2016 – Retarding Basin Portfolio Risk Assessment – Life in the Fast Lane

Kristen Sih, Richard Rodd

Melbourne Water currently manages over 235 stormwater retarding basins. The process of assessing the risk posed by these assets began in 2006, and at the end of 2015 full risk assessments were completed for around 30 of the basins that were estimated to pose the highest societal risk. However, when analysing the results of these risk assessments, there was some concern that the results were inconsistent and often too conservative, given the few incipient or actual failures that had been experienced.

It was found that one of the key areas causing the conservatism was poor documentation of design and construction details, and the fact that the tools used for assessing the Potential Loss of Life (PLL) were aimed at larger storages that cause much higher depths and velocities in dambreak events than these (generally) small storages. To remedy this situation, advice was sought from specialist practitioners to develop guidance notes on the assessment of PLL and failure likelihoods for retarding basins.

On the back of these guidance notes, Melbourne Water initiated an accelerated program of assessing the risk associated with 78 retarding basins over a 6 month period. This paper describes the key recommendations from the guidance notes, compares the results of the risk assessments performed pre- and post-guidance notes and provides a summary of the portfolio risk assessment outcomes, what they mean for Melbourne Water and what the organisation intends to do to manage this risk into the future.

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