Down in the flood

This is the third essay on the Maribyrnong River flood of October 2022 that I’ve posted on Open Forum.  This staged writing has been in response to how the reaction to the flood evolved and where we have ended up in terms of public reflection on the flood.

Firstly, there was this extreme event and the associated chaos and heartbreak that came with it.  Approaching 600 homes and businesses were flood affected.  What followed was a degree of recrimination regarding the efficacy of the crisis management, the impact of one high profile levee and some suggestions of what might have reduced the flood if such works had been built.  I decided to write about what I considered were the main factors causing this flood (incentivised by my son who lives close to the river and who kept asking me questions!).

Over the months that followed the flood a fairly constant stream of newspaper reports have kept us informed of the impact of the flood on individuals, the clean up and the intentions to review what happened.

Now, two reviews of the flood are in train.  The first is by Melbourne Water dealing with their responsibilities for flood forecasting and the impact of one levee that protects Flemington Racecourse (the home of the Melbourne Cup) from inundation.  A second more broadly based review will be carried out as a Parliamentary Upper House function.  This review is under the direction of the political opposition (the Greens and the Liberals).

In my second essay I criticised the Melbourne Water review as being too narrowly defined.  The now proposed Parliamentary review is a broader review but is also focused primarily on October 2022.  Neither review will lead us to a better understanding of the changing storm and runoff characteristics that are altering flooding characteristics in the Maribyrnong River.

The aspect that will be missed by these reviews is that flooding on the Maribyrnong River is changing in character because of climate change and catchment urbanisation.  I concluded in my first essay that these two factors, combined, were resulting in bigger, higher, quicker and more frequent flooding.  It is critical that we understand by how much the character of flooding is changing.

Climate change is actually resulting in a drier climate over South-Eastern Australia but counter-intuitively it is also producing more intense rainfall that is resulting greater storm event runoff into rivers and streams.  So, whilst low and moderate river flow is decreasing, high river flow is increasing.  For urbanisation, the impact is a substantial increase in impervious surfaces (roofs, driveways, etc).  This means that when there is rainfall it is not “captured” in the soil but rather immediately runs into a drain and quickly to a river.

In the Maribyrnong River catchment there are two aspects of this urbanisation impact.  Firstly, there is continuing expansion of the urban fringe into rural areas and secondly, there is redevelopment of old single dwelling houses in the inner and middle ring suburbs into multi-townhouse or apartment housing.

For both the urban fringe and redevelopment urbanisation the conversion of rainfall into runoff is moving from less than 50% up to as much as 95%.  This is why the floods are becoming larger, higher, quicker and more frequent. 

Why then is this change in flood characteristics not included explicitly in the reviews that are being undertaken? Why is this change given minimal commentary in the media?  There has been a small reference to climate change impact but none of urbanisation?  My explanation for this silence is that these impacts are considered immutable so why bother about them.

There is no particular individual or organisation responsible so there is no one to blame! However, this is just my conjecture.  Frankly it baffles me.  Regardless of the motive for this omission the important point is that we need to understand and quantify how flooding is changing and why it is changing.

Why is it important to have this understanding given these changes are so immutable?  The first part of the answer to this is that we owe it to those affected by flooding to provide an understanding of what is happening.  This is simply our collective social obligation.  Secondly, and more fundamentally, is that we need to understand how much impact the changes are having in order to formulate viable and effective long term flood mitigation measures.

Investigation

Hence, in my view, a major flood investigation is required.  This may well be an outcome of the Parliamentary Review and I very much hope it is.

This investigation must document what happened and explain the overall character of the October 2020 flood set against the character of all other major historic floods.

The investigation must include the use of hydrologic/hydraulic mathematical modelling of sufficient high quality and capacity.

The investigation should quantify as best as possible the increasing contribution of urbanisation to the flood size, height, speed and frequency.  It must be recognised that this impact is overlain on what is essentially a natural event (albeit altered by climate change).  Hence impact is blended into the range of natural weather-climate variability.  Making this assessment is not beyond the whit of hydrologists armed with the best scientific and modelling tools available.  Essentially this is a question of uncertainty and modern statistical tools enable such uncertainty analysis.

An example of the complexity challenges is that because of the very wet lead up to the October 2020 the soils in the catchment were fully saturated and runoff response was immediate.  This means the impact of urbanisation in this case was less than it otherwise would have been.

However as urbanisation increases, the significance of wet antecedent conditions in the making of a large flood is also less.  Said differently this means a smaller storm/antecedent rainfall event would produce a similar size flood to that of October 2022.  That is, the October 2022 flood becomes a less rare event with the ever increasing urbanisation.

It must also be remembered that October 2022 is just one flood and it is important that we understand its significance and rarity in terms of both rainfall and flood flow.  For any given storm, climate change is creating more intense rainfall and urbanisation is resulting in more runoff. It is a cocktail made in hell for anyone living near the river.

An element of scenario testing should include assessment of variation in storm epicentre locations.  One of the critical yet very unpredictable variables of any flood is the storm epicentre location.  During October 2022, the epicentre was in the upper still largely rural catchment.  A flood investigation should include assessment of storm epicentre variation.

One point that seems to be overlooked in the post flood commentary is that whilst the October 2020 flood was extreme in every way – antecedent rainfall, storm size, storm epicentre location (over the largest part of the catchment), it is possible a larger flood is not too far away.  The 1974 flood ranks with October 2022 but no one should comfort themselves with the idea that it will take another 48 years before we see a flood as big or bigger.

Mitigation

I will make a brief statement about mitigation options.

Mitigation options should be assessed as part of the investigation.  In doing so the extent of remnant risks must be considered.  Some would suggest a large flood retention storage is the go and others some substantial levees to protect everyone rather than just our favourite racetrack.  My view is one of strong reservation regarding these “hard” mitigation solutions.

This is because the more “protection” that is provided for flood prone properties the greater risk that that protection will fail and bring with it much greater damage than would be the case if the flood plain was fully in play during a flood.  Further to this, levees and retention basins encourage further development of the “protected” flood plain.  This only increases the size of the catastrophe when it occurs.

My preference would be for those already at high risk of significant flood damage to be offered adequate compensation to move off the flood plain.  I argue this because it isn’t really a case of caveat emptor for many of those who were flooded in October 2022.  Decades ago, there were no flood development limits and in any case both climate change and urbanisation have increased the flood risks beyond what anyone would have imagined.

This may be a question of degree or extent but it is an important option in the consideration of how to reduce future flood damage.  This is to say nothing of removal of deep personal trauma that comes with being affected by flooding.  No dollar value can be assigned to this trauma but the cost is beyond what value an economist might attempt to place on it.  It is much better to avoid it completely.

Equity

Finally there is a question of equity.

We have a situation where flooding impact is increasing due to climate change and urbanisation.  In both cases, the broader community is deriving benefit from our irresponsible and reckless consumption of fossil fuels and through the provision of millions of homes.

My equity question is; should not this broader community pay for those flood affected to be able to move to higher ground to a safer living environment? As you can see, there is more debate to be had about the consequences of the Maribyrnong River October 2022 flood.