Saturday, July 28, 2012

The sustainable and safe trail

What are the physical attributes of a sustainable and safe forest trail? If there are already some hiking trails in your local forest landscape, how do you assess their safety and sustainability. Can the existing trails be used in an expanded system of trails in the landscape? We will look at some of the physical aspects of a safe and sustainable trail.

Specifications of trail users: The human bipedal hiker walks with a flowing motion on gradients up to approximately 10% gradient (one up in ten along). On gradients above 10% the flowing motion is replaced by climbing steps that direct greater force into the ground. Depending on the cohesion or how well the soil material in the trail surface stick together, the climbing steps start to dig, loosen and erode the surface. Most hikers prefer trails that provide the marching flowing motion rather than climbing steps. On hard rock surfaces, humans can walk up gradients of 100%. Humans fall over with ease, and muscles from the feet all the way up through the body core and head, shoulders and arms work to maintain the delicate balance. Roots, projecting rocks in the trail tread or surface can undo the delicate balance.

Horses are much heavier than humans and their iron and steel shoes can chew up a trail surface. Gradient control and reduced maximum gradients are needed to accommodate horses. The same applies to mountain bikes. Braking can erode and damage a trail surface.

The physical sustainability of a hiking trail is affected by the inter-play of the following in the landscape:

  1. Gradient control 
  2. Soil conditions
  3. Control of water
In the case of trail gradient and soil conditions, a trail can be unsustainable in loose sand at under 10% gradient. On wet organic soils, a trail can be unsustainable at 0% gradient.  Soils with some cohesion will support foot traffic from 10 % to 15% gradient. Gradients approaching 20% or greater are problematic in most soils. Trails on solid rock can go up to 100%.

Water flow is eroding this steep trail
Unlike roads, trails seldom are supplied with ditches and culverts, so the issue of water control and trail sustainability is very important. It is easy to envision water flowing down a steep trail gradient causing substantial erosion. Yes, this does happen and is a problem. Water flow on a trail can be a problem as it moves through the topography in the landscape in some unexpected places. On smooth uniform gentle side slopes of 25% or less a trail can make the only crease in the surface and catch water. Trail gradients on such gentle slopes should have a gradient of one quarter the gradient of the side slope. Steeper side slopes are often more irregular in micro-topography and a combination of out sloping of trail surfaces and the incorporation of gradient breaks or dips can be effective in directing water from the trail surface.

Most forest landscapes have rather elusive water springs, intermittent seasonal streams or topography that tends to draw and direct water. Some of these spots, even appeal to the inexperienced trail locator as a good way to go. A considerable water supply at a storm event and a steep trail gradient can be a recipe for a major rapid erosion event. Erosion on a forest trail in a protected area or park can match the scale of an erosion event on a forest road in a timber producing forest.
Abandoned forest roads in trail networks
In BC it is not uncommon to find abandoned forest roads that have become part of a forest trail network. Most forest roads have had the benefit of some forest engineering and may have good gradient control and sensible placement on the landscape. Most BC forest environments are resilient and the disturbed soil along an abandoned road may re-vegetate quickly giving a natural forest trail appearance.The rather idyllic forest trail on the right was once a forest road with a gentle gradient. Abandoned forest roads in trail networks can be unsustainable. An abandoned forest road on a steep side slope with  gradients above 10% may present problems. Such a road has a considerable cut into the slope often capturing considerable water flow.  Once the road drainage fail for lack of maintenance, storm water flows down the trail, sometimes causing rapid erosion.

The safe trail
The trail in the photo above right has a safe tread or walking surface. There are few projecting rocks or roots to trip the hiker. The trail in the first or top photo is unsustainable through erosion. The erosion has left many loose or projecting rocks on the tread to trip or derail the hiker.

The trail on the right is unsafe but it is not unsustainable. It has a gradient of under 5% and has not been subject to erosion by water. It was constructed with no regard to a standard for the tread or walking surface. Most Parks or protected area jurisdictions follow the 4 by 4 inch rule: No rocks greater than 4 inches should be within 4 inches of the surface. Foot traffic will rub away smaller finer material surrounding the rock and the rock projects up from the trail and becomes a tripping hazard. This is perhaps the most important rule in trail construction. This rule has seldom been followed in BC. Coarse soils, or soils with a good measure of rocks and boulders are most common on the hills and mountains of BC. A trail built without regard to the standard of the tread in these soils ends up unsafe. Trail construction in some of BC's protected areas was done under the notion of light foot print and minimal disturbance. This idea seems to be favored by some ecologists but the end result is a heavy foot print. The trail in the photo widens in the foreground where several large rocks are exposed. Hikers choose different routes around these obstacles and cause the trail to widen and the foot print increases. Unsafe trails with many tripping hazards are not attractive to hikers because they have to spend most of the time looking at their feet rather than surrounding nature.

Unsafe trails in coarse soils can be maintained and improved. However, it often involves a rebuilding effort greater than initial trail construction to achieve a result that is less satisfactory than having built the trail properly in the first place. The necessary fine soil materials have been lost from the trail

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