  We adapt the fluid description of Fractional Quantum Hall (FQH) states, as
seen in (arXiv:2203.06516), to model a system of interacting two-component
bosons. This system represents the simplest physical realization of an
interacting bosonic Symmetry-Protected Topological (SPT) phase, also known as
the integer quantum Hall effect (IQHE) of bosons. In particular, we demonstrate
how the fluid dynamical boundary conditions of no-penetration and no-stress at
a hard wall naturally give rise to the two counter-propagating boundary modes
expected in these SPT phases. Moreover, we identify energy-conserving hydro
boundary conditions that can either create a gap in these edge modes or
completely isolate the edge states from the bulk, as described in (Physical
Review X 14, 011057 (2024)), where they are termed fragile surface states.
These fragile surface states are typically absent in K-matrix edge theories and
require bulk dynamics to manifest. By leveraging insights from hydrodynamical
boundary dynamics, we can further elucidate the intricate surface properties of
SPTs beyond the usual topological quantum field theory based approaches.
