Hydrology of five forest roads in the Oregon Coast Range Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/k0698b988

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  • Understanding the impact of low volume road networks on forested watersheds is important for future forest management and watershed restoration. This study characterized the hydrology of five segments of forest road in the Oregon Coast Range. Rainfall, infiltration, road surface runoff, and intercepted subsurface flow were measured at each road segment. Results indicate that these individual segments of forest road differ hydrologically, depending on how much subsurface flow they intercept from the hillslope. The first objective of this study was to compare and contrast hydrologic behavior of ditch flow resulting from infiltration-excess overland flow on the road surface with ditch flow that was intercepted subsurface flow from the hillslope. Overland flow and intercepted subsurface flow were physically separated in the ditch by a divider and routed through two trapezoidal flumes at the bottom of each road study segment. Runoff derived from infiltration-excess overland flow on the road surface was ephemeral, responding to high intensity rainfall, and it ceased within minutes to hours after rainfall. This was the only type of flow observed in road ditches at four of the five study sites. Subsurface flow intercepted from the hillslope was intermittent, occurring continuously during the rainy season with a more gradual, muted response to storms. This type of flow occurred, along with ephemeral ditch flow, at one of the five study sites. Ephemeral flow in the ditch of this site produced minimal runoff volume, no more than 4.5 m³ (16 mm / m² of road), for storms up to 140 mm in depth. In contrast, intermittent ditch flow intercepted from the hillslope produced up to 801 m³ (2800 mm / m² of road) for similar storms, 20 times more flow than all of the rainfall that had occurred on the road surface. Any ephemeral ditch flow derived from subsurface flow on the hillslope was not observed in this study, though it may exist. The second objective of this thesis was to quantify the relationship between rainfall intensity, infiltration capacity and road surface runoff at the study road segments. A rainfall simulator was used to measure road infiltration capacities. Estimates of infiltration capacity from the rainfall simulation averaged 4 mm/hr and ranged from 0 to 11 mm/hr. Despite the low infiltration capacities, runoff volumes from the road surface were on average only 5 percent of natural rainfall, because rainfall intensity remained lower than infiltration capacity during most of the duration of storms. Infrequent pulses of high intensity rainfall overwhelmed the infiltration capacity of the road and produced surface runoff. Median lag time from peak rainfall intensity to peak discharge of road-derived ditch flow was 10 minutes on road segments with up to 250 m² of surface area. Two other estimates of the infiltration capacity of a given road were 1) the maximum rainfall intensity that did not produce runoff, and 2) the minimum rainfall intensity that did produce runoff. These intensities ranged from 0.5 to 11 mm/hr, similar to the infiltration capacity estimates from rainfall simulation. Infiltration capacity and ephemeral road surface runoff were similar for all road segments in this study. Intermittent flow, intercepted from the hillslope, differed between roads and was two orders of magnitude greater than ephemeral runoff from the road surface. Intercepted subsurface flow has greater potential to cause erosive damage than ephemeral runoff from the road surface, because of its large peak discharges and flow volumes.
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