ABSTRACTSources of both systemic and parameter uncertintiesin flood alleviation cost‐benefit analysis are discussed. Cost‐benefit analyses are assessments of the future effects of different scheme options; consequently, there are inherent uncertainties. The omission of consideration of some impacts as ‘intangibles’, because it has not been possible to measure them, is a form of systemic uncertainty.A significant omission in the past from flood alleviation cost‐benefit analyses has been the non‐monetary impacts of flooding upon households. It is shown that these impacts are both large and more important to the households affected than are the direct mone