Driving in the winter is a fun-filled event. Between the uncontrolled skids across the snow, the sudden 360's on ice, and the mornings when the car simply won't start or when you forgot to plug in the block heater, it's almost enough to make one want to stay at home!
Then there is the propensity of our snow removal services to block driveways with drifts that make just getting on the road a major challenge. There are the rocks that get scattered with the dirt and gravel that is put down for traction. It really makes Winter the season for public transit!
But at this time of year, as if driving on the snow and ice are not bad enough, there are always those stretches of winter roads riddle with ripples.
I call this "washboard" because those ripples in the road are something akin to the surface of an old washboard and really do test the cold weather ability of your shock absorbers and your kidneys!
Of course, washboard is not just a winter phenomenon. Anyone who has driven a gravel or dirt road has more than enough experience with these menacing bumps.
The question is - where do they come from?
I was asked this the other day and my immediate thought was that they must somehow be related to the ripples that occur on sandy beaches. At a beach, the constant flow of water over sand picks up small particles for a brief moment. These get deposited again and result in a roughening of the surface.
Where surface is roughened, turbulent flow emerges which produces tiny horizontal whirlpools and these, in turn, pick up more sand. The scouring action digs out troughs and builds ridges which cause more turbulence and so on. The process is self-reinforcing or a feedback loop. Small ridges become large ridges; small valleys or dips become large valleys.
This might work well for a sandy beach under the influence of the tide and water but there isn't any water flowing over most washboard roads. If anything, the summer version of washboards occurs on really, really dry roads.
So, in search of an answer, I resorted to the internet, where I came across an article from the Geophysical Institute at the University of Alaska in Fairbanks.
A physicist, Keith Mather, has provided the "accepted" explanation. Washboard ripples start with an initial small bump or deviation in the road.
He built a small testing device which simulated various road conditions. His observations showed that the up-down motion of the tire as it goes over a bump results in a compression or dip right after the bump and a new bump. The next tire hits both the original bump and the new one from the dip and, in turn, it causes another dip and so on.
There is only one problem, though. He found that he could only create the washboard effect on very dry surfaces which explains summer washboard but not winter.
My thoughts are that the dirt and snow that covers our roads provide a similar but not identical medium. The covering of a snowy road is not a solid medium like the dirt of a summer road. Rather, it constitutes a fluid or "fluidized" medium.
It is a medium that is composed of particles of varying sizes which can move past one another. However, the motion in this case is restricted by the force applied to particles.
This form of movement is often seen in non-Newtonian liquids where the mobility of the molecules is limited. A classic example of this is corn starch and water or "magic mud".
The net result is that the snow/dirt mixture generates a wave action as a rolling wheel pushes down on it. The result is like the lump of dough that travels before a rolling pin when you roll out pastry.
But, because movement in dirt and snow is limited, the wave doesn't travel very far before giving out, creating a ripple. The formation of a new wave gives rise to the next one and so on. Each bump or ridge is self-reinforcing until we have a washboard winter road. Of course, this could use some experimental verification.
Hmmmm.... I guess I will have to do some driving this winter to check this out.
