Rigorous derivation of the Whitham equations from the water waves equations in the shallow water regime

TL;DR

This paper rigorously derives the Whitham equations from water wave equations in shallow water, establishing a direct link and comparing accuracy with Korteweg-de Vries models using two different methods.

Contribution

It introduces two novel methods for deriving Whitham equations from water wave models, one for unidirectional and one for bidirectional waves, with improved accuracy analysis.

Findings

Established a rigorous derivation of Whitham equations from water waves

Compared the accuracy of Whitham and Korteweg-de Vries models

Provided two methods: Riemann invariants and Birkhoff normal form

Abstract

We derive the Whitham equations from the water waves equations in the shallow water regime using two different methods, thus obtaining a direct and rigorous link between these two models. The first one is based on the construction of approximate Riemann invariants for a Whitham-Boussinesq system and is adapted to unidirectional waves. The second one is based on a generalisation of Birkhoff's normal form algorithm for almost smooth Hamiltonians and is adapted to bidirectional propagation. In both cases we clarify the improved accuracy on the fully dispersive Whitham model with respect to the long wave Korteweg-de Vries approximation.