[ghsc-seminars] GHSC Seminar—Sam McColl, GNS Science, Tuesday, May 31st, 2022, 10–11am MST

Mon May 30 20:29:40 UTC 2022

The causes and impacts of deep-seated, soft-rock landslides, New Zealand

Speaker: Sam McColl
               Department of Surface Geosciences
               Institute of Geological and Nuclear Science (GNS Science), New Zealand
Location: online/virtual only
Date/Time: REMINDER - Tuesday, May 31st, 2022, 10–11am MST

Abstract: Thousands of large (> 2 hectare) 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 regional-scale quantification of their activity, causes, and impact. This research brings together several local case-studies and regional analyses that document the geotechnical properties, causes, movement patterns, and the economic and environmental impacts of such rock slope failures in New Zealand’s central North Island.

At the local scale, for several large landslides that have affected infrastructure and farming operations we have (i) collated financial damages, (ii) monitored movement using GPS, time-lapse, and remote-sensing methods, and (iii) quantified sediment delivery loads through several independent methods. At the regional scale, for hundreds of large landslides mapped across the central North Island, we have (iv) assessed movement patterns using remote-sensing (InSAR and satellite pixel-tracking), (v) undertaken a regional-scale statistical analysis of the key landscape parameters explaining the spatial distribution of their occurrence and state of activity; and (vi) estimated regional-scale sediment contributions to catchment sediment loads.

>From the local case studies, we found that individual landslides (i) 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) about 9 % of >700 mapped large landslides have been actively moving over the past several year; (v) low slope angle, fluvial incision and dip slopes most strongly explain the distribution of all landslides, supporting previous site investigations that suggest strong structural control on weak planar clay layers; 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.

[cid:f4b9199d-876c-4ee1-b19d-d3450e327cb0]
The 11 km2 Poroa Landslide complex, one of several hundred soft-rock landslides in the central North Island, New Zealand. A particularly active part of the complex, in the lower left of the photo, is delivering >20,000 tonnes of sediment annually to the Rangitikei River. The landslide is imposing significant damage to farm operations. Photo: Graham Hancox.

Bio: Dr. Sam McColl is an Engineering Geomorphologist at GNS Science, New Zealand. Prior to joining GNS Science in March 2022 he had a 9 year lectureship in physical geography at Massey University, NZ. For his PhD research, at University of Canterbury, he examined paraglacial rock slope failure processes, and for his BSc (Honours) dissertation from Victoria University of Wellington, he studied earth flow hazard. Prior to his PhD he spent 2 years (2007-2008) working at GNS Science in the engineering geology team, on a variety of landslide projects. He is a quantitative geomorphologist with interests in landslide processes, hazards, and their geomorphic impacts. His research extends from mountain alpine environments to rural hill country and riverscapes.

GHSC Seminar Committee: Oliver Boyd <olboyd at usgs.gov>, Josh Rigler <erigler at usgs.gov>, Francis Rengers <frengers at usgs.gov>

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