![Erosion below a water bar. [erosion8086jpg 43 KB]](erosion8086.jpg)
Eroded tread directly below a water bar.
"I" am John Johnson; I have been building trail for over two decades, primarily in the foothills and mountains east of Seattle. I learned how to build trail from Bill Longwell and other members of the Issaqauah Alps Trails Club, from various classes, books, and pamphlets, and on my own by experimentation and study of trails over periods of years.
In the early days of the IATC we were not even trying to build "good" trail (what ever that might be). We were building a trail constituency, in part by leading hikes. For this we needed a lot of trails, fast. And so we built – a lot of trails, fast. We even had a motto that was quasi-official policy: "If [a certain trail] had been built to standards, it would be one mile long, instead of thirteen." Which was a bit of a joke, because (aside from some stuff about clearance heights and widths) I don't believe we had any standards. If we did they must have been confidential information, as I never saw any. One common trail maintenance handbook of the time said: "Know the class of trail you are maintaining, then adhere to the appropriate trail specifications"1 – but the only specifications given were for tread width and clearances.
But it seemed to me that we were spending a lot of time in repair and reconstruction of already built trail. And this wasn't ten or fifteen year old trail that was sagging a bit and needed some rejuvenation (at that time we didn't even have any ten-year old trail!), but, in some cases, trail just a year or two old. One year I kept a rough tally of of how much of my time was going into repair work, and was surprised to come up with a figure of 40%.
I also noticed a great deal of variability in how well our work lasted. In one particular case we rebuilt a switchback that had failed in less than a year of service. Several years later I saw the same switchback, and it was in excellent shape. So why had it initially failed in less than a year, but subsequently lasted for several years? It was the same folks, same techniques, same location. All that I could tell was we had made it a little longer and little wider – factors which back then seemed no more significant than the color of the tools we used. I suspected that routing – exactly where the trail was put on the ground – made a difference, but did not understand just why.
Eroded tread directly below a water bar.
Years later I figured it out. It turns out (here comes The Great Heresy) the cause of tread erosion is not running water, nor even (to any great extent) boot soles. (Water standing in puddles damages trail, but that is a different matter.) This needs to be explained at greater length elsewhere, but can be readily observed on any well-used trail: the severity of tread erosion correlates not with boot soles (which will be constant for a given trail segment), nor with running water (which may even be entirely absent for months at a time), but with the grade. Water is often blamed for erosion, but it is only the agent, not the cause. If a grade is not too steep water can, and does, flow across the tread, and even down, without causing erosion. No wonder our efforts sometimes failed: we were trying to fix drainage, when the real problem was grade.
Since I first wrote this page I have been somewhat amazed to see how strongly fixated many trailbuilders are on drainage. Folks, can we not see the evidence at our feet? Take a look at the adjoining picture, and note how loss of material has formed a trough about six inches deep. Is this due to water? Note also the rock waterbar just above. This waterbar is solid and competent, with an effective drain; there is no indication that water is flowing over it or around it. If this erosion is due to water, where did it come from?
It would seem more reasonable to attribute this loss of tread to boot soles – after all, this is on the very heavily used Mount Si trail. But the density of boot soles is pretty uniform along the length of a trail. So why isn't this trail uniformly eroded like this along its entire length? Why does loss of tread tend to happen on the steeper sections?
When I said at the top that I have studied trail, I mean that I have also measured it, and quantitatively correlated loss of tread (erosion) with steepness of grade. Yes, this is quite heretical, going against what all (?) of us were taught. But take a critical look, a close look, at the evidence at your feet, and see for yourself whether the sun orbits around "drainage".
Once it is accepted that the first requirement and principle of trail building is to avoid exceeding some critical grade (more on this in the main article), various difficulties in the conception and construction of switchbacks become amenable to a geometric approach. This is the basis of switchback theory, which I present in these pages.
Back to Switchback Theory and Principles.