Modeling Top Soil Erosion and Mass Failures in Steep Agricultural Slopes: Integrating WEPP and a Landslide Model

Abstract

Steep agricultural hillslopes are not only subjected to soil erosion, but also have a probability of failure. In hilly country were both soil erosion and landslide processes are active, the interaction between these processes is critical. A model called WEPP-SLIP was developed that integrates erosion modeling and landslide prediction to determine sediment delivery pre and post landslide failures. Initially, WEPP is used to estimate pre-failure erosion. The landslide model then predicts where a mass failure may occur along the slope. Changes in topography and soil structure are estimated from the predicted magnitude of the landslide. The WEPP model is then used again with the new topography to predict postfailure erosion. A flume based experiment was used to validate the modeling with loess and sandy type soil representative of hilly sheep pasture land in New Zealand. Results showed a good correlation between predicted and measured erosion and runoff. In fallow conditions, post-failure erosion was shown to be smaller than pre-failure erosion due to changes in slope and soil properties resulting from the failure. The opposite is true for hillslopes covered with grass, as slope failures disturb the cover resulting in greater erosion. Flume based results indicate that sediment yields during failures were high. WEPP-SLIP can be applied for individual hillslope profiles; however, efforts are on the way to create a spatially distributed model. The model will be used to improve management practices and calculate the long term implications of mass movements in hilly slopes.