Abstract:
Of all the geohazards, mass movement is one of the most destructive. Defined as nearly all types of gravity-driven, lateral mass movement of rock, soil or debris down-slope, this phenomenon can have large-scale impacts on both infrastructure and society. It is therefore essential, in sustainable development of alluvial fan areas subject to mass movement hazards, that the possibility and consequences of mass movements is investigated. The focus here is on the Te Puru Stream fan-delta on the western side of the Coromandel Peninsula in New Zealand, on which Te Puru Township is located. Te Puru Stream now follows an engineered course on exiting the mountain front, diverted north within levees around the fan apex into the Firth of Thames. Detailed landslide susceptibility mapping upstream of the Te Puru fan, both in the field and using a range of geographical information systems (GIS) indicates the catchment is vulnerable to both shallow soil landslides and subsequent debris flows. The study undertook a range of new hazard assessment techniques using the application of fuzzy logic to both map and statistically analyse the susceptibility of shallow soil landslides on a regional scale both within the Te Puru Catchment and greater coastal area. This was followed by assessment of the debris flow hazards using the flow simulation programs Titan2D and RAMMS to model and examine the impacts of a series of discharge estimates from extratropical rainfall events. Modelled flows indicate that while the discharge reaching the fan apex is somewhat constrained by the channel bend as the river exits the mountain front, debris flows are likely to super-elevate prior to reaching the fan apex. This would lead to avulsion of the Te Puru Stream down the centre of the fan-delta through the township. Hence, the application of numerical flow modelling to debris flow hazard vulnerability at Te Puru demonstrates the technique’s potential as a precise and powerful tool for hazard mapping and assessment.
