Engineering methods such as soil nails, geosynthetic reinforcement, retaining structures, gabions, and shotcrete are implemented to stabilize road cut slopes along mountainous areas. However, these structures are not environmentally friendly and, particularly in Ethiopia, it is impossible to address all road problems due to financial limitations. Nowadays, soil reinforcement with plant roots is recognized as an environmentally sustainable alternative to improve shallow slope failure along mountainous transportation corridors. The aims of this study was, therefore, to conduct slope stability analysis along a road corridor by incorporating the effect of plant roots. Five plant species were selected for the analysis based on their mechanical characteristics. Namely, Eucalyptus globules (tree), Psidium guajava (shrub), Salix subserrata (shrub), Chrysopogon zizanioides, and Pennisetum macrourum (grasses). The roots' tensile strength and soil parameters were determined through tensile strength testing and triaxial compression tests, respectively. The factor of safety of the slope was calculated by the PLAXIS-2D software. The study showed that when the slope was reinforced with plant roots, the factor of safety (FOS) improved from 22-34%. The decreasing effect of vegetation on slope stability was observed when soil moisture increased. The sensitivity analysis also indicated that: (1) as the spacing between plants decreased, the effect of vegetation on the slope increased. (2) Slope angle modification with a combination of plant roots had a significant impact on slope stabilization. Of the five-selected plant species, Salix subserrata was the promising plant species for slope stabilization as it exhibited better root mechanical properties among selected plant species.
Vegetation plays a vital role for sustainable rehabilitation of degraded lands such as badlands with active gully erosion. However, the establishment of plant species on badlands remains a long‐lasting challenge in most regions, including the subhumid tropics. To address this challenge, 18 multipurpose plant species (six trees, three shrubs, and nine grasses), which were preselected from the regional species pool in Southwest Ethiopia, were planted in a badland and monitored from July 2011 to June 2014. The experiment had a split‐plot design with farmyard manure (FYM) application as main plot and plant species as subplot factors repeated in three blocks. The study revealed that grasses were the most successful to survive and rehabilitate the gully within the monitoring period compared with trees and shrubs. The survival rate of the four most successful grass species, Chrysopogon zizanioides, Pennisetum macrourum, Pennisetum polystachion, and Pennisetum purpureum ranged from 61% to 90% with FYM application and from 20% to 85% without FYM, while most of the well‐known indigenous and exotic trees and shrubs failed to survive. For the grass P. purpureum, shoot height, shoot, and root biomass were enhanced by 300%, 342%, and 578%, respectively, due to FYM application, with a remarkably higher response to FYM compared with all the other studied species. The overall results demonstrate that badlands can be effectively restored by using early successional species such as locally adapted and selected grasses before the plantation of trees and shrubs.
Selection of appropriate plant species for rehabilitation of degraded lands while fulfilling socio‐economic interests of local communities is one of the decision‐making challenges. This research was undertaken to select multipurpose trees, shrubs and grasses to stabilize degraded lands in the Gilgel Gibe catchment of Southwest Ethiopia, situated in the sub‐humid tropics. Two multi‐criteria decision analysis methods, analytical hierarchy process and simple multi‐attribute rating technique, integrated in the excel‐based multi‐criteria tree selection tool, were used. Focus group discussions were held with experts and local communities to prioritize 40 plant species from a preselected regional pool of 129 species, using six criteria groups containing 47 individual criteria. Root characteristics of the top 9 ranked plants were studied for triangulating the multi‐criteria decision analysis results. Both local communities and experts gave priority to indigenous trees over shrubs and grasses as the best five species for multipurpose use, whereas the top 5 species prioritized for riverbank stabilization contained both trees and grasses. In contrast, communities preferred indigenous trees, and experts selected grasses as the best five species for gully stabilization. The root system characterization revealed that the five top‐ranked multipurpose species also have the required root characteristics for effectively reinforcing unstable slopes. However, communities prefer to plant Eucalyptus and Grevillea trees because of their short‐term economic benefits although they understood the multipurpose value of indigenous plants. The trade‐off between direct economic benefit and multipurpose benefits could be solved by awareness creation, incentives to communities and policy re‐enforcement. Copyright © 2017 John Wiley & Sons, Ltd.
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