Shallow mixing layers (SMLs) behind a splitter plate were studied in a tilted rectangular open-channel flume for a range of flow depths and the initial shear parameter
${\lambda = (U_{2}-U_{1})/(U_{2}+U_{1})}$, where
$U_1$ and
$U_2$ are streamwise velocities of the slow and fast streams, respectively. The main focus of the study is on (i) key parameters controlling the time-averaged SMLs; and (ii) the emergence and spatial development of Kelvin–Helmholtz coherent structures (KHCSs) and large- and very-large-scale motions (LSMs and VLSMs) and associated turbulence statistics. The time-averaged flow features of the SMLs are mostly controlled by bed-friction length scale
$h/c_f$ and shear parameter
$\lambda$ as well as by time-averaged spanwise velocities
$V$ and momentum fluxes
$UV$, where
$h$ and
$c_f$ are flow depth and bed-friction coefficient, respectively. For all studied cases, the effect of shear layer turbulence on the SML growth is comparatively weak, as the fluxes
$UV$ dominate over the spanwise turbulent fluxes. Initially, the emergence of KHCSs and their length scales largely depend on
$\lambda$. The KHCSs cannot form if
${\lambda \lessapprox 0.3}$ and the turbulence behind the splitter plate resembles that of free mixing layers. Further downstream, shear layer turbulence becomes dependent on the bed-friction number
$S = c_f \delta _v /(4 h \lambda )$, where
$\delta _v$ is vorticity thickness. When
$S \gtrapprox 0.01$, the KHCSs are longitudinally stretched and the scaled transverse turbulent fluxes decrease with increasing
$S$. The presence and streamwise development of LSMs/VLSMs away from the splitter plate depends on the
$\lambda$-value, particularly when
$\lambda > 0.3$, resembling LSMs/VLSMs in conventional open-channel flows when
$\lambda$ is small.