Trails in Wet Areas (continued)

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.

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.

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