  Gravity wave turbulence is studied experimentally in a large wave basin where
irregular waves are generated unidirectionally. The role of the basin boundary
conditions (absorbing or reflecting) and of the forcing properties are
investigated. To that purpose, an absorbing sloping beach opposite to the
wavemaker can be replaced by a reflecting vertical wall. We observe that the
wave field properties depend strongly on these boundary conditions. Quasi-one
dimensional field of nonlinear waves propagate before to be damped by the beach
whereas a more multidirectional wave field is observed with the wall. In both
cases, the wave spectrum scales as a frequency-power law with an exponent that
increases continuously with the forcing amplitude up to a value close to -4,
which is the value predicted by the weak turbulence theory. The physical
mechanisms involved are probably different according to the boundary condition
used, but cannot be easily discriminated with only temporal measurements. We
have also studied freely decaying gravity wave turbulence in the closed basin.
No self-similar decay of the spectrum is observed, whereas its Fourier modes
decay first as a time power law due to nonlinear mechanisms, and then
exponentially due to linear viscous damping. We estimate the linear, nonlinear
and dissipative time scales to test the time scale separation that highlights
the important role of a large scale Fourier mode. By estimation of the mean
energy flux from the initial decay of wave energy, the Kolmogorov-Zakharov
constant is evaluated and found to be compatible with a recent theoretical
value.
