Large landslides and their effect on sediment flux in South Westland, New Zealand

Abstract

Landslides and runoff are dominant erosional agents in the tectonically active alpine South Westland area of New Zealand, characterized by high uplift rates and extreme orographic precipitation. Despite a high density of shallow debris slides and flows, the geomorphic imprints of deep-seated bedrock failures are dominant and persistent. Over 50 large (> > > > >1 km 2 ) landslides comprising rock slide/avalanches, complex rotational and rock-block slides, wedge failures, and deep-seated gravitational slope deformation were detected on air photos and shaded-relief images. Major long-term impacts on alpine rivers include (1) forced alluviation upstream of landslide dams, (2) occlusion of gorges and triggering of secondary riparian landslides, and (3) diversion of channels around deposits to form incised meandering gorges. Remnants of large prehistoric (i.e. pre-1840) landslide deposits possibly represent the low-frequency (in terms of total area affected yet dominant) end of the spectrum of mass wasting in the western Southern Alps. This is at odds with high erosion rates in an active erosional landscape. Large landslides appear to have dual roles of supplying and retaining sediment. The implications of these roles are that (1) previous models of (shallow) landslide-derived sediment flux need to be recalibrated, and (2) geomorphic effects of earthquake-induced landsliding may persist for at least 10 2 years.

Distribution of landslides in southwest New Zealand

“…Large landslides are interpreted to involve complex deep-seated rotational rock slides, structurallycontrolled rockslide-wedge failures, and rock avalanches. There is also evidence of large-scale rock flow (Cruden and Varnes 1996) or sliding complexes of rock-mass creep in pelitic schist (Craw et al 2003), associated with ridge rents and counterscarps (Beck 1968), toe-slope bulging, or valley-floor occlusion (Korup 2005).…”

Paraglacial rock-slope stability

“…Furthermore, long-term estimates of alpine erosion rates suggest that glaciers are more effective agents of erosion than rivers (MacGregor et al, 2000;Montgomery, 2002;Brook et al, 2006). It would be unlikely for this to be true in the absence of significant landslide activity during glaciation considering mass-wasting is the most important form of erosion in many mountains (Korup, 2005).…”

Rock avalanches and other landslides in the central Southern Alps of New Zealand: a regional study considering possible climate change impacts

“…Landslides are a major process in hillslope evolution (Densmore and Hovius 2000;Korup et al 2005), impact upon rivers (Korup 2002;Korup 2005a) and glacial systems (Hewitt et al 2008;Shulmeister et al 2009) and are a serious hazard in many regions of the world (Nadim et al 2006). In glaciated Alpine terrain, landslides often involve large volumes and travel distances, owing to high local relief, enhanced travel over ice or snow surfaces, flow transformations and chain-reaction events (e.g.…”