MODELING EROSION FROM UNPAVED FOREST ROADS AT VARIOUS LEVELS OF GEOMETRIC DETAIL USING THE WEPP MODEL

Abstract

This study investigates the modeling question: What level of geometric detail is necessary for modeling surfaceerosion from a forest road network? We used the Water Erosion Prediction Project (WEPP) model to predict road sedimentyield from a road network in a timber sale on the Boise National Forest, Idaho. Using three levels of detail (high, intermediate,and low), the outsloped 4.4 km road was divided into road segments (145, 91, and 42) of different lengths. The model predictedthat 44, 44, and 42 metric tons per year of sediment would be produced from the 4.4 km road network based on the high,intermediate, and low detail methods, respectively. The model also predicted that a corresponding 0.036, 0.058, and0.061 metric tons of sediment per year would be delivered into the stream system. Most sediment was predicted to be depositedin the buffer. For the predicted sediment produced from the roads, the total difference among the three methods was less than5%, and regression analysis showed excellent correlation, with no practical differences among the three methods. For thequantity of sediment predicted to be delivered into the stream system from the road, the total difference among the methodswas 61%, 67%, and 4% for high vs. intermediate, high vs. low, and intermediate vs. low, respectively. There were no strongcorrelations between the three methods, and the regression analysis indicated significant differences. More deliveredsediment was predicted when using lower detail methods than when using higher detail methods. We conclude from the studyresults that the low detail method (i.e., considering only road grade reversal) can be used for input requirements of roadtraveled-way geometry, but that a high level of detail (i.e., dividing a road system into more road sections and consideringbuffer slope breaks and meanderings) is required for modeling the buffer slope geometry. Considering that the results werenot sensitive to road traveled-way but were sensitive to buffer geometry, we believe it is advisable to divide a road networkinto road segments based on buffer geometry as opposed to road traveled-way geometry.