Re: how are washboards (ripples) in dirt roads formed

How are washboards (ripples) in dirt roads formed?

I am not aware of any “scientific” study of the formation of dirt road 
washboarding. If we consider some of the common factors in how ground 
surface features are formed, however, I think we can speculate as to at 
least two of the major agents in washboard formation.

The main agent in the formation of washboards in dirt roads is running 
water.  When rain falls on the earth’s surface the majority of the initial 
water movement is over the ground surface in a process called sheet-wash. 
The initial water movement essentially has no channelization, however, as 
more water moves over the surface it tends to be focused in numerous small 
scale drainage channels.  When this water crosses a dirt road, these 
individual channels can be thought of as the initial washboards.  

After development of the washboards by rain water drainage, further 
development is probably caused by repetitive passage of vehicles and would 
mostly be due to the combined forces of vehicle weight and bouncing tires. 
 As can be imagined, the total effect could be significant. 
The actual process exerted by the vehicle wheels (such as erosion of high points and redeposition
in low points, as you have suggested in your question) may be road (or dirt
material) specific.