The Maribyrnong River flood review

In October 2022, an extreme flood occurred on the Maribyrnong River in suburban Melbourne.  A total of 520 homes were flooded along the lower reaches of the river.  In November 2022, I wrote an essay for Open Forum called Analysing the Maribyrnong River flood. In that essay I highlighted in general terms the impact of urbanisation including redevelopment on flooding characteristics.

A review of the flood is now underway and a terms of reference (TOR) for this review has been released.  There is a two-month period set for submissions from the public.  I have made a submission but it doesn’t comply with the TOR so I expect it will be ignored.

The TOR is for a review of how Melbourne Water (MW), which has principal responsibility for flood management, executed its duties.  MW carries out real time flood modelling and predictions.  The predictions of flood levels are then passed on to Emergency Services and the Police.  Such a review is necessary but it isn’t the critical issue for the future.

What then is the critical issue?  It is a better understanding the future flood risks.  Without this understanding mitigation measures cannot be properly assessed.  There are many families who were and could be flooded that are vitally interested in what is possible and even probable.

The following is my submission:

Submission to the Maribyrnong Flood Review

I have read the Terms of Reference and the website – yoursay.

My comments relate to:

• Any other matters that may have significantly contributed to the Flood Event, and
• Any impact of prior works or activities in the urban catchment on flood levels and extent during the Flood Event.

In particular, my comments relate to the historical urbanisation of the catchment, the effect of this on the October 2022 flood and the implications of this and future urbanisation on those affected by the flood.

As you will be aware, urbanisation including urban redevelopment increases runoff into the river for any given storm event.  This impact is fourfold – it increases flood volumes, flood peaks and the flood frequency and the speed of the rising flood levels (often called the time of concentration).

Over the past five or so decades, the Maribyrnong River catchment has experienced massive development and redevelopment.  This development and redevelopment extends over such established suburbs as Flemington, Maribyrnong, Essendon, Niddrie, Keilor, Melbourne Airport, Tullamarine and beyond to the fringe towns and suburbs of Sunbury, Gisborne, Riddells Creek, Macedon, Romsey and Lancefield.  This urbanisation is increasing the flooding in the lower reaches of the catchment.

This change in flooding character and impact is not reflected in the recent extreme flood of October 2022.  This is because of the weather patterns that produced the flood.  A very wet winter and the weeks and days leading up final flood storm meant that the catchment soils were fully saturated.  This meant that a very high proportion of the storm rainfall converted into runoff entering the river and this was regardless of whether there was urban development and the associated large percentage of impervious cover.  Furthermore, the storm event was centred on the upper catchment, which still retains a rural character.

This storm was a relatively rare event given the long lead-in time of very wet weather so some might say we can live with this because it is unlikely to reoccur for a very long time.  This is a reassuring but false assumption.  There are several reasons why I suggest greater caution.

The first reason is that climate change is resulting in more intense storm events.  At the same time, in South-Eastern Australia, overall rainfall is reducing.  So our rainfall character is changing and will continue along the path of greater intensity until we reach zero GHG emissions.

The second reason is that urbanisation means that the long lead-in wet period experienced in front of the October flood event is not as important in terms of the size of flood arising from the actual flood event storm.  Hence, increased urbanisation means that antecedent conditions become less important and therefore the frequency of extreme flooding will increase.

The third reason is that urbanisation is unstoppable and with each year more and more of the catchment is coming under impervious cover (if this isn’t quite clear, now is a good point to read my previous Open Forum essay).  Under Plan Melbourne 2017-2050, urban fringe growth will continue at least up to the existing Urban Growth Boundary and in those areas outside the Melbourne Metropolitan Area.  Sunbury and the Northern Growth Corridor and the peri-urban fringe towns of Romsey, Lancefield, Gisborne and Macedon will continue to expand.  Also, redevelopment in the form of the conversion of old suburban bungalow housing into multiple town houses and apartments is ongoing.

Finally, there is the unpredictability of the epicentre of an extreme storm event.  For October 2022, the rain was the most intense over the predominantly rural part of the catchment.  However a slight shift in the storm epicentre could bring it to a greater coverage of urban area and therefore result in a much larger and much faster rising flood.

Hence, this “cocktail” of climate change and urbanisation is an ever-growing time bomb for the homes and businesses in the Lower Maribyrnong reaches of the River.

It is likely that you consider this submission to be outside the TOR for this review.  However, this means the increasing flood risk will not be addressed.  I would be interested to know whether Melbourne Water plans to do this.

The Politics and the Community

Why has MW, with the endorsement of the Victorian State Government, chosen to do this limited review of the October 2022 flood?  As far as I am aware there has been limited criticism of what MW did in terms of predicting flood levels.  MW has a well-calibrated flood model and a highly qualified team of hydrologist-modellers and hence there is no reason to think they got it badly wrong.  I am aware of some criticism of late notification but also this issue is not included in this review.  Hence, I wonder why there is a review of something that was in all likelihood done quite well.  Is it the old story about never doing a review unless you know the outcome?

The Victorian State Government was recently re-elected for a third term.  It was a landslide victory against a weak and divided Opposition.  The Greens recently introduced a motion in Parliament calling for a more extensive review.  However this motion doesn’t acknowledge the need to consider what this flood might mean for a more urbanised catchment.

So, this doesn’t answer the question about the ostrich staying safely with its head in the sand.  This Victorian State Labor Government will go down in history as the big infrastructure builder.  I doff my hat to them on this.  However will they also do down in history as the government who ignored the plaintive cries of the Maribyrnong community for action to deal with the increasing threat of flooding?  With regard to this community, my sources tell me it is like a recent headline about the Lismore floods, “People are just worn down”.

Perhaps in the eyes of a supremely confident government this community don’t really matter.  This lack of sympathy may also permeate to the broader community.  Is it that most of us see it as a case of caveat emptor or “let the buyer beware”?  This is a false perspective that doesn’t apply for most of those affected by this flood.   It is mostly a very old community many of them post-war immigrants who brought their homes well before flood limits were established.  In any case these flood limits are obsolete because of climate change and urbanisation.  Furthermore, without corrective action the risks of flooding are growing.

There is no sign that the Victorian Government has any interest in the increasing risk of flooding from the Maribyrnong River.  The Premier is quoted as saying that we should let the independent experts do their job.  How can that be if this job has been so narrowly defined that the review has very little value?  I am tempted to accuse our Premier of being disingenuous or at least being poorly advised.  However, that would a case of turning bewilderment into accusation and that would be wrong wouldn’t it?

Still we absolutely need to understand the nature of this increasing flood risk so we can decide on a reasonable and appropriate course of mitigation.  I consider this to be what the community would most want.

[1] https://hdp-au-prod-app-mw-yoursay-files.s3.ap-southeast-2.amazonaws.com/2216/7633/9255/Terms_of_Reference_FINAL_January_2023.pdf

[2] https://www.planmelbourne.vic.gov.au/__data/assets/pdf_file/0007/377206/Plan_Melbourne_2017-2050_Strategy_.pdf