  Collective flow observables are known to be a sensitive tool to gain insights
on the equation of state of nuclear matter from heavy-ion collision
observations. Towards more quantitative constraints one has to carefully assess
other influences on the collective behaviour. In this work a hadronic transport
approach SMASH (Simulating Many Accelerated Strongly-interacting Hadrons) is
applied to study the first four anisotropic flow coefficients in Au+Au
collisions at $E_{\rm lab}=1.23A$ GeV in the context of the recently measured
data by the HADES collaboration. In particular, the formation of light nuclei
is important in this energy regime. Two different approaches are contrasted to
each other: A clustering algorithm inspired by coalescence as well as
microscopic formation of deuterons via explicit cross-sections. The sensitivity
of directed and elliptic flow observables to the strength of the Skyrme mean
field is explored. In addition, it is demonstrated that the rapidity-odd $v_3$
coefficient is practically zero in this energy regime and the ratio of
$v_4/v_2^2$ is close to the value of 0.5 expected from hydrodynamic behaviour.
This study establishes the current understanding of collective behaviour within
the SMASH approach and lays the ground for future more quantitative constraints
on the equation of state of nuclear matter within improved mean field
calculations.
