Multiscalar interaction between river erosion and landslide activity, and the implication for landslide hazards and fluvial sediment dynamics in soft rock landscapes.

Thousands of large (> 2 ha) rock slope failures affect the Neogene marine sedimentary cover rocks of Aotearoa New Zealand. These soft rock slope failures damage lifeline infrastructure, entire suburbs, agricultural land, and deliver disproportionate volumes of fine sediment to rivers. Most of the landslides are primed by, and adjacent to, major river corridors suggesting the interaction and coupling with rivers. The millennial-scale longevity of the landslides, their propensity to reactivate, and 10% being active today, provides an opportunity to explore the evolution of the landslides and their response to fluvial processes over a range of time scales from the late-glacial to present day. Here we present a range of case-studies along with results from local monitoring and regional statistical analyses that explore the relationship between fluvial erosion processes and that of landslide activation, reactivation, and active movement rates. We show that at regional scales, and millennial timescales, fluvial incision and stream power explain the density and position of landslides in the landscapes. At decadal scales, and for active landslides, undercutting by major storms can switch landslides between dormant and temporarily-active states. For active landslides, on daily to seasonal timescales, stream flow can control the rate of landslide movement and sediment delivery, the effect varies with the competency of the river. Our local and regional analyses suggest that the soft rock landslides are a highly disproportionate source of sediment delivery to rivers, contributing to some 10 – 30 % of the modelled catchment sediment loads (despite representing only ~0.2 % of the total area of these catchments). Soft rock landslides tend to deliver weak, fine-grained sediment which is readily eroded and suspended, while providing minimal contribution of coarse bedload. Consequently, their impact on river morphology is considerably different in geometry and more transient compared to that of rapid and hard rock landslides.