Vegetation altering hydrodynamic conditions of an open channel flow controls the exchanges of sediment, nutrients and contaminants. In this paper the turbulence structure of open-channel flow over a flexible grass vegetation is investigated. Velocity measurements were carried out, in a rectangular flume, using a 2D-ADV (Acoustic Doppler Velocimeter) for different values of stem concentration, water discharge, and bed slope. The experimental results showed that: 1) the maximum value of the longitudinal turbulence intensity occurs at a water depth close to the bent vegetation height; 2) the turbulence intensity damps for increasing values of the stem concentration; 3) above the vegetation height, an exponential distribution, similar to Nezu’s law, can be fitted to the measured longitudinal turbulence intensity profile. Finally, a new relative turbulence intensity distribution is proposed and fitted to the measured profiles. This relative turbulence intensity distribution needs the estimate of two parameters coinciding with the vegetation height and the water depth at which the maximum flow velocity occurs.