Building Trails Across Wetlands

A primer on wetland mitigation

ATV Trails, snowmobile trails, hiking and mountain biking trails.

Building new trails or connecting trails is not an easy task in Minnesota. We have water and lots of it, with over 10,000 lakes and 12.2 million acres of wetlands, the second highest in the lower 48 states behind Florida. When done properly, trails can be built across wetlands for motorized and non-motorized recreation.

The Value of Wetlands

Wetlands provide many values: improved water quality, sediment traps, wildlife habitat, carbon storage, floodwater retention, food production and groundwater filtering. In 1991, an important law went into effect called the Wetland Conservation Act (WCA). A key component of the WCA is the development of restored or created wetlands as replacement for wetlands that are drained or filled. 

In support of the WCA, the Ecological and Water Resources division of the Minnesota DNR has established a wetland review and conservation program to: 

• Coordinate the review of wetland replacement plans
• Provide technical assistance to local governments and landowners in developing wetland mitigatio
• Promote wetland conservation by providing science-based recommendations in the development of state wetland regulations, programs, and policies.

As an off-highway vehicle (OHV) rider, it is important to note that it is illegal to operate in a wetland. In Minnesota, conviction for carelessly and recklessly upsetting the natural and ecological balance of a wetland is a gross misdemeanor violation. Simply going off-trail, called trail braiding, to go around a water puddle in the middle of a trail could be impacting a wetland next to it. “Stay on Trail” is good advice, especially near wetlands.

Beginning the Trail Design Process

Beginning a trail design requires input from many resources. High on your list should be contacting a Certified Wetland Delineator. They will identify and map the wetland areas that your trail may impact. There are eight different types of wetlands in Minnesota, requiring a deep study in soils, vegetation, and hydrology to distinguish each type. The goal of every new trail is to avoid wetlands and consider all possible alternative routes. If you do need to impact a wetland, the Delineator will help work with you to develop a mitigation plan, or a plan that minimizes wetland impact. This plan will typically get reviewed by a Technical Evaluation Panel (TEP), that provides a forum to discuss site-specific interpretations of laws, rules, and technical data. They also provide an opportunity for gaining consensus on several primarily technical issues to make a recommendation to the decision-maker. 

Once your wetland mitigation plan is approved you will need to obtain a permit and purchase wetland credits, which is basically a program to create an equal amount of new or restored wetland to offset the wetland you are impacting.

The Puncheon Bridge Design

To connect trail segments and minimally impact wetlands, OHV clubs are choosing to construct puncheons. A “puncheon” is a type of rustic log or timber boardwalk that is built close to the ground. It’s a simple design consisting of sleepers, stringers, and decking used to sit on top of wetland vegetation, allowing the vegetation to grow over and through the design, acting as a tether to the surrounding surface. 

In some extremely wet areas, the puncheon design will float as the water rises. Sleepers are timbers set perpendicular to the direction of the walking or riding surface that contact the wetland surface. Stringers are placed on top of the sleepers, and act as support for the decking, and support for the load crossing the decking. The decking is the riding surface, which adds strength for the total load carrying capacity and structural integrity to the overall design.   

Puncheon wetland impact is calculated by the actual number of sleeper timbers that touch the wetland surface. For example, a full 6 in. x 6 in. x 10 ft. long timber has a surface area of 720 square inches or five square feet. For a 100-foot puncheon, using three sleepers every ten feet, or 30 total sleepers, your total wetland impact would be 150 square feet. Your wetland assessment credit would be based on 150 square feet. 

The Over the Hills Gang ATV Club in Outing, Minnesota, is in the process of constructing a new ATV trail that connects the City of Emily, located in Crow Wing County, to the Blind Lake Connector ATV Trail, located in Aitkin County. Aitkin County is well known for its many wetlands, which make up 51% of the overall county lands. The new 16-mile trail requires the construction of five separate puncheon-style boardwalks to connect trail segments, totaling 2,062 feet of new puncheon.

The puncheon design being used is a modified Aitkin County design. Instead of three stringers, it uses four to carry more weight, up to a 12,000 lb. machine such as a mini-excavator, skid steer, or two-yard gravel hauler. Clubs need to conduct routine maintenance on trail segments, sometimes only accessible by a puncheon, so load carrying capacity is a key design consideration. 

The sleeper and stringer timbers are 6 in. x 6 in. x 10 ft., made from S4S Southern Yellow Pine, which has the highest specific gravity of all common structural lumber species, providing superior fastener-holding properties and load-bearing capacity. S4S means it is ‘surfaced on four sides’ meaning actual dimensions are 5.5 in. x 5.5 in. x 10 ft. long. Foundation-grade, pressure-treated timbers are also recommended for longer life in a wet or submerged environment. The deck boards are 2 in. x 8 in. x 8 ft., pressure-treated lumber and spaced one inch apart to allow for vegetation to grow through the boards, and to allow water to run off. Each section is fastened together through the end sleeper timbers, using 3/8 in. diameter by 10 in. long Spax construction grade screws. Each stringer is fastened down to the end and middle sleeper timbers using a 5/16 in. x 10 in. long Spax screw. Pre-drilling may be needed depending on how wet the lumber is.

Learn More about Puncheons

Puncheons can be designed straight or with a curve, and with added sections for turnouts for ATVs that meet. This is a proven effective design that will last 25 years with minimal maintenance. Visit the ATV Minnesota website and look under the “Club Resources” tab to find puncheon design plans. You’ll also find the 2018 OHV Trail Workshop presentations discussing wetlands and wetland crossings.

We encourage all Trail Coordinators to send in their trail successes to share with others who are designing, maintaining, or improving our great Minnesota ATV trails.