Influence of log truck traffic and road hydrology on sediment yield in western Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9593tz041

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  • An understanding of the factors that influence surface erosion from roads is necessary to prevent and mitigate sediment production from forest roads. This study investigated the impacts of log truck traffic and road hydrology on sediment yield from ten forest road segments in the foothills of the Cascade Mountains of western Oregon. The hydrology of individual road segments was classified as either "ephemeral" or "intermittent." Ephemeral hydrology occurs when Hortonian overland flow from the road surface is the primary source of runoff. Intermittent hydrology occurs when subsurface flow from the adjacent hillslope is intercepted by the road ditch and is a significant component of the road runoff. Data for sediment yield calculations were collected using a Turbidity Threshold Sampling (TTS) system that used in situ measurements of turbidity and discharge to calculate runoff and sediment yield for each road segment. A linear mixed effects model was fit to log transformed estimates of sediment yield. Storm runoff volumes and log truck traffic were significant predictors of median sediment yield (t=17.05, DF=148, p<0.001). Road segments with ephemeral hydrology were more erosive than road segments with intermittent hydrology and produced more sediment per unit of runoff volume (t = -2.8, DF=148, p=0.006). However, roads with intermittent hydrology produced more runoff per mm of precipitation during a storm. Sediment yield increased by a median value of 3.3 times when truck traffic occurred on the road segment during the week prior to the storm (p=0.003, 95% CI: 1.93-4.69 times increase). This increase occurred irrespective of the type of hydrology of the road segment. The two road segments that produced the most sediment were a road segment with ephemeral hydrology and one with intermittent hydrology but, both were located on the heavily traveled Hinkle Creek Mainline Road. There was high variability in sediment yield among road segments and between storms. Sediment yield from an individual road segment, normalized by precipitation, ranged between 0.002 and 0.520 kg/mm. Sediment production occurred rapidly in response to precipitation, among all sites. Over ninety percent of sediment production occurred during storms. Physical attributes of the road segments such as road length and slope, that were previously reported to explain the variability in sediment yield, did not explain the variability in sediment yield in this study. The variability in sediment yield was best explained by storm runoff volume and the occurrence of log truck traffic. The results from this study indicate that to identify individual road segments with the greatest potential to produce sediment requires knowledge of the hydrology of the road segment and its exposure to log truck traffic.
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