Dam break inundation mapping is the first part of the assessment of the consequences of failure of a dam. Although there has been significant expenditure on research on the breach hydrograph, there has been little attention given to the influence of infrastructure embankments across the inundated area on flow paths, depth of inundation and consequences. This paper discusses the general issues and provides two case histories where the consequences of dam failure are significantly influenced by major infrastructure embankments across the valley downstream of the dam.