Flood Management and Drainage Strategy

Home , Dandenong Creek, Gardiners Creek, Maribyrnong River, Yarra River

Port Phillip and Westernport Region Flood Management and Drainage Strategy

i “Ideally society would like to be free of the risk of flooding, but this is neither practically nor economically feasible. What constitutes an acceptable level of flood risk however is a vexed question. The immediate risk is borne by the community, which must have a significant input into defining the acceptable level. To this end, public consultation and risk communication is very important.” Floodplain Management In Australia Best Practice Principles and Guidelines, (SCARM 2000) Development of this strategy has been guided by a steering committee headed by an independent chair, Rob Joy, with representatives from the following organisations: • Department of Sustainability and Environment • Office of the Emergency Services Commissioner • Shire of Macedon Ranges • Insurance industry • Department of Human Services • Municipal Association of Victoria • Stormwater Industry Association of Victoria • Institute of Public Works Engineers Victoria • Port Phillip and Westernport Catchment Management Authority • Melbourne Water. The strategy has been prepared following extensive consultation with flood management agencies and local government authorities in the Port Phillip and Westernport region. Stakeholder workshops were undertaken to identify issues of concern and submissions received in relation to a circulated discussion paper assisted in the formulation of future strategic actions.

iii Lollipop Creek, Werribee, February 2005 iv Flood Management and Drainage Strategy Contents

2 Introduction 6 Background 6 What is flooding? 8 Types of flooding in the region 12 History of flooding 14 Flood damage 20 Managing floods 22 Responsibilities for flood management across the region are shared 26 Flood management objectives Objective 1: Completing the knowledge base Objective 2: Potential long term future pressures on existing drainage systems Objective 3: An agreed approach to managing existing regional flood problems Objective 4: Enhanced community education, flood awareness and preparation Objective 5: Agreed responsibilities and improved collaboration between flood management agencies 44 Glossary Introduction

Floods are a natural and inevitable event. We cannot always control them. Therefore, we must learn how we can live with them while minimising risks to public health and safety, property and infrastructure.

The Port Phillip and Westernport Region faces significant Aside from the likely multi-billion dollar cost and extreme flood management and drainage challenges that require a disruption, the time frame to build such expensive coordinated and collaborative approach by flood managers infrastructure would exceed the lifespan of the properties and the community. they attempt to protect. There are currently more than 100,000 properties in the This strategy recognises that although it is not possible region that are known to be at risk from flooding of which to make the region completely free from flooding, there 82,000 properties are at risk of flooding from overland are some flooding risks that we may decide are intolerable flows. More than 40,000 of these properties contain and almost regardless of the cost, the consequence of buildings or dwellings that are at risk of flooding above flood should be reduced or eliminated. Consultation and floor level. On average, the annual damage caused by research will be undertaken to reach agreement on what flooding in the region has been estimated to be $245 constitutes intolerable flood risk. million. Our long term aim is to minimise all currently known The Flood Management and Drainage Strategy has been intolerable flooding risks to public health and safety, prepared following discussions between government property and infrastructure, and increase community departments, councils, emergency service organisations, understanding and preparedness for floods. and other agencies about how we can improve the This strategy proposes the development and management of flood risk in the Port Phillip and implementation of an accelerated program, in consultation Westernport Region. It complements other Melbourne with stakeholders, to identify and deal with intolerable Water programs that deal with river health, water flood risk across the region as quickly as possible. quality, floodplain environmental values, urban growth, stakeholder communications and cultural heritage. As part of this strategy, Melbourne Water will continue to map flood affected areas and will sponsor research into Melbourne Water and councils have traditionally used the intangible social and human health impacts of floods, engineering solutions to improve flood protection in to develop an improved flood risk assessment tool that identified high risk areas. takes these costs into account when determining flood risk Millions of dollars, funded primarily by council and priorities and responses. Melbourne Water drainage rates, are currently spent each Throughout this strategy there is broad recognition that no year on efforts to manage ongoing risks from overland single organisation and no single approach can deliver an flows and riverine flooding. effective response to flood management issues. While these efforts are effective at a local level they are While Melbourne Water and councils will continue to having little impact on the overall problem. deploy engineering solutions to mitigate flood risks, it is This is because it is not physically feasible to construct recognised that this approach must be accompanied by a required works to protect all properties in the region from range of non-structural responses. large events.

2 Flood Management and Drainage Strategy Richmond, July 1891

There are other tools that are equally effective and Objective 1: Completing the significantly less expensive, such as planning controls and public awareness programs that can be deployed knowledge base immediately to assist in treating the residual and future flood risks. These tools will be developed in partnership Flood mapping with other flood managers. Actions In particular, the strategy recognises the importance and 1.1 Melbourne Water will undertake and maintain ongoing benefits of improved education in assisting the community local flood extent, rural waterway and tidal mapping to prepare, respond and recover from floods. for the Port Phillip and Westernport region, taking Part of this focus on existing flood risks will also involve into account the potential long term pressures of working with councils to develop detailed local flood climate change and urban consolidation. This will management plans, and working with the State Flood involve consultation with local government and the Policy Committee, Office of the Emergency Services Department of Sustainability and Environment. Commissioner and other stakeholders to develop an 1.2 Melbourne Water will update and maintain flood integrated flood management planning framework. information for municipal planning schemes to These plans will outline roles and responsibilities, set ensure appropriate standards of flood protection for priorities, aims and goals, and establish work programs new development in flood affected areas. This will and targets to address flood management objectives involve consultation with local government and the identified in the strategy. Department of Planning and Community Development. The strategy defines five flood management objectives and outlines actions that will be undertaken to achieve Understanding social impacts each objective and guide priorities and expenditure by Melbourne Water. Actions 1.3 Melbourne Water will support collaborative research into the social and economic impacts of flooding on the community.

3 We all have an important role to play – Melbourne Water, emergency service organisations, local councils, government departments, research organisations, developers and other businesses, and the community.

Objective 2: Potential long-term future 2.2 Melbourne Water will develop flood risk management measures in consultation with Department of Planning pressures on existing drainage systems and Community Development, Department of Sustainability and Environment and local government. Development in established areas Solutions may include upgrading infrastructure, planning and building controls to limit imperviousness, Actions and onsite measures such as water sensitive urban 2.1 Melbourne Water will work with Department of design and stormwater capture and reuse. Planning and Community Development and local government to understand the likely impact of urban Climate change consolidation on runoff volumes and flood risk in existing urban areas. Actions 2.3 Melbourne Water will continue to support research programs to better understand the impacts of climate change on flood and drainage management. 2.4 Melbourne Water will develop adaptable flood management programs to deal with changing rainfall patterns, rising sea levels, and storm surge. These programs will be developed in consultation with the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and the Bureau of Meteorology.

4 Flood Management and Drainage Strategy Objective 3: An agreed approach to Objective 4: Enhanced community managing existing regional flooding education, flood awareness and problems preparation Actions Actions 3.1 Melbourne Water will sponsor research and consult 4.1 A community flood education, awareness and with stakeholders to develop criteria to establish flood preparedness program, including flood warning, will risk tolerability thresholds which take into account be developed and implemented for the Port Phillip and relevant social or intangible flood impacts. Westernport region in partnership with Victoria State Emergency Service, and other key stakeholders. 3.2 Melbourne Water will develop and implement a program in consultation with stakeholders to eliminate or reduce intolerable risks to an acceptable standard Objective 5: Agreed responsibilities through the combination of engineering works and non-structural measures. and improved collaboration between flood management agencies Actions 5.1 Melbourne Water will work with the State Flood Policy Committee, Office of the Emergency Services Commissioner and other stakeholders to develop and implement an integrated flood management planning framework for the region. 5.2 Melbourne Water will provide technical and financial assistance to councils that have identified flood risk for the preparation and review of flood management plans.

Left: Aitken Creek, Craigieburn, February 2005 Below: Princes Freeway, Little River, February 2005

5 Background

Melbourne Water The Port Phillip and Westernport Melbourne Water is the waterway manager for the Port Region Phillip and Westernport Region and as such is custodian The Port Phillip and Westernport Region covers an area of the health of 8,000 kilometres of rivers and creeks that of approximately 13,000 square kilometres and includes drain into Port Phillip Bay and Western Port. nearly all the land that drains to Port Phillip Bay and Melbourne Water is also the regional drainage and Western Port. It extends from high up in the Yarra Ranges floodplain management authority for the region and in the east, across to Ballan in the west, and from the is responsible for the management and maintenance Mornington Peninsula and Phillip Island in the south, of numerous retarding basins, water quality treatment to Lancefield and Kinglake in the north. It includes the wetlands, levee banks, pump stations, flood gates and greater Melbourne metropolitan area and is home to 3.8 1,500 kilometres of underground drains. million people. Melbourne Water’s area of responsibility matches the Prior to European settlement, the region’s rivers, creeks Port Phillip and Westernport Catchment Management and their floodplains formed a vital part of the Aboriginal Authority’s (CMA) operating area to ensure a whole of culture. They yielded many resources and were important catchment approach to the management of flooding, places of spiritual and community activity, including regional drainage and river health. birthplaces, burial sites, places of ceremony and transport routes. A range of European and Aboriginal heritage values The CMA plays an overarching strategic management role exist in the regions waterways and floodplains. in the development, co-ordination and delivery of the Port Phillip and Western Port Regional Catchment Strategy Melbourne Water acknowledges the traditional owners 2004 – 2009. of the land, rivers and creeks we manage, and pay our respects to their elders and ancestors. We recognise our Melbourne Water’s responsibilities, overall goals and long responsibilities to care for rivers and creeks in greater term aims for the management of waterways, floodplains Melbourne, mindful of the significance they represent for and regional drainage in the Port Phillip and Westernport our indigenous community. Region are outlined in the Waterways Operating Charter. Responsibilities for flood management across the region The Operating Charter sets out how we will work with are shared between all levels of government and the government agencies, community, local councils, rural community. There are 38 local government areas within landowners, developers and other stakeholders to deliver the region. an integrated and holistic approach to the management waterways, water quality and flooding. The region is made up of five primary drainage catchments – Westernport, Dandenong, Yarra, Maribyrnong and The Operating Charter outlines specific targets for a Werribee. In addition there are a number of smaller number of the initiatives contained in this strategy. catchments such as Elster Creek and Kananook Creek that The Operating Charter is required by government under drain directly to the bays. Each catchment has extensive our Statement of Obligations and is independently audited areas of flood affected land. and reported each year to ensure we meet our obligations to our stakeholders and the community.

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7 What is flooding?

Floods occur when flows generated by rainfall overtop the Floodplains dissipate the power of floodwater and reduce banks of a river, creek or constructed channel, or when associated damage. Floodwaters retain and replenish the amount of runoff exceeds the capacity of underground wetlands, and support the flora and fauna of floodplains drainage systems. and river systems. Floodplains have a fundamental role in managing the The majority of what is classed as problem flooding occurs flow of water by providing natural storage areas where when urban development and infrastructure (such as floodwaters can be retained and slowly released as stream roads) have constricted the floodplain or blocked natural heights recede. drainage lines and flow paths.

Likelihood of flooding Rainfall events and the floods that result from them are The 100 year ARI, or 1% AEP flood, is the generally categorised by the frequency at which events of a given accepted event on which guidelines and standards are size are likely to occur. based to protect new development. However flood risk assessment and emergency response require Annual Exceedance Probability (AEP) is the statistical consideration of a range of lesser and greater events. likelihood of occurrence of a flood of a given size or larger in any one year, usually expressed as a percentage. Rainfall events vary in duration and intensity. A storm of a certain duration and intensity will have different effects Average Recurrence Interval (ARI) is a statistical in different areas. As a result, there is no single 1% AEP estimate of the average period in years between the storm event for Melbourne, but rather a series of events occurrences of a flood of a given size or larger. For represented by the curve as shown in Figure 2. example, floods with a discharge as large as the 100 year ARI flood event will occur on average once every 100 This means that a 1% AEP event can occur in a small urban years. The ARI of an event gives no indication of when a drain following a rain storm that lasts an hour or so but flood of that size will occur next. has a very high intensity (48 mm of rain). Low intensity rainfall (less than 5 mm per hour) that lasts for days can Small events generally occur frequently (e.g. 50% AEP or generate a 1% AEP event in a major waterway such as the 2 year ARI) and large events quite rarely (1% AEP or 100 Maribyrnong River. year ARI). However, as rainfall events are randomly generated, much like the chances of a number coming up on a roulette wheel, it is possible to experience several large events in close succession.

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9 Types of flooding in the region Most flooding in the region is caused by rainfall either in the form of riverine flooding or overland flows. Other types of possible flooding are coastal tidal flooding, storm surge and tsunamis.

Riverine flooding Overland flows Riverine flooding is caused when the runoff from major Overland flows, or flash floods, occur when runoff from storms exceeds the channel capacity of a river or creek and severe storms exceeds the capacity of the underground overflows onto the surrounding floodplain. drainage system. When this happens, water begins to flow downhill over the surface of the land along natural flow Riverine flooding is relatively predictable in terms of paths or valleys towards the nearest creek or river. geography and timing. It can be known many hours or even days in advance when and where a river or creek is likely to Overland flows usually occur with little or no warning overflow its banks, so advance warnings and preparations following intense rainfall, often associated with short can be made accordingly. duration thunderstorm activity. They can be localised or widespread depending on the path or extent of Around 20,000 properties across the region are known to storm activity. be at risk of being affected by riverine flooding in a 1% AEP flood, and approximately 3,000 buildings could be flooded Flooding problems occur because the developers of many of above floor level. Many of these properties are located Melbourne’s older suburbs were unaware of these risks and within the Maribyrnong Township and the towns of the were not required to make provision for overland flow paths. Koo Wee Rup Flood Mitigation District, such as Cora Lynn, Flood mapping in catchments serviced by Melbourne Water Tooradin and Koo Wee Rup. drains has identified around 82,000 properties in the region Flood mapping for all rural waterways, especially those that are at risk of flooding from overland flows in a 1% AEP in the Werribee and Maribyrnong catchments is not yet flood with approximately 37,000 properties vulnerable to complete and so the total number of properties at risk of flooding above floor level from overland flows. flooding in these areas cannot be accurately determined. However, the total number of properties and public infrastructure at risk is likely to be greater, since this figure does not include properties subject to inundation in smaller local catchments serviced by council drains.

10 Flood Management and Drainage Strategy Coastal tidal flooding and storm surge Ocean tides can affect normal sea levels and cause flooding along the coastline and lower reaches of tidal rivers such as the Maribyrnong and Yarra Rivers, especially when combined with high rainfall. However, the tidal effects along an estuary or river diminish with distance inland. Storm surge is another mechanism that can affect water levels along coastal areas. Storm surge occurs through a combination of low barometric pressure, strong winds and large waves. It is uncommon for extreme rainfall events and extreme storm surges to occur simultaneously. To date there has been little mapping undertaken to indicate the extent of areas that would be subject to coastal tidal flooding or storm surge should such an event occur.

Left to right: Diamond Creek, Eltham North, February 2005 Corner of Normanby Avenue and Leinster Grove, Thornbury, December 2003 Kororoit Creek, Altona, February 2005

11 History of flooding

Riverine flooding The original settlement of Melbourne was situated The 1974 flood was the second largest on the Maribyrnong in a basin intersected by the paths of the Yarra and River, and occurred on 15 and 16 May when 110 mm of rain Maribyrnong Rivers. Many of the early recorded floods fell on the catchment over a 48 hour period, with 70 mm occurred when these rivers overflowed their banks and falling in 17 hours. inundated the surrounding floodplains. Approximately 385 hectares of land between the In the Great Flood of 1891, newspapers reported that the Maribyrnong Township and Dynon Road Bridge were Yarra River rose 14 metres above its normal level in some inundated by floodwaters up to one and a half metres areas. It flooded the riverside suburbs of Collingwood, deep. About 120 houses and 50 commercial and industrial Richmond and Prahran, and caused extensive damage. premises were flooded. Approximately 3,000 people were left homeless. Emergency services evacuated large numbers of people In 1934, 350 mm of rain fell in 48 hours over the Yarra from the Maribyrnong Township. catchment, with 140 mm falling over metropolitan areas. Installed in July 1975, the Maribyrnong Flood Warning Flooding was widespread across the region. Six thousand System continuously monitors catchment conditions, people were left homeless, and eighteen people died. rainfall and flows. This system has been improved to Since the 1891 flood, extensive remedial work has been provide automated telephone warnings to vulnerable carried out on the lower Yarra River, including widening residents. and redirecting its course to Port Phillip Bay. As a result, Widespread riverine flooding that occurred across the Port the impact of riverine flooding on the community has now Phillip and Westernport Region in February 2005 caused been substantially reduced. little property damage but did cause localised disruption to Major floods have also inundated the floodplains of transport infrastructure. the lower Maribyrnong River on numerous occasions. The Princes Highway was partially blocked at Werribee. The three largest floods occurred in September 1906, Traffic was also disrupted in Dandenong, Bulleen and September 1916 and May 1974. Diamond Creek.

12 Flood Management and Drainage Strategy Overland flows Almost every year, even during periods of prolonged drought, there is a thunderstorm somewhere across the Port Phillip and Westernport Region that causes localised flooding as a result of overland flows. A well-known example of overland flows occurred in Elizabeth Street on 17 February 1972, when 78.5 mm of rain fell in one hour over the Central Business District. This flooding caused significant disruption to the public and damage to businesses. In recent years a number of storms have occurred causing significant damage to private property and public infrastructure as well as significant disruption and stress to the community. On 3 December 2003, 106.5 mm of rain fell in 2.5 hours over the cities of Darebin, Banyule, Whitehorse and Manningham. The worst affected areas were Fairfield and Thornbury. On 29 January 2004, severe localised storms passed across the eastern and north eastern suburbs of Melbourne. In Kew, 66 mm of rain fell in 90 minutes, Elsternwick recorded 73 mm, Deer Park, 71 mm and Hawthorn, 70 mm. At Dunns Hill near Mount Dandenong, 31.6 mm fell in 30 minutes. Localised flooding occurred again on 25 February 2006 when 75.2 mm fell in 30 minutes in Northcote and Coburg.

Punt Road, Yarra River floodplain 1891

13 Flood damage

In December 2003 the City of Darebin was hit by “the worst storm in 100 years”.

While the whole of metropolitan Melbourne was affected, The complex and long lasting effects of flooding on areas within Darebin were hit hardest with homes, schools individuals, households and communities continue well and businesses severely damaged by flooding. after the floodwaters have receded. The following morning residents described the Fairfield Understanding the range of impacts of flooding is essential shopping strip as a “war zone” – the street was strewn with to the development of effective flood management. broken glass, debris and spoilt stock. Flood damage can be broadly classified as tangible and People were wandering around shocked and in a dazed intangible damage. state. In the aftermath of the crisis people had to deal with the stress of financial and emotional loss leading to the experience of grief for many.

“I went down to Station Street at 8 o’clock in the morning and many of the traders were just opening their doors in a shocked state. There was an unreality about it all.” After the Deluge, City of Darebin (2006) Tangible damage Tangible damage from flooding can be measured in Examples of indirect damage include: monetary or other quantifiable terms, i.e. dollar costs, • public clean-up costs numbers of houses or businesses affected or destroyed. • disruption to transport services (roads impassable This includes both direct physical damage to properties obstructing movement and the ability to provide and assets, and indirect damage arising from disruption of community services) normal social and economic activities. • disruption to utilities (water, sewerage, Data on tangible damage is relatively easy to determine communication) from insurance claims and replacement costs. • disruption to public and emergency services Examples of direct damage include: • economic impacts of health issues (medical costs and • damage to building and contents disruption to work activities) • external damage (pools, gardens, fences, shed • loss of business or trade as people shop elsewhere. contents) Losses attributed to emergencies are generally described • motor vehicles in monetary terms. • infrastructure (roads, bridges and other services) • clean-up costs for individual home owners • loss of stock • cost of re-instatement or rebuilding of houses and buildings.

From left: Darebin Creek, Alphington, February 2005 Darebin Creek foot bridge, February 2005

15 “I’ve never seen anything like it. Flash, flash, flash! When it rains now I think, ‘How heavy is it? Will it flood?’ I always go out and check the gutter.”

“After the floods there was a lot of anger. There was also an ugly side to things like stories going around about people getting flood relief money while others missed out who really needed help. People are still hurting.”

After the Deluge, City of Darebin (2006) Intangible damage Much of the impact of flooding can be described as ‘intangible’ – effects which cannot be evaluated in monetary terms. Intangible impacts include loss of life, perceived loss of security of the home, fear of continuing severe flood events, loss of memorabilia, and physical and psychological effects on human health and well being. Psychological impacts resulting from the experience of severe flooding are generally worse than the physical health impacts, and can persist well after the event. Intangible impacts are difficult to assess or quantify as a number of factors can affect an individual’s vulnerability or ability to cope and recover from a severe flood event. Other factors that affect an individual’s experience of flooding include current levels of health, networks of support and degree of financial security. An understanding and assessment of the full range and variability of intangible or social impacts is important for complete assessment of risks necessary to determine priorities for future flood management. Understanding intangible impacts is also important for ongoing development of emergency management and recovery programs, including education and awareness programs.

16 Flood Management and Drainage Strategy Flood damage, Maintree Road, Hawthorn, December 2003

17 Estimating the damage Large floods can cause damage that runs into the To understand the likely cost of flooding in any given year, hundreds of millions of dollars. However, they occur rarely. the total damage caused by all floods over a period of time Smaller floods that occur more frequently don’t cause as is divided by the number of years in that period. much damage individually, but collectively the damage is These likely losses are called Annual Average Damage significant. (AAD). To better understand the risks of damage to areas identified as subject to flooding, the likelihood and consequences of flooding need to be considered. By standardising or averaging damage, unit losses can be assigned to enable an estimation of damage.

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18 Flood Management and Drainage Strategy Economists use tools like the loss-probability curves shown By comparison, AAD associated with bushfires in the same in Figure 3 to provide an estimation of the likely damage period was relatively smaller at $77.2 million. However, in any given year. bushfires are the most hazardous type of disaster in terms of deaths and injuries.1 Flood damage increases significantly once flooding occurs above floor level, causing damage to household contents To understand the context of these figures, only the $30 such as floor coverings, curtains, furniture, plaster, million AAD figure for waterways flooding in the Port particleboard and painted surfaces. Phillip and Westernport Region is comparable to the national AAD for riverine flooding of $314 million. Annual Average Damage (AAD) figures for the Port Phillip and Westernport Region are shown in Figure 4. This is because in other major urban centres, or capital cities little if any overland flow path mapping and analysis This estimate takes into account all potential direct and has been undertaken. indirect tangible damage – that is, the level of damage that would occur if no remedial action of any kind were In the Port Phillip and Westernport Region, the AAD undertaken to reduce exposure to damage. It does not caused by overland flows identified by Melbourne Water include the intangible impacts of flooding. is approximately three times greater than the riverine flooding risk. In Australia between 1967 and 1999, riverine flooding ($314 million AAD) and severe storms ($284.2 million AAD) were If the estimated local overland flow impacts are considered, the most costly types of natural disaster. the impacts of these shorter duration events could be as much as seven times greater than the annual damage caused by riverine flooding.

1 Report 103 Economic Costs of Natural Disasters in Australia, Bureau of Transport and Regional Economics (2001)

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19 Managing floods

Commonwealth, State, regional and local authorities, as Response: Activities undertaken to combat emergencies well as local communities and individuals all have a vital and provide rescue and immediate relief services. role to play in flood management. Recovery: Activities aimed at providing assistance to The scope of flood management is described within the people and communities affected by emergencies to context of three overlapping activity clusters: prevention, enable them to achieve a proper and effective level of response and recovery.2 functioning. Prevention: Activities aimed at eliminating or reducing Activities required to manage a flood event, and the the incidence or severity of emergencies and the relationships between them, are outlined in Figure 5 below. mitigation of effects.

2 Victoria Flood Management Strategy, State Flood Policy Committee (1998)

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20 Flood Management and Drainage Strategy Lollipop Creek, Wyndham Valle, February 2005

21 Shared responsibilities for flood management

Australian Government Victorian State Government The Australian Government has a supportive role in State governments are recognised as the key level relation to flood management. of government responsible for natural resource and emergency management. Its primary role is in: The main responsibilities of the Victorian Government in • providing flood forecasting and warning relation to flood management are to: • supporting development of state emergency • develop standards and strategic approaches for management capabilities floodplain management • providing natural disaster financial support to state, • apply standards and strategies in a co-ordinated and local governments and agencies (e.g. Natural Disaster integrated manner Mitigation Program and Natural Disaster Relief and Recovery Arrangements) • provide technical support, planning advice, impact and needs assessments to aid emergency response • providing policy and legislation in relation to the and recovery planning and co-ordination of post availability of insurance emergency recovery support services • supporting the development and implementation of • develop the Victoria Planning Provisions national strategies. • develop the Victoria Building Regulations. Agencies include the Bureau of Meteorology, the Department of Transport and Regional Services, Groups include the Department of Sustainability and Emergency Management Australia and Geoscience Environment, the Department of Human Services, Office Australia. of the Emergency Services Commissioner, the Building Control Commission, the Victoria Emergency Management Council, the State Flood Policy Committee, and the Victorian Flood Warning Consultative Committee.

22 Flood Management and Drainage Strategy Emergency Services Regional authorities The State’s emergency services have a responsibility to Catchment Management Authorities and Melbourne assist the community during natural disasters and to: Water are designated floodplain management authorities. Primary flood management functions include: • support local agencies in emergency planning • undertaking flood studies and flood mapping • participate in emergency response and recovery planning, co-ordination and management • identifying flood affected land • develop and maintain community preparedness • undertaking flood prevention works such as retarding through awareness and education programs basins, levees and pipe augmentation (household, industrial and commercial safety plans, • assisting the Bureau of Meteorology with flood council community awareness programs, signage, forecasting and warning building materials advice) • advising planning authorities regarding appropriate • develop and maintain total flood warning systems land use and development of flood affected areas • undertake impacts and needs assessments. through their role as planning permit referral authorities Agencies include the Victoria Police and the Victoria State Emergency Service. • supporting community education and awareness programs. Throughout most of Victoria, Catchment Management Authorities are responsible for the preparation of catchment management strategies and floodplain management plans. Within this region, the Port Phillip and Westernport Catchment Management Authority is responsible for the development and monitoring of the Regional Catchment Management Strategy but the Minister responsible for the Water Act 1989 has delegated floodplain management functions to Melbourne Water. Melbourne Water is also responsible for: • planning regional drainage systems to ensure new urban development meets appropriate standards of flood protection and environmental performance • the ownership and maintenance of drainage assets where the catchment area is greater than 60 hectares • assessment of planning permit application referrals to subdivide land or develop flood affected land.

23 Local government Local government is primarily responsible for the planning In relation to flood management, local councils are and provision of services and facilities for the local expected to: community, and for providing and maintaining community • administer and enforce planning provisions and infrastructure. building regulations in relation to building and Although local government is not a delegated floodplain development on flood affected land management authority under the provisions of the Water • provide for the conservation of natural resources and Act 1989, it does have a critical role to play in flood areas of environmental significance management through its role as a planning authority and emergency management agency. • develop flood sub-plans as part of their municipal emergency management plans and participate in risk There are 38 councils in the Port Phillip and Westernport reduction activities Region that manage an estimated 25,000 kilometres of local underground drains and other drainage • provide the public with access to flood information infrastructure. • implement and maintain local flood warning systems • provide and manage local drainage infrastructure for catchment areas less than 60 hectares • support community education and awareness • manage local level emergency recovery support, clean-up and maintenance.

24 Flood Management and Drainage Strategy Local communities and individuals Managing flood risks is a shared responsibility between individuals and communities, government and non-government agencies, and the private sector. An informed and prepared community that understands the flood risk for its area and is familiar with local emergency management programs is well placed to protect itself from the impacts of emergencies. Supported by agencies at all levels, the community is responsible for: • knowing the level and extent of flood risk for its area • participating in risk assessment and risk reduction prioritisation processes • participating in reducing personal and community vulnerability Flood damage, Glenferrie Road, Hawthorn, December 2003 • managing its recovery from flood events • preparing personal emergency flood plan.

25 Flood management objectives

Flood managers face a series of challenges. • Emergency Planning and Preparation. Mapping identifies the areas at risk so that agencies can They must: implement flood warnings, plan flood evacuation, • identify and understand current and future flood risks relief and recovery strategies, prioritise community and the impacts they have on people and property information and awareness programs and develop • develop criteria to agree what is intolerable flood risk warning arrangements. • develop and implement a program to eliminate or • Mitigation priorities. Mapping identifies safety risks reduce intolerable risks and damage potential, and assists in the development of risk reduction targets, delivery programs and works • manage residual risks through improved community priorities. awareness, planning and preparedness. In addition to knowing where floods may occur, an understanding of flood depth, velocity, duration and rate Objective 1: Completing the of rise is also required to fully understand flood risk. knowledge base Such information is usually collated in the development of flood studies that map flood extent. Flood mapping to identify risks Challenges Melbourne Water’s floodplain management functions, outlined in the Water Act 1989, include finding out how Flood modelling and mapping is an expensive and far floodwaters are likely to extend and how high they are specialised activity. Additional flood mapping needs to likely to rise. be in a format and to a standard that is consistent with existing data held by Melbourne Water and suitable for Mapping throughout the region is currently incomplete the assessment of flood mitigation options and land use and some information has not been included in planning planning decisions. schemes or made easily accessible to the public. In addition, issues associated with the collection and In particular, mapping is not complete for many rural public release of such information, particularly through waterways for flooding where the catchment area is less the planning scheme amendment process, need to be than 60 hectares, or for areas that may be subject to carefully managed. coastal or tidal inundation. Not all agencies are appropriately resourced to The identification and accurate mapping of flood risk commission flood studies or to maintain and administer areas is considered to be a critical foundation for the flood information under current planning scheme development of sound drainage and flood management arrangements. Additional support and assistance to activities. complete these tasks may be required. A detailed understanding of flood affected areas gained Agencies have also expressed concerns regarding through mapping is important to the development of the potential liability issues and other obligations associated following aspects of flood management planning and with the collection of flooding information through response: mapping exercises. • Land use Planning Policy and Controls. These However, failing to identify these areas will not make the controls protect the environmental values of risk of flooding any less real, and can potentially increase floodplains and prevent current flood risks increasing the likely damage resulting from an event particularly if through inappropriate development. They also ensure further development is allowed to occur without taking that proposals that may be affected by flooding or flood risks into account. have an adverse impact on existing flood regimes are referred to floodplain management agencies. There are a variety of ways in which mapping studies and planning scheme amendments could be undertaken.

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27 Actions Challenges 1.1 Melbourne Water will undertake and maintain ongoing Floods affect people in different ways depending upon local flood extent, rural waterway and tidal mapping their health, well being, economic status and other for the Port Phillip and Westernport Region, taking personal circumstances i.e. their vulnerability or resilience into account the potential long term pressures of to flooding. climate change and urban consolidation. This will Determining the impact of an emergency on a community involve consultation with local government and the is a complex task. There is currently no agreed process Department of Sustainability and Environment. or tool for assessing flood damage and intangible social 1.2 Melbourne Water will update and maintain flood impacts of flooding that can be used to determine risk information for municipal planning schemes to ensure acceptability/tolerability thresholds. appropriate standards of flood protection are achieved Ongoing strategic research and further development of for new development in flood affected areas. This will our understanding of the social impacts of flooding is an involve consultation with local government and the issue that affects flood management agencies at all levels. Department of Planning and Community Development. There is currently no lead agency in Victoria with a clear responsibility for the development and implementation of Understanding social impacts a research program into the social impacts of flooding. Studies in emergency response and recovery management Actions have emphasised the importance of extending investigations into the impact of natural disasters to 1.3 Melbourne Water will support collaborative, research include psycho-social effects on individuals and the wider into the social and economic impacts of flooding on community. the community and use this information to assess flood management priorities. Understanding the relative vulnerability of different communities affected by flooding can also assist in the benefit cost analysis of flood mitigation works. It can also assist in the planning and development of education and awareness, flood warning and response and recovery programs. There is currently little evidence-based information available which captures the range of physical, mental health and associated impacts of flooding on communities in Victoria. Flood management programs which do not seek and utilise such knowledge could fail to meet community needs, lead to inappropriate targeting of resources, and increase community anxiety of an already traumatic event.

28 Flood Management and Drainage Strategy Urban consolidation, Parkville

29 It is projected that between 2006 and 2031, the population of Melbourne will increase by approximately 860,000 and adding over 510,000 new households to the metropolitan area.

Objective 2: Potential long term Increased numbers of dwellings in an area may result in increased hard surface coverage depending on the design future pressures on existing drainage of the development and existing conditions. systems If this occurs, an increase in runoff volumes and an increase in peak flows can be expected. Unless managed, Development in established areas this increased runoff could result in a reduction in drainage It is projected that between 2006 and 2031, the population infrastructure performance in some areas. of Melbourne will increase by approximately 860,000 and To a lesser degree, the replacement of older and smaller adding over 510,000 new households to the metropolitan structures by large new dwellings will also have an effect area. even when no increase in dwelling numbers occurs. Even if the population did not increase, the reducing Investigations into the impacts of development in average size of households means that more dwellings will established areas suggest that most of the metropolitan be necessary in the future to house the same number of area is able to cope with the forecast increase in dwellings people. and that any adverse impacts will be limited to a few The government’s metropolitan strategy, Melbourne catchments. 2030 – Planning for sustainable growth (Department In these catchments, there will be a range of outcomes that of Infrastructure 2002), aims to make the city more call for different forms of action. Increased runoff impacts sustainable while managing the increase in population and can be managed where new development is well planned households. for, such as in activity centres, major re-development sites One important way to help achieve this is to make the and around transport nodes. urban area more compact. The government is currently working with local councils Melbourne 2030 encourages a greater proportion of to develop strategy plans for principal and major activity new development to be built in and around strategic centres that will address, amongst other matters, redevelopment sites (such as principal and major activity drainage issues. centres) that are supported by existing infrastructure, Such planned growth enables authorities to better develop particularly public transport, and are best able to cope with strategies for infrastructure provision rather than ad hoc change. or uncoordinated redevelopment across the catchment. The share of new dwellings in ‘greenfield’ and dispersed The impacts of increased runoff resulting from such development areas will be reduced accordingly. dispersed infill developments are more difficult to manage If the Melbourne 2030 housing distribution aspirations because it is not easy to quantify or plan for the extent, are achieved, it would mean that 41% of new households timing and location of this form of development. will be located in strategic redevelopment sites and 28% The potential for infill development is influenced in some in the remainder of the existing urban areas through infill locations by factors such as heritage, neighbourhood redevelopment. character or environmental significance. This will generally cause the overall density of dwellings Similarly, such development in some catchments also to increase although most existing residential areas will needs to be sensitive to the capacity of the drainage experience very little change. system to accommodate additional runoff, or the ability of drainage authorities to undertake augmentation works.

30 Flood Management and Drainage Strategy Dandenong Creek, Dandenong North, February 2005

In some catchments, it may be necessary to undertake Challenges works to accommodate additional runoff to acceptable It is difficult to plan and fund infrastructure to cater for the levels. impacts of dispersed infill development as the exact timing In a very limited number of cases, there are parts of and location of such development is not always known. catchments where works are not possible, practical or There are some areas where it is not possible, practical or affordable. affordable to undertake works to accommodate additional In these areas, where infrastructure cannot be upgraded, runoff and a variety of flood management measures will be a more flexible range of activities may be appropriate, required to manage flood risk. such as: Action • education and awareness programs 2.1 Melbourne Water will work with the Department of • flood warning systems Planning and Community Development and local government to understand the likely impact of urban • revision of planning policies and controls (to limit development on runoff volumes and flood risk in the capacity for additional development or to control existing urban areas. impervious surface area) 2.2 Melbourne Water will develop flood risk management • water sensitive urban design measures in consultation with Department of Planning • stormwater capture and reuse. and Community Development, Department of Improved co-ordination between agencies responsible for Sustainability and Environment and local government. statutory and strategic planning and policy development, Solutions may include upgrading infrastructure and assessment of infrastructure capacity, will be required planning and building controls to limit imperviousness, to ensure a closer match between land use intentions in and onsite measures such as water sensitive urban the infill areas and infrastructure delivery. design and stormwater capture and reuse. Melbourne Water is preparing strategies for individual urban catchments where significant redevelopment is occurring. These strategies identify the potential impact of redevelopment on runoff levels. Where levels of drainage service may be affected, the strategy will identify the additional infrastructure, the costs associated with these works, and appropriate charges to be levied on developers to fund those works. While this approach ensures developers fund works to maintain existing levels of drainage service, it does not seek to resolve existing problems. However, the introduction of these strategies may provide opportunities to undertake flood mitigation works at the same time.

31 Climate change In areas where side entry pits can cope with the volume of water, the pipes fill quickly, again leading to overland flows. The Intergovernmental Panel on Climate Change’s Fourth Assessment Report 2007 concluded that warming of the CSIRO has suggested a preliminary estimate of a 5% earth’s climate is unequivocal. increase in rainfall intensity per degree of climate warming. The management of the impacts of climate change resulting from global warming is a worldwide challenge. A Melbourne Water sensitivity analysis of increasing rainfall intensity by 5% per degree of temperature However predicting the potential rate and extent of increase, suggests a similar increase in runoff volumes of climate change in Australia and the Port Phillip and 4% by 2020 to 25% by 2100. Westernport Region in particular, is a challenge as our climate is highly variable and there are limited historical Under these circumstances, what we currently consider climatic records to compare with the present. to be a 1% AEP event will occur more frequently as shown opposite in Figure 7. Based on the available climate change predictions, the interpreted trend for Melbourne is for increasing This could also mean more frequent flooding above existing temperature, reduced annual rainfall and more extreme floor levels. events.3 These more frequent events are also likely to stretch the Melbourne Water funded the CSIRO to study the impacts tolerance of the affected community and affect in-stream of climate change on water supply, sewerage and drainage values of urban waterways. functions. The study showed that while the amount of More frequent and larger flows have the potential to erode overall rainfall received each year is predicted to decrease, waterways and reduce the ability of flora and fauna living the rainfall may occur in more intense storm events rather within the waterways to endure the habitat disturbance than our normal winter and spring rains. associated with such floods. This change in rainfall patterns is more likely to result in Another predicted impact of global warming is a rise an increase in overland flows rather than riverine flooding in sea levels resulting from a number of factors such because the short, sharp nature of these intense storms as thermal expansion of the sea mass and melting ice results in sudden large volumes of runoff water. shelves. Side entry pits struggle to cope with these volumes of water, While the Port Phillip and Westernport region does not and overland flows result. experience significant coastal tidal flooding and storm 3 Melbourne Water Climate Change Study Implications of Potential Climate surge, there is a possibility that a change in sea level may Change for Melbourne’s Water Resources (2005), CSIRO increase these risks.

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Challenges We need to undertake sensitivity analysis of the impacts of different rainfall intensities on various sized catchments Knowledge regarding climate change is constantly to determine which areas will be most sensitive to the changing. Ongoing strategic research and understanding possible impacts of climate change. the impacts of climate change on drainage systems is a State and national issue that affects flood management Understanding these effects will assist in the development agencies at all levels. of appropriate adaptation strategies. The implications of changed weather patterns on the Action intensity and duration of storms in the region are not well 2.3 Melbourne Water will continue to support research understood. programs to better understand the impacts of climate We need to continue working with other key stakeholders change on flood and drainage management. such as the Bureau of Meteorology, CSIRO, Department 2.4 Melbourne Water will develop adaptable flood of Sustainability and Environment, State Flood Policy management programs to deal with changing rainfall Committee, and the Westernport Greenhouse Alliance to patterns, rising sea levels, and storm surge. These develop greater understanding of the likely timing and programs will be developed in consultation with the impacts of climate change, and the areas that will be most Commonwealth Scientific and Industrial Research sensitive to the impacts of climate change. Organisation, (CSIRO) and the Bureau of Meteorology. There are currently more than 100,000 known properties in the region at risk of flooding in up to a one in one hundred year storm event. More than 40,000 of these contain buildings or dwellings at risk of flooding above floor level.

Objective 3: An agreed approach Current efforts by Melbourne Water and councils to reduce the consequences of flooding in known flood affected to managing existing regional flood areas are effective at the local level, but they are having problems little impact on the overall problem due to the large number of properties at risk of flooding. Overland flows, or flash flooding, are by far the most common cause of this type of flooding. Most people living Even if it was physically feasible to construct all the in those areas are unaware of the flood risk. required works, it would cost billions of dollars (based on the cost of current mitigation projects), to protect all In most cases, flash flood events occur with little warning, properties in the region from a one in one hundred year last only an hour or less, are relatively shallow, slow moving flood. and pose little risk of drowning. Mitigation works are currently funded through council However, there are some areas where the combination of or Melbourne Water drainage rates and the magnitude the water depth and velocity pose threats to human safety. of works required may therefore exceed the capacity or A number of measures are used by agencies to manage willingness of the community to pay. existing flood risks in the region. In addition, at the current rate of construction, the However, it is clear that there is significant existing timeframe needed to build such works would extend exposure to flood risks in the region and not all of these beyond the design life of the properties they are intended risks can be managed in the same way. to protect. In many cases, reducing risk using structural measures is too costly, not physically practicable or necessary. In other cases, risks may be still unacceptable even with actions GifYXY`c`kp to provide warnings, emergency response and recovery

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34 Flood Management and Drainage Strategy Underground carpark, Hawthorn, December 2003

Intolerable risks For example, the long-term computed probability of flooding in a certain area may not be high but floods may Although we cannot remove all flood risk there are some still occur several times over a short duration. flood risks that we may decide are intolerable. Statistically the flood risk may not be intolerable, but the That is, a reasonable person would agree that almost combined impacts of multiple floods within a short time regardless of cost, the consequence of the flood should be frame may make this situation unacceptable. virtually eliminated or significantly reduced. Another example of unacceptable flooding may be where If that cannot be achieved by engineering works then even though the consequences of flooding are minor, a radical measures to reduce the consequences might be mitigation solution would be relatively inexpensive and easy required such as acquiring properties. to implement, such as regrading or the construction of a Factors which may make flood risks intolerable include: low levee. • significant threat to life health or safety Other factors that can influence the acceptability of risk • potential impacts to large numbers of people include. • significant environmental impacts • whether flooding is seen as natural (riverine) or human generated (development related) • significant disruption to economic activity or public infrastructure. • perception that risks are being managed through planning, preparedness, response (including warning) While many flood risks may not be regarded as ‘intolerable’, and recovery activities they may still be unacceptable to the community under certain circumstances. • awareness of flood risks so that exposure to risks is partly based on conscious individual decisions • cost of measures to reduce flooding relative to damage and impact on drainage rates.

35 Under current arrangements flood mitigation projects tend Action to be prioritised on the basis of the cost effectiveness of 3.1 Melbourne Water will sponsor research and consult protection from above floor flooding. with stakeholders to develop criteria to establish flood However, there is currently little formal understanding risk tolerability thresholds which take into account of what constitutes a socially acceptable level of risk relevant social or intangible flood impacts. or explicit use of this understanding in decision making 3.2 Melbourne Water will develop and implement a particularly in relation to flood mitigation capital program in consultation with stakeholders to eliminate expenditure. or reduce intolerable risks to an acceptable standard Further work will be undertaken to identify and understand through the combination of engineering works and both flood extents and flood damage, both tangible non-structural measures. (building and contents damage) and intangible (trauma, disruption, stress, anxiety). This work will improve assessment of flood risks required to undertake a benefit cost analysis of expenditure on flood mitigation projects.

Challenges Determining an acceptable balance between structural works and non-structural measures, and levels of protection within an agreed risk management framework that maximises community wellbeing and sustainability is a key challenge for flood management agencies in the region. In order to develop this framework we need to undertake research, consult and agree with stakeholders around what is intolerable flood risk so that we can determine a safe and acceptable level of flood protection for both general and particular situations. We need to develop a consistent and transparent decision- making framework and a program to prioritise works to eliminate or reduce risks to an acceptable standard. Melbourne Water has engaged the Royal Melbourne Institute of Technology to conduct research, including focus groups, into what the community regards as intolerable flooding, and to develop tools for assessing intangible flood damage. Greater flexibility and reliance on non-structural solutions, such as community education programs and planning controls, will also be required to manage both intolerable risk in the interim and the remaining or residual risk, which would still have varying degrees of acceptability.

36 Flood Management and Drainage Strategy Little River, Werribee, February 2005 Objective 4: Enhanced community Apart from general flood awareness and preparation information available on a small number of local education, flood awareness and government and agency web sites, there is little specific preparedness information available on flash flooding from extreme weather events. Understanding the extent and full impacts of flooding is essential for planning for potential future pressures on In addition, there is little or no information on flash flooding the drainage system arising from climate change and infill relating to specific areas. urban development. Few community education flooding awareness and Equally important is the capacity to share this information preparedness campaigns have been developed or and engage with other stakeholders and the community. implemented on a regional basis. Melbourne Water and councils have made flood information This lack of understanding limits the effectiveness of flood available to property purchasers and developers by warning systems and the community’s ability to prepare including information in municipal planning schemes and and respond to emergencies. updating property information statements. More community education and awareness programs would Once flooding information is shown on planning scheme assist in raising levels of preparedness, aid response and maps, it is also made available on planning certificates recovery activities, and reduce the impacts and trauma issued as part of any transfer of land documentation. associated with flooding. Melbourne Water and councils have individually notified The community education strategies adopted by Victoria’s approximately 120,000 affected landowners and occupiers fire response organisations (Country Fire Authority, as part of the planning scheme amendment process. Department of Sustainability and Environment, and the Metropolitan Fire Brigade) provide examples of the success However, there is still a significant gap between community of such awareness programs. and agency flood awareness and knowledge. These strategies reinforce the benefits of improved The risks associated with flooding, and urban flash flooding broad community awareness in enabling appropriate in particular, are poorly understood in the community. management actions to be undertaken by the community and individuals before, during and after an emergency situation. Challenges Floods are a natural event and although we will develop a program to reduce intolerable flood risk over time, flooding will continue to occur across the region. There is a need to increase the degree of flood awareness and preparation across all levels of the community. Further effort is also required in the development of warning systems for urban catchments affected by overland flooding. Work to be undertaken to understand the relative vulnerability of communities will assist in the development of these programs. However, historically there has been no single Victorian agency with clear responsibility for Melbourne Water fact sheet the development and implementation of community awareness, engagement and education programs focused on urban flooding. The Victoria State Emergency Service is currently exploring options for the development of an enhanced total flood warning system including community education in flood risk for the State. Action 4.1 A community flood education, awareness and preparedness program, including flood warning, will be developed and implemented for the Port Phillip and Westernport region in partnership with Victoria State Emergency Service, and other key stakeholders.

Detail of Greater Dangenong Planning Scheme – Local Provision Objective 5: Agreed responsibilities Examples of the key agencies providing specialised flood and improved collaboration between management skills and services include: flood management agencies • The Department of Sustainability and Environment has the primary responsibility for the development Historically, Melbourne Water has responded to flood of strategies and policy in relation to flood and events by undertaking engineering works to improve stormwater management at a State-wide level. flood protection. However it is not practical to build • The Victoria State Emergency Service are infrastructure capable of alleviating all flooding. responsible for flood response as well as assisting While intolerable risk areas will continue to be identified councils with their Municipal Emergency Management and addressed, there are many other flood management Planning, and can assist councils with education, tools that can be used in combination to reduce and awareness, warning systems and emergency manage the overall risks posed by flooding. management planning. These tools include planning controls, flood warning • The Department of Human Services impart expertise systems, and programs of education, response and in emergency recovery management, and can assist recovery. with the development of Municipal Emergency Management Plans and public health sub plans. The broad parameters defining and governing drainage and flood management mean that no single agency can • Councils undertake Municipal Emergency Management undertake all necessary activities on its own. Planning, (Flood sub plans), provide community support, damage cleanup, and are responsible for Collaboration between flood management agencies is administration of planning and building controls. essential for developing co-ordinated and integrated flood management programs and activities. • Melbourne Water has primary responsibility for identification of flood extents, providing flood warning Such collaboration must recognise that key agencies advice for major waterways, controlling development involved in flood management may differ in mandate. in floodplains, providing advice on flooding to local Some agencies such as councils deliver a spectrum of councils and the community, developing plans, and community services. Other agencies such as the Victoria undertaking actions to minimise flooding and flood State Emergency Service and Department of Human damage in the Port Phillip and Westernport region. Services have a more specialised focus and are uniquely positioned to provide specific skills and resources where needed. Melbourne Water, councils and flood management stakeholders have recognised the need for an integrated and sustainable approach where responsibility for activities is assigned on the capability of the agency to deliver.

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42 Flood Management and Drainage Strategy A storm approaches Goornong, north-east of Bendigo (picture by Clyve Herbert © Australian Sky & Weather) GLOSSARY

Annual Exceedance Probability (AEP) Flash flooding The likelihood of occurrence of a flood of a given size or Sudden and unexpected flooding caused by local heavy larger in any one year; usually expressed as a percentage. rainfall or rainfall in another area. Often defined as flooding For example, if a peak flood discharge of 500m3/s has an which occurs within six hours of the rain which causes AEP of 5%, it means that there is a 5% risk (i.e. probability flooding. of 0.05 or a likelihood of 1 in 20) of a peak flood discharge of 500m3/s or larger occurring in any one year. Flood awareness An appreciation of the likely effects of flooding and Average Annual Damage (AAD) knowledge of the relevant flood warning, response and The total damage caused by all floods over a period evacuation procedures. Flood awareness allows landholders of time divided by the number of years in that period. to defend themselves, their property and their community If the damage associated with various annual events, from flood threats and to effectively evacuate themselves (the consequences) is plotted against their probability and their possessions when necessary. In communities with of occurrence, the AAD is equal to the area under the a high degree of flood awareness, the response to flood consequence-probability curve. warnings is prompt and effective. In communities with a low degree of flood awareness, flood warnings are liable Average Recurrence Interval (ARI) to be ignored or misunderstood, and residents are often A statistical estimate of the average period in years confused about what they should do, when to evacuate, between the occurrence of a flood of a given size or larger what to take with them and where it should be taken. The (e.g. floods as big or larger than the 100 year ARI flood principal driver determining the degree of flood awareness event will occur on average once every 100 years). The ARI of a community is usually the frequency of moderate to of a flood event gives no indication of when a flood of that large floods in the recent history of the area. size will occur next. Flood damage Catchment The tangible (direct and indirect) and intangible costs The area of land draining to a particular site. It always (financial, opportunity costs, clean-up) of flooding. Tangible relates to a specific location and includes the catchments costs are quantified in monetary terms e.g. damage to of tributary streams as well as the main stream. goods and possessions, loss of income or services in the flood aftermath. Intangible damage is difficult to quantify Development in monetary terms and include the increased levels of The erection of a building or the carrying out of work, physical, emotional and psychological health problems including the placement of fill, the use of land or a building suffered by flood affected people and attributed to a or work, or the subdivision of land. flooding episode.

Infill development Flood emergency plan The development, within an existing subdivision, or An agreed set of roles, responsibilities, functions, actions vacant blocks of land that are generally surrounded by and management arrangements to deal with flood events developed properties; conditions may be imposed on infill of all sizes. Such plans describe flood warning, defence, development (e.g. minimum floor levels). evacuation, clean-up and recovery arrangements. A local flood emergency plan forms an essential component of a Redevelopment floodplain management plan. Rebuilding an area under the current or a similar land use Floodplain zoning – as urban areas age, it may become necessary to demolish and reconstruct buildings. Area of land adjacent to a creek, river, estuary, lake, dam or artificial channel, which is subject to inundation by the probable maximum flood.

44 Flood Management and Drainage Strategy Flood prone land Risk Land subject to inundation by the probable maximum Risk is defined (Standards Australia/Standards New flood. Floodplain management plans should encompass Zealand, 2004) as the chance of something happening all flood prone land, rather than being restricted to land that will have an impact on objectives. It is derived subject to defined flood events. from a combination of consequences and likelihood (or probability). Flood storage areas Those parts of the floodplain that are important for the Risk analysis temporary storage of floodwaters during the passage of a A systematic use of available information to determine flood. The extent and behaviour of flood storage areas may how often specified events may occur and the magnitude change with flood severity. of their likely consequences, in order to establish the level of risk. This is normally undertaken as part of a flood study. Hydraulic analysis The study of the flow of water in waterways. In particular, Risk assessment the evaluation of flow parameters such as water level, The process used to determine risk management extent and velocity. priorities by evaluating and comparing the level of risk against predetermined standards, target risk levels or Hydrologic analysis other criteria. The study of water and its constituents as they move through the natural processes that constitute the Risk identification hydrological cycle i.e. rainfall, runoff, evaporation, and The process of determining what can happen, why and how. infiltration. Risk management Overland flooding The systematic application of management policies, Inundation by local runoff caused by heavier than usual procedures and practices to the tasks of identifying, rainfall. Overland flooding can be caused by local runoff analysing, assessing, treating and monitoring risk. exceeding the capacity of an urban stormwater drainage system or by the backwater effects of mainstream flooding Risk treatment causing urban stormwater drainage systems to overflow. Selection and implementation of appropriate options for Probable Maximum Flood (PMF) dealing with risk.

The largest flood that could conceivably occur at a Runoff particular location. The PMF defines the extent of flood The amount of rainfall that enters the stormwater drainage prone land. It is difficult to define a meaningful annual system, (underground pipes, overland flow paths, floodway, exceedance probability for the PMF event. It is commonly and waterways) after losses such as infiltration have been assumed to be in the order of 1 in 10,000 to 1 in 1,000,000. taken into account. Residual flood risk The remaining level of flood risk that a community is exposed to after floodplain management measures to reduce risk have been implemented i.e. “untreated” flood risk. Residual risk varies with flood severity and may be substantial for flood events that are larger than the defined flood event adopted for planning purposes or for the design of structural works.

45 46 Flood Management and Drainage Strategy © Copyright November 2007 Melbourne Water Corporation. All rights reserved. No part of this document may be reproduced, stored in a retrieval system, photocopied or otherwise dealt with without the prior written permission of Melbourne Water Corporation.

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Cover photo by Clyve Herbert. 48 Flood Management and Drainage Strategy