A physically based model of the effects of forest roads on slope stability

Road networks in mountainous forest landscapes have the potential to increase the susceptibility to shallow landsliding by altering subsurface flow paths. In this paper a road interception/rerouting grid-based model is developed and coupled with a hydrogeomechanical, threshold-based model for slope instability. Model results allow quantification of the influence of roads on shallow landsliding hazard across a landscape and generation of hypotheses about the broader geomorphic effect of roads. The model is applied to three sites in northeastern Italy to study the influence of three different procedures for upslope drainage area computation on model results. The three procedures are a single-flow direction procedure (D8), a multiple-flow direction procedure (MF), and an algorithm based on proportioning flow between two downslope pixels (DINF). Model results are highly sensitive to upslope drainage area algorithm choice. It is shown that D8 and MF are unsuitable for road impact evaluation due to grid bias (D8) and excessive dispersion (MF). Model implementation with the DINF procedure results in a reasonable compromise between the artifacts that characterize the other two algorithms. Observed landslide patterns are broadly consistent with model estimates, a finding that underscores the utility of this simple approach for predicting the geomorphic effects of forest roads constructed on steep slopes. The approach used in this study may be useful for defining criteria for road design that reduce the effects of roads on geomorphic processes.