A DISTRIBUTED SLOPE STABILITY MODEL FOR STEEP FORESTED BASINS

A distributed, physically based slope stability model (dSLAM), based on an infinite slope model, a kinematic wave groundwater model, and a continuous change vegetation root strength model, is presented. It is integrated with a contour line-based topographic analysis and a geographic information system (GIS) for spatial data extraction and display. The model can be run with either individual rainfall events or long-term sequences of storms. These inputs can be either actual storm records or synthesized random events based on Monte Carlo simulation. The model is designed to analyze rapid, shallow landslides and the spatial distribution of safety factor (FS) in steep, forested areas. It can investigate the slope stability problem in both temporal and spatial dimensions, for example, the impact of timber harvesting on slope stability either at a given time or through an extended management period, the probability of landslide occurrence for a given year, and the delivery of landslide sediments to headwater streams. The dSLAM model was applied in a steep, forested drainage of Cedar Creek in the Oregon Coast Ranges using actual spatial patterns of timber harvesting and measured rainfall during a major storm which triggered widespread landslides in that area in 1975. Simulated volume and number of failures were 733 m(3) and 4, respectively. These values agreed closely with field measurements following the 1975 storm. However, the effect of parameter uncertainty may complicate this comparison. For example, when soil cohesion values of 2.0 and 3.0 kPa were used, the failure volume changed by factors of 2.04 and 0.41, respectively, compared with the average condition of 2.5 kPa used in the simulation. For soil depths 30% higher and lower than the standard condition, the failure volume changed by factors of 2.0 and 0.27, respectively. When maximum root cohesion changed from 12.5 kPa (average condition) to 10 kPa, the failure volume increased 1.73-fold; for the case of 15 kPa, the failure volume changed by a factor of 0.55. The simulated failures caused by the storm were mostly in hollows. The simulations show that the spatial distribution of FS is controlled mainly by topography and timber-harvesting patterns and is greatly affected by groundwater flow patterns during major rainstorms. Most areas with FS < 3.0 corresponded with the distribution of blocks clear-cut in 1968, and all elements with FS < 2.0 were in areas clear-cut in 1968. Areas with low FS (1.0-1.6) expanded dramatically during the rainstorm and decreased at a slow rate after the storm. Factors of safety in hollows declined sharply during the storm.