Recent findings related to measuring and modeling forest road erosion

Sediment is the greatest pollutant of forest streams. In the absence of wildfire, forest road networks are usually the main source of sediment in forest watersheds. An understanding of forest road erosion processes is important to aid in predicting sediment delivery from roads to streams. The flowpath followed by runoff is the key to understanding road erosion processes. On rutted roads, the flowpath follows ruts until a cross drain structure or change of grade is encountered, leading to considerable sediment delivery. Insloping roads to bare ditches can lead to ditch erosion, but if the ditch is graveled or vegetated, erosion is generally minimal. Outsloping a road minimizes the flow path length on the road, minimizing surface erosion, and runoff is dispersed along the hillside, minimizing delivery. If roads have low or no traffic, the road surface may become armored, reducing erosion rates by 70 to 80 percent. If there is no traffic, and a road becomes covered in vegetation, erosion may drop 99 percent, but the hydraulic conductivity of the road surface is only minimally affected. In many cases, forest buffers absorb road runoff, minimizing the delivery of road sediment to streams. Buffers are less effective in wetter climates in absorbing runoff and reducing sediment delivery. Cutslopes can erode, making sediment readily available to be transported from roads. Graveling reduces the likelihood of rut formation, generally leading to a significant decline in road erosion. Traffic, however, can reduce the effectiveness of gravel by pressing it into the subgrade, or breaking it down. Paving a road will reduce road surface erosion, but may increase erosion in road ditches and on the hillsides or channels in a buffer area. If water is delivered from road cross drains to a channel, the chances of delivering sediment increases, as does the chance of entraining additional sediment through channel erosion. Empirical (USLE and SEDMODL) and process-based (KINEROS and WEPP) models have been applied to road erosion. SEDMODL and WEPP have been specifically adopted to model road erosion, and to account for the important detachment and delivery processes. A version of WEPP is available online that is receiving widespread use in the USA and throughout the world. This tool can either analyze single segments of road between cross drains, or can analyze up to 200 segments in a single run. Areas needing to be improved in road erosion are modeling the armoring process within a storm, developing the probabilistic capabilities of WEPP for road applications, adding mass wasting to the WEPP technology and expanding the WEPP road soil database.