Coastal defenses must be upgraded to combat increasing flood risk due to climate change and other factors. Storm surge barriers, large movable hydraulic structures that close temporarily during storm surges to prevent coastal floods, play a vital role in protecting estuaries. Due to the complexity of their risk analyses, important improvements are sometimes overseen. Our objective is to develop a systematic approach which is more likely to find these important improvements. We tested the method to three historic cases where important improvements were initially overlooked. We anticipate that our method can be applied to other safety systems with a large number of failure modes as well.