicle induced turbulence which offsets deposition processes (Ball et al., 1998).
Egodawatta (2007) found that the pollutant build-up on roads is high for the first 2
days, while build-up on roofs was more gradual and took 7 days. After this rapid
increase it asymptotes to an almost constant value at a greatly reduced rate. Road
surfaces are the major source of pollutants in the urban environment. Typically
higher TSS concentrations are associated with stormwater runoff from roads, due to
their lower elevation and vehicular traffic (Duncan 1999).

2.9 Pollutant Wash-off
During storm events, rainwater first washes out atmospheric pollutants then, on
surface impact, picks up roadway deposits and flows into receiving water bodies.
Wash-off processes are principally driven by rainfall and runoff parameters (Duncan
1995). Rainfall intensity, rainfall volume, runoff rate and runoff volume all
contribute to the wash-off process (Egodawatta 2007). Rainfall events only remove a
certain fraction of pollutants. However, it is generally understood that with
increasing rainfall intensity and duration more pollutants will be washed from urban
surfaces (Sartor et al., 1974).
2.10 Water Quality Objectives
Moreton Bay Waterways and Catchments Partnership (2006) recently produced
WSUD guidelines for Southeast Queensland (SEQ) suggesting that implementing
WSUD practices should reduce environmental pollution based on a TSS reduction of
80%, a TP reduction of 60% and a TN reduction of 45%. No load objectives exist for
heavy metal reductions. However, the ANZECC (2000) Guidelines provide
concentration trigger values for both nutrients and heavy metals for the protection of
environmental waters.
Local and state authorities have realised the need to manage stormwater runoff to
protect environmental waters from degradation. Brisbane City Council has set water
quality objectives based on concentration values. These include TSS 15 mg/L, TN
0.65 mg/L, NH4 0.035 mg/L, NO3 0.07 mg/L, TP 0.07 mg/L, and PO4
3-
0.035 mg/L
(BCC 1999). Water quality objectives also exist for significant catchments in SEQ.

2.11 WSUD Philosophy
Traditional stormwater management aims to convey runoff as quickly as possible
through the local drainage infrastructure to the local stream. The more traditional
reasons for detaining stormwater in a large stormwater management facility at the
end of a subdivision have been to mitigate flooding. However, this practise of flood
mitigation is currently being questioned. Emerson et al., (2005) found that detention
basins reduce mainstream peak storm flows by an average of only about 0.3%, and
concluded that “on-site detention basins do not affect watershed-wide storm
hydrographs resulting from frequent storm events.” Unsurprisingly, detention
measures have proven ineffective in protecting stream health as the total quantity and
quality of water entering the stream remains unchanged (Argue 2004; Holman-
Dodds 2003; Booth and Jackson 1997; Hogan and Walbridge 2007).
In contrast, current stormwater management philosophy suggests that rainfall should
be managed where it falls, through the disconnection of impervious areas.
Stormwater should be infiltrated/detained through small neighbourhood bioretention
basins and/or small wetland areas which are aesthetically incorporated into the
landscape (Ellis 2000; Hogan and Walbridge 2007). The most innovative WSUD
application includes capturing and storing water and harvesting it for reuse and/or
infiltrating it to recharge groundwater (Argue 2004).
Argue (2004) suggests that the overriding principle in designing WSUD systems is
the ‘regime-in-balance strategy’. In practise this means utilising rainwater tanks to
harvest stormwater and building retention facilities to capture and infiltrate runoff. In
conjunction, these systems may have the potential to keep the post-development
water cycle similar to the pre-development cycle (Argue 2004; Walsh et al., 2009).
Argue (2004) is critical of detention systems as they do not generally reduce total
runoff volume. Also, they effectively increase the length of time that relatively high
flow of water discharges to the receiving water body. In contrast, retention systems
rely on natural processes (infiltration, percolation and evapotranspiration) and
stormwater harvesting via domestic and industrial use to reduce stormwater
discharge to environmental waters. Additionally, retention to facilitate infiltration is

promoted by Argue (2004) as it only takes 65% of the land required for detention and
is cost effective. Retention systems are usually easier to install as their depths are not
limited (as detention installations are) by street drainage invert levels, again reducing
installation costs (Argue 2004). For this purpose, ‘source control’ and utilising
infiltration techniques are seen as best practice for WSUD management by many
researchers (Argue 2004; Ladson et al., 2004; Van Roon 2007; Walsh et al., 2009).
However, ‘source control’ is a significantly complex design accomplishment. It
requires the designer to consider many WSUD devices such as rainwater tanks,
retention and detention systems, swales and wetlands, making the design process
complex (Ellis 2000). Researchers have noted the limitations of retention systems,
including clogging and potential groundwater contamination (Ellis 2000), poor
control of dissolved pollutants (Davis et al., 2006) and degradation of these devices
over time which may cause leaching of pollutants (Ellis 2000; Hunt et al., 2006;
Sharkey 2006).
Designing and successfully mitigating the impacts of urban runoff is made even
more difficult by the complex nature of stormwater quantity and quality. Stormwater
inflow to WSUD systems is highly stochastic in nature due to the intermittent nature
of rainfall and the varying shapes of the inflow hydrograph (Somes et al., 2000). The
physical changes made to urban catchments are also variable making it difficult to
determine how well structural methods will perform in attenuating urban runoff
(Egodawatta 2007). Similarly, the quality of urban stormwater is highly variable and
the generation and transport of pollutants in urban stormwater in not well understood
(Goonetilleke et al., 2005).
The inherent variability associated with urban stormwater quality gives rise to the
design of site specific structural measures (Goonetilleke et al., 2005). For example,
Taylor et al., (2005) reports that the dissolved fraction of nitrogen in stormwater in a
Melbourne study was 80%. The nitrogen composition of Melbourne stormwater was
different to international data and the authors urge caution when transferring
stormwater treatment designs from one location to another. This is consistent with
the findings of Goonetilleke et al., (2005) who reported that the majority of
pollutants at their study sites on the Gold Coast were in dissolved form. Goonetilleke

et al., (2005) also found that urban form significantly influenced the chemical
composition of stormwater. Due to this variation, they conclude that the effectiveness
of stormwater treatment designs would not be universal.
Experimental studies and modelling undertaken in Victoria have shown that the new
WSUD philosophy could provide a means to protect environmental values from
urban development (Fletcher 2002; Lloyd 2004). Out of this research, a model was
developed so that there was means of quantifying the water quality benefits provided
by WSUD technologies. The model was named the Model for Urban Stormwater
Improvement Conceptualisation (MUSIC) and is widely used throughout the
stormwater industry in Australia.
On the basis of encouraging research findings from southern sates in Australia and
the development of MUSIC, WSUD has been strongly promoted by the stormwater
industry and local governments throughout Australia. In Southeast Queensland
(SEQ) Moreton Bay Waterways and Catchments Partnership (known as Healthy
Waterways) has supported the uptake of Water Sensitive Urban Design, building
capacity and knowledge of WSUD in the region. Healthy Waterways Water by
Design Group has produced a number of detailed WSUD guidelines, accompanied
by training courses which have been running since early 2007.
Healthy Waterways have also developed guidelines for hydrologic management of
urban stormwater, noting a growing body of work showing the importance of flow
management on stream ecosystem health (Booth and Jackson 1997; Taylor et al.,
2005; Walsh et al., 2005; Walsh et al., 2009). This impetus saw the recently released
Southeast Queensland Regional Plan (Guideline No 7) requiring developers build
WSUD systems into their developments as a condition of approval for new
developments (QLD Government 2008).

2.12 Pollutant Removal Processes by WSUD Systems
An approach for ranking WSUD devices on their potential pollutant removal
efficiency has been suggested by Scholes et al., (2007). The authors suggest that
WSUD elements could be chosen by identifying the unit efficiencies of the various
removal processes. For this approach to be successful, key pollutant attenuative
mechanisms taking place in WSUD devices need to be understood. These include:
physical processes, physico-chemical processes and biological processes (Figure
2.7). These mechanisms are discussed in detail below.
Figure 2.7 - Fundamental unit process in relation to WSUD characteristics and
pollutant removal behaviour
(Adapted from Scholes et al., 2007)

2.12.1 Settling
Settling is the gravity settling of sediment particles to the base of a water column
(Ellis et al., 2004). Still conditions within ponds or wetlands promote settling.
Consequently, sedimentation ponds should be appropriately sized to reduce the
outflow rate to allow the settling velocity to occur. The size and weight of the
particle determines how fast it will settle. In SEQ, a target of settling 80% of
particles greater than 125 microns is recommended (Moreton Bay Waterways and
Catchments Partnership 2008).
2.12.2 Filtration
Filtration is physical sieving which removes particulate pollutants as they pass
through a porous substrate or hydraulic barrier (Ellis et al., 2004). Hence, the
potential for filtration to occur is considered to be the most effective treatment
mechanism in porous paving and porous asphalt (Scholes et al., 2007). Filtration is
also an important treatment mechanism for bioretention systems and recently
research has been conducted to refine filter media specifications to improve pollutant
removal (see Section 2.16.2).
2.12.3 Volatilisation and Photolysis
Volatilisation and photolysis are strongly dependent on surface exposure to the
atmosphere and are enhanced by open spaces within the treatment device (Scholes et
al., 2007). Photolysis is the process where molecules are broken down into smaller
units through the absorption of light and requires direct exposure to sunlight for
chemical decomposition to occur (Scholes et al., 2007). Volatilisation (vaporisation)
is increased by rising wind, air and water temperature and turbulent water conditions
(Bingham 1994).
2.12.4 Adsorption
Pollutants can be absorbed to media and vegetation, where charged particles are
electrically attracted to oppositely charged surfaces. Adsorption to suspended solids

is controlled by two main factors, the particulate surface area and surface
composition. The composition of the particulate surface affects the ability of
pollutants to adhere to the particle. For example, if the particle has a coating of
organic matter, organic pollutants will not adhere as readily to the particle (Scholes
et al., 2007). Increasing the contact time between the stormwater and vegetation/filter
media enhances adsorption.
The adsorption mechanisms of bonding and sorption are important for the removal of
heavy metals which adhere to organic matter, and the soil filter media through which
stormwater passes (Brydon et al., 2006). In bioretention systems, the type of filter
media can have a significant influence on the adsorption capacity. For example, sand
particles in a bioretention basin capture pollutants bound to particulates. However, due to
rapid infiltration of stormwater through the filter media, pollutants in solution and finer
particles are able to pass through the system (Sharkey 2006). Sand also offers limited
absorption sites to which pollutants can adhere. However, organic material and silt and
clay soils offer a high number of adsorption sites, and may capture pollutants associated
with smaller particle sizes.
2.12.5 Flocculation
Flocculation is the separation of solid particles from the water column by bonding
together in loose aggregates and fall out of solution by gravity. Particles are attached
to each other through electrostatic bonding. Chemicals and organic flocculants
produced by aquatic vegetation promote flocculation by causing suspended particles
to aggregate so they are heavy enough to settle out of the liquid.
2.12.6 Precipitation
Precipitation is mainly controlled by changes to the waters chemistry. Dissolved
elements such as metals form precipitates and settle out of solution depending on the
temperature and/or the chemical composition (pH and dissolved oxygen) of the water
(Scholes et al., 2007). Still conditions assist the potential for precipitation to take
place (Ellis et al., 2004).

2.12.7 Plant and Microbial Removal
Microbes take up nutrients and degrade pollutants in stormwater. The availability of
suitable attachment sites within the filter media and the aerobic and anaerobic state
of these sites determine if pollutants are being assimilated or degraded by microbes
(Scholes et al., 2007). Plants also contribute to the removal of pollutants from
stormwater and support the physical, chemical and microbiological processes that
take place (Read et al., 2008). Generally, the density and species of plants selected
will determine the effectiveness of the removal processes. Plants remove pollutants
through absorption and extraction of dissolved pollutants which become trapped in
the filter media as they pass through the plant-soil interface (Hatt et al., 2007).
2.13 Overview of WSUD Technologies
WSUD is considered best management practice (BMP) to mitigate the water quantity
and quality impacts resulting from urban development. Key WSUD stormwater
management practises include:
• detention of stormwater at source
• infiltration and retention of stormwater
• limitation of impervious surfaces
• disconnection of impervious surfaces and drainage infrastructure
Eriksson et al., (2007) suggest that WSUD systems should be classed into four
groups; filter strips (swales), infiltration systems (bioretention), storage facilities
(detention basins, ponds, constructed wetlands, rainwater tanks) and alternative road
structures (porous paving). WSUD systems can be classified as primary, secondary
and tertiary treatment measures in terms of their application and pollutant removal
efficiencies.
2.13.1 WSUD Treatment Measures
Primary treatment measures are important for protecting WSUD systems from litter
and clogging. These systems generally target litter and other gross pollutants, and
coarse sediments. In comparison, secondary treatment measures generally target

sediments, with partial removal of heavy metals, hydrocarbons and bacteria. Tertiary
WSUD techniques aim to remove nutrients, bacteria, fine sediments, hydrocarbons
and heavy metals (Figure 2.8 and Table 2.3).
Figure 2.8 - Three levels of WSUD treatment
(Adapted from IEAust 2006)
Table 2.3 - Devices within the WSUD treatment levels
(Adapted from Fletcher et al., 2004)
Primary treatment Secondary treatment Tertiary treatment
Trash rack Filter/Buffer Strips Constructed Ponds
Sediment Trap Grass Swales Constructed Wetlands
Gross pollutant Trap Extended Detention (Dry)
Basins
Urban Waterways
Oil Collector/Trap Sand/Bioretention Filters
Infiltration systems
The efficiency of WSUD treatment measures depend largely on hydraulic and
treatment capabilities, which vary with the treatment device. Once stormwater runoff
is stilled in a treatment device, coarse sediment may only take seconds to settle out,
whereas, dissolved pollutants may take several days. For the treatment of gross
pollutants and coarse sediment, a large amount of water can be passed through a
treatment device rapidly, with effective treatment of these pollutants. However, to

treat dissolved pollutants such as nutrients, a much smaller volume of water needs to
be treated as more time is needed for removal. To increase the detention time, an
increase in the size of the treatment device or a lower hydraulic loading rate is
required (Figure 2.9).
Figure 2.9 - WSUD treatment measures and acceptable hydraulic loading rates
(Adapted from BMT WBM 2007)
Among the structural WSUD systems, swales, infiltration systems and constructed
wetlands are the most commonly used treatment measures for stormwater
management, though seldom are they used in series (a treatment train). However,
current recommendations encourage placing WSUD elements in a treatment train to
achieve optimal flow management and pollutant removal (Lloyd 2004). Part of the
WSUD treatment train should include trash racks and gross pollutant traps to protect
‘soft’ engineering technologies from unsightly rubbish, early clogging and high TSS
loads (Ellis 2000).
Until recently, ponds and constructed wetlands have been favoured for stormwater
management as tertiary treatment devices. It is notable in Australian WSUD
guidelines, that water quality is still the main focus of practitioners rather than
hydrologic management. However, as discussed in Section 2.1, managing the
hydrologic impact of urban development is critical for ecosystem protection. Hence,
rainwater tanks and infiltration systems (which can provide both hydrologic and/or
water quality control) are seen as key treatment measures for ecologically effective
stormwater management (Walsh et al., 2009).

The following key WSUD technologies that are currently being incorporated into
residential developments in South East Queensland are reviewed below:
• Rainwater tanks
• Litter and gross pollutant traps
• Swales
• Bioretention Systems
• Constructed Wetlands
• Porous pavements
Particular attention is given to reviewing bioretention systems and constructed
wetlands as their performance was investigated in this research project.
2.14 Rainwater Tanks
In a WSUD treatment train, household rainwater tanks can play a key role in flow
management, provided they are connected to provide indoor and outdoor water
requirements (Coombes and Kuczera 2001; Walsh et al., 2009). This ensures that in
the majority of rain events, there is capacity in the tank to capture and store a
significant proportion of rainfall. Modelling of various scenarios in Croydon,
Melbourne confirmed that 5000 L rainwater tanks can reduce runoff frequency from
121 days per year to 59 days per year (Walsh et al., 2009). The use of household
rainwater not only provides important hydrologic benefits for the protection of
ecosystem health, but also reduces reliance and demand on a centralised water
supply. The Little Stringy Bark Creek project (started in 2008) is evaluating the
success of retrofitting rainwater tanks and bioretention systems into an urban
development in Melbourne, with the aim of restoring the ecosystem health of Little
Stringy Bark Creek, (http://www.urbanstreams.unimelb.edu.au/).
2.15 Swales (Vegetated filter strips)
Vegetated swales are usually designed to be broad and shallow, and used to convey
runoff in conjunction with or without underground piping systems. Swales generally
consist of vegetated zones that allow sheet flow through the vegetation (Figure 2.10).

Swales provide flow attenuation, temporary detention and removal of coarse to
medium sediments through infiltration through soil and filtration of shallow flow
though vegetation. Usually swales are designed to treat 80-90% of the flow volume
for a 3 month Average Recurrence Internal (ARI) event (Moreton Bay Waterways
and Catchments Partnership 2008).
Figure 2.10 - Swales in residential developments in SEQ
2.15.1 Swale Design
Swales are usually incorporated into the streetscape by having one on each side of
the road verge or down the centre of the road, with flush kerbing allowing road
runoff to flow into the swale. Incorporating trees and shrubs in grassed swales can
improve the aesthetics and define the road boundary. As vehicular traffic can cause
rutting and compaction in vegetated swales, planting with shrubs and trees, can act as
a barrier to traffic, and keep swales operating effectively.
An important consideration when designing a swale is to ensure that there is an even
distribution of inflow which is perpendicular to the direction of the swale (Moreton
Bay Waterways and Catchments Partnership 2006). Distributing the flow
appropriately in the swale will ensure that the water depth is kept to a minimum and
increase infiltration. Swales cannot be installed where the topography is too flat (as
they can become mosquito breeding areas) or too steep (as they can erode if flow
volumes and/or velocities are high).

Slopes should be between 1 and 4% grade (4 m in 100 m fall) to maintain stormwater
runoff at low velocities. Swales need to convey frequent storm events, such as the
ARI 3 month event. Manning’s equation in used for calculating the flow capacity of
a swale for preventing scour velocities. The flow velocity should be 0.5m/s for minor
flooding (ARI 2-10 y) and 2m/s for 50 years ARI (Moreton Bay Waterways and
Catchments Partnership 2006).
It is important to keep flow velocities low for public safety and to prevent erosion.
Consequently, swale designs must meet local requirements for velocity x depth of
water. Generally swales are limited to a length 40 - 50m as the velocity x depth value
can become too large. Most local authorities in SEQ have specified a velocity x
depth limit of 0.4m/s (Moreton Bay Waterways and Catchments Partnership 2008).
A discharge pit is placed at the point where the design discharge is attained and
runoff is conveyed to underground stormwater pipes.
2.15.2 Swale Effectiveness
Yu et al., (2001) evaluated the pollutant removal efficiencies of grassed swales with
synthetic runoff in Taiwan and Virginia and their work provides guidance for the
design of swales. Swales have improved performance with increased length. A slope
of 1% gives the best performance with a length of 50 m giving approximately 85%
removal of TSS, while slopes of greater than 3% had a TSS removal efficiency of
only 55%.
Yu et al., (2001) recommended that swales can be an important stormwater treatment
device for areas with low intensity storms. Yousef et al., (1985) recommended the
use of check dams which are constructed across the swale to temporarily detain the
water thus increasing infiltration as an important control to improve pollutant
removal. Yu et al., (2001) found that the inclusion of check dams to slow flows and
allow for greater infiltration was critical for improved pollutant removal.
In Australia, Lloyd (2004) investigated a grassed swale in Melbourne. Pollutant
removal efficiency was investigated by dosing the system with known concentrations
of TSS, TP and TN at two different flow rates (2 L/s and 4 L/s). The results showed

that a 35 m length of swale reduced TSS by 74 % for the 2 L/s flow rate, while
treatment efficiency reduced to 61% for the 4 L/s flow rate. A similar trend was
found for TP, which had removal efficiencies of 81% at 2 L/s and 61% for 4 L/s.
There was no effective treatment of TN by the grassed swale, which had removal
efficiency of 3% and -5% respectively for the low and high flow rate.
2.16 Bioretention Systems
2.16.1 Bioretention Treatment
Bioretention systems incorporate many designs, but can be defined as shallow dry
basins designed into the landscape to receive stormwater, and are commonly called
rain gardens (Figure 2.11). They are usually planted with shrubs, perennials or trees
which increase soil porosity and facilitate biological activity to remove pollutants
(Davis et al., 2001).
Figure 2.11 - Bioretention systems
Bioretention systems provide flow attenuation through detention/retention and
infiltration as stormwater percolates though a soil/gravel media. Bioretention can
play an important role encouraging groundwater recharge and enhance public space
(Argue 2004; Le Coustumer and Barraud 2007).
Bioretention systems can also be incorporated under swales (bioretention swales) to
enhance stormwater treatment (Moreton Bay Waterways and Catchments Partnership
2006; Leinster 2006; Lloyd 2004) as illustrated in Figure 2.12. For example, in
recent residential developments in Australia, such as Lynbrook estate (Lloyd 2004)

and Coomera Waters, swales have been incorporated with a filter zone of cross-
sectional area of about 1x1m to improve hydrologic and water quality treatment.
Figure 2.12 - Bioretention swale at Coomera Waters treating stormwater runoff
from the road and roofs
Pollutant removal from stormwater principally occurs through evapotranspiration,
absorption and biotransformation (Davis et al., 2006). As runoff percolates down the
soil mantle, it is treated and eventually drains to groundwater or is captured by a
perforated pipe and discharged to the stormwater network or stream. A point to note
is that the Wisconsin Department of Natural Resources has specifically
recommended not including an underdrain as potentially a large volume of water
could still be discharged to local waterways, thus reducing the effectiveness of the
system (Dietz and Clausen 2005).
Bioretention systems are designed to remove fine suspended solids and dissolved
pollutants (Hatt et al., 2007). It is typical of bioretention basins to incorporate an
overflow pit ensuring that water escapes rapidly enough to prevent overfilling of the
basin and subsequent damage to the infrastructure.

2.16.2 Bioretention filter media
The bioretention soil mix consists of varying depths of free draining granular fill
including mixes of gravel, sand, silt and organic matter (Hsieh et al., 2007). Recently
there has been a move away from using gravel based media to media with soil
properties (namely particle size distribution) similar to soil carried in stormwater
(Hatt et al., 2007). Filter media of the following ranges are considered to be optimal:
clay 5% -15%, silt <30%, sand 50% - 70%, assuming the following particle sizes
ranges, for clay <0.002 mm, silt 0.002 - 0.05 mm and sand 0.05 mm - 2.0 mm
(Moreton Bay Waterways and Catchments Partnership 2006). In contrast FAWB
(2008) suggest having less than <3% clay and silt as it can sustainably reduce the
hydraulic conductivity of the filter media (Table 2.4).
Table 2.4 - Recommended soil types for bioretention filter media
(Adapted from FAWB 2008)
Soil Type
% of filter
media
Particle size
distribution
Clay and Silt <3% <0.05 mm
Very Fine Sand 5-30% 0.05 - 0.15 mm
Fine Sand 10-30% 0.15 - 0.25 mm
Medium to Coarse Sand 40-60% 0.25 - 1.0 mm
Coarse Sand 7-10% 1.0 - 2.0 mm
Fine Gravel <3% 2.0-3.4 mm
Filter media controls the rate of stormwater infiltration and the water holding
capacity of the bioretention system and is a key process for determining the
effectiveness of the system (Figure 2.13). IEAust (2006) recommends that the
hydraulic conductivity of the filter media should range from 50-300mm h-1
, while
guidelines from FAWB (2008) recommend a range between 200-400 mm h-1
.
Hydraulic conductivity governs the rate of stormwater flow though the filter media,
as lower infiltration rates result in longer contact time between the stormwater and
the bioretention system and more opportunity is provided for treatment to take place.
Important characteristics of filter media which should be considered for pollutant
removal include; hydraulic conductivity, particle size and porosity, moisture, organic
matter, and nutrient sorption properties (Greenway 2008).

Figure 2.13 - Typical bioretention basin design
Care must be exercised when selecting the filter media. Culbertson and Hutchinson
(2004) found in a laboratory experiment that the bioretention media used had a
nutrient content that was too high to enable water quality improvement. Rather than
reducing nitrate and phosphorus concentrations, the leachate from the bioretention
columns was higher than the inflow concentration.
It is widely accepted that excess organic matter can be mineralised within
bioretention systems and result in nutrient export (Fletcher et al., 2007). A similar
finding was noted by both Davis et al., (2006) and Hunt (2003) who reported high
phosphorus content in soils resulted in high phosphorus concentrations in the
outflow. Hunt et al., (2006) recommended using soil with a Phosphorous index of
under 50, while FAWB guidelines (2008) recommend that filter media should have
no more than 100mg/kg of phosphorus and about 3% organic matter.
2.16.3 The Rhizosphere
The rhizosphere is the area of soil surrounding a plant root where microorganisms
such as bacteria, fungi and chemical processes in the soil are influenced by plant
roots. The plant and soil interactions in bioretention systems play an important role
in pollutant removal. Root growth is important in maintaining hydraulic conductivity
(Hatt et al., 2007). The rhizosphere provides important habitat for microbial
communities which consume nutrients, which is also a function provided by the roots
themselves (Figure 2.14).

The importance of the rhizosphere for bioretention effectiveness is becoming
apparent through recent laboratory studies (Fletcher et al., 2007; Greenway 2008;
Henderson et al., 2007). There is marked variation in pollutant removal in
bioretention systems depending on plant species chosen. Species currently
recommended for effective bioretention systems include; Carex appressa and Juncus
amabilis in Melbourne (FAWB 2008) and Callistmon pachyphllus and Dianella
brevipedunculata in SEQ. Research has found that these species have extensive root
systems that improve hydraulic conductivity and nutrient removal (Greenway 2008).
2.16.4 Bioretention Size
Design guidelines in the USA recommend that bioretention systems should occupy 5
- 7% of the drainage basin (USEPA 2000). In contrast Australian guidelines are not
specific but use modelling estimates of pollutant removal. These estimates are lower
than those from the USA, being approximately 1% of the drainage area for 90% TSS
removal in Melbourne and 3% for 90% TSS removal in coastal SEQ (Moreton Bay
Waterways and Catchments Partnership 2006). Recently released, ‘Deemed to
Comply solutions’ suggest that bioretention systems perform adequately if they
comprise of 1-1.5% of their catchment in SEQ (Dubowski et al., 2009).
The difference between the US and Australian WSUD sizing method is that the
Australian WSUD paradigm focuses on water quality improvement, with little regard
for effective hydrologic control. In comparison, in the USA there is a strong
emphasis on using WSUD for hydrologic control (capturing and treating 25 mm of
runoff from developments) in addition to water quality improvement (USEPA 2000).
For effective hydrologic control (reducing flow frequency and stormwater capture),
larger areas are required than when focused solely on water quality improvement.
2.16.5 Bioretention Effectiveness
There is growing interest in utilising bioretention systems for stormwater
management, particularly as ‘source control’ stormwater treatment devices (Argue
2004; Morzaria-Luna et al., 2004; Dietz and Clausen 2005; Le Coustumer and
Barraud 2007). This is understandable as these systems are aesthetically pleasing and

economical to incorporate into the urban environment (Morzaria-Luna et al., 2004;
Argue 2004). Although the number of theoretical and laboratory based studies
assessing the effectiveness of WSUD systems continues to grow, there are still
limited field studies undertaken to assess their effectiveness (Dietz and Clausen
2005).
Laboratory Studies
Column based laboratory studies using synthetic stormwater have been the principal
research method used to gain an understanding of bioretention system efficiency
(Davis et al., 2001; Hunt 2003; Kim et al., 2003; Davis et al., 2006; Henderson et al.,
2007; Hatt et al., 2007). These studies have shown good results for heavy metal
removal particularly for copper, lead, and zinc, with often >90% removal of metals
reported (Davis et al., 2001; Davis et al., 2003; Hunt 2003; Sun and Davis 2007; Hatt
et al., 2007; USEPA 2000).
Laboratory column studies have yielded varying results for phosphorus and nitrogen
removal. Hunt (2003) found 80% nitrogen removal rates in unvegetated laboratory
column studies with high (75%) removal of NO3
-
. In contrast, other studies (Hatt et
al., 2007; Henderson et al., 2007; Greenway 2008) found that soil based filter media
needed to be vegetated or it would act as a source, rather than a sink for some
pollutants, particularly for NO3
-
.
Barrett (2003) also found increased nitrogen export from sand filters treating urban
runoff which operate under similar principles to bioretention, but without vegetation.
Despite including vegetation in his experiments, Davis et al., (2006) still found that
NO3
-
removal was poor and consistently found outflow concentrations to be higher
than inflow concentrations.
To investigate ways to improve NO3
-
removal from bioretention systems, Kim et al.,
(2003) included an anaerobic zone and added organic carbon sources to anoxic sand-
packed bioretention columns. These systems resulted in high NO3
-
removal (70-80%)
due to denitrification, but increased concentrations of TKN and NH+
4.

Field Studies
Hunt (2003) and Davis (2007) explored the findings by Kim et al., (2003) in field
trials. They found variable NO3
-
removal, but did not find evidence that including a
dedicated anaerobic zone improved TN or NO3
-
removal in the outflow compared
with bioretention systems with conventional drainage (no anaerobic zone). Hunt
(2003) surmised that anaerobic zones occur throughout the soil media due to the
formation of a biofilm layer on soil particles which becomes anaerobic. Hence, the
inclusion of a saturated zone did not necessarily improve NO3
-
removal.
Davis et al., (2003) and (2006) assessed the performance of two bioretention basins
in Maryland, USA. In the first paper, the removal of metals is addressed with good
but variable removal between sites. The site at ‘Greenbelt’ reduced all metals by over
95%, while the site at ‘Largo’ reduced metals by 43-70%. In his second paper, the
results for nutrient removal are presented. For ‘Greenbelt’, phosphorus removal was
65% and at ‘Largo’ it was 87%, TKN removal was 52% at ‘Greenbelt’ and 67% at
‘Largo’. The removal for NO3
-
was poor, at only 16% and 15% for Greenbelt and
Largo respectively. The TN removal was 49% and 59%. As there were only two field
applications of synthetic stormwater to simulate rainfall events, the applicability of
these studies to real performance of bioretention basins is questionable.
Limited studies have assessed how well bioretention devices attenuate storm events,
and if this translates to a reduced outflow volume (Hunt 2003; Dietz and Clausen
2005). It is important to note that even if the inflow and outflow pollutant
concentrations are the same, load removal still occurs in bioretention systems if there
is a reduction in flow volume. Outflow volume reduction is a very important part of
bioretention function. Without outflow reduction, bioretention systems have been
found to actually increase pollutant loads (Hunt 2003; Hunt et al., 2006; Davis 2007;
Dietz and Clausen 2005; Sharkey 2006).
In recent years a few field studies conducted using natural rain events have reported
encouraging results for bioretention systems. A study at the Lynbrook Estate by
Lloyd et al., (2002) showed that under small intensity storm events, a bioretention
system successfully reduced flow by 51-100% and the loads of TSS, TP and TN
were reduced by 73%, 77% and 70% respectively.

Hunt et al., (2006) examined three bioretention systems in North Carolina under
natural rainfall events for pollutant removal capacity and hydrologic performance.
Over 2 years of monitored data, the studies found that unlined bioretention cells can
reduce the total outflow even in clayey soils, averaging 50% reduction. They also
reported that outflow concentrations of TN and TP exceeded those of the inflow
concentrations, with TN being 3 to 5 times higher and TP being 5 to 30 times higher.
However, due to reduction in flow volume (50%) total nitrogen load removal rate
was 40-50%. NO3
-
load removals were variable between 13% and 75%. Calculated
annual mass removal of zinc, copper, and lead were over 80%. However, iron
leached from the soil in one system which exported 13,000% more iron than the
inflow load. Phosphorus load reduction was variable ranging from -240% to 67%,
reflecting findings by Dietz (2007). Both researches noted that results were variable
and dependent on the concentration of phosphorus in soil used.
Dietz and Clausen (2005) evaluated the effectiveness of a small bioretention system
with an underdrain which treated roof runoff. In contrast to Hunt et al., (2006), they
found that though the bioretention system attenuated 99.2% of the runoff, nearly all
of this (98.8%) exited the system via the underdrain, resulting in little retention of the
stormwater. The bioretention system provided poor treatment of pollutants and
significant export of phosphorus was noted.
Davis (2007) reported on two field sites where bioretention effectiveness was
assessed under natural storm events. The combined results of the bioretention
systems showed that they successfully reduced the TSS EMC to 17mg/L in the
outflow which was a 47% load reduction. The TP EMC was reduced to 0.18mg/L
while heavy metals reduction was in the range of 57-83%. Though there was limited
data to assess TN and NO3
-
removal, the removal rates were found to be 62% and
83% respectively. Mass removals were higher than those based on concentrations,
due to flow reduction.
2.16.6 Clogging
The high volumes of suspended solids which can be carried in urban stormwater
runoff often lead to the clogging of bioretention systems. Lindsey et al., (1992) and

Mikkelsen et al., (1997) reported that the major cause of premature failure of
bioretention systems is due to clogging. This is supported by the review of
bioretention system performance by Ellis (2000) who found that 5 year failure rate
was 50% and that the cumulative TSS rate decreased by 70% within one year of
installation. There are still high failure rates for bioretention systems reported despite
the development of detailed guidelines for their design (Le Coustumer and Barraud
2007).
Hatt et al., (2006) hypothesised that it was possible that clogged bioretention systems
might actually increase the treatment efficiency. However, in a column experiment
they found that clogging did not improve removal efficiencies, and suggest that
prevention of clogging remains essential to extend the life of stormwater filtration
systems in terms of hydraulic capacity and treatment performance.
Leinster (2006) noted that the critical period for bioretention systems is the
establishment phase, due to poor sediment control during construction. A similar
finding was noted by Hatt et al., (2006) after discussions with developers and
operators. To manage sediment clogging of the bioretention basin during
construction, Leinster (2006) suggests that the surface of bioretention basins should
be covered with geofabric, topsoil (25 mm thick) and turf to protect the integrity of
the underlying filter media and prevent premature clogging.
Bioretention systems are typically designed to function effectively for at least 20
years before requiring desilting (Moreton Bay Waterways and Catchments
Partnership 2006). Similarly Davis et al., (2003) estimated from column based
laboratory trials that it would take approximately 20 years for metals to accumulate
within the bioretention filter media to reach toxic levels.
However, there is limited data on the longevity of bioretention systems in the field,
so it is still unknown how long these systems will remain efficient at removing
pollutants from stormwater. However, it is unlikely, given current evidence that
bioretention systems will function effectively for their entire design life (Ellis 2000
and Mikkelsen et al., 1997).

2.17 Constructed Wetlands
2.17.1 Constructed Wetland Treatment
Constructed wetlands are a common stormwater treatment device and in recent years
have grown in popularity over ponds for water storage and water quality
management. Unlike ponds, where macrophytes are usually absent, constructed
wetlands contain a diverse range of vegetation, including rushes, water lilies,
emergent reeds and submerged vegetation (Greenway 2005). However, because
constructed wetlands need to be shallow to support vegetation (less than 0.5 m), they
take up more land than ponds and their flood mitigation potential is relatively limited
(Figure 2.14).
Stormwater wetlands utilize a diverse range of processes to remove pollutants such
as sediments, nutrients, metals, hydrocarbons and pathogens. These mechanisms
include settling of particulate matter under gravity, filtration, adsorption, microbial
activity (nitrification and denitrification) and uptake of dissolved pollutants by biotic
communities (i.e. vegetation and bacteria).
Figure 2.14 – Constructed wetland cross section
(Adapted from Victorian Stormwater Committee 1999)
The vegetation in constructed wetlands can act as an important sink for nutrients
where plants, phytoplankton and periphyton uptake NO3
-
and PO4
3-
readily. It is also
one of the most important features in improving the hydraulic retention time (Jenkins
and Greenway 2005). Important characteristics of wetland design are:
the shape of the wetland

the hydraulic characteristics of the inlet and outlet
structures (pipes, outflow controls and overflow weir)
the wetland bathymetry
the vegetation type, density and spatial distribution
Constructed wetlands are essentially nutrient and metal traps and should not be
confused with natural wetlands whose primary role is to support a variety of biota
and not capture pollutants (Gaul and Goonetilleke 1999). As noted by Kadlec (1999),
wetlands are cyclic ecological systems. The sun (solar radiation) drives
photosynthesis and season cycles drive plant growth and litter fall (Figure 2.15).
When decomposition reaches its peak (typically in summer), nutrients taken up by
plants are countered by release (Kadlec 1999).
Hydraulic efficiency of the wetland system is critical for the successful treatment of
pollutants (Persson and Wittgren 2003). Many wetland management problems can be
attributed to poor hydrodynamic characteristics within the system (Jenkins and
Greenway 2005).
Figure 2.15 - Identification of the cyclic variables in a treatment wetland
(Adapted from Kadlec 1999)
Ideally the water entering a wetland stays as plug flow as it travels through the
wetland and eventually exits as a single plug of water. The time this plug flow
remains within the wetland is called the hydraulic retention time. A longer hydraulic
retention time allows more time for the plug of water to receive treatment, resulting
in improved water quality (Carleton et al., 2000; Jenkins and Greenway 2005). In

idealised plug flow conditions, water entering the detention system will only exit the
system when the entire permanent pool volume has been displaced by the inflow
(Figure 2.16) (Somes et al., 2000).
Figure 2.16 - Hydraulic efficiency of various wetland geometries; range is from
0 to 1, with 1 representing the best hydrodynamic conditions for
stormwater treatment
(Adapted from Melbourne Water 2005)
2.17.2 Wetland Design
For the sizing of constructed wetlands, BCC (1997) recommends that wetlands
comprise a minimum area of 1% and preferably up to 2.5% of their catchment.
Current WSUD design guidelines by Moreton Bay Waterways and Catchments
Partnership (2006) suggest sizing wetlands to be 5-6% of their catchment area is
more appropriate to achieve water quality objectives. However, in practise very few
wetlands are designed at such a large scale due to the land required. Typically
wetlands comprise 2% of the catchment in SEQ. For example, the constructed
wetland assessed in this study was 1.8% of the area of the catchment draining it.
Gaul and Goonetilleke (1999) make an important critique that a simple requirement
of 2% area is far too broad an approach. Wetland sizing is dependent largely on
climatic conditions and in subtropical areas such as SEQ, a more appropriate wetland
sizing maybe up to 5% of the catchment area or larger (Gaul and Goonetilleke 1999).
Similarly, Tilley and Brown (1998) report that for a 5 year design storm, effective

treatment of stormwater pollutants would require a minimum area of 2.3% and up to
10.8% in sub tropical areas, depending on the percentage impervious area and urban
class (i.e. residential or industrial). The differences in wetland size (percentage basis)
recommendations for constructed wetlands are due largely to the climatic conditions.
The ideal constructed wetland design is when the length to width ratio is large i.e. the
wetland is long and narrow (see Figure 2.16). This improves both the hydraulic
retention time and reduces the dispersion of pollutants, thus improving the treatment
efficiency (Persson and Wittgren 2003). Somes (2000) recommends incorporating a
permanent pool of 12% of total volume and using a riser outlet to double the
detention time of the inflow water. The author notes that risers allow more frequent
changes to water depth within the wetland, leading to a more diverse macrophyte
community.
2.17.3 Wetland Effectiveness
Wetlands have shown that they can provide adequate removal of pollutants, given
correct design (Rousseau et al., 2008). Brydon et al., (2006) reviewed constructed
wetland effectiveness and noted generally 50% removal rates for metals such as Cr,
Ni, Cu, Pb, and Zn. They also provide an estimate for the expected nutrient removal
in constructed wetlands of about 20%. However, pollutant removal percentages have
been reported to be higher with median removal found to be 76% for TSS 77% for
TN and 44% for TP (Shaver and Maxted 1993).
Carleton et al., (2000) found that though wetlands provided excellent water quality
improvement for some events there was high variability in the results. The authors
report TN removal was -193% to 99% and TP removal was 55% to 89%. Removal
percentages are dependent on wetland design, seasonal changes, temperature,
hydraulic residence time and loading rate, and are highly variable between systems
(Brydon et al., 2006; Gaul and Goonetilleke 1999; Rousseau et al., 2008).
The capacity for wetlands to continue removing incoming pollutants over the long
term is of significant concern. Gaul and Goonetilleke (1999) noted that as pollutant
inputs are increased, a steady state may be reached where system inputs equal system

outputs. Cartelon et al., (2000) noted the importance of including a sedimentation
pond (forebay) before the wetland proper, as it can improve constructed wetland
performance and extend its life, thus allowing periodic cleanout without disruption to
vegetation. Re-suspension of pollutants is also a concern and during dry conditions
water birds and wind create turbulence in the water, which is found to increase the
level of TSS in the outflow during inter-event periods (Greenway 2005).
Moustafa et al., (1996) and Cooke (1994) both recorded consistent reductions in
phosphorus, but export of nitrogen from wetlands during high flow. The loading rate
of nitrogen into constructed wetlands has been found to have significant influence on
retention of TN with outflow, with nitrogen levels becoming highly variable with
loading changes. A nine year study by Moustafa et al., (1996) reported a TN
retention of 26% in a constructed wetland which received variable discharges of river
water. Over the period of the study, there were 16 episodes of nitrogen export due to
increased flow.
Cooke (1994) noted nitrate export during periods of high flow and suggested that
changes in soil pH result in desorption of ammonium, which is subsequently flushed
from the system. Wetlands have a great ability to denitrify nitrite to nitrogen gas,
which is lost to the atmosphere, significantly reducing TN. However due to plant
senescence, algae and other organic material produced in the wetland, outflows can
have a high load of organic nitrogen, which can be higher or similar to the inflow
(Phipps and Crumpton 1994). NH4 can also be higher in outflow from wetlands due
to the ammonification of dead organic matter. However, large reductions in NO3
-
and
PO4
3-
have been noted (Greenway 2005).
Similarly, Birch et al., (2004) found variable treatment by a wetland in Sydney
draining an urban watershed. Most metals were removed efficiently (ca. 60%
removal). However TN and TP removal was poor at 16% and 9% respectively. TSS
removal was also poor, being between 9% and 46% during moderate flow events,
while for high flow events the TSS load in the outflow was 98% higher than inflow.
Phipps and Crumpton (1994) suggested that the largest retention efficiencies in
constructed wetlands are for suspended solids followed by nitrate, though often

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