GM4.15

EDI
Erosion, Weathering, and Sediment Transport in Mountain Landscapes

Mountain belts are characterized by the fastest rates of physical erosion and chemical weathering around the world, making them one of the best places to observe sediment production (e.g. erosion, weathering) and transport processes. In these settings, varied processes such as rockfall, debris flow, hillslope failure, glacial and periglacial erosion, fluvial erosion, transport and deposition, and chemical weathering operate, often simultaneously, over a wide range of temporal and spatial scales.

As a result, tracking the interactions between denudation, climatic forcing, tectonic activity, vegetation and land use is complex. However, these feedbacks affect both long- and short-term natural surface processes, landscape development, and human interactions with the environment. Many of these processes also pose serious threats to the biosphere, mountain settlements and infrastructure. Therefore, understanding and quantifying rates of erosion, weathering, and deposition within mountain landscapes is a challenging, but crucial research topic in Earth surface processes.

We welcome contributions that (1) investigate the processes of production, mobilisation, transport, and deposition of sediment in mountain landscapes, (2) explore feedbacks between erosion and weathering due to natural and anthropogenic forcings, and (3) consider how these processes contribute to natural hazards specific to mountain landscapes. We invite presentations that employ observational, analytical or modeling approaches in mountain environments across a variety of temporal and spatial scales. We particularly encourage early career scientists to apply for this session.

Co-organized by SSP3
Convener: Erica Erlanger | Co-conveners: Eric DealECSECS, Elizabeth DingleECSECS, Emma GrafECSECS
vPICO presentations
| Wed, 28 Apr, 09:00–10:30 (CEST)

vPICO presentations: Wed, 28 Apr

09:00–09:05
Sediment Production, Yield, and Transport
09:05–09:07
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EGU21-2007
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ECS
Tilahun Alemayehu Kasaye, Guchie Gulie, Margaret Chen, and Gert Verstraeten

Complex terrain tropical mountainous catchments are typically characterized by intense rainfall events, flash floods and high erosion rates with large variability over short distances. Whilst these processes are known, little quantitative information on the spatiotemporal variability in suspended sediment yield (SY) of African tropical mountain environments is available. Here, we provide such data for two catchments in the Southern Ethiopian Rift Valley characterised by annual rainfall of 700 to 1000 mm concentrated in the rainy season from April to October. In total 6 gauging stations were installed along Elgo (298 km²) and Shafe (191 km²) rivers which have their headwaters in the Gamo Highlands (max. elevation 3500 m) and run into the rift valley lakes of Chamo (1107 m) and Abaya (1169 m), respectively. For each river, a gauging station was installed where they enter the lakes as well as at the apex of extensive alluvial fans that developed in the graben lowlands, enabling to quantify the buffering capacity of the fans. For Elgo, two extra stations in the highlands were installed to monitor downstream changes in SY. At all stations, discharge (Q) was measured at 10-min resolution using a pressure diver during in 2018-2019. Additionally, 1542 samples were taken to measure the suspended sediment concentration (SSC), and these were used to establish sediment rating curves in order to calculate total suspended SY from the continuous discharge records. Observed SSC varies between 0.04 and 111.48 g/l for discharges ranging between 0.005 and 227.20 m³/s, whereas annual SY varies between 1133 and 6373 t/km²/year. Both SSC and SY values are in line with those reported for other highland rivers in Ethiopia and in line with SY values for other tropical mountain catchments in the world. A strong temporal variability in SSC and SY is observed and can be explained mainly due to changes in hillslope sediment supply throughout the seasons. Peak sediment transport is mostly concentrated in the first two months (May to June) of the rainy seasons accounting for about 60% of the total SY of the season. At the start of the rainy season, topsoil is loose because of tillage operations that prepare the soil for cultivation. Furthermore, vegetation cover is at its lowest value. Throughout the rainy season, vegetation cover increases and hence soil erosion and sediment yield declines.  Comparing the SY of the various gauging stations shows that total sediment load increases in downstream direction, up to the apex of the alluvial fans. Whereas agricultural top soil erosion is most important in the upper parts of the landscape, gully erosion and river bank erosion also contribute much sediment in downstream direction. However, total suspended SY delivered to the lake-based gauging stations is 32 to 53% lower compared to the total suspended SY measured at the gauging station situated near the apex of the alluvial fans. This implies that a significant proportion of the sediment load is buffered by the fans and points to an important dis-connectivity between eroding mountains and rift valley lakes.

How to cite: Alemayehu Kasaye, T., Gulie, G., Chen, M., and Verstraeten, G.: Spatial and Temporal Variability in Suspended Sediment Yield for two tropical mountain catchments draining into the Southern Ethiopian Rift Valley., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2007, https://doi.org/10.5194/egusphere-egu21-2007, 2021.

09:07–09:09
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EGU21-16067
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ECS
Anatoly Tsyplenkov and Valentin Golosov

Processes linked to climate change and intensified anthropogenic pressure influence the environment, the hydrology and by extent the denudation processes in the Caucasus mountain belt. Quantitative assessments of sediment fluxes and their temporal evolution in this mountain region required for various environmental and engineering purposes, including the planning and maintenance of water reservoirs and other structures. This paper presents an analysis of the suspended sediment load data from almost 40 gauging stations located in the mountain part of the Terek river basin (North Caucasus, Russia). The collected dataset include river basins with various glacier cover (0%-20%) and human impact. All river basins show consistent decreases in mean annual suspended sediment load (SSL, kg/s) up to 1–2% per year during 1925–2018 (according to Mann-Kendall test). The cumulative deviation curve of the mean annual SSL for the last 60 years indicates that SSL has increased significantly from ca. 1970-1980 to 1990-2000 for the most North Caucasus rivers. However, after the 2000s mean annual values of the SSL show a stable decrease in all observed rivers. Possible mechanisms of observed changes are discussed. This study provides the data on climate-related changes in the sediment yield for a previously not investigated region.

How to cite: Tsyplenkov, A. and Golosov, V.: How does the suspended sediment yield change in the North Caucasus during the Anthropocene?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16067, https://doi.org/10.5194/egusphere-egu21-16067, 2021.

09:09–09:11
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EGU21-12062
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ECS
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Highlight
William Rapuc, Julien Bouchez, Pierre Sabatier, Kim Genuite, Jérôme Poulenard, Jérôme Gaillardet, and Fabien Arnaud

Soil erosion is one of the main environmental threats affecting the Critical Zone (CZ) and thus ecosystem services and human societies. This represents an emerging concern considered as one of the geosciences/society central issues. Through time, the physical erosion is linked to both, precipitation amounts induced by climate fluctuations, and the evolution of vegetation cover and land-use. Understanding these forcing factors is key to improve our management of this resource, especially in mountainous areas where CZ erosion is highest. Only studies combining large spatial and temporal approaches allow to assess the effect of these forcing factors on soil erosion rates. Here, we apply a retrospective approach based on lake sediments to reconstruct the long-term evolution of erosion in Alpine landscapes. Lake Iseo located in northern Italy at the downstream end of the Val Camonica acts as a natural sink for all the erosion products from a large watershed (1777 km²). This watershed is representative of the southern Italian Alps, where Holocene human activity and climate fluctuations are well known. The approach combines a source-to-sink method, using isotopic geochemistry (εNd, 87Sr/86Sr), with a multiproxy study of a lacustrine sediment section covering the last 2000 years. The applied methodology allows us to disentangle the role of climate and land use as erosion forcing factors through their differential impact on the various rock types present in the watershed. Indeed, the high-altitudinal part of the Val Camonica, the erosion of which is dominated by glacier advances and retreats, presents isotopic signature different from those of the sedimentary rocks located in the lower part of the watershed, where both human activities and precipitations impacted erosion through time. A chronicle of glacial erosion over the last 2000 years was produced. Once the climatic trend was highlighted, the signal of erosion of sedimentary rocks was investigated to understand the influence of humans. From the Roman Period to the Industrial Age several period of deforestation and increased human pressure were documented. The past sediment yield inferred for sedimentary rocks exhibits the highest values (> 80 t.km-2.yr-1) at periods of intense human practices. Hence, since the late Roman Period, human activities seem to be the dominant forcing factor of the physical erosion in mountainous environment of northern Italy. This study presents the first reconstruction through time of sediment yield derived from lake sediment associated with sediment sources identification and quantitative evaluation of the erosion forcing factors.

How to cite: Rapuc, W., Bouchez, J., Sabatier, P., Genuite, K., Poulenard, J., Gaillardet, J., and Arnaud, F.: Quantitative evaluation of human and climate forcing on erosion over the last 2000 years in northern Italy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12062, https://doi.org/10.5194/egusphere-egu21-12062, 2021.

09:11–09:13
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EGU21-2913
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ECS
Coline Ariagno, Caroline Le Bouteiller, Peter Van der Beek, and Sébastien Klotz

At the interface between the lithosphere and the atmosphere, the critical zone records the complex interactions between erosion, climate, geologic substrate and life, and can be directly monitored. The sparsely vegetated, steep marly badland catchments of the Draix-Bléone Critical Zone Observatory (CZO), SE France are characterised by high quantities of exported sediment and rapid morphologic changes. Characterizing and understanding the physical weathering processes in this area are key to predict the temporal variability of regolith production and sediment flux, as well as their evolution under changing climate conditions.

Long data records collected in the Draix-Bléone CZO allow analysing long-term regolith dynamics and climatic control on sediment export. Although widely accepted as the first order control, rainfall variability does not fully explain the observed yearly variability in sediment export, suggesting that regolith production and its controls may contribute to the observed pattern of sediment export. Within the several factors that can influence marls weathering (soil moisture, density, chemical weathering), this study focuses on continuous temperature data, recorded at different locations over multiple years, and aims to highlight the role of frost cracking in regolith production. Several proxies for frost cracking intensity have been calculated from these data and compared to the sediment export anomalies, with careful consideration of field data quality. Our initial results suggest that frost-cracking processes have a significant impact on catchment sediment response and should be taken into account when building a predictive model of sediment export from these catchments under a changing climate.

How to cite: Ariagno, C., Le Bouteiller, C., Van der Beek, P., and Klotz, S.: Sediment export in marly badland catchments controlled by frost cracking intensity, Draix-Bléone CZO, SE France, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2913, https://doi.org/10.5194/egusphere-egu21-2913, 2021.

Erosion and Weathering
09:13–09:15
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EGU21-3121
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ECS
Daniel Draebing and Till Mayer

Mechanical weathering by freezing and thermal processes are influenced by climate. Topography modulates this climatic influence due to altitudinal decrease of temperature, modifying insolation due to rockwall aspects and insulation by snow cover. In this study, we (i) quantify rock fracture damage in the field, (ii) monitor rock surface temperature and snow cover, (iii) model frost weathering processes, (iv) quantify fracture kinematics and (v) assess how these processes contribute to rockwall erosion. For this purpose, we conducted measurements on rockwalls with different aspects along an altitudinal gradient ranging from 2,500 to 3,200 m in the Hungerli Valley, Swiss Alps, between 2016 and 2019.

(i) The geology of the Hungerli Valley comprises schisty quartz slate with inclusions of aplite and amphibolite. We conducted Rock Mass Strength (RMS) measurements and used fracture spacing and uniaxial compressive strength (UCS) measurements as proxies for mechanical weathering. RMS ranges from 62 to 77 for schisty quartz slate rockwalls, up to 73 for aplite and 74 for amphibolite. Fracture spacing and UCS reflect lithological differences of the catchment area suggesting a geological control on weathering efficacy. 

(ii) Rock surface temperatures (RST) were monitored using temperature loggers. RST decreases with elevation from 2,500 to 2,900 m, however, increases again at 3,150 m potentially due to higher insolation on ridges. Snow cover duration shows a similar altitudinal trend. Due to aspect, RSTs are 2 to 4 °C warmer on south facing rockwalls with significant shorter snow cover period.

(iii) We used measured RST to drive frost cracking models by Walder and Hallet (1985) and Rempel et al. (2016). Both models show near surface frost weathering at lower altitudes, which should results in lower UCS. The models show significantly higher frost cracking at higher altitudes with peaks at rock depths between 0.5 and 2 m suggesting a higher fracture spacing.

(iv) Rockwalls between 2,500 and 2,900 m were equipped with crackmeters and show higher daily temperature changes and crack deformation at lower altitudes or south facing aspects due to higher insolation compared to higher located rockwalls. Seasonal crack displacement depends on dipping of monitored blocks and is controlled by both thermal and cryogenic processes (Draebing, 2020).

(v) In summary, low-altitudinal rockwalls show a higher weathering at the surface due to a combination of thermal processes and near surface frost weathering resulting in release of small blocks and lower erosion rates. In contrast, rockwalls at higher altitudes reveal higher seasonal thermal changes propagating deeper into the rock in combination with frost cracking in higher depths, which results in larger blocks and higher erosion rates.

 

Draebing, D.: Identification of rock and fracture kinematics in high Alpine rockwalls under the influence of altitude, Earth Surf. Dynam. Discuss., 1-31, 2020.

Rempel, A. W., Marshall, J. A., & Roering, J. J.: Modeling relative frost weathering rates at geomorphic scales. Earth and Planetary Science Letters, 453, 87-95, 2016.

Walder, J., and Hallet, B.: A Theoretical-model of the fracture of rock during freezing, Geological Society of America Bulletin, 96, 336-346, 1985.

How to cite: Draebing, D. and Mayer, T.: Topographic controls on frost and thermal weathering processes and implications for rockwall erosion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3121, https://doi.org/10.5194/egusphere-egu21-3121, 2021.

09:15–09:17
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EGU21-528
Masahiro Chigira and Yasuto Hirata

Typical cooling joints of granite have been believed to be orthogonal, and characteristic topography of granitic rocks like tors and boulder fields are interpreted in combination with the cooling joints and weathering. However, most of the previous studies were performed by the observation on the ground, and the 3D observation of cooling joints and the topographic features was not sufficient. We observed tors and boulder fields of granitic rocks using UAV and 3D modelling and found that columnar joints are typical for the granite that forms tors and that boulder fields are the accumulations of rock columns as well as boulders made by the spheroidal weathering of rock columns. Tors we observed were Mt. Kinabalu of Borneo, Mt. Mizugaki, Mt. Jizo, Mt. Gozaisho, Mt. Konze and 5 other locations in Japan. We observed that tors consist of polygonal rock columns with undulating joints, more irregularly shaped than the columnar joints of volcanic rocks. The cross-sectional areas of rock columns varied from 1 to 130 m^2, much larger than typical rock columns of volcanic rocks. The rock columns of granite are typically polygonal dipyramids, of which shapes may be dependent on the cooling history of granite. Boulder fields we observed was the Kui boulder fields in Hiroshima. We found that the boulder field is the accumulation of prismatic rock columns as well as rounded rock boulders. The prismatic rock columns had basal cross-sectional areas of 0.8 m^2 on average. The rock columns had chamfering cracks at corners, which are assumed to be made during cooling and to form preliminary outlines of core stones. Core stones had surface crusts or rindlets, which exfoliate and leave more rounded core stones.

  Rainstorm-induced landslides of weathered granite reflect weathering styles of granite: Landslides that occurred recently in Japan had three types, landslides of loosened layers of decomposed granite (or micro-sheeted granite), landslides of core-stone bearing materials, and landslides of saprolite. Landslides with core-stones were particularly destructive because of their inertia. Potential sites of such landslides could be predicted using columnar joints in fresh rocks as a clue.

How to cite: Chigira, M. and Hirata, Y.: Cooling joints of granite as a structural cause of the tors and boulder fields of granite, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-528, https://doi.org/10.5194/egusphere-egu21-528, 2021.

09:17–09:19
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EGU21-5908
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ECS
David Mair, Alessandro Lechmann, Romain Delunel, Serdar Yeşilyurt, Dmitry Tikhomirov, Christof Vockenhuber, Marcus Christl, Naki Akçar, and Fritz Schlunegger

Rock fall processes of various size and magnitude control retreat rates of high alpine rock-walls. For millennial time scales, these retreat rates can be quantified in-situ from concentrations of cosmogenic nuclides along bedrock depth profiles (Mair et al., 2019). We measured cosmogenic 36Cl and 10Be along several such profiles at Mt Eiger in the Central Swiss Alps to study the local rock-wall retreat on this time scale (Mair et al., 2019; 2020). The resulting spatial pattern shows that rock-wall retreat rates are low (0.5 to 0.6 ± 0.1 mm/yr) in the higher region of the NW rock-wall, in contrast to both the lower part of the NW rock-wall and the SE face, where rates are high (1.7 ± 0.4 to 3.5 ± 1.4 mm/yr). We link these retreat rates to differences in local temperature conditions, because the patterns of faults and fractures and the lithology of the bedrock are similar at all sites, and thermo-cryogenic processes are known to weaken the bedrock through fracturing, thereby preconditioning the occurrence of rock fall (e.g., Draebing and Krautblatter, 2019). However, it is still unclear how effective and at which rate individual thermo-cryogenic processes contribute to the preconditioning through fracturing. Therefore, we investigate several processes and estimate the probability of bedrock fracturing through the employment of a theoretical frost-cracking model, which predicts cracking intensity from ice segregation. The model results infer a low efficiency in the higher region of the NW rock-wall, but a relatively high one in the lower section of the NW wall and on the SE rock face of Mt. Eiger. Although the model is rather generic, the results disclose a significant control of temperature conditions on the erosional processes and rates. Furthermore, temperature conditions for the last millennia have been similar to present day conditions, as our reconstructions disclose, therefore the cosmogenic-nuclide-based long-term differences in rock-wall retreat rates predominantly stem from large contrasts in the microclimate between the NW and SE walls of Mt. Eiger. Accordingly, the site-specific differences in microclimate conditions could explain the lower retreat rates in the upper part of the NW rock-wall and the rapid retreat in the SW face and in the lower part of the NW rock face.

References

Draebing, D. and Krautblatter, M.: The Efficacy of Frost Weathering Processes in Alpine Rockwalls, Geophys. Res. Lett., 46, 6516–6524, doi:10.1029/2019GL081981, 2019.

Mair, D., Lechmann, A., Yesilyurt, S., Tikhomirov, D., Delunel, R., Vockenhuber, C., Akçar, N. and Schlunegger, F.: Fast long-term denudation rate of steep alpine headwalls inferred from cosmogenic 36Cl depth profiles, Sci. Rep., 9, 11023, doi:10.1038/s41598-019-46969-0, 2019.

Mair, D., Lechmann, A., Delunel, R., Yeşilyurt, S., Tikhomirov, D., Vockenhuber, C., Christl, M., Akçar, N. and Schlunegger, F.: The role of frost cracking in local denudation of steep Alpine rockwalls over millennia (Eiger, Switzerland), Earth Surf. Dyn., 8, 637–659, doi:10.5194/esurf-8-637-2020, 2020.

How to cite: Mair, D., Lechmann, A., Delunel, R., Yeşilyurt, S., Tikhomirov, D., Vockenhuber, C., Christl, M., Akçar, N., and Schlunegger, F.: High alpine rock-wall retreat rates over millennia through thermo-cryogenic pre-conditioned rock fall (Mt. Eiger, Switzerland), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5908, https://doi.org/10.5194/egusphere-egu21-5908, 2021.

09:19–09:21
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EGU21-16253
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ECS
Blen Taye and Heather Viles

Weathering of rock-cut structures exposed to the environment is strongly influenced by fluctuations in climatic variables. Both macro and microclimate data are needed to identify key weathering types and rates likely to affect rock-cut structures in a specific region. The aim of this paper is to study the macro and micro climatic conditions affecting the rock-cut churches in Lalibela, Ethiopia to determine how the climate influences weathering at this site. Macro climate data collected over a 26-year period and microclimate data monitored on the north, east, south and west walls at one of the churches in the Lalibela church complex (Bete Mariam) are used to make these assessments. Microclimate data was monitored during the long rains (Kiremt), short rains (Belg) and dry (Bega) seasons in 2018 and 2019. The results showed a high seasonal variation in macro climatic conditions like rainfall and ambient relative humidity. The micro climatic (rock surface) conditions also tended to vary seasonally. The diurnal range of rock surface temperature during Bega varied significantly depending on which cardinal directions the walls were facing, with south and west facing walls having high diurnal thermal ranges. The influence of aspect was less pronounced in Belg and Kiremt, but cloud cover played an important role in varying the range of diurnal thermal and humidity cycles from day to day during these seasons. These climate trends are likely to cause seasonal variations in wetting and drying cycles, deep wetting, increased time of wetness and thermal cycling. These wetting/drying and heating/cooling characteristics affect weathering processes. During Kiremt, biological weathering, salt weathering and clay swelling are more likely to occur than in Belg and Bega. High diurnal thermal ranges in Bega are likely to cause thermal fatigue in this season. This is the first paper to address the macro and micro climatic trends that influence rock weathering at the rock-cut churches in Lalibela. The results of this study also have implications for rock-cut structures in northern Ethiopia having similar environmental conditions as Lalibela.

How to cite: Taye, B. and Viles, H.: Impacts of climatic seasonality on weathering of rock-cut structures at Lalibela, Ethiopia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16253, https://doi.org/10.5194/egusphere-egu21-16253, 2021.

09:21–09:23
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EGU21-9221
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ECS
Aaron Bufe, Niels Hovius, Robert Emberson, Jeremy K.C. Rugenstein, Albert Galy, Hima J. Hassenruck-Gudipati, and Jui-Ming Chang

The supply of fresh minerals to Earth’s surface by erosion is thought to modulate global climate by removing atmospheric carbon dioxide (CO2) through silicate weathering. In turn, weathering of accessory carbonate and sulfide minerals is a geologically-relevant CO2 source, which may dampen or reverse the effect of silicate weathering on climate. Although these weathering pathways commonly operate side by side, we lack quantitative constraints on their co-evolution across erosion-rate gradients. Using stream-water chemistry across a 3 order-of-magnitude erosion-rate gradient in shales and sandstones of southern Taiwan, here, we demonstrate that silicate, sulfide, and carbonate weathering are linked: Increasing sulfide oxidation generates sulfuric acid and boosts carbonate solubility whereas silicate weathering kinetics remain constant or even decline, perhaps due to buffering of the pH by carbonates. On timescales shorter than marine sulfide compensation, CO2 emission rates from weathering in rapidly-eroding terrain are more than twice the CO2 sequestration rates in slow-eroding terrain. On longer timescales, CO2 emissions are compensated, but CO2 sequestration rates do not increase with erosion, in contrast to assumptions in carbon cycle models. We posit that these patterns are broadly applicable to many Cenozoic mountain ranges that expose dominantly siliciclastic metasediments.

How to cite: Bufe, A., Hovius, N., Emberson, R., Rugenstein, J. K. C., Galy, A., Hassenruck-Gudipati, H. J., and Chang, J.-M.: Co-variation of silicate, carbonate, and sulphide weathering drives release of CO2 with erosion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9221, https://doi.org/10.5194/egusphere-egu21-9221, 2021.

09:23–09:25
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EGU21-14281
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ECS
Timothée Jautzy, Gilles Rixhon, Régis Braucher, Laurent Schmitt, and Aster Team*

The Vosges Mountains in NE France belong to the belt of Variscan massifs located in the foreland of the Alps. Despite its rather limited extension barely reaching 6000 km², this range of low mountains peaking at ~1425 m presents three contrasting primary characteristics. Firstly, a bipartite N-S subdivision can be achieved based on the geological basement: whereas the southern part, traditionally referred to as the crystalline Vosges, is composed of a mosaic of Palaeozoic rocks, including igneous (mostly intrusive and secondarily extrusive), metamorphic, and sedimentary rocks, the northern part is much more homogeneous given its Triassic sandstone cover (“sandstone Vosges”). Secondly, a clear E-W topographic gradient characterises the mountain range. By contrast to the steep hillslopes and elevation drops regularly exceeding 600 m (sometimes reaching 900-1000 m) between the summits and the valley floors on the eastern side (Alsace; south-western border of the Upper Rhine Graben, URG), the western side exhibits more gently-sloping hillslopes along with a longer extension (Lorraine; eastern border of the Parisian Basin). This results from the sharp E-W contrast in Late Cenozoic tectonic activity between sustained subsidence in the URG to the east and weak rock uplift characterising the Parisian Basin to the west. Finally, the imprint left by Quaternary climatic fluctuations yielded a N-S gradient: whereas the southern part (roughly covering 80-90% of the crystalline Vosges) hosted abundant valley glaciers and still bears traces of significant glacial erosion (cirques and U-shaped valleys), the northern part (mostly the sandstone Vosges) was void of ice cover.

In spite of these advantageous characteristics, very little is known about the Quaternary evolution of the massif, in particular regarding the long-term interactions between denudation, lithological control, climatic forcing and tectonic activity. Against this background, this contribution aims to present the first data of long-term, massif-wide denudation. Modern stream sediments from 21 river catchments of different size draining the whole massif were thus sampled for in situ 10Be concentration measurements at the outlet of their mountainous reach. Catchment-wide denudation rates inferred from cosmogenic 10Be will be combined with the analysis of morphometric parameters and structural connectivity resulting from the processing of a high-resolution DEM (5 m). Catchment selection was operated according to the threefold subdivision above: i.e. heterogeneous vs homogenous petrography, tectonically-active eastern side vs “quiescent” western side and glaciated vs unglaciated catchments. We thus test the main hypothesis that the four NE, NW, SE, SW quarters of the Vosges massif shall be characterised by contrasting denudation rates, reflecting the respective role played by the controlling factors on long-term denudation. To our knowledge, this contribution is the first attempt to quantify denudation at the massif scale of a European low mountain range. This is especially relevant as long-term landscape evolution in the Variscan belt, by contrast to the numerous works focusing on denudation in high-mountains ranges (e.g. the Alps), has been regularly disregarded in recent geomorphological studies.

*Georges Aumaître, Didier L. Bourlès and Karim Keddadouche

How to cite: Jautzy, T., Rixhon, G., Braucher, R., Schmitt, L., and Team*, A.: Massif-wide denudation of the Vosges Mountains (NE France) inferred from in situ 10Be concentrations in stream sediments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14281, https://doi.org/10.5194/egusphere-egu21-14281, 2021.

09:25–09:30
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EGU21-3278
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ECS
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solicited
Chelsea Willett, Keith Ma, Mark Brandon, Jeremy Hourigan, Elizabeth Christeleit, and David Shuster

The topography, climate, and geology of the central Patagonian Andes provide an auspicious natural laboratory to track long-term rates of erosion in a dynamic mountainous landscape. Herein, we report a mountain-scale record of erosion rates in the central Patagonian Andes from >10 million years (Ma) ago to present, which covers the transition from a fluvial to alpine glaciated landscape. Apatite (U-Th)/He ages of 72 granitic cobbles from alpine glacial deposits show slow erosion before ~6 Ma ago, followed by a two- to three-fold increase in the spatially averaged erosion rate of the source region after the onset of alpine glaciations and a 15-fold increase in the top 25% of the distribution. This transition is followed by a pronounced decrease in erosion rates over the past ~3 Ma. We ascribe the pulse of fast erosion to local deepening and widening of valleys, which are characteristic features of alpine glaciated landscapes. The subsequent decline in local erosion rates may represent a return toward a balance between rock uplift and erosion.

How to cite: Willett, C., Ma, K., Brandon, M., Hourigan, J., Christeleit, E., and Shuster, D.: An auspicious landscape: Quantifying transient glacial incision in the Patagonian Andes from ~6 Ma to present, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3278, https://doi.org/10.5194/egusphere-egu21-3278, 2021.

09:30–09:32
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EGU21-6571
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ECS
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Highlight
Duna Roda-Boluda, Taylor Schildgen, Hella Wittmann-Oelze, Stefanie Tofelde, Aaron Bufe, Jeff Prancevic, and Niels Hovius

The Southern Alps of New Zealand are the expression of the oblique convergence between the Pacific and Australian plates, which move at a relative velocity of nearly 40 mm/yr. This convergence is accommodated by the range-bounding Alpine Fault, with a strike-slip component of ~30-40 mm/yr, and a shortening component normal to the fault of ~8-10 mm/yr. While strike-slip rates seem to be fairly constant along the Alpine Fault, throw rates appear to vary considerably, and whether the locus of maximum exhumation is located near the fault, at the main drainage divide, or part-way between, is still debated. These uncertainties stem from very limited data characterizing vertical deformation rates along and across the Southern Alps. Thermochronology has constrained the Southern Alps exhumation history since the Miocene, but Quaternary exhumation is hard to resolve precisely due to the very high exhumation rates. Likewise, GPS surveys estimate a vertical uplift of ~5 mm/yr, but integrate only over ~10 yr timescales and are restricted to one transect across the range.

To obtain insights into the Quaternary distribution and rates of exhumation of the western Southern Alps, we use new 10Be catchment-averaged erosion rates from 20 catchments along the western side of the range. Catchment-averaged erosion rates span an order of magnitude, between ~0.8 and >10 mm/yr, but we find that erosion rates of >10 mm/yr, a value often quoted in the literature as representative for the entire range, are very localized. Moreover, erosion rates decrease sharply north of the intersection with the Marlborough Fault System, suggesting substantial slip partitioning. These 10Be catchment-averaged erosion rates integrate, on average, over the last ~300 yrs. Considering that the last earthquake on the Alpine Fault was in 1717, these rates are representative of inter-seismic erosion. Lake sedimentation rates and coseismic landslide modelling suggest that long-term (~103 yrs) erosion rates over a full seismic cycle could be ~40% greater than our inter-seismic erosion rates. If we assume steady state topography, such a scaling of our 10Be erosion rate estimates can be used to estimate rock uplift rates in the Southern Alps. Finally, we find that erosion, and hence potentially exhumation, does not seem to be localized at a particular distance from the fault, as some tectonic and provenance studies have suggested. Instead, we find that superimposed on the primary tectonic control, there is an elevation/temperature control on erosion rates, which is probably transient and related to frost-cracking and glacial retreat.

Our results highlight the potential for 10Be catchment-averaged erosion rates to provide insights into the magnitude and distribution of tectonic deformation rates, and the limitations that arise from transient erosion controls related to the seismic cycle and climate-modulated surface processes.

 

 

How to cite: Roda-Boluda, D., Schildgen, T., Wittmann-Oelze, H., Tofelde, S., Bufe, A., Prancevic, J., and Hovius, N.: Erosion rates of the New Zealand Southern Alps reflect long-term tectonics and transient climate, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6571, https://doi.org/10.5194/egusphere-egu21-6571, 2021.

09:32–09:34
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EGU21-3439
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ECS
Larissa de Palézieux, Kerry Leith, and Simon Loew

Fluvial systems provide a primary means of sediment transport in alpine environments. This transport potential is regulated by precipitation, a fundamental contributor to stream power. Here, we investigate the legacy of reduced sediment transport capacity during dryer glacial intervals to fundamentally affect the diverse long-term evolution of 2 alpine catchments in the NW of Bhutan. The landscape in this region is characterized by three distinct geomorphic domains including transport-limited alluvial plains in the Inner Valleys, detachment-limited regimes in narrow valleys with steep hillslopes and high relief, and glacially overprinted low-relief landscapes at the foot of the High Himalayan peaks. The two major drainage basins, the Wong Chhu basin in the West and the Puna Tsang Chhu basin in the East, both traverse these geomorphic domains, yet show marked differences in their river profiles, with the alluvial plain of the Wong basin located 1000 m higher than the Puna Tsang Chhu basin. 

The characteristic difference in elevation and extent of the alluvial infill between the two main basins of NW Bhutan, points to a systematic difference in relative erosional efficacy. For the effects of baselevel fall due to differential uplift on the Himalayan rangefront to be expressed as increased erosional potential in the High Himalaya, sediment deposited in alluvial planes of the Inner Valleys during low stream power glacial periods must first be evacuated during interglacial intervals.  Through a combination of geomorphological mapping and chi analysis of river profiles, we demonstrate that while post-glacial incision today is approaching the upper limit of the alluvial plain in the Puna Tsang Chhu (and can therefore drive bedrock incision through much of a ~40 kYr interglacial interval), the present-day erosional limit in the Wang Chhu is less than half-way through the alluvial fill in the Wang Chhu. It is therefore unlikely rivers in the Wang Chhu have been able to access bedrock or propagate the effects of baselevel fall to the upper extents of the catchment since the mid-Pleistocene transition (MPT). 

The evolution of the fluvial system appears to be reflected in rock mass weathering and hillslope evolution throughout the study area. Engineering geological descriptions of rock mass properties, in particular weathering, recorded at 295 sites over a period of 8 weeks in the region closely associated with alluvial valleys demonstrates a range of weathering grades from fresh rock to residual soil. A progressive increase in weathering degree with increasing elevation above river channels is evident in both catchments, indicating the signal reflects the time since active fluvial erosion ceased (as opposed to a pre-existing rock mass property). We observe higher degrees of weathering in the Puna Tsang Chhu valley, corresponding to the more humid climate in this valley supporting more rapid bedrock weathering. This more efficient transition from rock to soil at lower elevations may hint at a positive feedback in which despite indications of an additional 1000 m of cumulative incision since the MPT, hillslopes have evolved to erode at a rate which approaches that of fluvial incision.

How to cite: de Palézieux, L., Leith, K., and Loew, S.: Buffering of interglacial landscape evolution due to reduced sediment transport during glacial periods in Bhutan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3439, https://doi.org/10.5194/egusphere-egu21-3439, 2021.

Landslides and Mass Movements
09:34–09:36
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EGU21-8508
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ECS
Karolina Naranjo Bedoya, Edier Aristizábal, Daniel Hölbling, John García, Asaf Aguilar, and David Ortiz

Colombia is an equatorial country located in the northwestern corner of South America with characteristic and complex climatic and geologic settings, which contribute to a great diversity of landforms in the Colombian Andes. 65% of the Colombian population is concentrated in this mountainous terrain, where landslides and torrential flows are common. These natural hazards led to several tragic events over time. Their occurrence is favored by a very dynamic landscape made up of weak and highly weathered materials and affected by tectonic stress. In this study, we aim to gain a better understanding of morphometric control on the occurrence of landslides and torrential flows through process geomorphology and information derived from Digital Elevation Models (DEMs). Several morphometric indices related to drainage network, basin geometry, drainage texture, relief characteristics, asymmetry factor and others were calculated over 168 drainage basins in the northern Colombian Andes. We used quantitative geomorphology to find patterns of anomalies associated with landscape evolution and the occurrence of landslides and torrential flows. Understanding morphodynamics from morphogenesis is important to assess landslide and torrential flow hazard conditions in relation to landscape characteristics and evolution, to support hazard assessment, and consequently to reduce human and economic losses.
Keywords: Landslide, torrential flow, morphometric indices, mountainous terrains.

How to cite: Naranjo Bedoya, K., Aristizábal, E., Hölbling, D., García, J., Aguilar, A., and Ortiz, D.: Approach for analyzing landslide and torrential flow hazard conditions in relation to landscape evolution in the northern Colombian Andes , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8508, https://doi.org/10.5194/egusphere-egu21-8508, 2021.

09:36–09:38
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EGU21-15605
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ECS
Benedetta Dini and Georgina L. Bennett

 

Landslides from mountainous bedrock hillslopes often contain boulders, the presence of which has been shown to influence landscape evolution by altering hillslope geomorphic processes and river erosion. Furthermore, the presence in various proportions of large grain sizes on hillslopes can amplify both landslide and flood hazards in largely unquantified ways. Boulders can have an immediate destructive potential on properties and infrastructure and can hinder response and recovery by blocking access routes, posing a challenge for removal. On entering the river network, they might have far reaching effects if remobilised in high flows, damaging or destroying key infrastructure such as hydropower plants and inducing significant knock-on effects on local economies. A fundamental step towards quantification of increased hazard potential is the understanding environmental controls on boulder production. Despite their potential to enhance hazard, the probability of large boulders being produced within different landslide types has not been directly accounted for in landslide hazard mapping.

Our study focuses on the upper Bhote Koshi catchment, northeast of Kathmandu (Nepal), characterised by extreme topographic gradients, seismicity and monsoonal climate, and subjected to frequent landslides and floods. This, coupled with increased population pressure and infrastructure growth, makes the area prone to natural disasters.

We used high resolution optical imagery to map more than 11300 boulders and analysed this large dataset in combination with lithology and topography, and well as structural and landslide data, to investigate controls on boulder production and grain size distributions in different lithological, structural and geomorphic settings of the landscape.

Lithology appears to exert a significant influence on boulder sizes, with statistically significantly larger boulders observed in crystalline rocks of the Higher Himalaya Sequence than in metasedimentary rocks of the Lesser Himalaya Sequence. We also observe that the spacing of the most pervasive fracture set, parallel to foliation, influences boulder size distributions in some lithologies, whilst other dominant regional fracture sets appear not to strongly correlate with mapped boulder sizes.

Although recent studies have shown the importance of structural control on boulder sizes, our large dataset reveals that for complex, high-relief landscapes, with high erosion rates, fracture characteristics do not fully explain grain size distribution.

The type of processes involved in boulder production and transport on slopes, before reaching the river network, also appears to exert a control over grain size distributions and boulder density, with rockfall processes appearing to be responsible for producing boulders with largest sizes as opposed to rockslides, where the high energy and mode of transport is likely associated with increased fragmentation.  

Analysing lithological and structural characteristics alone may not be sufficient to explain the observed distribution and would thus only give a limited insight in the enhanced hazard levels posed by boulders across different sectors of a landscape and other factors, such as distance from source and mode of transport at shorter temporal scales, must be taken into account.  

How to cite: Dini, B. and Bennett, G. L.: Analysis of controlling factors on boulder production on the hillslopes of the upper Bhote Koshi catchment, Nepal, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15605, https://doi.org/10.5194/egusphere-egu21-15605, 2021.

09:38–09:40
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EGU21-10795
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ECS
Odin Marc, Jens Turowski, and Patrick Meunier
The size of grains delivered to rivers by hillslopes processes is thought to be a key factor to better understand sediment transport, long-term erosion as well as sedimentary archives. Recently, models have been developed for the grain size distribution produced in soils, but they may be irrelevant to active orogens where high erosion rates on hillslopes are driven by landsliding. Still, until now relatively few studies have focused on measuring and explaining the variability of landslide grain size distributions.
Here we present grain size distribution obtained by the grid-by-number method on 17 recent landslide deposits in Taiwan, and we compare it to the geometrical and physical properties of the landslides, such as their width, area, rock-type and strength, drop height and estimated depth. All landslides occurred in slightly metamorphosed sedimentary units, except two which occurred in younger unmetamorphosed shales, with rock strength expected to be 3 to 10 times weaker from their metamorphosed counterparts. We found that 4 deposits displayed a strong grain size segregation on their deposit with grains at the toe (downslope) of the deposit 3 to 10 times coarser than the one at the apex. In 3 cases, we could also measure the grain size distribution inside the landslides that presented percentiles 3 to 10 times finer than the surface of their deposits. Both observations could be due to either kinetic sieving or deposit reworking after the landslide failure but we could not explain why only some deposits had a strong segregation.
Averaging this spatial variability we found the median grain size (D50) of the deposits to be strongly negatively correlated to drop height, scar width and depth. However, previous work suggests that regolith particlesvand bedrock blocks should become coarser with increasing depth (Cohen et al., 2010; Clarke and Burbank, 2011), opposite to our observation. Accounting for a model of regolith coarsening with depth, we found that the ratio of the original bedrock blocksize and the D50 was proportional to the potential energy of the landslide normalized to its bedrock strength. Thus the studied landslides agree well with the simple fragmentation model from Locat et al. (2006), even if it was calibrated on much larger and much stronger rock avalanches. This scaling may thus serve for future model of grain size transfer from hillslopes to river, trying to better understand landslide sediment evacuation and the coupling between hillslopes and river erosional dynamic.
 
References:
Clarke, B. A. and Burbank, D. W.: Quantifying bedrock-fracture patterns within the shallow subsurface: Implications for rock mass strength, bedrock landslides, and erodibility, Journal of Geophysical Research: Earth Surface, 116(F4), F04009, , 2011.
Cohen, S., Willgoose, G. and Hancock, G.: The mARM3D spatially distributed soil evolution model: Three-dimensional model framework and analysis of hillslope and landform responses, Journal of Geophysical Research: Earth Surface, 115(F4), , 2010.
Locat, P., Couture, R., Leroueil, S., Locat, J. and Jaboyedoff, M.: Fragmentation energy in rock avalanches, Canadian Geotechnical Journal, 43(8), 830–851, , 2006.
 

How to cite: Marc, O., Turowski, J., and Meunier, P.: Controls on the grain size distribution of landslides in Taiwan: the influence of drop height, scar depth and bedrock strength, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10795, https://doi.org/10.5194/egusphere-egu21-10795, 2021.

09:40–09:42
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EGU21-16062
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ECS
Yohei Arata, Takashi Gomi, Rasis Putra Ritonga, and Roy C. Sidle

The spatial variability of landslides and associated sediment deposits induced by earthquakes alters both short- and long-term sediment dynamics in watersheds. Linkages between landslide occurrence and sediment accumulations within channels are important for evaluating spatial and temporal dynamics of sediment from headwaters to downstream. To evaluate spatial variability of landslides, we examined landslide-area density (LAD: landslide area divided by watershed area) in different sub-watersheds (areas 0.01 to 4.4 km2) of Habiugawa watershed (40 km2), which was affected by the 2018 Hokkaido Eastern Iburi earthquake, Japan. The watershed is located 13 km north of the epicenter and is covered by secondary conifer and deciduous forest. The topography is hilly associated with long-term landform development by paleo-glacial erosion. Altitude ranges from 30 to 440 m; mean hillslope and channel gradients are 30° and 10°, respectively. Landslides mostly occurred at depths from 1 to 2 m below pumice layers formed by the Mt. Tarumae eruption 9000 yr ago (Ta-d), with total soil depths from 2 to 3 m. The 0.5 m LiDAR-based DEM and 0.2 m post-earthquake orthophotos were used to calculate LAD by GIS analysis. To examine spatial variability of in-channel sediment deposited by landslides, we used deposit-length ratio (DLR: total length of sediment accumulations within channels divided by total channel length within sub-watersheds). Sediment deposition in channels was assessed as rough surface topography by DEM and orthophotos.

We identified 2941 landslides: mean area=1620 m2; range from 20 to 34710 m2. LAD in the entire Habiugawa watershed was 0.12 km2 km-2, which is high compared to the other earthquake-induced landslides (e.g., Wenchuan earthquake: 0.03 km2 km-2). Sub-watersheds < 0.1 km2 had wide ranges in LADs (0.0 to 0.8 kmkm-2), while sub-watersheds from 0.1 to 0.5 km2 ranged from 0.2 to 0.5. Sub-watersheds > 0.5 km2 had LADs from 0.1 to 0.3. Seventy-four percent of small watersheds (< 0.5 km2) with high LADs (> 0.3) also had high sediment accumulations within gentle channels (DLR ≥ 0.8). This suggests that poorly mobilized sediments that initiate in headwaters rapidly deposit in channels. Conversely, the other small watersheds (26%) had lower sediment accumulation within steeper channels (DLR < 0.8), suggesting that these high-mobilized sediments traveled longer and were evacuated from watersheds to some extent. Such differences in sediment mobility in small sub-watersheds (< 0.5 km2) may cause sporadic sediment accumulations within channels of larger watersheds (> 0.5 km2). Our findings suggest that geomorphic features of watersheds associated with long-term legacies of geomorphic evolution possibly affect the spatial variability of landslide occurrence and the associated in-channel sediment accumulation induced by the earthquake.

How to cite: Arata, Y., Gomi, T., Ritonga, R. P., and Sidle, R. C.: The spatial variability of landslide occurrences and transported sediments induced by the 2018 Hokkaido Eastern Iburi earthquake, Japan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16062, https://doi.org/10.5194/egusphere-egu21-16062, 2021.

09:42–09:44
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EGU21-3143
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ECS
Sharon Pittau, Giovanna Daniele, Marco Pizziolo, and Francesco Brardinoni

In mountain environments, landslides are dominant geomorphic processes of sediment transfer and as such, they play a fundamental role in landscape evolution and sediment management at the watershed scale. While monitoring of landslide dynamics at the scale of the single slope failure provides precious site-specific information, an appraisal of landslide-driven sediment dynamics at more representative spatial scales is rarely pursued. In this context, the compilation of multi-temporal, high-resolution landslide inventory represents a challenging but critical task.

In the Emilia-Romagna region (Italy), landslides cover about the 24% of the hilly and mountain areas within the Northern Apennines. Here, the most common types of landslides are earth slides and earthflows that mainly involve the terrain of clay Ligurian Units and usually are the re-activations of preexisting mass movements. Since the mid ’80, the Geological Survey of Emilia-Romagna Region (RER) has started compiling and updating a region-wide landslide inventory, which includes all movement types, as well as both active (n = 44,377) and dormant (n = 36,608) landforms. The inventory update is customarily performed in selected areas, mainly where landslides have created damages or pose risk to infrastructures, or where ad hoc land planning is needed. In this context, a systematic multi-temporal approach that could provide robust information on landslide occurrence and recurrence is missing.

To address this gap, in this contribution we propose a multi-temporal inventory prototype, which includes a set of attributes aimed at characterizing landslide sediment transfer across decades. The prototype is developed in the mountain portion of the Sillaro River basin (139 km2). The basin is chiefly underlain by argillites of the Ligurian domain, where earth slide and earthflow activity is pervasive.

The compilation of the multi-temporal landslide inventory is conducted through visual inspection of 10 sequential aerial photo sets (1954, 1969, 1976, 1985-88, 1996, 2000, 2006, 2008, 2011, and 2014), as well as Google Earth satellite imagery (2016 and 2018). In particular, each landslide polygon encloses the total disturbed area, which includes initiation, transport and deposition zones. Polygon planimetric changes are then recorded across sequential photosets. In this way, it is possible to record recurring landslide movements.

Landslide planimetric geometry includes length, width, and area. Landslide attributes include movement type, photo year of occurrence, morphologic position at initiation (source), and sediment delivery target (sink). Subsequently, for each landslide we subdivide total disturbed area into initiation-transport and deposition polygons. For recurring landslides, we note whether the movement involved: (i) the whole landslide body or only part of it; (ii) headscarp migration; and (iii) advance of the landslide terminus. Finally, we note whether the landslide deposition zone displayed headward incision by means of gully development and/or revegetation.

This work, as part of the projects BEDFLOW and BEFLOW PLUS, is partially funded by Fondazione Cassa di Risparmio in Bologna.

How to cite: Pittau, S., Daniele, G., Pizziolo, M., and Brardinoni, F.: A prototype, high-resolution multi-temporal landslide inventory for the Sillaro River basin, Northern Apennines, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3143, https://doi.org/10.5194/egusphere-egu21-3143, 2021.

09:44–10:30