What causes the even pattern of ripples in a “washboard” gravel road?

Question

I finally stopped to look more closely. These ripples are about 10 inches wide ands look to be almost exactly uniform through long stretches of road. Where does this precision come from?

In: 879

Answer 1

The wavy appearance resembles ribs that form when fluids flow over finely distributed solids and relocate them superficially.