Analysing the Maribyrnong River flood

In mid-October 2022, the urbanised lower reaches of the Maribyrnong River in Inner-Western Melbourne experienced major flooding.  Since then my eldest son, who lives not far from the river and more importantly the limit of this flooding, has been texting a range of “quick questions” as he wrestles with the idea that his first home may be at risk of inundation.  Others, no doubt, are much more traumatised by what happened.

The Bureau of Meteorology (BOM) reported that October was the wettest month on record and was preceded by a very wet winter.   These are the classic conditions for major flooding in South-Eastern Australia.  I can do no better than repeat BOM’s description of the storm that generated the flooding in October 2022 along the Maribyrnong River:

“A cold front and a low pressure trough interacting with humid air over northern Australia, brought rain and thunderstorms to Victoria on the 6th and 7th. Widespread rainfall and showers resulted in daily rainfall totals between 10 and 30 mm at many sites. There were isolated higher falls, mostly associated with thunderstorms. 

Showers and rain developed across most of Victoria on the 12th in a moist northerly airflow, with the heaviest falls in central parts of the state; through these areas, widespread daily rainfall totals between 20 and 60 mm were recorded, with isolated totals exceeding 100 mm. A cold front crossed Victoria on the 13th, bringing widespread rainfall and strong winds, with the highest daily totals in the central and north-eastern parts.”

In the wake of the Maribyrnong flood, the Government has appointed Melbourne Water, the responsible drainage and flood management authority, to investigate the flood and the various causes and consequences of the flooding.

However, before this investigation is complete it is quite clear that this flood was an extreme event both in terms of the climatic forces at play and the extent of damage.

In this environment, it is the nature of our media to quickly seek a guilty party or two.  One such accused guilty party or culprit is the premier racecourse in Australia, Flemington Racecourse, which is the home of the Victoria Racing Club and the world renowned Melbourne Cup.  The Flemington Racecourse is mostly located on the Maribyrnong River floodplain.

A few years ago, a concrete levee was constructed to protect the track.  I extracted the following from one of the press offerings at the time of recent flooding:

“While homes are in ruins, residents displaced and fingers pointed regarding the recent Maribyrnong floods, one question is everywhere: did a wall built to protect Flemington Racecourse make it worse?”

Of course, this is a question but the tone is quite accusative.

The answer to this question is very likely it did cause an increase in flood level but it is also likely to have been a small increase.

During the post-flood debate, run of accusations and denials, there was to my mind, excessive attention given to this one encroachment on the Maribyrnong River flood plain.  Of course, this does not mean it was a good or a bad thing to have built a levee to protect the famous home of the Melbourne Cup.  I will side step this debate by stating I have written several essays about the perils of flood plain encroachment for urban development and would be the last to endorse the VRC action.

However in this case, I consider the guilty party lies largely elsewhere.  There are in my judgement three primary suspects.  In no particular order of significance, these suspects are climate change, normal weather and climate patterns and urban development.  Of these, it appears that the debate about the recent Maribynong flood has overlooked the last and this will be my focus in the following paragraphs.  However before that a few words on both climate change and the weather and climate patterns.

Climate change in South Eastern Australia has resulted in less rainfall overall.  However the rain that does fall is more intense.  This means that river flows are declining over the long term but large floods are becoming more frequent.  This may appear counter-intuitive but it is the case.

The normal weather and climate patterns over most of Australia but particularly South Eastern Australia are under several influences that result in giving us the most variable climate and particularly rainfall in the world.  This naturally high variation is due to Australia’s location between the Indian and Pacific Oceans and astride the Tropic of Capricorn, the lack of a strong orographic (mountain uplift) impact and a lack of warm current in adjacent oceans.

Amongst all this we have El Niño/La Niña and the Indian Ocean Dipole.  The influences on Melbourne’s climate are very diverse and the reason why we often refer to four seasons in one day. At a more significant scale it means that major droughts and major floods can follow in quick succession.  Dorothea Mackeller, in comparing the dreary yet predictable English weather to Australia’s offering, got it pretty right with:

I love a sunburnt country, a land of sweeping plains, of rugged mountain ranges, of droughts and flooding rains. 

So, let’s move on to urban development in the catchment.  Until relatively recently, the middle and upper parts of the Maribyrnong River catchment were primarily forest, woodland and grassland.  This is increasingly less so as Melbourne’s fringe growth continues to encroach on this rural land.  Additionally, the redevelopment growth within established suburbs is now significant.  This development and redevelopment has accelerated over the last decade.

This development and redevelopment results in more storm rainfall entering the river system and it does so much more rapidly than before.  The hydrologists call this a reduction in the time of concentration for any given flood. In lay terms this means the floods are more “peaky and flashy” than in the past.

Let’s take a journey with a raindrop as it falls from the sky and hits the ground.  In the past, with a largely rural catchment and even with the typical 20^th Century low-density suburban development, this raindrop would more likely fall on vegetation-covered soil than it would on an impervious surface.  Hence, the raindrop is intercepted and dissipated via evapo-transpiration. This leaves a fraction of the raindrop to take a slow journey via the earth to eventually discharge into a river or stream.  Hydrologists call this attenuation.

Modern higher density urbanisation has changed this significantly for our raindrop.  Now, over large parts of the middle and upper Maribyrnong catchment, our raindrop is more likely to hit a roof or a concrete driveway and go directly into a stormwater system and then, quite quickly, pass into the nearest stream or river.  Urbanisation has increased the speed our raindrop (and all the other raindrops) enters the Maribyrnong River.

Over the last few decades, the extent of urban area has increased considerably within the Maribyrnong catchment.  If you think of high growth urban fringe areas, you might think of places like Craigieburn, Sunbury, Romsey, Lancefield, Meadow Heights, Macedon, Mount Macedon, Gisborne, Riddles Creek and so on.  All these towns and suburbs are within the Maribyrnong River catchment.  These newer urban developments are contributing to this fast response flooding.

This trend will continue because the Maribyrnong catchment contains large areas of zoned urban growth land.  Even within the lower reaches of the Maribyrnong River, in the older urban areas, the river inflow characteristics are being changed by the steadily increasing town house and apartment development.

What was once a “quarter-acre” house lot might now have five or more townhouses.  Here, the chances of our raindrop falling on vegetated soil reduce from 60% to 70% down to less than 5%.  Our raindrop is now falling on a radically changed ground surface.

We await the findings of the Melbourne Water investigation to see exactly what are the major guilty parties for the recent flooding along the lower Maribyrnong River.  Urbanisation is my pick as the short odds favourite for this but for the triella you can also add climate change and the “natural” climate.