Decay of small energy solutions in the ABCD Boussinesq model under the influence of an uneven bottom

Abstract: The Boussinesq $abcd$ system is a 4-parameter set of equations posed in $\mathbb{R}_t\times\mathbb{R}_x$, originally derived by Bona, Chen and Saut as first order 2-wave approximations of the incompressible and irrotational, two dimensional water wave equations in the shallow water wave regime, in the spirit of the original Boussinesq derivation. Among many particular regimes, depending each of them in terms of the value of the parameters $(a,b,c,d)$ present in the equations, the generic regime is characterized by the setting $b,d>0$ and $a,c<0$. If additionally $b=d$, the $abcd$ system is hamiltonian. Previously, sharp local in space $H^1\times H^1$ decay properties were proved in the case of a large class of $abcd$ model under the small data assumption. In this paper, we generalize [C. Kwak, et. al., The scattering problem for Hamiltonian ABCD Boussinesq systems in the energy space. J. Math. Pures Appl. (9) 127 (2019), 121--159] by considering the small data $abcd$ decay problem in the physically relevant variable bottom regime described by M. Chen. The nontrivial bathymetry is represented by a smooth space-time dependent function $h=h(t,x)$, which obeys integrability in time and smallness in space. We prove first the existence of small global solutions in $H^1\times H^1$. Then, for a sharp set of dispersive $abcd$ systems (characterized only in terms of parameters $a, b$ and $c$), all $H^1\times H^1$ small solutions must decay to zero in proper subset of the light cone $|x|\leq |t|$.