The use of vegetation to mitigate rainfall induced landslide hazards is a form of a Nature Based Solution. Rainfall induced landslides occur when the pore-water pressure builds up in the soil in response to infiltrating rainwater. The vegetation can create a stabilising effect by the removal of soil water via transpiration. The removal of water keeps the soil in an unsaturated state and decreases pore-water pressure in the soil, thus reducing hydraulic conductivity. Lowering the hydraulic conductivity before a rainfall event will reduce the downward infiltration of water and, hence, hinder pore-water pressure build-up (drop in suction) that could result in a landslide.

To improve this stabilizing effect the transpiration during a drying period should be maximized. This study looks at implementing biological processes that occur in the root zone with the aim of engineering the transpiration process. A common biotic interaction that occurs in the soil is the symbiotic relationship between mycorrhizal fungi and plant roots. The fungus has been found to improve nutrient and water uptake in plants. This research looks at the effect of mycorrhizal fungi on transpiration induced soil suction.

A controlled laboratory experiment was carried out to determine if the application of arbuscular mycorrhizal (AM) fungi can increase transpiration of the herbaceous plants Medicago sativa and Lolium perenne. The plants were grown in mini-lysimeters with and without AM fungi and then exposed to drying conditions. The evapotranspiration was monitored via measurements of the change in mass of the mini-lysimeter and the soil water content was then back calculated from the final water content. Periodic soil volumetric water content measurements were also carried out using TDR-probes. The suction during drying was determined from the volumetric water content assuming that the water retention in the vegetated soil is the same as the bare soil, whose water retention properties were fully characterised in a separate experiment.

The inoculation with AM fungi in M. sativa plants increased the potential evapotranspiration during soil drying, this is likely due to the increase in the aboveground biomass. There was no significant difference between inoculated and non-inoculated L. perenne plants. The soil suction in the M. sativa plants increased by almost twice when inoculated with AM fungi in the same drying period. Inoculating with AM fungi can increase plant transpiration rates and generates a higher suction in the soil for M. sativa vegetation, although there was little difference with L. perenne plants. This suggests that the effect of AM fungi on plant water uptake can depend on root functional groups.

How to cite: Roberts-Self, E. and Tarantino, A.: Engineering transpiration-induced suction using mycorrhizal fungi with application to slope stability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5759, https://doi.org/10.5194/egusphere-egu23-5759, 2023.

Plant roots provide mechanical reinforcement to soil and improve soil shear strength. How root decay upon mortality may affect the root biomechanical properties and the subsequent changes in root reinforcement to soil have rarely been systematically studied. The aim of this study is to provide new experimental evidence and quantify the influences of root growth and decomposition on the temporary variations in root breakage strength, root Young’s modulus and the shearing behaviour including dilatancy of compacted soils vegetated with a grass species, Cynodon dactylon L. In this study, C. dactylon was cultivated for six months in 33 columns of compacted lateritic soils (90 mm diameter and 115 mm height), and then either burned or treated with herbicide to introduce root mortality and decay. At different durations of plant growth (60, 120 and 180 days), decay after plant burning (30, 60, 120, 180 and 360 days) and after herbicide application (15, 30 and 60 days), each column was split into two parts; the top part was used for direct-shear tests, whilst root samples were collected from the bottom part for the measurements of root diameter as well as root biomechanical and chemical properties (including the cellulose and lignin contents) (n = 303). Our results showed that all the tensile strength-diameter relations of the roots of C. dactylon followed a negative power law relation (R² > 0.6) regardless of the treatment applied. Growth effects had significant effects on the increase in median tensile strength, which was consistent with the increase in cellulose and lignin contents. As a result, the vegetated soils displayed greater shear strength and larger dilatancy, which were attributable to the growth-induced increase in the root cellulose content and thus the root tensile strength and modulus. The predominant root failure mode at all growth durations was pull-out (rather than breakage); thus, the soil shear strength was better explained by root modulus (36.0%; which defines root extension) than root strength (25.8%; which defines root breakage capacity) and root biomass (1.6%; which defines root content). On the other hand, root decay due to burning or herbicide application caused significant reductions in cellulose and lignin contents, accompanied by a drop in root tensile strength. This explained the significant reductions in soil dilatancy and soil shear strength. The predominant root failure mode switched from pull-out to breakage. Compared with burning, herbicide application introduced a greater and faster degradation of cellulose and lignin con-tents, which explained the more significant and faster root weakening. Reducing the shear strength of the vegetated soils to the level of fallow soil took approximately 2 months of herbicide application and 4 months of burning.

How to cite: Leung, A., Kamchoom, V., Likitlersuang, S., and Boldrin, D.: Losses of biomechanical properties and soil reinforcement upon the decomposition of the roots of Cynodon dactylon, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10147, https://doi.org/10.5194/egusphere-egu23-10147, 2023.

Poplar (Populus sp.) is an important species for preventing shallow, rainfall-triggered landslides and hydraulic bank erosion in New Zealand. However, quantifying the spatial root distribution pattern and reinforcement remains challenging. This study aimed to find the Root Bundle Model with the Weibull survival function (RBMw), a root distribution model (RDM), and a root reinforcement model for the implementation in models such as BankforMAP and SlideforMAP. Our study was conducted within a 26-year-old “Tasman” poplar stand at Ballantrae Hill Country Research Station in the North Island of NZ. We measured root distribution at distances of 1.5, 2.5, 3.5, and 4.5 m from the stem of four poplar trees whose diameters ranged from 0.41 to 0.56 m and from eleven soil profiles along a transect located in a sparse to a densely planted poplar stand. This created a unique database of root distribution. 124 laboratory tensile tests and 66 field pullout tests on roots with diameters up to 0.04 m were carried out. The root distribution model well predicted spatial root partition in trenches of single tree root systems with R² = 0.78 and in the transect with R² = 0.85. The model tends to overestimate root distribution when planting density was higher than 200 stems per hectare. The maximum lateral root reinforcement model tends to underestimate forces in single tree root systems with R² = 0.64, but it well performs along the transect within the stand with different planting densities. The basal root reinforcement model performed well in predicting its vertical distribution as a function of soil depth. In conclusion, our study provided a detailed dataset for the quantification of root distribution and reinforcement of poplars on a hillslope for the purpose of increasing slope stability and mitigating hydraulic bank erosion. The implementation of these data in models for the simulation of shallow landslides and hydraulic bank erosion is fundamental for the identification of hazardous zones and the prioritization of bio-engineering measures in NZ catchments. Moreover, the results are used to formulate a general guideline for the planning of bio-engineering measures considering the temporal dynamics of poplar’s growth and their effectiveness in sediment and erosion control.

How to cite: Ngo, H. M., van Zadelhoff, F. B., Gasparini, I., Plaschy, J., Flepp, G., Dorren, L., Phillips, C., Giadrossich, F., and Schwarz, M.: Analysis of “Tasman” poplar’s (Populus deltoides x Populus nigra) root systems for the quantification of bio-engineering services in New Zealand pastoral hill country., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14299, https://doi.org/10.5194/egusphere-egu23-14299, 2023.

High-tensile steel nets have established themselves as a solution for stabilising landslide-prone areas. To return an engineered slope as much as possible back to a natural look, thoughts of revegetation should always accompany such stabilisation projects. Different factors play a role in natural revegetation, such as the inclination and orientation of said slope and the erosion potential through water percolation. Sometimes it is necessary to help revegetation when a combination of unfavourable factors is present. This can be achieved by adding a geotextile.
When vegetation might take several years to grow, a long-term geotextile is required, which does not degrade in the first years. In such a long-term geotextile, 3D mats out of polypropylene were quite successful. Unfortunately, when it comes to its degradation, sometimes after decades, microplastics are released into the environment. This added an essential aspect to the constantly growing need for substituting petrol-based polymers with nature-based materials. This work highlights the different trials with materials to obtain a controlled degradation of the geotextile material into non-hazardous components. By applying biobased, biodegradable polymers for such applications, the need for material recollection is eliminated, and plastic waste’s impact on the environment is significantly reduced. In this work, biopolymer blend fibres were produced and evaluated in terms of mechanical properties and biodegradability. The blends were based on bio-polyester blends, and the properties were tailored by adjusting the composition of the blend fibres to resemble the existing polypropylene product. The degradation rate of the blend fibres was observed by artificial weathering and hydrolytic degradation tests. As a result, a relation between material structure, fibre strength and durability period was obtained and a complete 3D mat could be produced. Once the material was thoroughly tested in the laboratory, a first small-scale field test was set up in 2020, followed last summer with a large-scale 1:1 field test in southern Switzerland, where careful monitoring assesses the stability of the material as well as the stability of the soil beneath it. The development of the material in the laboratory as well as the first results from the large-scale field test will be presented.

How to cite: Hofmann-Lanter, H., Prambauer, M., and Czerski, D.: Development of bio-based, biodegradable geotextiles for the revegetation of stabilized landslide-prone areas, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14574, https://doi.org/10.5194/egusphere-egu23-14574, 2023.

Harvesting of steepland forests results in a short period where the landscape is particularly susceptible to rainfall-triggered shallow landslides between crop rotations. This is known as the ‘window of vulnerability’ (WoV) and has been considered to occur up to 6-8 years following clearfelling. The WoV represents a period in the forest's rotation where the interplay between declining root strength from the previous crop coincides with changes in soil hydrology creating conditions where soil strength is at its lowest, and the slope is vulnerable to failure.

This project aimed to focus on when maximum susceptibility to rainfall triggered landslides occurs within the WoV. We examined three areas in New Zealand where significant rain events (AEP's  < 1%) had resulted in many landslides on forest land harvested in the years immediately preceding those events.

Using forest company imagery, LiDAR and satellite information we manually discriminated rainfall-triggered landslides for each study area. In all three areas, landslides were 'tagged' to vegetation cover, time since harvesting and whether associated with forest infrastructure such as roads and landings or not.

Maximum landslide number and density occured on land clear-felled 2-4 years prior to the event and was slightly different for each study area. Landslides also occurred in older forest age classes and on areas with different vegetation covers, i.e., mature indigenous forests, pasture, scrub, etc. There were fewer landslides associated with forest infrastructure such as roads and landings than those deemed to be ‘natural’ slope failures.  

Better information on the period of susceptibility to rainfall-triggered landslides following forest removal may help forest managers and regulators better understand the nature of this hazard and what can and can’t be done to mitigate the effects of rain events that result in landslides and in some cases often ‘disastrous’ off-forest impacts.

How to cite: Phillips, C.: Examining the "Window of Vulnerability" following steepland plantation forest removal in New Zealand, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1026, https://doi.org/10.5194/egusphere-egu23-1026, 2023.

According to the European Federation of Soil and Water Bioengineering ( EFIB ) Soil and Water Bioengineering (SWB) is a specific discipline that combines technology and biology in which native plants and plant communities are used as living building material to solve erosion and conservation problems, contributing to the regeneration of degraded ecosystems due to natural or anthropic causes, toregenerate the dynamics of ecological and geomorphological processes and to the recovery of Biodiversity.

The use of Nature Based Solutions (NBS)  as SWB is becoming more and more widespread in public administrations, for example, looking for specific solutions  for landslide stabilization(SWB )

Due to the orographic, edaphic, and climatological conditions of the Basque Country, landslides occur frequently. In the last decade, several landslide stabilizations projects have been carried out on road slopes using SWB solutions as an alternative to traditional engineering solutions. the Department of Road Infrastructures of the Basque administration  requested a series of technical projects  in which alternatives to traditional engineering solutions are presented using SWB, choosing a series of standard reports. The communication will present several examples realized  in the last fifteen years  in different conditions and the evolution of this work during this period.

The measures taken focused on:

• The restoration of the slope morphology and its natural gradient and vegetation
• The stabilization of landslides by using live structures like living cribwall or living
• Improvement of the drainage of the slope with living drains

More recently , public entities have detected the need to have high quality technical manuals that support them in both the drafting of projects and definition of this type of works. To give answer to this situation, the Provincial Council of Bizkaia has commissioned the companies IDOM and SCIA SL  to form a working group for the definition and development of the technical manual of SWB works applied to the case of linear infrastructures. This manual is including the latest advances in the soil and water bioengineering discipline (description and constructive details of the techniques, root reinforcement models, work design methodologies in the short and long term, deterioration models of the utilized materials, etc.). The full manual will be completed and published throughout 2023.

This paper shows concrete examples realized in the Basque country and the progress and contents developed to date as well as the topics index that will be included throughout this year

.

How to cite: Sangalli, P. and Tardío, G.: Landslide stabilization using Soil and Water Bioengineering  in linear Infrastructures in Basque country  and technical manual   , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16844, https://doi.org/10.5194/egusphere-egu23-16844, 2023.

Within the biodiversity hotspot of the West Indies, Guadeloupe presents a remarkable plant biodiversity. The strong urbanization along its rivers leads to the alarming degradation of their banks through artificialization and proliferation of invasive alien species.

Since 2015, the "PROTEGER" project, led by the Guadeloupe National Park and involving INRAE and the University of the West Indies, and funded by the European Union and the French Biodiversity Agency, has been promoting and developing the use of native species and copying natural models to restore riparian ecosystems using soil bioengineering techniques. The first phase of the project (2016-2018) characterized 12 types of Guadeloupe's riparian forests and identified 80 native species potentially suitable for riparian restoration. The second phase of the project (2019-2022) focused on the control of the multiplication of 26 of these species, and their use on bank protection and restoration sites. An experiment on cuttings was conducted on 21 native species (9 tree 4 shrub species; and 8 herbaceous species), another on the germination and growth of seedlings of 5 species of tree legumes. The first soil water bioengineering demonstration projects was implemented along a riverbank through a training course. The third phase to come aims to disseminate the knowledge to local stakeholders and the development of a local economic sector of nurseries and specialized companies.

This project allows the improvement of scientific knowledge of Guadeloupean riparian environments and the development of innovative techniques for the restoration and protection of these environments using local plant species. This project associates managers and researchers in a global approach for the development of a sustainable sector of ecological engineering on the Guadeloupean territory.

How to cite: Evette, A., Mira, E., Robert, M., Rousteau, A., Tournebize, R., Labbouz, L., and Raymond, P.: “Protéger” an integrated project to promote soil bioengineering to protect riverbanks of Guadeloupe, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1695, https://doi.org/10.5194/egusphere-egu23-1695, 2023.

This project is part of a wider EU-funded investment totalling 5,8 M€, that generally aimed for the sustainable flood control using bioengineering solutions in a total of 32 km. It represents the biggest investment ever made in Portugal regarding river restoration.

For this case, the objectives were to stabilize both river margins of a 250 m stretch of river Póvoa in Loures city and to restore the native vegetation.

The site is located in a highly populated area near Lisbon and is prone to recurring (and severe) floods.

This is due to several reasons: it falls within the floodplain of river Trancão; part of its basin drains water from steep hills located at a distance of less than 1 km; it suffers the influence of the Atlantic Sea tides via river Tagus.

The problems observed were both geotechnical and ecological. In the first case, several mass movements and cracks with more than 20 cm depth and 30 m long where present. The ecological problems were mainly related to the dominance of the giant reed (Arundo donax), an invasive species which the superficial roots do not contribute to in-depth slope stabilization.

The major aim of the latter was to develop a global solution for sustainable water management in the event of floods. During this process, the hydrology and hydraulics of river Loures basin was studied. The interventions included not only several works of civil engineering – culvert widening, installing passive tide gates, etc. – but also soil and water-bioengineering techniques (SWBT). These would promote erosion control, slope stabilization and consolidation as well as the ecological recovery of the site.

Regarding the Póvoa stretch, the hydrological and hydraulic model concluded that a maximum velocity of 1,8 m/s and a maximum shear stress of 17,068 N/m² was expected for the 100-year flood, this including a high-tide scenario. Taking this into consideration along with the site characteristics and a thorough review of SWBT literature, the solution adopted to stabilize the streambanks was a vegetated crib wall. This structure is able to withstand water velocities between 3 and 6 m/s and 200-300 N/m² of shear stress, thus suitable for the existing conditions.

The design included a retaining structure in two terraces in both riverbanks and a total height of 2,6 m. The bottom crib wall was to be filled with drain rock and the upper structure with soil and live willow stakes.

The works started in June 2022 and were at a good pace. The contractor was following the Project’s drawings as well as the Project team’s recommendation – a team with several years of experience in implementing SWBT in Portugal and abroad.

A few months later, the site was subject to heavy rainfall that caused 3 major floods, including the nearby area. Although the vegetated crib wall was not totally finished, the structure remained intact.

This Project and specifically the constructed vegetated crib wall showed that a careful conception and design of SWBT allied with a solid construction is vital for a successful outcome.

How to cite: Freitas, A., Oliveira, A. M., Sousa, R., Silva Santos, R., and Sousa, S. F.: Streambank Stabilization in River Póvoa, Loures, Portugal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16051, https://doi.org/10.5194/egusphere-egu23-16051, 2023.

Due to the steep relief of the mountain massifs, the valleys are strategic locations for the establishment of human activities. Over last decades, the floodplain of alpine rivers has been highly anthropized. However, mountain rivers have a strong erosive power that can threaten human infrastructures. To protect them, riverbanks are regularly protected with civil engineering works. Yet, this environment is home to a high level of biodiversity. Thus, soil water bioengineering techniques appear sustainable by allowing both to protect the anthropogenic assets and to welcome the rich riparian biodiversity and the associated ecological services. To implement these Nature-Based Solutions, it is necessary to draw inspiration from the biotic and abiotic components of natural riverbanks, which are still poorly understood at high altitude due to several pressures (climatic, hydrologic, hydraulic or morphologic). In order to understand the structure and functioning of natural models of alpine riverbanks, a protocol has been implemented on 21 mountain banks with mature vegetation and no signs of erosion. The sites were located in the Vanoise massif (France) on an altitudinal gradient (1331 – 2131 m) ranging from montane to subalpine belts with a wide range of channel slope (0,8 – 28,4 %). By focusing on the bank toe, biotic (vegetation cover and species biological features) and abiotic (altitude and hydrogeomorphology) components have been measured. First observations showed that the grain size distribution of the sediments forming the bank was coarser and more homogeneous than those forming the channel bed. For the step-pool channels, bank grain sizes were coarser than for plane-bed channels. Shields parameters for the 21 banks were well below the thresholds for sediment motion, and were therefore relatively stable. Furthermore, on all the sites the shear stresses were relatively high but according to the literature values, classic bioengineering structures could be implemented on 16 of the sites and would withstand a 100-year return period flood, which is consistent with our observation of mature, stable and healthy vegetation patches. The mineral component of the bank toe is closely linked to the woody plant structure. The vegetation cover was dominated by 12 species of willow and the green alder. Green alder cover increased with the altitude and the bed slope. Willow species cover varied with altitude and was less important for steep river. Thus, the natural models of banks were: (i) for the rivers with steep slope, models mixing mineral and plant units which both took part jointly in the stability of the bank; and (ii) for the rivers with lower slope the stability of the bank was mainly ensured by the vegetation. This work provided additional knowledge on the structure and functioning of the banks of stable and mature high-altitude rivers. Accordingly, these results will allow to design soil water bioengineering techniques more adapted to mountain environments (plant composition and structure, mineral unit size).

How to cite: Juliette, R., Guillaume, P., Marie, D., Adeline, F., and André, E.: Natural streambank structure assessment in mountain rivers: an approach combining ecology and hydromorphology, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5403, https://doi.org/10.5194/egusphere-egu23-5403, 2023.