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Trail Construction and Maintenance Notebook

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Trails in Wet Areas

Very few critters like to get their feet wet. There are a few exceptions, of course. Otters, beavers, goofy retriever dogs, motorcyclists, and most young children like to jump right in. But the rest of us--horses, llamas, and stodgy adult hikers--will often go to great lengths to avoid getting our feet wet or going for an unplanned swim. This section deals with a range of options for getting trail traffic from one side of wet ground to the other.

Because nearly every technique for fixing trails in boggy areas is expensive and needs to be repeated periodically, relocating the problem section of trail should be considered first. Scouting for suitable places to relocate trails and reviewing soil maps will be time well spent. The alternative route should bypass extensive boggy areas, be on a slope for better drainage, and have mineral rather than organic soil for its tread. Don't reroute a problem section of trail to another boggy piece of ground. If you do, the result will be two problem sections instead of one.

Sometimes, improved drainage will cure the problem. If so, this is a much less costly solution than other alternatives. Placing stepping stones is another technique for crossing bogs and streams. Stepping stones should be large, fairly flat on top, and partially buried in the streambed. Space the stones for the average stride, remembering that trails are for kids, too. It shouldn't be necessary to jump from stone to stone.

Moving up in cost and complexity, two types of structures--turnpike and puncheon--are commonly constructed to provide dry trails through wet or boggy areas. Using geosynthetics in combination with these techniques can often result in a better tread with less fill. Rock and fill causeways are popular in some areas where hardened trails are needed to cross fragile alpine meadows.

In situations where long spans are needed high above the ground, or for crossing streams, a trail bridge is usually needed instead of puncheon. Bridges require special designs fitted to each application. Engineering approval is needed before constructing either a standard or special design bridge.

Boardwalks are common in some parts of the country, particularly parts of Alaska and in the Southeast. These can range from fairly simple structures placed on boggy surfaces, to elevated boardwalks over marshes or lake shores, as are sometimes found at interpretive centers (Figure 25).

Photo of a group of people on a boardwalk
Figure 25--This boardwalk relies on pilings for support.
Helical earth anchors can also support the structure.

Let's look at some of these alternatives in more detail.

Improving Drainage

Although an area may appear perfectly flat, often it will have a slight gradient and flow of water. Drainage ditches and culverts can help ensure that water drains off the trail.

Generally, ditches are at least 300 mm (12 in) deep, have flat bottoms, and side slopes of 1:1. In many cases, the ditch can be extended beyond the wet area to capture water that might flow onto the trail (Figure 26).

Image of a ditch.
Figure 26--Ditches are a simple and effective way to drain
wet areas. Slope angle and depth vary with soil and
water conditions.

The simplest way to get water across a trail is to cut a trench across it. These open-top cross drains (Figure 27) can be reinforced with rocks or treated timbers to help keep them from caving in. These structures are not usually a good alternative because people and stock stumble on them. One way to reduce this risk is to make the ditch wide enough, at least 600 mm (2 ft), so stock will step in it rather than over it (Figure 28).

Image of a open-top cross drain culvert.
Figure 27--Open-top cross drains of culverts are not
often chosen because they are a hazard to
livestock, hikers, and bikers.

Photo of a wide cross drain and causeway.
Figure 28--Wide cross drain and causeway

An open drain can be filled with gravel. This is called a French drain. Start with larger pieces of rock and gravel at the bottom, topping off with smaller aggregate (Figure 29). French drains are often used to drain a spring or seep from under a trail bed.

Drawing of French Drain or Rock Underdrain: end view
Drawing of French Drain or Rock Underdrain: top view
Figure 29--Wrapping French drains with geotextile
helps prevent clogging. These are used to drain
low-flow springs and seeps.

Culverts are probably the best way to move small volumes of water across a trail. They have a big advantage over open top cross ditches because the tread extends over the culvert without interruption (Figure 30). Metal or plastic culverts can be installed easily, or the culverts can be constructed out of rock. Dig a ditch across the trail as wide as the culvert and somewhat deeper.

Photo of a culvert.
Figure 30--Culverts need to be installed at a sharp
enough angle to prevent sediment from being deposited.

Bed the culvert in native soil shaped to fit the culvert. There also needs to be sufficient drop, about 3 percent, from one side to the other so water will flow through the culvert without dropping sediment. The culvert needs to be covered with 150 mm (6 in) or more of fill. Cut the culvert a little longer than the trail width, and build a rock facing around each end to shield it from view and prevent it from washing loose. Often a rock-reinforced spillway will reduce headcutting and washouts.

The local trail manager may have definite preferences for metal, plastic, wood, or rock culverts. Synthetic materials may be taboo in wilderness. Plastic is often preferable to metal because it is lightweight, easy to cut, and less noticeable. Painting the ends of aluminum or steel culverts helps camouflage them. Use a culvert with a diameter large enough to handle maximum storm runoff and to be accessible for cleaning with a shovel or combination tool. Usually this means at least a 260-mm (9-in) diameter culvert.

Rock culverts offer a chance to display some real trail skills. Begin by laying large, flat stones in a deep trench to form the bottom of the culvert. In some installations, these bottom rocks may not be necessary. Then install large, well-matched stones along either side of the trench. Finally, span the side rocks with more large, flat rocks placed tightly together, enough to withstand the expected trail use. Cover the top rocks with tread material to hide and protect the culvert. These culverts, too, need to be large enough to clean out easily. The rocks should not wiggle (Figure 31).

Image of a rock culvert.
Figure 31--Rock culverts may also have stones laid along
the bottom of the culvert. The perfect rocks shown here
are seldom found in nature, except reportedly in
Southwestern sandstone.

Water flowing toward a culvert often carries a lot of silt. If the water slows as it goes under the trail, the silt may settle out and clog the culvert. A good way to help prevent this from happening is to construct a settling basin at the inlet to the culvert (Figure 32). This is a pit at least 300 mm (1 ft) deeper than the base of the culvert. It can be lined with rocks as desired. The idea is that sediment will settle out here, where it is much easier to shovel away, rather than inside the culvert.

Image of a settling basin.
Figure 32--Settling basins help prevent culvert clogging.


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Updated: 04/14/2014
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