2021 – Risk analysis and safety upgrade of Mangrove Creek Dam; SFAIRP assessment under Dam Safety NSW regulations

Vicent Espert, Peter Buchanan, Colleen Baker, Malcolm Barker, Mark Locke

Mangrove Creek dam is an 80 m high CFRD constructed between 1976 and 1982 for water supply to the NSW Central Coast area, and is currently operated by Central Coast Council (CCC). The dam is classified as a ‘High A’ Consequence Category dam for both Sunny Day and Flood breach in accordance with ANCOLD guidelines.

Previous assessments of the dam identified that it would not be able to safely pass the ANCOLD Fallback flood capacity of the PMP flood in its current configuration. As such, the dam has been operated at a restricted full supply level for many years.

In 2020, GHD was engaged by CCC to develop a concept and detailed design to increase the spillway capacity using a standards-based approach to achieve the flood capacity fallback position. The first phases of this contract also required GHD to undertake additional investigations and analyses of various aspects of the dam and spillway to confirm the scope of works for the upgrade. During this review, it became evident that although the spillway capacity does not meet the ANCOLD fallback position, the Annual Exceedance Probability (AEP) of the existing capacity was relatively low and could potentially be deemed acceptable from a risk-based position.

A Risk Assessment was subsequently undertaken, with a SFAIRP assessment developed based on the new Dam Safety NSW guidelines. This assessment may be the first one to be completed for a major dam using the Dam Safety NSW guidelines. This paper discusses the different outcomes for a standards-based ‘Fallback’/’Simplified’ criteria and risk criteria based on DS NSW regulations, as well as the investigations developed to maintain confidence in the assessment. In addition, it describes a practical case for the application of SFAIRP criteria to a major dam.

In the case of Mangrove Creek Dam, the application of the new DS NSW Guidelines resulted in the dam being assessed as acceptable in its current state, with the FSL returned to the original design level. The outcome provided significant savings to the client, by avoiding costly upgrade works and avoiding disruption to the operation of the storage – a real success story.