CryoWALL - Steep permafrost slopes in Norway

The stability of steep slopes is a serious consideration in relation to a changing climate, and critical for society and infrastructure. Many studies indicate that permafrost (continuously frozen ground) is a major factor for the stability of steep slopes. Permafrost stabilises rock walls as water in joints and cracks remains frozen throughout the year. The CryoWALL project evaluates the distribution of steep permafrost rock walls in Norway, evaluates stability and helps determining risk areas. CryoWALL is a co-operation between the University of Oslo, the Norwegian Geological Survey in Trondheim, the Technical University in Munich, Germany, the Norwegian Meteorological Institute and the Norwegian Road Authorities. Rock surface temperatures are measured continuously at 25 locations in 7 regions of Norway, covering a latitudinal profile from Hordaland to Finnmark (Alta canyon). Most sites have a certain importance for risk due to their vicinity to roads or other building structures. Rock wall surface temperature data were used for statistical modelling of a map of permafrost distribution in the steep slopes of Norway. We show that in northern Norway permafrost is present above 600 m a.s.l in north-facing and 800 m a.s.l in south-facing rock walls. In southern Norway, the lower limit of permafrost varies between 1200 m a.s.l and 1600 m a.s.l in northern and southern slopes, respectively. Many of recorded rock falls and active rockslides lie in such elevations, indicating that permafrost has possibly a role in these destabilizations. The study indicate that at least 11% of the mapped unstable slopes in Norway lie in the mountain permafrost area, where permafrost processes influence the stability of the slopes. The project triggered a close collaboration with NVE which survey unstable rock slopes, e.g. on Mannen in Romsdal and on Gamanjunni i Kåfjorden/Troms. At the Technical University of Munich, a range of rock mechanical tests have been carried out on rock samples from these sites to investigate thermo-mechanical implications of warming mountain permafrost on rock wall stability. The laboratory test results provide a surprisingly clear evidence for a pronounced decrease in rock mass strength between frozen and unfrozen conditions. At these high-risk sites, we used temperature-calibrated 3D geo-electric profiling to map the permafrost in the steep slope. This was supplemented by provided geophysical soundings along the back scarps of the unstable rock walls within a collaboration with the University of Savoy-Mont Blanc during 2018 and 2019. During the project phase we contributed with numerical modelling of the Veslemannen rock wall instability, along with analysis for all other monitoring places. These study for Veslemannen clearly demonstrated the influence of ground thermal processes on the dynamics of Veslemannen, which failed fall 2019. We could demonstrate that on the other sites clear warming trends are modelled since the end of the LIA. The ERT soundings in the steep rock wall could reproduce the thermal pattern from the numerical modelling exercise, and demonstrated a pattern of frozen and thawed regions in the unstable rock wall, clearly indicating that thermal regime has to be considered when evaluating these areas with respect to stability. To evaluate past rock slide activities, 38 samples from rock wall and associated deposits for Cosmegenic Nucleide (CN) dating were taken and prepared and analysed in Halifax/Canada. 35 of those represent rock slope failure deposits and 3 were taken from near vertical sliding surfaces. In a subsequent study, we compared all CN datings taken from NGU in unstable rock slopes with climate variation since deglaciation in terms of Holocene climate variation. These samples show that the initial failure of three rock slope instabilities in Norway coincide with periods of strong temperature changes, indicating that permafrost thawing or increased precipitation could have been one of the triggering factors. When it comes to dating of sliding surfaces of rock slope failures at the permafrost boundary there is a strong link of initiating of sliding during the warmest millennia within the Holocene, while no dated slide initiated in the colder millennia. In summary, in addition to clear scientific progressions of the possible role of permafrost in large instabilities, the project increased the awareness of the responsible governmental institutions on this issue. We have produced a base line for Norway, showing in which slopes we can expect permafrost conditions. We could indicate how temperatures may distribute in steep slope setting and clear demonstrate that warming has occurred during the last century, and that this warming may influence slope stability.

I Norge er det stort interesse om hvordan klimaendringer påvirker skred og ras. Mye av fokuset er knyttet til ekstremvær som f. eks. store nedbørhendelser, som ventes å øke i frekvens med klimaendringene. CryoWALL har rettet søkelyset mot sakte endringer, slik som endret temperatur i bratte skråninger og fjellvegger. Vi vet f.eks. at bevegelser som måles nå i de overvåkete fjellsider er langt raskere i dag enn noen gang etter siste istid. En akseptert hypotese i fagmiljøet i Norge er at dette kan skyldes oppvarmingen av fjellet. Prosjektet har også sementert et godt samarbeid mellom ulike aktører i Norge (NVE, NGU) på dette feltet, samt internasjonalt samarbeid, hovedsakelig til miljøer som arbeider i Alpene (TU München, Tyskland, Universitet Savoie-Mont Blanc, Frankrike). Prosjektet har finansiert en PhD (Paula Hilger) og en PostDoc-stilling (Florence Magnin). Begge er nå ansatt i faste stillinger i henholdsvis Høgskulen på Vestlandet og Universitet Savoie-Mont-Blanc, Frankrike.

The stability of steep slopes and rock walls is a major point of concern in relation to a changing climate, and important for society, transport etc. It is well known that permafrost can be a major factor for the stability of steep slopes, covered either with debris or in rock walls. In Norway permafrost is widespread all over the country. Many steep rock walls lie in the mountain permafrost zone, and in Jotunheimen monitoring of rock wall temperatures documents this fact. In northern Norway, permafrost is attributed to be a major cause for the deformation pattern of a steep rock slope, which is a large threat if failure occurs. However, there is little to no knowledge about this topic in Norway in terms of spatial and temporal distribution of ground thermal regime in steep slopes, and its possible impact on slope stability. In the past decade, many thousand rock fall or slide events were recorded which affected the national road network and transport infrastructure, and many of those might originated from steep slopes underlain by permafrost. Cryowall will therefore evaluate the spatial distribution of steep permafrost rock walls in Norway, instrument selected sites of special interest both in southern and northern Norway, and help addressing risk areas by employing empirical and physical rock?fall run out models from selected site. We will install rock wall temperature loggers, and use these data to model the thermal regime in rock walls depending on climate forcing and snow conditions. We will employ stability modelling of selected rock falls, along with laboratory testing of rock samples at the TU Munich, Germany. Rock fall and rockslides have happened also earlier during the Holocene, and to understand future responses of permafrost in rock walls, it is important to address former conditions. We will therefore use advanced dating techniques to evaluate the development of slip planes in permafrost rock walls.