Spatial and temporal changes in terrain resistance to shallow translational regolith landsliding

Abstract

Landslide episodes in the New Zealand hill country involve processes of displacement, evacuation, transport and redeposition of material throughout a catchment. Responses to these processes include mechanisms that have geomechanical implications and are thought to influence terrain resistance and subsequent erosional behaviour, thus affecting inherent susceptibility to future landsliding. This thesis describes a change over time in overall susceptibility to landsliding for the catchment of Lake Waikopiro on the east coast of New Zealand's North Island - specifically as a result of a series of large rainstorm-triggered landslide events. The response to these events is detectable as a change in the state of the landsurface. The contemporary landsurface condition of a unit area is both a product of the erosional processes that have already occurred, and a control on the susceptibility of that area to subsequent erosion. This is because the contemporary landsurface condition is an expression of the various slope and material parameters that determine strength and stress relationships, and hence susceptibility to landsliding. Change in landsurface condition of a given area is therefore paralleled by a change in stress/strength parameters, and hence in the inherent stability of that area. A classification has been developed for Contemporary Landsurface Condition with respect to the erosional and depositional processes pertinent to translational regolith slides and flows. Eight classes of Contemporary Landsurface Condition are recognised. These include surfaces defined as: "Undisturbed", "Old Landslide Scars", "Colluvial", "Alluvial", "New Landslide Scars" and three distinct classes of "Recent Landslide Debris" distinguished by their occurrence on either Undisturbed, Colluvial or Old Scar surfaces. These classes are characterised on the basis of geomechanical properties, such as regolith depth, material densities and strength parameters. Using aerial photography, the distribution of Contemporary Landsurface Condition classes has been mapped for two historical occasions, covering a 23-year period in which at least six large landslide-triggering events occurred. Inherent susceptibility to failure of discrete slope units has been modeled within a Geographic Information System on the basis of geomechanical properties associated with the different Contemporary Landsurface Condition units. Change in slope stability over time as a function of ongoing erosional activity is thus demonstrated. As a result of ongoing erosional and depositional processes, the catchment of Waikopiro exhibited reduced susceptibility to landsliding in 1988, as compared to 1965. The distribution of strength and susceptibility indices in 1965 are correlated with the pattern of landslide occurrence in 1988. It is argued that landsurface condition is therefore worthy of inclusion in probabilistic models of landslide occurrence. Modeling erosion solely on the basis of frequency/magnitude of exogenic triggers ignores a real source of variability in the probability of landslide occurrence.