Corrugating or washboarding occurs on a wide variety of surfaces that are exposed to moving objects: overhead electric tram lines can become corrugated, steel railway lines (known as ‘roaring rails’), well traveled ski trails, even concrete- or bitumen roads, and in our case of interest gravel roads. In all cases it is a form of mechanical erosion that creates of pattern of periodic, transverse ripples in the surface.
Road corrugations can form on any surface that is dry and granular, all it takes is a wheel that travels faster than 7 kph. Since it is impossible the build a perfectly smooth road surface, there will inevitably be a bounce at some point, sending the wheel upwards. As it comes down and hits the surface, it sprays some material both forwards and sideways off the track, leaving behind a little crater. This crater then forms the valley of a corrugation. As the wheel comes up out of the valley, it jumps into the air again forming a ridge, and so the pattern of valley-and-mountain repeats itself.
There are many factors affecting the formation of the pattern, i.e. the suspension and hardness of the wheel, speed, wind etc. The pattern can also move in both directions along the surface over time, and the spacing of the pattern is determined by the speed of traffic, whereas higher speeds cause the ridges to be spaced further apart (increased pitch) and the valleys to deepen. As the speed increases, the wheel travels further with each jump, creating a more widely spaced pattern. As there is more energy in the faster moving wheel, the depth and spacing of the pattern increases. The opposite is true for lower speeds. Hard tires exacerbate the effect of corrugation as they break and scatter the aggregate more than soft tires do. De- and acceleration seems to have an effect too, which is why there are usually more corrugations around a bend in the road, or an intersection. Since frequent rainfalls reduce the formation of corrugations, they appear more frequently during the dry season or drought
As corrugations form on any kind of dry and granular surface subjected to vehicular traffic, there is unfortunately little that can be done to prevent them. They can be leveled out by grading the surface with a grader, a set of tyres chained together or even a tree dragged over the road acting as a giant broom, all of which are time and cost intensive exercises. Only setting the speed limit to 5 kph would prevent corrugations from forming - which isn't feasible either considering the size of the park.
To claim this Earthcache you have to send me the answers to the following questions:
1. Find corrugations and estimate the length of the corrugated section. Take a reading with your GPSr and add the location to your log.
2. Measure or estimate the distance between the ridges of the pattern as well as the depth of the valleys
3. Describe and measure or estimate the size of the material that makes up the ridges of the corrugations
4. Give other examples of erosion forming patterns in geological features on earth.
Please note:
As you may not encounter corrugations directly at the given coordinates, you may increase your area of observation to the length of the S36 or adjacent gravel roads.
Please be advised that you may not leave your vehicle except for demarcated get-out points and that the speed limit on gravel roads in the park is 40 kph
