Bridges, Boardwalks and Other Wetland Crossings
Sooner or later, many trails must cross rivers, streams, marshes or other wetland areas. For trails designed to provide hikers with primitive conditions or challenges, natural trail crossings can be left unbridged or undeveloped. However, most trails will be used by people seeking safer, or at least drier, recreational pursuits. Crossing structures increase the likelihood the trail will be used, and they protect stream banks and wetland environments. Trails through wet meadows and marshes are prone to a common pattern of breakdown in the local soil and vegetation. As trail use in these areas increases, trails become soggy and muddy and hikers tend to walk on the edge of the trail on drier ground. Gradually the trail widens as increasingly larger muddy holes develop.
All stream crossings have several common requirements. Trail crossings should be located at the narrowest, and most stable, spots along a river or stream. Straight sections of the waterway, away from curves where stream banks tend to erode, are the most stable areas. The stream channel needs to be well defined at the point of crossing, and a flat stream gradient is desirable to avoid high velocity water flow against the bridge. Slopes leading to the crossing on both sides of the stream need to be stable and well vegetated. Careful location of crossings is critical because of high construction and maintenance costs.
Boardwalks provide interesting access routes into swamps and similar large wetlands, and are often designed for aesthetic and recreational purposes. Elsewhere, crossing structures may be used to avoid poorly drained spots in trails. In the latter situation, you should consider relocating the trail around the wet spot if it cannot be drained into adjacent vegetation.
All crossings represent limitations to certain trail users, such as horses, bikes and motorized vehicles. Constraints include weight limits and slippery or poor (for tire or hoof) surfaces. Potential users must be considered in designing all crossing structures, and weight limits should be well in excess of maximum expected loading.
Crossing structures must be well anchored to stream banks, dry spots, or in the underlying soil in the case of boardwalks. Footings or mud sills are usually perpendicular to the crossing logs or planks, and anchored into stream banks with large rocks, long spikes or bars, or even concrete if the site is easily accessible. Wherever logs and planks are connected, spikes should be driven at least 3 in. into the second board or log, and countersunk if the spike would be a tripping hazard.
Railings should be used on all crossings that are more than 2 feet off the ground or above water, and are advisable on many lower structures. (See picture of crossing with railing) They are usually 36 in. high and firmly attached to the crossing structure. The trail tread on any crossing should be at least 12 in. wide, although some narrower treads are acceptable for single log bridges in marshes (where a slip will only muddy your feet).
Although a variety of structures might be used for crossing distances of up to 15 feet, the three most common are log crossings, short bridges and various rock structures.
Log crossings are basically one or more log stringers laid side by side, with a flattened top surface for the treadway. Stringers are usually at least 8 to 10 ft long. In "topped log" bridges, one side of logs 6 inches in diameter or larger is removed to provide a flat surface. Up to 1/3 of the diameter of the log is removed in this process. In "split log" crossings, larger logs are split in half to produce a flat surface. For both types, the ends of the stringers are supported by mud sills that are short logs (3 to 4 ft long, 8 to 10 in. diameter) or squared timbers. The sills are notched to hold the rounded lower surface of the logs, and the logs are attached to the sills with pins or spikes with at least 4-in. penetration into the sill.
Simple log crossings
(Courtesy of the Appalachian Mountain Club)
Small log cribs (logs stacked in a square pattern) can replace one or both mud sills if additional height is needed at one or both ends of the bridge. For better traction, tread surfaces can be roughened with pieces of wood nailed to the log surfaces, with tacked down chicken wire, or with tar and sand.
Short bridges, or catwalks (See Illustration), may be as simple as two logs laid across the water, 2 to 4 feet apart. They are anchored directly to the crossing banks at both ends, or may be attached to mud sills. Planks or small logs are then attached perpendicularly to the logs to form the walking surface. Large planks or beams can be used in place of the logs, and handrails may be added for safety or convenience.
Rock structures are frequently used for shallow stream and gully crossings. At stream crossings, a barrier of large rock is usually placed on the downstream side of the crossing, level with the trail tread. This barrier slows water flow over the crossing immediately upstream from it. Small rock, gravel, or flat stepping stones spaced no more than 2 feet apart, serve as the crossing tread. The rock barrier should be extended at least 12 inches into the banks on both sides to prevent water flowing around the ends and undercutting the crossing. A peeled log, extending 4 feet into each bank, could also be used to anchor the gravel tread. Large native rock placed on the downstream side of the log will help hold it in place. For shallow crossings, rock barriers may not be needed and stepping stones or a bed of gravel or rock may suffice for all except those who want a 100% guarantee of dry feet.
(Illustration of a Rock base shallow stream crossing)
Spans up to 40 feet can be crossed with log stringer bridges. Although single logs have been used, bridges more commonly use two or three logs supported at both ends by single base (mud sill) logs or cribs. Longer distances can be bridged with the use of midspan cribs. Minimum diameters of the stringers depends on the length of the span and tree species used for the logs. Tables in the Forest Service Trails Management Handbook and the Trail Manual of the Florida Trail define acceptable log diameters.
(Illustration of a simple log stringer bridge)
Once the logs are in place, which may require significant manual labor, winches or small pieces of equipment, the top surfaces are patterned or shaved to provide a bearing surface at least 2 in. wide. Decking from small split logs or rough sawn 2-inch planks are spaced to allow drainage and spiked to the logs with a minimum 3-inch penetration. If cribs are used for supporting the ends they should be constructed with logs that are at least 8 to 10 in. diameter, and the cribs should be filled with rock or soil as they are built. They must also be anchored on the outside with large rocks. Stringers are anchored to cribs or sills with large spikes or galvanized bolts. Handrails are advisable on all long bridges. One end of the stringers can be cabled to nearby trees to save the bridge in case of a flood. If both ends are cabled the pressure of both flood water and debris in it may destroy the bridge more quickly than if one end broke loose and the bridge swung against the bank.
For shorter spans (up to 24 feet), log stringers can be replaced by pressure treated 2 x 10 in. lumber, bolted together. Pressure treated 2 x 8 in. planking may be used for the walking surface. Diagonal braces (2 x 10's) on the underside of the frame will provide additional support and reduce lateral movement. Other bridge designs are described in various trail manuals. If you are considering construction of any large bridge it would be wise to contact local engineers, contractors, utility companies or even National Guard units for possible assistance with design or materials.
The two most common uses of boardwalks in Florida are for nature trails through swamps and marshes, and for beach ramps across sandy areas to waterfront activities. They may be constructed as permanent fixtures or in easily transportable sections to facilitaterelocation when necessary. Permanent boardwalks require pressure-treated posts, poles or piers be sunk into underlying soils or rock for stability.
(Courtesy of Dr. Dirk DeWaard)
Cross braces or beams connect pairs of posts (see end view in next figure). Planks laid perpendicular, and connected, to the cross braces may serve as the walkway (top view). Using this pattern with boards of 1- or 2-inch thickness, Dr. Dirk DeWaard, a Stewardship landowner in north Florida, constructed a 560-foot boardwalk at a cost of about $2 per linear foot. Working alone, he built the boardwalk in 12 ft. sections, completing two sections a day.
Basic boardwalk construction plan
(Courtesy of Dr. Dirk DeWaard)
Alternatively, only two planks might be used to connect adjacent pairs of posts and cross braces. Short planking laid perpendicular to the long planks would provide the walkway, and side rails could be added for safety. Size of posts, timbers and planks used in boardwalk construction will depend on the intended use of, and numbers of hikers on, the boardwalk.
Portable sections are constructed in a similar pattern except that the supporting vertical posts rest on the soil or sand surface and are only as long as necessary to raise the boardwalk the desired distance above the sand or water. Individual sections can be connected end to end with planks and large bolts, and should be small enough that two to four people can move them when necessary.
Boardwalks are especially useful where trails traverse large areas with year-round standing water. Elsewhere, trails may cross small areas that are only intermittently wet or have saturated soils without deep standing water. If a small wet area cannot, or should not, be drained, and trail relocation would be impractical, various types of rock trails will help stabilize the trail surface. Flat rocks set into mud no more than 2 feet apart provide dry and stable stepping stones. If a trail is to receive fairly heavy traffic, a more intensive use of rocks may be necessary. Rock treadways (illustrated in "Trail Construction"/ Maintenance section) are created with large flat rocks laid side by side.
If flat rock is not available, rock boxes can be constructed as log frames filled with rock. Boxes only need to be high enough to allow the trail to pass over a wet spot. If possible, spread soil as a fill material before rock placement. Finally, if no rock is available, drainage ditches can be dug along the side of the trail through the wet spot. Soil dug from the ditches is spread in the center of the trail to raise the tread out of the wet zone and create a "turnpike" in trail lingo.
Another simple crossing for saturated, but not regularly flooded, wetlands is a "bog bridge", consisting of logs extended end to end across the wet area. Logs should be topped or split to provide a flat walking surface and can lay flat on the ground or be mounted on notched mud sills. The crossings may be one or two logs wide. They do not require handrails since the tread is barely above the ground surface. Life expectancy may be 7 to 10 years with softwoods such as pine or cypress, and shorter with most hardwoods.
As with other parts of the trail system, all bridges and crossings should be checked at least annually; more frequent evaluation of bridges is recommended. Many of these structures represent the greatest safety hazard on a trail, and careful inspection and maintenance are essential. All connections should be checked for tightness. Lumber, planks and logs should be monitored for cracking or rot and must be replaced if there is any likelihood of the structural component failing in the near future. Periodic treatment of wood components with paint or preservatives every three to five years should substantially prolong the life of the structure. Steel beams, cables, or other hardware should be treated with rust-resistant paint.