Root Reinforcement in Slope Stability Models: A Review

The influence of vegetation on mechanical and hydrological soil behavior represents a significant factor to be considered in shallow landslides modelling. Among the multiple effects exerted by vegetation, root reinforcement is widely recognized as one of the most relevant for slope stability. Lately, the literature has been greatly enriched by novel research on this phenomenon. To investigate which aspects have been most treated, which results have been obtained and which aspects require further attention, we reviewed papers published during the period of 2015–2020 dealing with root reinforcement. This paper—after introducing main effects of vegetation on slope stability, recalling studies of reference—provides a synthesis of the main contributions to the subtopics: (i) approaches for estimating root reinforcement distribution at a regional scale; (ii) new slope stability models, including root reinforcement and (iii) the influence of particular plant species, forest management, forest structure, wildfires and soil moisture gradient on root reinforcement. Including root reinforcement in slope stability analysis has resulted a topic receiving growing attention, particularly in Europe; in addition, research interests are also emerging in Asia. Despite recent advances, including root reinforcement into regional models still represents a research challenge, because of its high spatial and temporal variability: only a few applications are reported about areas of hundreds of square kilometers. The most promising and necessary future research directions include the study of soil moisture gradient and wildfire controls on the root strength, as these aspects have not been fully integrated into slope stability modelling.

Biomechanical properties of growing and decaying roots of Cynodon dactylon

AimWe measured the effects of growth and decay on the tensile strength of roots of Cynodon dactylon, considering different mortality causes common to agricultural land conversion (i.e., burning and herbicide application). Drivers of root strength changes have also been studies, including root chemical composition (i.e., cellulose and lignin).MethodWe applied three treatments to C. dactylon grass: (i) growth duration (60, 120 and 180 days), (ii) decay duration after burning (30, 60, 120, 180 and 360 days) and (iii) decay duration after herbicide application (15, 30 and 60 days). After each treatment, diameter, tensile strength, cellulose content and lignin content from root samples were measured (n = 303).ResultsIrrespective to the treatments, strength-diameter relations followed a negative power law (R2 > 0.6). Increase in the median strength values due to grass growth was consistent with the increases in both the cellulose and lignin contents. Root decay caused by herbicide applications caused significantly greater and faster reduction in strengths than by burning treatment, because of the faster reduction of both the cellulose and lignin contents.ConclusionRoot decay due to different causes of plant mortality can increase the susceptibility to erosion and slope instability during conversion of agricultural land. Measures on slope safety and erosion are vital when applying herbicides for weed clearance in farmlands because of faster deterioration of root chemical composition and root strength (compared to burning).

Will the Pyrenees suffer less rainfall-triggered landslides in the future? Results of regional-scale stability modelling in the Val d’Aran focussing on land cover and rainfall predictions.

<p>The occurrence of rainfall-induced landslides in high-mountain areas will be affected by future environmental changes. We analysed the influence of climate changes as well as land use and land cover (LULC) changes on shallow slope failures in the Val d’Aran region (Central Pyrenees) applying the simplified physically-based susceptibility model FSLAM. In this study, the event rainfall as well as the root strength were defined as the two input parameters that will be affected by the future changes.</p><p>On one side, the climate changes were analysed by the rainfall projections that are defined in the 26 regional climate models available at the moment in the EURO-CORDEX database using RCP 8.5 scenarios. Future precipitation return periods up to 2100 were calculated by a simplified peaks-over-threshold method based on storm events frequency analysis. Finally, daily rainfall scenarios for the entire study were estimated by weighting current rainfall extremes using a multiplier factor. On the other side, the LULC changes were calculated by the IDRISI TerrSet software suite. All the predictions were performed for three time periods (near, mid and far future).</p><p>The results of the climate change prediction showed that the daily rainfall will increase between 15 and 27 % assuming a return period of 100 years. In addition, the LULC predictions foresee a strong increase of the forest area, while in particular grassland, but also shrubs, decrease in area. Using the different rainfall and LULC predictions, multiples scenarios were defined and the corresponding susceptibility maps calculated. The stability calculations by the FSLAM model indicate that the overall stability conditions in the study area reduces when only the future rainfall prediction is considered. In contrast, the overall stability largely improves when only considering the LULC predictions (due to the increase of forest area and the corresponding higher root strength). However, the effect of LULC-changes is more important than the influence of rainfall-changes. Therefore, the overall stability conditions will improve in the future.</p><p>Many simplifications were incorporated in this susceptibility assessment and there are many uncertainties. Nonetheless, these results may help future studies to improve our knowledge on the impacts of future environmental changes on landslide occurrence in high-mountain areas.</p>

The tensile root strength of Spartina patens declines with exposure to multiple stressors

Coastal wetlands may be subjected to numerous biotic and abiotic stressors from natural and anthropogenic forces in the landscape. The influx of nutrients, inorganic compounds and xenobiotics are suspected of degrading the belowground biomass of coastal macrophytes. Spartina patens acts as an ecosystem engineer for lower salinity coastal marshes and its biomechanical properties are vital to the stability and resilience of coastal wetlands. S.patens was exposed to one natural (flooding) and two anthropogenic stressors (atrazine and nutrient addition) in a greenhouse experiment to test the hypothesis that these three stressors reduce the tensile root strength of S. patens. A one-way Welch’s analysis of variance revealed that the tensile root strength S. patens significantly declined after exposure to two flood duration regimes, three levels of atrazine exposure, and two levels of nutrient addition that consisted of nitrogen-phosphorus combinations. A one-way ANOVA of tensile root strength with an atrazine-flood duration-nutrient addition combination treatment as the main effect resulted in a 52 to 63% loss in tensile strength, while the individual atrazine, flooding, and nutrient treatments produced 40, 39, and 37% losses in tensile root strength, respectively. These results indicate that the effects of multiple natural and/or anthropogenic stressors may degrade the tensile root strength of S. patens, which could facilitate coastal erosion and subsequent collapse of the wetland ecosystem.

Mapping Landslide Prediction through a GIS-Based Model: A Case Study in a Catchment in Southern Italy

Shallow landslides are an increasing concern in Italy and worldwide because of the frequent association with vegetation management. As vegetation cover plays a fundamental role in slope stability, we developed a GIS-based model to evaluate the influence of plant roots on slope safety, and also included a landslide susceptibility map. The GIS-based model, 4SLIDE, is a physically based predictor for shallow landslides that combines geological, topographical, and hydrogeological data. The 4SLIDE combines the infinite slope model, TOPMODEL (for the estimation of the saturated water level), and a vegetation root strength model, which facilitates prediction of locations that are more susceptible for shallow landslides as a function of forest cover. The aim is to define the spatial distribution of Factor of Safety (FS) in steep-forested areas. The GIS-based model 4SLIDE was tested in a forest mountain watershed located in the Sila Greca (Cosenza, Calabria, South Italy) where almost 93% of the area is covered by forest. The sensitive ROC analysis (Receiver Operating Characteristic) indicates that the model has good predictive capability in identifying the areas sensitive to shallow landslides. The localization of areas at risk of landslides plays an important role in land management activities because landslides are among the most costly and dangerous hazards.

Root strength comparison between early and late successional trees in a subtropical forest

<p><strong>ABSTRACT    </strong></p><p>Slope stability of forested areas is often determined by tree root strength. After landslides, the early successional species emerged first, followed by the late successional species. This study aimed to examine whether tree root strength varies as tree species change along with the succession sequence. The study site is in the Lienhuachi Experimental Forest in central Taiwan, where multiple landslides happened in 2008. Three dominant early (Mallotus paniculatus, Sapium discolor, and Schefflera octophylla) and three late successional species (Cryptocarya chinensis, Engelhardtia roxburghiana, and Randia cochinchinensis) were sampled to conduct the single-root-pull-out tests in the field. Root strength which varies with root diameters was estimated with the Root Bundle Model with the root-failure Weibull survival function (RBMw). Results showed that the overall root strength of the early successional tree species were higher than that of late successional species only when root diameter was lower than 5.44 mm. However, among the six species, the root strength of Sapium discolor, an early successional species, was highest and the species with the lowest root strength was a late successional species (Engelhardtia roxburghiana). To precisely estimate tree effects on slope stability, our results highlighted the need to collect root strength data specifically for each species, even though it will be a daunting task for areas rich in tree diversity.</p><p><strong>Keyword: landslide, Root Bundle Model, vegetation succession </strong></p>

Estimating additional root cohesion by exploiting a root topological model based on Leonardo’s Rule

<p>Root topological models are schematic representations of the root structure based on a defined topology graph theory. In the context of hillslope stabilization modeling against rainfall-induced shallow landslides, the root topological models may be used in combination with root strength models assessment, such as the Root Bundle Model (RBM), to estimate the ultimate root reinforcement. The effect of plant roots on slope stabilization is determined by the interaction between soil and the hydrological processes (within the root zone) and the biotechnical characteristics of the root system, such as root length, root density, root tensile strength, root area, root diameter profile and the total number of roots. Describing adequately the root architecture of a plant species is useful, for example, to evaluate how the root structure may change in different soil and/or climatological conditions and, ultimately, as an example, to assess the most suitable plant species to be adopted.<br>This study exploits the potentiality of a root topological model based on Leonardo’s rule in describing root architectures of (i) different species (and tree individual) at given growth conditions, (ii) same species at different environmental conditions, e.g., exposure to light, water and nutrient availability. The former is supported by field campaign measurements from Tuscany region, the latter are reproduced starting from a reference case and imposing growth assumptions. Next, the information of the root system, in terms of root length, density, root diameter profile, total number of roots, are used to estimate, through a RBM approach, the additional root tensile force, deriving it from the force-deformation theory of linear elasticity in a rigorous framework aimed to derive the additional shear resistance from the Mohr-Coulomb’s failure plane. <br>The preliminary results demonstrated the capability of the root topological model of reproducing different types of root system; additional data are required to further validate the model, with regard to the growth conditions simulation. Similarly, laboratory test of root strength would allow to quantify the improvement derived from the rigorous method adopted to estimate the additional root strength.</p>