Level of detail necessary for predicting forest road surface erosion

Rhee, H.; Foltz, R.B.; Elliot, W.J.; Fridley, J.L.; Bolton, S.M. 1999. Level of detail necessary for predicting forest road surface erosion. Presented at the American Society of Civil Engineers 1999 International Water Resources Engineering Conference, August 7-11, 1999, Seattle, WA. Also, Appendix C in Rhee, H.; Fridley, J.L.; Bolton, S.M. 1999. Modeled outsloped forest road surface erosion using the Water Erosion Prediction Project (WEPP) model. Final Report for Contract INT-95124-RJVA. USDA Forest Service Rocky Mountain Research Station, Moscow, ID. September 1999.

Keywords: road

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Abstract: Increasing pressure on natural resources has extended forest land management activities into remote moutainous areas. Forest roads are an essential element for forest and watershed management. However, forest roads are also a major source of sediment production.

We used the Water Erosion Prediction Project (WEPP) model to predict sediment from the roads in a timber sale on the Boise National Forest, Idaho. The outsloped 4.4 km road was divided into road segments of different lengths, using three levels of detail (high, intermediate, and low). Results from each set of runs were assessed to determine what level of detail was necessary. For the high detail method, the road segments were identified by changes in road grade and azimuth angle. This resulted in 145 segments with an average length of 31 m. Buffer slope was determined from 40 ft (12 m) contours along the flow path, with an average buffer length of 370 m. For the intermediate detail method, the criterion was less than 2 percentage points of road grade difference between, resulting in 91 segments with an average length of 49 m. Buffer slope was measured where major slope breaks occurred along the flow path, with an average buffer length of 360 m. For the low detail method, the road segments were identified by road grade reversal. This resulted in 41 segments with an average length of 110 m. The horizontal distance and elevation difference between two points, road and stream, were measured for each segment. The average buffer length was 320 m.

The results of this study indicated that road grade reversal was a sufficient criterion for input information for the sediment produced from the roads. Buffer slope information required considering major slope breaks.

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