  Threads are the main constituents of prominences and are subjected to
oscillations that might be interpreted as MHD waves. Moreover, the
Kelvin-Helmholtz instability (KHI) has been reported in prominences. Both waves
and KHI may affect the thermodynamic state of the threads. We investigate the
triggering of turbulence in a thread caused by the nonlinear evolution of
standing torsional Alfv\'en waves as well as possible observational signatures
of this dynamics and the plasma heating. We modeled the thread as a radially
and longitudinally nonuniform cylindrical flux tube with a uniform axial
magnetic field embedded in a coronal environment. We perturbed the flux tube
with the longitudinally fundamental mode of standing torsional Alfv\'en waves
and numerically solved the 3D MHD equations to study the temporal evolution in
both ideal and dissipative scenarios. We also performed forward modeling to
calculate the synthetic H{\alpha} imaging. Standing torsional Alfv\'en waves
undergo phase-mixing owing to the radially nonuniform density. The phase-mixing
generates azimuthal shear flows that eventually trigger the KHI and, later,
turbulence. If nonideal effects are included, plasma heating is localized in an
annulus region at the thread boundary and does not increase the temperature in
the cool core. Instead, the average temperature in the thread decreases owing
to the mixing of internal and external plasmas. In the synthetic observations,
first we find periodic pulsations in the H{\alpha} intensity caused by the
integration of the phase-mixing flows along the line of sight. Later, we find
fine strands that may be associated with the KHI vortices. Turbulence can be
generated by standing torsional Alfv\'en waves in a thread after the onset of
KHI, but this mechanism is not enough to heat globally the structure. The
dynamics could be seen in high-resolution H{\alpha} observations.
