Modeling IG waves with SWAN : final report

To assess the safety of the coast against extreme storm impacts a switch from empirical modeling to process-based models is underway. This allows for more robust testing for coastal configurations that deviate from prototype flume experiments used to calibrate these models. An important element in both dune and dike safety is the runup that determines dune erosion and potential overtopping. It is well known that close to the water line the InfraGravity (IG) motions to a large extend control the maximum runup. Process models predict the transformation from Sea-Swell (SS) waves to the IG waves within the coastal zone and associated runup. However, currently these models ignore the presence of free incident IG waves at the offshore boundary, instead assuming all IG waves are locally forced, and therefore potentially underestimate the runup, overtopping and dune erosion. Hence, for a proper prediction both the incident Free IG (FIG) waves as well as the forced waves have to be described. The objective of this study is therefore to predict the total IG wave field, i.e. both forced and free, along the Dutch coastal zone at the depth of the offshore boundary of the process-based models. To that end the SWAN model is stepwise extended to be able to describe the generation, propagation, and evolution of both the forced and free IG waves.

• Step 1: Prediction of free IG waves within the coastal zone.
• Step 2: Prediction of the free IG waves at the onset of the storm.
• Step 3: Prediction of the forced IG waves by SWAN.
• Step 4: Prediction of the combined free and forced IG waves .

The following report presents an overview of the new prediction capabilities of free and forced IG waves using SWAN which were developed as part of Step 4. Additionally, the new prediction capabilities of SWAN for estimating the total IG signal (free and forced) are verified against IG observations which were recently collected as part of the REFLEX project.

Er is ook een Nederlandse samenvatting opgenomen naast het rapport ‘Modeling IG waves with Swan’ door Gal Akrish, Ad Reniers en Marcel Zijlema waarin het werk beschreven is in het kader van de opdracht van Rijkswaterstaat om via Kennis voor Keringen SWAN een impuls geven op het gebied van de InfraGravity golven (IG), om meer inzicht te krijgen in de opwekking, transmissie en invloed op de keringen van zowel vrije als golfgroep geforceerde IG-golven. Zowel dieper water als de kustnabije (shoaling- en brekings) zones zijn dus van belang. Om de infragravity-energie langs de Nederlandse kust goed te kunnen voorspellen in het kader van de kustveiligheid, is het noodzakelijk om de methode van Rijnsdorp et al. [2021] voor de vrije infragravity (Free InfraGravity afgekort als FIG)-golven verder te valideren, en uit te breiden voor de generatie en aanwezigheid van golf-groep geforceerde infragravity (Wave Group Forced InfraGavity afgekort als WGF-IG)-golven die bestaan uit een combinatie van gebonden infragravity (Bound InfraGravity afgekort als BIG)-golven en lokaal vrijgemaakte IG golven.