Sydney - Middle East Times Int’l: Four times from 2020-22 in the Hawkesbury-Nepean region, there were big floods which reached a level not seen at Windsor since 1978. 

During those years there was constant debate about whether to raise Warragamba Dam a further 14m to alleviate the risk of further floods, a plan scrapped by the new Labor government. 

However, there are now fears that the community is not ready if an even bigger flood arrives. 

Flooding rainfall in eastern Australia is not merely linked to La Nina in the Pacific Ocean.  

There is a longer-term pattern known as the Interdecadal Pacific Oscillation (IPO) that has an outsized influence on Australian weather.  

There have been three wet phases since 1900: 1916-25, 1942-76 and a strengthening spell that arguably began in 200. There were dry phases in between that were associated with some of Australia’s worst droughts. 

Western Sydney’s Hawkesbury-Nepean Valley has a long history of flood dominant phases alternating with periods of minimal flooding. These periods line up quite closely with the phases of the IPO.  

During the 61 years of negative “wet” IPO since 1900, there were 51 floods that exceeded the moderate level at Windsor. 

Across the remaining 62 years of neutral and positive “dry” phases there were only 24 moderate or major floods. 

The current negative IPO phase therefore substantially increases the flood risk.  When La Nina occurs within a negative IPO phase the flood risk increases even more. 

Historically, 70% of years with La Nina and a negative IPO had at least one flood – some with many while just 30% of the remaining years had flooding. 

When looking at large floods above 10m, including the most recent three floods, there was a similar trend - with about double the flood rate in years influenced by La Nina, which is likely to make an appearance in 2024, and a negative IPO. 

The previous government wanted to raise Warragamba Dam a further 14m but promised minimal resources on other measures.