The shoreline problem for the one-dimensional shallow water and Green-Naghdi equations

TL;DR

This paper investigates the shoreline problem for the one-dimensional Green-Naghdi equations, addressing the challenges posed by free boundaries and degeneracies at the shoreline, and develops new analytical tools for this complex setting.

Contribution

It extends the well-posedness analysis of Green-Naghdi equations to cases with a free boundary where water depth vanishes, introducing novel methods for degenerate dispersive hyperbolic systems.

Findings

Established local well-posedness without positive lower bound on water depth

Developed new analytical tools for degenerate dispersive boundary problems

Analyzed the behavior of dispersive smoothing at the shoreline

Abstract

The Green-Naghdi equations are a nonlinear dispersive perturbation of the nonlinear shallow water equations, more precise by one order of approximation. These equations are commonly used for the simulation of coastal flows, and in particular in regions where the water depth vanishes (the shoreline). The local well-posedness of the Green-Naghdi equations (and their justification as an asymptotic model for the water waves equations) has been extensively studied, but always under the assumption that the water depth is bounded from below by a positive constant. The aim of this article is to remove this assumption. The problem then becomes a free-boundary problem since the position of the shoreline is unknown and driven by the solution itself. For the (hyperbolic) nonlinear shallow water equation, this problem is very related to the vacuum problem for a compressible gas. The Green-Naghdi equation include additional nonlinear, dispersive and topography terms with a complex degenerate structure at the boundary. In particular, the degeneracy of the topography terms makes the problem loose its quasilinear structure and become fully nonlinear. Dispersive smoothing also degenerates and its behavior at the boundary can be described by an ODE with regular singularity. These issues require the development of new tools, some of which of independent interest such as the study of the mixed initial boundary value problem for dispersive perturbations of characteristic hyperbolic systems, elliptic regularization with respect to conormal derivatives, or general Hardy-type inequalities.