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Probability of Sediment Yields from Surface Erosion on Granitic Roadfills in Idaho

Intermountain Research Station, Forest Service, USDA, 316 East Myrtle Street, Boise, ID 83702;

Stephen B. Monsen

Intermountain Research Station, 735 North 500 East, Provo, UT 84606;

Monte D. Wilson

Dep. of Geology and Geophysics, Boise State Univ., Boise, ID 83725.

^* Corresponding author.

ABSTRACT

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.