Long wave instability in a three-layer stratified shear flow

In inviscid fluid flows instability can occur due to a resonance between two wave modes. For the case when the modes remain distinct at the critical point where the wave phase speeds coincide, then in the weakly nonlinear, long wave limit there is an expectation that the generic outcome is a model consisting of two coupled Korteweg-deVries equa- tions. This situation is examined for a certain three-layer stratified shear flow.