The application of geo-mechanical models for mapping soil flow hazard

Abstract

Methodologies for the mapping, over small geographical areas, of the hazard from shallow regolith failures are considered with a view to the general availability of the necessary data for applying the techniques in New Zealand. Geo-mechanical models of the failure process are commonly used for mapping the landslide hazard on slopes and aspects of these models are applied to a particular landslide event which occurred on Pukehou hill, near Otaki, in October 1998. Rainfall data from local stations in the national climatological network are considered in order to determine a rainfall record representative of the study area. The erosion history of Pukehou, which has been grazed for over 100 years, is assessed in order to determine rainfall thresholds for landsliding. Antecedent soil moisture was found to be a significant factor in the landslide event. A low resolution 10m digital elevation model (DEM) was developed from readily available topographic data and the landslides were mapped on this model. Slope aspect was the only topographic control found to be a significant factor in the Pukehou landslides, with slips being prevalent on slopes with a northerly aspect. The influence of slope angle and slope curvature were not strong and this is believed to be because of the coarse DEM. The accumulation of water in the terrain was found to depend on the assumptions and the model used, with a dynamic wetness index showing the most promise. The failure surface of all the landslides on Pukehou was identified as being on an impervious layer of weathered greywacke bedrock with low cohesion. Nearly all the soil water movement was found to be through the macro-pores. The soil on slopes with a northerly aspect was found to have a low liquid limit and low plastic index. The water content on these slopes, throughout the winter, was found to be above the liquid limit for the material, which is believed to weaken the strength from particle-particle contact. The debris runouts cover a greater area than the failure scars and therefore cause a greater cost in lost production. These costs are quantified. Attempts to model the trajectory of runouts found a poor relationship between the length of the runout and the channel confinement along its length. This was explained by the low resolution of the DEM. It is concluded that a DEM at the scale readily available in New Zealand is not adequate for use in geo-mechanical modelling of the hazard from shallow regolith failures.