A sign announcing the construction of a backflow valve at the mouth of Western Creek. It is erected on the gate to the footbridge under Coronation Drive. (Photo: S. Cowley)

So, it’s really happening. If this sign is to be believed, then Western Creek is finally going to have its floodgate. The photo was snapped at the entrance to the footbridge under Coronation Drive at the mouth of Western Creek (otherwise known as the Milton Drain) a few weeks ago by Steven Cowley, who has been my eyes on the ground at Western Creek ever since I moved to the southside last year.

The sign doesn’t mention a floodgate, but that is essentially what the ‘devices to be installed’ as part of the ‘Backflow Program’ are. They are contraptions of varying designs that ‘mitigate river water from flowing back up stormwater pipes when the river is in flood’. Such devices have already been installed just downstream near Cribb Street and the Go-Between Bridge. The City Council’s website has a page listing all of the places where backflow devices have been, or might soon be, installed. The page also provides links to the technical reports about these devices that the council commissioned in the wake of the 2011 flood.

A floodgate at Western Creek has been a long time coming — nearly 125 years, in fact. Calls for one to be built here can be traced back as far as 1889, when a Miltonite named Hugh Bell wrote to the Brisbane Courier explaining that the ‘lakes of water at Milton and Rosalie’ that resulted from the flood in July that year ‘would have been prevented by a proper arrangement of tidal valves’.¹ You can read all about the history of this idea, and of Brisbane’s curious relationship with flood mitigation more broadly, in my previous essay, Backflow to the Future.

A flap-gate valve installed on a drain by the river at Milton.

As I noted in that essay, sealing off Western Creek from a flooding Brisbane River presents something of an engineering challenge. The space beneath Coronation Drive is large and irregularly shaped, and there is a walkway there that needs to be maintained. The technical report (see page 19) to the council proposed two stages of backflow prevention: one up to the level of the footbridge, and another up to the level of Coronation Drive. The first, which would only guard against a very modest flood or an unusually high tide, could be achieved by simply installing a row of flap-gates under the new footbridge that runs beside the bikeway. Preventing backflow to the height of the road, however, will require a barrage of gates called penstock valves (see the picture below) constructed so as to accommodate the walkway. Or rather, the walkway may need to be re-designed to accommodate the barrage, possibly by having a section that can be removed when the gates are raised.

An example of a penstock valve (Australian Water Engineers)

I should point out that I am not certain that the council is building a floodgate to the height of Coronation Drive, rather than just to the footbridge. But if they are, then the device will protect against a flood as high as the 2011 flood, but not much higher. If the river breaches the road as it did in 1974, a floodgate will only provide some extra time for people to move their belongings.

Even so, the construction of a floodgate at Western Creek will be welcome news to the many residents and businesses who were flooded in 2011 or 2013. But there is a possibility that this floodgate, and the many others being installed along the river, will put other areas at greater risk. Flood mitigation is not a zero-sum game. Water that doesn’t go to one place must instead go to another. As the council’s own technical report points out in relation to Western Creek’s floodplain,

In a major flood event, this system provides storage for a significant volume of flood water. It is recommended that Council investigate the impacts of removing the flood storage provided by this system on the river system as a whole.

I don’t know whether the council has followed this up. But if I lived in a flood-prone area downstream that does not have a backflow prevention device already planned or installed (if any such areas exist), I would be asking some questions.

The Milton Drain at high tide, seen from just near Milton Road.

The question that is most on my mind, however, is whether Western Creek’s backflow device(s) will lock out the water that ordinarily flows into the Milton Drain on a daily basis — the water that you can see rising and falling with the tides in the open sections of the drain. If flap-gates are to be installed under the bikeway bridge, then this would seem to be an inevitable result. On the other hand, a penstock valve under Coronation Drive might be able to be retracted fully when it is not needed.

If the normal flow of water is cut off, then I will be disappointed, even a little sad. For as unappealing as the Milton Drain is, I think it adds a lot to the area simply by filling up with water (yes, even icky brown water) twice a day. It brings a natural rhythm — that of the tides — to this part of Milton that would otherwise be absent. It’s no substitute for a real creek, but it is better than nothing. And if it is ugly now, imagine how depressing it would be if it were empty all the time.

Hopefully, my concerns are misplaced. If there is a council engineer reading out there, perhaps you can shed some light on what is being planned. In any case, I suppose we will find out by mid 2014.

UPDATE – August 2014

Western Creek now has its floodgate. Read all about it here.