  We present an experimental investigation of multiple scattering in
photonic-crystal-based topological edge states with and without engineered
random disorder. We map the spatial distribution of light as it propagates
along a so-called bearded interface between two valley photonic crystals which
supports both trivial and non-trivial edge states. As the light slows down
and/or the disorder increases, we observe the photonic manifestation of
Anderson localization, illustrated by the appearance of localized
high-intensity field distributions. We extract the backscattering mean free
path (BMFP) as a function of frequency, and thereby group velocity, for a range
of geometrically engineered random disorders of different types. For relatively
high group velocities (with $n_g < 15$), we observe that the BMFP is an order
of magnitude higher for the non-trivial edge state than for the trivial.
However, the BMFP for the non-trivial mode decreases rapidly with increasing
disorder. As the light slows down the BMFP for the trivial state decreases as
expected, but the BMFP for the topological state exhibits a non-conventional
dependence on the group velocity. Due to the particular dispersion of the
topologically non-trivial mode, a range of frequencies exist where two distinct
states can have the same group index but exhibit a different BMFP. While the
topological mode is not immune to backscattering at disorder that breaks the
protecting crystalline symmetry, it displays a larger robustness than the
trivial mode for a specific range of parameters in the same structure.
Intriguingly, the topologically non-trivial edge state appears to break the
conventional relationship between slowdown and the amount of backscattering.
