Proper Road Design Minimizes Stream Impacts
Roads that have been properly located and constructed need not threaten the water quality of streams. In fact, the entire process of designing, constructing, and maintaining forest roads can be looked at as a creative challenge that combines technical knowledge, experience, common-sense, and good judgment to achieve the desired goal: an efficient, low-cost, low-impact road that has a minimal effect on a watershed.
Poorly planned roads, on the other hand, can cause excess runoff and erosion, leading to sedimentation that can pollute water supplies, increase flooding potential, and trigger landslides. Aquatic life may be threatened and riparian vegetation damaged, resulting in loss of food and cover for fish and wildlife.
In choosing the best road design, a number of factors need to be considered. The physical environment (slope, soils, and drainage) will determine the best choice for road location. Avoid wetlands as they are highly sensitive to disturbance and require special protective measures. Efforts should also be made to minimize stream crossings (bridges are the least damaging choice here). Buffer strips of undisturbed vegetation should be left between roads and nearby streams to act as a filter to catch any sediment from road runoff. Time construction activities for the dry season to reduce soil erosion and allow vegetation to become established before the rains. Other important factors include safety, intended use, cost, ability to maintain the road, and regulatory constraints.
Adequate road drainage is a vital consideration in good road design. Road drainage includes runoff from both the road surface and hillslope.
Culverts are only one of several alternatives for providing adequate road drainage. A culvert that is not properly maintained or is undersized can easily blow out, leading to greater problems.
Insloped and crowned roads drain runoff to the inside of the road bed, often into a ditch. This concentrated flow is then discharged through culverts.
Outsloped roads are typically less expensive to construct and less difficult to maintain than insloped roads. Rolling dips, depressions in the road grade, are most frequently used on outsloped roads to drain and disperse road surface runoff. Rolling dips are installed in the road bed as needed to drain the road surface and prevent erosion. They require very little maintenance if properly constructed.
Waterbars (or waterbreaks) are dips built at an oblique angle across the road with a berm at the end. Waterbars are high maintenance drainage structures that are effective only on roads with little or no traffic.
A thoughtful approach to road planning and maintenance will be rewarded with a healthier watershed.
Road Design Resources
- Handbook for Forest and Ranch Roads
This is the classic resource that has just about everything you need to know about roads. There is also a video and a DVD.
- A Landowner's Guide to Building Forest Access Roads
- Forest Roads Manual (Oregon Department of Forestry)