Effect of geologic variability on sediment yields from yarding and forest roads

Abstract

We investigated sediment yields from yarding roads and their grain size characteristics, and studied the effect of differences in geology on yarding and forest roads. The study site included yarding roads within an abandoned former plantation forest that had been clear-cut, located near Kuma-mura, Kumamoto Prefecture, Japan. The geology of the study site consists of sedimentary rocks. To clarify sediment yields from yarding roads and grain size characteristics of the sediment yield, two experimental catchments were installed. Yearly sheet erosion rates in R1 (experimental catchment with an 86-m² area and 8° average slope) and R2 (experimental catchment with a 111-m² area and 6° average slope) were about 3.3 mm and 0.9 mm, respectively. These rates were about 33% and 9%, respectively, compared to those measured in a weathered granite catchment. The reason for this difference is that cohesion in sedimentary rocks was greater than that in weathered granite. The median grain diameter for individual sediment yields in R1 was between 0.27 mm and 0.75 mm, and for R2 was between 0.67 mm and 5.45 mm. Grain size distributions of sediment yields in R2 were variable in comparison with grain size distributions of sediment yields in R1.