BMP Field Manual
Chapter 6 - Forest Roads
- Background
- Planning, Location and Design
- Stream Crossing Design and Construction
- Road Construction and Drainage
- Drainage Structures
- Soil Stabilization
- Road Maintenance
Roads, skid trails and landings are all part of a forest transportation system. (Skid trail and landing BMPs are covered in Timber Harvesting). Roads connect the forest land to existing public roads. They provide forest access for such activities as managing timber, improving fish and wildlife habitat, fighting fires, and recreation.
Forest roads that are poorly located, constructed or maintained are the largest source of nonpoint source pollution from forest management activities. Roads over steep slopes, erodible soils or stream crossings hold the greatest potential for degrading water quality.
There are three types of forest roads. Identify the type of road system you need during your planning phase.
- Temporary roads. These are the most common type of forest road. They are designed and constructed for short-term use during a specific project such as timber harvesting. These roads are used only when the ground is frozen or firm. When the project is done, the temporary road is closed, all stream crossing structures are removed, and the road is naturally or artificially revegetated.
- Permanent seasonal roads. These are maintained as part of the permanent road system but are designed for use only when the ground is frozen or firm. These roads are generally narrower than permanent all-season roads, are built to lower engineering standards, and have minimal surface gravel.
- Permanent all-season forest roads. These roads usually have gravel surfaces and are designed for year-round use. However, there may be some restrictions on use at various times of the year.
For specific information on roads in wetlands, see Wetlands.
Decisions made at the planning stage will affect a road's construction costs, long-term maintenance needs, service life, and the amount of nonpoint source pollution it causes. Loggers and landowners should plan, locate and design the road system together.
- Plan road systems that minimize the number, width, and length of roads to limit the total area of the site disturbed. Remember to:
- consider future uses of the road system;
- coordinate development with adjoining landowners when possible; and
- use temporary roads where practical.
- Use existing roads when they provide the best long-term access. Consider relocating existing roads if doing so improves access and reduces environmental impacts. Reconstruct existing roads to the extent necessary to provide adequate drainage and safety. Do not disturb stable road surfaces.
- Select road locations that allow for drainage away from the road.
- Minimize the number of stream crossings.
- Identify optimum stream-crossing locations before locating the rest of the road. Optimum locations include straight and narrow stream channels with l ow banks and firm rocky soil. Roads should approach streams at the least gradient possible.
- Where possible, locate roads on well-drained soils.
- Locate roads outside riparian management zones except at stream crossings. For more information, see Riparian Management Zones.
- Road grades should not exceed 10%. If road grades greater than 10% are necessary, limit grade length to minimize erosion, or break the grade using drainage structures. See the Drainage Structures section. Graveling the road surface on steep grades can also help maintain stability. Note: Optimum road grades should be less than 5%.
- Locate roads to follow natural contours and to minimize cut and fills. Balance cut and fills to minimize the need for fill or removing excess materials.
A side-hill cross-section illustrating how cut materials, A, equals fill materials, B.
Road Grade % = Rise/Run x 100.
Stream Crossing Design and Construction
Operating equipment in or near perennial or intermittent stream channels may add sediment directly to streams. Stream crossings that are poorly located or constructed may erode streambanks.
As roads approach a stream crossing, proper road drainage is critical to avoid sedimentation in streams. Three common stream crossing structures are culverts, bridges and fords.
Stream crossings must be designed, constructed, and maintained to safely handle expected vehicle loads and to minimize disturbance of streambanks, channels and, ultimately, aquatic organisms. Consider streambed material, stream size, storm frequency, flow rates, intensity of use (permanent or temporary), and the passage of fish when planning crossings. The USDA Natural Resources and Conservation Service, your local land conservation department, or a private consultant can help with designing fords and culverts. To design a bridge, contact a private consultant or experienced contractor.
- General BMPs for Stream Crossings
- Use soil stabilization practices on exposed soil at stream crossings. Use seed and mulch and install temporary sediment control structures such as straw bales or silt fences immediately following construction to minimize erosion into streams. Maintain these practices until the soil is permanently stabilized. Refer to the Soil Stabilization section.
- Design, construct and maintain stream crossings to avoid disrupting the migration or movement of fish and other aquatic life. Bridges or arch culverts that retain the natural stream bottom and slope are preferred for this reason.
- Install stream crossings using materials that are clean, non-erodible and non-toxic to aquatic life.
- Install stream crossing structures at right angles to the stream channel.
- Minimize channel changes and the amount of excavation or fill needed at the crossing.
- Limit construction activity in the water to periods of low or normal flow. Keep use of equipment in the stream to a minimum.
- Construct a bridge or place fill directly over a culvert higher than the road approach to prevent surface road runoff from draining onto the crossing structure and into the stream.
- Divert road drainage into undisturbed vegetation, preferably outside the RMZ so that the drainage does not directly enter the stream. See Diversion Structures.
- Stabilize approaches to bridge, culvert, and ford crossings with aggregate or other suitable material to reduce sediment entering the stream.
- Anchor temporary structures on one end with a cable or other device so they do not float away during high water. Install them so they can be easily removed when no longer used, regardless of the season.
- Pipe Culverts for Stream Crossings
- Install pipe culverts long enough so that road fill does not extend beyond the ends of a culvert.
- Install permanent culverts that are large enough to pass flood flows and are a minimum of 12 inches in diameter. Culverts that are too small can plug up with debris and result in the road washing out or in flooding upstream. Wisconsin law states that the landowner and /or contractor are responsible to obtain a flood easement from any affected property owners upstream of culvert crossings that are not designed to pass the 100-year flood. The USDA Natural Resources and Conservation Service, your local land conservation department or a private consultant (i.e., engineer or registered land surveyor) can help with sizing culverts.
- Install culverts so there is no change in the stream bottom elevation. Culverts should not cause damming or pooling.
- Firmly compact fill material around culverts, particularly around the bottom half. Cover the top of culverts with fill to a depth of one-third of the pipe diameter or at least 12 inches, whichever is greater, to prevent crushing.
- Use riprap around the inlet of culverts to prevent water from eroding and undercutting the culvert. For permanent installations, use filter fabric under the riprap. In addition, consider using flared-end culvert sections for inlets.
- Keep culverts clear and free of debris so that water can pass unimpeded at all times. This is especially important in areas where beaver are present.
- Fords
- Use fords for crossing dry streambeds or where fording would cause minimal water quality impacts.
- Locate fords where streambanks are low.
- Streambed should have a firm rock or gravel base. Otherwise, install stabilizing material such as reinforced concrete planks, crushed rock, riprap or rubber mats on streambeds.
A permit is required to construct a ford or install a culvert or bridge across any navigable intermittent or perennial stream. A stream is navigable if it has bed and banks, and it is possible to float a canoe or other small craft in the waterway on a regular reoccurring basis - even if only during spring runoff. Streams identified on current U.S. Geological Survey (USGS) topographical maps (7.5 minute/1:24,000 scale) should be considered navigable. Other lakes and streams may be determined to be navigable by a Wisconsin DNR water management specialist. If you have a question about navigability, contact a Wisconsin DNR water management specialist.
For temporary stream crossings for skid trails, see the Stream Crossings for Skidding section in Timber Harvesting.
Place fill directly over a culvert higher than the road approach to prevent surface road runoff from draining onto the crossing structure and into the stream.
Diversion ditch to divert road runoff away from the road so the runoff does not directly enter the stream.
Install culverts so there is no change in the stream bottom elevation.
Installation of culverts. (Adapted from Montana Department of State Lands, 1992.)
Use riprap around the inlet of culverts. Also use geotextile filter fabric for permanent installations.
Road Construction and Drainage
The most effective method to control erosion on forest roads is to keep water from accumulating on the road surface. Fast-moving water can easily erode soil from road surfaces and ditches, but road erosion can be controlled when water drains off the road surface and is dispersed into vegetation and ground litter.
- Design and construct roads to remove water from road surfaces to keep the road dry and structurally sound. The following figure shows three common road designs: crowned, outsloped, and insloped. Install insloped roads with ditches and adequate cross drainage. Outsloped roads (usually outsloped 1-2%) are less expensive to construct and maintain; use them on roads with moderate gradients and stable soils.
- Construct stable cut-and-fill slopes that will revegetate easily, either naturally or artificially.
- Do not bury debris in the road base. It causes uneven settling that can lead to erosion and frost-heaving that creates mud holes.
- Compact the road base material or allow it to settle before using the road to reduce the amount of water that soaks into it. This will increase the road's carrying capacity, reduce road maintenance and reduce erosion.
- Surface the road with gravel where steep grades, erodible soils or high-traffic volume make the potential for surface erosion significant.
- Locate gravel pits outside RMZs, using proper location, development, and soil stabilization practices to minimize erosion from the pits.
Typical road designs for drainage and stability.
Road-drainage structures include cross drains (pipe culverts, open-top culverts, broad-based dips and water bars) and water-diversion structures. Cross drains allow water from roadside ditches to move from one side of the road to the other.
- Where necessary to protect water quality, install road drainage structures to remove storm water or seepage from the road surface and ditches. Space these structures at intervals close enough to minimize waterflow volume and speed, avoiding ditch erosion. As road grades increase, use drainage structures more often.
- Where necessary, provide erosion protection for outflows from road drainage structures to minimize erosion and disperse the water, allowing it to soak into the soil. Riprap, mulch and/or seeding may be necessary (see the Soil Stabilization section).
| Recommended distances between drainage structures on forest roads and skid trails. | ||
| Road Grade % | Distance between waterbars (feet) | Distance between broad-based dips and cross-drain culverts (feet) |
|---|---|---|
| 1 | 400 | 500 |
| 2 | 250 | 300 |
| 5 | 130 | 180 |
| 10 | 80 | 150 |
| 15 | 50 | 130 |
| 25+ | 40 | 110 |
- Pipe Culverts for Cross Drains
- Install pipe culverts to provide cross drainage on road grades at regular intervals immediately above steep grades, below bank seepages, and where water will run onto log landings or forest roads.
- Install pipe culverts long enough so that road fill does not extend beyond the end of a culvert.
- Install cross drain pipe culverts at grades at least 2% more than the ditch grade and angled 30 to 45 degrees to improve inlet efficiency.
- Select the size of cross-drain culverts according to the size of the road and area drained by the ditch. To avoid clogging, permanent culverts should be at least 12 inches in diameter. The USDA Natural Resources and Conservation Service, your local land conservation department, or a private consultant (i.e., engineer or registered land surveyor) can help with sizing culverts.
- Install pipe culverts on a surface of compacted granular material. Firmly compact fill material around culverts, particularly around the bottom half. Cover the top of the culvert with fill to a depth of one-third of the pipe diameter, or at least 12 inches (whichever is greater) to prevent crushing.
- Use riprap around the inlet of culverts to prevent water from eroding and undercutting the culvert.
- Open-Top Culverts
- Install open-top culverts to provide cross drainage immediately above steep grades, below bank seepages, where water will run onto log landings or forest roads, and on road grades at regular intervals.
- Clean open-top culverts frequently since they easily fill in with debris.
- Broad-Based Dips
- Construct broad-based dips deep enough to provide adequate drainage and wide enough to allow trucks and equipment to pass safely.
- Place a surface of crushed stone or gravel on the dip and mound for soils and conditions where rutting may occur.
- Water Bars
- Place water bars at a 30 to 45 degree angle with a cross drainage grade of 2%.
- Diversion Structures
- Construct diversion ditches so they intersect the roadside ditch at the same depth and are outsloped 1% to 3%.
Cross-drain culvert.
Open-top culverts provide cross drainage and road surface drainage and are usually installed on seasonal or temporary roads.
Open-top log culvert.
Broad-based dips can provide cross drainage and road-surface drainage for roads and skid trails with a gradient of 15% or less. Broad-based dips can be used instead of culverts, usually at lower cost and with lower maintenance. Dips are not used for draining seeps, or for intermittent or permanent streams.
Broad-based dip.
A water bar is a shallow trench with a mound (or berm) which provides cross drainage and intercepts runoff from skid trails, recreational trails, firebreaks, or inactive or closed roads. Constructing a water bar will minimize erosion and provide conditions for natural or artificial revegetation.
Water Bars.
Diversion ditches, or berms, divert water away from roads and side ditches, and channel it into vegetation. These structures are often used before stream crossings to ensure that water will be diverted into vegetation and not directly into a stream, lake or wetland.
Soil stabilization practices are used where soil is exposed and natural revegetation is inadequate to prevent soil erosion and subsequent sedimentation into streams, lakes and wetlands. This occurs during road construction and when the road system is being used (active) or is closed (inactive). Practices include mulching, seeding, and installing sediment control structures. References include the Wisconsin DNR's Wisconsin Construction Site Erosion Control Handbook and Chapter 10 in the Wisconsin Department of Transportation's Erosion and Sediment Control Facilities Development Manual. To obtain these references, see Sources for Help. Contact a Wisconsin DNR forester or the USDA Natural Resources and Conservation Service for more information.
It is always more efficient and cost effective to prevent erosion than it is to repair damage after the fact.
- Mulching and Seeding
- Use mulch and/or seed where necessary to minimize soil erosion into streams, lakes and wetlands.
- Sediment Control Structures
- Install sediment control structures where necessary to slow the flow of runoff and to trap sediment until vegetation is established at the sediment source. Sediment control structures include straw bale fencing, silt fencing, and sediment traps.
- Maintain, clean, or replace sediment-control structures until areas of exposed soil are stabilized.
Mulch, such as straw, woodchips or bark, retains soil moisture, important for seed germination, and protects the soil surface from erosion due to runoff and raindrop impact. Mulch can be used to: (1) promote natural revegetation or (2) protect seeds that have been spread over an area. If you seed, apply mulch immediately afterward. Netting may be necessary to hold mulch in place on steep slopes or on areas where water flow concentrates.
Seed mixtures should include fast growing species for quick soil protection plus perennial species for longer soil protection until native vegetation returns to the site. Recommendations for seed mixes that are best for specific regions in Wisconsin are available from your local Wisconsin DNR wildlife management specialist and USDA Natural Resources and Conservation Service office. However, a suggested generic seed mixture, applied at the rate of 31 lbs./acre, is as follows:
| White Dutch clover | 8 lbs./acre |
| Perennial rye grass | 5 lbs./acre |
| Annual rye grass | 8 lbs./acre |
| Creeping red fescue | 10 lbs./acre |
| Total | 31 lbs./acre |
An acre equals 43,560 square feet.
A list of native seed mixtures, including prices and availability, is available from the Wisconsin DNR's Bureau of Endangered Resources.
Straw bale fencing to slow runoff and trap sediment for sheet flow or channelized flow.
Silt fencing to slow runoff and trap sediment primarily for sheet flow, not channelized flow.
A sediment trap to slow runoff and trap sediment for channelized flow.
Roads must be well maintained or water quality protection structures may quickly degrade. For both active and inactive roads, follow BMPs in the Soil Stabilization section.
- Active Roads
- Inspect the road system at regular intervals, especially after heavy rainfall, to detect problems and to schedule repairs.
- Clear debris from culverts, ditches, dips and other drainage structures to decrease clogging that can lead to washouts. Place the debris where it cannot be washed back into these structures or into open water.
- Keep traffic to a minimum during wet periods and spring breakup to reduce maintenance needs.
- Shape road surfaces periodically to maintain proper surface drainage. Fill in ruts and holes with gravel or compacted fill as soon as possible to reduce erosion potential.
- Remove berms along the edge of the road if they will trap water on the road.
- When dust control agents are used, apply them in a manner that will keep these compounds from entering lakes, streams and groundwater. Consult a qualified road engineer from the County Highway Commissioner's office or Wisconsin Department of Transportation for assistance in selecting the appropriate chemicals and amounts. Note: It is illegal to spread oil on roads, land or water in Wisconsin.
- Inactive Roads
- Remove all temporary drainage and stream crossing structures.
- Shape all road system surfaces to maintain proper surface drainage, if necessary.
- Install water bars where necessary. See the Water Bars section and follow the recommendations in Drainage Structures.
- Inspect and maintain road surfaces, permanent drainage and stream-crossing structures (ditches, culverts, bridges, etc.) to minimize erosion.
When forest roads are inactive for extended periods, closing the system will help to protect the road surface and the water quality protection structures. Consider erecting a barrier to traffic such as a gate or berm, and post "Closed" signs at the entrance of temporarily closed roads. Stating the length of time and/or reason for closure, and inviting acceptable uses may be helpful to assure compliance.
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