Community-oriented strategies to prevent New South Wales dam overflow

The wettest Spring on record across the Murray Darling Basin and New South Wales has brought tremendous hardship for thousands of people, and extreme vigilance by WaterNSW in safely managing the state’s inland dams.

The recent exceptional weather has helped drive the devastating flooding from which communities across much of regional New South Wales are still recovering, while others brace for their turn for floodwaters to arrive.

As manager of the state’s major supply dams, including the dozen or so across regional New South Wales, WaterNSW operations have been centred on flood operations for many months.

Dam personnel on site – often living in flooded communities themselves – and expert planners have been working to manage huge volumes of water that have flowed into dams over the past 12 months, reaching a peak in spring.

As back-to-back rain events and major storms created these large inflows into the dams and generated equally high levels in the creeks and rivers downstream, WaterNSW staff worked hard to grab the severely limited windows of opportunity to protect local communities.

RECORDS BREAKING ALL AROUND

Here is a brief snapshot of the challenges WaterNSW dam operators faced in 2022:

Copeton Dam, near Inverell, exceeded capacity and spilled more times in the latter part of 2022 than the previous 40 years combined

Throughout 2022, Keepit Dam – the main storage on the Namoi River near Gunnedah that ran dry during the most recent drought – received and released water equivalent to near 2.5 times its storage capacity

If dams like Burrendong, on the Macquarie River near Wellington, and Burrinjuck on the Murrumbidgee River near Yass, had both been empty in July 2022, they would have received enough water – since winter - to fill and spill almost three times

The spectacular footage of Wyangala Dam spilling into the Lachlan River in November 2022 became an iconic image of the devastating flood throughout the region.

However, the image could not convey the months of careful calculations and success WaterNSW dam operators had in creating space in the dam between rainfall events to hold back huge amounts of water, before the frightening November deluge finally pushed the dam beyond capacity.

Over the previous six months, Wyangala – which can expect average annual inflows of 560GL – had received almost four times that amount (2,071GL), with WaterNSW operators managing to hold the majority of the water back during flood peaks.

Once the dams are at capacity, the water must be released, but this is carefully managed so it rarely reaches the rate of what’s flowing into the dam. By holding the water back while uncontrolled downstream tributaries peak and begin to recede, the full intensity of the flood height is reduced.

EMPTYING THE BATH WITH THE TAP RUNNING

A common suggestion from flood-weary communities is that dam storages should be lowered to capture the flood. While these sentiments are entirely understandable, there are many pieces to this difficult puzzle.

Firstly, dams only capture water flowing from one part of the upstream catchment. The rivers on which our dams are situated are fed from multiple creeks and rivers. Some of these flow into the dams, but many don’t. Instead, they flow downstream of the dam and into its catchment area. It is these uncontrolled tributaries that have caused the worst of the flooding across inland New South Wales.

At times, these tributary flows have been enormous.

For example, in 1971 the previous highest rate of flow ever recorded in the Horton River, a tributary which feeds into the Gwydir River downstream of Copeton Dam, was almost 250GL per day – more than the spectacular spill from Wyangala Dam in November 2022.

In October 2022, the same Horton River easily broke that record of half a century, by flowing at a peak rate of 330GL per day.

Secondly, even if we had a 15 per cent or 20 per cent storage buffer, the huge volume of inflows experienced in the later part of 2022 would have filled this up in little more than a weekend in several cases, and some dams have received more water than the equivalent of their entire storage, in a few short months.

To mitigate flooding, WaterNSW’s dam managers set out to create as much capacity as possible in the storage between rain events to receive the next round of inflows, a relatively modest accomplishment in most circumstances, but a very challenging one during the rolling La Ninas of 2022.

Catchments saturated from months of rain not only flood rivers by generating high downstream flows; the volume is larger and it flows for longer, instead of soaking into the earth. In those conditions, reducing the storage is like trying to empty a bath with the taps running.

Dams are often the unsung heroes during floods, even where extraordinary weather has pushed them to the limit. WaterNSW, in partnership with local communities and emergency agencies, has worked hard to hold water back and protect communities living with a dam located upstream.

CARCOAR DAM