Abstract:
Forest roads produce fine sediment with traffic during wet weather. If the forest road is connected to a stream it can be a source of turbidity and fine sediment that may be detrimental to aquatic organisms especially salmonids.
The goal of this work was to investigate turbid runoff during wet-weather use from the
pavement of forest roads that were designed to reduce sediment production. This research explored the opportunity costs associated with upgrading forest roads for environmental performance, determined a
method to design an unbound aggregate pavement to reduce sediment production, and tested alternatives for road pavements that were designed specifically to minimize turbid runoff during wet weather hauling.
The opportunity costs associated with restricted timber hauling and harvesting are potentially a resource that could be made available to improve aggregate road surfaces to minimize hauling restrictions during wet-weather. In this study the opportunity costs were 1.7 to 15 percent of the total
net revenue for McDonald-Dunn Research Forest.
A method of design for the pavement structure for unbound aggregate roads was developed. This "reduced stress" design method designs against subgrade mixing by reducing stresses on the subgrade to allow for strain hardening of the subgrade. The method recommends depths of surface aggregate that are greater than traditional pavement design methods but is an appropriate design method to reduce sediment production from subgrade mixing.
Alternative designs of the pavement for unbound aggregate roads influenced the production of sediment, but results were not consistent; the pavement treatments produced different results across different research locations. The results suggest that fine sediment in surface runoff does not originate
from the subgrade but rather from the surface aggregate. Road managers that want to minimize the production of sediment from forest roads should be concerned with the unbound aggregate pavement rather than the subgrade. Managers should design the aggregate pavement with consideration to the
availability of fine sediment in the aggregate and should design the pavement to resist rut formation.
