The Missoula Technology and Development Center (MTDC) was asked to evaluate the use of floating bridges for trail crossings in very wet areas. This report outlines the basic designs of floating structures. It includes information about floating boat docks, floating bridge designs, anchorage systems, and devices that allow the dock to adjust itself to varying water levels.
Floating bridges are generally not feasible for equestrian and livestock use because the bridges move. Floating bridges should generally be a structure of last resort, used only for crossing ponds, marshes, swamps, bogs, or similar areas that are too wet for more traditional, less costly trail bridges or for other wetland trail construction techniques.
Floating bridges have been used in military operations for the past 2,500 years. Floating docks are more modern, but they are common in all parts of the United States. A dock is essentially a bridge connected to land at only one end. Design concepts and applications for floating docks are applicable for floating bridges. In this report the term "floating structures" will apply to either a floating bridge or a floating dock.
Floating docks are far more common than floating trail bridges. Many commercially produced floating docks are available (figure 1). These products are easy to purchase and assemble and are usually less expensive than building a dock from scratch. Most floating dock systems are easily adapted for use as a floating bridge and can be installed by a trained trail crew.
Figure 1--This commercially available modular
dock system is easily assembled using common tools
(photo by Paul Schrooten, USDI Fish and Wildlife Service).
Floating trail bridges can provide a viable, but limited, alternative to the traditional means of crossing wet areas. Traditional dock designs and quality building methods will produce an enduring and esthetically pleasing structure. Anchorage is a vital part of any design.
Floating structures are supported by buoyancy of the construction materials. The submerged portion of the structure must be significantly lighter than water. Any material that is impervious to water with a specific gravity lower than water could be used to support a floating structure. Anything from a piece of plywood nailed to floating logs to a $150 million interstate highway bridge supported by reinforced concrete pontoons qualifies as a floating structure. The size and complexity of a floating structure depends on its intended user and on the environment in which it is placed.
Floating trail bridges are appropriate in relatively few locations. A traditional bridge, puncheon, or boardwalk will be the preferred option in most locations. This report will describe situations where floating bridges may be a practical solution.
Floating bridges are usually installed to provide recreational access to a bay, lake, or river, or as a floating boardwalk for wildlife viewing. Floating dock material can be used to build a floating bridge, provided you have solid shoreline approaches and a secure anchoring system.
The site might have special needs:
- Rapidly changing water levels may require frequent dock adjustments, such as in a reservoir or in a river below a dam.
- Areas may be intermittently dry.
- Areas may be exposed to ice flows, debris, high winds, and heavy currents.
- Areas may be too wet for traditional methods but may not be suited for true floating structures.
Before considering a floating bridge, ensure that traditional methods of crossing wet areas will not work. The MTDC reports, Wetland Trail Design and Construction (0123-2833-MTDC) and Trail Construction and Maintenance Notebook (0023-2839-MTDC), provide information about conventional wetland trail construction. These reports discuss construction using bridges, boardwalks, turnpikes, causeways, puncheons, bog bridges, corduroys, and geosynthetics. In most cases, one of these techniques is more appropriate and will cost less than a structure that floats on the water.
If there is too much water for traditional techniques, floating structures should be considered. A floating bridge can be designed to carry trail bridge loads for relatively short distances. The types of environments where floating bridges are feasible include bogs, swamps, lakes, bays, and very slow-moving bodies of water. Conventional bridges are needed to span streams with substantial currents. Floating bridges work best in the following conditions:
- Low flow velocities, usually less than 0.25 feet per second. The upper limit of velocity depends on the structure's design and the anchorage. Many manufacturers can provide information to calculate the upper limit of flow velocity for a particular design. Military documents listed in the Additional Information section provide detailed discussion on this subject.
- No significant debris flowing in the stream.
- Exposure to no more than gentle wave action. Most designs with proper anchorage can withstand typical waves on lakes and waves caused by boat wakes.
- Minimal ice formation. Unless floating structures are designed specifically to withstand the rigors of icy conditions, most will need to be removed before freezeup.
The site should be mapped and inventoried before a floating structure is constructed. The inventory should include:
- Possible floating bridge locations
- Types and amount of expected use
- Existing recreational, commercial, scientific, or other forest uses
- Water levels (high and low), water depth, wave heights, and current direction and velocity
- Ownership boundaries
- Navigational and recreational hazards
- Prevailing wind direction and strength
- Land and aquatic flora and fauna (especially endangered or threatened species)
- Existing Recreational Opportunity Spectrum classification of the trail and surrounding area
- Environmental concerns
- Anchorage points
To locate the floating bridge in the best possible site, select the trail location last. The bridge is likely to be more expensive and difficult to site than the trail.
Trails need to be accessible to people of differing abilities. All trails do not have to be accessible to all people, but accessibility is to be considered for new trail construction and reconstruction of trails managed for pedestrian use. If a segment of a pedestrian trail has a bridge, and that trail segment is intended to be accessible, the bridge also needs to be accessible.
The Federal Access Board's draft report on Accessibility Guidelines for Outdoor Developed Areas provides general guidance on trail accessibility. It is available at: http://www.access-board.gov/outdoor/outdoor-rec-rpt.htm. The USDA Forest Service also has trail accessibility guidelines, intended for use on national forests. These draft guidelines are available at: http://www.fs.fed.us/recreation/programs/accessibility/.
Boat docks are required to be accessible. Accessibility guidelines for boat docks are available through the Federal Access Board at http://www.access-board.gov in Recreation Facilities Accessibility Guidelines, under the Boating and Fishing Facilities subsection. Call the Access Board's technical assistance unit, at 800-877-2253, for further assistance in locating these guidelines. If the dock is adjacent to a parking area, toilet facility, or other constructed element of the site, those elements must be accessible and connected by accessible routes.
The United States military has a wide variety of prefabricated bridge designs ready for deployment. Most military bridges are designed for rapid setup and disassembly so troops and equipment can cross rivers or streams.
For the most part, military floating bridges are big, heavy, and expensive. These structures would not be a good choice for Forest Service recreational use because of their limited availability and need for maintenance. In addition, these bridges would not fit well in a backcountry setting. More information is available in the Additional Information section.
Floating docks can be easily modified to serve as a floating bridge. Resorts use floating docks to allow access to swimming platforms. In Alaska, the Forest Service and U.S. Department of the Interior (USDI) National Park Service use bridges floating on calm water to access bear-viewing platforms. In addition, many State recreation departments use floating docks as fishing platforms.
Docks are an efficient way of crossing water barriers. They are designed for pedestrian traffic. With the right modifications (width, buoyancy, stability, and anchorage) they can be adapted for other types of traffic, such as off-highway vehicles (OHVs).