Spontaneous wavefronts in a flat flow

[gbell]

Hi everyone,

I'm puzzled by this phenomenon - when there's a thin sheet of water flowing in the street, it forms little wavefronts which appear periodic. Obviously the water gets thin enough to interact with the rough asphalt surface, but how does the random texture manifest an organized, periodic wave?

Can someone explain this?

 

[cesiumfrog]

The lower energy state for a plane of water is to be flat rather than rippled (unlike a line of water, for which forming droplets is energetically favourable) so it seems a reasonable question why it invariably forms coherent waves in nature.

 

[gbell]

Here's a theory...

The water spreads out so thinly that it stops flowing (sticks) to the surface. Like a drop of water on a mild slope that doesn't roll.

But the water is still flowing so it piles up behind the sticking point until it builds up enough weight to cascade over. This little pulse rushes forward and unsticks the next point down stream, reinforcing the wave.