Implications of the USGS analysis of slope stability at Sulphur Creek

of comments and calculations The slope stability equation and values for material properties recommended by USGS geologist Dr. Raymond Wilson were used to map the stability regime of the four units of THP 1-97-307 HUM and the two units of THP 1-96-413 HUM. When calculations are carried out for conditions without trees, results indicate that each unit includes significant areas that would be expected to fail during storms of a size that occurs regularly in the area. With the exception of unit 413-A, over 50% of each unit has a factor of safety of less than 1.0 for fully saturated conditions without trees; each also includes at least one area with a factor of safety of less than one even under dry conditions. Recurrence intervals for 1-day rainfall at the Kneeland 10 SSE rain gauge indicate that 1-day rainfalls sufficient to saturate a 2.5-foot depth of soil (considered characteristic of the Sulphur Creek slopes by Dr. Wilson) occur on average once every 95 years, and such storms are expected to occur more frequently at Sulphur Creek because it has a 60% higher annual rainfall. These slopes thus would be expected to fail during storms of a size that occurs regularly in the area if trees are not present. Calculations for a 10-year recurrence-interval storm further indicate that over 20% of units 413-B, 307-4, and 307-1 would be expected to fail during even these relatively frequent storms. That the slopes persist despite their calculated instability is due largely to the presence of tree roots, which add cohesion. Recalculation of the stability regimes assuming the presence of trees indicates that the area susceptible to failure during 10-year storms is significantly reduced. However, the high density of landslides already mapped on the units indicates that even root cohesion is not sufficient to fully stabilize the slopes. Decreased root cohesion due to logging would further decrease the stability of these already unstable slopes (R. Wilson, personal communication, 5 August 1998). Data from nearby Bear Creek demonstrate this effect: rates of landsliding increase by a factor of 9.6 on hillslopes logged less than 15 years previously on the same rock type and soil type as present at Sulphur Creek. Tree roots take several years to decay, and regeneration of conifers and hardwoods on a logged slope begins to restore a component of root strength after several years. Soil cohesion thus rarely reaches the minimum level …