GEOMORPHIC MAPPING AND SUBSURFACE CHARACTERIZATION OF A DEBRIS FLOW COMPLEX IN BANKS PENINSULA, NEW ZEALAND

Debris flows represent a significant geologic hazard in hilly and mountainous regions throughout the world, and may pose a threat to human life and property. Here we use a multi-technique approach to investigate a debris flow complex in Purau, Banks Peninsula (BP). Geomorphic mapping, ground penetrating radar (GPR), and stratigraphic studies reveal a debris flow complex composed of at least two temporally distinct events - based on differences in texture, degree of induration, and the presence of bounding soil horizons. The source drainage catchment has a Melton R value of ~0.65 and watershed length equal to ~1.5 km, indicating a catchment prone to debris flows. The resolution of the GPR is limited to the upper 2 meters, but appears to capture a potential debris flow contact and the location for several buried boulders. The studied debris flows demonstrate the ability to raft large volcanic boulders (max. volume = ~15-25 m 3 ) from the source area and highlight the potential hazard to a nearby and recently built residential home, as well as any future development. We propose that any new development in the study catchment (or nearby catchment areas) should design hazard mitigation strategies assuming future events of similar magnitude will occur. Our findings suggest the potential debris flow hazard in BP should be given a high priority as building and improvements to infrastructure continues. While these results indicate past debris flows have occurred in the studied catchment, the timing of the debris flows remains unknown. Although we were unable to find organic material for radiocarbon dating, surface exposure dating of rafted boulders and/or OSL (optically stimulated luminescence) dating of loess-colluvial sediments could prove successful and should be considered.