Vegetation as nature-based solution for mitigating climate induced geo-hazards and associated consequences along slopes and streambanks

Climate induced geo-hazards are known to increase due to climate change causing more intense rainfall and more frequent extreme weather events. Use of vegetation on potentially unstable slopes and along stream banks is an example of Nature-Based Solutions (NBS) that can mitigate climate induced geo-hazards due their role at the soil-atmosphere interface. Vegetating slopes or stream banks are also key for riparian buffers for agricultural nutrient uptake, CO2 storage, improvement of water quality, ecological restoration and rewilding. However, researchers in different fields of science do not easily communicate, even though they are addressing aspects of the same problem.
Interdisciplinary research is needed to document the effects of vegetation in hazard-prone areas in a measurable and quantifiable manner. These NBS must have an ecological approach, where in the long-term perspective, a multiple approach for biodiversity and ecosystem services will give mutual synergies.

This session aims to stimulate interdisciplinary and international communication, knowledge exchange and dissemination on plant-soil-atmosphere interaction, with focus on vegetation based NBS for mitigating climate-induced geohazards, with a special focus on shallow landslides and erosion, and associated consequences.
Contributions documenting how vegetation and associated roots can be beneficial in landslide mitigation as well as in land use planning, restoration ecology, climate change adaptation are welcome within the fields of geotechnical engineering, plant ecology, root-soil biophysics, biodiversity, alpine timberline, hydrogeology and agronomy.

Specific topics are related to the following questions:

• How the plant-soil-atmosphere interaction affects the unsaturated zone and thus the slope stability?
• Measuring and quantifying the effects of vegetation as NBS for mitigating climate induced geo-hazards and associated consequences at or near instable slopes and along stream banks – case studies at full scale or laboratory scale.
• How to ensure interdisciplinary interaction for sustainability and mutual synergies for studies containing vegetation as NBS for mitigating climate induced geo-hazards and associated consequences at or near instable slopes and along stream banks.

Convener: Vittoria CapobiancoECSECS | Co-conveners: Sabatino Cuomo, Dominika KrzeminskaECSECS, Anders Solheim
vPICO presentations
| Thu, 29 Apr, 11:00–12:30 (CEST)

vPICO presentations: Thu, 29 Apr

Chairperson: Vittoria Capobianco
Anthony Leung, Davide Boldrin, Ali Akbar Karimzadeh, Zhaoyi Wu, and Suriya Ganesan

Plant roots affect soil water regime through root-water uptake upon transpiration. This process induces soil matric suction, which affects soil hydraulic conductivity, soil shear strength and hence shallow soil stability. This is referred to as plant hydrological reinforcement in the soil bioengineering application. Recent experimental evidence put forward by the authors has demonstrated that plant hydrological reinforcement should not be exclusively limited to the effects of root-water uptake and plant transpiration. The presentation will provide some new evidence of other potential aspects of plant hydrological reinforcement, namely (1) root-induced changes in soil hydraulic properties, (2) root water-dependent bio-hydro-mechanical properties. In aspect (1), laboratory test results on how root growth dynamic alter the soil pore size distribution and hence affect both the soil water retention curve and hydraulic conductivity will be presented. To highlight the effects of these root-induced changes in soil properties on slope water regime and slope stability, numerical simulation employing a dual-permeability water transport model in unsaturated rooted soil will be discussed. In aspect (2), a new concept, hysteretic root water retention curve (relationship between root water content and root water potential), will be introduced with support of some preliminary data. How root water retention affects the root biomechanical properties including not only tensile strength and Young’s modulus that have received wide attention in the soil bioengineering literature but also breakage strain will be presented. New data will be provided in order to attempt to use root water content to explain the large variability of biomechanical properties observed in the literature.

How to cite: Leung, A., Boldrin, D., Karimzadeh, A. A., Wu, Z., and Ganesan, S.: Root bio-hydro-mechanical reinforcement of unsaturated vegetated soil: experiments and modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8126, https://doi.org/10.5194/egusphere-egu21-8126, 2021.

Catalina Lozada and Yurleidy Rocha

The developments in infrastructure require adequation and construction of new roads and therefore stable slopes adapted to the weather and its variations associated with the current climate change. In Colombia, many of the slopes are reinforced with vegetation of different species, which are selected depending on the climatic conditions related to the altitude (between 0 to 3000 msl in Colombia). The vegetation contributes to the slope stability in two manners: (i) mechanically, as the roots act as small anchors in tension increasing the shear strength to the soil, and (ii) plant transpiration, which contributes to the increase of suction in the soil and therefore increasing the shear strength. Despite that this practice is very common in the country, the design continues to have a very important empirical component.

The objective of this research is to study through physical models the mechanical contribution of the root of three different grass species (Vetiver, Brachiaria, and San Agustin) on the deformation field, the shape of the failure surface, and the increase of the factor of safety in a clayey slope. In order to do this, physical models in the geotechnical centrifuge of the Colombian School on Engineering Julio Garavito were performed. For the root of grass simulation, glass fiber was selected considering scaling laws for physical modeling in the geotechnical centrifuge for an acceleration field of 100 x g. To model each grass, the glass fiber was mixed with clay with a percentage in mass that depends on the tensile strength specific to each root. The contribution of the grass-reinforced soil in the undrained shear strength was obtained through triaxial tests in samples of clay and grass-reinforced clay. The theoretical increase of the factor of safety of each grass-reinforced slope was computed using the finite element software Slide by Rocscience. Finally, physical models of slopes in the geotechnical centrifuge with and without reinforcement equivalent to each grass were performed. Deformation fields in each model were analyzed through particle image velocimetry technique using the software GeoPIV_RG.

As a result, the numerical and the physical models show that the movement produced in the slope reinforced with vetiver grass is lower than the movement obtained with the Brachiaria grass and San Agustin grass, respectively. This is because the root morphology generates a specific size, number, and depth of roots that affects the global stresses of the slope and therefore consequent deformations. The results obtained in the physical models allow the designers to predict the behavior of the reinforced slope and to estimate the order of magnitude of the reinforcement that should be reached in the field for each species of grass. It is important to continue investigating the effects of vegetation on slope stability as a solution that can reduce the environmental impact compared to other solutions that also involve higher construction costs.

How to cite: Lozada, C. and Rocha, Y.: Physical modeling of the effect of roots of grass in the slope stability , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3499, https://doi.org/10.5194/egusphere-egu21-3499, 2021.

Anthony Leung, Ali Akbar Karimzadeh, and Zhaoyi Wu

Plant roots have been considered to be effective to reinforce shallow soil slopes under rainfall conditions. Recent evidence from geotechnical centrifuge modelling shows that plant roots could improve earthquake-induced slope stability and reduce slope crest settlement. However, the underlying fundamental mechanisms of soil-root mechanical interaction against seismic loading are unclear. Although there has been a large volume of studies focusing on root reinforcement, cyclic soil-root mechanical interaction has rarely been investigated. Moreover, whether plant roots could reduce the liquefaction potential of rooted soil. This presentation will present some new test data and evidence about (1) cyclic root biomechanical behaviour and (2) cyclic responses of root-reinforced soil. In part (1), results of cyclic uniaxial tensile tests on roots of a wide diameter range will be presented, including any root hardening or softening and change in the size of hysteresis loops under displacement-controlled loading condition. Special attention will be paid on any observation of cyclic-induced root mechanical fatigue. In part (2), results of a comprehensive set of monotonic and cyclic triaxial tests on rooted soil will be presented. The cyclic behaviour observed will be interpreted through the monotonic behaviour observed along both the triaxial compression and extension paths. Any change in soil failure mechanism from limited flow failure to cyclic mobility due to plant roots, and how/when this change occurs at different root volume and cyclic stress ratio, will be discussed in detailed. A new attempt to interpret the liquefaction resistance through an energy-based approach will be made to evaluate the energy dissipation mechanism in rooted soils.

How to cite: Leung, A., Karimzadeh, A. A., and Wu, Z.: Cyclic soil-root mechanical interaction, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8127, https://doi.org/10.5194/egusphere-egu21-8127, 2021.

Alessandro Fraccica, Enrique Romero, and Thierry Fourcaud

Soil tensile strength plays an important role in the hydro-mechanical behaviour of earth structures and slopes interacting with the atmosphere. Shrinkage-induced cracking may be generated by drying/wetting cycles, with consequent faster water infiltration from the top of slopes and reduction of the safety factor. Vegetation roots were proven to increase soil shear strength, but less is known about their effects on soil tensile strength. For this purpose, new equipment has been designed and used to induce plant growth in compacted soil samples and to perform uniaxial tensile tests on the reinforced material. The equipment is composed of two cylindrical moulds linked by a soil bridge in which the tensile crack is induced due to geometrical restraints.

For this study, silty sand was chosen and compacted at a low dry density (ρd = 1.60 Mg/m3) and at a water content w = 15%. After compaction, samples were gently poured with water up to a high degree of saturation (Sr ≈ 0.95) and low suction (s ≈ 1 kPa). Then, six of them were seeded with Cynodon dactilon, adopting fixed seeding density and spacing. Plants were irrigated and let to grow for three months: during this period, suction was monitored by a tensiometer. Seven fallow specimens were prepared following the same procedure, for comparison purposes.

When ready, samples were dried in a temperature/relative humidity-controlled room and left in the darkness for three hours, to attain and equalise the desired value of initial suction. Finally, the tensile stress was induced on the soil by a displacement rate of 0.080 mm/min. For each test, suction was continuously monitored by a tensiometer while the water content was checked at the beginning and at the end. Moreover, the void ratio and the root volume and area ratio were assessed close to the crack generated, at the end of each test.

The hydraulic state affected the soil mechanical response upon uniaxial extension: an increase of strength and a more brittle behaviour were observed as suction was increasing. At the same suction, a higher strength was systematically observed in the vegetated soil. In fact, even at very low suction (i.e. s = 1 kPa), vegetation roots induced a considerable increase in soil tensile strength (i.e. 10 kPa). The soil hydraulic state also affected the root failure mechanism. In wet soil, the roots subjected to tension were stretched and pulled-out whereas in dry soil they experienced a more immediate breakage (i.e. in concomitance with the cracking of the surrounding soil). Some preliminary PIV (Particle Image Velocimetry) analyses showed differences among dry/wet and fallow/vegetated soils. Indeed, a more diffuse strain field was observed in vegetated samples, thanks to the redistribution of stresses induced by the roots.

Results were successfully interpreted by a well-established shear strength criterion for partially saturated soils, considering the degree of saturation, suction and soil microstructure. An increase of the soil shear strength was observed and correlated to the presence of roots and to their geometrical and mechanical features. Moreover, good consistency was detected with results coming from other equipment.


How to cite: Fraccica, A., Romero, E., and Fourcaud, T.: Vegetation effects on the tensile strength of a partially saturated soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16367, https://doi.org/10.5194/egusphere-egu21-16367, 2021.

David Boldrin, Anthony Glyn Bengough, Jonathan Knappett, Kenneth Loades, and Anthony Kwan Leung

Climate change is expected to introduce increasing threats to human health and the urban built environment, due to extreme events such as heavy precipitation. In the urban environment, impermeable hard-engineered surfaces may exacerbate climate change effects and increase the risk of floods. Adaptation solutions are essential to limit the climate change impacts on the urban environment. Research is needed to design new environmentally friendly multi-layer earthen barrier systems that can mimic the natural hydrological processes (e.g., plant-soil interaction) removed by urbanization.

In this study, potential barrier materials were selected from both natural soils and recycled waste materials (e.g., recycled concrete aggregates). Contrasting herbaceous species (legumes, grasses and forbs) were selected and grown for five months in compacted soil columns and saturated hydraulic conductivity (Ksat) was tested for each soil column. Following Ksat tests, all soil columns were saturated and left for evapo-transpiration. Plant water uptake, matric suction and soil strength (penetration resistance) were measured.

Among the materials tested in this study, recycled concrete aggregate (RCA) was the most suitable material for the barrier drainage layer, having a Ksat equal to natural gravel, but with 14% lower dry density (2.3 Mg/m3) and seven-fold greater water holding capacity (0.08 g/g). However, a portion of the water stored in the RCA was strongly bound to micropores and not available for plants. Plant growth in soil columns increased Ksat. On average Ksat of four-month old vegetated soil (3.2e-5 ± 2.0e-6 m/s) was four times larger than that of control fallow soil (6.9e-6 ± 1.4e-6 m/s). However, tested species differed in their effect on Ksat, ranging from 9.9e-6 ± 1.3e-6 m/s of Festuca ovina (Grass) to 4.1e-5 ± 3.7e-6 of Lotus pedunculatus (Legume). In the fallow soil, daily evaporation led to an average water loss of 0.49 ± 0.04 g per 100 g of soil, evapo-transpiration led to a daily water loss up to 2.58 ± 0.10 g per 100 g of soil in Lotus corniculatus columns. Thus, soil drying and induced matric suction strengthened the vegetated soil and further increased its ability to store water. For instance, soil vegetated with L. corniculatus had seven times faster water absorption and twenty-five times greater strength compared with control fallow soil. Plants affected the hydraulic conductivity and water relation of the barrier system. Root systems can increase soil hydraulic conductivity through root-induced channels. This may enable faster drainage during floods, but we found large differences between species. Transpiration restored the water holding capacity of barrier systems after heavy rain events and induced strengthening of soil.

We suggest that vegetation should not be simply selected for aesthetically “greening” the barrier system, but specifically selected for its role in improving soil engineering function. There is a substantial scope to choose species to manipulate hydrological properties of the barrier system and improve its performance during extreme climate events.

How to cite: Boldrin, D., Bengough, A. G., Knappett, J., Loades, K., and Leung, A. K.: Plant-soil interactions can enhance earth barrier systems in urban spaces , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9626, https://doi.org/10.5194/egusphere-egu21-9626, 2021.

Stine Olsen and Vittoria Capobianco

Biodiversity loss (including land degradation) and climate change are the biggest challenges of our time, and they have a strong two-way interaction. Climate induced geo-hazards, such as landslides and floods, are likely to to increase in the near future due to climate change causing more rainfall and more frequent extreme weather events. On the other hand, biodiversity is continuously threatened by climate-induced geo-hazards.

PlaNet is a multidisciplinary network that gathers experts in the field of nature-based solutions (NBS) that use vegetation for mitigating rainfall-induced geohazards, with a focus on shallow landslides and erosion. Vegetating slopes or stream banks are also key for ecological restoration and rewilding, as well as buffer-zones for agricultural lands.

PlaNet pools expertise from leading research organisations within geosciences and provides a collaborative environment for addressing these problems using sustainable, nature-based solutions.

The Norwegian Geotechnical Institute (NGI) coordinates PlaNet, and the network has 5 research organisations and 7 universities as initial core partners. PlaNet is multidisciplinary, encompassing a wide range of expertise areas, such as geotechnical engineering, hydrology, soil science, plant ecology, biodiversity, and agronomy.

The objectives of PlaNet are to share research on how vegetation-based solutions can be used to mitigate climate changes, influence policy nationally, internationally prepare the grounds for a European policy to be adopted by future research programmes and to foster multidisciplinary and international research-exchange and facilitate participation of industry and entrepreneurs.

PlaNet provides an appropriate knowledge exchange forum that is urgently needed for future implementation of vegetation and nature-based solutions for mitigating climate-related geohazards, while protecting biodiversity. This forum does not only benefit researchers but also provide the knowledge needed for policy makers, entrepreneurs and suppliers, and to the general public for education and information.

Activities in the network include knowledge dissemination, through filming and distributing virtual laboratory/site tours to real case study sites where NBS have been implemented, to generate interest and enhance the impact of PlaNet beyond the "research" boundaries. As well as to promote the international presence of Norwegian PlaNet partners. Activities also include publishing articles in national and international professional journals and magazines.

PlaNet contributes to encourage Norwegian researchers to participate more widely in, and exert greater influence on, global research on climate and the environment and it contributes to Norway gaining a leading position in Europe for development and use of nature-based solutions. It also contributes to strengthen the international relationship among Norwegian research institutes and universities developing expertise in different aspects of nature-based solutions. It promotes and supports research on vegetation-based solutions for mitigating climate-induced geohazards and contributes to the use of nature-based solutions and ecological restoration.

How to cite: Olsen, S. and Capobianco, V.: PlaNet – An international research network on plant-based solutions to mitigate climate-induced geo-hazards, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10844, https://doi.org/10.5194/egusphere-egu21-10844, 2021.

Sabatino Cuomo and Mariagiovanna Moscariello

Mountain tracks and slope cuts are important sources of runoff and sediment transport in a watershed. Some slope instabilities are also observed nearby mountain roads and tracks. Most of the current literature points out as relevant the modifications of the slope topography, and the concentration of runoff at the bends of the trackways. However, quantitative analysis of runoff generation and sediment delivery are still uncommon. Moreover, the role of vegetation removal or modification along/nearby tracks is not addressed. A physically-based distributed modelling of water runoff, soil erosion and deposition on a natural slope is performed considering the impacts of a mountain track, either in terms slope topography modifications or for the infiltration-runoff patterns. The erosion scenarios for a 30° steep slope are computed with different rainstorms and initial soil suction considered. The numerical analyses provide a comprehensive set of erosion scenarios. Particularly, the numerical results outline the bend of the mountain roads as a major confluence path for water runoff, consistently with the in-situ evidences. The highest loss of soil is found besides and downslope the bends. Very unfavorable combinations of vegetation removal and change in slope topography may finally lead to extensive rill erosions and/or shallow slope failures.

How to cite: Cuomo, S. and Moscariello, M.: Distributed modeling of runoff and soil erosion in vegetated slopes with man-made mountain tracks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14306, https://doi.org/10.5194/egusphere-egu21-14306, 2021.

Carmine Gerardo Gragnano, Guido Gottardi, and Elena Toth

One of the principal source of vulnerability for riverbanks is given by slopes instabilities, which is triggered on the riverside by fluvial erosion. In order to mitigate such erosion, the establishment of a dense herbaceous cover aims at promoting the slope protection and reducing the likelihood of embankment failure. In fact, the aerial parts of vegetation reduce the mechanical impact of river level fluctuations and rainfall on the embankment and retain sediment transported, while the belowground parts reinforce mechanically the materials forming the top of the embankment, facilitating drainage in the topmost layers and promoting plant water uptake, thus contributing to the regulation of the drying/wetting cycle.

Plating deep-rooting perennial, herbaceous species on earth embankments therefore represent a sustainable, green intervention for the protection of a riverbank susceptible to fluvial erosion, contributing to the preservation of the fluvial ecosystem environment and avoiding a wide use of grey solutions. The European research project OPERANDUM is testing also this typology of NBS, with an experimental site selected on the river Panaro, one of the main tributary of the main Po River, Italy. To investigate the effect of vegetation on the riverbank soil, a monitoring system has been installed at shallow depths. The system estimates soil water content, matric suction and pore water pressure, in order to quantify the effects of the growth of different vegetation species, which have been recently seeded on site, for analyzing the plant-soil-atmosphere interaction. The work will present the site preparation and the system implementation. The analysis of the first collected data and the outcomes of the preliminary investigations, including site and laboratory experiments, will then be discussed. Monitoring data collected along the entire vegetation growth cycle, that is expected to take around two years, will allow to quantify the influence of vegetation in the soil-atmosphere interaction processes and, on the long-term, verify its effective contribution in riverbank protection.

How to cite: Gragnano, C. G., Gottardi, G., and Toth, E.: Monitoring soil retention properties in a riverbank susceptible to fluvial erosion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6233, https://doi.org/10.5194/egusphere-egu21-6233, 2021.

Vittoria Capobianco, Kate Robinson, Bjørn Kalsnes, and Øyvind Høydal

Vegetation is used as a nature-based solution (NBS) to restore rivers and mitigate water triggered processes along stream banks, such as soil erosion or floods. Furthermore, roots are well-known to improve the overall stability of slopes through hydro-mechanical reinforcement within the rooted zone. Vegetation based solutions require selection of species which are most suitable for specific locations, aimed at restoring the natural state and function of river systems in support of biodiversity, flood management and landscape development. Selecting a combination of different species (trees, shrubs and grasses) along different zones of the riverbank (upper part, along the slope, at the toe of the slope) can improve the conditions for the river system regarding biodiversity and flood management. However, how the combination of different plant species can improve the stability of the stream bank needs to be further studied. Relevant factors are both related to the improved mechanical strength of the soil from the roots and the changed pore pressure conditions. This work presents a methodological approach for slope stability modelling including vegetation. We present the results obtained from a series of slope stability analyses carried out by using the proposed methodology, for different topographical conditions (slope inclination), and different plant combinations for species typically found along streams in south-eastern Norway.

In this study, two types of tree species were selected, respectively Norway Spruce (Picea Abies) and Downy birch (Betula pubescens). The Goat willow (Salix caprea) was selected as shrub while a common mixed-grass was chosen as grass. Vegetation features were obtained from the literature. The plant combinations considered were: i) only grass, ii) grass and shrubs, iii) only trees, and iv) trees, shrubs and grass. The commercial software GeoStudio (GEO-SLOPE International, Ltd.) was used. The module SEEP/W was used for the hydrological modelling and the calculation of pore-water pressure distribution while SLOPE/W was used for the slope stability modelling and calculation of the safety factor through the rotational failure model proposed by Bishop.

Although one of the main outcomes is that the purely mechanical contribution of vegetation to slope stability could not be decoupled from the hydrological reinforcement (as the most critical shear surface occurred outside the rooted zone), the combinations including trees (both only trees and trees, shrubs and grass) gave the highest mechanical improvement to the stability. To assess the hydro-mechanical reinforcement played by the combined vegetation, two seasons of the year were analysed (spring and autumn) and it was found that the main reinforcement occurs in the spring season, due to the favourable weather (more days of drying and lower rainfall intensity), and for combinations including low height vegetation ( i.e. grass and shrubs) because of their better aboveground vegetation features. In conclusion, a mixed combination of vegetation (trees, shrubs and grass) is the most suitable for reaching the highest hydro-mechanical reinforcement of streambanks, and in the meantime boosting the ecosystem biodiversity. 

How to cite: Capobianco, V., Robinson, K., Kalsnes, B., and Høydal, Ø.: The effects of combined vegetation on the stream bank stability– numerical analyses of benchmark cases for a catchment in south-eastern Norway , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8653, https://doi.org/10.5194/egusphere-egu21-8653, 2021.

Vito Tagarelli, Federica Cotecchia, and Osvaldo Bottiglieri

The soil-vegetation-atmosphere interaction is becoming more and more the subject of intense scientific research, motivated by the wish of using smart vegetation implants as sustainable mitigation measure for erosive phenomena and slope instability processes. 
The use of novel naturalistic interventions making use of vegetation has been already proven to be successful in the reduction of erosion along sloping grounds, or in increasing the stability of the shallow covers of slopes, whereas the success of vegetation as slope stabilization measure still needs to be scientifically proven for slopes location of deep landslides, whose current activity is climate-induced, as frequent in the south-eastern Apennines. Recently, though, peculiar natural perennial grass species, which develop deep root systems, have been found to grow in the semi-arid climate characterizing the south-eastern Apennines and to determine a strong transpirative flow. Therefore, their peculiar leaf architecture, their crop density, combined with their perennial status and transpiration capacity, make such grass species suitable for the reduction of the net infiltration rates, equal to the difference between the rainfall rate and the sum of the runoff plus the evapotranspiration rate. As such, the grass species here of reference have been selected as vegetation measure intended to determine a reduction of the piezometric levels in the slope down to large depths, in order to increase the stability of deep landslide bodies. 
At this stage, only preliminary field data representing the interaction of clayey soils with the above cited vegetation species are available. These have been logged within a full scale in-situ test site, where the deep-rooted crop spices have been seeded and farmed. The test site (approximatively 2000 m2) has been set up in the toe area of the climate-induced Pisciolo landslide, in the eastern sector of the Southern Apennines.
The impact of the vegetation on the hydro-mechanical state of the soil is examined in terms of the spatial and temporal variation of the soil water content, suction an pore water pressure from ground level down to depth, both within the vegetated test site and outside it, where only spare wild vegetation occur, in order to assess the effects of the implant of the selected vegetation. The soil water contents, suctions and pore water pressures have been also analyzed taking into account of the climatic actions, monitored by means of a meteorological station. 

How to cite: Tagarelli, V., Cotecchia, F., and Bottiglieri, O.: Preliminary field data of selected deep-rooted vegetation effects on the slope-vegetation-atmosphere interaction: results from an in-situ test , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15582, https://doi.org/10.5194/egusphere-egu21-15582, 2021.

Raül Oorthuis, Jean Vaunat, Marcel Hürlimann, Antonio Lloret, José Moya, Càrol Puig-Polo, and Alessandro Fraccica

Slope-mass wasting like shallow slides and surficial erosion are mostly triggered by climatic actions, where rainfall plays the most important role. The number of extreme weather events such as droughts and intense rainfalls has increased in the past decades due to climate change. Consequently, slope-mass wasting has become in recent years one of the most important environmental problems with many socio-economic repercussions. Slope mass-wasting may be the most dangerous geo-hazard in many mountainous regions and represents as well an important threat in artificial or man-made slopes like infrastructure and transportation embankments.

The erosion and stability of slopes depend on the soil-vegetation-atmosphere (SVA) interactions and the thermo-hydro-mechanical soil conditions. Therefore, understanding  the SVA interactions and the processes leading to slope-mass wasting is crucial to promote sustainable, low cost, and environmental-friendly mitigation strategies on potentially unstable slopes, such as the use of vegetation. Moreover, it is necessary for developing a correct land-use planning strategy. In this study, SVA interactions are assessed by a full-scale monitored embankment divided into four partitions with North and South-facing slopes and with bare and vegetated slope covers at each orientation. Monitoring is a fundamental task for understanding the physical mechanisms related to SVA interactions and for calibrating and validating models. The monitoring started in 2017 and includes 60 sensors recording 122 variables every 5 minutes. These devices provide accurate information on the thermo-hydraulic response of bare and vegetated slopes at both North and South orientations. In addition to hydraulic variables like suction and soil moisture, which are measured at several depths, thermal and atmospheric variables are monitored: soil heat flux, soil temperature at different depths, air temperature, rainfall, wind speed/direction, solar radiation, etc. 

The results show that vegetation has a strong effect on both thermal and hydraulic slope response. On one hand, vegetation increases rainfall infiltration and induces a faster saturation of the soil, which may reduce slope stability (this effect should be counterbalanced with other phenomena not considered in this work, like raindrop impact protection and root soil reinforcement, among others). Such an increase in turn suggests that vegetation decreases runoff and hence reduces slope surficial erosion. On the other hand, vegetation increases in-depth suction by plant transpiration, which may increase soil strength and stability on slopes. Regarding thermal aspects, vegetation strongly reduces the incidence of solar net radiation. As a result, soil heat flux, daily temperature fluctuations and evaporation decrease. In addition, this research shows that North-vegetated slopes develop dryer soil conditions when compared to South-bare slopes. This shows that the vegetation transpiration induces higher soil drying rates than the solar radiation effects on a bare surface with full solar exposition (i.e. southward orientation). Therefore, these results recommend the implementation and maintenance of vegetated slopes as a sustainable solution for preventing soil erosion especially in sparse vegetated or bare areas and in present and forthcoming semi-arid regions.

How to cite: Oorthuis, R., Vaunat, J., Hürlimann, M., Lloret, A., Moya, J., Puig-Polo, C., and Fraccica, A.: Effect of vegetation and slope orientation on water infiltration in a monitored embankment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7599, https://doi.org/10.5194/egusphere-egu21-7599, 2021.

Luca Piciullo, Vittoria Capobianco, and Håkon Heyerdahl

In unsaturated steep slopes, the upper unsaturated zone may have a crucial role in the slope stability. In this work we studied a natural slope located adjacent to a railway track in Eastern Norway. Due to its steep inclination, the factor of safety should be always kept under observation. In addition, the climate in Norway is expected to become wetter and warmer, with increased snow melting. Thus the rainfall/snowmelt infiltration play an important role and needs to be monitored properly to avoid any failure. The slope is instrumented since 2016 and both volumetric water content (VWC) and the pore water pressure regime are monitored.

The modulus SEEP/W of the commercial software GeoStudio (GEO-SLOPE International, Ltd.) was used to model the transient seepage conditions of the slope for a 7-month monitoring period (from June 2019 to December 2019). Several analyses were carried out by changing the initial conditions and the boundary climate conditions of the slope. Regarding the initial conditions, two series of simulations were carried out, one with an initial calibration of the VWC distribution, another one without calibration, hence, by only locating the ground water table at a specific depth and by indicating the maximum negative head (as required by the model). The calibration, instead, consisted in starting the simulation considering a VWC distribution as closer as possible to the in-situ value.

For each series, a total of three simulations were carried out with different boundary climate conditions, respectively considering only rainfall/snowmelt (R), considering both rainfall/snowmelt and evaporation (Cl), and considering rainfall/snowmelt and evapotranspiration due to vegetation (V). Indeed, the slope is all covered by relatively dense vegetation, with shrubs and birch trees. For the simulations including evaporation and vegetation (Cl and V), the land-climate interaction boundary condition was adopted. Climate functions, such as the pairs temperature-time, relative humidity-time, wind speed-time, were obtained from a close meteorological station. The evaporation was determined by using the Penman-Monteith equation, including vegetation features in the case of vegetated slope. Preliminary results show that the initial calibration is important for the correct back-analyses of the measured data, and that the model is more accurate when accounting for climate boundary conditions and vegetation, which influence also the slope stability conditions.

The work has been part of the working package "Landslide triggered by hydro-meteorological processes" within the Center for Research-based Innovation (CRI) programme KLIMA2050 (), financed by the Research Council of Norway.

How to cite: Piciullo, L., Capobianco, V., and Heyerdahl, H.: The importance of accounting vegetation on unsaturated slopes prone to rainfall-induced instabilities – a case study in Norway, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16136, https://doi.org/10.5194/egusphere-egu21-16136, 2021.

Sabatino Cuomo, Mariagiovanna Moscariello, Valerie Baumann, and Costanza Bonadonna

Vegetation has an important role on slope stability and erosion through hydrological and mechanical processes. Especially plants transpiration and roots uptake can preserve a large amount of matric suction during and after a rainfall event. Soils properties as the water infiltration rate, the moisture content, the organic matter content and the aggregate stability are affected by plants cover as well. The presence of vegetation and its effect on soil moisture also implies an increase of shear strength. Moreover, plants roots increase the tensile strength and the overall shear strength of the vegetated soils, because of their ability to sustain tension, and to occupy the space of soil pores. Trees and shrubs roots produce significant cohesion-like aliquots of strength into shallow soil deposits and increase subsurface drainage, impacting the pronenesse of shallow landslides. Thus, vegetation acts on most of the soil properties which control the slope instability.

Volcanic eruptions can drastically change hillslope hydrology by removing or burying the vegetation in large areas. Events of rainfall-induced slope instability and erosion in differently vegetated recent volcanic deposits are here investigated using a distributed physically-based numerical model. The model considers the effect of vegetation through an additional amount of cohesion due to plants roots, the leaf area index, the average height of plants, the storage capacity of plant cover. Several sets of parameters are considered, and the effect of differently aged vegetation covers on the stability recent volcanic deposits stability is analysed.

How to cite: Cuomo, S., Moscariello, M., Baumann, V., and Bonadonna, C.: Slope instabilities in differently vegetated recent volcanic deposits, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14786, https://doi.org/10.5194/egusphere-egu21-14786, 2021.

Christopher D'Amboise, Matthias Plörer, Anne Hormes, Marc Adams, Karl Kleemayr, and Michaela Teich

We present object protection forest maps for rockfall, shallow landslides and snow avalanches, which were generated within the Interreg Alpine Space project GreenRisk4ALPs with the runout model Flow-py. Six Alpine regions with varying sizes from 45 km2 to 2250 km2, and topographies, from steep valleys of Val Ferret in Italy to the German Alpine foothills in Oberammergau, were modeled. 

The term direct object protection forest is used for forests that protect objects in developed areas against gravitational natural hazards. That is, a direct object protection forest can only be assigned, if an object is endangered and a direct link between the precise locations of the hazard process area and the object can be established. The two main protective effects forests can have against gravitational natural hazards are 1) to reduce the release probability, or 2) to reduce the magnitude of an event, the effectiveness of both is dependent on forest structure. In addition, the degree to which the forest reduces the energy (magnitude) of the hazard also depends on the speed of the mass. If the magnitude/speed of a hazard process is too high, the forest will be destroyed. The location of a forest therefore determines its protective effect in two ways. First, high elevations and steep terrain (over 45°) will produce a weaker structure and be less effective against gravitational natural hazards compared to lower elevation gentle sloped terrain. Second, the energy of the hazard will be lower closer to the hazard’s release and runout areas than in the middle of the process path. 

Based on these relationships, we generated two types of object protection forest maps:  

(i) maps that highlight existing direct object protection forest 

(ii) maps that show where direct object protection forests have or would have the highest potential to either reduce release probability or reduce the energy of the hazard 

The Flow-py model was used to model the routing and stopping of the three hazards and to establish the link between endangered objects and the hazard process areas. Input data are digital elevation models (10 m resolution) and locations of release areas as well as a GIS layer containing locations and types of objects, which is required for a custom plugin. The Back-calculation plugin was used with the Flow-py model to identify areas on the terrain (release areas, transit paths and deposition areas) that are associated with endangered infrastructure. To obtain the first maps, the model outputs were overlaid with digital maps of existing forest areas to identify direct object protection forest. The second map was produced by using the same model outputs and digital terrain models to identify areas in the process paths where the modelled hazard energy was low and effective protection forest can grow. 

The presented maps can help to support decisions and prioritize interventions in risk-based protection forest and ecosystem-based integral natural hazard risk management in the Alpine Space. 


How to cite: D'Amboise, C., Plörer, M., Hormes, A., Adams, M., Kleemayr, K., and Teich, M.: Identifying object protection forest for natural hazards with the model Flow-py , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14315, https://doi.org/10.5194/egusphere-egu21-14315, 2021.

Ilenia Murgia, Filippo Giadrossich, Marco Niccolini, Federico Preti, Yamuna Giambastiani, Gian Franco Capra, and Denis Cohen

SlideforMAP and SOSlope are part of a suite of software available through ecorisQ (www.ecorisq.org), an international, non-profit association promoting solutions for risk reduction of natural hazards. SlideforMap is a probabilistic model that quantifies the stabilizing effect of vegetation at the regional scale and localizes potential areas where forest protection could be improved. SOSlope is a hydro-mechanical model that computes the factor of safety at the slope scale, using a strain-step discrete element method, which includes the effects of vegetation root structure and composition. The research aims at investigating the landslide susceptibility at two different spatial scales, using both models. 

The study area is located on a vegetated slope near an interregional connecting road (Tuscany, Emilia-Romagna, central Italy), which crosses the Foreste Casentinesi National Park (Tuscany) an important natural area for both touristic and recreational activities. 

A shallow landslide susceptibility analysis was performed at two different spatial scales, combining the use of the two previously mentioned models. In particular, SlideforMap was applied to identify the main susceptible areas to landslides at regional scale. Next, the identified unstable areas were investigated at detailed scale using SOSlope which simulated an intense rainfall event. Specifically, both distributions of root and soil forces along the slope were analyzed; for the sake of comparison, beech (Fagus sylvatica L.) and spruce (Picea abies L.) parameters were used. Finally, a back-analysis was performed on real landslides. 

The results showed the activation of root reinforcement spatially distributed in the studied slope. The basal root reinforcement map highlights significant differences, with beech showing higher reinforcement values compared to spruce. According to the factor of safety map, landslides may occur along the investigated unstable area. 

SlideforMap and SOSlope may be useful tools to support land and forestry planning, allowing the localization and quantification of the protective effects of forests, root reinforcement included. Results demonstrated that the factor of safety can be used as benchmarks for silvicultural interventions, thus improving the whole planning activities in both forest and surrounding natural and man-made systems.

How to cite: Murgia, I., Giadrossich, F., Niccolini, M., Preti, F., Giambastiani, Y., Capra, G. F., and Cohen, D.: Using SlideforMAP and SOSlope to identify susceptible areas to shallow landslides in the Foreste Casentinesi National Park (Tuscany, Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14454, https://doi.org/10.5194/egusphere-egu21-14454, 2021.