|MadSci Network: Engineering|
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.
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