Abstract:
Forest roads alter hillslope hydrologic processes by intercepting, concentrating, and rerouting storm runoff. Current road drainage guidelines are based on minimizing erosion and do not take into account the impact of forest roads on hillslope hydrology. This work monitors ditch flow and rainfall for 10
road segments over the course of one winter in the central Oregon Coast Range. The objective was to determine rainfall/runoff relationships and quantify metrics of runoff for the flow of water in roadside ditches. Road and hillslope characteristics were also recorded and related to the metrics of runoff of ditch flow. Five large discrete storms were selected from the record for analysis. Two distinct ditch flow behaviors were identified from field observations and
hydrograph inspection and were termed intermittent and ephemeral flow. Road segments that had intermittent flow had higher peak flows and greater storm runoff
volumes than road segments with ephemeral flow. Rainfall/runoff relationships such as the lag time from the maximum rainfall intensity to the peak flow and the percent of rainfall seen as ditch flow were also significantly different between the two flow behaviors. Road and hillslope characteristics were not related to runoff peak flows or storm volumes. The best predictors of runoff were rainfall intensities and amounts. Evidence suggested that road segments with intermittent flow were being driven by the interception of upslope subsurface flow and that road segments with ephemeral flow were being driven by road surface runoff.
