A two-layer, shallow-water frontal model on an f-plane is used to study the nonlinear evolution of frontal waves. The fluid is confined to a periodic channel with parallel vertical walls. It is found that, at an advanced stage in the evolution of frontal waves, small-scale disturbances develop along the cold front while the warm front evolves in a smooth fashion. It is shown that the motion field associated with the primary low advects kinetic energy and low potential vorticity into the cold-frontal region. That kinetic energy is transferred by barotropic processes to the secondary disturbances at locations along the cold front where advection of low potential vorticity results in an enhancement of the horizontal shears. On the other hand, kinetic energy is removed from the warm-frontal region, which remains undisturbed.