2022 – Beyond 100% is meaningless and unsustainable for public communications on the flood status of dams

Michel Raymond – Independent Engineer

Over the past decade or more a practice has emerged in public communications on the flood status of dams being reported as percent of full supply volume. With this form of communication, a dam level over 100% indicates that a dam is in flood and the water supply storage is over full. While reporting dam levels over 100% does indicate the full supply volume is exceeded, the author considers there is minimal further value in this approach to define flood status. It becomes meaningless when compared across different dams if it is interpreted for safety concern or flooding. It is confusing to the extent it could be dangerous, and it is not sustainable to continue this form of flood status reporting. The community and emergency management stakeholders cannot meaningfully understand whether a dam is safe when it is above 100%. Some dams may fail below 150% and other dams may be safe well above 200%.

This paper presents information for a selection of dams to demonstrate the wide variance and poor correlation between dam level percentages relative to interests for the safety of dams or outflows and downstream flooding. Potential situations are then discussed where misunderstanding of a metric of dam level percentage relative to full supply volume for flood status could result in complacency or danger. Further situations are explained where the future change of full supply volume, for example due to reservoir sedimentation or updated survey of the reservoir capacity, may add further confusion. This context supports a position that the percentage of full supply volume approach to communicate flood status is not sustainable.

Alternative more consistent metrics are suggested for the flood status of dams; for example, defining the utilisation of a dam’s flood compartment for interest in the safety of the dam and the use of flood classifications for flood impacts.