NSW flooding and Jerrara Dam

By looking at rainfall amounts occurred during 25-26 August and comparing the amount with the standard design rainfalls for nearby areas, the rainfall event that occurred is not a huge one. Basically, it is an event of less than five years ARI (Average Recurrence Interval), i.e. probability of occurrence of this sort of rains is less than one in five years. Usually, a dam should be designed to handle a flood/rain of magnitude more than 100 years ARI event.  It is surprising that still this dam is overtopping with this rain event. We should look at original design criteria for this dam, also the contributed catchment area considered during the design phase and amount of urbanisation that occurred in the catchment area during the period since the dam was built. In general, a dam should not overtop for a rainfall event of one in five years magnitude. From State Emergency Services records, it looks like this dam is getting flooded and residents are receiving frequent warning messages every now and then. It needs thorough investigation.

The potential failure of the Jerrara Dam actually highlights a rare example of good management of smaller dams in Australia. While the structural safety of Australia’s large dams are rigorously regulated by state government agencies, smaller structures are not.

There are thousands of small dams scattered around the landscape that are decades old. Hundreds of these were built with environmentally damaging designs, are no longer used and are often abandoned or poorly maintained and unsafe. For instance, there are a great many weirs in NSW that were built to supply water to steam trains that no longer grace our few remaining train lines. These dams inflict great ecological damage, for example, by blocking the migration of indigenous fish.

In the United States, private owners of small hydropower dams are subject to periodic re-licensing, where the owners must publicly demonstrate every 50 to 30 years that their structures are safe, economically valuable, and that environmental and social impacts are minimised.

We need similar periodic relicensing in Australia so as to better manage unsafe, poorly performing and redundant small dams. The Jerrara Dam is a rare and positive example in Australia of an unsafe dam being removed. While the timing of this flood is unfortunate, this sort of pro-active decommissioning should be a regular event in Australia.