Shallow landsliding repeatedly causes loss of life, physical damage to public and private property, and extreme habitat degradation in mountainous regions. As an example of the timeliness of this problem, the Oregon Forest Practices Act was modified (in response to five deaths, resulting from two 1996 debris flows) to adopt and enforce forest practice rules that will reduce the occurrence, timing, and effects of rapidly moving landslides. The proposed research will focus on a physics-based characterization of near-surface hydrologic response and slope stability initiation processes for steep hillslopes at the catchment scale. This characterization will be based upon sophisticated numerical simulations for the Coos Bay experimental catchment (CB1). The 860m 2 CB1 is located in the Oregon Coast Range. CB1 is an unchanneled valley, with a north-facing aspect, that has an average slope of 43. In the area of CB1, the Coast Range exhibits steep, finely dissected hillslopes where debris flows are the dominant geomorphic process. The instrumentation at CB1, used to characterize the spatial and temporal variability in near-surface hydrologic response for three sprinkler/tracer experiments (in 1990 and 1992), includes an exhaustive grid of rain gages, piezometers, tensiometers, TDR wave-guide pairs, lysimeters, meteorological sensors, and weirs. Continuous measurements of rainfall, discharge, and total head are available for CB1 from 1990 through 1996. Rigorous simulation of near-surface hydrologic response will be conducted with the Integrated Hydrology Model (InHM). The comprehensive InHM quantitatively simulates, with a first-order coupled approach, 3D variably-saturated flow and solute transport in porous media and macropores and 2D flow and solute transport over the surface and in open channels. There is no a priori assumption of a specific hydrologic-response mechanism with InHM. The two slope stability models to be employed in the proposed study are the infinite slope model and a 2D finite-element solution to the linear elastic deformation equations. The proposed research would be conducted in three phases. In phase I, event-based InHM simulations of fluid flow and solute transport will be conducted for the three CB1 sprinkler/tracer experiments to characterize hydrologic-response and pore water pressure distributions. In phase II, long-term InHM simulations of hydrologic response (for the seven year CB1 record) will be conducted to generate continuous pore water pressure distributions. In phase III, slope stability simulations (driven by hydrologic-response simulations in phases I and II), with the two slope stability models, will be conducted to assess slope failure initiation for CB1. The CB1 data sets (discharge, pressure head, soil-water content, and solute concentration) will be used in a rigorous assessment of model performance. The coupled simulation of hydrologic response and slope stability will be used to evaluate slope failure at times of known stability and times of known instability (i.e., the November 1996 CB1 failure). Related to intellectual merit, the proposed simulation effort will effectively demonstrate the utility and/or limits of hydrologic-response models less comprehensive than InHM for field conditions similar to CB1 and beyond. It will also be possible guide the structure of new models (simpler than InHM) for use in the decision-management arena based upon dominant response generalizations of InHM results for what if scenarios. Related to broader impact, results from the proposed study will be useful for the optimization of data collection networks for both future studies of hydrologic response/landslide initiation and real-time hazard warning systems. A major contribution of the proposed work is the training of a PhD student.
