Sediment generation and delivery from large historic landslides in the Southern Alps, New Zealand

There is much variability of sediment yield in mountain river systems because it is influenced by both periodic (e.g., snowmelt, monsoonal circulation) and episodic (high-intensity rainstorms, earthquakes) controls. In the Southern Alps of New Zealand, landslides are dominant mechanisms of conveying large amounts of debris to river channels. This paper attempts to quantify catastrophic modes of sediment delivery by presenting three recent examples of large landslides on the western slopes of the Southern Alps. A combination of ground surveys, DEM-based volume calculations, and air photo interpretation attributes immediate postfailure sediment yields in excess of 70,000 t km−2 a−1 in the short term. Such landslide-derived sediment pulses pose significant hazards to downstream settlements and infrastructure on alluvial fans, where they cause massive aggradation, increased flooding frequency, and large-scale channel avulsion. In addition to intramontane valley floor obliteration or channel metamorphosis, these geomorphic impacts may thus occur far beyond the initial locations of slope failure. These catastrophic off-site effects involve apparent net sediment delivery rates in the order of 105–106 m3 a−1. The accurate quantification of the relative contribution of debris from the disturbing landslide to overall sediment flux is extremely difficult. Nonetheless are quantitative estimates of landslide-derived sediment production and delivery important data to develop a more comprehensive understanding of the variability in alpine sediment flux. Such data are essential not only for use in hydraulic engineering or catchment management, but also in gauging potential future impacts from rainstorm- and earthquake-induced landsliding in the Southern Alps.