Crossing Streams and Rivers
Stream and river crossings present a challenge to trail managers grappling with a mix of user challenge, safety, convenience, cost, and esthetics. At one end of the use spectrum, a bridge can allow people with disabilities, toddlers, and those new to the outdoors to experience the trail with little risk. But bridges are expensive. Wilderness visitors who expect a challenge may prefer a shallow stream ford. During high water these folks may opt for a tightrope walk across a fallen log. Each kind of water crossing has consequences for the recreation experience and the lands being accessed. Choose wisely from the spectrum of options before committing present and future resources to any given crossing.
The Minimum Tool philosophy suggests that we get the job done with the least long-term impact while still meeting management objectives for an area. A few Minimum Tool questions for crossings are:
A shallow stream ford is a consciously constructed crossing that will last for decades with a minimum of maintenance (barring major flood or debris torrent) and will provide a relatively low challenge to users.
The idea behind a shallow stream ford is to provide solid footing, at a consistent depth from one bank to the other. Most fords are not designed to be used during high runoff, but are intended to be used when flows are moderate to low. A ford for hikers and "nontraditional" packstock (like llamas and pack goats) should not be more than 400 to 600 mm (16 to 24 in) deep (about knee high) during most of the use season. A horse ford (Figure 53) shouldn't be deeper than 1 m (39 in).
Figure 53--Build fords when the water is low.
Place stepping stones for hikers.
Fords should be located in wider, shallower portions of the stream. The approaches should climb a short distance above the typical high water line so that water isn't channeled down the tread. Avoid locations where the stream turns, because the water will undercut approaches on the outside of a turn.
The tread in the ford is level, ideally made of medium-sized gravel. This provides solid footing. The plan is to even out the flow through the ford so the gravel-sized material isn't washed away, leaving only cobble or boulders.
You can do this by arranging a level riprap of big rocks (like a miniature dam) or anchoring a log about 1 to 2 m (3 to 7 ft) downstream from the trail centerline. The idea is to evenly slow the water as it goes across the ford. This slowing effect can be enhanced by placing several rows of stepping stones or rocks upstream from the tread. These slow the water entering the ford and begin to even out the flow. Be sure these upper rocks are not too close to the trail to avoid a scouring effect.
On trails receiving motorized use, concrete planks or blocks placed in soft stream bottoms can strengthen the trail tread for a solid crossing.
Well-constructed shallow stream fords are almost maintenance free. Watch for deep spots developing in the crossing. Floods or seasonal runoff can wash away the approaches or parts of the dam. Debris can catch in the dam or stepping stone line and alter flow characteristics. Approaches can erode into jumpoffs or turn into boggy traps. Maintenance consists of retaining or restoring the design criteria of an even shallow flow with solid footing.
Bridges range from a simple foot log with handrails to multiple span, suspended, and truss structures (Figure 54).
Figure 54--Typical log stringer trail bridge.
|On national forests, all bridges require design approval from engineering before being constructed. A national standard trail bridge drawing is available, and some regions have standardized, approved designs for simple bridges.|
On hiking trails, foot logs can be used to cross streams where safe fords cannot be located or to provide access during periods of high runoff. Constructed foot logs consist of a log, sills, and bulkheads. The foot log should be level and well anchored. Notch the sill, not the log. The top surface should be hewn to provide a walking surface at least 250 mm (10 in) wide. Don't let the log or rails touch the ground. Remove all bark from logs and poles (Figure 55).
Figure 55--Log footbridge. The sill can be
notched to accommodate the logs, not vice-versa.
If the foot log is associated with a shallow stream ford, be sure to position the log upstream or well downstream of the ford. Logs immediately below the crossing can trap travelers who lose their footing in the ford. If you have handrails, construct them according to plan. Improperly constructed handrails are a big liability, because they are not strong enough.
Choosing the materials for a bridge is not a simple process. Even the use of native material for a simple foot log has consequences. For example, most untreated logs of a durable wood (like coastal Douglas-fir) have a useful life of less than 20 years. Yet a log that is big enough to support visitor traffic and winter snow loads may be 100 years old. Plus, the typical bridge has three to four stringers. Multiply this replacement-to-growth ratio by several replacement cycles and you see how it's possible to create a slow motion clearcut around the bridge site.
Imported materials are often used to extend a bridge's life. Pressure-treated wood, metal, concrete, wood laminates, and even "space-age" composites are being used in bridges. Many of these materials must be trucked or flown to a bridge site and the old materials hauled out. All this is really expensive. Yet, these costs may be less than the more frequent and more dangerous replacement of structures made from native materials. It's possible to mix-and-match steel or other "unnatural but hidden" components with wood facing and decking to achieve a natural appearance.
Unless your bridge is preassembled and flown right onto a prepared set of abutments, you'll end up moving heavy materials around the bridge site. Be careful to avoid trashing the site by allowing winch guylines and dragged logs to scar trees and disturb the ground. The damage done in a moment can last for decades.
Other types of bridges include multiple span, suspended, and truss structures (Figure 56). A two-plank-wide suspended foot bridge with cable handrails is more complex than it looks. Midstream piers for multiple spans need to be professionally designed to support the design loads and withstand expected flood events. It does no one any good to win the National Primitive Skills Award for building a gigantic bridge by hand--only to have it fail a year later due to a design or construction oversight.
Figure 56--Suspension trail bridge typical
of the Northern Rockies.
Bridges are expensive, so it makes sense to take good care of them. Check foot logs and bridges annually for problems. Loose decking, planking, curbs, or handrails should be repaired as soon as possible. Clean debris and organic material from all exposed wood surfaces on the bridge or supporting structures. Structural members should be checked for shifting, loose, or missing spikes or bolts. Approaches need to be well drained so water does not run onto the bridge.
Report any of the following problems to a qualified bridge inspector who can determine whether the bridge should remain open to traffic: rot; bent, broken, or disconnected steel members; large checks, splits, crushed areas, or insect damage in wood members; permanent sag or excessive deflection; erosion around abutments; broken concrete; concrete with cracks larger than 3 mm (1/8 in); or exposed rebar.
All bridge structures need to be inspected by a qualified bridge inspector at least every 4 years.