Process-based simulations of near-surface hydrologic response for a forested upland catchment: the impact of a road

Forest management practices, specifically logging road construction, create seepage faces and hydraulic conductivity contrasts that can significantly affect the hydrologic response, accelerate the near-surface evolution of a hillslope catchment, and increase erosion and mass wasting events. The objective of this study is to understand the physical, topographic, and hydrologic controls that drive subsurface flow at Watershed 3 (WS3) in the H.J. Andrews Experimental Forest, Blue River, OR and the role logging roads play in altering the hydrologic processes. This study combines a field component, process-based numerical simulation of near-surface hydrologic response, and slope stability analyses to address the impact of a road in a typical steep forested catchment.
The fieldwork conducted at WS3 characterized hillslope morphology, soil depth, and soil properties of a single hillslope (C3) within WS3. The modeling component of this study had four objectives: (i) identify what part of the WS3 and C3 hillslope systems were most drastically effected by the presence of a road using NUM5, a simple 2-D finite-difference model; (ii) isolate that portion of the flow domain for more detailed simulation with VS2DT, a 2-D transient, saturated-unsaturated, finite-difference model; (iii) compare the results from the VS2DT simulations with observed piezometric levels and hydrograph data for precipitation events in 1995; (iv) use the factor of safety criterion to assess if pore pressure buildup in specific areas of C3 during a series of hypothetical events would be enough to trigger a mass movement event.
In general, the results from this study show that slope stability decreases as a function of rainfall intensity and/or duration, with increased slope angle, with higher antecedent soil-water content conditions, and with increased hydraulic conductivity contrast between the high conductivity soil and the compacted soil beneath the road. The vertical hydraulic conductivity contrast below the road acts to impede the slope-parallel subsurface flow and water builds up behind the low permeability zone and creates a region of groundwater mounding. The increased pore pressure leads to an increased risk of failure that propagates upslope from the road.