What makes laminar cascade break? Near my house there is a mall that have a cascade, which has a pratically constant flow, and doesn't seem to have perturbations (at least near the edge where water falls), between its two levels.
What botters me is thar that laminar flow that seem unperturbed, break in the middle after falling for a height of approximately 1.1m (it falls another 0.5m, before being colected by a pool, and, I think, repumped up). I don't know exactly how to explain that behavior but I think it must have to do with waves propagating on that filament of water and that make it break after falling a height $h$. Part of that flow is directed forward, and part is directed backwards (see the front view of the sketch below). From this view I can conclude also that there is a perfect left-right simetry (in relation to the vertical red line).
Update1:
I incluided also what I expect from the velocity vectors due to viscosity (with purple arrows on the sketch), similar of what we can see on Flow velocities due to viscosity
of the Laminar flow text. Unfortunately I don't have a real picture, or vídeo, of the local.

What is causing this effect? How can it be explained?


 A: As you indicate in your drawing, the velocity of the water is not homogeneous in the spanwise direction: because of inflow conditions, and also because of the friction with air, which will vary as an air circulation is created around the fall. Thus there are gradients of velocity in the system.
Because the water is in free fall, it is accelerated. At some height, a threshold is reached above which the flow becomes turbulent: the shear profiles destabilize. It is of course in the direction normal to the liquid sheet thats this shows most. And because of momentum conservation, you'll have an approximately equal mass of water going to the front and back of the sheet.
A: I think that the water in the sides of the cascade are denser than the middle because the water sources are closer to there. The sides begin to expand. The expansion of the sides sucks water from the middle (the density of the middle allows the expansion of the sides to take affect more rapidly outward) and the cascade parts.
