Why Does Rain Create Waves on My Street? - Investigating Fluid Mechanics

[Trying2Learn]

TL;DR Summary

Why does water running down a hill come in waves

Hello

(Perhaps this should be in physics, but perhaps it should be mechanical engineering (fluid mechanics).

I attach a picture of water running down the street near my house.

The street is a nearly constant 15 degree incline that goes on for about 1 kilo-meter with no speed bumps.

The pictures is taken during a light rain.

I realize the indentations (the channels in which the water is running) is caused by the tire indentations over the years.

But why do we see waves? (You can see them in the picture here). The water is falling as a constant, relatively thin rain, and yet I see these waves.

Could someone explain why this happens?

 

[anuttarasammyak]

It is soliton of non linear physics. If I remember correctly KdV equation describes it. Ref. https://en.wikipedia.org/wiki/Korteweg–De_Vries_equation

 

[Trying2Learn]

It is soliton of non linear physics. If I remember correctly KdV equation describes it. Ref. https://en.wikipedia.org/wiki/Korteweg–De_Vries_equation

OK... I see there is an explanation... It is beyond me.

But, in the spirit of Feynman (one must be able to explain it in words) can someone explain it in words?

 

[Baluncore]

But, in the spirit of Feynman (one must be able to explain it in words) can someone explain it in words?

For thin sheets of water the flow is laminar. As the depth of the sheet increases the upper layers travel faster. An irregular thickness results in the peaks traveling faster and increasing in depth as the peak overtakes and collects slower water. The peak becomes steeper until the wave breaks and the flow becomes turbulent. That turbulent wave tumbling down the slope is what your eye will see. Behind the wave only a thin film remains, until another wave overtakes it again.

 

[Trying2Learn]

For thin sheets of water the flow is laminar. As the depth of the sheet increases the upper layers travel faster. An irregular thickness results in the peaks traveling faster and increasing in depth as the peak overtakes and collects slower water. The peak becomes steeper until the wave breaks and the flow becomes turbulent. That turbulent wave tumbling down the slope is what your eye will see. Behind the wave only a thin film remains, until another wave overtakes it again.

thank you very much!

 

[Keith_McClary]

There was nothing obvious at the upstream end of the culvert to account for these "waves".

 

[Baluncore]

I suspect what you are seeing is the water analogy of a microwave "corrugated waveguide", fed by a steady flow. The corrugations can store water in each trough, which makes what is called a "slow wave structure". The depth of water increases until overflow of a corrugation triggers a single fast wave that forms and dumps water at a faster rate than the slow wave structure would normally flow.
https://en.wikipedia.org/wiki/Slow-wave_coupler
https://www.microwaves101.com/encyclopedias/slow-wave-structures
https://www.jstage.jst.go.jp/article/pfr/5/0/5_0_S2091/_pdf
https://digital-library.theiet.org/content/journals/10.1049/piee.1969.0043