Hillslope-channel coupling, hazards, and environmental impacts of large landslides in the marine sedimentary rocks of Aotearoa New Zealand.

Thousands of large (> 2 ha) rock slope failures affect the Neogene marine sedimentary cover rocks of Aotearoa New Zealand. These slope failures are known to damage lifeline infrastructure, entire suburbs, agricultural land, and deliver substantial volumes of fine sediment to rivers. Despite their prevalence in the landscape and potential impacts, there has been little systematic quantification of their activity, causes, and impact. This research brings together several local case-studies and regional statistical & monitoring analyses that document the causes, movement patterns, and economic and environmental impacts of such rock slope failures in New Zealand’s North Island. At the local scale, for several large landslides that have affected infrastructure and farming operations we present i) financial damage estimates, ii) movement data collected using GPS, time-lapse, and remote-sensing methods, and iii) sediment delivery loads estimated from several independent methods. At the regional scale, for hundreds of large landslides mapped across a region of the North Island, we iv) assess movement patterns using remote-sensing (InSAR and satellite pixel-tracking), v) present a regional-scale statistical analysis of the key landscape parameters explaining the spatial distribution of their occurrence and state of activity; and vi) estimate regional-scale sediment contributions to catchment sediment loads. From the local case studies, we found that: i) individual landslides can impose hundreds of thousands of dollars of damage and seriously hinder infrastructure and farming operations; ii) can move at rates of 10^-2 to 10¹ m per year (or fail catastrophically) with movement patterns strongly influenced by fluvial erosion; and iii) deliver tens of thousands of tonnes of sediment annually. At the regional scale, we found that iv) approximately 9 % of the ~730 mapped large landslides have been actively moving over the past several years; v) low slope angle, fluvial incision and dip slopes most strongly explain the distribution of all landslides, whereas low slope angle and rainfall best explain the distribution of the active landslides; and vi) the active landslides contribute 10 – 30 % of modelled catchment sediment loads (despite representing only 0.2 % of the total area of these catchments). Our findings suggest that large rock slope failures within the marine sedimentary rocks of Aotearoa New Zealand are a significant, but hitherto poorly recognised, hillslope erosion process, source of fine sediment to rivers, and natural hazard. Moreover, our analyses demonstrate an intimate connection between hillslope and fluvial processes, with rivers influencing slope destabilisation across multiple scales and as an efficient receiver of landslide sediment.