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How ‘drought infrastructure’ can help us get through floods

South East Queensland’s grid of water treatment plants are meant to drought proof the region, but could help in floods too. AAP Image/Dave Hunt

Over the last six years, there have been major investments made in “climate independent” water supplies and other measures to help “drought-proof” most of Australia’s capital cities. These have included a number of desalination and advanced water treatment/recycling plants, but also the construction of large water supply networks linking different water sources and users across a whole region.

One of the largest such systems has been constructed in South East Queensland, where the 250km-long water distribution “grid” now connects all major water treatment plants, including the Tugun desalination plant, with over 2 million users in the entire region from the Gold Coast to Noosa.

The main purpose of this major infrastructure has been to secure water supplies to our growing cities during severe droughts. But it is not well known that this has some very significant benefits in periods of flooding as well. This has been demonstrated in the major flood events in Brisbane and South East Queensland in January 2011 and recently over the Australia Day weekend 2013.

One of the major challenges during such major floods is obviously not the lack of water, but the quality of the water that is used for the drinking water treatment. Most of Australia’s water supplies are relying on surface water (generally dams catching run-off from more or less protected environments). Major rainfall, then, has a very rapid and often highly detrimental effect on the water quality in the rivers and creeks flowing into the dams and hence the dam water itself.

Further challenges are created when the water extraction points for the drinking water plants are not directly at the dam, but from rivers downstream of the dam. The direct inflows into these rivers are usually even more heavily loaded with sediments and other pollutants than the dam water itself.

This latter situation is exactly what created a potentially serious water supply shortage in Brisbane during the recent flood. The large amount of run-off from the Lockyer Valley, a major agriculture area west of Brisbane, was flowing into the Brisbane River downstream of Wivenhoe dam, but upstream of the region’s largest water treatment plant at Mt Crosby. The resulting “dirty water” event reached turbidity levels up to four times as high as during the January 2011 flood, therefore drastically reducing the drinking water production capacity for several days. This temporarily eliminated nearly 50% of the region’s water treatment capacity, and hence created the very real potential that parts of Brisbane would literally run out of water – in the middle of a flood!

In the end, a major supply crisis was avoided thanks to the infrastructure built as part of the drought response in 2007-2010. With water treatment plants still operating effectively both at the Gold Coast and north of Brisbane, the water distribution grid enabled the supply of significant amounts of treated water to Brisbane from the north and the south. The Tugun seawater desalination plant on the Gold Coast was also put into full production to add further supply.

Together with public calls for temporary water saving measures, the overall supply for Brisbane was successfully maintained at all times during and after this flood event.

Without the additional capacity through the desalination plant and the grid’s ability to source water where available across the region and supply it to the main demand areas, a significant fraction of Brisbane residents could have lost their water supply for a couple of days. This would have been a very serious additional difficulty on top of the widespread electricity cut-offs due to the severe storm that swept down the whole of Queensland.

However, there was also a major part of the drought-proofing infrastructure that could not be used in this situation at all, even though it had the capacity to make a major contribution with relatively minor (technical) changes.

South East Queensland’s three Advanced Water Treatment Plants (aka water recycling plants) have a combined water production capacity of 232ML/day. They use treated wastewater as their supply and generate very high quality water that can be pumped via a 140km pipeline to Wivenhoe dam.

There was certainly no shortage of supply during this period as all wastewater treatment plants received very high inflows due to the widespread rain. However, there was clearly no point in adding more water to an already full dam, and it would have not helped the water supply situation anyway as the dam release was limited to avoid adding further water to the already flooding Brisbane River.

So how could it have helped then in this situation? The key change would be to add the high quality water from the Advanced Water Treatment Plants directly into the water treatment plant, in this case at Mt Crosby, rather than the dam. This would not only allow the plant to generate up to 50% of its usual water production directly from the recycled water, but some of the “dirty” river water could have been taken in and treated with the dilution from the purified recycled water.

This would only require a relatively small engineering modification: the recycled water pipeline runs within 10km of the Mt Crosby plant. This change has significant advantages not just in flood situations, but also during drought. The pumping energy would be substantially less by not going to the dam, the high water quality could be maintained, and it would avoid losses through evaporation and infiltration from the dam.

A direct flood mitigation benefit could even be achieved with this modification as well. Given that the recycled water can supply a significant fraction of the typical production of the water treatment plant, this water does not need to be supplied from the dam. Therefore, to maintain the same level of supply security as currently, less dam capacity is needed for water supply purposes, hence leaving more available for flood mitigation storage. This would have a direct and significant impact on the ability to reduce or prevent future floods, with all of the associated major benefits for the region.

However, to achieve all of these outcomes we would need a shift in public views and particularly political will to consider this direct potable reuse option as a suitable strategy for improved water supply security. This shift may well be warranted if we want to gain the maximal benefits of the very capable, but also very expensive “new water” infrastructure.

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