Inferring traffic induced sediment production processes from forest road particle size distributions.

摘要

Forest roads are considered major sediment sources, and it is often observed that forest roads with heavy traffic have high surface erosion. However, the exact processes that create erodible materials haven't been clearly identified and documented. The objective of this study is to better understand how traffic affects forest road aggregate so as to contribute to a better understanding of forest roads, erosion, and their relationship. It is hoped that this will help minimize forest roads' impacts on aquatic systems while maintaining the same level of management efforts from forest managers.; There are three processes to change physical properties of aggregate, i.e. particle size, related to traffic: (1) crushing---break down of the surfacing materials, (2) piling down---forcing upward of fine-grained particles from subgrade, and (3) sweeping migrating large loose particles on the surface toward outside of roads by tire actions. Rather than addressing the processes themselves, their results, aggregate particle size changes were studied.; Aggregate was collected from three forest roads within the same road network in the Clearwater National Forest, Idaho, including a no traffic, a light traffic, and a heavy traffic road with three road locations (tire track, shoulder, and between them) and three depths (surface to 1.3 inch depth, 1.3 to 2.7 inch, and 2.7 inch to below). Twenty seven samples with five replicates each, for a total of 135 aggregate samples were collected. Particle size distributions (PSDs) were generated and compared.; On the no traffic road no changes in the PSDs from the original construction material were observed. On the light traffic road, the results indicate that crushing occurred in the top aggregate layer except on the shoulder, and no changes in the PSDs were observed in the middle and bottom layers; no evidence of piling down was found; and sweeping occurred on the surface. On the heavy traffic road there is strong evidence that crushing occurred everywhere except at the bottom layer on the shoulder where piling down occurred; and no sweeping occurred.; With increasing amount of traffic, crushing is the dominant process in this road network, and forest managers should consider focusing on strengthening the surface materials.