MORE INFORMATION- LA NINA

The 1955 – 1956 Floods

The classic example of flooding associated with a La Niña in Australia is the infamous Hunter River flood of 1955 and the subsequent widespread flooding of the entire Murray-Darling system in 1956.

Table 1 records the Southern Oscillation Index values for the years 1954,1955 and 1956.
 

Table 1. Monthly values of the SOI for the years 1954 to 1957
(Source: Bureau of Meteorology, Australia)

Year

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

1953

2.2

-6.0

-5.8

-0.5

-31.9

-2.3

-1.0

-17.2

-13.0

-0.1

-2.0

-4.0

1954

6.0

-3.6

-0.9

6.9

4.4

-1.5

4.2

10.4

4.5

1.8

3.9

12.8

1955

-5.4

15.2

2.9

-3.0

13.1

16.4

19.2

14.9

14.1

15.2

15.1

9.3

1956

11.3

12.4

9.4

11.1

17.9

12.3

12.6

11.0

0.2

18.3

1.9

10.3

1957

5.6

-2.2

-0.9

1.2

-12.2

-2.3

0.9

-9.5

-10.6

-1.3

-11.9

-3.5

From July 1954 onwards, the monthly Southern Oscillation Index (SOI) values were positive, peaking at +15.2 in February and then rising again in the second half of1955. A large part of Northern and Eastern Australia experienced a cool wet summer. In the north the monsoons were particularly active while in the southeast many areas received record high rainfall especially during the month of February. For the Hunter region of NSW in particular, rainfall was above average throughout the summer of 1954-55. During the month of February, several weather stations in the region recorded their highest ever February rainfall totals, some in excess of 350 mm for the month.

The full brunt of the La Niña fed depressions was experienced in central NSW during the week from 23rd February to 29th. Beginning around the 20th February 1955, an intense monsoonal low developed over central Queensland moving southwards. Another low pressure trough developed off the coast.  By the 22nd and 23rd, these two systems had merged over Northern NSW bringing a strong,  extremely moist north easterly airflow into the Hunter River catchment and the western slopes of the Great Dividing Range feeding into the Darling River system in central NSW.  
Rainfall intensities reached record levels over the next few days, overwhelming the capacity of river systems to drain the water away. Streamflow in the Namoi's River for example, reached 9000 cubic metres per second compared with a normal flow of 25 cubic metres per second. The Castlereagh and Macquarie rivers feeding into the Darling system were similarly affected.

But it was on the eastern side of the Great Dividing Range, in the Hunter River basin, that the impacts of these enormous volumes of water left their most devastating legacy. In excess of 30,000 residents in the catchment had to be evacuated. In the town of Maitland alone, where the River level exceeded its previous record flood height by nearly a metre, 15000 people had to be evacuated. On 25th of February, 6 residents in various parts of Maitland were swept to their depth. On the 26th, the entire population of Hexham down stream from Maitland had to be evacuated when the residential areas were completely submerged.


 
Figure 1: A deserted farmhouse on the outskirts of Maitland N.S.W., during the infamous 1955 floods
(Source: National Library of Australia -  nla.pic-vn3308235)      

By the time the worst of the flood waters had begun to abate around the 29th February, a total of 25 persons had lost their lives in the Singleton-Maitland area of the catchment. 7000 homes had been damaged. The cost of the flood in today’s terms is estimated to have been about $1.3 Billion.


  
Figure 2: Victoria Street, Maitland N.S.W., during the infamous 1955 floods
(Source: NSW State Emergency Service)

It is worth noting that the month of February did not see the last of the torrential rain associated with the 1954-1956 La Niña.  For 21 months straight from May1955 through to January 1957, the SOI remained in the positive. During the first half of 1956, floods occurred repeatedly along almost the entire Murray-Darling river system. Leaving aside the short-term damage produced by these flood events, the recharging of the extensive inland river system associated with the La Niña brought enormous environmental and economic benefits to the region that were  felt for many many months afterwards.