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

Onderzoek naar de impact van lange golven (infra-gravity golven) op dijk oploop en overslag. Deze zijn nu niet opgenomen in het Beoordelings- en Ontwerpinstrumentarium (BOI). Het advies is deze wel op te nemen in het BOI.

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.

The range of IG waves on the shallow foreshores in front of Frisian and Groningen dikes and their effect on the failure probability of these dikes has been estimated.
Use has been made of available SWASH hindcasts of the November 2007 storm and an extreme scenario in which the water level of that was increased by two meters. The IG wave height showed a clear evolution from offshore to the ebb-tidal delta, where IG waves that were bound to the sea-swell at the North Sea were released, and propagated towards the dike, gradually losing some energy. Apart from the sea swell component free IG waves reach the Wadden Sea, arriving from adjacent and distant coasts. The free IG wave height offshore was determined by extreme value analysis by TU Delft (2025) and translated from the ebb-tidal delta to the dike assuming the same evolution as the sea-swell component. Both IG component are of the same order. For the extreme case the total IG wave height varies between 0.4 and 0.9 m.

For this case the effect on the wave runup height z2% is 0.8 - 2 m.
The failure probabilities for the mechanisms GEKB and GEBU-Golfklap increase with a factor in the range of 1.5-2 and 2-5 respectively, which can be considered as a lower boundary for the more extreme cases.
For the storm hindcast and the extreme scenario the same offshore wave boundary conditions were imposed. If offshore wave boundary conditions, including IG waves, would have been considered that were consistent with the extreme water level considered, the failure probabilities for both mechanisms would have been larger. For the less sheltered eastern part of the Wadden Sea, the Eems-Dollard excluded, the IG wave heights are likely to be of the same order. The same holds for the IG wave heights in front of the Eastern Scheldt barrier, in the Western Scheldt they are probably smaller.

The uncertainty in the effect of including IG waves on the failure probability has been reduced. It can be concluded that IG waves do have a significant effect. It is recommended to implement IG waves in the assessment and design instruments program “BOI”, so that a more realistic estimate of the run-up and overtopping of dikes is obtained.