Water NSW reveals the story of flood mitigation after Wyangala took nearly four years' water in 2022

Two months on, WaterNSW says that footage could not convey the months of careful calculations and success their operators had in creating space in the dam between rainfall events to hold back huge amounts of water, before the frightening deluge pushed the dam beyond capacity.

Wyangala received almost four times its average annual inflows in 2022, Water NSW has revealed in an explainer published to their website on January 25, 2023.

A whopping 2,071GL - that's billion litres - flowed in, the average year's inflows are 560GL.

"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," the Water NSW document states.

"By holding the water back while uncontrolled downstream tributaries peak and begin to recede and only letting it out once this has happened, the full intensity of the flood height is reduced, even when the dam exceeds capacity."

The water authority says there are many pieces to the difficult puzzle that is catchment management in such a year.

"Firstly, dams only capture water flowing from one part of the upstream catchment," the piece states.

"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 and instead flow in downstream of the dam and its catchment area.

"It is these uncontrolled tributaries that have caused the worst of the flooding across inland NSW.

"And these tributary flows have at times been enormous."

Dam managers worked to create as much capacity as possible in storages including Wyangala between rain events, but the rolling La Ninas of 2022 were challenging.

"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," the report states.

"In those conditions reducing the storage is like trying to empty a bath with the taps running.

"In the Lachlan system, floodwaters that devastated communities such as Forbes in mid-November were still making their way downstream to communities such as Condobolin and Lake Cargelligo many weeks later.

It was an incredibly "delicate balance" and involved months of regional advisory panel meetings in river valleys with landholders, councils and agencies such as the Bureau of Meteorology, and the NSW State Emergency Service.

"Water released from dams must go somewhere, and a 10 per cent reduction risks pre-flooding downstream when water from the vast surface area of the dam storage is released into the narrow confines of the river, with potentially more wet weather forecast," WaterNSW said.