Abstract:
Landslide dams are a common but complex geomorphic phenomenon, which present direct and indirect mass movement hazards. Two landslide dammed lakes situated in the North Island, New Zealand are investigated in this thesis to help understand the controls on landslide dam formation. The Tutira landslide dam is a large slope failure (200Mm³) which is situated in soft interbedded sedimentary rocks of northern central Hawke‟s Bay. It has resulted in the formation of three lakes; Tutira, Waikapiro, and Orakai. The Mangapapa landslide dam, which is located in the weak rock hill country near the Rangitikei Valley, is a smaller slope failure which has impounded a headwater of the Mangapapa River. The aim of this project was to assess the likely failure mode and triggers of each landslide. This was done using a combined approach of field, laboratory and modelling analysis. The field investigation involved traditional engineering geologic mapping, use of the geological strength index, and terrestrial photogrammetry. Samples were taken from the field and strength tested in the laboratory. Limit equilibrium method of slices technique was used to assess the variables associated with failure. The Mangapapa slope failure occurred during a high intensity precipitation storm, via a flexural toppling-circular failure mechanism. The flexural toppling weakened the rock mass, allowing failure to occur. The Tutira landslide dam was a more complex event formed as a result of a large magnitude earthquake, with the possibility of precipitation and river incision playing a role in instability. Failure was controlled by the rock mass conditions of each slope, with the structural features playing a role in the mechanism movement. Both landslide dams illustrate the influence of tectonic and climate controls on preconditioning the slope for failure.
