Infragravity waves in Dutch tidal basins and estuaries : implications for flood risk assessment

During storm conditions infragravity (IG) waves significantly affect runup and potential overtopping of dikes with shallow foreshores. However, it is unclear how much impact these IG waves will have on the potential flooding due to failure of the Dutch flood defences during normative conditions. Initial explorations suggest that they can be significant but with a wide range in uncertainty. Currently, the presence of IG waves is not included in the instrumentation that is used to assess the safety level of flood defences, the so-called “Beoordelings- en Ontwerp instrumentarium” (BOI), introducing uncertainty in the determination of flood probability. The uncertainty is particularly large in the shallow Eastern Wadden Sea and the entrance of the estuaries of the Eastern Scheldt and Western Scheldt.