A series of 75 non-bordered plots was used to measure surface erosion on granitic road cuts on forest roads in the mountains of Idaho. Erosion data were collected for four years following road construction. Erosion rates for the first winter period after construction averaged about five times greater than the average of erosion rates for subsequent seasons. Both mass and surface erosion processes were observed on road cuts with mass erosion particularly important during the first season after construction. Regression analysis showed slope gradient, slope aspect, ground cover density and snow-free period rainfall erosivity had statistically significant effects on erosion. Slope gradient was by far the most influential site factor affecting erosion but slope length had no affect. Three erosion control treatments -dry seeding, hydroseeding plus mulch, and terracing with hydroseeding plus mulch -were evaluated. Two treatments -dry seeding and hydroseeding plus mulchcaused statistically significant reductions in erosion. Dry seeding was the most cost-effective treatment on sites with deep alluvial soil. Elsewhere, hydromulching was the most cost-effective treatment. Further testing is needed to evaluate the effectiveness of erosion control treatments during the first period after construction. We were unable to discriminate between erosion rates on the moderately to highly weathered granitic rock included in this study. A discussion of the application of the study results is presented.

A series of 29 bordered plots, 1.8 m wide by 4.6 m long, was used to measure sediment yields from granitic roadfills on forest roads in the mountains of Idaho. Slope gradients on the plots ranged from 34 to 41°. Sediment yield data for the snowfree season were collected for 3 yr following road construction. Various site factors were tested by regression analysis for their effects on sediment yield, but only ground cover density and snowfree period rainfall erosivity were statistically significant. Analysis of 22 yr of snowfree period rainfall erosivity data showed that erosivity was log‐normally distributed and established the parameters for the probability density function. These data, coupled with the prediction equation from the regression model, were then used in a Monte Carlo simulation model to define the probability of occurrence of sediment yields from granitic roadfills given various levels of ground cover density. Recently published studies that update the slope gradient and slope length components of the Universal Soil Loss Equation are used to extrapolate the results of the present study to all lengths and gradients of granitic roadfills. A discussion of the application of the study results is presented.

Cost effectiveness of six alternative road embankment construction practices and 16 alternative fill-slope stabilization practices was evaluated as means for controlling erosion on forest roads built on granitic soils in Idaho (USA). A total of 63 bordered erosion plots 1.8 m wide by 4.6 m long, were used. Eroded material was collected in troughs at the bottom of each plot in the spring and fall. Measurements began in the fall of 1981 and continued until the fall of 1983. Costs were obtained by tabulating actual labor, equipment, and supplies used to install the treatments. Rainfall erosivity values accumulated for the snow-free periods between erosion measurements proved to be a statistically significant covariate for the analysis of treatment effects. There were no significant differences between the three embankment construction practices (sidecast, layer placed, and controlled compaction). But surface rolling did significantly increase erosion in all cases. Nine of the post-construction soil stabilization measures significantly reduced erosion, and one measure significantly increased erosion ( P < 0.5). Stabilization measures that combined mulches with revegetation appeared to be more beneficial for erosion control than either mulches or revegetation alone. Rankings of the erosion control effectiveness of individual soil stabilization practices were different from the rankings of the cost effectiveness of the practices. Application of alternative treatments is discussed.

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