On the rigid-lid approximation for two shallow layers of immiscible fluids with small density contrast

TL;DR

This paper rigorously justifies the rigid-lid approximation for two shallow, immiscible fluid layers with small density contrast, showing it accurately captures the slow mode while the fast mode remains small over time.

Contribution

It provides a mathematical validation of the rigid-lid approximation in the small density contrast limit for two-layer shallow water flows.

Findings

Rigid-lid approximation accurately predicts the baroclinic mode.

Fast barotropic mode remains small over time.

Explicit first-order surface deformation behavior described.

Abstract

The rigid-lid approximation is a commonly used simplification in the study of density-stratified fluids in oceanography. Roughly speaking, one assumes that the displacements of the surface are negligible compared with interface displacements. In this paper, we offer a rigorous justification of this approximation in the case of two shallow layers of immiscible fluids with constant and quasi-equal mass density. More precisely, we control the difference between the solutions of the Cauchy problem predicted by the shallow-water (Saint-Venant) system in the rigid-lid and free-surface configuration. We show that in the limit of small density contrast, the flow may be accurately described as the superposition of a baroclinic (or slow) mode, which is well predicted by the rigid-lid approximation; and a barotropic (or fast) mode, whose initial smallness persists for large time. We also describe explicitly the first-order behavior of the deformation of the surface, and discuss the case of non-small initial barotropic mode.