The flood risk at Kensington Banks

In October 2022 ‘a wall of water’ swept down the Maribyrnong River with destructive effect.  Some 500 homes suffered flood damage but the 900 homes in Kensington Banks were not affected.  The residents might have thought they had dodged a bullet. It now appears that the bullet is on its way and will arrive some time later.

I begin this story with some background on Kensington Banks.  When I was a child I would come with my father to what was then Newmarket Saleyard, Victoria’s premier location for the buying and selling of sheep and cattle.  It was an exciting place, full of weather beaten men clad in driza-bone coats.  There was lots of bustle and shouting as the auctioneers moved from pen to pen.

Perhaps the highlight for me was lunchtime when we would adjourn to the Newmarket Hotel.  I would get a lemon squash and would sit listening to all those hard working, hard drinking stockmen tell their stories of good and bad sheep and good and bad prices.

Sadly those days are gone.  Kensington is only two train stations from the City and over the decades what was a very working class suburb became gentrified.  As we know the gentry wouldn’t have taken kindly to the smell and the hustle and bustle of this major market place.  In the late 1980s, the government closed down the Newmarket saleyards and proceeded to build a brand new medium density suburb.

The Major Projects Group

A dedicated government project management group called the Major Projects Group was established and amongst many broadly socially aligned projects (including Federation Square) they built Kensington Banks.  The suburb is unusual in a sense that it was built by government to a certain style.  If it was in England we might call the modern terraces mews houses.  The homes are predominantly terraces and have a uniform set of pale colours.  It could be in regional France.

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.  I was delighted when my son and his family decided to buy a home there.  Each visit would remind me of the old days and all those kind weather beaten men.

Of the many special features of Kensington Banks is that it is built on the Maribyrnong River flood plain and is protected by flood mitigation works including levees and a flood retention storage ‘disguised’ as a wetland, a park and tennis courts.  When I first saw this, I thought, ‘very clever’.

So, you might have the picture of suburban bliss of a very unique form.  At least that is my intention.  Has this now turned into a nightmare?  During the October 2022 flood my son kept asking me questions as water kept advancing up the main walking path.  It stopped well short of their place so I reassured him that his home would be okay.  In any case his Section 32 had no mention of them being within a flood affected area.  All was well I thought but then I started to think about what might be happening.

Over the next two years the debate rose and fell over what happened and who was to blame or not to blame.  There have been two enquiries and recently the responsible authority, Melbourne Water, has released new flood limits.  These limits are called the 1% Annual Exceedance Probability flood extent.

The surprise and shock for Kensington Banks residents is that they now find that they all lie within the new flood limits and have been advised that they may experience flooding depths of up to two metres.  What has gone wrong you might ask?  That is exactly what many of them did ask at a public meeting held recently.

In a strange process, managed by Melbourne Water, the flood limits. based on reassessment of the flood risk arising from climate change and catchment urbanisation have been released without any consideration of the scope for upgrading of the flood mitigation works.  Melbourne Water has essentially ignored that the estate was built with flood mitigation integral to it.

Further investigations of flood mitigation works all along the river have been announced.  However notwithstanding any possible upgrading of the existing Kensington Banks flood mitigation, homeowners are being left with skyrocketing flood insurance premiums, falling market values and wondering how they might deal with up to two metres of water in their homes.  In technical terms this might be called an ‘arse about’ process.

Melbourne Water’s Response

What are Melbourne Water and the Victorian Government thinking?  To get an idea I listened to an ABC interview with the CEO of Melbourne Water:

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 ho 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.

The Reporter is silent and looks askance.

This may have been a ‘gotcha’ moment but I am inclined to think it was an attempt to control the parameters of the conversation.  Melbourne Water has been at pains supply predicted flood levels for each home and to inform owners how they might protect themselves from the damages arising from this.  The web site points to a detailed guideline on how to do this.  This looks to me like an attempt to shift responsibility onto homeowners.

The ABC reporter also asked the CEO why the 1% flood extent has changed so significantly.  She replied:

“The technology at the time that Kensington Banks was developed was the late 90s,” Ms Di Lorenzo said.  “What we’re able to do today is so much more granular.

I take it that by ‘granular’ she means precise and accurate.  Again, if I was polite I might say this was an attempt to control the conversation.  I have read the flood modelling report and acknowledge that it contains high quality analysis using a proven mathematical modelling suite combining RORB, a rainfall-runoff model for the catchment, and TUFLOW, a two dimensional hydraulic model for the river downstream of Keilor Gauge.

However, there is a certain reality about flood investigation work that I will share with you – This is that 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 often considerable uncertainty with predictions particularly of extreme flooding.  The Maribyrnong flood investigation is no exception to this.

Future Assumptions

There is a second point of uncertainty about flood modelling and that relates to assumptions made about the future.  For example there is considerable uncertainty about the future impact of climate change.  The following is what the ‘bible’ for Australian hydrological practice – Australian Rainfall and Runoff(ARR) says about predicting the impact of climate change:

“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. 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” (author bolding)

For this investigation the following is what the consultants did:

“To assess the impact of climate change, the flood modelling included a year 2100 scenario. In this scenario, rainfall intensity was increased by 18.4% based on a high emissions scenario, and sea levels were increased by around 0.8 meters, both as per current Melbourne Water and Australian Rainfall and Runoff guidance, to reflect climate change predictions. It should be noted that revised Australian Rainfall and Runoff climate change guidance is due to be published later in 2024 and when this is released, revised climate change considerations may need to be incorporated into the Project.”

My point here is that, whilst the consultants have followed sound professional practice, the estimates of future flood risk are merely that – estimates with considerable uncertainty.  The community need to understand more about this uncertainty and not be given a message of high power modelling and resulting precision.

Another issue that I have been banging on about since 2022 is the matter of urban expansion into the upper rural catchment.  Until about 2020, the upper rural catchment, which makes up about 90% of the whole catchment and has the highest rainfall, was predominantly rural.  Hence, under ‘normal’ conditions (by the way the Oct 2022 flood event was not normal) the upper catchment would retain a high percentage of the rainfall until at least the peak flood had passed.  Effectively it acts like a giant sponge.

However, since 2000, the urban areas of Sunbury, Gisborne, Macedon, Wallan and even Kilmore have grown considerably and the proportion of urban catchment has steadily increased.  This means a much higher proportion of the rainfall becomes ‘runoff’ that directly contributes to the flood peak.  The consultants have accounted for the existing urban impact but it appears (from my reading at least) that there is no allowance for future urbanisation.

This is an issue and adds to the uncertainty around the investigation outcomes.  We know that the Victorian Government is desperate for more homes to be built and the flat basalt high plains of the upper Maribyrnong River catchment are the perfect place for them.  Who would resist plonking millions of homes up there in order to met the demand of our ever-growing population?

So far, much of the debate about the past flooding and future risk of flooding along the Maribyrnong River has been a pursuit of ‘guilty parties’.  Within the community this has fed distrust of officialdom.  As you can see I have even indulged a little in this. This is unfortunate and distracting to what would be a more constructive endeavour; this a better understanding of the growing flood risk and the finding of solutions to this flood risk.

Flood Mitigation

In this respect, I am comforted that Melbourne Water is now turning its attention to flood mitigation options.  I understand that investigations of possible improved flood mitigation are about to be commissioned. Whilst I consider the release of the new 1% AEP maps to be premature and irresponsible, presumably if feasible flood mitigation can be identified and enacted, these maps can be revised.

It is now appropriate that Melbourne Water, through the process of investigating flood mitigation options along the Maribyrnong River engage fully and openly with the affected communities that will be impacted.  It is clear that each area has different issues and hence there will be different solutions.  With respect to Kensington Banks there are a few things to highlight.

Firstly, it is important to remember that the Government of Victoria constructed Kensington Banks and that it constructed it with flood mitigation works.  No one would have chosen to live there without those works.  Kensington Banks was and is a showcase of how medium density communities can be created.  It is a model for how Melbourne might be reshaped.

Hence it isn’t reasonable or sensible to simply hand a booklet on how to flood proof your home to the nine hundred families that live in Kensington Banks.  It seems to me that the Victorian Government has an obligation to upgrade the existing flood mitigation works to meet the increased flooding risk arising from climate change and catchment urbanisation.

Secondly, it is worth considering that the reasons why the flood risk has occurred is not the doing of those who live in Kensington Banks or any of the other flood affected communities.  It is due to climate change, a consequence of world wide excessive consumption and of catchment urbanisation, a consequence of the growth of Melbourne’s population.  Should these existing riverside communities alone bear the cost of these impacts?

Thirdly, it appeared that the CEO of Melbourne Water, in her interview with the ABC reporter was arguing caveat emptor (let the buyer beware).  Doesn’t she realise how much nonsense that is for this situation?  I very much hope so.

I wonder what those hard working and drinking stockman having their pub lunches back in the 1950s and 1960s would have thought of all this?  It is hard to say but I wouldn’t want to have had a disagreement with them.  This quality might be something they share with the present residents of Kensington Banks if there isn’t reasonable action on improving the flood mitigation that serves the area.