2012 – Applying joint probabilistic techniques to the determination of downstream coincident flows and incremental consequences of dam failure

David Stephens, Peter Hill, Rory Nathan

The estimation of incremental consequences of dam failure often requires consideration of coincident flows in downstream tributaries. In the past overly simplistic assumptions have often been adopted. Examples include an assumption that flows in downstream tributaries are negligible, equivalent to the 1 in 100 Annual Exceedance Probability (AEP) flood, the mean annual flood or the flood of record. Experience has shown that these assumptions often underestimate coincident flows, particularly for extreme events approaching the AEP of the Probable Maximum Precipitation. Additionally, the justification for adopting these techniques is usually driven by ease of use rather than the degree to which they represent the relevant physical processes at play. For some dams, these techniques may have a negligible influence on the overall consequence assessment. However, there are many dams for which an improved understanding of coincident flows using a joint probabilistic framework can result in significantly altered estimates of the natural flood and dambreak flood inundation zone. This can frequently lead to the consequences of the natural flood being larger than would otherwise have been the case, leading to a reduction in incremental consequences. Two examples of such situations are presented, including a description of the techniques used to estimate coincident flows and a discussion on likely influence of these flow estimates on incremental consequences. These examples are then used to draw some general principles for the types of dams at which an improved understanding of coincident flows is warranted.
Keywords: dam failure, coincident, joint probability, consequence assessment