A New Way to Look at Forest Roads: the Road Hydrologic Impact Rating System (RHIR)

Posted on October 10, 2006
One of the greatest impacts of roads and (especially motorized) trails is their effect on the hydrology of natural landscapes, including the flow of surface and ground water and nutrients. These hydrologic effects are re­sponsible for changes to geomorphic processes and sediment loads in roaded areas (Luce and Wemple 2001).

Assessing Roads
I developed the Road Hydrologic Impact Rating (RHIR) system to provide a summary of forest road data and to compare forested ecosystems throughout the intermountain West. Forest roads are generally characterized as narrow, not covered with asphalt, lightly traveled, and remote (Forman and Alexander 1998). The RHIR system falls between a quali­tative environmental index and a quantitative, data-intensive procedure to assess the impacts of forest roads.

The RHIR form (see next page) provides a means to summarize and rate selected habitat variables. For each vari­able, levels are categorized as excellent, good, fair, or poor, with a corresponding score. The scores are added to give an overall rating ranging from 0-110, with zero being the worst and 110 being the best. Variables in the RHIR were based on the following metrics: road features, verge conditions, mass movements, and use. I selected variables that could be easily measured and used to identify features that may require man­agement. Precise definitions, and optimum levels based on a composite of several sources, are described below:
  • Width – The width of the road/trail from verge to verge, or the actual road size. A mean value of < 1 m is optimal.
  • Gradient – The difference in road elevation divided by distance between measuring points. Steeper gradients con­centrate surface runoff, create gullies and increase erosion.
  • Cut Slope Gradient – Cut slope erosion is related to soil stability and the amount of subsurface water intercepted by the road cut. Cutslope gradient may be the most influential site factor affecting mass movements.
  • Fill Slope Gradient – Fillslope erosion is also related to soil stability and the amount of subsurface water that is inter­cepted by the road cut. Fillslope failures are the most com­mon road-related mass movements and are directly related to this gradient.
  • Verge Zone – Width of the verge zone comprised of con­tiguous undisturbed vegetation. A vegetated verge provides overstory, scatters runoff, and reduces sediment transport.
  • Surface Material – Composition of the roadbed in gravel, sand, silt, and/or clay. Fine sediments are detrimental to aquatic systems due to highly erodible tendencies and ease of transport.
  • Construction Method – Most forest roadbeds consist of the soil materials from which they were cut. On improved roads, an overlay of gravel is put down to decrease rutting, decrease surface runoff speed, and limit erosion.
  • Use – The type of use: recreational use from mountain bikers, horseback riders, and off-road vehicles; or industrial use from heavy logging or mining vehicles.
  • Overstory Vegetation – Vegetation intercepts precipita­tion, reduces soil impacts, and helps maintain an open pore structure at the soil surface, promoting infiltration and inhib­iting runoff and erosion (Manning, 1997).
  • Density – Forman and Alexander (1998) propose road density (in km/km2) as a useful index of roads’ effects on faunal movement, population fragmentation, human access, hydrology, and fire patterns. For hydrologic effects, such as altered groundwater, the road density maximum is 2-3km/km2. However, for a naturally functioning landscape with large carnivores, the maximum is 0.6km/km2.
  • Erosion – All of the above listed items are factors that influence th degree of erosion. Erosion can lead to stream sedimentation which can greatly impact aquatic health.


Conclusion
Assessing all the impacts of roads is difficult, and many of their far-reaching influences are still to be observed, studied, or considered. While the RHIR system does not ad­dress all conditions or road/trail types encountered, inherent biases will be consistent, so a rating can provide a relative basis for comparison over time. Another value of this index is that variables with a poor rating are isolated and can be targeted for management. This index should not stand alone but should be used in conjunction with all available informa­tion to arrive at conclusions about the forest road network.

Road removal projects have been undertaken to restrict motorized access, increase hillslope stability, minimize ero­sion, restore natural drainage patterns, protect endangered plants and wildlife, and restore aquatic and wildlife habitat (Switalski et al. 2004). Road removal and recontouring can be the simplest and best option for the full recovery of hydro­logic function within a watershed.

 

— Ron Malecki is a graduate student in Environmental Studies at the University of Montana. For a full copy of his report, plese visit: http://www.wildlandscpr.org/roads/road_resources.htm.

To download the Road Hydrologic Impact Rating System score sheet, please go to page 13 of the PDF of the Autumn Equinox 2006 Road-RIPorter.

Literature Cited
Forman, R., and L. Alexander. 1998. Roads and their major ecological effects. Annual Review of Ecology and Systematics 29: 207-231.
Luce, C.H., and B. Wemple. 2001. Introduction to Special Issue on hydrologic and Geomorphic Effects of Forest Roads. Earth Surface Processes and Landforms 26, 111–113
Manning, J.C., 1997. The Applied Principles of Hydrology. Upper Saddle River, New Jersey: Prentice Hall, Inc.
Switalski, T.A., J.A. Bissonette, T.H. DeLuca, C.H. Luce, and M.A. Madej. 2004. Frontiers in Ecology and the Environment 2(1): 21–28.