title: Modeling slope failure in the Jones Creek Watershed, Acme, Washington
Creators: name: Brayfield, Brandon M.
Publisher: name: Western Washington University
Year: 2013

description: <p>Mountain watersheds in the Pacific Northwest are particularly susceptible to shallow landslides and debris flows during periods of intense precipitation. The Jones Creek watershed near Acme, WA, is a 6.7 km2 basin that hosts several active landslides. Shallow mass wasting on the unvegetated landslide toes, and deep-seated rotational slide movement can lead to landslide dam outburst floods and debris flows. There are approximately 100 buildings constructed on a 0.75 km2 alluvial fan deposited by debris flows sourced in the watershed. Predicting the occurrence of mass wasting and deep-seated movement events as they relate to the duration and intensity of antecedent precipitation conditions is important for land-use planning and emergency preparedness in the surrounding Acme community. The Distributed-Hydrology-Soil-Vegetation Model (DSHVM) simulates a water and energy balance at the pixel scale of a digital elevation model (DEM). I use DHSVM hydrology simulations, coupled with an infinite-slope failure model, to determine the probability of shallow mass-wasting events for a variety of historical precipitation scenarios. The infinite slope model uses a stochastic approach to predict the probability of slope failure on a cell-by-cell basis. Following the methods of Godt (2004), I use the simulated failure probabilities, paired with antecedent precipitation and intensity, to define a series of predictive antecedent precipitation thresholds for slope failure probability in the Jones Creek watershed. Although basin hydrology is not well-constrained in this study, the failure probability thresholds compare favorably with similar, more rigorous studies performed in the Pacific Northwest. Timber harvest can increase the rate of slope failure in steep basins due to reduced evapotranspiration and root strength loss. In order to supplement current logging prescriptions in the Jones Creek basin, I use DHSVM to model slope failure probability for a design storm event under a number of hypothetical harvest scenarios. DHSVM simulations suggest that root strength is the most important factor for the stabilization of slopes in the Jones Creek basin, and that a total basin harvest would significantly increase the susceptibility to slope failure. Based on the results of this study, I recommend expansion of the current logging prescriptions to include more harvest-restricted area. I also use RocScience SLIDE© version 6.0 software to model the influence of groundwater and soil mechanical properties on deep-seated slope stability for four deep-seated landslides in the Jones Creek watershed. Slide uses a comprehensive suite of tools for probabilistic modeling of complex failures, and incorporates a standalone finite element model for groundwater flow. SLIDE results indicate that the transition from unconsolidated material to weak bedrock on the toes of the deep seated landslides is likely to occur at a depth of less than two meters, which agrees with observed conditions in the basin.</p>