Maribyrnong River blues

This is my fourth essay devoted to the Maribyrnong River floods.  I wonder if anyone is taking any notice of the social and economic dangers that exist.  It seems like “the boy who cried wolf” in reverse.

The floods along the Maribyrnong are changing for the worse.  They are getting higher, bigger, quicker and more frequent.   The causes of this are urban development in the catchment and climate change.  In October 2022, a major flood brought extensive damage to some 600 houses and recent modelling indicates at least 1,000 more homes and businesses are at grave risk.

I have written more fully about this increasing risk but here is a quick potted version:

Climate change is producing more intense storm rainfall and therefore greater runoff into rivers and streams.  This means floods, particularly the larger ones, are increasing in size and frequency.

Urbanisation and urban redevelopment (re-urbanisation) is increasing the amount of rainfall that becomes runoff into rivers and streams.  Urban development means increased coverage by impervious surfaces (roofs, driveways, etc.).  This results in more of the rainfall reaching rivers and streams more rapidly via urban drainage.  In rural and older suburban areas, much of the rainfall is “intercepted” by vegetation or is absorbed into the soil and does not contribute to the size of the flood but flows into the river later or not at all.

Whilst the lower catchment of the Maribyrnong River has been urbanised for more than one hundred years, the proportion to the total catchment that this old urban area takes up is quite small.  Until the 1970s, the upper catchment was predominantly rural.  Subsequently urban growth has encroached into this area.

Also, this modern suburban housing and the higher density redevelopment in existing urban areas have far greater impervious areas.  Overall, these increases in impervious areas mean that the percentage of rainfall becoming inflow to the Maribyrnong River has increased for these areas from less than 40% to more than 90%.  It also means that the inflows from the urban areas reach the Maribyrnong more quickly via the pipe and channel urban drainage networks.

This urban expansion and redevelopment has accelerated over the last two plus decades. The new urban areas are around Melbourne Airport, Sunbury, Taylors Lakes, Gisborne, Macedon, Romsey and as far north as Kilmore.

So far, the debate about the October 2022 flood has centred on one levee – the Flemington Racecourse levee.  River hydraulics suggested to me that this would not be a significant impact and subsequent detailed hydraulic modelling has confirmed this to be the case; this levee had only minor impact on the flood height.  Unfortunately, given human nature, there remains an inclination to attribute more blame to this high profile levee than it deserves.  This is unfortunate because this attention is misplaced and is a distraction from considering the major factors contributing to the change in flood regime.

The real issues are that because of urban development a greater proportion of the storm rainfall over the catchment is entering the river more quickly than ever before and because of climate change, storms are becoming larger and more intense.  Essentially through human interference the nature of flooding has changed for the worse; it is worth repeating – the floods are becoming larger, higher, quicker and more frequent than ever before.

Melbourne Water

On May 19, 2024, the Melbourne Age newspaper reported that Melbourne Water (the responsible flood and drainage authority) had reviewed the flood extent maps for the Maribyrnong River.  These revised maps show a major extension of the 1 in 100 year flood event for the lower reaches of the river.  The 1 in 100 year return period is a “standardised” nomenclature for a flood risk of one per cent in any given year.  It is the usual reference point for consideration of significant flood risk.

This extension caught the attention of the media because it now envelops a high profile medium density estate constructed by the Victorian Government in the 1990s.  This estate called Kensington Banks Estate contains around 1,000 townhouses and apartments.  This is a major change to Melbourne Water predictions on flood level and could result in a major change in political perspective.  I will get to that later in this piece.

The Age article reports Melbourne Water on their flood modelling revision in the following terms:

A Melbourne Water spokesman said the new flood maps were influenced by factors including climate change, urban development and new technology that used millions of data points to ensure a result that was of much higher quality than before.

The maps will be used to update flood overlays in municipal planning schemes, inform emergency management and investigate new potential works to prevent flooding.

“These new models give us the information we need to best prepare our communities for this risk,” the spokesman said.

The Age report also states that Melbourne Water has now considered both the impact of climate change and urban development in the process of revising the estimates of flood extent.

As the boy who has been crying wolf for two years now, I have several questions I would like answered by the responsible authorities – Melbourne Water and the Victorian Government.  These questions are in two parts.  The first set of questions relate to equity and fairness:

1. What is meant by, ‘we need to best prepare our communities for this risk’? This implies that each home or business owner within the flood extent area is going to have to own these new flood risks arising from causes beyond their control.
2. Are not the principle causes of increased flooding risk the result of “interference” with the natural flow regime for the Maribyrnong River? In a legal sense such interference might attract claims for damages by affected parties.
3. How should the question of social equity be handled? Urban development in the catchment is causing the flooding of homes and businesses on the lower Maribyrnong flood plain.  Is it fair that this urban development for the many should so badly impact on the few?
4. Should caveat emptor or “let the buyer beware” apply to those located in the recently extended flood extent areas? Kensington Banks Estate owners would have bought homes on the understanding they were outside the 1 in 100 year flood extent. Might they have grounds for compensation for a loss of property value?
5. Does Victorian Government have a policy position on “buy backs” and/or appropriate flood mitigation for the homes and businesses affected by the flooding caused by urban development in the catchment and climate change? Questions 1 to 4 are relevant to what Government might determine in this respect.

The second part of this inquiry relates to the Melbourne Water modelling:

1. What has been assumed in terms of urban development? There several aspects to this – the extent of urban expansion in the upper rural part of the catchment, the extent of re-development of the old “quarter-acre” blocks into medium and high density town houses and apartments, the rainfall-runoff assumptions applied, the “times of concentration” in key sub-catchments and the assumptions around storm location and intensities.  These assumptions are critical in the determination of flood characteristic prediction and need to be carefully scrutinised and justified.
2. How has the dynamic nature of climate change been accommodated in the determination of 1 in 100 year flood frequency under 2024 and 2100 conditions? The traditional methodology for flood frequency determination implies a “static” climate regime – that is past climate is a good guide to future climate.  With climate change we know that the climate is continuing to “heat up”.  It will continue to do so, not only to the point when the world  achieves zero carbon emissions but beyond for centuries until such time as the accumulated carbon is dissipated.

Based on the assumptions made, we need to understand how certain the Melbourne Water estimates of flood extent are.  It is possible that these estimates are an underestimate of the size of the 1-in-100 year flood and its associated extent of inundation.

It is time for this boy to stop crying and hope for a fully transparent reaction from those in the know and those who are responsible for decision making for this very significant problem.  With the October 2022 flood there were 600 flood affected homes and businesses but for a future 1-in-100 year flood the number will grow to around 2,000.

It probably doesn’t need to be mentioned but the political scale of the flooding along the Maribyrnong River is ratcheting up.  It would be a mistake for the Government to not take a proactive and transparent stance on addressing the increasing flood problems along the lower Maribyrnong River.