Wave kinetics in an integrable model -- the Kaup-Boussinesq system

Abstract: We study wave turbulence in one-dimensional (1-D) bidirectional shallow water waves described by the Kaup-Boussinesq (KB) equation, which is known to be an integrable system. In contrast to the generally accepted empirical belief that an integrable system yields no kinetic theory, we derive and validate a non-trivial wave kinetic equation (WKE) for the KB system with a non-zero interaction coefficient on the four-wave resonant manifold. This WKE is non-homogeneous in nature due to the non-homogeneity in the dispersion relation of the KB system; however, approximate Kolomogrov-Zakharov (KZ) solutions can be derived in a novel way under certain approximations. We numerically verify the theoretical findings in two cases: (i) In free-evolution cases, although the discrete (nonlinear) spectrum remains unchanged as guaranteed by an integrable system's isospectrality, an initial arbitrary wavenumber spectrum quickly evolves into a thermo-equilibrium state, demonstrating the kinetic aspect of the system; (ii) in forced-dissipated cases, we find stationary power-law spectra that agree with the theoretical predictions.