Author: Gregory DiSanto, M.S. Candidate, Environmental Science & Policy
While this might be the first time you have heard of “trail science,” it is not a novel idea. Scientists and trail builders have been studying erosion on hiking trails for a long time, and a lot of thought goes into designing, constructing, and maintaining trails. With over 1200 miles of hiking trails, the White Mountain National Forest is a great laboratory for studying erosion.
The trails in the White Mountains – many of them built in the early 20th century – transport thousands of hikers per year from roadside parking areas to the summits of our ancient mountains, often taking the most direct route possible. Anybody who has hiked a few four-thousand-footers in New Hampshire is familiar with the impacts of erosion and the methods which are often used to limit erosion. The impacts that are most readily apparent are exposed rocks, roots, and soil. Besides being aesthetically unappealing, these impacts can create a trail surface that is loose or slippery. Once erosion on a trail becomes excessive, there are a few possible solutions – stop the flow of water by installing a water bar or other drainage feature, re-routing a section of trail, or armoring the surface of the trail with rock.
Familiar sights to hikers in the White Mountains: erosion on the Old Bridle Path hiking trail (left) and a rock water bar installed on the Dicey’s Mill trail to divert flowing water off of the trail.
Once the shape of a trail becomes concave, it acts as a channel for water, and erosion begins to make this channel deeper and wider. The following factors increase the rate of this process:
- Slope – the rise over run of a trail.
- Trail traffic can increase the rate of erosion – when hiking boots, bicycle tires, and horse hooves lose traction and slip, they detach soil particles which can then be transported downhill by flowing water. The steeper the slope, the more intense this factor is.
- The amount of water that enters the trail and flows down the trail increases erosion. Water can enter the trail by overland flow, groundwater seepage, or from a nearby stream during extreme flooding events – such as Hurricane Irene, which destroyed many miles of trails in the White Mountains in the summer of 2011.
- The uninterrupted length of the trail down which water flows – often equal to the distance between two water bars – increases the power of the water to transport soil.
- Some soil types resist erosion better than others. Rocks and roots embedded in the soil also help resist erosion.
Cross-section of an eroded hiking trail. The width (horizontal red arrow), depth (vertical red arrow), and cross-sectional area (shaded red area) are measurable dimensions of a trail that can be used to quantify erosion.
Preventing erosion from occurring in the first place requires an understanding of the factors that cause erosion. It is not always possible to limit all of these factors, so it is helpful to understand which factors are the most important and which factors feed off each other.
For my thesis project, I am examining the relationships between erosion on trails and the factors that cause the erosion. I want to understand the nature of each factor – is it linear or exponential? Is its effect compounded by other factors? This summer, I made measurements of erosion at over 100 sample locations on hiking trails and measured or estimated some of the factor variables – slope, length, traffic, soil type, and the amount of water which enters the trail.
Design and construction practices for trails are constantly evolving and improving. Trail building is both an art and a science – new techniques and practices are often developed based on anecdotal observations and trail builders’ experience. The small, but growing, field of trail science can help inform the official guidelines and best management practices that are published by organizations such as the Appalachian Mountain Club (AMC), U.S. Forest Service, and the International Mountain Bike Association (IMBA).
My thesis project has given me many great opportunities to engage with others in the trail building community about design and construction practices and to share anecdotal observations about what works and what does not. Hopefully all of this gives you something to think about next time you’re out hiking or mountain biking!