Geomorphic Effects of Roads

Roads affect geomorphic processes by four primary mechanisms:

• Accelerating erosion from the road surface by both mass and surface erosion processes.
• Directly affecting channel structure and geometry.
• Altering surface flow paths, leading to diversion or extension of channels onto previously unchannelized portions of the landscape.
• Causing interactions among water, sediment, and woody debris at engineered road-stream crossings.

These mechanisms involve different physical processes, have various effects on erosion rates, and are not uniformly distributed either within or among landscapes.

Mass Erosion

On steep forest land prone to landsliding, the greatest effect of roads on erosion rates is from increased rates of mass soil movement after road building. Mass soil movements affected by roads include shallow (three to several feet deep) debris slides, deep-seated (depths of tens of yards) slumps and earth flows, and debris flows (rapid channelized and fluidized movements of water, sediment, and wood). Of these, effects of roads on debris slides and flows have been the most extensively studied. Typically landslides have been inventoried using some combination of sequential aerial photography and ground verification. Accelerated erosion rates from roads because of debris slides range from 30 to 300 times the normal rate. The magnitude of road-related mass erosion differs with climate, geology, road age, construction practices, and storm history. Several studies in the Eastern United States show that landslides are driven more by storm magnitude and geology than by land use. A threshold of 5 inches of rain per day (Eschner and Patric 1982) and meta-sedimentary geology are associated with large debris slides in the Appalachians. Road drainage can cause small slides in road fills, but some major landslides originate on undisturbed forest land (Neary and Swift 1987, Neary and others 1986).

Road-related mass failures result from various causes: improper placement and construction of road fills and stream crossings; inadequate culvert sizes for water, sediment, and wood during floods; poor road siting; modification of surface or subsurface drainage by the road surface; and diversion of water into unstable parts of the landscape (Burroughs and others 1976, Clayton 1983, Furniss and others 1991, Hammond and others 1988, Larsen and Parks 1997, Larsen and Simon 1993). Effects of roads on deep-seated mass movements have been much less extensively studied, but cases of road building apparently accelerating earth-flow movement have been documented. Such movement can be caused by destabilizing the toe area or diverting water onto the earth-flow complex (Hicks 1982). Little is documented about the potential for increased mass failures from roads resulting from decay of buried organic material that has been incorporated into road fills or landings during road building. Anecdotal evidence, however, shows that failures occur after decay of the organic material.

Although mass erosion rates from roads typically are one to several orders of magnitude higher than from other land uses based on unit area, roads usually occupy a relatively small fraction of the landscape. Hence, their effect on erosion may be comparable to other activities, such as logging. Studies by Swanson and others (1981) in the Oregon Coast Range, for example, showed that although the unit-area increase in erosion from roads was 30 times greater than the increase from clearcutting, road-related landslide erosion accounted for just three times as much accelerated slide erosion in the watershed when the areas in roads and clearcuts were taken into account. Road and clearcut erosion were nearly equal in a study in the west side of the Cascade Range in Oregon (Swanson and Dyrness 1975). In the Klamath Mountains of southwest Oregon, erosion rates on roads and landings were 100 times those on undisturbed areas, but erosion on harvested areas was 7 times that of undisturbed areas (Amaranthus and others 1985). A related point is that only a few sites can be responsible for a large percentage of the total erosion. For example, major erosional features occupied only 0.6 percent of the length of roads studied by Rice and Lewis (1986).

Although road location, design, construction, and engineering practices have improved markedly in the past three decades, few studies have systematically and quantitatively evaluated whether these newer practices result in lower mass erosion rates (McCashion and Rice 1983).

Surface Erosion

Erosion from road surfaces, cut banks, and ditches represents a significant and, in some landscapes, the dominant source of road-related sediment input to streams. Increased sediment delivery to streams after road building has been well documented (Kochenderfer and others 1997, Swift 1985, Swift 1988). Rates of sediment delivery from unpaved roads are highest in the first years after building (Megahan and Kidd 1972) and are closely correlated with traffic volume on unpaved roads (Reid and Dunne 1984, Sullivan and Duncan 1981). Surface-erosion problems are worst in highly erodible terrain, particularly landscapes underlain by granite or highly fractured rocks (Megahan 1974, Megahan and Ketcheson 1996). In the eastern United States, poorly designed and managed forest access and county roads are major sources of sediment input to streams (Hansen 1971, Patric 1976, Van Lear and others 1995). Roads were identified as the major source of sediment in the Chattooga River Basin, where 80 percent of the road sources are unpaved, multipurpose roads (forest and county) (Van Lear and others 1995).  Sediment losses were largest during road building and before exposed soils were protected by revegetation, surfacing, or erosion control materials (Swift 1985, Swift 1988, Thompson and others 1996, Vowell 1985). Soil loss from skid roads in West Virginia ranged from 40 tons/acre during logging, to 4 tons/acre the first year after logging, to 0.1 ton/acre 1 year after logging was completed (Hornbeck and Reinhart 1964). Raw ditch-lines and roadbeds are continuing sources of sediment (Miller and others 1985), usually because of lack of maintenance, inadequate maintenance for the amount of road use, excessive ditch-line disturbance, or poorly timed maintenance relative to storm patterns (Swift 1984, Swift 1988).

Extensive research has demonstrated that improved design, building, and maintenance of roads can reduce road-related surface erosion. Key factors are road location, particularly layout relative to stream systems (Swift 1988, USDA Forest Service 1999), road drainage (Haupt 1959), road surfacing (Burroughs and King 1989, Kochenderfer and Helvey 1987, Swift 1984), and cut slope and fill slope treatments (Burroughs and King 1989, Swift 1988). Many studies show that surfacing materials and vegetation measures can reduce the yield of fine sediment from road surfaces (Beschta 1978, Burroughs and others 1984, Kochenderfer and Helvey 1987, Swift 1984).

Interaction of Roads with Stream Channels

Roads interact directly with stream channels in several ways, depending on their orientation to streams (parallel, orthogonal) and their landscape position (valley bottom, midslope, ridge). The consequences of these interactions, particularly during storms, include increased erosion rates and direct and off-site effects on channel morphology and drainage network structure, but these effects are complex and often poorly understood. Encroachment of forest roads along the main stream channel or floodplain may be the most direct effect in many watersheds. Poorly designed channel crossings also may affect the morphology of small tributary streams, as well as limit or eliminate fish passage. Indirect effects of roads on channel morphology include the contributions of sediment and altered streamflow that can alter channel width, depth, local gradient, and habitat features (pools, riffles) for aquatic organisms (Harr and Nichols 1993).

Roads in midslope and ridgetop positions may affect the drainage network by initiating new channels or extending the existing drainage network. By concentrating runoff along an impervious surface, roads may decrease the critical source area for headwater streams (Montgomery 1994). In addition, concentrated road runoff channeled to roadside ditches may extend the channel network by eroding gullies or intermittent channels on hillslopes and by linking road segments to small tributary streams (Weaver and others 1995, Wemple and others 1996). These effects of roads on the channel network have implications for slope stability, sedimentation, and streamflow regimes.