Will big storms threaten Melbourne’s “Big Build”?

Introduction

Melbourne’s rapid growth over recent decades is presenting many challenges for the authorities that deliver transport, electricity, gas, water supply, sewerage and drainage services to citizens across this huge and evolving city.  All of these services are in need of extension, augmentation and renewal and for each the story is different.

The provision of adequate stormwater drainage infrastructure may be missing the boat in terms of the huge investment that is labelled Melbourne’s Big Build.  Stormwater drainage doesn’t have the profile of the other services.  The stormwater drainage networks are largely out of sight and out of mind.  Also, for days, weeks, months and even years, the storm drains may not carry any or very little water.  Then, all of a sudden, these drains are full to overflowing and only then does anyone care if they have adequate capacity or not.

The massive growth of the city means increasing paved surfaces and in turn this means more runoff flowing directly into the stormwater drains and down the various rivers and streams that run through the city.  Added to this is the impact of climate change, which is resulting in more intense storms and hence even more runoff.

In 2019, following a major review of the principle national design manual [1] for stormwater drainage systems, the responsible authority, Melbourne Water initiated a reassessment of system capacities.  The October 2022 Maribyrnong River flood drove more specific investigation of that drainage system.  The net effect of these investigations has been a significant enlargement of the flood risk extent [2] for substantial parts of Melbourne.  However, there has been no associated metropolitan wide commitment to augment stormwater drainage systems to reduce the increasing risk of flooding.

The new risks are certain but not the extent of their impact.  Unfortunately Melbourne Water has a proclivity to represent the findings of their investigations with a precision that isn’t there.  They argue incorrectly that the ‘best practice’ modelling tools and investigation processes are yielding this precision. One might question the motives for taking this position and ignoring the inherent uncertainty is to deny optimum solutions to meeting the threats.

In this essay I explore in detail why stormwater drainage – this Cinderella without glass slippers – has no place in Melbourne’s Big Build program.

The relevant authorities and the principle stormwater issues

Melbourne Water, as the principal responsible authority for stormwater drainage in the Melbourne metropolitan area, manages all the natural watercourses and some 1,400 kilometres of trunk (major) stormwater drains.  It also has oversight of local government responsibilities for 25,000 kilometres of local smaller street drainage networks.

This is how Melbourne Water sees the emerging threats [3]:

Rain, storms and floods are a natural part of life in Melbourne but can have a big impact on our way of life. The impacts of climate change bring more unpredictable weather and as our city grows more buildings and streets are built. As a result, we’re facing more challenges than ever in ensuring that rainfall that falls on the ground and flows through the landscape is safely carried away from built-up areas into rivers and creeks. [4]

A little context – Melbourne’s growth pangs

In 1950, Melbourne was sedate little city of around 1.3 million living mainly in crowded Victorian terraces, Edwardian weatherboards and Californian bungalows and within walking or tram distance from their place of work.  The span from edge to edge of the city was generally little more than 10 kilometres. However change was on the way.  This change came in three distinct phases.

The first phase was the creation of the suburban city that has been such a key part of our culture for most of the seven decades since World War II.  It might be called the ‘backyard’ culture.  As the baby boomers and immigrants arrived in the hundreds of thousands, the city expanded rapidly to effectively double in size and more than double in population by the mid-1970s when the long boom [5] came to an end.

Over the next two plus decades, growth slowed but the city remained much as it was – a single large centre of business and employment – the Central Business District (CBD), flanked on almost all sides by a vast suburbia of detached homes.

After 2000, Melbourne, chameleon like, switched from the premier manufacturing city of Australia to the premier medical and education centre of the country.  With this transition came a new more densely populated urban form.  Immigration surged again and in consort so did the suburban fringe but by now the backyards had disappeared.  In the established suburbs, the old bungalow homes were being torn down and replaced by multiple townhouses and apartments.

Close to the CBD, high-rise apartments sprang up overnight and in the vast suburbia several mini-CBDs were evolving. The transformation has been rapid and caught the politicians and authorities by surprise.  Essential infrastructure, particularly transport infrastructure was not being built.  Melbourne began to choke as the population approached 5 million and the suburbs extended for more than 50 kilometres in some directions.

In 2014, the Andrews Labor Government was elected with a mandate to upgrade transport infrastructure to meet the changing needs of Melbourne.  This Government considers an upgrading of transport infrastructure to be the key mechanism for transforming Melbourne into a ‘city of centres’ [6] projected to double in population in just a few more decades. This is the ‘Big Build’ program.

Source: Plan Melbourne 2017 t0 2050. Note: dark and light purple to 1927, dark and light brown to 1971 and dark and light blue to 2015.

As the city creeps ever outward and the redevelopment of the old suburbs moves into overdrive, the natural watercourses and the stormwater drains are taking more and more storm runoff. Melbourne Water and the municipalities have responded by re-calculating the flood risk extents but without a commitment to upgrading the storm drainage networks.

Two cases of authority response to changing flood risk

To understand the impact of the ‘re-drawing’ of flood risk mapping I present two recent examples – Kensington Downs and the City of Yarra.

1. Kensington Banks

Kensington Banks is a nine hundred home estate constructed by the Victorian government [7] during the 1990s.  It is a medium density development of terrace (mews) houses and apartments built on the site of the old Newmarket saleyards.  Only two kilometres from the CBD, Kensington Banks is threaded with shady laneways lined with ancient peppercorns, parks and playgrounds with borders marked by ‘sleeper’ railings rescued from the old saleyards.

The entire estate is constructed on the flood plain of the Maribyrnong River but is protected by a dedicated flood mitigation system consisting of levees, a retention basin for local runoff and a protective drainage outlet to the river.  During the October 2022, Maribyrnong River flood the flood mitigation system prevented significant flooding within the estate.

A subsequent flood investigation [8] has shown the existing flood mitigation works are not sufficient for protecting the estate in the event of a 1% AEP flood [9].  Subsequently Melbourne Water has declared a new 1% AEP flood extent (in a planning terms – Land Liable to Flooding (LLF)) that now extends across the entire estate. This means that these owners and occupiers of nine hundred high value homes are, at the stroke of a pen, subject to significantly higher flood insurance premiums and falls in the value of their properties.

Kensington Banks – extent of the 1% AEP flood (Land Liable to Flooding)

Understandably, the community reaction to this change has been one of dismay and a quiet anger that is resounding into the political arena. The Victorian Government has responded to this anger by announcing that there will be an investigation of future requirements to meet the increased flood risk.  However, so far, there has been no commitment to an upgrading of the flood mitigation works.

It is clear from the actions so far that the declaration of an extended flood risk zone was the priority rather than an upgrading of flood mitigation infrastructure.  In a meeting with the local Member of Parliament, Melbourne Water indicated that any upgrading works would be in the order of five to eight years away and there was no commitment to undertaking such works.  In the meantime Melbourne Water are seeking to engage with individual homeowners on how they can protect their homes in the event of such a flood.

2. The City of Yarra

The City of Yarra, in association with Melbourne Water recently undertook a flood investigation [10].  The investigation culminated in a significant enlargement of the LLF zone for the whole municipality.   Around one third of all properties in the City of Yarra are now within the LLF.  The estimated increase in storm runoff means the trunk stormwater drains across much of the City of Yarra are now not fit for purpose.

The LLF (1% AEP flood) extent – the light blue extent is the result of trunk main (dark blue line) capacity limitations and the hatched brown is local street generated inflows that are ‘backed up’.

The City of Yarra is located on a flat volcanic (basalt) plain where natural drainage paths would have consisted of a series of shallow marshes.  Storm water drainage systems first constructed as early as the late 19^th Century effectively drained these lines of marshes and made the area suitable for urban development.  These drainage trunk mains have served the City of Yarra well over a century or so but are now inadequate for meeting the increased flows.

The community reaction to the revised LLF extents is one of dismay.  One councillor responded [11]:

Nothing in the paperwork tells us how bad the stormwater drains are, can any be repaired, what would the cost be to fix this. It was inappropriate for the problem to be thrust immediately onto residents through no fault of their own.  Insurance premiums will go through the roof, property values will drop and landlords will pass on costs to renters.

Even more succinctly a City of Yarra resident expressed the concern as:

Because they can’t be bothered or can’t afford to fix stormwater drains, suddenly it becomes our responsibility.

Behind the declarations of increased flood risk – ideology and investment priority

The communities are angered by the redrawing of the flood risk mapping because they assume the provision of adequate drainage is a core responsibility of the State in existing urban areas. In fact, this is part of the charter of Melbourne Water and the municipalities.

Why then the lack of commitment to augment the existing stormwater drainage systems? I proffer two reasons – ideology and investment priority.

The ideological motivator – shifting responsibility

There is a well-defined trend in contemporary society to shift responsibility for a range of risks from representative authority to the individual. The State is not the deliverer of social protection and certainty it was in the mid-twentieth century.

In a book titled Risking Together [12], the authors express this trend as follows:

Something is changing in wealthy societies such as Australia, to create a sense that economic security and its affordability are becoming beyond reach.  Life is now full of financial costs, risks and uncertainties.

Whether the issue is paying the rent or mortgage, ballooning health care and utility bills, childcare, after-school fees and university fees, or managing superannuation and buying into a retirement village, we are increasingly exposed to risks and costs that are getting more complex and harder to manage.  To access life’s necessities, we are required to sign financial contracts, which embed exposure to risks, but at the same time our sources of income to pay for those costs are also getting more uncertain and volatile.  We are told we just have to manage those risks and it’s the way life now is [13].

In Risking Together, the authors refer to this as risk shifting, which amounts to moving risk from both public and private collectives onto the individual.

This intent is evident in the views of the CEO of Melbourne Water expressed in an interview [14] [15], with an ABC journalist after the new flood risk extent mapping was announced for the areas near the Maribyrnong River:

Reporter – Would you buy a place in one of these areas (i.e. Kensington Banks)?

CEO of MW – I would be checking my property certificates.  I would be checking my Section 32.  I would be looking at the risk and how I would manage that.

Reporter – Do you accept that people didn’t have the right information to refer to because you have only just released the 2024 modelling?

CEO of MW – Well we released it as soon as we had it.  So we released it well before it went into planning schemes.

Reporter – is silent and looks askance.

This is a ‘let the buyer beware’ message and is of no help to those who bought their home based on the previous flood risk extent mapping, which excluded the entire Kensington Banks precinct.  These buyers did do their due diligence in buying their homes.  For them the redrawing of the flood risk extents is a case of risk shifting.

The immediate responses by both Melbourne Water and the responsible government ministers have focussed on flood awareness and individual action during a flood. Whilst these responses acknowledge ‘the frustration, anxiety and anger of many residents’, there is no commitment to upgrade the existing flood mitigation works. The letters are aimed at disarming the dismay rather than dealing with it.

The cost priorities

The Big Build major road and rail projects dominate the Victorian infrastructure expenditure of hundreds of billions of dollars.  This leaves little room for improved stormwater drainage works.  Whatever is left over from the Big Build is going into new hospitals and housing.  After the Metro tunnel, the level crossing removals, the West Gate tunnel, the North-East Link, the Suburban Rail Loop and Melbourne Airport Rail how could anyone seriously consider reconstruction or upgrading of such unspectacular infrastructure as stormwater drainage networks?  Cinderella without her glass shoes and gilded carriage is a no body.

Uncertainty is reality

Melbourne Water has continually stressed the accuracy of the flood extent estimates [16].  This is a kind of mythology that authorities are inclined to present to the general public – precision rules over uncertainty.  It is understandable that Melbourne Water is proud of the quality of the Flood Study.  I agree that it is of a high quality but ‘best practice’ should not be confused with a high level of ‘accuracy’.  There is a certain reality about flood investigation work that I will share with you – hydrology and hydraulics are precise sciences that must use imprecise and often inconsistent (yes you can say very messy) data.  However much the tools have improved and how rigorous are the processes there is considerable uncertainty in the prediction of extreme flood risk.

Australian Rainfall and Runoff (ARR) [17] recommends an uncertainty analysis to determine the probability distribution that matches the most likely flood magnitude estimate.  ARR lists the various sources of uncertainty and provides guidelines for undertaking an uncertainty analysis.  These guidelines cover data analysis, modelling tools, calibration assumptions and all key hydrological and hydraulic assumptions.  The independent and peer reviews should have asked for an uncertainty analysis.

There are two aspects of uncertainty of particular note given they represent most of the increasing risk of flooding – catchment urbanisation and climate change.

On climate change Australian Rainfall and Runoff (ARR) [18] is quite explicit:

Quantifying the effects of climate change on the factors that affect flood estimation is a difficult task, and any estimates of impacts of future climate on the inputs to flood assessments will include large uncertainties [19]. The fact that the occurrence of flood events, and their associated causal factors, is rare limits the data available to assess changes in their frequency or intensity.

On urbanisation there is little certainty about its ultimate extent.

Essentially ARR is cautioning users of the manual against such accuracy claims.

The authorities highlight the importance of transparency.  A key element of transparency needs to be an uncertainty analysis.

The power of water – a reminder

On 17 February 1972, a ‘freak’ storm wreaked havoc along Elisabeth Street in the CBD. The Melbourne Age [20] reported this storm as follows:

Three inches of rain deluged the city area between 4 p.m. and 5 p.m. — just as the peak hour rush was starting. The storm caused one of the worst traffic jams ever.

By 4.45 p.m. waves were spraying over the tops of parking meters in Elizabeth Street. They caused hundreds of thousands of dollars’ damage to shops and stock.

The waves were driven by 34 mph winds. In parts of Elizabeth Street the water was four feet deep.

White-capped waves lifted cars bodily, turned them about, and swept them away.

 

This photograph highlights two facts about intense rainfall.  Firstly, the obvious one, the destructive power of water and secondly there are times when, regardless of the stormwater drainage capacity, nature can overcome what is provided.

The storm drain beneath Elizabeth Street has long replaced the stream of pre-European settlement.  Its catchment is not big – it covers much of the CBD and extends only up into North Melbourne and South Carlton but is big enough provide the raging torrent that is depicted above.

The Elisabeth Street drainage catchment

What then is the appropriate capacity of a drainage system?  There is no definitive answer to this question.  The answer is always going to be a trade off between the cost of the infrastructure and the frequency and damage that occurs when the drainage system is exceeded.  Whilst Melbourne Water and other responsible authorities have developed design ‘standards’ these should be reviewed for the changing circumstances.  Importantly such review needs to involved engagement and input with the potentially impacted communities.  This is particularly so now as the threats due to climate change and urbanisation continue to grow.

Conclusion – where might this end?

Recent investigations overseen by Melbourne Water show that some of the stormwater drainage systems across the Melbourne metropolitan area are now inadequate for the job at hand – the protection of built property from major flooding.  The new threats are coming from climate change and urbanisation.

In response to these increasing threats Melbourne Water and municipalities have acted quickly to redraw the flood extent maps for the metropolitan area.  This is not the right direction – these flood extent maps are primarily for the control of new development.  This rapid redrawing of the flood extent maps in existing built up areas, where the social and economic impacts are so high, is neither reasonable nor just.

There are several issues unaddressed by this approach. Firstly, there is no transparency.  Secondly, the investigations do not include assessment of uncertainty.  This omission means the delineation between areas at risk of flooding and those that are not is far too binary.  Finally, there is a noticeable absence of commitment to augmenting stormwater drainage system capacity.

I have proffered two reasons for this absence of commitment.  Firstly there is no money left after all the Big Build commitments and secondly governments are favouring a shifting of responsibility for flood protection from the State to the individuals.  This is called risk shifting.

So, now to what should be done.

Firstly, all the recent flood risk extent changes that have been made by Melbourne Water and various municipalities should be withdrawn until uncertainty analyses is carried out in accordance with the ARR specification.

Secondly, the locations of actual significant risk of flooding should be identified.  In this respect the “bands of error” that are determined via the uncertainty analysis will provide guidance as to where the significant risks actually are.  It is self evident that there is a big difference between the predictions of water lapping at your doorstep and water that is two plus metres up your living room wall.

Thirdly, a review and prioritisation should be done of required stormwater drainage works augmentation. This needs to be done on a metropolitan area wide basis.

Fourthly, the Government should commit to funding these works that may need to be on-going until the new threats are dealt with.

Finally, as recommended by ARR, there should be regular review of the state of both climate change and urbanisation to ensure the storm drainage networks remain fit for purpose..

Let’s be frank about it – we all want Cinderella to go to the ball.  Don’t we?

References

[1] Australian Rainfall and Runoff, A Guide to Flood Estimation, Books 1 – 9, Commonwealth of Australia, 2019

[2] The flood risk extent maps show the area estimated to be inundated by a flood with a probability of 1% of being exceeded in any year – 1% annual exceedance probability (1% AEP).

[3] https://www.melbournewater.com.au/water-and-environment/flooding-advice/drainage-system

[4] My bolding

[5] Economists refer to this post war period as the Golden Age.

[6] Plan_Melbourne_2017-2050_Melbourne Metropolitan Strategy, Victorian Government

[7] The Victorian Government established the Major Projects Group to build broadly socially aligned projects including Federation Square.  Kensington Banks is one of its most significant projects.

[8] Jacobs, 2024 Maribyrnong River Flood Model Report, V9, Melbourne Water Corporation, 13 June 2024

[9] 1% AEP flood has a one per cent chance of occurring in any year.

[10] Engeny, Identification of Land Liable to Flooding, Technical Report, VC2036_006-REP-001-3, City of Yarra, 27 June 2024

[11] Sophie Aubrey, Flood risk for more than 22,000 inner Melbourne properties, new modelling shows, The Melbourne Age, July 9, 2024

[12] D Bryan and M Rafferty, Risking Together – How Finance is Dominating Everyday Life in Australia, Sydney University Press, 2018

[13] My bolding

[14] Rhiana Whitson and Lucy Kent, Experts say thousands of home owners face property value loss due to new flood modelling. In Melbourne, the process has begun, ABC News, 13 June 2024

[15] https://www.abc.net.au/news/2024-06-13/kensington-banks-melbourne-water-flood-mapping-value-loss-fears/103960736

[16] This claim is presented in the responses from ministers and various Melbourne Water statements to the affected communities.  For example – Maribyrnong River Flood Model Questions & Answers, 11 July 2024

[17] Australian Rainfall and Runoff, A Guide to Flood Estimation, Book 1- S2.8 – Commonwealth of Australia, 2019.

[18] Australian Rainfall and Runoff, A Guide to Flood Estimation, Book 1 – S4.16.1, Commonwealth of Australia, 2019

[19] my bolding

[20] The Melbourne Age, February 18, 1972