It is not uncommon for a flood defence to be combinedwith other societal uses as a multifunctional flood defence, from housing inurban areas to nature conservation in rural areas. The assessment of thesafety of multifunctional flood defences is often done using conservativeestimates. This study synthesizes new probabilistic approaches to evaluatethe safety of multifunctional flood defences employed in the Netherlands andexplores the results of these approaches. In this paper a case representinga typical Dutch river dike combining a flood safety function with a natureand housing function is assessed by its probability of failure for multiplereinforcement strategies considering multiple relevant failure mechanisms.Results show how the conservative estimates of multifunctional flooddefences lead to a systematic underestimation of the reliability of thesedikes. Furthermore, in a probabilistic assessment uncertainties introducedby multifunctional elements affect the level of safety of the dikeproportional to the reliability of the dike itself. Hence, dikes with higherprotection levels are more suitable to be combined with potentially harmfuluses for safety, whereas dikes with low protection levels can benefit mostfrom uses that contribute to safety.
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