  An emerging long obstacle placed in a boundary layer developing under a
free-surface generates a complex horseshoe vortex (HSV) system, which is
composed of a set of vortices exhibiting a rich variety of dynamics.
  The present experimental study examines such flow structure and characterizes
precisely, using PIV measurements, the evolution of the HSV geometrical and
dynamical properties over a wide range of dimensionless parameters (Reynolds
number $Re_h \in [750, 8300]$, boundary layer development ratio $h/\delta \in
[1.25, 4.25]$ and obstacle aspect ratio $W/h \in [0.67, 2.33]$).
  The dynamical study of the HSV is based on the categorization of the HSV
vortices motion into an enhanced specific bi-dimensional typology, separating a
coherent (due to vortex-vortex interactions) and an irregular evolution (due to
appearance of small-scale instabilities).
  This precise categorization is made possible thanks to the use of vortex
tracking methods applied on PIV measurements,
  A semi-empirical model for the HSV vortices motion is then proposed to
highlight some important mechanisms of the HSV dynamics, as (i) the influence
of the surrounding vortices on a vortex motion and (ii) the presence of a phase
shift between the motion of all vortices.
  The study of the HSV geometrical properties (vortex position and
characteristic lengths and frequencies) evolution with the flow parameters
shows that strong dependencies exist between the streamwise extension of the
HSV and the obstacle width, and between the HSV vortex number and its
elongation.
  Comparison of these data with prior studies for immersed obstacles reveals
that emerging obstacles lead to greater adverse pressure gradients and
down-flows in front of the obstacle.
