3. Basic Principles

Fortunately, the erosional impacts of road construction need not be passively accepted; there are a variety of practices available to reduce impacts. These can be summarized as our basic principles:

1. Minimize the amount of disturbance caused by road construction by: (a) controlling the total mileage of roads; and (b) by reducing the area of disturbance on the rod that are built.
2. Avoid construction in high erosion hazard areas.
3. Minimize erosion on areas that are disturbed by road construction by a variety of practices designed to reduce erosion.
4. Minimize the off-site impacts of erosion.

All four factors must be weighed to reduce total erosional impacts. This is important because stress on individual factors may not meet this goal. For example, a shorter road may have to be lengthened to avoid high erosion hazards. In this case, total erosional impacts may be minimized although the area disturbed is increased. Erosion control practices are certainly beneficial and considerable effort has been and should be devoted to their development and implementation. However, prevention, rather than control, usually is by far the most efficient means to reduce erosional impacts. Prevention can have an added benefit by avoiding possible irreparable damages or costly repairs that may exceed original construction costs.

The first basic principle emphasizes measures designed for erosion prevention rather than control. Minimizing road mileage and areas of disturbance help reduce erosional impacts considerably. This is particularly true on forested lands where the total length of road required is often regulated by the distance capabilities of logging systems and the silvicultural practices prescribed for the timber stands (Table 1).

1. Jammer = a small, truck-mounted skidder-loader, maximum reach about 6 meters.

2. High lead = a cable system that drags logs to the loading area, maximum reach about 200 meters.

3. Skyline = a cable system that suspends logs during transport to the loading area, maximum reach about 800 meters.

4. Estimated by Virgil W. Binkley, Pacific Northwest Region, U.S. Forest Service, Portland, Oregon, based on a maximum flying distance of about 1.5 kilometers.

Reductions in the area disturbed by road construction can also be made by careful road location and design. For example, use of flexible horizontal and vertical alignment standards during road location to avoid steep slopes can decrease the width of area disturbed considerably. To illustrate, total width of disturbance by a road 4 meters wide increases from about 7 meters on a 4% slope to 16 meters on a 60% slope; on a 65% slope the width increases to 32 meters (Figure 1). For a given slope, additional reductions in area disturbed can be made by minimizing road and ditch width and by maximizing the gradient of cut and fill slopes (assuming the steeper slopes do not increase other erosion hazards).

Figure 1. Width of disturbance (projected to a horizontal plane) caused by road construction as a function of hillslope gradient. Assumptions: road width, 4 meters; fill slope gradient, 67% (1.5:1); cut slope gradient, 200% (0.5:1); volume of material removed from cut = volume of material in fill (“balanced construction”).

The second basic principle for reducing road erosion impacts is another matter of prevention rather than control and consists simply of avoiding high erosion hazard areas. Examples of serious erosion problems caused by road construction in high erosion hazard areas are common, especially where landslide hazards are high. Here even minor location changes of 10 or 20 meters may eliminate a major erosion problem. Usually, problems of this type arise from adoption of, and strict adherence to, traditionally accepted road standards (such as, alignment standards for speed purposes) rather than providing some flexibility in standards to allow the road location to be adjusted to the site properties of the particular landscape in question.

The third basic principle is to reduce erosion on the areas that are disturbed by road construction. This is the traditional approach using a multitude of practices to help reduce erosion. Successful design of erosion control practices requires considerable knowledge of erosion processes, including the major type of erosion that is occurring and the individual factors that control erosion. To illustrate, little benefit results from attempting to stop mass erosion by mulching or surface erosion by installing subsurface drains. Likewise, mulching a road fill slope may have little value if improper design, failure of the road drainage system, or both, cause large quantities of water to flow over the fill.

The fourth basic principle for reducing erosional impacts is to minimize the off-site impacts of erosion that does occur. Essentially this amounts to reducing sediment delivery to stream channels by: (a) keeping disturbed areas as far from channels as possible, (b) providing a maximum of obstructions to catch and retain sediment before it reaches the drainage system, and (c) recognizing that the efficiency of a downslope area to deliver sediment varies considerably depending upon its form and structure.

< Back to Reducing Erosional Impacts of Roads

Last Modified: 08/20/2008