VARIATIONAL DERIVATION OF THE GREEN–NAGHDI SHALLOW-WATER EQUATIONS

Abstract

We consider the two-dimensional irrotational water-wave problem with a free surface and a flat bottom. In the shallow-water regime and without smallness assumption on the wave amplitude we derive, by a variational approach in the Lagrangian formalism, the Green–Naghdi equations (1.1). The second equation in (1.1) is a transport equation, the free surface is advected by the fluid flow. We show that the first equation of the system (1.1) yields the critical points of an action functional in the space of paths with fixed endpoints, within the Lagrangian formalism.

Keywords:

• Green–Naghdi equations
• shallow-water waves
• variational methods