It seems they are called "roll waves" and happen naturally in this circumstance: http://cee.usc.edu/assets/027/88403.pdf http://cee.usc.edu/assets/027/88403.pdf

It seems they are called "roll waves" and happen naturally in this circumstance: http://cee.usc.edu/assets/027/88403.pdf http://cee.usc.edu/assets/027/88403.pdf "The uniform, free surface, turbulent water flows down an inclined, wide, rectangular open channel may become unstable spontaneously, if some sufficient conditions are satisfied. As a result, steady hydraulic conditions are interrupted, including the water depth (or pressure) and flow velocity. When the channel is sufficiently long, the small instabilities on the water surface will develop to a series of intermittent shock-like waves separated by sections of the gradually varying flow."