Credit NSW SES Kiama Unit

NSW flooding and Jerrara Dam

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The NSW State Emergency Services has issued an alert for Jerrara Dam suggesting dam failure is imminent. The NSW State Emergency Service has directed local residents to evacuate. The NSW SES is urging Lake Conjola residents to also prepare immediately as the lake level continues to rise rapidly. Below flood experts comment:

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Expert Reaction

These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.

Dr Monzur Imteaz is Associate Professor of Water Engineering at Swinburne University

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.

Last updated: 03 Nov 2016 7:18pm

Dr Jamie Pittock is Associate Professor at the Fenner School of Environment and Society. He is a UNESCO Chair in Water Economics and Transboundary Water Governance

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.

Last updated: 03 Nov 2016 4:13pm

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