| Shallow landslides that usually involve only the colluvial soil mantle, are a widespread phenomenon in the United States and the world. Often triggered by extreme precipitation events, they can be the primary sources of debris flows, and are generally a threatening source of hazards, causing loss of life, destruction of property, and affecting communities all across the nation. Shallow landslides also play an important role in landscape evolution, dominating erosion in steeper landscape, unleashing debris flows that carve valley networks, and delivering sediment to rivers. The two primary aspects affecting the impact of shallow landslides, both in terms of downstream hazard and their geomorphic significance, are their location and size.;Theoretical and observational research has provided some insight on the controls on the size of shallow landslides. It has been observed that landslide exhibit a smaller size in grasslands than in forested areas and that landslides were smaller in areas where root strength decreased as a result land use change. The parameters that are most relevant for the occurrence of rainfall-triggered shallow landslides are slope, pore pressure, root and soil strength, and soil depth. Theoretical analyses have suggested that a decline in root strength results in failures having lower minimum lengths and widths, while low gradients, low pore water pressures, or high soil friction result in failures having higher lengths and widths. However, few if any studies examine the controls on both landslide location and size across a landscape. I hypothesize that the co-organization of landscape properties, such as slope, soil depth, pore pressure, and root reinforcement, controls the size and location of shallow landslides.;We currently lack mechanistic models for specifically predicting shallow landslide size across landscapes, thus reducing the effectiveness of landslide hazard delineation, and inhibiting our ability to formulate and apply mechanistic models for landslide flux and surface erosion. One reason for this is the one-dimensional representation of slope stability, generally applied in existing regional scale applications. Such a representation cannot produce discrete landslides and thus cannot make predictions on landslide size. Furthermore, one-dimensional approaches cannot include lateral effects which are known to be important in defining instability. These limitations can be addressed by a three-dimensional slope stability model, but its application to a landscape is challenging. Whereas the one-dimensional slope stability at a location can be determined independently of its dimensions and surroundings, multi-dimensional analyses require the treatment of discrete shapes. As these shapes are not known a priori, a search algorithm is required. This is a non-trivial problem, whose naive solution (i.e. an exhaustive search) is of exponential complexity, rendering the problem effectively intractable at any relevant scale. Any new procedure must be sufficiently general to evolve with current understanding, but with a parsimonious parameterization in order to be compatible with available data. The procedure must be computationally efficient to be applicable at scales large enough to be relevant for geomorphological and hazard related questions, yet at sufficiently fine resolution to capture the fundamental mechanics of slope failure.;In this dissertation I develop a procedure which couples a novel slope stability model that captures the basic physics of shallow landsliding, with a new and efficient search algorithm based on spectral graph theory that can predict discrete shallow landslides. In order to apply this procedure at the regional scale, I define sub-models to produce the required data, when they are not available at the necessary resolution. These sub-models extract topographic attributes, compute the spatial distributions of soils, and estimate the root reinforcement and pore pressure fields. I define formal framework to evaluate the performance of the procedure, based on information retrieval theory. This procedure should advance our understanding and prediction capability, enabling me test the hypothesis that the co-organization of landscape properties, such as slope, soil depth, pore pressure, and root reinforcement, controls the size and location of shallow landslides. As these properties are mostly dictated by topography, I hypothesize that topography exerts a first order control on both location and size. (Abstract shortened by UMI.). |
