2024 – Bringing Background into the Foreground: An Examination of the Usefulness of Considering Background Flood Risk Impacts due to Dam Upgrades

Ella Nicholas, Samuel Holden and Nick Glover

There is a trade-off in some risk-based dam upgrade projects between reducing the incremental societal risk of dam failure, and, increasing non-breach or background flood risks in downstream (or upstream) catchments as a result of risk mitigation works. An example of this compromise would be increasing flood capacity (and reducing incremental risk) of a dam through a spillway enlargement that results in a greater discharge for events which may have higher frequency than the improved flood capacity of the dam. In such a situation there is a transfer of risk from contingent incremental dam safety risk to inherent background flood risk, after the upgrade is completed. This transfer in risk could be viewed as part of the ‘sacrifice’ required to reduce incremental dam safety risk and hence should be considered in assessment and justification of the As Low As Reasonably Practicable (ALARP) or So Far As Is Reasonably Practicable (SFAIRP) principles.

This paper proposes a methodology to consider this trade-off by quantifying and comparing background flood risk between dam upgrade options using F-N charts. The proposed methodology utilises information which is already required to conduct a dam risk assessment, enabling it to be incorporated into the risk assessment process. Explicitly evaluating background flood risk enhances the dam upgrade process by enabling more informed decision-making when considering sacrifices required to reduce incremental dam safety risk to satisfy ALARP/SFAIRP.