Flow sliding of submerged slopes in front of dikes can affect the reliability of flood defences. The occurrence of flow sliding may result, sometimes with delay, in a failure mode and consequent flooding. The current safety assessment method for flow sliding does not consider the interaction between flow sliding and failure modes which can cause a breach. Therefore it is unclear whether the safety assessment method is sufficiently safe or too conservative. This paper presents a method to derive a simple safety assessment and design rule based on a number of advanced probabilistic analyses made for four Dutch cases, in which both flow sliding and the relevant direct failure modes have been incorporated. Two methods for the safety assessment of flow sliding are presented: as separate failure mode and coupled to direct failure modes.