Skip to content U.S. Department of Transportation/Federal Highway AdministrationU.S. Department of Transportation/Federal Highway Administration
Office of Planning, Environment, & Realty (HEP)

Wetland Trail Design and Construction

USDA Forest Service logo   Table of Contents

Back | Next
Forest Service Technology & Development logo

Wetland Trail Structures - (continued)


A more environmentally friendly relative of the turnpike is the causeway, essentially a turnpike without side ditches (figure 18). Causeways filled with broken rock have been successfully used throughout the Sierra Nevada and elsewhere to create an elevated, hardened tread across seasonally wet alpine meadows. Often, multiple parallel paths are restored and replaced with a single causeway. Causeways create less environmental impact than turnpikes because they do not require ditches that lower the water table. In highly saturated soils the causeway could sink into the ground, a problem that geotextiles can help prevent.

Image of a causeway
Figure 18-Causeways create an
elevated, hardened tread across seasonally wet areas.

Improving Drainage

Dips or Ditches

Turnpikes and causeways interrupt the flow of water along and across the trail. You need to take measures to ensure that water flows away from turnpikes instead of saturating them. The tools to ensure that water flows away form turnpikes include: dips (or ditches), open drains, French drains or underdrains, and culverts.

Generally, dips are at least 12 inches deep, have flat bottoms, and sideslope ratios of 1:1. In many cases, the dip can be extended beyond the wet area to capture water that might flow onto the trail.

The simplest way to get water across a trail is to cut a trench across it. These open- top cross drains, or dips, can be reinforced with rocks or treated timbers to prevent cave-ins. These structures are not usually a good alternative because people and pack-stock stumble on them. One way to reduce this risk is to make the dip wide enough (at least 2 feet) so that packstock will step into the drain rather than over it (figure 19).

Image of a dip
Figure 19-Dips and ditches are a simple and effective way to drain wet areas.
The slope angle and depth vary with soil and water conditions. Stones help reinforce the dip.

An open drain can be filled with gravel. Such a drain is called a French drain. Start with larger pieces of rock and gravel at the bottom, topping the drain off with smaller aggregate (figure 20). French drains are often used to drain a seep or spring underneath a trail bed. A perforated or slotted pipe in the bottom of the drain reduces the amount of fill material needed and drains the area more effectively.

Image of a French drain
Figure 20-Wrapping French drains with geotextile helps prevent clogging.
French drains or underdrains are used to drain springs and seeps that have a low flow.


Culverts provide better and safer drainage across turnpikes than open drainage gaps or ditches.

Historically, culverts were built as small bridges, using stone or logs. Stone culverts require large stones and a skilled mason. Finding large stones is difficult. Today, dry stone masonry is almost a lost art. Well-built stone culverts can be extremely durable. Some stone culverts that are at least 100 years old are still in use.

Log culverts were used where stone (and stone masons) were not as readily available as logs. Construction crews may also have been more familiar with log construction. Log and timber culverts require less skill to build than stone culverts, but need maintenance. Expect to replace log culverts in 20 to 40 years, although they can last much longer.

Building a timber culvert is simple. We typically use 6 by 6 and 4 by 6 timbers, cutting them to any length suitable for the site and trail condition. Although old railroad culverts sloped the invert of their culverts, it may be difficult to do so in a wetland. In fact, it may be wise to build the invert level. This way the rising water from a creek or river can easily flow through the culvert as the water rises and recedes during a flood.

To build a timber culvert, two timber sills are placed in a shallow trench on each side of the trail, parallel to the trail centerline. The sills are pinned to the ground with at least two driftpins (figure 21). A timber wall is constructed on each side of the invert, resting on both sills. These walls can be as high or low as is suitable for the site condition. Notched 4 by 6 timbers are placed on the top of the walls to become the trail tread. The 4 by 6 timbers are spiked or pinned to the walls. Depending on materials available, the invert may be lined with stone or with planks resting on the sills.

Image of a timber culvert
Figure 21-Timber culvert. The invert (bottom of the culvert)
is often built level in wetlands because it is less likely to wash away in floods.

Normally, timber walls require deadmen going back perpendicular from the face of the wall into the earth behind the wall. The deadmen help keep the wall from collapsing. Because the walls for a timber culvert are only 6 to 8 feet long, installing deadmen is impractical. However, some bracing is needed to keep the walls from collapsing. For timber culverts with walls retaining 12 to 24 inches of earth, 4 by 6 timbers can be used for each side of the tread surface. The bottom of each 4 by 6 can be notched 1 inch deep at each end to fit over the two timber walls, forming a brace to support the walls. The area between these two 4 by 6s can be filled with 3-inch-wide planks (figure 22). Timber walls retaining more than 24 inches of earth should have notched 4 by 6s the full width of the tread surface.

Image of deck planks over timber culvert
Figure 22-Notching the deck planks on both ends of the culver (two planks
with notches are adequate for a wall up to 24 inches high) helps to brace the walls.

Most lumberyards carry 4 by 6s only in 8-foot lengths. For efficient use of the wood, the 4 by 6s should be used to span distances of 8 feet, 4 feet, or 2 feet, 8 inches. The 2-foot, 8-inch length would provide a 20-inch-wide open area and is the minimum width recommended for timber culverts. The minimum clear height of the culvert should be 7 inches. Rough sawn 2-inch -wide lumber is adequate for the entire tread surface of the 2-foot, 8-inch culvert (figure 23).

Image of wood culvert component sizes
Figure 23-Minimum size recommendations for a wooden culvert
designed to use standard timber sizes efficiently. Often larger culverts may be needed.
This culvert has no constructed invert.

The trail tread can be the surface of the top timbers of the culvert, or a curb can be added on each side and the space between the curbs can be filled with earth. The height and width of a timber culvert can be adjusted to fit site conditions and the expected volume of water. The spacing and number of culverts can also be adjusted to reduce the concentration of runoff and potential erosion problems. Timber culverts have an advantage over round pipe because the top timbers can be quite low and still provide the cross-sectional area of a large round pipe. Round pipe also concentrates runoff, while timber culverts spread the same volume of water over a wider area. Timber culverts work well with turnpike construction. Round pipe requires taller structures, a disadvantage.

Pipe is not a traditional or visually compatible material for some backcountry trail culverts. Also, it is difficult to clean a small-diameter pipe with a shovel. A typical shovel blade is 9 inches wide and requires many passes to clean out a 12-inch-diameter pipe. You can do the job more easily with the smaller-diameter combi (combination tool). Make the rectangular opening of a timber culvert 20 inches wide and it will be much easier to clean than a round pipe. Pipes as small as 2 inches in diameter have been used to carry surface runoff beneath turnpike (Appendix C compares round and rectangular culverts). Such pipes plug up within weeks and are impossible to clean. They should never have been installed.

Corrugated plastic culverts are sturdy, lightweight, and easy to cut. Although the culverts are not natural, the colors usually blend in with their surroundings better than steel. Plastic culverts have become quite popular for trail work. However, some trail designers feel corrugated plastic culverts look out of place and they may not meet Recreational Opportunity Spectrum guidelines in remote sites.

A timber culvert was part of the flood-damaged turnpike described earlier in this section. The turnpike and the culvert were diagonally pinned with drift pins. Both survived the flood.

Timbers or logs used in culvert construction should be naturally rot-resistant or treated wood. Using rot-resistant logs or treated wood will help meet sustainability criteria.

USDA Forest Service logo Top

Back | Next

Table of Contents

Cover Page
Forest Service Technology & Development logo
Updated: 4/14/2014
HEP Home Planning Environment Real Estate
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000