Analysis of topographic and climatic control on rainfall-triggered shallow landsliding using a quasi-dynamic wetness index

A model for the prediction of both topographic and climatic control on shallow landslide initiation processes in hilly mountainous terrain is proposed. The model develops upon a theory for coupled shallow subsurface flow and landsliding of the soil mantle previously proposed by Montgomery and Dietrich [Water Resour. Res, 30 (1994) 1153], The model uses a 'quasi-dynamic' wetness index to predict the spatial distribution of soil saturation in response to a rainfall of specified duration. The rainfall predicted to cause instability in each topographic element is characterised by duration and frequency of occurrence. The incorporation of a rainfall frequency-duration relationship into assessment of landslide hazard provides a practical way to include climate information into estimation of the relative potential for shallow landsliding. The model is applied to a mountain experimental basin where high-resolution digital elevation data are available: the Cordon catchment (5 km(2)), in north-eastern Italy. An inventory of landslide scars is used to document sites of instability and to provide a test of model performance by comparing observed landslide locations with model predictions. The model reasonably reproduces the observed distribution of landslides, although spatial variability of soil properties and hydrologic complexities not accounted for by the model complicate prediction of where landslides occur within areas of similar topographic-climatic control. Model predictions from the quasi-dynamic model are compared with those provided by the steady-state model [Water Resour, Res. 30 (1994) 1153], These results suggest that the quasi-dynamic model offers significant improvement over the steady-state model in predicting existing landslides as represented in the considered landslide inventory. (C) 2002 Published by Elsevier Science B.V.