  We discuss the hydrodynamic approach to the study of the time evolution
-induced by a quench- of local excitations in one dimension. We focus on
interaction quenches: the considered protocol consists in creating a stable
localized excitation propagating through the system, and then operating a
sudden change of the interaction between the particles. To highlight the effect
of the quench, we take the initial excitation to be a soliton. The quench
splits the excitation into two packets moving in opposite directions, whose
characteristics can be expressed in a universal way. Our treatment allows to
describe the internal dynamics of these two packets in terms of the different
velocities of their components. We confirm our analytical predictions through
numerical simulations performed with the Gross-Pitaevskii equation and with the
Calogero model (as an example of long range interactions and solvable with a
parabolic confinement). Through the Calogero model we also discuss the effect
of an external trapping on the protocol. The hydrodynamic approach shows that
there is a difference between the bulk velocities of the propagating packets
and the velocities of their peaks: it is possible to discriminate the two
quantities, as we show through the comparison between numerical simulations and
analytical estimates. In the realizations of the discussed quench protocol in a
cold atom experiment, these different velocities are accessible through
different measurement procedures.
