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How Melbourne Water manages wet weather sewer spills

By S Watkins and R Considine.

First published as an Ozwater'18 Conference Paper.



Waterways play a central role in many aspects of daily life and hold significant environmental, cultural, social and economic value for almost every city on earth. Sewerage systems are also an important (though often hidden) foundation of the liveability of any city. All Australian states and territories have devised mechanisms to ensure sewerage networks manage the risk of sewerage spills to society. Following a lengthy review, there is now momentum in Victoria toward a preventative, accountable and flexible approach to environmental regulation. This paper presents a trial of a new approach to sewerage management consistent with Melbourne Water’s vision of enhancing life and liveability and the emerging regulatory approach in Victoria.

It is Victorian environmental policy that sewerage systems will be managed to protect the beneficial uses of Victorian waters. The statutory requirement is that sewerage systems will contain all flows in rainfall events up to a 1 in 5 year recurrence interval. Measures adopted to achieve this should be cost effective and proportional to the significance of the problems being addressed.

The current policy allows for a departure from rigid flow containment standards though a ‘comparable design standard’; instead focussing on improving environmental and community outcomes. These outcomes could be achieved through a more cost effective approach of alternative measures which provide greater benefit to both the environment and community when compared to upgrading the capacity of a non-compliant sewer. This paper outlines a framework developed for Melbourne Water’s application of an alternative, risk-based approach to wet weather spills management.

Year case study was implemented

2013 to 2018.

Case study summary

Melbourne Water manages and protects Melbourne’s major water resources on behalf of the community, guaranteeing the supply of affordable, high-quality water, reliable sewerage, healthy waterways, integrated drainage and flood management. Since 2011, Melbourne Water has been working with the Environment Protection Authority (EPA) to pilot and develop a new approach to better manage the impacts to the environment for low-risk, non-compliant wet weather sewer overflows.

The Enhancing Our Dandenong Creek pilot project was an innovative program identified by the Victorian Water Industry (VicWater)implemented in partnership between Melbourne Water and EPA. Following a review of the program, Melbourne Water has developed a framework for management of WWSOs, which will be formalised and embedded in the Melbourne Sewerage Strategy.

The framework contains the following:

  • A risk assessment and prioritisation process, consistent with EPA guidance on wastewater risk assessment underpinned by genuine partnerships and dialogue between the sewerage manager, regulator and waterway manager;
  • Development of alternative options, guided by a set of principles;
  • Assessment of options against beneficial uses, through a participatory multi-criteria process with input with input and decision-making by stakeholders and community;
  • Collaboration in implementation, built through transparent and adaptive governance, decision-making and delivery models; and
  • An adaptive management approach, including a comprehensive and targeted monitoring program to inform primary stressors and resultant activities.

Case study detail


Environmental regulation is essential to prevent harms from pollution and waste. The discharge of sewage, including overflows from controlled release points, is regulated to manage the risks and impacts to communities and the environment. In Victoria, this occurs at the state level, via the Environment Protection Act 1970 (Vic) (1970) and the subordinate State Environment Protection Policy (SEPP) (Waters of Victoria) (1988)). Whilst the policy applies to State and Local government agencies, business and communities, it sets out specific responsibilities for water corporations like Melbourne Water.

The intent of the SEPP is to protect the beneficial uses of the receiving environment, as defined in the Environment Protection Act 1970 (Vic) and including current or future environmental value or use of surface waters that communities want to protect. The regional waterway strategy or Healthy Waterways Strategy, defines the current and future environmental value in more detail.

The SEPP recognises that containment of all sewage flows within the system is not practical, and instead sets a standard that is a balance of risk to beneficial uses, and cost to community. Obligations for sewerage authorities under the SEPP require sewerage system containment of flows resulting from rainfall events up to a 1 in 5 year average recurrence interval. Victoria and New South Wales are the only Australian States that adopt a frequency target in the regulatory framework; the majority of other states provide for a general environmental duty or similar code.

Whilst prescriptive, inclusion of a containment standard in the policy aids sewer planners and engineers in determining the capacity design of sewers and provides a clear legal liability on water corporations. The 1 in 5 year standard has been adopted in all new sewerage systems constructed in Melbourne since the policy was introduced. However, the majority of the sewerage network predates the containment standard, introduced in 1988. Over the past two decades, Melbourne Water has progressively upgraded the bulk sewerage system, investing in larger sewers reduce non-compliance (Figure 1) whilst managing the impact on customer bills. Between 1992 and 2012, close to $1B was invested, reducing non-compliant spills from 180 annually, to the point where the last non-compliant spill from a Melbourne Water network was in 2011/12. This has resulted in improved environmental outcomes across Melbourne.

With the projected changes to climate (increased storm intensity) and population growth the regulatory burden of wet weather containment is expected to increase. Costs in the order of $200M could be incurred in the next decade, and modelling has shown that there could be at least 45 spill sites in the Melbourne Water sewerage system by 2050.

While the number of non-compliance wet weather sewer spills is expected to increase, the Ringwood South Branch Sewer, located near Dandenong Creek in Melbourne’s South East, remains one of the last few sites that is currently non-compliant in the Melbourne Water network. To achieve the containment standard, a sewer tunnel would be required, with extensive works estimated to be around $140M.

Excluding its forested headwaters, Dandenong Creek is a highly modified waterway which flows through urban, industrial and rural areas on its 53km course from the Dandenong Ranges to Port Phillip Bay, near Carrum. Streamside vegetation is often poor, particularly near the non-compliant sewer spill discharge point, in Bayswater. Despite modifications to large sections of the waterways, threatened plant and animal species including Yarra gums, dwarf galaxias, swamp skinks and powerful owls are present, representing important environmental values.

However, overall the in-stream ecological value of the waterway is poor, due to significant modification to the flood plain and the long-term impacts of urbanisation and industrial land use in the catchment. Considering this, the resulting environmental benefits from investing $140M in the conventional sewer upgrade were brought into question.

Figure 1: Number of sewage spills and Melbourne Water capital investment for sewer spill management Figure 1: Number of sewage spills and Melbourne Water capital investment for sewer spill management

Early development of an alternative approach

Similar questions around viability and value raised with upgrading the Ringwood South Brach Sewer also arose in other locations across the Victorian water sector and in 2012 the Victorian Water Industry (VicWater) identified the need for a framework for future decision making regarding sewerage spills abatement and environmental benefit. The framework needed to take a holistic approach to spills management, including diffuse source pollution management, social and environmental outcomes.

Together with the EPA and VicWater, a pilot project was proposed for the Ringwood Branch Sewer. A team of ecologists, river health managers, sewerage planners, water chemists and biologists came together to understand the impact of wet weather sewer spills on the waterway health of Dandenong Creek.

Independent ecological research was undertaken by the University of Melbourne, adopting a comprehensive multiple lines of evidence approach. This work demonstrated that wet weather sewer spills are not the dominant threat to ecological values in Dandenong Creek. Pollutants such as heavy metals and pesticide that enter the Creek during dry-weather flow from stormwater drains are instead the major limiting factor on waterway health. The relatively low risk posed by sewer spills, combined with the relatively high risk posed by other sources of pollution warranted a broader assessment of pollution management, as an alternative to conventional sewer augmentation.

Enhancing our Dandenong Creek (EODC) pilot project

The aim of the Enhancing our Dandenong Creek program was to enable Melbourne Water to provide cost-effective pollution management in Dandenong Creek to comply with Clause 35 of the SEPP (WoV) for Ringwood South Branch Sewer by 2018.

A ‘comparable design standard’ to 1:5 must account for the protection of beneficial uses, necessitating a program approach that addresses both significant sources of pollution and significant impacts of pollution.

A management group was established to oversee the program, consisting of representatives from Melbourne Water, EPA, Yarra Valley Water and South East Water. A Melbourne Water working group involving of river health managers, ecologists, sewerage planners, stormwater quality experts and environment managers identified a pollution management program to protect these relevant beneficial uses of Dandenong Creek, being public health, amenity, water quality and the impact on biota.

The approach adopted for the Enhancing our Dandenong Creek pilot was built around a risk-based methodology to support decision making, with six core principles:

  1. Understanding of policy intent
  2. Genuine partnership between regulator, sewerage manager and waterway manager
  3. Identification of threats and risk posed including development of a program to address these risks
  4. Targeted and local science based evidence
  5. Strong stakeholder engagement and collaboration in program design and decision making
  6. Adaptive management, underpinned by multiple lines of evidence and ongoing monitoring and evaluation

The objectives of the program are to provide habitat for threatened species, improve water quality and increase the natural amenity value of the creek corridor, because these are the beneficial uses impacted by sewerage spills. Over 60 potential options were developed and assessed using a triple-bottom line approach to result in four project areas. All of the objectives must be met by at least one of the projects when delivered as a comprehensive program.

The pollution management program (i.e. the comparable design standard) involved 6 projects (Table 1):

Table 1: Summary of Enhancing Our Dandenong Creek projects Table 1: Summary of Enhancing our Dandenong Creek projects

Importantly all projects delivered under the EODC project were not “business-as-usual” activities, and were thus funded through the same allocation as sewerage services. It is a requirement of the agreement with EPA that no funding from waterways and drainage be directed to the above program.

Pilot program outcomes

The EODC program has been in implemented since 2013, and has delivered significant works to improve amenity and native fish populations, source and manage dry-weather pollution, and increase public health protection.

Over the 5 year pilot program, approximately $14.5M has been invested to deliver the following works:

  • Installation of penstocks to manage uncontrolled customer spills;
  • Removal of 830m of drainage pipe to “daylight” and naturalise Dandenong Creek;
  • Installation of 15 interpretative signs and amenities, including seating, drinking fountains and gym equipment;
  • Revitalisation of 6 billabongs and wetlands along the creek corridor;
  • A behaviour change program targeting industrial catchments to improve stormwater quality;
  • Construction of 20 fish habitats, including breeding areas, a fish-stocking program and reintroduction of two threatened fish species; and
  • On-going monitoring for pollution identification, tracing and program verification.

All of the above activities have been undertaken and funded by Melbourne Water as a sewerage project. Some project components have been delivered in partnership with Councils (including a number of billabongs and amenities) with sewerage funding from Melbourne Water.

In addition to the measurable and tangible on-ground environmental and social benefits, the project has also achieved significant social benefits among the organisations involved. The complexity and multiplicity of stakeholders and institutions makes delivering collaborative projects a challenge. The role of champions and trusted experts to advocate and provide a credible voice of support was critical to the success of the project. This, coupled with increased transparency developed genuine buy-in from stakeholders. These benefits have enabled the successful and timely completion of the program, but will continue to yield in future collaborative efforts.

Framework for application of an alternative, risk-based approach

The Enhancing our Dandenong Creek pilot project is in the final year of the 5-year program. Reflecting the adaptive management and partnership approach of the program, Melbourne Water and EPA are undertaking a review of the Enhancing our Dandenong Creek pilot. Drawing on learnings from the program and review Melbourne Water has developed a framework for adopting an alternative approach to wet weather sewage spill management.

In addition to being embedded in the Melbourne Sewerage Strategy and future planning and management of these assets, the framework is intended to provide guidance to industry on how and when to adopt an alternative approach, to ensure consistency and best practice in achieving the desired outcomes for waterways and community

At its core, the framework adopts an integrated water management approach, that is collaborative and coordinated, and which promotes participation by impacted stakeholders, including community, in the decision-making process. This aligns with environmental policy in Victoria, which is underpinned by the principles of integration of economic, social and environmental considerations to guide decision-making in the protection and management of surface waters in Victoria.

Framework for application of alternative, risk-based approach

Risk assessment and prioritisation – when to adopt an alternative approach?

Legacy infrastructure, population grown and climate change are factors contributing to non-compliant assets within the sewerage network across Melbourne. The SEPP recognises that achieving system-wide compliance takes time, and therefore advocates for actions to be implemented on a priority driven and progressive basis. It calls for the sewerage manager should develop a program of continuous improvement to be implemented to ultimately contain flows to the required standard.

Melbourne Water has demonstrated a concerted commitment to improving compliance through the Wet Weather Spills Reduction Program. As part of this program development, significant work was undertaken to understand the frequency and location of overflow points, the sensitivity and values of the receiving waterways, and the impacts on beneficial uses. Additionally, the feasibility to manage these threats through sewerage augmentation, and the resultant benefits needs to be considered.

Where wet weather sewer overflows are having a high impact to the environmental, social and economic values of the waterway, upgrades to alleviate this are required. However, where the impact is low, an alternative approach could be considered.

Existing data on spill frequency, volumes, waterway water quality, and waterway values should be reviewed to develop a conceptual understanding of the stressors, threats and impacts to beneficial uses for the specific waterway. Combined with the relevant waterway strategy (Healthy Waterways Strategy), this will give an indication to the level of risk posed by sewer spills. Where this evidence suggests that sewer overflows are not likely to be a limiting factor to waterway health, further investigation should be undertaken to validate this to the satisfaction of the relevant authorities.

A number of quantitative and qualitative approaches are available to determine the level of impact or risk posed by wet weather sewer spills, to varying levels of detail, depth and cost. The approach adopted should be consistent with EPA guidance on wastewater risk assessment, with evaluation of the interaction between environmental values, the stressors to these and management actions for protecting the values. Reflecting the principle of genuine partnerships, the risk assessment approach should be developed between the sewerage manager, regulator and waterway manager.

In the case of Dandenong Creek, a targeted, site-specific investigation was undertaken to verify the initial risk assessment from sewage spills. This multiple lines of evidence approach provided rigorous evidence required for informed decision-making. Additionally, it also provided a baseline data set and was able to identify key stressors on to ecological endpoints.

Depending on available data, financial constraints and the risk-appetite of various authorities, this level of scientific evidence may not be required. Although, as on-going monitoring, including baseline data, is required as part of an adaptive management approach, investment site-specific investigations upfront is useful.

Risk treatment

Despite the impacts of urbanisation and modification, almost all waterways provide significant social, economic and cultural benefits, in addition to ecological values. This might include provision of water supply, flood protection, aesthetic enjoyment, recreation or habitat for iconic species or significant cultural values.

As such, even where ecological function is not limited by sewerage spills, there will likely be some residual impact to social, economic or cultural values. Aligned with principles for best practice management, the solution for sewerage managers is not “do nothing”, but instead the delivery of alternative, compensatory measures to manage for these residual impacts.

Risk treatment involves estimating the effectiveness of existing and alternative risk treatments, and identifying the most cost-effective approach to managing the risk of sewer overflows. Significant skill is required, since it involves estimating the effectiveness of the controls at influencing the beneficial use endpoints. This may involve one or more detailed analyses from specialised technical experts may also be appropriate depending on the complexity of the end-points (such as ecologists, hydrologists, eco toxicologists, microbiologists or other human health experts). Sharing of such investigations across the water industry would help lower the costs of investigations and promote consistent decision making.

Building on the evidence gathered in the first phase, water corporations should identify the importance and effectiveness of existing controls that manage the risk posed by sewer overflows. These controls might include the capacity of the sewer to contain sewage, maintenance of the sewerage infrastructure and incident response procedures. These measures need to address both significant sources and impacts of pollution. The effectiveness of the controls needs to be estimated by reference to the end-points. The unit costs of the existing controls should (where possible) be monetised.

By collaborating with stakeholders, water corporations should identify new options that may change the risk of sewer overflows. These options might include:

  • Sewerage infrastructure upgrades that enable containment of sewerage flows under a range of rainfall event probabilities;
  • Constructed wetlands;
  • Habitat restoration works;
  • Water sensitive urban design;
  • Recreational controls;
  • Customer compensation; etc.

The effectiveness of the alternative options should be estimated by reference to the end-points. Hybrids of the alternative options may also be appropriate. As the intent is to protect beneficial uses of the receiving environment, mapping potential options against beneficial uses within the system ensures consistency, transparency and rigour in the decision-making process.

Drawing from the EPBC environment offsets policy principles, the development of options should be governed by the following principles:

  • Options development should be a participatory process including a range of stakeholders
  • No net increase to risk or impact to any beneficial use
  • Options must deliver an overall improved outcome (net benefit)
  • Options must be in addition to activities already required or agreed to by any authority
  • The suite of options should include mostly direct measures
  • Options should be site specific, and of a size and scale proportionate to the residual impacts to values, as determined through appropriate criteria developed and agreed with stakeholders
  • All options should include the required sewer augmentation to achieve compliance, at an appropriate point in the future (deferral of sewerage infrastructure).
  • All options must be funded by the sewerage manager, with collaborative implementation and delivery
  • Activities should be achievable and have measurable results within the project timeframe

Assessment of options against beneficial uses

Benefit cost analysis is the preferred assessment framework for many public sector decision-making bodies (including the UK Treasury, the Canadian EPA and the US EPA). Benefit cost analysis is a method for organising information to aid decisions about the allocation of resources. True benefit-cost analysis extends to monetising both benefits and costs, however, given the difficulties that may be anticipated in estimating the effectiveness of risk treatments for sewer overflows in terms of end-points. It is proposed to limit the analysis to cost-effectiveness analysis, which is appropriate where there are alternative options to achieving estimated outcomes, but where the outcome itself cannot be monetised. The EPA has indicated support previously for cost effective measures in proportion to the significance of environmental problem.

The cost-effectiveness analysis needs to account for the timing of benefits and costs on a consistent and sound basis through discounting processes. Thus, in order to undertake the cost-effectiveness of risk treatment options:

  • End-points must be estimated over a time period (say 25 years)
  • Costs must be estimated over the same time period

In discounting values into present day equivalents in a cost-effectiveness analysis a different discount rate may need to be used than in a financial assessment to reflect the social rate of time preference or the social opportunity cost of capital. The government’s discount rate for public sector investments should provide the standard.

The results of the cost effective analysis should include:

  • The cost-effectiveness of the existing controls
  • The cost-effectiveness of alternative controls

The results should be used to select the most cost-effective risk treatments.

Assessment of the options should also consider the risk or uncertainty of the measure not achieving the desired benefits.

A suite of key performance indicators should be agreed for each measure to ensure transparency, accountability and evaluation.

Collaboration in implementation

Adopting a holistic, integrated approach to wet weather sewer overflows also includes considering the stakeholders and actors involved in the development and delivery of an effective and successful program. This systems-thinking approach is necessitated by the multi-disciplinary and trans-jurisdictional nature of contemporary urban water management. This approach focuses on coordinated decision making across sectors, scales and stakeholders.

For activities to address specific beneficial uses, such as amenity or water based recreation, engagement and collaboration with the impacted community is critical to ensure that the adopted measures achieve the desired outcomes. The Enhancing our Dandenong Creek project adopted a high level of engagement intent from the outset, aiming to “collaborate” with key stakeholders (IAP2). By actively seeking to include a variety of stakeholders, across broad disciples, sewerage managers, and other stakeholders benefit from a greater understanding of issues (Mosterat et al., 2011), increased social capital (Tippet and Griffiths, 2006) and improved trust and acceptance of the outcomes (Dobbie et al., 2016) , leading to greater success of projects.

Trans-jurisdictional delivery may be required for some project elements – for example, it may be easier for the delivery of biodiversity habitat or amenity measures to be undertaken by the waterway manager, or local government – which requires close collaboration between sewerage manager, waterway manager and other relevant agencies

Adopting an outcomes-based approach to wet weather spills management is also adopting a multidisciplinary approach, and with that comes capacity building, increased social learning and networks (Morison et al., 2010). Working collaboratively and across sectors will enable the breakdown of socio-institutional barriers (Brown and Farralley, 2009).

Adaptive management and governance

Foundational to this process, are clear governance arrangements, involving (at a minimum) the sewerage manager, regulator and waterway manager. This may take the form of a plan, or MoU that is formally endorsed by the sewerage manager and regulator. Ultimately, some form of contractual governance needs to be in place to drive accountability. The plan should describe the program of actions identified and include the allocation of resources to fund the implementation of the plan, including any review and amendment of the plan. The plan should also set objectives which are specific, measurable, achievable, realistic and time-bound, with progress monitored and reported to stakeholders.

To manage risks and uncertainty, and ensure the program achieves the desired benefits and improvements to beneficial uses, an adaptive management approach should be embedded in the program. An evidence based approach provides for an important understanding of baseline condition of the receiving environment, and provides a foundation on while to build monitoring and compliance with the plan objectives and actions.

Aligned with adaptive management principles, the plan must include an evaluation and review process, which provides for the participation of the stakeholders involved in the plan development.


Building on the learnings from the Enhancing our Dandenong Creek pilot program, Melbourne Water has developed a framework for the application of an alternative, risk-based approach to wet weather sewer spills management. In addition to achieving greater community benefits, through a more cost-effective and outcomes-based approach, the framework also provides an opportunity for Melbourne Water to improve their transparency and community empowerment in decision making. By delivering an alternative suite of measures that have been developed and delivered collaboratively with stakeholders the environment and community can benefit from greater and more meaningful impacts.

The lessons learned from the pilot have informed the development of a framework for the application of alternative, risk-based approaches in sewerage management. This framework presents a demonstrable shift towards new governance models which involve participatory dialogue with community and empowerment in decision-making. It moves away from top-down conventional approaches in response to command-and control regulation. The success of the Enhancing our Dandenong Creek pilot program demonstrates that adopting a participatory approach to decision making does not expose authorities to the risk of community backlash from “over promising and under-delivering”, but instead builds trust, engagement and capacity amongst stakeholders and the community.

This approach also provides water authorities with the opportunity to improve their transparency and community empowerment in decision making. Adopting a participatory approach with communities could lead to greater community awareness regarding sewerage, waterway and water management issues, with increased trust and reputational benefits for water utilities. As such, there is significant scope to achieve multiple beneficial outcomes – reputationally, financially, socially and environmentally - through the adoption of an alternative approach to wet weather spills management.

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