United States 

Environmental Protection 

Agency 

Office of Research 

and Development 

Washington DC 20460

EPA/600/R-09/128 

August, 2009 

Innovative 

Approaches for 

Urban Watershed 

Wet-Weather Flow 

Management and 

Control: State-of-theTechnology 

INTERIM REPORT 
 

 

EPA/600/R-09/128

August, 2009 

Innovative Approaches for Urban 

Watershed Wet-Weather Flow 

Management and Control: 

State-of-the-Technology 

INTERIM REPORT 

By 

Scott D. Struck, Ph.D., PWS 

Tetra Tech, Inc. 

350 Indiana St., Suite 500 

Golden, CO 80401 

A. Charles Rowney, Ph.D., P. Eng., D.WRE 

ACR, LLC 

184 Tollgate Branch 

Longwood, FL 32750 

and 

Linda D. Pechacek, PE 

LDP Consultants, Inc. 

2115 Chantilly Lane 

Houston, TX 77018 

FOR 

NATIONAL RISK MANAGEMENT RESEARCH LABORATORY 

OFFICE OF RESEARCH AND DEVELOPMENT 

U.S. ENVIRONMENTAL PROTECTION AGENCY 

CINCINNATI, OH 45268
 
Notice 

The U.S. Environmental Protection Agency (EPA) through its Office of Research and Development 

performed and managed the research described here. It has been subjected to the Agency‘s peer 

and administrative review and has been approved for publication as an EPA document. Any 

opinions expressed in this report are those of the author and do not, necessarily, reflect the official 

positions and policies of the EPA. Any mention of products or trade names does not constitute 

recommendation for use by the EPA. 

iii 
Abstract 

This report documents initial efforts to identify innovative strategies for managing the effects of 

wet-weather flow in an urban setting. It served as a communication tool and a starting point for 

discussion with experts. As such, the document is a compilation of literature reviews from prior 

work and of content based on interviews with experts, but stops short of a definitive final analysis. 

Investigations of wet-weather flow treatment approaches through source control (and treatment at 

the source) make up its setting. Innovative systems that treat stormwater as a beneficial resource 

through reclamation and reuse are also explored. The effort focused on practices and technologies 

that can be implemented at the urban watershed management and infrastructure interface to 

combine cost-effective, integrated solutions. The result is a document containing urban watershed 

wet-weather flow management and control approaches from a national and international 

perspective. 

Specific tasks included a global information search to identify wet-weather flow management 

approaches that represent the current state-of-the-technology. The topics were subjected to review 

by international experts who provided comment and feedback. This document is targeted for the 

user community of regulators, academics, consultants, and municipalities investigating options to 

control the high costs of water, wastewater, and stormwater management and treatment. Case 

examples are included along with conclusions and recommendations to guide future research, 

development, and demonstration initiatives. 

iv 
 

Foreword 

The U.S. Environmental Protection Agency (EPA) is charged by Congress with protecting the 

Nation’s land, air, and water resources. Under a mandate of national environmental laws, the 

Agency strives to formulate and implement actions leading to a compatible balance between human 

activities and natural systems. To meet this mandate, EPA’s research program is providing data 

and technical support for solving environmental problems today and building a scientific 

knowledge base necessary to manage our ecological resources wisely, understand how pollutants 

affect our health, and prevent or reduce future environmental risks. 

The National Risk Management Research Laboratory (NRMRL) is the Agency’s center for 

investigation of technological and management approaches for preventing and reducing risks from 

pollution that threaten human health and the environment. The focus of the Laboratory’s research 

program is on methods and their cost-effectiveness for prevention and control of pollution to air, 

land, water, and subsurface resources; protection of water quality in public water systems; 

remediation of contaminated sites, sediments and ground water; prevention and control of indoor 

air pollution; and restoration of ecosystems. NRMRL collaborates with both public and private 

sector partners to foster technologies that reduce the cost of compliance and to anticipate emerging 

problems. NRMRL’s research provides solutions to environmental problems by: developing and 

promoting technologies that protect and improve the environment; advancing scientific and 

engineering information to support regulatory and policy decisions; and providing the technical 

support and information transfer to ensure implementation of environmental regulations and 

strategies at the national, state, and community levels. 

This publication has been produced as part of the Laboratory’s strategic long-term research plan. It 

is published and made available by EPA’s Office of Research and Development to assist the user 

community and to link researchers with their clients. 

 Sally C. Gutierrez, Director 

 National Risk Management Research Laboratory 

v 
 

Contents 

Notice .......................................................................................................................................................................... iii
 

Abstract ...................................................................................................................................................................... iv
 

Foreword ..................................................................................................................................................................... v
 

Contents...................................................................................................................................................................... vi
 

Figures ........................................................................................................................................................................ ix
 

Tables.......................................................................................................................................................................... xi
 

Acknowledgements .................................................................................................................................................. xiii
 

Chapter 1. Global Perspectives on Wet-Weather Technology............................................................................... 1
 

References ................................................................................................................................................................ 3
 

Chapter 2. Integrated Water Management ............................................................................................................. 5
 

Water and Energy..................................................................................................................................................... 7
 

Challenges of Total Water Management .................................................................................................................. 7
 

Technological Innovation......................................................................................................................................... 7
 

One Approach - Water Balance................................................................................................................................ 7
 

Chapter 3. Framing the Challenges Ahead.............................................................................................................. 9
 

Notion 1: Prediction of the Future Needs More than Records of the Past............................................................... 9
 

Notion 2: Water Quality is Not an Adequate Basis for Evaluating Impacts and Solutions...................................... 9
 

Notion 3: Regional Planning is Not Always the Best Basis for Decisions. ............................................................ 10
 

Notion 4: Planning for the Long Term is Not Always the Best Choice. ................................................................ 10
 

Notion 5: The Control Technologies are Not All Proven. ..................................................................................... 10
 

Notion 6: The Ultimate Problem is Broader than Engineering.............................................................................. 11
 

Notion 7: Owner/Operators May Not be Able to Handle the Solutions That are Needed..................................... 11
 

Notion 8: Our Analytical Capability is Less Than What is Needed........................................................................ 11
 

Workshop Results: Innovative Approaches for Urban Watershed Wet-Weather Flow Management and Control:
 

State-of-the-Technology ......................................................................................................................................... 12
 

Overview............................................................................................................................................................ 12
 

vi 
 

Key Findings to Date ......................................................................................................................................... 12
 

Methods and Outcomes...................................................................................................................................... 13
 

Appendix A - Literature Review ............................................................................................................................... 1
 

Section 1: CSO Technology Development - Design and Operation of Sewerage Systems................................... 1
 

CSO Technology Alternatives................................................................................................................................... 2
 

Collection System Controls.................................................................................................................................. 2
 

Storage Facilities.................................................................................................................................................. 3
 

Treatment Technologies....................................................................................................................................... 3
 

Technology Combinations ................................................................................................................................... 4
 

References ................................................................................................................................................................ 5
 

Section 2: CSO Control – Real-Time Control and Storage Approaches .............................................................. 6
 

Real-Time Control.................................................................................................................................................... 6
 

Tanks and In-line Storage ........................................................................................................................................ 7
 

In-pipe Sediment Processes...................................................................................................................................... 8
 

Tank Processes......................................................................................................................................................... 9
 

References .............................................................................................................................................................. 10
 

Section 3: Watershed Management Strategies - The US Baseline...................................................................... 13
 

Historical Approach and Legacy............................................................................................................................ 13
 

Emerging Trends in US Practice............................................................................................................................ 14
 

Conservation Design .......................................................................................................................................... 14
 

Infiltration .......................................................................................................................................................... 15
 

Runoff Storage ................................................................................................................................................... 15
 

Runoff Conveyance............................................................................................................................................ 15
 

Filtration............................................................................................................................................................. 16
 

Low Impact Landscaping................................................................................................................................... 16
 

Recent US Policy Developments............................................................................................................................. 16
 

References .............................................................................................................................................................. 17
 

Section 4: Water Sensitive Urban Design (WSUD)............................................................................................... 18
 

Case Studies ........................................................................................................................................................... 20
 

Low Impact Development in Tyngsborough, Massachusetts, US...................................................................... 20
 

Cluster Development in Ipswich, Massachusetts, US........................................................................................ 21
 

A Holistic Approach to CSO Control: Green Streets, Ecoroofs, and Rain Gardens in Portland, Oregon.......... 22
 

References .............................................................................................................................................................. 22
 

Section 5: Litter Traps and Swales......................................................................................................................... 23
 

Litter Traps............................................................................................................................................................. 23
 

Swales..................................................................................................................................................................... 24
 

Case Studies ........................................................................................................................................................... 26
 

Swales for Stormwater Pollution Control in Northern Sweden ......................................................................... 26
 

BMP Developments in Scotland ........................................................................................................................ 27
 

Buffer Strip Development in the US .................................................................................................................. 27
 

Summary of Grassed Swale Research in the US................................................................................................ 28
 

vii 
 

 

 

 

 

 

References .............................................................................................................................................................. 30
 

Section 6: Rooftop Greening................................................................................................................................... 32
 

Green Roof Benefits................................................................................................................................................ 33
 

Water Quality..................................................................................................................................................... 33
 

Thermal Insulation ............................................................................................................................................. 33
 

Urban Heat Island .............................................................................................................................................. 34
 

Biodiversity........................................................................................................................................................ 34
 

Cost .................................................................................................................................................................... 34
 

Case Studies ........................................................................................................................................................... 35
 

Runoff Detention Effect of a Sedum Green Roof .............................................................................................. 35
 

Toronto Green Roof Study................................................................................................................................. 37
 

A Review of 18 Green Roof Studies.................................................................................................................. 38
 

Green Roofs in Sweden...................................................................................................................................... 39
 

Green Roof Research and Solutions in Belgium................................................................................................ 40
 

Stormwater Monitoring of Two Ecoroofs in Portland, Oregon, US................................................................... 40
 

Performance of Two Green Roofs in North Carolina, US ................................................................................. 42
 

References .............................................................................................................................................................. 45
 

Section 7: Porous Paving......................................................................................................................................... 47
 

Case Studies ........................................................................................................................................................... 48
 

Review of the Performance of Permeable Pavers in Australia........................................................................... 48
 

Effects of a Porous Pavement with Reservoir Structure on Runoff Water in France......................................... 49
 

Heavy Metal Retention by Porous Pavement in Germany................................................................................. 50
 

Monitoring Permeable Pavement Sites in North Carolina, US .......................................................................... 50
 

Testing Permeable Pavements in Renton, Washington, US............................................................................... 51
 

References .............................................................................................................................................................. 53
 

Section 8: Infiltration Trenches, Bioretention Systems, and Rain Gardens ....................................................... 54
 

Infiltration Trenches............................................................................................................................................... 54
 

Bioretention Structures and Rain Gardens............................................................................................................. 55
 

Case Studies ........................................................................................................................................................... 56
 

Heavy Metal Removal in Cold Climate Bioretention in Norway ...................................................................... 56
 

Two Bioretention Cells at the University of Maryland, College Park, Maryland, US....................................... 57
 

Rain Garden Performance in Haddam, Connecticut, US ................................................................................... 58
 

Performance of Several Bioretention Facilities in North Carolina, US.............................................................. 60
 

Implications of Bioretention System Implementation in Wisconsin.................................................................. 62
 

Mt. Airy Rain Catchers: Testing a Reverse-Auction Incentive in Cincinnati, OH ............................................ 62
 

10,000 Rain Gardens: Involving the Public in Combined Sewer Overflow Management in Kansas City, 

Missouri, US ...................................................................................................................................................... 63
 

References .............................................................................................................................................................. 64
 

Section 9: Rainwater Harvesting and Reuse ......................................................................................................... 65
 

Case Studies ........................................................................................................................................................... 65
 

Residential Area with Raintanks and Aquifer Recharge Area in Newcastle, Australia ..................................... 65
 

Inkerman Oasis, Development with Integrated WSUD Techniques in Australia .............................................. 65
 

........................................................................................................................................................................... 66
 

A Greywater System of Rainwater Reuse at the Royal Melbourne Institute of Technology (RMIT) in Australia
 

References .............................................................................................................................................................. 67
 

viii 
Figures 

Figure 1. A schematic of transitioning from stormwater only management to total integrated water 

management.................................................................................................................................8
 

Figure 2. Desirable design ranges for treatment measures and pollutant sizes (Melbourne Water, 2008). 

.............................................................................................................................................. A-18
 

Figure 3. The contextual significance of an outlet approach vs. a distributed approach (Melbourne Water, 

2008)..................................................................................................................................... A-20
 

Figure 4. Example of a litter trap structure good for high litter load areas (Melbourne Water, 2008). . A-23
 

Figure 5. Example of a litter trap for urban drainage system (Melbourne Water, 2008). ...................... A-24
 

Figure 6. Modeled total nitrogen removal of varying swale slopes and surface area (Melbourne Water, 

2008)..................................................................................................................................... A-25
 

Figure 7. Modeled total suspended solids removal of varying swale slopes and surface area (Melbourne 

Water, 2008)......................................................................................................................... A-26
 

Figure 8. A terraced roof garden building in Fukuoka City, Japan (MetaEfficient, 2008). .................. A-33
 

Figure 9. Inflow and outflow hydrographs for a sedum green roof and traditional roof. Green roof (black 

line) and traditional roof (grey line) runoff for (a) two-year average (b) “test” rainfall of 0.8 

mm/min for 22 min (c) rainfall event on August 2, 2002, (d) rainfall event, July 22, 2001. A-36
 

Figure 10. Typical cross-sectional layout of a green roof. ..................................................................... A-36
 

Figure 11. Schematics showing components and the sensor locations in the roofing systems (Liu and 

Minor, 2005)......................................................................................................................... A-37
 

Figure 12. Daily maximum (dark) and minimum (light) membrane temperatures on the green roofs. . A-38
 

Figure 13. Annual runoff of intensive green (int), extensive green (ext), gravel-covered (gravel) and 

traditional (trad) roofs as a percentage of the total annual rainfall. Data range (whiskers), 25% 

and 75% percentiles (box boundaries), and the median (line within box) are reported. ...... A-39
 

Figure 14. Rain (dark line) and runoff (grey line) and water storage on a thin green roof in Augustenborg,
 

Sweden. ................................................................................................................................ A-40
 

Figure 15. Storm peak intensity attenuation for the Hamilton West Ecoroof: (a) high intensity, short 

duration winter storm; (b) high intensity, short duration summer storm (Hutchinson et al., 

2003)..................................................................................................................................... A-41
 

Figure 16. Storm peak intensity attenuation for the Hamilton West Ecoroof: (c) low intensity, high 

volume winter storm; and (d) low intensity, low volume winter storm (Hutchinson et al., 

2003)..................................................................................................................................... A-42
 

Figure 17. (a) WCC green roof in Goldsboro, NC (April 2003). (b) B&J green roof in Raleigh, NC 

(August 2004) (Moran et al., 2006)...................................................................................... A-43
 

Figure 18. Monthly retention rates of the WCC green roof from April 2003 to September 2004. ........ A-44
 

Figure 19. Peak flow reduction of green roof runoff at WCC green roof on April 7, 2003................... A-44
 

Figure 20. Concentrations of total nitrogen (top) and total phosphorus (bottom) from April 2003 to 

September 2004 from WCC green roof runoff..................................................................... A-45
 

Figure 21. Reinforced gravel (top left), reinforced grass pavement (middle), and 90% impervious blocks 

with gravel (U.S. Environmental Protection Agency, 2000)................................................ A-47
 

Figure 22. An example of a tanked permeable pavement system being built (Frederico, 2006). .......... A-48
 

ix 
Figure 23. An example of a permeable pavement infiltration................................................................ A-49
 

Figure 24. Photographs of Cary (left), Goldsboro (center), and Swansboro (right) PICP sites (Bean et al., 

2004)..................................................................................................................................... A-50
 

Figure 25. Infiltration trench diagram (Akan, 2002).............................................................................. A-54
 

Figure 26. Construction of a bioretention cell: (a) excavation; (b) placement of underdrains and gravel 

envelope; (c) spreadng of soil media; and (d) planting vegetation at surface of bioretention 

bed. ....................................................................................................................................... A-55
 

Figure 27. Schematic of a bioretention/rain garden (Davis, 2008). ....................................................... A-56
 

Figure 28. Input and output hydrographs for University of Maryland bioretention facilities................ A-58
 

Figure 29. Precipitation, inflow, and outflow (underdrain) for one event, Haddam rain garden (Dietz and 

Clausen, 2005)...................................................................................................................... A-59
 

Figure 30. Bioretention cell C1 in Chapel Hill 8 months after construction (Hunt et al., 2006). .......... A-60
 

Figure 31. Hal Marshall bioretention cell 16 months after the cell was revegetated and the study 

commenced (Hunt et al., 2008). ........................................................................................... A-61
 

Figure 32. Inflow and outflow hydrographs for HMBC for January 13–14, 2005 rain event (Hunt et al., 

2008)..................................................................................................................................... A-62
 

Figure 33. Overview of water sensitive design elements at Figtree Place. ............................................ A-65
 

Figure 34. Inkerman Oasis Sand Filter................................................................................................... A-66
 

Figure 35. Schematic drawing of the proposed greywater system showing how the water is captured and 

stored for future reuse........................................................................................................... A-67
 

x 
Tables 

Table 1. pH, suspended solids (SS), and heavy metal concentrations in snow and melt water in three 

roadside swales in Lulea (March–April 2000)....................................................................... A-27
 

Table 2. Life-cycle costs of different types of roofs in Germany........................................................... A-35
 

Table 3. Comparison of Substrate layer depth and percent runoff of intensive green (int), extensive green 

(ext), gravel-covered (gravel) and traditional (trad) roofs. .................................................... A-39
 

Table 4. Summary of water retention and peak flow reduction for each research site........................... A-43
 

Table 5. Comparison of event pollutant loadings for porous pavement relative to a reference site....... A-49
 

Table 6. Heavy metal concentrations and percentage of metal retained in runoff from porous pavement 

infiltration of 50 years of equivalent loads compared to permissible limits for seepage....... A-50
 

Table 7. Hydrologic summary of results from Cary PICP site (Bean et al., 2004)................................ A-51
 

Table 8. Mean pollutant concentrations and factors of significance for Cary site (Bean et al., 2004). . A-51
 

Table 9. Pollutant summary for Goldsboro site (Bean et al., 2004)....................................................... A-51
 

Table 10. Mean concentrations of detected constituents from storm samples in 1996 and 2001-02. .... A-52
 

Table 11. Average total metal inflow and outflow concentrations from bioretention box..................... A-57
 

Table 12. Flow mass balance for rain gardens, Haddam, CT. Depth values are based on total rain garden 

area (Dietz and Clausen, 2005). ............................................................................................. A-59
 

Table 13. Reduction from Peak Inflow to Peak Outflow at HMBC (Hunt et al., 2008)........................ A-61
 

xi 
ANN Artificial Neural Network 

ASCE American Society of Civil Engineers 

ASTM American Standard Testing Methods 

BMP Best Management Practice 

BSD Better Site Design 

BOD Biochemical Oxygen Demand 

COD Chemical Oxygen Demand 

CSO Combined Sewer Overflow 

CSS Combined Sewer System 

CWA Clean Water Act 

DWF Dry-Weather Flow 

EMC Event Mean Concentration 

EPA U.S. Environmental Protection Agency 

EU European Union 

I/I Infiltration and Inflow 

LEED Leadership in Energy and Environmental Design 

LID Low Impact Development 

NPDES National Pollutant Discharge Elimination System 

NTU Nephelometric Turbidity Units 

ORD Office of Research and Development 

PICP Permeable Interlocking Concrete Pavement 

POTW Publicly Owned Treatment Works 

RTC Real-Time Control 

SCADA Supervisory Control and Data Acquisition 

SM Standard Methods 

SS Suspended Solids 

SSC Suspended Sediment Concentration 

SSO Sanitary Sewer Overflow 

SSP Stormwater Site Plan 

SUDS Sustainable Urban Drainage Systems 

TMDL Total Maximum Daily Load 

TN Total Nitrogen 

TOC Total Organic Carbon 

TP Total Phosphorus 

TSS Total Suspended Solids 

TWM Total Water Management 

UK United Kingdom 

US United States of America 

UV Ultra Violet 

WWF Wet-Weather Flow 

WSUD Water Sensitive Urban Design 

Acronyms and Abbreviations 

xii 
 

Acknowledgements 

An undertaking of this type requires the dedication and cooperation of a team. The technical direction 

and coordination for this project was provided by the technical project team of the Urban Watershed 

Management Branch, under the direction of Mr. Richard Field, PE, Task Order Manager. Special 

recognition is extended to Mr. Anthony Tafuri, PE, Branch Chief who provided critical editorial review 

and technical and managerial guidance at key points. Also acknowledged is the support during these 

investigations provided by Linda Pechacek, PE, who provided literature search, written content and 

critical review to the project, and by Christopher Rowney, who was a substantial help in tracking down 

references and remedying bugs in the text. Finally, the manifold contributions of the many authors and 

professionals who were contacted or cited in this work are acknowledged, as it is their efforts that 

underlay the discussion and advances contained in this report. 

xiii 
 

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Interim Report xiv October 09 
 

 

 

Chapter 1. Global Perspectives on Wet-Weather Technology 

The intent of this project was to explore emerging topics in wet-weather control, those that have passed 

the point of being pure research, but have not yet reached the point of common practice. A starting point 

for this effort was in the form of an earlier report produced by Rowney et al. (2009) which addressed 

advanced technologies in water resources and which provided a foundation. Some of the content in this 

work borrows from that foundation where it is relevant, but the primary interest was to extend the 

thinking in that earlier work into new areas. 

As the present work progressed, the authors found that the emerging topics do not provide an easy area to 

evaluate, because the zone of interest that lies between these two conditions is somewhat subjective in 

extent and is by definition a moving target. If a technology passes muster at a research level, and begins a 

shift towards common practice, it will if successful, eventually reach the point of prevailing practice and 

therefore move out of our zone of interest. Further, no objective benchmark as to what constitutes 

prevailing practice was found. Ultimately, it was concluded that discussions with prominent leaders in 

the industry coupled with an examination of the content emerging in global conference publications was a 

reasonable way to discover what might be termed leading edge notions in technology. Supported by 

reviews of the literature, including research journals, trade publications and internet searches, this 

examination gave rise to a number of instances where there are technologies or technical perspectives that 

seemed to be promising but that are new to prevailing industrial practice. Much of this search also 

focused on studies that contained cost information to provide comparisons of promising technologies. 

Time will tell whether or not these ideas become mainstream or simply fade, but in the mean time there 

are a number of intriguing notions as to alternative ways to approach wet-weather technology that have 

been selected for consideration. 

New technology emerges in various ways, but as the authors went through this process a number of 

drivers or pressures that seem to have prompted new thinking surfaced. Climate change, population shifts 

and economic pressures all have the potential to affect control requirements, design considerations, and or 

solution opportunities. Increases in precipitation could lead to overflows where none existed. Increases 

in population could do the same. Economic pressures, however, could reduce the ability to implement 

potential solutions or could make less costly solutions (and perhaps less effective) preferable. The 

technologies to detect flows or predict them, and the ability to manage data and communicate 

information, are also changing. Taken together, this means that the water resources world is in a state of 

flux, and the strategies for mitigating the impacts of wet-weather runoff in an urban environment are 

numerous. They are so diverse, in fact, that common practices on a global scale are as yet not a reality 

even though professional society interactions make it clear that common interests abound1

. Even a 

common vocabulary has yet to emerge. A recent attempt by a well-founded research program in France, 

1

 The recent 11 International Conference on Urban Drainage, held in Edinburgh in September of 2008, underscored 

this point. Participants from all over the world were present, and the authors observed first hand the spirit of mutual 

interest and divergent methodologies that are discussed in this report. 

Interim Report 1 October 09 
 

 

intended to develop a simple multilingual (French, German, English, Spanish) thesaurus illustrated this, 

because it was found that about one third of the technical terms identified were in common use in one 

language had no definable translation into another2

. The theoretical underpinnings may be common, as 

they are defined at their root by the physics of precipitation, hydraulics, chemistry and so on, but the 

realization of theory in practice is very divergent and seems to be a function of the chance effects of 

historical local needs and practices. 

This divergence complicates a task already made challenging by the diversity in technical solutions that 

can be employed. Wet-weather control systems can range from a city-wide rainwater catchment and 

reuse system such as that employed in Knittlingen, Germany, (Smrcka, 2007) to a community-based 

organization implementing an incentive program to install ultra-low-flush toilets in East Los Angeles, 

United States (US) (Dickinson, 2003). Stormwater management is regionally specific and runs the gamut 

from city-wide technological updates to single-family residential renovations. Even the goals of these 

systems and programs can vary substantially from one point to another. One perspective is that water 

management is intended to make water resources more sustainable. Another goal is to simply remove 

flood water with some reasonable degree of certainty. Yet another goal is to provide potable water. The 

intent of water management is clearly regionally dependent. 

Nevertheless, there are common underlying themes. Urban areas tend to increase imperviousness (at 

least, in unfrozen conditions) wherever the location, with numbers like 85% impervious coverage in 

commercial areas, 20% in residential areas (Novotny and Olem, 1994) typical, and imperviousness alters 

the natural flow regime of stormwater in predictable ways. Also predictable is the fact that during wet 

weather events, water becomes more polluted, increases in velocity, and cannot be slowly released into 

the ground before it reaches storm sewer systems and open waterways. These are issues that cross 

national and state boundaries and that do not change due to international political situations. Every 

country is faced with these issues. Whether they are priorities and how they express themselves depends 

on context, but the means of coping with them depends on physical principles. So translation of solutions 

from one point to another may be more a matter of inclination and prevailing practice than anything else. 

This means that solutions elsewhere may be new to the US and candidates for adoption here if they fit our 

regulatory, climatological and infrastructure needs. At the same time, it seems implicit in the above 

notions that developing countries focused on flood protection and water supply may have priorities that 

are not conducive to solutions of interest in the US, while those that are well developed may have a 

preoccupation with sustainability, habitat, aesthetics or other factors that are of material US potential. 

The detailed discussion in the following chapters probe this point further, but several examples of global 

priorities provide a useful backdrop. 

The United Nations has a division for sustainable development whose mission statement promotes 

“development that meets the needs of the present without compromising the ability of future generations 

to meet their own needs,” (United Nations, 2008; The Economist, 2007). 

Australia has in recent years had severe drought conditions with stormwater conservation developing out 

of necessity. This force has been strong enough that changes in behavior were prompted, including such 

simple social changes as altered practices in bathing, car washing, and other common activities. The 

result persists as a focus on conservation and resource management (Grubel, 2007). 

In Europe, stormwater control initiatives are being championed as part of what is called Sustainable 

Urban Drainage Systems (SUDS), also known as Water Sensitive Urban Design (WSUD) (SUDSWP, 

2007). Programs that use such technologies are only as strong as the partnerships forged to implement 

them. Scotland has recognized this as a necessity because there is no single organization with the control 

to implement successful SUDS (SUDSWP, 2007). A successful partnership between state and local 

2

 Personal communication, Bernard Chocat, Groupe de Recherche Rhône Alpes sur les Infrastructures et l'Eau., 

November 6, 2008. 

Interim Report 2 October 09 
 

governments, non-governmental organizations, city planners, and developers is necessary to ensure that 

technologies are implemented effectively and consistently. 

The United Kingdom (UK), especially, has seen climate change as its rallying cry for new technologies, 

including green infrastructure, to make its way into the mainstream of urban development (SUDSWP, 

2007). The British government has made commitments to address the serious future water issues that will 

arise from the impacts of global climate change. These commitments include conservation of water to 

keep it sustainably within its replenishment cycle, addressing diffuse water pollution problems, and 

managing existing and future of flood risks (SUDSWP, 2007). Again, the onus is on policymakers and 

local organizers to develop partnerships to implement these programs. 

More broadly, in the European Union (EU), these kinds of national efforts are backed by a broader 

program. The Water Framework Directive (2000) is a program beginning in 2002, and drives an 

integrated and far reaching approach to water management that is different from anything prevailing in 

US practice. This program alone is likely to drive research and evolution of practice in ways central to 

this research. 

The directive has been taking steps legislatively in the UK since 2003 on measures to ensure the health of 

local ecologies, drinking water, and particular habitats (SUDSWP, 2007). URBACT is a European 

program whose goal is to “develop exchanges between towns, disseminate expert knowledge across 

Europe and design the urban policies of the future” (URBACT, 2008). Scotland, Wales, and Northern 

Ireland have championed the Sustainable Development Commission, which acts as their Government’s 

watchdog on sustainable development (SDC, 2008). 

The conclusion is that there are differences in practice internationally, but that water across the world is a 

major preoccupation not just in developing countries but in well-established nations as well. The changes 

in practice across the globe are extensive and continuing, and the need to track these changes and seek out 

lessons learned or practical solutions that may be of use in the US is clear not only now but as a part of 

any future program. 

The following sections explore innovative approaches and design and operation of sewerage system that 

consider potential cost saving opportunities that have yet to be tried or proven or those that hold promise. 

Also included are opportunities for watershed management strategies that improve wet-weather flow 

(WWF) management in a cost-effective manner. Exploration of technologies will apply to new 

development, redevelopment and retrofit situations in US and International locations including reviews of 

case studies. 

References 

Dickinson, Mary Ann. 2003. Water Efficiency Case Studies from California: The reservoir that toilets 

built. California Urban Water Conservation Council, USA. 

The Economist, April 2007. The Big Dry-Australia's Water Shortage. Gale Group, Farmington Hills, MI, 

USA. 

European Commission. 2000. Official Journal (OJ L 327). EU Water Framework Directive. 

http://ec.europa.eu/environment/water/water-framework/index_en.html 

Goode, David. 2006. Green Infrastructure. Report to the Royal Commission on Environmental Pollution. 

http://www.rcep.org.uk/urban/report/green-infrastructure-david-goode.pdf 

Grubel, James. 2007. Parched Australia becomes a nation of water misers, Reuters. 

Interim Report 3 October 09 
 

Novotny, V. and Olem, H. 1994. Water Quality: Prevention, Identification, and Management of Diffuse 

Pollution. Van Nostrand Reinhold, New York. As cited in Dietz, Michael E., and J.C. Clausen. 2005. A 

Field Evaluation of Rain Garden Flow and Pollutant Treatment. Water, Air, and Soil Pollution 167: 123­

128. 

Smrcka, Karel. November 30, 2007. German Scientists Develop Water-Recycling System. Creamer 

Media’s Engineering News Online. http://www.engineeringnews.co.za/article.php?a_id=121541 

SUDS Working Party (SUDSWP). 2007. Sustainable Urban Drainage Systems – Setting the Scene in 

Scotland. 

http://www.sepa.org.uk/pdf/publications/leaflets/suds/setting_the_scene.pdf 

Sustainable Development Commission (SDC). Accessed August 4, 2008. http://www.sdcommission.org.uk/index.php 

United Nations Department of Economic and Social Affairs, Division for Sustainable Development. 

2008. About the United Nations Division for Sustainable Development. 

http://www.un.org/esa/sustdev/about_us/aboutus.htm. 

URBACT, European Programme for Sustainable Development. Accessed August 4, 2008. 

http://urbact.eu/no_cache/home.html 

Woods-Ballard, B., Kellagher, R., Martin, P., Jeffries, C. Bray, R. and P. Shaffer. 2007. The SUDS 

Manual. Classic House, London, UK. 

Interim Report 4 October 09 
 

 

 

 

Chapter 2. Integrated Water Management 

In recent years, it has become increasingly evident that the water problems of a country can no longer be 

resolved by the water professionals and/or the water agencies alone. The water problems are becoming 

increasingly more interconnected with other development-related issues and also with social, economic, 

environmental, legal, and political factors at local and national levels and sometimes at regional and even 

international levels. Already, many of the water problems have become far too complex, interconnected 

and large to be handled by any one single institution, irrespective of the authority and resources given to 

it, technical expertise and management capacity available, level of political support, and all the good 

intentions. 

The current and the foreseeable trends indicate that water problems of the future will continue to become 

increasingly more complex, and will become more intertwined with other development sectors like 

agriculture, energy, industry, transportation, and communication, and with social sectors like education, 

environment, health, and rural or regional development. The time is fast approaching when water can no 

longer be viewed in isolation by one institution or any one group of professionals without explicit and 

simultaneous consideration of other related sectors and issues and vice versa. In fact, it can be argued that 

the time has already come when water policies and major water-related issues should be assessed, 

analyzed, reviewed, and resolved within an overall societal and development context; otherwise the main 

objectives of water management, such as improved standards and quality of life of the people, poverty 

alleviation, regional and equitable income distribution, and environmental conservation cannot be 

achieved. One of the main questions facing the water profession is how this challenge can be successfully 

answered in a socially-acceptable and economically-efficient manner. 

In the 1990s, many in the profession began to appreciate that water problems have become multidimensional, multi-sectorial, and multi-regional and filled with multi-interests, multi-agendas, and multicaused. Also, the 1990s brought in the introduction of ‘watershed-based’ approaches and ‘low impact 

development (LID)’. Both of these issues require proper multi-institutional and multi-stakeholder 

coordination. The issue at present then, is how can this be achieved in the real world in a timely and a 

cost-effective manner? Often these approaches are defined through Integrated Water Resource 

Management or Total Water Management (TWM). 

The definition that is most often quoted at present for TWM is “a process which promotes the coordinated 

development and management of water, land and related resources, in order to maximize the resultant 

economic and social welfare in an equitable manner without compromising the sustainability of vital 

ecosystems.” 

The definition of TWM is an important consideration. When the definitional problem can be successfully 

resolved in an operational manner, it may be possible to translate it into measurable criteria, which can 

then be used to assess the degree to which the concept of integration has been applied in a specific case, 

and also the overall relevance and usefulness of the concept. 

Interim Report 5 October 09 
 

In addition, a fundamental question for which there is no clear-cut answer at the present state of 

knowledge is what are the parameters that need to be monitored to indicate that a water resources system 

is functioning in a TWM manner? In the absence of both an operational definition and measurable 

criteria, it is difficult to identify what constitutes an integrated water resources management at present, or 

how should water be managed so that the system remains inherently integrated on a long-term basis. 

Depending upon the viewpoint, TWM can mean the integration of: 

•	 Objectives that are not mutually exclusive (economic efficiency, regional income redistribution, 

environmental quality, and social welfare); 

•	 Water supply and water demand; 

•	 Surface water and groundwater; 

•	 Water quantity and water quality; 

•	 Water and land related issues; 

•	 Different types of water uses: domestic, industrial, agricultural, navigational, recreational, 

environmental, and hydropower generation; 

•	 Rivers, aquifers, estuaries, and coastal waters; 

•	 Water, environment, and ecosystems; 

•	 Water supply and wastewater collection, treatment, and disposal; 

•	 Macro, meso and micro water projects and programs; 

•	 Urban and rural water issues; 

•	 Water-related institutions at national, regional, municipal, and local levels; 

•	 Public and private sectors; 

•	 Government and non-gover nment organizations; 

•	 Timing of water release from the reservoirs to me et domestic, industrial, agricultural, 

navigational, environmental, and hydropower generation needs; 

•	 All legal and regulatory frameworks relating to water, not only d irectly from the water sector, but 

also from other sectors that have implications on the water sector; 

•	 All economic instruments that can be used for water management; 

•	 Upstream and downstream issues and interests; 

•	 Interests of all different stakeholders; 

•	 National, regional, and international is sues; 

•	 Water projects, programs, and policies; 

•	 Policies of all different sectors that have impli cations for water, both in terms of quantity and 

quality, and also direct and indirect (sectors include agriculture, industry, energy, transportatio n, 

health, environment, education, gender, etc.); 

•	 Intra-state, interstate, and international rivers; 

•	 Bottom-up and top-down approaches; 

•	 Centralization and decentralization; 

•	 National, state, and municipal water policies; 

•	 National and international water policies; 

•	 Timings of water release for municipal, hydrop ower, agricultural, navigational, recreational, and 

environmental water uses; 

•	 Climatic, physical, biological, human, and environmental impacts; 

• All social groups, rich and poor; 

•	 Beneficiaries of the projects and those who pay the costs; 

•	 Present and future generations; 

•	 All gender-related issues; 

•	 Present and future technologies; and 

•	 Water development and reg ional development. 

Interim Report 6 October 09 
 

 

Theab ve list, which is by no means comprehensive, o id entifies many sets or combinations that should be 

integrated under the aegis of TWM. Even at a conceptual level, all of these issues simply cannot be 

achieved. Therefore fundamental to future research would be a prioritization where the greatest potential 

for integration on a social, environmental, and economic level can be achieved. 

Water and Energy 

Consider the issue of water and energyinterrelationships. The water profession in the past has ignored 

energy for the most part, eve n though in many ways water and energy are closely interlinked. For 

example, water not only produces energy (hydropower), but also the water sector is a prodigious user of 

energy. Accordingly, in a country like India, hydropower accounts for slightly over 20 % of electri city 

generated, but the water sector in turn “consumes” a similar amount of India’s electricity. Furthermore, 

no large-scale electricity production, be it thermal, nuclear, or hydro, is possible without water. In 

countries like France, the biggest user of water is not agriculture, but the energy industry. Thus, it simply 

is not possible to consider water resources management in an integrative manner without reference to 

energy, or integrated energy resources management without considering water. 

Challenges of Total Water Management 

In the real world, integrated water resources management, e ven in a limited sense, becomes difficult to 

achieve because of extensive turf wars, bureaucratic infightin g, and legal regimes (like national laws and 

constitutions) even within the management process of a single resource like water, let alone in any 

combined institution covering two or more ministries which have been historic rivals. In addition, the 

merger of such institutions produce an enormous organization that is neither easy to manage nor con trol. 

It should also be noted that water has linkages to all development sectors and social issues like poverty 

alleviation and regional income redistribution. It is simply unthinkable and totally impractical to bring 

them under one roof in the guise of integration, irrespective of how it is defined. Such integrations are 

most likely to compound the complexities of the problems, instead of solving them. 

Technological Innovation 

Technology and adaptation of technologies are key components of many efforts within water sectors. At 

the conceptual level technologies such as models and forecasting systems are being improved, particularly 

as a result of advances in computer technology, to allow better predictions of temporal and spatial 

variations in the quantity and quality of available water resources. This may help to reduce uncertainties 

and risks in the use and management of the resources. Water saving technologies in irrigation (e.g. , drip 

irrigation), improved and cost-effective methods for the treatment and reuse of wastewater in industries 

and domestic systems, aquifer recharge technologies, human waste disposal systems that require no or 

extremely small quantities of water, and cheap but effective water purification systems for villages are 

other examples of promising innovations which can promote the sustainability of future water resources . 

One Approach - Water Balance 

The water balance approach promotes and facilita tes sustainable approaches to water use, land use and 

water resource management at all levels – from th e region to the household; and in all sectors – from 

domestic, resource, industrial and commercial, to recreational and ecosystem support uses. The water 

accounting methodology is based on an accounting or pre-development versus post-development wate r 

distribution weighed with other water resource needs. Water balances consider inflows and outflows 

from basins, sub-basins, and service and use levels 

Conceptually, the water balance approach is straight forward but may require a change in perception from 

stormwater management (waste product) to rainwater management (source of water resource) (Figure 1). 

Often though, some of the components of the water balance concept can be difficult to estimate or are not 

Interim Report 7 October 09 
Stormwater Management to Rainwater Management 

From Traditional to Integrated TWM 

 Drainage Systems Ö  Ecosystems` 

 Reactive (Solve Problems) Ö  Proactive (Prevent Problems) 

 Engineer-Driven Ö  Interdisciplinary Team-Driven 

 Protect Property Ö  Protect Property and Habitat 

 Pipe and Convey Ö  Mimic Natural Processes 

 Limited Consultation Ö  Extensive Consultation 

 Local Government Ownership Ö  Partnership with Others 

 Extreme Storm Focus Ö  Rainwater Integrated with Land Use 

 Peak Flow Thinking Ö  Volume-Based Thinking 

 

available. For example, groundwater inflows and outflows to and from an area of interest may be 

difficult to measure. 

Figure 1. A schematic of transitioning from stor mwater on ly management to total integrated water 

management. 

Interim Report 8 October 09 
 

Chapter 3. Framing the Challenges Ahead 

The foregoing chapters present potential opportunities for future research to innovatively manage urban 

WWF flows. To advance the body of knowledge and to communicate a consensus to the expert 

community to be polled in this project, it was necessary to go beyond a simple recitation of the literature, 

much of which is known to those experts, and establish a series of perspectives and questions that would 

guide a further and more forward-thinking discussion. This section provides a summary of the notions 

developed during this part of the process. The content of this section closely follows the form and 

sequence of a white paper that was developed for delivery at a forum of experts scheduled as a part of this 

project. These notions are then followed by the key findings from a technological perspective and a 

summary of workshop outcomes from participating experts. 

Notion 1: Prediction of the Future Needs More than Records of the 

Past 

An underlying need in water resource management is to predict the future and assess the consequences of 

change (for example to some developed condition). The literature reviewed made it clear that most 

analyses consider hydrologic systems to be statistically stationary. Changes to a watershed or drainage 

system are almost universally depicted in terms of changes in conveyance capacity, infiltration capacity 

and so on. The precipitation in the future, however, is assumed to be the same as has been experienced 

for decades, as are temperature, evaporation and so on. The literature describes in detail the various 

approaches for the interpretation of rainfall, flow and other records as a basis for projections of the future. 

We estimate such things as flood frequencies, geomorphic drivers and habitat characteristics on this basis. 

Yet it is now generally accepted that for some things, notably including water resources engineering, the 

past is becoming a poor predictor of the future. Changes in our climate are upon us, and a scan of the 

projections over the last decade makes it tempting to suggest that the predictions of our future are 

changing faster than the reality. 

The question that is therefore reasonable to explore is to probe whether or not current predictive methods 

are useful. If changes are going to affect the way precipitation is received, but precipitation is uncertain, 

what is the value of the simulation? On the other hand, if forecasting is to be eliminated, where does that 

leave us? What should we do instead? And if we do, how do we defend ourselves in a legal system 

wedded to the idea of precedents? Even if we get all that right, what do we do to predict the human 

response to the massive impacts we will experience, the movement of populations to high ground or 

cooler climates or some other preferred region? 

Notion 2: Water Quality is Not an Adequate Basis for Evaluating 

Impacts and Solutions 

The preponderance of the literature shows that climate change, for whatever reason, is upon us. Less 

obvious in discussions, but just as clear, is the implication that major changes in our ecosystems coming 

(a prospect that a moment's reflection on recent reports on the polar ice cap underscores). A large part of 

Interim Report 9 October 09 
 

 

our effort is targeted at preserving native species and the conditions that gave rise to them. It is generally 

assumed that such an ecosystem will be protected if water quality and quantity are preserved. If massive 

changes in climate or other factors are on their way, the question of water quality and quantity could 

become moot. So a position could be taken that water quality standards cannot truly be targeted at 

anything we are certain will come to pass. They then become abstract ideals, no longer reflective of value 

propositions based on habitat. That being the case, should we ensure that we invest our limited resources 

in pursuit of something more determinate, such as flood protection perhaps coupled with epidemiology, 

and limit our pursuit of wider objectives to truly gross pollutant capture? 

Notion 3: Regional Planning is Not Always the Best Basis for 

Decisions. 

For years it has been accepted that the more globally we plan and build drainage solutions, the better 

chance we will have to develop a generally optimum solution. The idea has been that by looking at 

interactions at a large scale, decisions can be better made at a small scale. Opportunities of scale are the 

essence of this idea. It seems reasonable, for example, to set recharge and runoff requirements for a small 

parcel based not on its local impact, but on the net effect of many such choices. It is also reasonable to 

question if this is always true, however. In some cases, it probably is. If one is considering major 

infrastructure elements that are only replaced or altered with difficulty, such as highways, regional 

planning may be essential. But in smaller scale problems it may not be. If the future is driven by 

indeterminate needs, where the population is mobile and solutions are evolving, then drainage 

requirements are inherently uncertain. In such a case, what is really accomplished with regional planning 

practices if we can not predict regional behavior? Add to that the recognition that meaningful regional 

planning implies solutions of regional extent, and we encounter the implication we will invest in advance 

of need, possibly far in advance. Do we then end up with structures and practices either undersized or 

never used? Should we push for local solutions locally planned and built, and recognize they not only can 

but must change in the future? 

Notion 4: Planning for the Long Term is Not Always the Best Choice. 

The above notions also directly raise the question of just how far ahead we should attempt to plan. If a 

system will change over a scale of years, what is the benefit of planning building for conditions 

anticipated to be operative over a scale of decades? Given that we are in a time of significantly changing 

needs and horizons, how can we justify major planning studies, and how do we make a convincing case 

for long term investments? A balance seems to be the point, here. Planning ahead makes sense. 

Planning beyond what we can predict is questionable. There is a need to explore the actuarial realities of 

working with long-term designs that have high risk of obsolescence. It may be that we should be thinking 

in terms of short-term solutions on the expectation that that they will be superseded, and plan and build 

accordingly. 

Notion 5: The Control Technologies are Not All Proven. 

As noted in the literature review (Appendix A), there is a range of physical devices currently popular, 

including such things as porous paving, green rooftops, hydraulic controls, and all manner of devices or 

methods intended to remove undesirable materials from effluents either at the point of discharge or 

somewhere in the catchment. Yet, at the same time, as we are building these things, we are still doing 

research on how they work and in some cases if they work. The question must be asked whether or not 

we truly know how these devices will work, and whether we are working largely based on assumptions. 

Results from the National BMP database (www.bmpdatabase.org) in particular make this a relevant 

question, because the evidence in that program is not persuasive that the BMPs we are installing are all 

effective. A look at long-term empirical evidence of BMP performance does not present a compelling 

Interim Report 10 October 09 
 

case that we can predict how these various things will perform. An examination of the literature shows 

that we are still at odds when it comes to fundamentals as basic as indicator organisms, their meaning and 

the way they are affected by BMPs. Should we therefore reconsider our design approaches, and build 

solutions with the expectation of immediate failure a central design principle? Rather than trying for 

designs that are robust, should we just make them easy to remove and re-build when we finally figure out 

how well they work? 

Notion 6: The Ultimate Problem is Broader than Engineering. 

Civil engineers are well equipped to plan, design and build a pipe network, or indeed any physical 

expression of a drainage infrastructure. It is uncertain that they possess all the skills necessary to deal 

with the wide range of biological, physical and institutional problems that are also part of the solution. 

Some industries with different skill sets are only marginally involved in planning and solution but might 

offer value if engaged in the solution process. As an example, control systems are increasingly evident in 

water resources practice, and these inherently involve information technology, something not always 

included on BMP planning teams. As another example, industrial engineers think in terms of facility life 

cycles as a start point, not a collateral factor, and this might be a useful mind set in the future planning 

problem. It may be that progress might be found outside the bounds of traditional engineering as applied 

to this problem. 

Notion 7: Owner/Operators May Not be Able to Handle the Solutions 

That are Needed. 

It is reasonable to question if the future solutions, those that are adaptive, are truly value-centric and an 

expression of what is known about stormwater imply service demands beyond the ability of those 

responsible for managing them. The day-to-day labor force may not have what it takes to manage the 

results. Will new solutions be hampered by the inability of regulators, constructors and managers to 

respond to new realities? Can we change our practices meaningfully if our pace of progress is limited by 

the adaptability of our service population? 

Notion 8: Our Analytical Capability is Less Than What is Needed. 

Delivering sustainable solutions truly requires the integration of widely varied technical considerations. 

To date, however, our sector has been marked by a mix of detailed assessments of compartments of the 

problem, and only cursory examinations of behavior in an integrated form. There are currently efforts 

under way to remedy this, notably in such things as the EU efforts embodied by the Water Framework 

Directive, but despite such examples, the problem still is approached piece meal. We offer for example 

the gulf between the groundwater and surface-water communities, physically two sides of the same coin 

but often isolated from each other on projects. 

The above questions are quite specific, but consideration of them leads to several points that need to be 

answered. These points, were to be addressed in an expert workshop and presented in the following 

questions: 

o What are the criteria by which new solutions should be judged? 

o What solutions are emerging? 

o Of the emerging solutions, which ones seem best positioned to fulfill the criteria? 

The results of these discussions are presented in the next section. 

Interim Report 11 October 09 
 

Workshop Results: Innovative Approaches for Urban Watershed WetWeather Flow Management and Control: State-of-the-Technology 

Overview 

The overall project objective is to document a range of innovative and emerging technology and 

management strategies for dealing with urban watershed management control and failing infrastructure 

from within and outside of the US. The intent is to establish areas where external information can benefit 

EPA research. This includes gaining an understanding on developing priorities, research breakthroughs 

elsewhere, potential overlaps or duplications, and common needs. Specific technologies and topics that 

can be implemented or researched for implementation in the US are targeted. 

The proposed approach for obtaining this information included: a worldwide search and review of the 

literature, the convening of an (or multiple) international forum to supplement the literature and provide 

additional case examples, and provide a report of forum results and findings that consider the state-of-thetechnology literature and case studies. 

Interest from the international community in the project was high. Sessions designed to elicit information 

from experts in the field were well attended. There was unforeseen synergy with other agencies, and 

matching funding was applied to this work by European sources (see below). The work is still under 

way, but findings of value have emerged and will be consolidated and documented over the next months. 

As well as research reports, journal papers in association with several European entities are planned. 

Key Findings to Date 

Researchers and practitioners in the field contacted to date have shown a pervasive interest in revisiting 

some of the principles of planning and design now applied in water resources engineering. A finer 

granularity for decision making, adaptable solutions and technologies, new targets (shifting away from 

chemical parameters to ecosystem based evaluations) and methods that address the inherent uncertainty of 

predictions in an evolving multivariate context are all high priority items. 

Specific technologies that have emerged as areas of interest include: 

o	 Intelligent materials – pipes and tanks with built-in sensing and communications links that 

enable auto-detection of leaks or other conditions of interest so as to support proactive and timely 

management. 

o	 Virtual management systems – software visualization and manipulation tools that enable 

managers and operators to make better founded decisions in real time, by facilitating the retrieval 

and manipulation of data in a more comprehensible way, including such things as 3D 

visualization and virtual manipulation of system components. 

o	 IT/IM data and decision systems – technologies that enable secure and reliable storage, retrieval 

and manipulation of data, including and real-time prediction of intervention scenarios, so that 

response lags and data degradation are minimized and management functions improved. 

o	 Emerging detection and response systems – technologies that enable real-time monitoring of 

water quality constituents and/or surrogate parameter measurement rapidly and accurately enough 

to enable real-time control based on ambient conditions. 

o	 Control algorithms suitable for uncertainty analysis in a water system – mathematical 

methods and associated software that enable decision making in real-world systems where 

Interim Report 12 October 09 
 

conditions, targets and system responses are only approximately known, improving on existing 

predominantly used pseudo-deterministic methods that are only weakly able to deal with 

uncertain systems. 

Methods and Outcomes 

The approach used in this work has been to i) build on earlier EPA ORD Advanced Topics research 

(which among other things evaluated emerging needs with a focus on CSOs), ii) complete a literature 

review to establish a body of knowledge that contains state-of-technologies that have associated data 

from outside and within the US, and iii) conduct workshops to enable focused review that is beyond dayto-day preoccupations. Two workshops have been held, with a total attendance of 89 international 

experts. One workshop was held in Edinburgh, Scotland and one in Lyon, France in September and 

November of 2008, respectively. Discussed below are pre-workshop ideas that were used to spark debate 

in these forums, and synopses of the outcomes of each. The sequence was such that the results of the 

Edinburgh workshop were made available to participants in the Lyon workshop. 

The two-day Lyon Workshop was well attended by notable engineers and researchers, government 

directors, planners, architects, sociologists and water operators from France, the UK and other European 

representatives, as well as many experts from the US. To facilitate a focused discourse on emerging 

evolutions of urban water management at the Lyon workshop, a document detailing new concepts in 

sustainable urban water management was provided to the participants. Workshop objectives focused on 

identifying a clear vision of the stakes and difficulties in surmounting probable evolutions of urban water 

management. Reflections from workshop discussions focused on eight proposals to enhance successful 

implementation of water sustainability strategies. These proposals included: 

• Restoring water visibility in the city 

• Designing adaptable and time-sliding planning procedures 

• Integrating solutions’ diversity, redundancy and adaptability 

• Analyzing, understanding and integrating individual and collective behaviors 

• Defining new multi-objective and multi-criteria assessment tools 

• Taking into account the global cost of economic assessment 

• Taking advantage of the cultural and urban related opportunities 

One prominent theme woven throughout the workshop was the acknowledgement that future planning is 

predicated on an analysis of the past. This planning process is typically rigid and straightforward, and 

does not allow for adaptable solutions, which are needed to meet present challenges, such as predicting 

climate change for long term implementation of stormwater and wastewater systems. Feedback 

mechanisms were proposed within the planning process that integrated uncertainty in planning 

procedures, allowing for structured review as knowledge and forecasting abilities evolved. Rather than 

precisely defining the works to be constructed, it was proposed that the planning process be more 

strategic in nature, whereby solutions are designed that enable the time required for implementation of 

solution(s) to be shortened. Solutions could include a shorter life span or diversified solutions that 

include redundancy and a higher exchange of information, thereby facilitating adaptability. 

Another underlying concept related to ‘adapting the organizations’ whereby multidisciplinary project 

teams or simultaneous engineering groups are the norm, as opposed to the current partitioning of technical 

services. Such a notion integrates new indicators or systems of reference. Such evaluation criteria would 

be measurable in a continuous manner (cyclic or regular) to assess the relevance and efficacy of the 

strategy implemented. The real costs of solutions could be better assessed if such elements as investment, 

Interim Report 13 October 09 
 

 

operation and amortization as a function of effective service life were integrated into a cost evaluation 

system. In addition, developing new uses of existing assets should be considered. Real time management 

tools offer new horizons in this topic. Time will be needed to shift from the current planning process 

system toward the development of dynamic management systems. Transition management strategies are 

needed so that the system functions appropriately in the intermediate period. 

Results of the workshops were positive. Interaction and engagement by both assembled communities was 

forward thinking, intense and substantial. In both cases, interest in follow-up discussions was substantial. 

Two sponsors expressed interest in this kind of follow-up, including the Pennine Water Group in the UK, 

and GRAIE in France, with the France workshop to become a reality. It is a significant indicator of the 

value of this initiative that was perceived by other agencies to commit to matched funding. This was 

made possible for the Lyon effort by the French Water Agency, INSA of Lyon, the Rhone-Alps Region, 

and GRAIE. A multiplier effect on EPA ORD funds was therefore achieved. 

The outcome of this effort is still being assembled, but some of the early findings are that there is a very 

substantial need for improvements in practice in a number of areas and for implementation or application 

of existing technologies on other areas. These include a listing of change agents likely to affect practice 

in water engineering: 

o	 Regional impacts of climate change 

o	 Changing demographics and their impact on service infrastructure needs 

o	 New control technologies 

o	 New control targets (those beyond quality and quality, such as ecosystem sustainability and other 

factors) 

o	 Prevailing infrastructure development competencies (construction capability, maintenance 

limitations, service industry preparedness etc.) 

o	 Granularity (the appropriate size of planning, design and operating units) 

o	 Materials technology (intelligent materials, alternative materials etc.) 

o	 Information technology (control systems, predictive systems, sensing technology etc.) 

Specific findings from the groups involved included: 

•	 Reconsider our basic approach to setting targets: We desire minimization of costs and ecosystem 

impacts, but focus on surrogate water quality indicators that may not safeguard these things. 

•	 Climate change provides stormwater practice with a most important possibility for new targets 

and procedures: For example, heat island effects and green buildings/infrastructure ideas are 

increasingly important. We should consider a microclimate focus as we set objectives for control. 

•	 Reintroduce planners and architects to natural ecosystems: We need a systematic way to move 

forward that incorporates these ideas in our thinking. 

•	 Incorporate redundancy: We are currently forced to design without building in redundancy. Our 

approach to optimizing the design results in a bare minimum standard without redundancy 

provided in the system. 

•	 Introduce urban sociology into the curriculum: The success and failure of solutions can depend on 

social reactions that engineers are not well equipped to motivate and manage, so specialist skills 

in this area may be valuable. 

Interim Report 14 October 09 
 

 

•	 Aesthetics: Should these be a part of the criteria, as the outcomes clearly have an aesthetic 

impact? Ugly parking lot ‘holes in the ground’ as opposed to water features with intrinsic 

aesthetic value were cited, as was the impact of perceived value on outcomes. 

•	 Economic Equilibrium. As change is pursued, it should be guided by economic equilibrium, so 

that a balance is achieved. 

•	 Tailored solutions: There is no one-size-fits-all solution in water management. Solutions need to 

be developed and designed against local requirements as opposed to some sweeping set of 

national criteria. 

•	 Some specific technical development opportunities were also elicited. 

•	 Pipe as indicator as well as network element: It may be possible to incorporate intelligent 

materials and sensing systems in pipe networks so that they have a dynamic ability to sense and 

control flows. Implementing this might be different in retrofit and new systems, but it would in 

principle be possible either way. 

•	 Capitalize on available IT and model capabilities: Instead of building a pipe network system and 

letting it sit statically, with uniform operating capabilities, move to individual operations. This 

could happen all through the treatment train. Some of this is known to be possible, but not widely 

adopted; for example real-time control. It was voiced that the idea might have relevance in 

agricultural contexts as well as urban drainage systems. RTC scalability was discussed– Micro vs. 

small vs. integrated systems approaches are possible, but not often considered in the urban 

context. It might be possible to tailor the size to the application (intelligent house with IT linked 

solar elements as an example). 

•	 Emerging contaminants (bioactive components): These may be detectable in stormwater, but are 

not always addressed. Germany and Switzerland were said to be leading on the issue and have 

developed most of the data to date. Research is being conducted in water and wastewater; there 

seems to be no data collection counterpart or detection process for stormwater. 

•	 Multi-functional strategies are important: Consider more than a single control approach, but 

mesh treatment, re-use and optimization. There remain questions in performance and design, for 

example with biofilters and infiltration. Available computer simulation models to deal with these 

problems were questioned. The need to address solutions over wider scales was noted. The 

implication of wider temporal and spatial scales that would accompany this shift was 

acknowledged. 

•	 Virtual asset management/materials repository: The technology exists to build virtual 

management systems that provide an expanded ability for managers to ‘see’ the system as a 

whole and make better informed decisions. This is completely possible with present technologies 

and applications, but not in the forefront of our thinking. We still tend to build systems and 

manage them at the inlet and outlet, with little consideration of what lies between. This could be 

updated and changed if pursued. 

•	 Innovative detection technologies. It was noted that intelligent materials are harder to deploy 

than might meet the eye because of the challenge of getting an electric signal from a monitoring 

point to a point where it can be processed and interpreted. It may be, however, that alternative 

means of detecting problems could be used. Resonance techniques have been used, for example, 

to detect pipe fractures or irregularities remotely. 

•	 Water balance: It was suggested that technologists use a local water balance approach as a basis 

for design. Linkage between surface and groundwater was also noted as an important element of 

the problem that is at present poorly addressed. 

Interim Report 15 October 09 
 

 

General ‘blank slate’ options for development, we might do when there is no old infrastructure (i.e., new 

development) to contend with were also brought up for discussion. In some cases, this could also include 

areas where wholesale changes in infrastructure were a possibility. Points of discussion included: 

•	 Volume as a criterion: Consider shifting the paradigm from looking at flow rate to focus on 

volume, as this can better relate to quantity and loading, instead of only the flow rate paradigm . 

•	 Elimination of the network: It was questioned why we think in terms of containment and why 

have a system, when other options may be available. 

•	 Seek beneficial urbanization: Urbanize in a way that solves not just drainage and stormwater , but 

other problems. Seek ways to develop that consider contributions to e nergy, the food cycle, 

drinking water, and results in reduced footprint in all of these areas. 

• A water neutral city: This is a concept that focuses on water balance. It also enables an overall 

balance instead of a point to point balance, for example it might be prudent or environmentally 

best if one can choose to degrade one location inorder to preserve another and thereby achieve 

larger benefits over all. 

The next challenge is to shift from the very broad considerations that the collective experts developed, 

towardsa more focused set of actionable technologies and advances. This will be done in the next project 

stages. 

Interim Report 16 October 09 
 

 

 

Appendix A - Literature Review 

Section 1: CSO Technology Development - Design and 

Operation of Sewerage Systems 

There is an increasing pressure on municipalities, emerging from National policy related to water quality 

management, to reduce discharges of contaminants to receiving waters. This is a costly direction to take, 

although a crucial element of environmental management, because it touches on infrastructure nationwide. The systems that are now in place to collect and convey wastewater and combined flows is 

intimately associated with the municipal system, and it is disruptive and expensive to undertake major 

revisions of the constituent pipes, tunnels and tanks of which the system is comprised. In order to 

establish a basis for exploring the cost-effectiveness and environmental consequences of this area of 

activity, a review of these situations is necessary. It may be that truly meeting the newer discharge goals, 

total maximum daily loads (TMDLs) and other requirements can only be achieved with holistic system 

changes in collection and treatment approaches. If that is the case, there is a significant benefit to 

identifying treatment technologies that can accomplish what is needed more readily or with potential use 

and maintenance of older infrastructure components. This raises the question as to what alternatives 

should be considered in this research, and in particular what experience elsewhere might shed light on this 

question. As noted in Chapter 1, this work focuses on those technologies that are not in common practice 

but are beyond the point of basic research. This does not immediately imply that wholly new 

technologies are the only candidates for incorporation in this work. In some cases, the context of 

application may be considered to fit this area of interest. For example, BMPs are well established in the 

stormwater field, and have been applied in combined sewer systems (CSS), but it may be that there are 

techniques in the stormwater arena that can be re-purposed to positive effect in achieving combined sewer 

overflow (CSO) control. 

It did not prove to be a simple matter to find truly new ideas or applications in the area of CSO control. 

Much of the published literature seemed to echo ideas that had been in the literature for some time, with 

refinements case by case but not in fundamental approach. Perhaps this is because the fundamental 

technologies have been used for years, promoted by the fundamental problems they address. Such things 

as source controls, inflow controls, optimization methods (real-time control, storing combined sewage in 

existing sewers, or revision to facility operations), improved treatment technologies, and in-situ 

remediation such as may be accomplished by aeration and flow augmentation are well known and provide 

proven approaches. Each technology has differing potential for success when considered from the 

perspectives of regulatory compliance, cost-effectiveness, remedial efficacy, public acceptance, collateral 

impact, and other factors, but these are the outcome of the circumstances of the situation, not perhaps of 

novel thinking. The authors sought to find in the literature new or innovative ways of applying old 

technologies, such as storage of WWF as it accumulates, and bleeding it back into either treatment plants 

or other advanced facilities. High-rate treatment methods offered another possible area of investigation, 

and other avenues were sought. What was discovered is that the grey literature, containing claims of 

Interim Report A-1 October 09 
 

effectiveness that are sensitive to the case at hand or are of unproven or uncertain merit, makes it difficult 

to determine with confidence whether or not an innovative re-purposing of technology truly has merit. A 

recent systematic review of options in this area is not available, and this literature is difficult to apply in 

balance as a result. A review of practice, in terms of what has been accomplished, particularly outside the 

US, is presented. Given the recent experiences of the EU and the massive effort into analysis on 

watershed management options in that area, it was found that this topic was of particular interest and 

requires further exploration. 

CSO Technology Alternatives 

It is useful to provide a context from which to evaluate the potential application areas of candidate 

technologies. Four key principles are presented in CSO Policy to ensure controls are cost-effective and 

meet the objectives of the CWA (33 U.S.C. § 1251 et seq.). These principles include: 

1.	 clear levels of control that would be presumed to meet objectives; 

2.	 flexibility to consider the site-specific nature of CSOs and to determine the most cost-effective 

means to reduce pollutants and meet CWA requirements; 

3.	 allowance for a phased approach given a community’s financial capability; and 

4.	 review and revision of water quality standards and implementation procedures when developing 

CSO control plans to reflect the site-specific wet weather impacts of CSOs. 

Suggestions for evaluating control option alternatives include performance-based options, such as setting 

a maximum allowance of overflow episodes permitted per year, providing controls that achieve a 

designated capture rate, or expansion of the POTW secondary and primary capacity. 

Given that the final long-term CSO plan will become the basis for NPDES permit limits and 

requirements, the selected controls should be sufficient to meet CWA requirements. Examples such as 

enlarging a sewer trunk line or adding storage tanks would be an acceptable CSO control alternative. 

Both alternatives increase the storage capacity of the sewer system, thereby decreasing the sanitary and 

stormwater flow volume that could otherwise overflow prior to discharging into the treatment plant. 

Several EPA documents on CSOs have summarized characteristics of these complex systems, including 

impacts and a description of the resources spent and technologies used by municipalities to reduce 

impacts. It establishes a baseline of related data concerning sewerage management and describes typical 

technologies and operational practices to reduce CSO impacts. Summaries of major discussion topics 

follow. 

Collection System Controls 

Collection system controls maximize the capacity of the sewer system to store or transport wastewater 

through hydraulic control point adjustments to maximize system storage capacity while minimizing the 

volume of infiltration and inflow (I/I) into the system undergoing treatment. The controls may include 

maximizing flow delivered to the plant for treatment, disconnecting stormwater discharges into the 

collection system, developing a more effective system using real-time controls to monitor flow rates and 

more effectively manage the system’s storage capacity while maximizing the flow volume directed to the 

plant during WWF, and sewer system rehabilitation. 

Interim Report A-2 October 09 
 

 

Storage Facilities 

In-line or off-line storage options provide additional capacity when a sewer system is unable to transport 

or provide full treatment for WWF. In-line storage of WWF is provided within the sewer system and 

includes the use of flow regulators, in-line tanks or basins and parallel relief sewers. DWFs pass directly 

through these facilities. Flow regulators allow for in-line storage by adjusting the flow into or out of the 

facility during wet weather. In-line storage capacity can be supplemented by the installation of parallel 

relief sewers or replacing older pipes with larger diameter pipes. Field et al., (2004) note that areas of 

mild slopes provide the best opportunity for in-line storage facilities, while observing that this method can 

potentially increase wastewater basement back-up and street flooding. The mild slope may also promote 

sedimentation and debris accumulation within the sewer. The traditional solution to prevent solids 

deposition within the collection system is to have design wastewater flow velocities high enough to flush 

sediments and prevent solids accumulations within the pipe. 

Off-line storage facilities store WWF in near-surface tanks and basins or deep tunnel locations. Off-line 

facilities can be adapted to numerous site-specific designs and settings relating to basin volume, inlet and 

outlet structure, and disinfection process. Flows are routed around the off-line facility during dry 

weather, whereas during wet weather, wastewater discharges are pumped and/or flow by gravity into the 

storage facility. Overflows can arise if capacity is exceeded. The primary utility of the facility is storage 

of WWF discharges and treatment by solids settling when stormflow volume exceeds storage capacity. 

On-site storage at the wastewater treatment plant can also be used as a control where the capacity of the 

wastewater collection system exceeds that of the treatment facility. The two most common types of onsite storage are flow equalization basins and the conversion of abandoned treatment units, such as 

clarifiers or lagoons. 

In areas where in-line storage is not attainable or unavailable, the cost of creating off-line storage may be 

very high. The costs associated with on-site storage are typically lower than the construction of near 

surface off-line facilities because the on-site storage facility is typically located on land already owned by 

the facility. Expanding conveyance capacity is usually the most expensive storage development option. 

Treatment Technologies 

In those collection systems where WWF exceeds the sewer conveyance and treatment facility capacity, 

end-of-pipe controls may be used in lieu of storing excess flows. Different pollutants, such as solids, 

bacteria or floatables, use specific treatment technologies. The disinfection of excess WWF is used as an 

end-of-pipe treatment for microorganism removal, whereas vortex separators are used for solids removal. 

Given the assumption that dry-weather flows are treated at the wastewater treatment facility, these 

technologies are assumed to operate only during wet weather or storage dewatering conditions. 

Supplemental Treatment 

These technologies supplement treatment during wet weather conditions. An example of such a 

supplement would be the installation of a parallel treatment process at a wastewater treatment plant that is 

only operated during wet weather conditions. Potential supplemental treatment technology options for 

excess WWF include ballasted flocculation and/or chemical flocculation to accelerate the settling of 

solids, deep-bed filtration using anthracite and sand, and microscreens. These technologies must be 

dependable and able to respond to intermittent and variable flow regimes and influent pollutant 

concentrations. 

Plant Modifications 

Plant modifications to existing treatment process configurations or process operations can increase the 

facility’s ability to handle and treat WWF. Such examples include providing an even flow distribution 

Interim Report A-3 October 09 
 

between process treatment units (blending), baffle installation to prevent hydraulic surges in clarifiers, 

adding flocculants to accelerate suspended solids removal, switching a portion of flow delivery from the 

primary to bypass the secondary units, and switching from series operation during dry weather to a 

parallel operation of unit processes during wet weather. Performance evaluations are necessary to 

confirm whether additional treatment capacity developed for WWF blending may adversely impact the 

pollutant removal and treatment process for dry-weather flow. 

Disinfection 

The application of a disinfection process to CSO discharges has been limited, when compared to the 

disinfection unit process used in wastewater treatment plants. High flow rates and partially treated 

wastewater may adversely impact the disinfection process if the exposure of the disinfection agent to the 

wastewater undergoing treatment is reduced. Chlorine disinfection is the method most often used to 

disinfect WWF. Toxic residual chlorine and disinfection byproducts limit the usefulness of chlorine 

disinfection in those areas that have high organic solids in their effluent. It is suggested that UV light 

may be an alternative disinfection method as long as WWF receives some secondary treatment (to remove 

larger particulate matter) beyond settling that occurs during primary treatment. 

Vortex Separators 

Vortex separators are designed to separate and concentrate solids and floatables from the flow undergoing 

treatment. The separated effluent with floatables and with some solids removed can then undergo further 

treatment or discharged to the receiving stream. These separators have limited capability to reduce small 

and light particles and dissolved contaminants and should not be placed in a treatment train downstream 

of other units that provide the same or a higher level of pollutant removal. 

Low Impact Development Techniques 

Low Impact Development (LID) techniques can be used to attenuate stormwater runoff discharging into 

the sewer collection system, thereby potentially reducing the volume or occurrences of CSO events and 

capacity of downstream control facilities. LID controls provide runoff volume storage opportunities and 

include technologies such as porous pavement, bioretention facilities, rain gardens, green roofs and water 

conservation practices. Incorporating LID controls into the footprint of urban developments decrease the 

storage volume capacity required in sewer collection and CSO control. More of this technology will be 

covered later in this report. 

Technology Combinations 

Some technologies work well when applied together. Some of the combinations that have been suggested 

by Field et al., (2004) are: 

LID Designs Coupled with Structural Controls 

Both controls reduce the peak flow rate and quantity of runoff that enters the sewer collection system. 

The runoff volume and peak flow reductions allow for the size of downstream storage control structures 

to be reduced or eliminated. Again, see later chapter on Total Water Management (TWM). 

Disinfection Coupled with Solids Removal 

Numerous pollutants in wastewater discharges can interfere with and reduce the effectiveness of 

disinfection processes. These pollutants include high concentrations of BOD5, ammonia, and iron, which 

consume or prevent the disinfectant from interacting with the microorganisms. Larger solid particles can 

shield microorganisms located in the particle’s interior from the effect of all disinfectants, including 

chlorine, ozone, chlorine dioxide, and UV. 

Solids removal enhances disinfection by settling out and removing shielding particles and the clad 

Interim Report A-4 October 09 
 

pathogens, and reduces chlorine demand. Using effective solids removal controls can improve the 

performance of disinfection process units treating CSO discharges. Off-line storage facilities, vortex 

separators and supplemental treatment facilities have demonstrated additional benefit at removing solids 

out of the wastewater stream. 

Sewer Rehabilitation Coupled with Sewer Cleaning 

Sewer cleaning techniques should be conducted or at least considered before scheduling the rehabilitation 

of the sewer collection and retrofitting CSO control facility systems; so that needless and expensive 

infrastructure replacement is not implemented when simple maintenance and cleaning are all that is 

necessary. 

Real-Time Control (RTC) Coupled with In-line and Off-line Storage Tanks 

RTC technology is used to maximize flow to the treatment plant and storage within the sewer. Both 

outcomes serve to reduce the volume and frequency of untreated overflows. RTC uses operating rules, 

monitoring data, software (SCADA systems) to dynamically operate system components to optimize 

wastewater routing, treatment and storage. System components include weirs, gates, pumps, valves and 

dams. RTC is most often employed in sewers that have considerable in-line storage using large pipes 

designed for excess WWF. Off-line storage facilities, such as tunnels or basins, can also be operated by 

RTC. The dynamic operation resulting from RTC features optimizes the sewer storage volume available 

for excess WWF. 

References 

Field, R., Sullivan, D. and Tafuri, A.N., editors, (2004) Management of Combined Sewer Overflows, 

CRC Press LLC, Lewis Publishers (ISBN 0-56670-636-X) 

Interim Report A-5 October 09 
 

 

 

Section 2: CSO Control – Real-Time Control and Storage 

Approaches 

In principle, real-time control (RTC) and storage options provide a promising way to resolve CSO 

overflow problems. This is dependant, however, on a well developed capability to model the system, and 

that in turn depends on an equally well developed understanding of the mechanics that govern that 

system. A useful review of progress on modeling and the mechanics of in-line retention as a means of 

combating CSO releases that exceed regulated limits was recently provided by Ashley et al. (2002). As a 

part of this, they identified models suitable to represent the range of phenomena, but they concluded that 

no truly comprehensive model existed. They also evaluated solids buildup and behavior in the line and 

flushing effects at the plant, along with a number of other consequences of grit and material build up and 

removal along the length of the system. It was not concluded that the systems involved are fully 

understood. Nevertheless, it is clear from this work that a significant knowledge base exists and that 

modeling and control approaches might have merit as avenues of continued development. 

Real-Time Control 

One approach to RTC is to directly predict the ability of a plant to provide treatment adequate for a 

particular event, and to adjust flows to meet that need or otherwise cope with it. Recent work by 

Seggelke et al. (2008) introduced an integrated control approach. The intent is to reduce CSOs and 

collect early information on the plant’s critical process conditions. In their method, treatment processes 

are continuously monitored and models are used to predict the treatment capacity of a plant in contrast 

with the materials that need to be controlled. A controller is used to adapt the plant’s inflow rate 

according to that assessment. The intervention is rule-based. This means that the system is evaluated in 

detail, and responses to stimuli are determined a priori. Such a system uses the input conditions to 

determine the appropriate response based on the rules developed this way. In their particular application, 

Seggelke et al. (2008) determined that a fixed value of the maximum wastewater treatment plant 

(WWTP) inflow does not provide the best use of treatment capacity at the plant, which lends support to 

the need for RTC methods. 

Another hydraulics based effort that approaches the problem on a wider scale is being pursued by Guillon 

et al. (2008). Their work involves numerous on-line tanks in the HAUT-de-Seine sewer system, and the 

author explained3

 that the project had been based on independent pre-determined rule curves. Guillon et 

al. also indicated that those responsible were only recently considering movement to a more global 

approach to discharge management that involved comprehensive system-wide operations methods. She 

noted that a difficulty was in finding simulation and/or predictive tools that had the needed capabilities for 

this. A conclusion of this observation is that the benefits of system-wide operation are recognized and 

sought, but that there remains a need to further develop the necessary predictive and intervention systems. 

A more complex approach is to have the system ‘learn’ from experience what the best possible operating 

mode may be in order to control a particular event. This provides an ability of the control system to 

adjust dynamically over time on the basis of known sequences of events and their outcomes. Kurth et al. 

(2008) described an Artificial Neural Network (ANN) to predict the hydraulic characteristics of CSO 

discharges. In order to predict hydraulic performance, they developed a comprehensive (monitoring, 

modeling and operational strategy were all incorporated) system. Their system uses input from weather 

radar to predict hydraulic performance of CSO assets. System data in three UK drainage areas were used 

to train, validate, and test what was described as a hidden layer feed-forward multilayer perception. The 

3

 Personal communication, Lyon, France, November 5, 2008. 

Interim Report A-6 October 09 
 

 

 

approach was hydraulically oriented. They used local rainfall data and predict consequences on water 

depth and weir crest elevation. Given the much more intractable nature of the problem of constituent 

prediction, this seems like a reasonable surrogate for performance evaluation. It may eventually be 

possible to distinguish between causes of observed water patterns such that discrimination of input 

conditions can lead to effective determination of responses. At the time of publication, prediction over a 

time horizon of 15 minutes was reported as being effective. 

This kind of predictive approach may, in due course, be amended to incorporate constituents other than 

hydraulic, and this may be an avenue for further exploration. Hydraulic behavior brings with it quality 

impacts, and controlling the degree of quantity overflow has an impact on the degree of quality overflow 

so this is a useful step to take. However, it seems reasonable to consider the option of quality control as a 

direct effect instead of an implicit result of an intermediary phenomenon such as quantity. 

There has been work which does step towards direct use of water quality simulation to determine 

operating conditions. Blumensaat et al. (2008) evaluated the implications of on-line quarter quality data 

on the reduction of model uncertainty. They applied water quality modeling and applied simulation 

results to explore system optimization options. The researchers concluded in part that water quality 

constituents associated with sediments were significant, and that long-term conclusions made without 

incorporating this effect were limited. 

The elements of the problem are therefore all in place to enable predictive approaches as an integral part 

of RTC based on quality constituents. RTC of CSO discharges is not new, but there are elements of the 

problem that are certainly feasible, but not yet in common practice. The work currently being reported 

suggests that there is an opportunity in the extension of RTC to include quality constituents, in the 

development of predictive tools, and in the integration of RTC at a system-wide level. 

Tanks and In-line Storage 

A tried and tested component of CSO management is the use of storage to buffer flows and minimize 

overflows. This storage can be on-line or off-line, and the best approach depends on the context. These 

have a significant history but are nevertheless relatively recent on the CSO scene. As noted by Brombach 

et al. (2008) pretreatment of excess WWF not flowing to the treatment works was introduced in the 1970s 

and consists of a combination of detention, sedimentation, and floatable debris removal. These are a 

popular solution, and it is noted by Brombach et al. (2008) that more than 30,000 decentralized CSO 

tanks are in operation in Germany. Even so, knowledge about the efficiency of the tanks is imperfect and 

research is need in this area, particularly as regards sedimentation and resuspension of sewer sediments. 

Practice continues to evolve. In the 1970s tanks were designed for first flush, but recently retention 

treatment basins have accepted as CSO treatment structures. This section examines the current state of 

this evolution. 

On-line facilities, commonly accomplished by in-sewer storage and oversizing pipe but also possible 

through ‘point’ volumes, is provided within the sewer system. Since they are on-line, DWFs (undiluted 

sewage) passes through them. Simple geometry suggests that these facilities are readily implemented in 

flat areas. Unfortunately, this together with the flows during dry weather means that sedimentation and 

buildup are potentially problematic. Further, the intimate association with the conveyance system may 

predispose such a system to backups or other problems during extreme events. Some of this can be 

countered with careful design, for example by maintaining velocities such that scouring minimizes 

deposition. 

Off-line storage facilities store WWF in volumes connected to the conveyance system but not in a way 

that requires flow to pass through them on a continuing basis. Instead, the larger events only are 

captured. They may be placed by using near-surface tanks and basins or deep tunnel locations. They are 

Interim Report A-7 October 09 
 

 

designed so that they drain back into the system, or are pumped back into the system, when the event has 

passed. Because they are situated off the main system, they can be cheaper and easier to retrofit than an 

on-line system, and are often used in that context as a result. 

Shepherd et al. (2008) provide a full history of settling basin design. It is noted that according to classic 

theory, for an ideal rectangular basin, the selection of volume implies a surfaced overflow rate that is a 

driver for removal under steady conditions. Sediment removal under dynamic conditions has also been 

addressed by the EPA.4 

Shepherd et al. (2008) also noted that about half the inflow is routed into storage tanks, and the rest is 

treated at the plant. Flows beyond what the tanks can manage are discharged to the receiving water. 

Flows diverted to the tanks are sent back to the plant when conditions moderate and capacity for this is 

recovered. The main benefit of the tank, therefore, is to hold volume that would otherwise be simply 

discharged so that it can be treated when conditions moderate. Since some materials settle out in the 

tanks, the depth and other characteristics have an impact on how much material is re-suspended and 

flushed into the receiving stream when overflow events are occurring. 

Although the principles of storage tanks and CSO systems are universal, standards in implementation 

differ in various jurisdictions. It is interesting to note that UK design practices are stated in terms of 

volume only, with aspect ratio characteristics not specified, and that this means design characteristics that 

would tend to promote or minimize re-suspension are left up to the knowledge and judgment of the 

particular team managing the particular case. As noted by Shepherd et al. (2008), flows entering 

European facilities are limited to six times the mean DWF, and CSOs function above that level, at the 

entrance to the treatment works. This compares to typical US practice, which operates at a lower level. 

Other elements also vary. For example, Germany specifies a surface loading rate of 10 m/h and a 2 to 1 

length to width ratio for sizing a rectangular tank. In the US, the specified loading rates range from 0.5 

m/h for small populations (US Army Corps of Engineers, 1984) to 5 m/h (Tchobonoglous, et al. 2003) for 

larger populations. 

In-pipe Sediment Processes 

Temporary storage methods commonly concentrate on managing volume and minimizing overflow as a 

result, but are not designed to actively treat the volumes that are captured. This may provide an 

opportunity for future enhancements. 

The processes that affect WWF quality in the system are complex. Materials can fall out, or be 

resuspended, and biologically active materials can be transformed as well. These processes are 

imperfectly understood, even though some elements of solids in sewer systems have been under 

investigation by US and European researchers for over the past four decades (see for example Ashley et 

al. 2003). Some aspects of research are targeted at maintaining the hydraulic capacity of the system, 

which can be impaired by deposition. In more recent cases, European researchers target fluid-solids 

interactions as processes that lead to water quality changes during storm events. (Ashley, 2005). 

Some results speak to the nature of these transformations. McIllhatton et al. (2002) reported that much of 

the observed suspended load originates from solids eroding from the sewer solids bed. They recognized 

that large changes in water quality result from the re-entrainment of materials during an event and could 

propagate to cause releases to the receiving waters accepting discharges from the facility. Other research 

supports the notion that in-pipe effects on quality can have an impact on system performance. Using 

4

 Currently available at http://www.epa.gov/ednnrmrl/publications/reports/epa440587001/index.htm 

Interim Report A-8 October 09 
 

 

physical models, Leung et al. (2005) concluded that solids originating from resuspended bed material 

exhibited higher bacterial activity than the solids originally present in the sewage stream, implying that 

the sediments can concentrate materials and/or are active. It has been suggested by Schellart et al. (2005) 

that microbial activity can have an impact on the release of in-pipe sewer sediments. Banasiak et al. 

(2005) reports results which add to this notion, in that biological processes were seen to weaken the 

strength of sediments, and in which it was suggested that weakened shear strength may be a contributor to 

the foul flush. Biggs et al. (2005) provided results that suggest that temperature may also be a factor in 

concentration and possibly sediment erodability. With a substantial data base (turbidity, conductivity and 

flow for one year at two sites at one minute intervals in a Paris CSS) Lacour et al. (2008) found it was not 

possible to develop a relationship between hydraulic flow dynamics and turbidity. On the other hand, 

Abda et al. (2008) was able to show good agreement for velocity, water height and suspended solid 

concentrations, which suggest that given the right conditions, the problem may not be intractable. 

Although this may suggest that the problem of prediction may be more challenging than is immediately 

apparent, an implication of this work is that it may be better to simulate or measure and act on quality 

parameters than quantity parameters. Taken together, the evidence is persuasive that sediments are 

indeed active, and that prediction of their mobility will require substantial and complex analytical tools. 

Given the importance of sediment mobility on water quality, this would seem to be a promising area for 

further research, but given the limited practical ability to use these results in real-world applications, it 

may be that this area offers little benefit for the present project. 

Some progress has been made in measuring build-up that may have value in real-time or at least operating 

contexts. Bertrand-Krajewski et al. (2008) described a marine sonar unit with an attached laser meter on 

a floating frame was successfully tested for measuring sediment profiles in a large sewer. Sections were 

measureable in a time scale of seconds and to a resolution of 1 cm, and telemetry made the results 

immediately available. This suggests that the ability to implement operating decisions based on real-time 

sediment build-up behavior may be possible. 

There is also research that explores some of the smaller scale and more local effects of structural elements 

of the system on sediment buildup and disturbance. Campisano et al. (2007) simulated the sediment resuspension effect of flushing waves produced by hydraulic flushing gates and developed insights into the 

design and positioning of the flushing gates. Williams (2008) investigated the cause of sediment 

deposition when using a generic flow control device, and suggested that sediment load affects the deposit 

formed. He also explored the relationship of a flushing device to sediment size, noting that finer 

sediments may be more easily removed by such a device. 

Tank Processes 

Research is actively pursuing the implications of storage volumes on CSO behavior. As noted above, 

Guillon et al. (2008) evaluated on-line tanks in the HAUT-de-Seine sewer network to reduce CSOs to the 

River Seine. This is reported to be a large system, with 100 CSO points of which 22 have been outfitted 

with automatic gates to regulate flow. The overflows with automatic gates are independently operated, in 

real time, manage on-line storage. The remainder of the gates (fixed) are being considered for 

replacement with automatic gates. The main criterion for control was annual overflow volume, and the 

remaining storage capacity of the whole network was a decision factor as well. In this case, the location 

of the first overflow and of the onset of flooding was identified. Other factors important in establishing a 

plan included smaller pipes or steeper slopes that made them unsuitable for control of this type. 

Numerical methods are sometimes used to size tanks. Paoletti et al. (2008) used numerical simulation to 

assess the filling and emptying of cycles for off-line CSO storage tanks associated with combined sewers 

Interim Report A-9 October 09 
 

using different operating rules. The results were expressed as multiple regression relationships between 

intercepted volume, mean annual overflows, and numbers of filling-emptying cycles of tanks. It was 

noted that the simulation was limited by available rainfall data. Schroeder et al. (2008) reported on the 

early stages of a long-term program to manage a system, in which numerical modeling was used to size a 

CSO tank. It is intended in this system to place the storage tanks within a river, and it is an interesting 

aspect of this that the surfaces of the tanks are to be developed as platforms by implementing designs that 

enable this. As a part of this, factors considered included peak inflows, average number of annual 

filling/emptying cycles, average and maximum duration of empty tanks, and duration of CSOs staying in 

the tank. 

There is also current research into overflow hydraulic design as a performance factor. The hydraulic 

designs of clarifier-type CSO tanks was evaluated by Brombach et al. (2008) They examined tank 

geometry and surface loadings. It was noted that as well as a regular overflow, an emergency overflow is 

needed to bypass flows exceeding acceptable clarifier hydraulic limits, and that improper selection of the 

overflow weir can cause excessive through flow and re-suspend settled solids. 

There is some effort being devoted to evaluation of receiving-water effects as a direct element of CSO 

management through RTC. Achleitner and Rauch (2007), in a program consistent with the EU’s Water 

Framework Directive for basin-wide improvement approaches, used a RTC system to examine costs 

(including direct costs, energy and spilled water) associated with adjusting receiving-water baseflows to 

meet the dilution needs of upstream CSO discharges. In some ways this seems to be a dilution approach 

comparable to methods abandoned in North America, and it does little if anything to manage loads, but it 

does demonstrate the option to manage the receiving water concurrent with the CSO system. 

Not all modeling approaches are detailed and/or complex. Simplified probabilistic methods have also 

been used in the CSO context. Balistrocchi et al. (2008) assessed the long-term efficiency of CSO 

capture tanks as represented by first flush mass reduction. The tanks were represented as buried with an 

overflow device. In this effort a new probabilistic rainfall model was calibrated using five sets of 

continuous simulation time series data. Reasonable agreement on runoff results was achieved when this 

approach was tested on an urban catchment. 

Solids have also been evaluated in the suspended phase, in some detail, and that research suggests that 

this aspect, too, is imperfectly understood. Maus (2008) examined the effectiveness of settling velocity of 

suspended solids in the WWF context. This included testing of sedimentation tanks, separation 

efficiency, and real-time particle size distribution measurement using a submersible field instrument. The 

research demonstrated that sizing differed between the inlet and outlet, which implies a removal process 

that is sensitive to size. It was suggested that lighter organic materials are transported differently from 

denser inorganic materials. It was also noted that sample storage had an impact on results. Over all, this 

work exemplifies efforts to better understand removal processes in the treatment system. 

Some basic research is being done to provide insights into the effects of various design conditions on 

deviations from ideal mixing conditions. Shepherd et al. (2008) describe full-scale and lab-scale physical 

models that were used to look at the difference between actual residence times and theoretical ideal 

residence times. It was noted that the fifty-percentile residence times are always less than the theoretical 

residence times. This can have implications on design because it implies that tanks designed without this 

effect accounted for will tend to be under-designed. 

References 

Abda, F., Azbaid A., Ensminger, D., Fischer, S., Francois, P., Schmitt, P., Pallares, A. 2008. “Ultrasonic 

Interim Report A-10 October 09 
 

device for real time sewage velocity and suspended particles concentration measurements”, Proceedings 

of the 11th International Conference on Urban Drainage. Edinburgh, Scotland, UK. 

Achleitner, S., Rauch, W. 2007. Increases of River Base Flow by Hydropower Gate Operation for 

Mitigation of CSO Impacts – Potential and Limitation. Water Resources Management. Vol. 219), pp 

1487-1503. 

Ashley, R.., Bertrand-Krajewski, J., Hvitved-Jacobsen, T. 2005. Sewer solids- 20 years of investigation. 

Water Science & Technology. Vol. 52( 3), pp 73-84. 

Ashley, R.M., Dudley, J., Vollertsen, J., Saul, A.J., Jack, A. and Blanksby, J.R. 2002. The effect of 

extended in-sewer storage on wastewater treatment plant performance. Water Science and Technology. 

Vol. 45(3), pp 239–246. 

Balistrocchi, M., Grossi, G., Bacchi, B. 2008. “Assessment of the long term efficiency of CSO capture 

tanks by semi probabilistic methods”, Proceedings of the 11th International Conference on Urban 

Drainage. Edinburgh, Scotland, UK. 

Banasiak, R., Verhoeven, R., De Sutter, R. 2005. The erosion behavior of biologically active sewer 

sediment depositions: Observations from a laboratory study. Water Research. Vol. 39(20), pp 5221-5231. 

Bertrand-Krajewski, J.L., Gibello, C. 2008. “A new technique to measure cross-section and longitudinal 

sediment profiles in sewers”, Proceedings of the 11th International Conference on Urban Drainage. 

Edinburgh, Scotland, UK. 

Biggs, C., Prall, C., Tait, S., Ashley, R. 2005. Investigating the effect of storm events on the particle size 

distribution in a combined sewer simulator. Water Science & Technology. Vol. 52(3), pp 129-136. 

Blumensaat, F., Tranckner, J., Krebs, P. 2008. “Reduction of model structure uncertainty by detailed 

online water quality monitoring”, Proceedings of the 11th International Conference on Urban Drainage. 

Edinburgh, Scotland, UK. 

Brombach, H., Weiss, G., Pisano, W. C. 2008. “Clarifier-type CSO Tanks: Hydraulic Design for 

Optimum Sedimentation Efficiency”, Proceedings of the 11th International Conference on Urban 

Drainage. Edinburgh, Scotland, UK. 

Campisano, A., Creaco, E. and Modica, C. 2007. Dimensionless Approach for the Design of Flushing 

Gates in Sewer Channels. Journal of Hydraulic Engineering. Vol. 133(8), pp. 964-972. 

Guillon, A., Kovacs, Y., Pascal, O. 2008. “Evaluating on-line storage in the Haut-de-Seine Department 

sewer network in order to reduce overflows to the river Seine”, Proceedings of the 11th International 

Conference on Urban Drainage. Edinburgh, Scotland, UK. 

Kurth, A. Saul, A., Mounce, S., Hanson, D. 2008. ”Application of Artificial Neural Networks (ANNs) 

for prediction of CSO discharges”, Proceedings of the 11th International Conference on Urban Drainage. 

Edinburgh, Scotland, UK. 

Maus, C. 2008. “Measurement method for in situ particle settling velocity”, Proceedings of the 11th 

International Conference on Urban Drainage. 

McIllhatton, T., Sakrabani,R., Ashley, R. and Burrows, R. 2002. Erosion mechanisms in combined 

Interim Report A-11 October 09 
 

sewers and the potential for pollutant release to receiving waters and water treatment plants. Water 

Science and Technology. Vol. 45(3) pp 61-69. 

Lacour, C., Joannis, C., Gromaire, M. C., Chebbo, G. 2008. “Potentail of continuous turbidity 

measurements for improving management of pollutant flows during wet weather”, Proceedings of the 11th 

International Conference on Urban Drainage. Edinburgh, Scotland, UK. 

Leung, H., Chen, G., Sharma, K. 2005. Effect of detached/re-suspended solids from sewer sediment on 

the sewage phase bacterial activity. Water Science & Technology. Vol. 52(3), pp 147-152. 

Paoletti, A., Sanfilippo, A., Becciu, G. 2008. “Filling and Emptying Cycles for Stormwater Storage 

Tanks in Combined Systems”, Proceedings of the 11th International Conference on Urban Drainage. 

Edinburgh, Scotland, UK. 

Seggelke, K., Fuchs, L., Tranckner, J., Krebs, P. 2008. “Development of an integrated RTC system for 

full-scale implementation”, Proceedings of the 11th International Conference on Urban Drainage. 

Edinburgh, Scotland, UK. 

Schellart, A., Veldkamp, R., Klootwijk, M., Clemens, F., Tait, S., Ashey, R., Howes, C. 2005. Detailed 

observation and measurement of sewer sediment erosion under aerobic and anaerobic conditions. Water 

Science & Technology. Vol. 52(3), pp 137-146. 

Schroeder, K., Sonnenberg, H., Barjenbruch, M., Gantner, K., Steeg, R., and K. Joswig. 2008. 

“Dimensioning of a river-based stormwater tank by means of numerical long-term simulation”, 11th 

International Conference on Urban Drainage. Edinburgh, Scotland, UK. 

Shepherd, W., Saul, A., and J. Boxall. 2008. “Quantifying the Performance of Storm Tanks”, 

Proceedings of the 11th International Conference on Urban Drainage. Edinburgh, Scotland, UK. 

US Army Corps of Engineers. 1984. Engineering and Design - Domestic Wastewater Treatment. Report 

# EM 1110-3-172. 

Williams, K. J. 2008. “In-Sewer Sedimentation Associated with Active Flow Control”, Proceedings of 

the 11th International Conference on Urban Drainage. Edinburgh, Scotland, UK. 

Interim Report A-12 October 09 
 

Section 3: Watershed Management Strategies - The US 

Baseline 

A synthesis of practice in the US was not a target of this research, but it was useful to provide a synopsis 

for communication with the international community that was assembled as a part of this project. The 

following pages provide a brief outline of what was assembled for this purpose. 

Historical Approach and Legacy 

WWFs in the US have historically been mitigated to ensure flood control, property protection and public 

safety. The mid-20th century initiated an era of rapid conveyance of stormwater through hardened 

structures, such as curbs, gutters, pipes, and concrete-lined channels. Flood control was achieved using 

stormwater detention practices, particularly wet and dry detention ponds. Standards were written to 

control peak stormwater flows from new developments without consideration of the changes in volume, 

timing, or duration of flows from the site. 

This type of development led to water body degradation, particularly stream channel alteration, due to 

both hydrologic changes that altered stream geomorphology and from pollutant impacts. Increased 

regulation resulting from the CWA, shifted the focus of wet-weather management from flood control to 

water quality and water resource protection. Initial efforts focused on pollutant control with less 

emphasis on stormwater flow control. In the 1990s, research began in earnest to identify a new approach 

for stormwater management. The terms Low Impact Development (LID), as coined by Prince George’s 

County, Maryland, and Better Site Design (BSD), coined by the Center for Watershed Protection, 

emerged to describe two similar approaches that focused on maintaining and restoring the natural function 

of a development site. 

Most of the existing infrastructure in the US as it exists today is based on the flood protection, rapid 

removal approach. In older areas of the country, particularly in the Eastern and Midwestern US, drainage 

infrastructure was not built with adequate capacity for today’s flows, and the condition of the 

infrastructure itself has deteriorated as a result of inadequate attention to maintenance and replacement. 

Current regulations focus on new development controls, which in many heavily developed areas do little 

to address the impacts from decades of prior development and degradation. Some communities have 

embraced retrofitting of existing infrastructure to improve water quality and flow control performance. 

Policy approaches include incorporating water quality features into municipal maintenance and right-ofway projects, requiring additional stormwater management for redevelopment projects, and incorporating 

water quality and flow retrofits into maintenance and repair activities. However, most communities are 

not pursuing retrofits due to the lack of requirements to do so, and because retrofits can be difficult and 

costly to implement. 

For developing areas where new development regulations are appropriate for addressing current and 

future stormwater impacts, there are a wide range of performance standards ranging from the basic floodcontrol, peak flow mitigation approach of the mid-20th century to more innovative requirements that strive 

to mimic a site’s natural, pre-development hydrology (i.e., LID). This variability in policies can be 

attributed to a number of factors, including the extent to which the state or federal government imposes 

regulations on the community to control WWFs, the amount of funding available for stormwater research 

and program implementation at the local level, and the general attitude of the populace or government 

officials toward developing more sustainable communities. 

Research sponsored by the Water Environment Research Foundation (2008) showed that the primary 

regulatory drivers for strong local wet weather and stormwater management programs were the National 

Interim Report A-13 October 09 
 

 

Pollutant Discharge Elimination System (NPDES) municipal stormwater management requirements 

(http://www.epa.gov/npdes/stormwater), the need to reduce or eliminate CSOs 

(http://www.epa.gov/npdes/cso), and a general desire by citizens and municipal leaders to promote a 

sustainable, “green” ethic, which includes sustainable stormwater management. 

Emerging Trends in US Practice 

Perhaps the most notable broad thrust in the US at present is denoted by the LID approach that has 

become a strong element of practice in the water resources community. LID is a stormwater management 

strategy that has been adopted in many localities across the country in the past several years (EPA, 2007). 

It is not really new, in the sense that the principles involved have been well understood for decades, but it 

represents a consensus understanding in the community and to that extent is a significant development. 

LID is a stormwater management approach and set of practices that can be used to reduce runoff and 

pollutant loadings by managing the runoff as close to its source(s) as possible. A set or system of smallscale practices, linked together on the site, is often used. LID approaches can be used to reduce the 

impacts of development and redevelopment activities on water resources. In the case of new 

development, LID is typically used to achieve or pursue the goal of maintaining or closely replicating the 

predevelopment hydrology of the site. In areas where development has already occurred, LID can be 

used as a retrofit practice to reduce runoff volumes, pollutant loadings, and the overall impacts of existing 

development on the affected receiving waters. 

In general, implementing integrated LID practices can result in enhanced environmental performance 

while at the same time reducing development costs when compared to traditional stormwater management 

approaches. LID techniques promote the use of natural systems, which can effectively remove nutrients, 

Indicator bacteria, and metals from stormwater. Cost savings are typically seen in reduced infrastructure 

because the total volume of runoff to be managed is minimized through soil infiltration and 

evapotranspiration. By working to mimic the natural water cycle, LID practices protect downstream 

resources from adverse pollutant and hydrologic impacts that can degrade stream channels and harm 

aquatic life. 

It is important to note that typical, real-world LID designs usually incorporate more than one type of 

practice or technique to provide integrated treatment of runoff from a site. For example, in lieu of a 

treatment pond serving a new subdivision, planners might incorporate a bioretention area in each yard, 

disconnect downspouts from driveway surfaces, remove curbs, and install grassed swales in common 

areas. Integrating small practices throughout a site instead of using extended detention wet ponds to 

control runoff from a subdivision is the basis of the LID approach. 

When conducting cost analyses of these practices, examples of projects where actual practice-by-practice 

costs were considered separately were found to be rare because material and labor costs are typically 

calculated for an entire site rather than for each element within a larger system. Similarly, it is difficult to 

calculate the economic benefits of individual LID practices on the basis of their effectiveness in reducing 

runoff volume and rates or in treating pollutants targeted for best management practice performance 

monitoring. 

The following is a summary of the different categories of LID practices, using US terminology, including 

a brief description and examples of each type of practice. 

Conservation Design 

Conservation designs can be used to minimize the generation of runoff by preserving open space. Such 

designs can reduce the amount of impervious surface, which can cause increased runoff volumes. Open 

space can also be used to treat the increased runoff from the built environment through infiltration or 

evapotranspiration. For example, developers can use conservation designs to preserve important features 

on the site such as wetland and riparian areas, natural vegetation, forested tracts, and areas of porous soils. 

Interim Report A-14 October 09 
 

Development plans that outline the smallest site disturbance area can minimize the stripping of topsoil 

and compaction of subsoil that result from grading and equipment use. By preserving natural areas and 

not clearing and grading the entire site, less total runoff is generated on the development parcel. Such 

simplistic, nonstructural methods can reduce the need to build large structural runoff controls such as 

retention ponds and stormwater conveyance systems and thereby decrease the overall infrastructure costs 

of the project. Reducing the total area of impervious surface by limiting road widths, parking area, and 

sidewalks can also reduce the volume of runoff. Residential developments that incorporate conservation 

design principles also can benefit residents and their quality of life due to increased access and proximity 

to communal open space, a greater sense of community, and expanded recreational opportunities. 

Infiltration 

Infiltration practices are engineered structures or landscape features designed to capture and infiltrate 

runoff. They can be used to reduce both the volume of runoff discharged from the site and the 

infrastructure needed to convey, treat, or control runoff. Infiltration practices can also be used to recharge 

ground water. This benefit is especially important in areas where maintaining drinking water supplies and 

stream baseflow is of concern because of limited precipitation or a high ratio of withdrawal to recharge 

rates. Infiltration of runoff can also help to maintain stream temperatures because the infiltrated water 

that moves laterally to replenish stream baseflow typically has a lower temperature than overland flows, 

which might be subject to solar radiation. Another advantage of infiltration practices is that they can be 

integrated into landscape features in a site-dispersed manner. This feature can result in aesthetic benefits 

and, in some cases, recreational opportunities; for example, some infiltration areas can be used as playing 

fields during dry periods. 

Runoff Storage 

Impervious surfaces are a central part of the built environment, but runoff from such surfaces can be 

captured and stored for reuse or gradually infiltrated, evaporated, or used to irrigate plants. Using runoff 

storage practices has several benefits. They can reduce the volume of runoff discharged to surface waters, 

lower the peak flow hydrograph to protect streams from the erosive forces of high flows, irrigate 

landscaping, and provide aesthetic benefits such as landscape islands, tree boxes, and rain gardens. 

Designers can take advantage of the void space beneath paved areas like parking lots and sidewalks to 

provide additional storage. For example, underground vaults can be used to store runoff in both urban 

and rural areas. 

Runoff Conveyance 

Large storm events can make it difficult to retain all the runoff generated on-site by using infiltration and 

storage practices. In these situations, conveyance systems are typically used to route excess runoff 

through and off the site. In LID, conveyance systems can be used to slow flow velocities, lengthen the 

runoff time of concentration, and delay peak flows that are discharged off-site. LID conveyance practices 

can be used as an alternative to curb-and-gutter systems, and from a water quality perspective they have 

advantages over conventional approaches designed to rapidly convey runoff off-site and alleviate on-site 

flooding. LID conveyance practices often have rough surfaces, which slow runoff and increase 

evaporation and settling and removal of solids. They are typically permeable and vegetated, which 

promotes infiltration, filtration, and some biological uptake of pollutants. LID conveyance practices also 

can perform functions similar to those of conventional curbs, channels, and gutters. For example, they 

can be used to reduce flooding around structures by routing runoff to landscaped areas for treatment, 

infiltration, and evapotranspiration. 

Interim Report A-15 October 09 
 

Filtration 

Filtration practices are used to treat runoff by filtering it through media that are designed to capture 

pollutants through the processes of physical filtration of solids and/or cation exchange of dissolved 

pollutants. Filtration practices offer many of the same benefits as infiltration, such as reductions in the 

volume of runoff transported off-site, ground water recharge, increased stream baseflow, and reductions 

in thermal impacts to receiving waters. Filtration practices also have the added advantage of providing 

increased pollutant removal benefits. Although pollutant build-up and removal may be of concern, 

pollutants are typically captured in the upper soil horizon and can be removed by replacing the topsoil. 

Low Impact Landscaping 

Selection and distribution of plants must be carefully planned when designing a functional landscape. 

Aesthetics are a primary concern, but it is also important to consider long-term maintenance goals to 

reduce the amount of labor, water, and chemicals needed. Properly preparing soils and selecting species 

adapted to the microclimates of a site greatly increases the success of plant establishment and growth, 

thereby stabilizing soils and allowing for biological uptake of pollutants. Dense, healthy plant growth 

offers such benefits as pest resistance (reducing the need for pesticides) and improved soil infiltration 

from root growth. Of particular importance is the avoidance of heavy earth-moving equipment and soil 

compaction during construction thus enabling post-construction soil-infiltration to occur in an uninhibited 

manner. Low impact landscaping can thus reduce impervious surfaces, improve infiltration potential, and 

improve the aesthetic quality of the site. 

Recent US Policy Developments 

With the drafting of the Managing Wet-weather with Green Infrastructure—Action Strategy 2008 

(http://www.epa.gov/npdes/pubs/gi_action_strategy.pdf), the US formalized a collaborative effort to 

consider development with water management issues in mind (EPA, 2008). The stated purpose of the 

action strategy is to coordinate the efforts of several organizations to “promote the benefits of using green 

infrastructure in mitigating overflows from combined and separate sewers and reducing runoff, by 

encouraging the use of green infrastructure as prominent components of CSO and sanitary sewer overflow 

(SSO) plans, municipal stormwater programs, and nonpoint source and watershed planning efforts.” The 

action strategy itemizes a number of important efforts to bring “green infrastructure technologies and 

approaches into mainstream wet-weather management.” The Green Infrastructure Action Strategy is also 

intended to address projected impacts of climate change. 

In September of 2007, the Environmental Council of the States passed a resolution 

(http://www.ecos.org/content/policy/detail/2861/) outlining its plan to use green infrastructure to reduce 

the negative environmental impacts associated with CSO and SSO events (Brown 2007). Another 

resolution was passed in 2006 by mayors across the US 

(http://www.usmayors.org/urbanwater/policyres_06c.asp) to show their interest in seeing the array of 

environmental benefits in wet-weather technology applied to their cities (U.S. Conference of Mayors, 

2006). This resolution stated in part: 

Green infrastructure naturally manages stormwater, reduces flooding risk and improves 

air and water quality, thus performing many of the same functions as traditionally built 

infrastructure, often at a fraction of the cost. - U.S. Mayors’ Resolution on Green Infrastructure 

These steps by US political entities and lawmakers may signal a pending significant increase in the 

demand for solutions, and therefore for persons with the technical knowhow, that will accomplish these 

ambitious and broad reaching goals. 

Interim Report A-16 October 09 
 

The technology outlined in this report has been tried and tested, and research findings demonstrate their 

usefulness in mitigating wet-weather control problems. The case studies presented in the following 

sections provide a snapshot of projects that have been completed or are ongoing across the globe. Data 

from these studies will go a long way in providing guidance and assurance to city planners and policymakers in all parts of the world. 

References 

Brown, Stephen S. 2007. Supporting Green Infrastructure. Environmental Council of the States (ECOS). 

http://www.ecos.org/content/policy/detail/2861/ 

U.S. Conference of Mayors. 2006. “Promoting “Green” Infrastructure in the Nation’s Communities”, 

Proceedings of Policy Resolutions: 74th Annual Conference. Las Vegas, NV, USA. 

http://www.usmayors.org/urbanwater/policyres_06c.asp. 

U.S. Environmental Protection Agency. 2007. Reducing Stormwater Costs through Low Impact 

Development (LID) Strategies and Practices. www.epa.gov/owow/nps/lid/costs07/. 

U.S. Environmental Protection Agency. 2008. Managing Wet-weather with Green Infrastructure Action 

Strategy 2008. http://www.epa.gov/npdes/pubs/gi_action_strategy.pdf 

Water Environment Research Foundation. 2008. Using Rainwater to Grow Livable Communities. 

http://www.werf.org/livablecommunities/. 

Interim Report A-17 October 09 
 

Section 4: Water Sensitive Urban Design (WSUD) 

The following chapters highlight the approaches of many US and International case studies primarily for 

surface water/stormwater management. Case studies were selected based on having a suite of data 

available on performance as well as costs. 

The primary goals of Water Sensitive Urban Design (WSUD) are similar to the core values of any water 

conservation practice. WSUD should follow the following principles (Melbourne Water, 2008): 

• Protect natural systems 

• Integrate stormwater treatment into the landscape 

• Protect water quality 

• Reduce runoff and peak flow 

• Add value while minimizing development costs 

What is new is the integrated approach to urban wastewater management that WSUD proposes. The 

integration goes by the term “treatment trains,” which denotes the staggered and graduated approach to 

sediment and pollutant sequestration, as shown in Figure 2. 

Figure 2. Desirable design ranges for treatment measures and pollutant sizes 

(Melbourne Water, 2008). 

WSUD is an Australian term applied to the approach of using all available technologies in concert with 

each other to achieve the maximum wastewater and pollutant reduction (Melbourne Water, 2008). It is 

also known as SUDS, mostly in the UK, and LID, mainly in the US, but will be called WSUD in this 

report. This is a philosophy in which the end product should mirror, as closely as possible, the natural 

drainage conditions prior to development (Woods-Ballard, 2007). Minimizing sewage discharges (thus 

Interim Report A-18 October 09 
 

reducing combined sewer overflow events) and demand on water supply systems are other key benefits to 

this philosophy (Brisbane City Council, 2006). 

The legislative backbone to the propagation of WSUD projects in Queensland, Australia, is the Integrated 

Planning Act (IPA). The IPA is centered on the goal of ecological sustainability by utilizing natural 

resources in such a way as to minimize the impacts on the environment (Brisbane City Council, 2006). 

Part of what makes this approach successful is its ability to be integrated into an urban environment to 

achieve multiple benefits. The technologies used in WSUD use the topography of an area to maximize 

the aesthetic and recreational qualities of the landscape. Building systems into parks and along walking 

paths, as well as in riparian corridors, emphasizes the natural watershed drainage system and its functions 

(Melbourne Water, 2008). In the upcoming chapters we will explore the various technologies that 

contribute to a wastewater treatment train. 

The following are the steps involved in implementing the WSUD approach (Brisbane City Council, 

2006): 

•	 Step 1 – Site Assessment: Assess the natural assets of the site and appropriate measures to 

minimize water impacts. 

•	 Step 2 – Establish Design Objectives: Determine required design objectives based on local 

authority requirements. 

•	 Step 3 - Device Selection: Determine short list of suitable WSUD measures or series of devices 

that can be incorporated within the site to meet design objectives. 

•	 Step 4 - Determine Conceptual Design: The optimal suite of WSUD measures based on 

performance in meeting design objectives and life-cycle cost. Computer modeling may be 

required to demonstrate compliance with design objectives. 

•	 Step 5 - Undertake Detailed Design: Detailed design of selected measures. 

•	 Step 6 - Operation and Maintenance: Implement operation and maintenance plan for construction 

and operational phases. 

There are two main types of treatment for urban drainage systems: outlet and distributed (Figure 3). The 

outlet method of treating at one site located at the catchment outlet is a more centralized approach, which 

is beneficial when there is a sizeable pollutant load located in one generalized area. Maintenance of 

centralized treatment structures can be relatively straightforward; however, the treatment structure would 

need to sequester a larger pollutant load than distributed practices. Also, to have the desired impact on 

wastewater treatment, the practice would have to cover a larger land area to facilitate a larger amount of 

infiltration (Melbourne Water, 2008). The benefits of a “distributed” system are many and the risks are 

small. Dividing a large urban landscape into manageable catchment areas makes it possible to target 

many areas of moderate to high pollution, extending water quality protection along a greater length of 

waterway. A technology may be chosen to best suit the topology of the site and the catchment area. The 

flow velocities passing through “distributed” treatment structures are lower and therefore provide more of 

an opportunity for infiltration and pollutant removal. 

Interim Report A-19	 October 09 
 

 

Figure 3. The contextual significance of an outlet approach vs. a distributed approach (Melbourne 

Water, 2008). 

If programs implemented by the city, state/provincial, or local organizations have staged funding, then 

individual sites can be built one by one as the funding becomes available (Melbourne Water, 2008). Also, 

if one of the distributed treatment systems fails, the entire effort is not lost. Treatment systems fail for a 

number of reasons, which include inappropriate site topography (steep slopes), instability of soils or 

geology (highly erosive area), limited space to implement the technology effectively, and social 

constraints. Social constraints affecting the success of wet-weather technologies can range from odor 

associated with sequestered pollutants and detritus to contamination from pollutants (infection, etc.) and 

vermin such as mosquitoes or rats (Melbourne Water, 2008). 

Ultimately, when the correct treatment option is chosen along with the correct size of the treatment area, 

there can be as much as a 90% or more reduction in gross pollutant load in water leaving the treatment 

site (Brisbane City Council, 2006). This is in comparison with traditional urban development practices, 

in which stormwater is not treated at all and is simply directed as efficiently as possible into a drainage 

pipeline system, which drains directly into the nearest water body. 

When more techniques are used in series, the overall system becomes increasingly effective. Conveying 

stormwater from one treatment system to the next is an important consideration in the management of 

WWFs. During extreme storm events, having a well established overland flow route is necessary to 

convey water safely (Woods-Ballard, 2007). Proper maintenance and enhanced public perception of 

treatment structures will ensure their long-term success as pollution and water management systems. 

Case Studies 

Low Impact Development in Tyngsborough, Massachusetts, US 

Marla Circle in Tyngsborough, MA is a subdivision with five lots that originally called for a conventional 

development and as such was submitted to the town’s planning board for approval in June 2003. The 

required stormwater practices took the space planned for the fifth lot. The project was withdrawn and 

redesigned, with LID elements added. The new design was approved for construction in spring 2004. 

The major LID change was to retain all of the stormwater in the road, in a bioretention cell within a culInterim Report A-20 October 09 
 

de-sac. This change allowed space for the fifth single-family home to be built. Water-quality swales 

were installed along the uncurbed road’s right-of-way. The last home was completed in September 2005 

(Buranen, 2008). 

The bioretention cell in the cul-de-sac and the swales will require periodic removal of sediments. Trees 

and shrubs were planted within the cul-de-sac, so routine landscape maintenance will also be required. 

Tyngsborough’s Department of Public Works agreed to take on both of these maintenance responsibilities 

when the town accepted the road. 

A narrower street (24 instead of 28 feet) and narrower driveways reduced paved areas and runoff. The 

homes were located closer to the street, reducing the length of driveways as well as sewer and utility 

lines. Street proximity also meant fewer disturbances to the land, and fewer trees needed to be removed. 

Each of the five homes has its own rain garden. Owners are required to maintain their gardens, including 

removal of debris and sediment, remulching, and replanting vegetation as needed. With the public and 

private LID elements in place, rainwater will overflow into the town’s drainage system only in extreme 

storm events. 

This innovative project took longer to achieve—three years—than would a conventional development. 

City officials, unfamiliar with LID elements, required additional review time before granting final 

approval. The developer cited that time and money was saved by meeting with the Tyngsborough 

planning board to discuss the ideas before engineered plans were developed. 

Cluster Development in Ipswich, Massachusetts, US 

In Ipswich, MA, the Partridgeberry Place project involves 20 innovative home sites built on 38 acres in 

the Ipswich River watershed (Buranen, 2008). By clustering the single-family homes on lots of 8,000 to 

12,000 ft 2

, 74% of the site was kept as woods and open space. Hiking trails lead to a nearby state park. 

Meridian Associates of Beverly, MA, did the design and engineering work for the project. The Martins 

Companies of Danvers, MA, developed and built the subdivision. The main LID features were 

constructed by December 2006. 

The Massachusetts Department of Conservation and Recreation (DCR) chose Partridgeberry Place as a 

LID subdivision demonstration site. The DCR also selected an adjacent conventional subdivision as a 

basis for comparison. Runoff percentages for both subdivisions will also be compared to those found in 

the literature for LID and conventional development for a yearlong monitoring of stormwater runoff 

relative to rainfall. The US Geological Survey designed the monitoring plan and installed the equipment 

for the study in the winter and spring of 2007. 

LID features of Partridgeberry Place include minimal land disturbance; reduced pavement areas and a 

subdivision road that is only 18 ft wide; reduced setbacks resulting in shorter driveways and smaller front 

yards and backyards; grass pavers for visitors’ parking; an open grass swale that drains to a central 

bioretention area; rain gardens on each homeowner’s lot; less space for lawns and more landscapes of 

native vegetation; and infiltration of roof runoff through drywells. Installation of a shared septic system 

made the clustering of the homes work and preserved more open space. The system allows on-site 

recharge of wastewater. 

Interim Report A-21 October 09 
 

 

A Holistic Approach to CSO Control: Green Streets, Ecoroofs, and Rain Gardens in 

Portland, Oregon 

Portland’s stormwater officials are faced with runoff from an average annual rainfall of 37 in. Portland’s 

Green Streets program combines rain gardens with such LID features as permeable pavement, green 

roofs, curb extensions with plantings, and planters that allow water to infiltrate (Buranen, 2008). With 

these features, Portland can reduce peak stormwater flows into their combined sewer system (CSS) by as 

much as 85%, stormwater volume by 60%, and pollution in runoff by up to 90%. But there’s still a lot of 

runoff and more impervious surfaces from new developments. The City Commissioner stated that 

Portland’s goals include 3,700 green streets; 250 acres of ecoroofs; and 250 acres of planters, swales, and 

rain gardens. An impressive example of a Portland project with multiple LID features is the retrofit at the 

Mt. Tabor Middle School. This innovative joint project of the Portland Public Schools and the 

Department of Environmental Services manages runoff from a total area of approximately 2 acres. It 

includes a swale, six planters, three drywells, a curb extension adjacent to the school, and, of course, a 

rain garden. In 2007, the American Society of Landscape Architects presented a national award to the 

project’s rain garden, which replaced 4,000 square feet of asphalt. 

Decentralizing stormwater management through on-site projects means that rain gardens, even those 

installed through community projects, become the homeowner’s responsibility to maintain. Requiring 

homeowners to attend workshops to learn about their rain gardens and other LID features connects the 

homeowners to the programs over the long term. 

References 

Brisbane City Council, Moreton Bay Waterways, and Catchment Partnership. 2006 

http://www.healthywaterways.org/FileLibrary/wsud_tech_guidelines.pdf 

Buranen, M. 2008. Rain Gardens Reign. Stormwater - The Journal for Surface Water Quality 

Professionals. Vol 9 (4). http://stormh2o.com/may-2008/rain-gardens-management-4.aspx 

Goode, David. 2006. Green Infrastructure. Report to the Royal Commission on Environmental Pollution. 

http://www.rcep.org.uk/urban/report/green-infrastructure-david-goode.pdf 

Melbourne Water. 2008. WSUD Key Principles. 

http://wsud.melbournewater.com.au/content/wsud_key_principles/wsud_key_principles.asp 

Smrcka, Karel. 2007, November 30. German Scientists Develop Water-Recycling System. Creamer 

Media’s Engineering News Online. http://www.engineeringnews.co.za/article.php?a_id=121541 

Woods-Ballard, B., Kellagher, R. Martin, P., Jeffries, C., Bray, R., and P. Shaffer. 2007. The SUDS 

Manual. Classic House, London, UK. 

Interim Report A-22 October 09 
 

Section 5: Litter Traps and Swales 

Litter Traps 

Urban litter is one of the major contributors to water quality degradation in urban watersheds. The 

primary pretreatment method is to remove large debris so downstream treatment of wet-weather runoff 

occurs more effectively (Melbourne Water, 2008). There are numerous control technologies available to 

address litter problems, all with the designs to allow access for operation and maintenance. As seen in 

Figure 4, structures such as this litter collection basket are simplistic in nature and generally require 

maintenance intervals based on the amount of gross solids and floatables within each drainage system. 

An advantage of these systems is that they can be incorporated into existing drainage systems with little 

visual impact (Melbourne Water, 2008). 

Figure 4. Example of a litter trap structure good for high litter load areas (Melbourne Water, 2008). 

Release nets are another form of the litter collection and removal technique. As demonstrated in Figure 5, 

release nets are easy and inexpensive to install and maintain. The desired pore size of the netting is 

chosen based on the size and pollutants common to the area. They are not visually appealing if in a high 

traffic area out in the open, but they do allow for trash collection and removal (Melbourne Water, 2008). 

Interim Report A-23 October 09 
 

 

Figure 5. Example of a litter trap for urban drainage system (Melbourne Water, 2008). 

Large sediment can be trapped also. Opening the channel in a sediment catchment slows the flow and 

allows for sediment to collect at the bottom of the catchment before the flow is passed on down the 

treatment train. Another option is to install sediment booms in high flow areas to capture sediment as it 

travels downstream. This option is less attractive because it does not capture finer sediments and is more 

area intensive (Melbourne Water, 2008). 

The costs associated with the implementation of litter traps are contingent upon the following 

considerations (Melbourne Water, 2008): 

• Installation – design, size, capacity, etc. 

• Maintenance – contingent on installation factors 

• Disposal – disposal costs should be estimated based on gross pollutant load 

Swales 

Whether natural or man-made, swales can significantly reduce overland pollutant conveyance. Swales 

are wide, shallow, grassy channels with dense vegetation covering the sides and bottom. The advantage 

of utilizing swales as a stormwater treatment option is that they manage stormwater flows, capture 

particulate pollutants (suspended solids and associated pollutants), and promote infiltration (U.S. 

Environmental Protection Agency, 2006). In urban settings, swales can effectively replace traditional 

road medians and be strategically placed to filter and help infiltrate parking lot storm runoff (Melbourne 

Water, 2008). They provide a number of benefits, including (Clar, 2004): 

• Stream channel protection 

• Peak discharge control 

• Water quality control 

• Ground-water recharge 

• The reduction of urban runoff impacts 

Interim Report A-24 October 09 
 

It is important to note that there is an inherent seasonal functionality associated with any vegetative 

filtering system. During dormant months the effectiveness of the vegetation to filter pollutants is reduced. 

In addition to this annual inconsistency, there are a few possible negative impacts of swales; these include 

(U.S. Environmental Protection Agency, 1999): 

• They are impractical in very flat and very steep topography 

• They may erode in high water velocities 

• Human health risks include drowning hazards and mosquito breeding areas 

• Regular inspections are required 

Figure 6 represents the total nitrogen removal and Figure 7 represents total suspended solids percentages 

associated with different swale slopes. 

Figure 6. Modeled total nitrogen removal of varying swale slopes and surface area (Melbourne 

Water, 2008). 

Interim Report A-25 October 09 
 

Figure 7. Modeled total suspended solids removal of varying swale slopes and surface area 

(Melbourne Water, 2008). 

Case Studies 

Swales for Stormwater Pollution Control in Northern Sweden 

Swedish researchers used existing grassed swales to determine the pollutant reduction capabilities of 

swales from snow and snowmelt water sources (Backstrom, 2003). The pollutants measured were 

suspended solids, particulates, and total and dissolved metals (Table 1). Grassed swales proved to even 

out both peak flows from urban stormwater runoff as well as peak pollutant loading. During high flows 

and high pollutant rates, grassed swales did very well in reducing the amount of pollutants in water 

flowing out of the swale area. This was primarily due to sedimentation of particulate matter. However, 

the research also showed that during low flow rainfall events pollutants may be released rather than 

retained in grassed swale areas. This is likely a result of the inability of vegetation and soil to 

permanently sequester pollutants with the possible transfer to of dissolved constituents to ground water. 

Due to their limitations, grassed swales should be considered as primary treatment devices and should be 

used in combination with other treatment practices. 

Interim Report A-26 October 09 
 

 

 

Table 1. pH, suspended solids (SS), and heavy metal concentrations in snow and melt water in three 

roadside swales in Lulea (March–April 2000). 

BMP Developments in Scotland 

The purpose of this study was to assess which wet-weather BMPs were most common already and which 

would be the most useful when planning future urban development (Scholz, 2007). The counties in 

which wet-weather BMPs were tested were Edinburgh and Glasgow, Scotland; 182 sites were identified 

and evaluated for their management effectiveness across these two counties. The results from this study 

were that the most common BMPs are ponds used in conjunction with permeable pavements. The 

indication of this study is that the combinations of ponds combined with shallow swales are the most 

effective BMPs for wet-weather technology implementation. 

Buffer Strip Development in the US 

The Roadside Vegetated Treatment Sites Study was a two-year water-quality monitoring project 

undertaken to evaluate the removal of stormwater contaminants by existing vegetated slopes adjacent to 

freeways (Barrett et al., 2004). The objectives of this study were to determine if standard roadway design 

requirements result in biofilter strips with treatment equivalent to those specifically engineered for water 

quality performance, and to generate design criteria. Variables such as width, slope, vegetation density, 

and hydraulic loading were evaluated by studying the runoff through existing vegetated slopes at four 

locations in northern California and four locations in southern California. 

At each location, concrete channels, approximately 90 ft long, were constructed to capture freeway runoff 

after it passed through existing vegetated strips of varying widths. The quantity and quality of the runoff 

discharged from the buffer strip was compared to freeway runoff collected at the edge of pavement. The 

study found that buffer strips consistently reduced the concentration of suspended solids and total metals 

in stormwater runoff. The strips were generally less effective at removing dissolved metals and 

essentially no change in concentration was observed for nitrogen and phosphorus. Concentrations of 

organic carbon, dissolved solids, and hardness were observed to increase. For the constituents exhibiting 

a decrease in concentration, steady state levels were generally achieved within 15 ft of the pavement edge 

for slopes commonly found on highway shoulders and when the vegetation coverage exceeded 80%. 

Slope, vegetation type and height, highway width, and hydraulic residence time had little or no impact on 

the final concentrations. 

Interim Report A-27 October 09 
 

 

Summary of Grassed Swale Research in the US 

Alabama Highway Drainage Conservation Design Practices—Particulate Transport in Grass 

Swales and Grass Filters 

The objective of this project was to demonstrate how a common Alabama Department of Transportation 

design and maintenance practice—the use of grass drainage swales—can help meet the requirements of 

the EPA’s new Phase II Stormwater Regulations (Nara and Pitt, 2005). As part of the study, 69 sediment 

samples were collected at an outdoor grass swale located adjacent to Tuscaloosa City Hall, Alabama, 

during 13 storm events from August to December 2004. The samples were analyzed for turbidity, total 

solids, total suspended solids, total dissolved solids and particle size distributions. The total suspended 

solids concentrations observed during different rain events showed significant sediment reductions as a 

function of the length of the swale. The particle size distributions of the suspended solids at the swale 

showed preferential transport of small particles for all lengths of the swale and preferential trapping of 

large particles. 

California - BMP Retrofit Pilot Program: Final Report 

Litigation between Caltrans and the Natural Resources Defense Council, Santa Monica BayKeeper, the 

San Diego BayKeeper, and the U.S. Environmental Protection Agency resulted in a requirement that 

Caltrans develop a Best Management Practice (BMP) Retrofit Pilot Program in Caltrans Districts 7 (Los 

Angeles) and 11 (San Diego) (CALTRANS, 2004). The objective of this program was to acquire 

experience in the installation and operation of a wide range of structural BMPs for treating stormwater 

runoff from existing Caltrans facilities and to evaluate the performance and costs of these devices. Each 

BMP was designed, constructed and maintained at what was state-of-the-art at the time the project began 

(in 1997). 

Biofiltration swales and biofiltration strips were among the BMP types included in the study. These 

practices are considered technically feasible depending on-site-specific considerations. Overall, the 

reduction of concentration and load of the constituents monitored was comparable to the results reported 

in other studies, except for nutrients. Nutrient removal was compromised by the natural leaching of 

phosphorus from the salt grass vegetation used in the pilot study. This condition was not known at the 

start of the project but was discovered later in the program. 

Biofiltration swales and strips were among the least expensive devices evaluated in this study and were 

among the best performers in reducing sediment and heavy metals in runoff. Removal of phosphorus was 

less than that reported by another study but may be related to leaching of nutrients from the salt grass 

during its dormant season. The swales are easily sited along highways and within portions of 

maintenance stations, and do not require specialized maintenance. In addition, the test sites were similar 

in many ways to the vegetated shoulders and conveyance channels common along highways in many 

areas of the state. Consequently, these areas, which were not designed as treatment devices, could be 

expected to offer water quality benefit comparable to the engineered sites. More research is needed to 

investigate this possibility. 

Maryland - Grassed Swale Pollutant Removal Project 

In Maryland, grassed swales are routinely incorporated into highway medians and right-of-way as an 

aesthetically pleasing method for conveying highway runoff (MDOT, 2005). This project evaluated the 

performance of grassed swales as a stormwater management technology that can remove surface runoff 

contamination through sedimentation, filtration by the grass blades, infiltration to the soil, and some 

likely biological processes. Three storm events were monitored to evaluate pollutant removal effects of 

two grassed swales receiving highway runoff, and of the pollutant removal efficiencies of grassed swales. 

The researchers observed significantly reduced concentrations of many pollutants. 

Interim Report A-28 October 09 
 

 

Results confirm the positive event mean concentration (EMC) removal (35 to 84%) of most pollutants of 

interest, including total suspended solids, nitrate, nitrite, total Kjeldahl nitrogen, copper, lead and zinc. 

The EMC was calculated by combining the flow and concentration data for a total pollutant mass. 

However, the swales demonstrated some export of phosphorus and chloride. Export of phosphorus in a 

natural system like a grassed swale is understandable because this element is present in all organic 

material. 

Minnesota - Improving the Design of Roadside Ditches to Decrease Transportation-Related 

Surface Water Pollution 

This project involved a field monitoring program that began in spring 2000 to test the ability of a grassy 

roadside swale to remove pollutants in stormwater (Biesboer and Elfering, 2003). A check dam was 

designed and installed into the vegetative swale. The check dam system incorporated some unique design 

features, including a peat filter to trap nutrients and metals and a low rock pool to trap water for the 

settling of suspended solids and for biological processing. The check dam was cost-effective and simple 

to install. The system was quantified and evaluated hydrologically and qualitatively before and after the 

check dam installation. Pollutants monitored included total suspended solids, total phosphorus, and 

orthophosphorus. The average pollutant removal rates for three storms were 54% for total phosphorus, 

47% for orthophosphorus, and 52% for total suspended solids. Metals were also analyzed for two storm 

events, one before and one after installation of the check dam. Peat soil samples were analyzed for 

nutrients, organic content, water capacity, metals, and pH before and after check dam installation. The 

results suggest that properly designed short vegetative strips and swales can reduce pollutant levels from 

the stormwater that drains off roadways. 

Further research is also needed to determine the efficiency of the check dam at removing large pollutant 

loadings similar to those exiting highly traveled roads. The pollutant loadings in this research were 

relatively small. Greater removal efficiency values may be realized with the addition of a series of check 

dams in longer roadside swales. Also, more research needs to be performed analyzing the check dam 

efficiency at reducing heavy metals. 

Texas - Use of Vegetative Controls for Treatment of Highway Runoff 

This study investigated the capability of two vegetative controls—grassed swales and vegetated buffer 

strips—to treat highway runoff (Walsh et al., 1998). A grassed swale was constructed in an outdoor 

channel to investigate the impacts of swale length, water depth, and season of the year on pollutant 

removal efficiency, and two vegetated strips treating highway runoff in the Austin area were monitored to 

determine removal capabilities. 

A grassed swale constructed in a steel channel removed over 50% of the suspended solids, zinc, and lead 

after 120 ft of swale treatment. COD (chemical oxygen demand) concentrations decreased 25 to 79% 

after 120 ft of treatment, while the reduction of nutrient concentrations varied from negative to 45%. In 

general, the majority of pollutant removal occurred in the first 60 ft of swale. Increasing the water depth 

and velocity of surface flow of runoff in the swale reduced the removal efficiency of the swale. More 

suspended solids were removed in the channel swale in the growing season than in the dormant season. 

During the growing season, new grass stood alongside dormant grass that increased the grass blade 

density in the swale. This increase in removal is attributed to the combined filtering capacity of the dead 

material and live grasses. The removal of nutrients and organic material may decline in the growing 

season, when decay of vegetation from the previous season contributes to the constituents in the runoff. 

The concentrations of constituents in runoff that had percolated through the soil in the swale were 

generally lower than the concentrations in surface runoff after 120 ft of treatment by the swale. However, 

Interim Report A-29 October 09 
 

 

the impact of swales on ground-water quality in the field will vary with thickness of soil to ground water, 

permeability of the soil, and the constituents in the highway runoff. 

Texas - Evaluation of the Performance of Permanent Runoff Controls: Summary and Conclusions 

This study found that pollutants in runoff from highways may produce adverse impacts in receiving 

waters under some conditions (CRWR, 2007). The Edwards Aquifer is particularly vulnerable to this 

type of nonpoint source pollution and concern about the potential impact on the aquifer has led to the 

construction of stormwater controls on highways in the Austin area. This study was designed to help the 

Texas Department of Transportation (DOT) identify the types of runoff control systems that are most 

applicable for highways in this area. The study investigated the capability of vegetative controls (grassed 

swales and vegetated buffer strips) and sedimentation/filtration systems for treating stormwater runoff. 

For conclusions and recommendations for the study, data from channel swales indicated: 

•	 Removal of total suspended solids, chemical oxygen demand, total phosphorus, total Kjeldahl 

nitrogen, zinc and iron was highly correlated with swale length. No trend was observed for 

nitrate. 

•	 Most of the reduction in the concentration of constituents in runoff occurred in the first 60 ft of 

the swale. Little improvement in water quality was observed during the last 60 ft. 

•	 The removal efficiency for suspended solids, organic material and most metals decreased with 

increased water depth. No relationship between water depth and removal efficiency was 

observed for nitrate and total Kjeldahl nitrogen. 

•	 The removal efficiency of the grassed swale was about the same during the dormant and growing 

season for all constituents except for total suspended solids. Total suspended solids experienced 

the highest removal during the growing season, when there is a combination of new grass and 

remaining dormant grass. 

•	 Percolation of runoff through layers of soil and gravel into the underdrain reduced concentrations 

of all constituents except nitrate. 

•	 Excellent pollutant removal occurred in the channel swale when the hydraulic residence time was 

approximately nine minutes. The removal was similar to that of a site monitored in Seattle that 

had about the same residence time, but differed in other aspects. Hydraulic residence time 

appears to be an appropriate design criterion for grassed swales. 

References 

Backstrom, M. 2003. Grassed Swales for Stormwater Pollution Control During Rain and Snowmelt. 

Water Science and Technology.Vol. 48(9). pp 123-134. 

Barrett, M.; Lantin, A.; Austrheim-Smith, S. 2004. Stormwater pollutant removal in roadside vegetated 

buffer strips. Transportation Research Record. Vol. 1890. pp 129-140. 

Biesboer, David, and Jodi Elfering. 2003. Improving the Design of Roadside Ditches to Decrease 

Transportation-Related Surface Water Pollution. For Minnesota Department of Transportation Research 

Services Section, June 2003. http://www.mrr.dot.state.mn.us/research/pdf/200411.pdf 

Interim Report A-30	 October 09 
 

 

 

Center for Research in Water Resources (CRWR), University of Texas, Austin. 1997. Evaluation of the 

Performance of Permanent Runoff Controls: Summary and Conclusion. CRWR Online Report 97-3. 

http://www.crwr.utexas.edu/reports/pdf/1997/rpt97-3.pdf 

California Department of Transportation (CALTRANS). 2004. California BMP Retrofit Pilot Program: 

Final Report. CALTRANS - Division of Environmental Analysis, Report CTSW-RT-01-050. 

http://www.dot.ca.gov/hq/env/stormwater/special/newsetup/_pdfs/new_technology/CTSW-RT-01­

050.pdf 

Clar, Michael L. 2004. Stormwater Best Management Practice Design Guide Volume 2: Vegetative 

Biofilters. U.S. Environmental Protection Agency. 

http://www.epa.gov/nrmrl/pubs/600r04121/600r04121a.pdf 

Walsh, F., Barrett, M.E., Malina, Jr.,J.F., Charbeneau, R.J. 1998. Use of Vegetative Controls for 

Treatment of Highway Runoff. Federal Highway Administration (FHWA), Report No. FHWA/TX-7­

2954-2. http://www.utexas.edu/research/ctr/pdf_reports/2954_2.pdf 

Highways Agency. 2006. Vegetated Drainage Systems for Highway Runoff. Design Manual for Roads 

and Bridges. http://www.standardsforhighways.co.uk/dmrb/vol4/section2/ha10306.pdf 

Maryland Department of Transportation (MDOT). 2005. Grassed Swale Pollutant Removal Project 

Presented at AASHTO Region 1 RAC Research Highlights 2005 

http://www.nh.gov/dot/org/projectdevelopment/materials/research/documents/2005_rac.pdf 

Melbourne Water. 2008. Litter Traps: Litter Trap Types. 

http://wsud.melbournewater.com.au/content/treatment_measures/litter_traps/litter_trap_types.asp 

Melbourne Water. 2008. WSUD Swales: Treatment Performance. 

http://wsud.melbournewater.com.au/content/treatment_measures/swales/treatment_performance.asp 

Nara, Y., and R. Pitt. 2005. Alabama Highway Drainage Conservation Design Practices—Particulate 

Transport in Grass Swales and Grass Filters. University Transportation Center for Alabama. 

http://rpitt.eng.ua.edu/Publications/UTCA/UTCA%20final%20swales%20report.pdf 

Scholz, Miklas. 2007. Development of a Practical Best Management Practice Decision Support Model for 

Engineers and Planners in Nordic Countries. Nordic Hydrology. Vol. 38(2). pp 107-123. 

U.S. Environmental Protection Agency. 1999. Storm Water Technology Fact Sheet, Vegetated Swales. 

EPA-832-F99-006. http://www.epa.gov/owm/mtb/vegswale.pdf 

Interim Report A-31 October 09 
 

 

Section 6: Rooftop Greening 

Green roofs provide myriad environmental benefits at an economical life-cycle cost. Since the discovery 

of their varied uses and contribution to environmental quality over 40 years ago, they have been utilized 

first by European nations, later by the US, Japan, and other nations (Figure 8). Although most of the 

advances in technology and public policy regarding green roofs have occurred in Germany, they have 

served as a case study and catalyst for the rest of Europe and the world to follow. In Melbourne, 

Australia around 90% of its central business district consists of green rooftops (Melbourne Water, 2008). 

With this opportunity for water sensitive project developments, Australia has become a leader in the use 

of WSUD strategies. One such project involved the use of a rooftop garden to contain stormwater and 

use it for toilet flushing (see Case Studies). 

The UK has seen an increased interest in green roofs in the past decade, even though it remains without 

incentives, standards, or policies to propagate the use of green roof technology. This could be due to 

misinformation and myths that still surround green roofs. Many developers mistakenly see green roofs as 

an obstacle, given the myth that vegetated roofs cause structural problems. There is a preference among 

professionals in the UK to use pitched roofs instead of flat ones; this gives concerns that green roofs are 

more liable to leak than traditional roofs. Green roofs will actually introduce a number of measures that 

protect the roofs from degradation. These include waterproofing systems and protection against 

ultraviolet light, frost, erosion, and other forms of weathering. Urban regeneration schemes have brought 

a renewed focus on getting green roofs installed in UK cities. Government incentives and a strong 

regulatory framework are needed for green roof technology to really gain ground in any country. 

In 1997 the Swiss undertook green roofing research as it relates to biodiversity. The concerns in 

Switzerland came from the development of brownfield land in urban settings; land which is important 

habitat for a number of scarce beetles and spiders. The following design principles were arrived at in 

regards to green roofs and biodiversity: 

•	 The use of local substrates as growing mediums on green roofs helped replicate the conditions at 

ground level. 

•	 Varying the depth of the substrate provided microhabitats for rare spiders and beetles associated 

with brownfields in the city. 

•	 Planting with a local seed mix. 

•	 The placing of objects associated with natural habitats such as dead wood and old branches 

increased the biodiversity of the roofs. 

The goal of biodiversity with the installation of green roofs gave way to the coining of the term “brown 

roof” by the same Swiss researchers. Brown roof refers to a green roof that has certain specifications 

deliberately added to ensure biodiversity for development in brownfield areas. Designing roofs to be 

suitable habitat for specific indigenous species is just another adaptation of the flexibility of this 

technology for urban landscapes. 

The US has benefitted from the efforts of the US Green Building Council (USGBC), which has been at 

the forefront of environmental design. The Council has introduced a new way to certify (and therefore 

publicly endorse) buildings that have the initiative to use green building techniques, green roofs being a 

big part of it. This certification is called Leadership in Energy and Environmental Design (LEED). This 

Interim Report A-32	 October 09 
 

 

program creates levels of certification that gives people incentive to think and build green (USGBC, 

2008). 

Figure 8. A terraced roof garden building in Fukuoka City, Japan (MetaEfficient, 2008). 

Green Roof Benefits 

Water Quality 

The hard, impervious surfaces that cover urban landscapes have made stormwater control an important 

issue to both conservationists and municipal sewer districts. During storm events wastewater and 

stormwater flows spike from the inability of sidewalks, buildings, and roads to control and slow the 

release of rainfall runoff. Green roofs reduce this runoff and store the water temporarily to release it 

slowly over time. The burden that storm events have on wastewater treatment plants is also mitigated by 

green roofs. The occurrences of CSO and SSO events declines in correlation to the percentage of green 

roof cover a city has. Green roofs return moisture into the atmosphere by evapotranspiration, and unlike 

other land intensive stormwater control technologies, green roofs use existing roof space. Depending on 

substrate depth, green roofs can retain 25 to 100% of rainfall (Beattie and Berghage, 2004), and reduce 

total building runoff by 60 to 79% (Köhler et al., 2002). 

Thermal Insulation 

Green roofs are an effective way to regulate energy flow in a building. The primary ways in which 

vegetation provides an insulating effect are weather dependent (Porsche, 2003). The first way is by heat 

transfer and the way in which vegetation works to prevent energy losses in the summer from air 

conditioning. The cavities in the soil and the absorption of heat by the plants, as well as the water 

retention by the soil provide excellent insulation in hot weather. Of course depending on the size of the 

Interim Report A-33 October 09 
 

building and the type of green roof application (extensive, intensive, strata depth, etc.) the savings in 

energy will change with respect to these variables. Environment Canada used a Micro Axess Simulation 

model to assess the energy saving in a typical one-story building with a grass roof and 3.9 in. of growing 

medium. The model found that this type of installation would have a 25% reduction in summer cooling 

needs (The Cardinal Group, 2003). The sod layer on a green roof with its rough surface as opposed to 

smooth tar roof also decreases convection losses. 

Urban Heat Island 

The effects of urban heat islands have been widely documented. Replacing vegetation with dark, 

impervious surfaces has led to temperature increases between 2 to 10 ̊F in urban areas in contrast to 

surrounding rural areas. This effect impacts city dwellers by increasing energy costs (e.g., cost for air 

conditioning), air pollution levels, and heat-related illness and mortality. This effect is especially 

significant at night, when dark surfaces, absorbing heat from the sun during the day, radiate that heat into 

the air at night. The surface temperature of a traditional rooftop can be up to 90 ̊F hotter than a vegetated 

roof (EPA, 2008). A regional simulation model showed that a total of 50% green roof coverage 

distributed throughout Toronto would reduce temperatures by as much as 2 ̊C (Bass et al., 2002). 

Biodiversity 

In addition to the advantages green roofs have in protecting important native species as described earlier, 

these roofs are used by nesting and native birds, insects, spiders, beetles, plants, and lichens which have 

all been found on green roofs. For the human city dwellers, the benefits are seen in the relaxation, 

aesthetics, and stress-reduction effects that green roofs provide. 

Cost 

Developers are driven by the market forces of both lowering overhead costs to build and consumer 

demand. With greater consumer demand for urban roof gardens, more developers will have to make it a 

standard practice wherever green roofs are applicable. They can be fitted for new buildings and 

retrofitting for old buildings. In Europe, green roofs have had the attention of public policy makers for 

decades, making them much more affordable now compared to the US green roof industry, which is just 

getting off the ground. In Europe, a green roof will cost approximately $4/ft2

 to 13/ft2, whereas in the US 

it can cost upwards of $10/ft2

 to $25/ft2 (depending on the type of roof chosen). In general a green roof 

will have an initial cost roughly three times higher than a traditional low-priced prefabricated or welded 

roof (Porsche, 2003). The trick, as with many emerging environmental technologies and renewable 

energies, is to look past initial cost to what is now driving city planners and public policy makers across 

international borders; the long-term sustainability of a green roof infrastructure. Lower priced traditional 

roofs often need replacement or significant repair every 15 years. Green roofs, however, will survive 

thirty years or more. Table 2 explores the life-cycle costs of green roofs in Germany over a typical 90­

year building lifecycle for green roofs (Porsche, 2003). 

Interim Report A-34 October 09 
 

Table 2. Life-cycle costs of different types of roofs in Germany. 

As mentioned previously there is a substantial practical gain in utilizing green roofs as thermal insulators. 

Green roofs also provide an economic gain as well. With energy prices what they are today, cutting 

cooling costs in the summer up to 25% would contribute to vast cost savings over both the short and long 

term. 

Case Studies 

Runoff Detention Effect of a Sedum Green Roof 

This study was conducted at Lund University in Sweden by the Department of Water Resources 

Engineering (Villareal, 2007). A green roof plot was constructed to test the infiltration and peak 

attenuation rate of a sedum species of plant as the media for the green roof plot. The experiments were 

made in comparison to a traditional impervious roof as the control. The end result, as seen in the 

hydrographs in Figure 9, is that the green roof had a higher detention rate and lower peak flow for wetweather events. The volume reduction observed for the green roof was up to 65% for some storms. A 

typical cross-section can be seen in Figure 10. 

Interim Report A-35 October 09 
 

 

Figure 9. Inflow and outflow hydrographs for a sedum green roof and traditional roof. Green roof 

(black line) and traditional roof (grey line) runoff for (a) two-year average (b) “test” rainfall of 0.8 

mm/min for 22 min (c) rainfall event on August 2, 2002, (d) rainfall event, July 22, 2001. 

Figure 10. Typical cross-sectional layout of a green roof. 

Interim Report A-36 October 09 
 

Toronto Green Roof Study 

Research was completed to provide technical data on the performance of green roofs in the City of 

Toronto to illustrate their benefits in an urban context (Liu and Minor, 2005). Two extensive green roof 

systems were installed on a community center in Toronto. Both systems contained the same components 

but differed in materials and designs (Figure 11). Green Roof System G consisted of a composite semirigid polymeric drainage and filter mat and a root-anchoring mat. It had 100 mm of lightweight growing 

medium containing small light-colored granules. Green Roof System S consisted of expanded 

polystyrene drainage panels and a geotextile filter fabric. It had 75 mm of lightweight, dark-colored 

growing medium containing porous ceramic granules. The green roofs, and a reference roof, were 

instrumented to provide thermal performance at the underlying impermeable membrane (moisture barrier) 

and energy efficiency data, as well as runoff measurements. Although the vegetation was not well 

established in the first year of monitoring, the extensive green roofs reduced the building’s energy 

demand by lowering the heat flow through the roof, especially in the summer (Figure 12). The green 

roofs were shown to be effective in delaying and reducing stormwater runoff. The ability of roofs to 

retain volume depended upon the characteristics of the rain event (intensity and amount) and the wetting 

history of the growing medium. Preliminary observations and membrane temperatures recorded also 

suggested that green roofs could likely improve membrane durability by reducing heat aging, thermal 

stresses, ultra-violet radiation and physical damage. 

Figure 11. Schematics showing components and the sensor locations in the roofing systems (Liu and Minor, 

2005). 

Interim Report A-37 October 09 
 

Figure 12. Daily maximum (dark) and minimum (light) membrane temperatures on the green roofs. 

A Review of 18 Green Roof Studies 

Researchers analyzed the data that was reported in 18 different green roof publications (Mentens et al., 

2006). They obtained rainfall-runoff relationships for an annual and seasonal time scale from the analysis 

of the available 628 data records. They derived empirical models that allowed them to assess the surface 

runoff from various types of roofs, when roof characteristics and the annual or seasonal precipitation were 

given. Their analysis showed that the annual rainfall-runoff relationship for green roofs was strongly 

determined by the depth of the substrate layer (Figure 13 and Table 3). The retention of rainwater on 

green roofs was lower in winter than in summer. 

Interim Report A-38 October 09 
 

Figure 13. Annual runoff of intensive green (int), extensive green (ext), gravel-covered (gravel) and 

traditional (trad) roofs as a percentage of the total annual rainfall. Data range (whiskers), 25% and 75% 

percentiles (box boundaries), and the median (line within box) are reported. 

Table 3. Comparison of Substrate layer depth and percent runoff of intensive green (int), extensive green 

(ext), gravel-covered (gravel) and traditional (trad) roofs. 

Green Roofs in Sweden 

An experimental station in Augustenborg, Sweden, was chosen as a site to test the hydrologic function of 

an extensive green roof (Bengtsson et al., 2005). The site was chosen because of the strain on the area’s 

combined sewer systems and frequent flooding. The study found that during a short storm, runoff in 

excess of field capacity is temporarily stored in soil and roof vegetation reducing and delaying peak flow 

(Figure 14). The maximum storage capacity for the thin, extensive green roof was found to be 9 to 10 

mm. 

Interim Report A-39 October 09 
 

Figure 14. Rain (dark line) and runoff (grey line) and water storage on a thin green roof in 

Augustenborg, Sweden. 

Green Roof Research and Solutions in Belgium 

Using Brussels as an example, researchers ran a simulation to determine the effectiveness of green roofs 

in reducing rainfall runoff for an entire city (Mentens et al., 2006). The results indicated that extensive 

green roof systems on 10% of the buildings would result in a reduction of 2.7% of annual runoff for the 

region, and 54% of annual runoff for individual buildings. This function is particularly important given 

that urbanization in developed countries is expected to have 83% of the population living in urban areas 

by 2030. The study came to the conclusion that green roofs are especially important tools because they 

do not take up “open space” on the ground to function. They also point out that using green roofs in 

conjunction with other infiltration technologies will have the impact needed to make cities sustainable 

into the future. 

Stormwater Monitoring of Two Ecoroofs in Portland, Oregon, US 

When the City of Portland’s Bureau of Environmental Services (BES) began considering ecoroofs for 

stormwater management, no applicable performance data could be located (Hutchinson et al., 2003). To 

generate region-specific data, BES initiated a monitoring project of the Hamilton West apartment building 

vegetated with two different ecoroofs. More than two years of water quality monitoring and more than a 

year of flow monitoring have been measured. Precipitation retention has been calculated at 69% for the 4 

to 5 in. ecoroof substrate section, and nearly all of the rainfall is absorbed during dry period storm events. 

Stormwater detention and peak intensity attenuation has also been impressive even when the roof was 

saturated during winter months (Figures 15 and 16). Some water quality benefits have proven more 

difficult to quantify, but important water quality lessons have been learned. In situations where a 

receiving water system may be sensitive to certain pollutants, substrate composition will be an important 

consideration in the ecoroof design. The researchers’ work to date has proven that ecoroofs can be an 

effective urban stormwater management tool. Their next major endeavor will be to apply this information 

to system modeling efforts to determine hydrologic and hydraulic infrastructure and stream benefits that 

Interim Report A-40 October 09 
 

may be achieved. This information is also expected to assist bureau managers, planners, engineers and 

elected officials with policy decisions, such as zoning density bonuses, infrastructure designs, drainage 

fee discounts, and code compliance. 

(a) 

(b) 

Figure 15. Storm peak intensity attenuation for the Hamilton West Ecoroof: (a) high intensity, short duration 

winter storm; (b) high intensity, short duration summer storm (Hutchinson et al., 2003). 

Interim Report A-41 October 09 
 

(c) 

(d) 

Figure 16. Storm peak intensity attenuation for the Hamilton West Ecoroof: (c) low intensity, high volume winter 

storm; and (d) low intensity, low volume winter storm (Hutchinson et al., 2003). 

Performance of Two Green Roofs in North Carolina, US 

Two green roofs were constructed and monitored in North Carolina’s Neuse River Basin (Figure 17) 

(Moran et al., 2006). The green roof at Wayne Community College (WCC) in Goldsboro was 

constructed in May 2002 and is approximately 750 ft2

. The original, flat rooftop of this storage building 

was divided into two equal halves for research purposes; one half remained unchanged and became the 

Interim Report A-42 October 09 
 

Table 4. Summary of water retention and peak flow reduction for each research site 

Total Average Average 

Green Roof Total Rainfall Percent Peak Green Roof Percent 

Location Rainfall Retained Retained Rainfall Runoff Reduction 

Goldsboro, NC 59.6 in. 37.8 in. 63% 1.4 in./hr .18 in./hr 87%
 

Raleigh, NC 12.4 in. 6.8 in. 55% 1.7 in./hr 0.75 in./hr 57%
 

control for the experiment and the other half was transformed into the WCC green roof. The average soil 

media depth at this site is 3 in. Hydrodrain 300TM was used on the WCC roof; it has a non-woven filter 

fabric system incorporated into its design and has negligible storage. 

Figure 17. (a) WCC green roof in Goldsboro, NC (April 2003). (b) B&J green roof in Raleigh, NC (August 

2004) (Moran et al., 2006). 

The 1400 ft2 green roof atop the Brown & Jones Architects, Inc (B&J) office in downtown Raleigh was 

constructed in February 2003 as a retrofit project. The rooftop was divided into two halves; one half 

restored as a control roof and the other half was constructed into a green roof. The green roof has a 7% 

slope and an average soil media depth of 4 in. AmergreenTM 50RS was used at the B&J green roof; this 

drainage material has a water storage capacity of 0.06 gal/ft2

. 

The hydrologic and water quality performance of each green roof were investigated. Each green roof 

retained a significant proportion of the rainfall (Table 4 and Figure 18). Peak outflows were significantly 

reduced from the green roofs and each green roof had substantial delays in runoff (Figure 19). Runoff 

coefficients from the WCC green roof were calculated for storms at least 1.5 in. in size. The rational 

coefficient averaged 0.50 for ten storm events. Both the concentrations and amounts of total nitrogen and 

total phosphorus increased from rainfall to green roof outflow and from the control roof outflow to green 

roof outflow (Figure 20). It was determined that the soil media, composed of 15% compost, was leaching 

nitrogen and phosphorus into the green roof outflow. This field study demonstrated the importance of 

green roof media selection in locations where nutrients are a concern. 

Interim Report A-43 October 09 
 

Figure 18. Monthly retention rates of the WCC green roof from April 2003 to September 2004. 

Figure 19. Peak flow reduction of green roof runoff at WCC green roof on April 7, 2003. 

Interim Report A-44 October 09 
 

Figure 20. Concentrations of total nitrogen (top) and total phosphorus (bottom) from April 2003 to 

September 2004 from WCC green roof runoff. 

References 

Bass, B., E.S. Krayenhoff, A. Martilli, R.B. Stull, and H. Auld. 2003. “The Impact of Green Roofs on 

Toronto’s Urban Heat Island”, Proceedings of the First North American Green Roof Conference: 

Greening Rooftops for Sustainable Communities; Chicago, IL USA. 

Interim Report A-45 October 09 
 

 

Beattie, D., and R. Berghage. 2004. “Green Roof Media Characteristics: The Basics”, Proceedings of the 

Second Annual Greening Rooftops for Sustainable Communities Conference, Awards and Trade Show; 

Portland, OR, USA. 

The Cardinal Group. 2003. Green Roofs for Healthy Cities. www.greenroofs.ca. 

Harvard Green Campus Initiative. 2007. Green Roofs at Harvard. 

http://www.greencampus.harvard.edu/hpbs/green_roofs.php#Costs 

Hutchinson, D., Abrams, P., Retzlaff, R., and T. Liptan. 2003. Stormwater Monitoring Two Ecoroofs in 

Portland, Oregon, USA. Presented at the Conference “Greening Rooftops for Sustainable Communities’ 

Chicago, IL, USA. 

Liu, K., and J. Minor. 2005. Performance Evaluation of an Extensive Green Roof. In Greening Rooftops 

for Sustainable Communities, Washington, D.C., USA. 

http://www.greenroofs.com/projects/pview.php?id=59 

Köhler, M., M Schmidt., F.W. Grimme, M. Laar, V.L. de Assunção Paiva, and S. Tavares. 2002. Green 

Roofs in Temperate Climates and in the Hot-Humid Tropics—Far Beyond the Aesthetics. Environment 

and Health Vol. 13. pp 382–391. 

Melbourne Water. 2008. Water Sensitive Urban Design (WSUD). http://wsud.melbournewater.com.au/ 

Mentens, J., Raes, D., and M. Hermy. 2006. Green Roofs as a Tool for Solving the Rainwater Runoff 

Problem in the Urbanized 21st Century? Landscape and Urban Planning. Vol. 77. pp 217–226. 

MetaEfficient: A Guide to Highly Optimal Things. No date. Amazing Green Building: The ACROS 

Fukuoka. http://www.metaefficient.com/architecture-and-building/amazing-green-building-the-acrosfukuoka.html 

Moran, A, W. Hunt, and J. Smith. (Masters Thesis). 2006. Hydrologic and Water Quality Performance 

from Greenroofs in Goldsboro and Raleigh, North Carolina. North Carolina State University, Department 

of Biological and Agricultural Engineering, Raleigh, North Carolina. 

Porsche, Ulrich., Kohler, and Manfred. 2003. Life Cycle Costs of Green Roofs. University of Applied 

Sciences Neubrandenburg, Germany. pp 461-467. 

Villareal, E.L. 2007. Runoff Detention Effect of a Sedum Green Roof. Nordic Hydrology. Vol. 38(1). pp 

99-105. 

U.S. Environmental Protection Agency. 2008. Heat Island Effect. http://www.epa.gov/hiri/ 

U.S. Green Building Council. 2008. Welcome to USGBC. http://www.usgbc.org 

Interim Report A-46 October 09 
 

Section 7: Porous Paving 

Using existing infrastructure as much as possible while lowering maintenance costs are key components 

of watershed management. Using eco-paving, or porous pavement, instead of conventional impervious 

pavement allows for both benefits to coexist. Porous paving started being widely used as a flood 

mitigation technique in Europe during the early 1980s (Frederico, 2006). The needs of urban areas to 

plan effective water management systems comes out of the sprawling development that has characterized 

cityscapes around the world over the past few decades. More urban development means more impervious 

surfaces being laid down, such as ever-widening roads, sidewalks, roofs and parking lots. This means an 

ever-decreasing ability for natural rainfall to infiltrate back into the ground. Porous pavements are a 

method to help infiltrate storm runoff that fits well into urban landscapes (Figure 21). There is a wide 

range of permeable pavement types, each with its own benefits and drawbacks based on how the paved 

area will be used. No matter which type is utilized, they all can be expected to have the following 

benefits for urban ecology and wastewater management: 

• Help decrease the frequency of CSO events 

• Mitigate thermal pollution of neighboring sensitive waterways 

• Provide water for groundwater recharge 

• Control erosion of streambeds and riverbanks 

• Facilitate pollutant removal 

• Reduce imperviousness while maximizing land use 

• Retain highway pollutants within the substructure of permeable pavements 

• Improve aesthetics over conventional impervious pavements 

Figure 21. Reinforced gravel (top left), reinforced grass pavement (middle), and 90% impervious 

blocks with gravel (U.S. Environmental Protection Agency, 2000). 

Although originally a method to control flooding, recent drought conditions in Australia have forced 

scientists and developers to use porous pavement in a different way. Using porous pavement in tandem 

with tanked systems can provide water-thirsty cities an option to reuse rainwater runoff instead of losing 

this resource (Figure 22). Outflow water from the tanked permeable pavements can be used for 

everything from gardening to flushing toilets (Federico, 2006). 

Interim Report A-47 October 09 
 

 

Figure 22. An example of a tanked permeable pavement system being built (Frederico, 2006). 

There are few studies showing the price competitiveness of permeable pavements and conventional 

pavements. Construction costs are relatively comparable with permeable pavements, but they have a 

somewhat higher initial cost. However, as with all sustainable infrastructure projects the cost savings will 

be measured with time. When the overall savings to stormwater infrastructure are considered, porous 

paving can be up to three times less expensive than traditional measures to manage the urban water cycle 

(Melbourne Water, 2008). 

Case Studies 

Review of the Performance of Permeable Pavers in Australia 

In addition to the stormwater management benefits that controlling and slowing storm runoff provides; 

Rankin and Ball (2004) developed an experiment to see if permeable pavements filter the runoff and 

improve water quality as well (Figure 23). This case study in Sydney, Australia reviewed the 

performance of permeable pavements as pollutant reduction technologies. The results were a reduction of 

42% imperviousness on the street. The porous pavement did reduce the pollutant loadings by reducing 

the overall flow from the street. However, the permeable pavement did not have a significant 

independent effect on water quality. The total load of phosphorus and other heavy metals were reduced 

because the total flow of runoff was decreased. 

Interim Report A-48 October 09 
 

 

Figure 23. An example of a permeable pavement infiltration. 

Effects of a Porous Pavement with Reservoir Structure on Runoff Water in France 

A study by Legret and Colandini (1999) in Rezé, France was conducted to determine the fate and 

transport of heavy metals in stormwater runoff through porous pavement. The effluent from a traditional 

catchment with impervious pavement in a separate sewer system was compared to that of a reservoir 

structure under permeable pavement. The metals studied were: copper, zinc, cadmium, and lead. 

Suspended solids were also measured. As seen in Table 5, the authors report that the quality of the water 

passing through the permeable pavement and flowing out of the reservoir structure is significantly 

improved compared to a traditional separate sewer system outflow, with the exception of copper which 

remained relatively similar. 

Table 5. Comparison of event pollutant loadings for porous pavement relative to a reference site. 

Interim Report A-49 October 09 
 

 

 

Heavy Metal Retention by Porous Pavement in Germany 

Porous pavements as a means of pollutant load reduction were tested in Germany by Dierkas et al. 

(1999). Different base materials under porous concrete blocks were tested and the results were 

extrapolated for 50 years. The research showed that the porous pavement sequestered most heavy metals 

(Table 6). Those metals tested were lead, cadmium, copper and zinc (common urban street runoff 

pollutants). The German limits for seepage water were not exceeded within the structures. 

Table 6. Heavy metal concentrations and percentage of metal retained in runoff from porous 

pavement infiltration of 50 years of equivalent loads compared to permissible limits for seepage. 

Monitoring Permeable Pavement Sites in North Carolina, US 

Three permeable interlocking concrete pavement (PICP) sites were monitored across North Carolina in 

Cary, Goldsboro, and Swansboro (Figure 24) by Bean et al. (2004). 

Figure 24. Photographs of Cary (left), Goldsboro (center), and Swansboro (right) PICP sites (Bean et al., 

2004). 

The Cary site was located in clay soil and flow rates and samples of exfiltrate and rainfall over 10 months 

were collected and analyzed for pollutant concentrations (Tables 7 and 8). 

Interim Report A-50 October 09 
 

 

Table 7. Hydrologic summary of results from Cary PICP site (Bean et al., 2004). 

Table 8. Mean pollutant concentrations and factors of significance for Cary site (Bean et al., 2004). 

The Goldsboro site was constructed to compare the water quality of asphalt runoff to exfiltrate of 

permeable pavement. The site was located on a sandy soil and samples were analyzed for pollutants over 

a span of 18 months. PICP exfiltrate from the Goldsboro site had significantly lower concentrations of 

Total Phosphorus and Zinc compared to asphalt runoff (Table 9). Total Nitrogen (TN) concentrations 

were close to significantly lower in exfiltrate, but did show an increasing trend of TN removal. 

Table 9. Pollutant summary for Goldsboro site (Bean et al., 2004). 

The Swansboro site was constructed and instrumented to monitor runoff flow and rainfall rates and 

collect exfiltrate and runoff samples from the permeable pavement lot over ten months. The site was 

located on very loose sandy soil and experienced no runoff. 

Testing Permeable Pavements in Renton, Washington, US 

Research by Brattebo and Booth (2003) examined the long-term effectiveness of permeable pavement as 

an alternative to traditional impervious asphalt pavement in a parking area. Eight stalls were constructed 

with four types of commercially available permeable paving systems, with two neighboring stalls covered 

Interim Report A-51 October 09 
 

with each of the four permeable paving systems. The permeable pavement systems used in this study 

were: 

•	 Grasspave2®, a flexible plastic grid system with virtually no impervious area, filled with sand 

and planted with grass. 

•	 Gravelpave2®, an equivalent plastic grid, filled with gravel. 

•	 Turfstone®, a concrete block lattice with about 60% impervious coverage, filled with soil and 

planted with grass. 

•	 UNI Eco-Stone®, small concrete blocks with about 90% impervious coverage, with the spaces 

between blocks filled with gravel. 

Each test parking stall was 3 m wide by 6 m long. A series of gutters and pipes were used to collect both 

surface runoff and subsurface infiltrate. The stalls were evaluated after six years of daily parking usage 

for structural durability, ability to infiltrate precipitation, and impacts on infiltrate water quality. 

Visual inspection of the permeable pavement systems showed varying, but generally minor, signs of wear 

and tear after six years. In two small areas, the interlocking sheets of the Grasspave2® and the 

Gravelpave2® plastic matrix had shifted slightly and partly lifted out of the soil in the area where the rear 

wheels of the parked cars typically rest. The Turfstone® and UNI Eco-Stone® showed no areas of 

rutting, settling, or shifting. Grass was growing uniformly across the Turfstone® surface, but more spotty 

(and locally quite sparse) in the Grasspave2® stalls. 

Virtually all rainwater infiltrated through the permeable pavements, with almost no surface runoff. The 

infiltrated water had significantly lower levels of copper and zinc than the direct surface runoff from the 

asphalt area (Table 10). Motor oil was detected in 89% of samples from the asphalt runoff but not in any 

water sample infiltrated through the permeable pavement. Neither lead nor diesel fuel were detected in 

any sample. Infiltrate measured five years earlier displayed significantly higher concentrations of zinc, 

copper, and lead. 

Table 10. Mean concentrations of detected constituents from storm samples in 1996 and 2001-02. 

Note: 1996 results from Booth and Leavitt (1999) in square brackets. 

Interim Report A-52	 October 09 
 

References 

Anon. 2002. Research into Effective Life. of Permeable Pavement Source Control Installations. Urban 

Water Research Centre, Division of IT, Engineering and the Environment, University of South Australia. 

Bean, E.Z., Hunt, W.F., and D.A. Bidelspach. 2004. A Monitoring Field Study of Permeable Pavement 

Sites in North Carolina. 8th Biennial Conference on Stormwater Research & Watershed Management. 

North Carolina State University, Raleigh, NC. http://www.bae.ncsu.edu/info/permeablepavement/SWFWMD.pdf 

Brattebo, B.O., and D.B. Booth. 2003. Long-Term Stormwater Quantity and Quality Performance of 

Permeable Pavement Systems. Water Research. Vol. 37(18). pp 4369-4376. 

Dierkes, C, Holte, A., and W. F. Geiger. 1999. ”Heavy Metal Retention Within a Porous Pavement 

Structure”, Proceedings from the 8th International Conference on Urban Drainage. Sydney, Australia. 

Field Evaluation of Permeable Pavements for Stormwater Management. Olympia Washington. U.S. 

Environmental Protection Agency. Washington D.C. 841/B00/005B. 2000 

EPA 2000, http://www.epa.gov/owow/nps/pavements.pdf 

Frederico, Gino. 2006. IPWEA NSW Division Annual Conference 2006 

http://www.ipweanswconference.com.au/downloads/Presentations/GinoFederico.pdf 

Legret, M., and V. Colandini. 1999. Effects of a Porous Pavement with Reservoir Structure on Runoff 

Water: Water Quality and Fate of Heavy Metals. Water Science and Technology Vol. 39(2). pp. 111-117. 

Permeable Pavements. Low Impact Development – Urban Design Tools. 2008. 

http://www.lid-stormwater.net/permpavers_benefits.htm 

Porous Paving: Maintenance & Costs. 2008. Water Sensitive Urban Design (WSUD). Victorian 

Government- Melbourne Water. 

http://wsud.melbournewater.com.au/content/treatment_measures/porous_paving/maintenance_and_costs. 

asp 

Rankin, K. and J.E. Ball. 2004. A Review of the Performance of Permeable Pavers. In Water Sensitive 

Urban Design 2004 Conference. The University of New South Wales. Sydney, Australia. 

Interim Report A-53 October 09 
 

Section 8: Infiltration Trenches, Bioretention Systems, and 

Rain Gardens 

Infiltration Trenches 

“Generally, an infiltration structure is designed to store a ‘capture volume’ of runoff for a specified 

period of ‘storage time’,” (Akan, 2002). 

Stream ecology alterations can begin when surfaces are only a few percent impervious. In addition, 

significant water quality and habitat degradation can occur when imperviousness reaches only 8-12% 

(Wang et al. 2001). Facilitating infiltration along multiple lines of stormwater interception gives a higher 

rate of overall success in achieving the benefits associated with wet-weather management. Infiltration 

trenches are excavated trenches, 3 to 12 ft deep, backfilled with a stone aggregate, and lined with filter 

fabric (Figure 25) (Bell, 1999). 

Figure 25. Infiltration trench diagram (Akan, 2002). 

In addition to its ability to manage water, the pollutants that are targeted by infiltration trenches are: 

suspended solids, particulate pollutants, coliform bacteria, organics, trace metals, and nutrients from wetweather runoff (Bell, 1999). Ground-water recharge and baseflow in nearby streams increase with the 

effective use of infiltration trenches. There are a few limitations that exist with the use of infiltration 

trench structures when planning urban infrastructure projects. Some of these limitations include: the 

potential for pollutants and sediment to clog the gravel and infiltration surface, ground-water 

contamination, and low dissolved pollutant removal. Additional limitations may include the preclusion of 

locating infiltration trenches on steep slopes and loose or unstable areas (Melbourne Water, 2008). The 

removal efficiency of an infiltration trench is based on the inspections and maintenance it receives. If not 

properly maintained many infiltration trench designs have a high likelihood of failure and potential for 

groundwater contamination (Bell, 1999). Regular maintenance and inspections also prevents clogging of 

Interim Report A-54 October 09 
 

 

 

the porous media. Clogs can be removed by washing the porous material and replacing the fabric layer 

on top (Melbourne Water, 2008). 

When considering sites for any infiltration project, ground-water contamination must be closely watched. 

Sites that store chemicals or hazardous materials should not be chosen as a good place to put an 

infiltration trench. If this is the case, diversion structures can also be implemented to keep spills and 

potential spills away from infiltration systems (Bell, 1999). 

Bioretention Structures and Rain Gardens 

On the street-scale level, bioretention systems can be introduced into source control methods to treat road 

and roof runoff before these waters end up in streams and rivers (Melbourne Water, 2008). These 

systems can be lined, using underdrains if in situ soils are not well drained (Figure 26). Generally, 

bioretention structures contain 2 ft to 3 ft of porous media (sand/soil/organic mater mixture) covered by a 

thin layer of hardwood mulch, grasses, shrubs, and small trees (Figure 27). This layering and vegetation 

diversity promotes evapotranspiration, maintains soil porosity, and encourages biological activity (Davis, 

2008). 

Figure 26. Construction of a bioretention cell: (a) excavation; (b) placement of underdrains and gravel 

envelope; (c) spreadng of soil media; and (d) planting vegetation at surface of bioretention bed. 

Interim Report A-55 October 09 
 

 

Rain gardens work best under the most decentralized conditions. They are a kind of bioretention structure 

and as a bioretention structure they reduce peak flows, recharge aquifers, and allow stormwater to 

Figure 27. Schematic of a bioretention/rain garden (Davis, 2008). 

infiltrate (Dietz, 2005). Rain gardens can be landscaped into residential lawns and around houses. They 

can be designed to landowner requests, because structurally they require the least design specifications 

compared to other infiltration structures. The low maintenance requirements (especially when planted 

with native species) for rain gardens is an appealing aspect to those seeking wastewater management on a 

localized level. They oftentimes do not require watering, mowing or fertilization after establishment, 

depending on rainfall quantities and timing (Melbourne Water, 2008). 

Case Studies 

Heavy Metal Removal in Cold Climate Bioretention in Norway 

A study by Muthanna et al. (2007) set out to investigate the correlation between temperature and the 

heavy metal removal capabilities of bioretention media. The experiment was undertaken in Trondheim, 

Norway, in a constructed bioretention box. The sample box was put through runoff events corresponding 

to historical data of storms and rainfall. The months chosen for this experiment were April (average 

temperature of 5.8°C) and August (average temperature of 13.4°C) of 2005. It was shown across all 

heavy metal parameters that this type of pollutant removal is more efficient in warmer months. The 

authors indicate that this is due primarily to the partially frozen soil strata and less active above-ground 

biomass affecting infiltration and evapotranspiration rates in colder months. Overall, the bioretention 

system proved to reduce heavy metal pollutants significantly. Zinc was reduced by 90%, lead up to 89%, 

and copper between 60 and 75% (Table 11). The mulch layer was determined to be the major sink for 

metals removal. The authors also found that metal retention was independent of the selected hydraulic 

loading rates (equivalent to 1.4mm/h to 7.5 mm/h precipitation) showing that variable inflow rates during 

the tested set of events did not affect the treatment efficiency of the system 

Interim Report A-56 October 09 
 

Table 11. Average total metal inflow and outflow concentrations from bioretention box. 

Two Bioretention Cells at the University of Maryland, College Park, Maryland, US 

Flow Attenuation 

Flows into and out of two bioretention facilities constructed on the University of Maryland campus were 

monitored for nearly 2 years, covering 49 runoff events (Davis, 2008). The primary objective of this 

work was to quantify the reduction of hydrologic volume and flow peaks and delay in peak timing via 

bioretention. 

A bioretention research and education-site was constructed on the University of Maryland campus in 

College Park, Maryland, in Fall 2002/Spring 2003. The site contains two parallel bioretention cells that 

capture and treat stormwater runoff from an approximately 0.24-ha section of an asphalt surface parking 

lot. An asphalt curb was constructed along the perimeter of the parking lot to funnel sheet flow to the 

corner of the lot where the facilities were located. The parking area is high-use, employed year-round for 

commuter students and athletic events. Each bioretention cell is rectangular, with a width of 2.4 m and a 

length of 11 m. The resulting bioretention surface area is about 28 m2

 for each cell, producing a drainageto-bioretention area ratio of about 45. 

One of the bioretention cells (the shallow cell) was constructed according to the standard bioretention 

design outlined in the Prince George's County, Maryland, and Bioretention Manual. In addition to the 

standard media, the second cell incorporates an experimental anoxic zone at the bottom to encourage 

denitrification of runoff that passes through the cell (the deep cell). 

Overall, results indicate that bioretention can be effective for minimizing hydrologic impacts of 

development on surrounding water resources (Figure 28). Eighteen percent of the monitored events were 

small enough so that the bioretention media captured the entire inflow volume and no outflow was 

observed. Underdrain flow continued for many hours at very low flow rates. Mean peak reductions of 

49% and 58% were noted for the two cells. Flow peaks were significantly delayed as well, usually by a 

factor of two or more. Using simple parameters to compare volume, peak flow, and peak delay to values 

expected for undeveloped lands, it was found that the probabilities for the deep bioretention cell to meet 

or exceed volume, peak flow, and peak delay hydrologic performance criteria were 55%, 30%, and 38%, 

respectively. The probabilities were 62%, 42%, and 31%, respectively, for the shallow bioretention cell. 

Interim Report A-57 October 09 
 

Figure 28. Input and output hydrographs for University of Maryland bioretention facilities. 

Water Quality 

The same two bioretention facilities described above were monitored from Summer 2003 through Fall 

2004 to quantify water quality improvements to parking lot stormwater runoff (Davis, 2007). Twelve 

inflow/outflow water quality data sets were successfully collected and analyzed for total suspended solids 

(TSS), phosphorus, and zinc. Nine sets were collected for copper and lead, and three for nitrate. In two of 

the events, all of the runoff flow was attenuated by the bioretention media and no flow exited the cells, 

resulting in zero pollutant discharge. In all cases, the median pollutant output is lower than the input, 

indicating successful water-quality improvement through the bioretention media. Statistically 

insignificant differences were noted between the two cells for all pollutants examined. Median values for 

the effluent event mean concentrations and percent removals based on combined data sets (both cells) 

were TSS 17 mg/L and 47%; total phosphorus 0.18 mg/L and 76%; copper, 0.004 mg/L and 57%; lead 

0.004 mg/L and 83%; zinc 0.053 mg/L and 62%; and 0.02 mg-N/L and 83% removal of nitrate (based on 

limited data). Mass removals were higher than those based on concentrations due to flow attenuation. 

These values are in reasonable agreement with those previously published from bioretention field and 

laboratory studies. 

Rain Garden Performance in Haddam, Connecticut, US 

Replicated rain gardens were constructed and monitored in Haddam, Connecticut, US, to capture 

shingled-roof runoff (Dietz and Clausen, 2005). The gardens were sized to store the first 1 in. of runoff. 

Influent, overflow, and percolate flow were measured using tipping buckets and sampled passively. 

Precipitation was also measured and sampled for quality. All weekly composite water samples were 

analyzed for total phosphorus (TP), total Kjeldahl nitrogen (TKN), ammonia-nitrogen (NH3-N), and 

nitrite+nitrate-nitrogen (NO3-N). Monthly composite samples were analyzed for copper (Cu), lead (Pb), 

and zinc (Zn). Redox potential was measured using platinum electrodes. Poor treatment of NO3-N, TKN, 

organic-N, and TP in roof runoff was observed. Many Cu, Pb, and Zn samples were below detection 

limit, so statistical analysis was not performed on these pollutants. The only pollutants significantly 

lower in the effluent than in the influent were NH3-N in both gardens and total-N in one garden. 

The design used for these rain gardens worked well for overall flow retention, but had little impact 

pollutant concentrations in percolate. Most of the influent left the rain gardens as subsurface flow 

Interim Report A-58 October 09 
 

(98.8%). Only 0.8% of the inflow water overflowed during the entire study period (Table 12), which 

included an unusually cold and snowy winter with frequent frost in the soil. 

Table 12. Flow mass balance for rain gardens, Haddam, CT. Depth values are based on total rain garden area 

(Dietz and Clausen, 2005). 

Overflow occurred four times for garden 1 and three times for garden 2 during the 56-week study period. 

The residual volume (0.4%) was assumed removed by evapotranspiration (ET) from the gardens. 

Precipitation and flow data for one event on October 1, 2003, demonstrate the ability of the rain garden to 

reduce peak flow rates and increase lag time (Figure 29). The timing and shape of the inlet (roof runoff) 

hydrograph follow the precipitation hyetograph closely. However, the underdrain outflow shows a lower 

peak flow rate and a delayed response to the precipitation event. No overflow occurred during this 42mm 

event. 

These results suggest that if an underdrain is not connected to the stormwater system, high flow and 

pollutant retention could be achieved with the 2.54-cm design method. 

Figure 29. Precipitation, inflow, and outflow (underdrain) for one event, Haddam rain garden 

(Dietz and Clausen, 2005). 

Interim Report A-59 October 09 
 

Performance of Several Bioretention Facilities in North Carolina, US 

Three bioretention field sites in North Carolina were examined for pollutant removal abilities and 

hydrologic performance (Hunt et al., 2006). The cells varied by fill media type or drainage configuration. 

The field studies confirmed high annual total nitrogen mass removal rates at two conventionally drained 

bioretention cells (40% reduction each). Nitrate-nitrogen mass removal rates varied between 75% and 

13%, and calculated annual mass removal of zinc, copper, and lead from one Greensboro cell were 98%, 

99%, and 81%, respectively. All high mass removal rates were due to a substantial decrease in outflow 

volume. The ratio of the volume of water leaving the bioretention cell versus that which entered the cell 

varied from 0.07 (summer) to 0.54 (winter). There was a significant (p<0.05) change in the ratio of 

outflow volume to inflow volume when comparing warm seasons to winter. Cells using a fill soil media 

with a lower phosphorus index (P-index), Chapel Hill cell C1 (Figure 30) and Greensboro cell G1, had 

much higher phosphorus removal than Greensboro cell G2, which used a high P-index fill media. The 

authors concluded that fill media selection is critical for total phosphorus removal, as fill media with a 

low P-index and relatively high cation exchange capacity appear to remove phosphorus much more 

readily. 

Figure 30. Bioretention cell C1 in Chapel Hill 8 months after construction (Hunt et al., 2006). 

Researchers examined the Hal Marshal bioretention cell (HMBC) in an urban setting in Charlotte, North 

Carolina, US from 2004 to 2006 (Hunt et al., 2008). The HMBC is a retrofit BMP treating runoff from 

an asphalt parking area adjacent to the Hal Marshall Municipal Services Building in the City of Charlotte, 

North Carolina (Figure 31). The drainage area was 0.37 ha (0.92 ac) of an aging asphalt parking lot, 

which was last paved at least ten years prior to the study. The traffic load on the watershed was a mix of 

private vehicles and service vehicles. During office hours, use of the parking spaces was observed to be 

near 100%. The design of the bioretention area followed the recommendations made by the state of North 

Carolina Division of Water Quality Stormwater BMP Design Manual. 

Interim Report A-60 October 09 
 

 

 

Figure 31. Hal Marshall bioretention cell 16 months after the cell was revegetated and the study commenced 

(Hunt et al., 2008). 

Flow-weighted, composite water quality samples were collected for 23 events and analyzed for TKN, 

NH4-N, nitrate/nitrite-N, TP, TSS, BOD5, Cu, Zn, Fe, and Pb. Grab samples were collected from 19 

storms for fecal coliform and 14 events for E. coli. There were significant reductions (p<0.05) in the 

concentrations of TN, TKN, NH4-N, BOD5, fecal coliform, E. coli, TSS, Cu, Zn, and Pb. Iron 

concentrations increased significantly (p<0.05) by 330%. NO2-3-N concentrations were essentially 

unchanged. Efficiency ratios for TN, TKN, NH4-N, TP, and TSS were 0.32, 0.44, 0.73, 0.31, and 0.60, 

respectively. Fecal coliform and E. coli efficiency ratios were 0.69 and 0.71, respectively. Efficiency 

ratios for Zn, Cu, and Pb were 0.77, 0.54, and 0.31, respectively. 

The peak outflow of the bioretention cell for 16 storms with less than 42 mm of rainfall was at least 

96.5% less than the peak inflow, with a mean peak flow reduction of 99% (Table 13 and Figure 32). 

These results indicated that in an urban environment, bioretention systems can reduce concentrations of 

most target pollutants, including pathogenic bacteria indicator species. Additionally, bioretention can 

effectively reduce peak runoff from small to midsize storm events. 

Table 13. Reduction from Peak Inflow to Peak Outflow at HMBC (Hunt et al., 2008). 

Interim Report A-61 October 09 
 

Figure 32. Inflow and outflow hydrographs for HMBC for January 13–14, 2005 rain event (Hunt et al., 2008). 

Implications of Bioretention System Implementation in Wisconsin 

Morzaria-Luna et al. (2004) analyzed the implementation of various bioretention systems, including rain 

gardens, vegetated swales, trenches, and infiltration basins in the St. Francis subdivision, Cross Plains, 

Wisconsin. Through the examination of archival data and interviews with key participants, it was found 

that although regulatory and political pressures encouraged the inclusion of bioretention, current 

standards for stormwater management prevailed. The developers had to meet both existing requirements 

and anticipated rules requiring infiltration. As a result, bioretention systems simply supplemented, rather 

than replaced, traditional stormwater practices. The confusion surrounding dual standards contributed to 

substantial delays in the negotiations among relevant stakeholders in the watershed. The authors 

concluded that the St. Francis subdivision serves as both a cautionary tale and a bioretention success 

story. As a caution, the situation demonstrated the need for careful review and refinement of existing 

stormwater ordinances to incorporate water quality improvement technologies, such as bioretention. The 

demonstrated success of the St. Francis development was that it became a positive prototype for best 

management stormwater practices elsewhere in the region. In addition, the water quality monitoring data 

from the site contributed to development of a new county ordinance, the first in Wisconsin to address both 

quantity and quality of storm water runoff. These findings suggest that any policy aimed at modifying the 

approach to stormwater management must originate within the governance structure. The authors believe 

that attention must be concentrated on reviewing and reconfiguring existing storm water regulations and 

ordinances. 

Mt. Airy Rain Catchers: Testing a Reverse-Auction Incentive in Cincinnati, OH 

The Mt. Airy Rain Catchers Project, a joint venture of interested and motivated homeowners, the US 

Environmental Protection Agency, Horticultural Asset Management Inc., and Tetra Tech Inc, is the 

largest Economic Incentive and monitoring program of its kind in the country (Buranen, 2008). It is a 

pilot program to test a reverse-auction-based method of encouraging participation by homeowners, an 

idea of staff members in the EPA’s Sustainable Environments Branch. 

Interim Report A-62 October 09 
 

The Mt. Airy Rain Catchers Project began with the creation of a demonstration-site consisting of a rain 

barrel and two rain gardens in December 2006. Signs were added to educate the public on how rain 

gardens and rain barrels function. Information about the project, in an attractive, easy-to-understand 

brochure, was mailed to property owners in the spring of 2007. Each house in the Mt. Airy neighborhood 

was eligible to receive up to four rain barrels and a rain garden. Homeowners could choose to receive 

either or both. Installation, planting, and hardware costs were paid for by the U.S. Environmental 

Protection Agency. 

The novel reverse-auction approach called for homeowners to submit bids with a dollar amount they 

wished to be paid for permitting the installation and maintenance of rain gardens and/or rain barrels on 

their property. Those who submitted the lowest bids were the most likely to be selected. Most of the bids 

were for $0; of the bids that asked for payment, most were less than $200. 

In the summer of 2007, designated contractors installed 50 rain gardens, each measuring 150 to 160 

square feet, and 101 rain barrels at the selected homes. Each property owner selected for the program 

received an owner’s manual. The contractors will maintain the rain barrels and rain gardens and monitor 

the water quality in local streams through 2010. Homeowners are asked only to empty the rain barrels 

after each rainfall and to close the valve before the next rainfall. 

10,000 Rain Gardens: Involving the Public in Combined Sewer Overflow Management in 

Kansas City, Missouri, US 

Kansas City needed to address its aging infrastructure, not only to help prevent catastrophic flooding, but 

also to improve stormwater quality, reduce the incidence of combined sewer overflows, and meet 

environmental regulations (Buranen, 2008). The price tag for addressing all of the region’s infrastructure 

needs through engineered solutions was staggering: approximately $2 billion to address water quantity 

issues alone. 

With early support from the Mayor, Kansas City’s stormwater managers were able to develop and 

implement the 10,000 Rain Gardens Initiative fairly quickly. The initial media campaign roll-out 

occurred in January and February of 2006 and included television and radio spots showing interviews 

with garden experts, as well as print ads, stories, and editorials in local newspapers. The City’s key 

messages for the media campaign were to stress the importance of keeping water on the property, 

reducing turf areas and the maintenance and inputs associated with them, and creating an attractive 

landscape feature. The City was aware of statistics that showed gardening was a common pastime, so 

they thought the Initiative would appeal to a broad audience. 

A major part of the 10,000 Rain Gardens Initiative involved training for professionals. Early in the 

Initiative, the City sponsored three, day-long sessions for private landscaping businesses and retailers as 

well as municipal employees, and each event was sold out. How-to workshops continue to be held on a 

regular basis for both citizens and landscape professionals, and brief presentations are offered by request 

to provide an overview of the Initiative, rain gardens, and water quality concerns. 

One of the most cost-effective tools employed by the City is the web site for the Initiative, 

www.rainkc.com, which is designed to offer citizens and other audiences a comprehensive suite of 

information about rain gardens in particular and stormwater management in general. The site acts as a 

clearinghouse of information pertaining to the 10,000 Rain Gardens Initiative: resources, news items, 

technical information, examples and photographs, background information, and more. The site reaches a 

broad audience “on demand” and can be maintained and augmented at a relatively low cost. Even though 

Interim Report A-63 October 09 
 

 

the media campaign has ended, the site still has more than 2,000 hits per week and more than 100,000 

visits per year. 

References 

Akan, O.A. 2002. Sizing Stormwater Infiltration Structures. Journal of Hydraulic Engineering Vol. 128 

(5). pp 534-537. http://www.uvm.edu/~awra/Charrette/sw%20infilt%20-%20akan2002.pdf 

Bell, W., and Covington, M., 1999. Storm Water Technology Fact Sheet Infiltration Trench. EPA/832/F99/0191999. Washington D.C. U.S. Environmental Protection Agency, 1999. 

http://www.epa.gov/OWM/mtb/infltrenc.pdf 

Buranen, M. 2008. Rain Gardens Reign. Stormwater - The Journal for Surface Water Quality 

Professionals. Vol 9 (4). http://stormh2o.com/may-2008/rain-gardens-management-4.aspx 

Davis, A.P. 2007. Field Performance of Bioretention: Water Quality. Environmental Engineering 

Science. Vol. 24(8). pp 1048-1064. 

Davis, A.P. 2008. Field Performance of Bioretention: Hydrology Impacts. Journal of Hydrologic 

Engineering. Vol 13(2). pp 90-95. 

Dietz, M.E., and J.C. Clausen. 2005. A field evaluation of rain garden flow and pollutant treatment. 

Water, Air, and Soil Pollution Vol. 167. pp 123–138. 

Hunt, W.F., A.R. Jarrett, J.T. Smith, and L.J. Sharkey. 2006. Evaluating Bioretention Hydrology and 

Nutrient Removal at Three Field Sites in North Carolina. Journal of Irrigation and Drainage 

Engineering, 132(6) pp 600-608. 

Hunt, W.F., J.T. Smith, S.J. Jadlocki, J.M. Hathaway, and P.R. Eubanks. 2008. Pollutant removal and 

peak flow mitigation by a bioretention cell in urban Charlotte, N.C. Journal of Environmental 

Engineering. Vol. 134(5). pp 403-408. 

Melbourne Water. 2008. Rain Gardens Stormwater Sensitive Homes. 

http://library.melbournewater.com.au/content/wsud/sustainable_urban_design/Raingardens.pdf 

Morzaria-Luna, H.N., Schaepe, K.S., Cutforth, L.B., and R.L. Veltman. 2004. Implementation of 

Bioretention Systems: A Wisconsin Case Study. Journal of the American Water Resources Association 

Vol. 40(4). pp 1053-1061. 

Muthanna, T.M., Viklander, M., Gjesdahl, N., and S.T. Thorolfsson. 2007. Heavy Metal Removal in Cold 

Climate Bioretention. Water, Air, & Soil Pollution. Vol. 183 (1-4). pp 391–402. 

Wang, L., J. Lyons, P. Kanehl, and R. Bannerman. 2001. Impacts of urbanization on stream habitat and 

fish across multiple spatial scales. Journal of Environmental Management 28(2). pp 255–266. 

WERF. 2007. Kansas City, Missouri: Sharing Stormwater Management Responsibility with the 

Community. http://www.werf.org/livablecommunities/studies_kc_mo.htm. 

Interim Report A-64 October 09 
 

Section 9: Rainwater Harvesting and Reuse 

Arid and semi-arid regions can benefit greatly from a stormwater management system that includes 

stormwater capture and aquifer recharge to replenish and help ensure the long-term sustainability of water 

supplies. Rainwater harvesting systems can also provide benefits for wetter climatic areas, as well. 

Rainwater can be collected for reuse in rain barrels, cisterns, and underground storage tanks. This type of 

rainwater capture constitutes stormwater retention and removes a set volume of stormwater from offsite 

runoff, protecting downstream areas from some of the ill effects of urban runoff. Property owners can use 

this water for irrigation, toilet flushing, or other non-potable uses. 

Practices that promote stormwater infiltration, such as those mentioned previously, will contribute to 

ground-water recharge, reducing water temperatures (subsurface flows have lower temperatures than 

surface flows) and enhancing baseflow to streams during dry weather. 

Case Studies 

Residential Area with Raintanks and Aquifer Recharge Area in Newcastle, Australia 

The project at Fig Tree Place demonstrated what can be accomplished with regards to water reuse in a 

suburban residential neighborhood (Figtree Place, no date). Results from this project can be applied to 

any number of sustainable urban design areas. Started in 1998, Figtree Place, a neighborhood in 

Newcastle, Australia was designed with an aquifer recharge area. Roof runoff from 27 inner city 

residences is stored in cisterns for personal use in each household (for hot water, toilet flushing, etc.). Any 

overflow from the personal cisterns flows to a grassed infiltration zone for aquifer recharge. When onsite demands for water (irrigation or an adjacent bus-washing facility) increases, the aquifer can be 

accessed for use. Total water savings in this development are nearly 60%. Figure 33 below shows a 

diagram of the neighborhood as a conceptual model of how the water conservation designs work. 

Figure 33. Overview of water sensitive design elements at Figtree Place. 

Inkerman Oasis, Development with Integrated WSUD Techniques in Australia 

This community near Port Phillip Bay, Australia, combined a number of WSUD techniques (Inkerman 

Oasis, no date). This community is the first Australian community to integrate greywater and stormwater 

for reuse in a high-density residential development. These residents use recycled and treated domestic 

greywater for baths, showers, and toilet flushing. Secondary systems for stormwater, second to raintanks, 

Interim Report A-65 October 09 
 

includes a native wetlands and sand filter area (part of which can be seen in Figure 34). The reuse of 

water reduces potable water demand by 45% and keeps up to 7 tons of nitrogen and phosphates from 

draining to Port Phillip Bay each year. 

Figure 34. Inkerman Oasis Sand Filter. 

A Greywater System of Rainwater Reuse at the Royal Melbourne Institute of Technology 

(RMIT) in Australia 

Estimated at $228,500, this project to use rooftop and basement storage tanks for beneficial use of 

rainwater for toilet flushing is an example of innovation to capture runoff from urban impervious surfaces 

(Melbourne Water, 2008). In addition to providing much needed water conservation in Australian cities, 

this practice has converted 720 m2

 of outdoor space into a garden plaza attractive to students and 

aesthetically pleasing in a heavily urban environment (Figure 35). The maintenance considerations are 

based on results from periodic monitoring to achieve maximum success. 

We have evicted nature from the city, much to our cost. But nature has all the answers 

it’s the only sustainable system we know, and we need to learn from it. It’s time we 

invited nature back in. 

-Terry White, developer of this project in Melbourne 

Interim Report A-66 October 09 
 

 

n 

Figure 35. Schematic drawing of the proposed greywater system showing how the water is captured 

and stored for future reuse. 

References 

Figtree Place. (no date). City of Newcastle, NS, Australia 

http://www.eng.newcastle.edu.au/~cegak/Coombes/FIGTREE%20LAST21.htm 

Inkerman Oasis. (no date), Port Phillip Bay - St. Kilda, Melbourne, Victoria,Australia. 

http://www.melbournewater.com.au/content/library/wsud/case_studies/inkerman_oasis.pdf and 

http://www.portphillip.vic.gov.au/print_sustainable_case_studies.htm 

Melbourne Water. 2008. WSUD Greywater Reuse to support Rooftop Greening: Case Studies. 

Interim Report A-67 October 09