Block Stability in 3 Dimensions (BS3D) is an incremental-iterative algorithm and code to analyse general failure modes of rock blocks subjected to generic forces, including non-conservative forces such as water pressure. This algorithm is used to investigate the effect of high-velocity water jet impact on the stability of single rock blocks in plunge pools. After reviewing the literature, an approach is proposcd to estimate the dynamic water pressure fluctuations at the pool bottom and in rock fractures. The approach is validatcd using an experimental study. In addition, by comparing the results of these analyses to additional experimental results, a scour threshold is introduced for plunge pools. It is found that if the maximum upward block displacement is larger than a quarter of the block height, the block is most probably removed from its mould. The proposed approach and threshold are validated using case histories that indicate excellent match between prediction and observed scour depth.