Extreme watershed rainfall prediction for east coastal Australia

Coastal watersheds along the east coast of Australia are highly significant in terms of water resources and their management. This is because the area is the most heavily populated part of Australia and its uniformly high rainfall is subject to severe and extreme rainfall events. The coastal plain is very narrow along almost the entire length of the state of New South Wales (NSW). As a result of the heavy rainfall events, the coast and adjacent hinterland experience a range of natural disasters including flash floods, mud slides and soil erosion from terraced, agricultural areas on steep slopes. Storm surges, waves and swells are also responsible for coastal erosion. Infrastructure along the NSW coastline is frequently disrupted by severe weather events.

In this study we have focused on the important regional city of Coffs Harbour (population approx. 30,000). Although many other cities along the NSW coast are badly affected by these natural disasters, the main reason for our choice is the fact that the watershed for Coffs Harbour is very small (less than 100 square km) and it is prone to extreme rainfall rates. The resulting run-off passes through the city itself. Coffs Harbour has been subjected to many flash flood events, the worst recorded being that of 23 November 1996, when four hours of rainfall produced about 400 mm on the hills just to the west of the city. The runoff swept into the city, resulting in one death and insurance costs of AUS$30 million dollars.

In this paper, we use a simple perturbation technique to generate an envelope of possible initial model states that are within those of the current climate spread. Specifically, the range of perturbation values for each variable considered appropriate for current climate conditions was within 2 standard deviations of the mean, for each variable. The variables that were perturbed were those that affect the position, intensity and duration of the meteorological systems. As examples, sea surface temperatures (SSTs) were perturbed within the range ± 2.5deg. C of mean values and winds were perturbed in the upper tropospheric levels in the range ± 15% of mean values.

It was found that the 24 hour rainfall for November 23, 1996 was increased, in the most extreme scenario, by almost 50%. This translates to about an extra metre to the maximum recorded height of the water through Coffs Harbour in the flood event of November 1996. The design allowance for the river system was exceeded in the 1996 flood so the extreme scenario generated in this study is a catastrophic event, within current day climate values.