  Large scale coherent streaks are artificially forced in a well developed
turbulent boundary layer at $\Redstar \approx 1000$ using an array of
cylindrical roughness elements. Measures of the velocity field with particle
image velocimetry reveal the presence of well reproducible, streamwise
oriented, steady coherent streaks. We show that the amplitude of these coherent
streaks transiently grows in space. The position of the maximum amplitude is
proportional to the spanwise wavelength of the streaks and the most amplified
spanwise wavelength is of very large scale $\lz \approx 6 \dblo$. These results
are in good agreement with the recent predictions based on the optimal
transient growth analysis of turbulent mean flows.
