By Erik Mickelson, Field Manager
The trail I maintain has issues with water channeling down the tread. What can be done to fix this?
It sounds like your trail has a lack of grade reversals. A “grade reversal” is not what happens when trail builders reduce the quality of their work by playing too many video games instead of studying for that test on bog-bridging. A grade reversal literally refers to a reversing, or changing the grade of a trail—going downhill to uphill, and then back downhill again (or vice-versa). In one word, a grade reversal is all about drainage.
Ideally, when you lay out a trail, grade reversals are built in. Just how frequent these reversals occur is a subject of debate, and I won’t try to answer that here, but I will talk about two types of post-hoc, or post-construction, add-on grade reversals: grade dips and rolling grade dips. The idea of both reversals is to shed water off the trail before a rill (a shallow channel cut into the soil by erosion) becomes a gully, and the trail washes into, or becomes, a stream.
A grade dip is the construction of a depression in the prevailing, or running, grade.
A rolling grade dip involves a grade dip as well as a “speed bump” of soil piled up on the downhill side of the dip. Ramp heights should be kept close to dip depths.
Grade dips with a half-circle shape are called “knicks.”
Rather than abrupt channels, grade reversals should be soft and smooth undulations that are almost unnoticeable while walking through them. Of course, achieving this is often easier said than done here in the rocky Northeast, where dips may be more practical than rolling grade dips. It’s important to also note that when grades start to exceed 15 percent, dips alone are more practical, as rolling grade dip ramps will only increase the steepness of the trail.
If grade reversals are built on side-hilled trails, the outflow can drain down the mountain. If the outflow is steeper than 15 percent, there’s a good chance it will be self-cleaning. Otherwise, like most after-the-fact drains, they’ll have to be monitored and cleared of debris. If there are large volumes of water exiting the drain, and/or grades exceed 25 percent, then adding some rip-rap (rock) to the outflow will help reduce the chances of the outflow eroding. For non-side hill trails, a channel and/or pit or sump can be added to the outflow so drainage water has a place to collect or disperse away from the trail.
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