Stabilizing badlands, gullies and riverbanks with multipurpose trees, shrubs and grasses in the sub-humid tropics

Land degradation due to deforestation, erosion and landslides is one of the major challenges of the 21st century. Particularly in Africa, where awareness about the problem is low, it reaches an alarming level. The root cause of land degradation is deforestation due to rapid population growth and ever increasing demands for farm land, fire wood, charcoal, timber and other subsistence needs. To alleviate the problem and to transform the livelihoods of the population, one of the sub-Saharan countries, Ethiopia, is expanding the number of hydroelectric power plants. Currently, Gilgel Gibe I is the largest hydroelectric dam in the country, and its power plant contributes 29% of the country’s electric energy demand. However, it was estimated that the volume of Gilgel Gibe reservoir will reduce by half within 12 years due to siltation, while the design life span was expected to be 50-70 years (Devi et al., 2008, Bioresource Technology: 99: 975–979). Gullies, unstable riverbanks and landslides are identified to be major sources of the suspended sediment that is transported to the reservoir of Gilgel Gibe I dam. One of the measures to control soil erosion and the ensuing sedimentation of reservoirs is plantation of trees, shrubs and grasses along areas that are susceptible to erosion. Vegetation can stabilize riverbanks, landslides and gullies by roots; roots increase the apparent soil cohesion by binding the soil together and by decreasing the soil matric potential. Vegetation can moreover protect river banks and gully bottoms by reducing the water flow velocity. However, which approach and which species is most suited to tackle the problems depends on the agro-ecological conditions of the region, the socio-economic benefit of the species, and its acceptability by the local community. The objective of this research is therefore, to develop a comprehensive methodology for selecting plant species that are most suitable for stabilizing riverbanks, landslides or gullies in humid tropical regions based on an integrated assessment of shoot and root architecture, and mechanical and hydrological plant properties. To achieve the intended objective, experiments are undertaken on selected multipurpose trees, shrubs and grasses to identify easy-to-measure plant characteristics that are most suited to assess the suitability of a species for increasing the apparent soil cohesion and for reducing flow velocities. Existing conceptual or mechanistic models will be used to assess the relative importance of mechanical and hydrological effects of vegetation for stabilizing a given sediment source type and thus to make an integrated assessment of the effectiveness of a species. Multicriteria methods will be adapted to select the most suitable vegetation type and species for a specific sediment source, taking into account both its effectiveness for stabilization, ease of establishment and benefits/acceptability to local communities. Key words: Land Degradation, Stabilization, erosion, grasses, treesDedication i Preface-foreword ii Abstract vi Samenvatting viii List of Abbreviations x List of Figures xvii List of Tables xix Chapter I: Introduction 1 1.1. Land degradation 1 1.2. Soil erosion by water 3 1.3. The role of vegetation in controlling erosion and siltation 7 1.4. The challenge of plant species selection for land rehabilitation 11 1.5. Land degradation in SW Ethiopia 12 1.6. Problem statement 14 1.7. Research questions 19 1.8. Research hypotheses 19 1.9. Aim and objectives 20 1.9.1. Overall aim 20 1.9.2. Specific objectives 20 1.10. Research approaches 21 Chapter II: Study area 23 2.1. Biophysical characteristics and population of Ethiopia 23 2.2. Land degradation and the soil and water conservation practice in Ethiopia 25 2.3. River basins and the status of watershed management in Ethiopia 27 2.4. Omo-Gibe basin 27 2.5. The Gilgel Gibe catchment (GGC) 29 2.5.1. Location and climate 29 2.5.2. Geology, geomorphology and soils of GGC 29 Chapter III: Multi-criteria-based plant species selection for gully and riverbank stabilization in a sub-humid tropical area 31 3.1. Introduction 31 3.2. Materials and methods 33 3.2.1. Study area 33 3.2.2. The multi-criteria decision process 33 3.2.3. Data analysis 41 3.3. Results 41 3.4. Discussions 46 3.4.1. Plant species selection dilemma for land rehabilitation and multipurpose 46 3.4.2. Comparing experts’ and communities’ species preferences 47 3.4.3. Evidence from root characteristics supporting the MCDA results 50 3.5. Conclusions and recommendations 51 Chapter IV: Survival and growth analysis of multipurpose trees, shrubs and grasses to rehabilitate badland in the sub-humid tropics 53 4.1. Introduction 53 4.2. Materials and Methods 55 4.2.1. Study site 55 4.2.2. Soil description and analysis of the experimental site 56 4.2.3. Experimental design and treatments 58 4.2.4. Response variables 62 4.2.5. Data analysis 63 4.3. Results 64 4.3.1. Survival rate 64 4.3.2. Height-based relative growth rate (RGRht) 65 4.3.3. Canopy/contact cover 66 4.3.4. Shoot and root biomass 68 4.4. Discussions 71 4.4.1. Factors affecting the suitability of plant species for badland rehabilitation 71 4.4.2. Relative growth rate (RGR) and plant biomass influence on the species suitability 72 4.4.3. Indication of the effectiveness of plants in erosion control 73 4.4.4. The role of facilitation by grasses to establish trees and shrubs on degraded lands 74 4.4.5. The influence of FYM on the survival and growth of the species 75 4.4.6. Potential for invasiveness of grasses 76 4.4.7. The overall cost of gully exclosure and rehabilitation 77 4.4.8. Sustainability of badland and gully rehabilitation and impact of climate change 79 4.5. Conclusions and recommendations 81 Chapter V: Effects of vegetation types on flow velocity and turbulence characteristics in rivers: a field observation study 83 5.1. Introduction 83 5.2. Materials and methods 86 5.2.1. Study site and the open channel setup 86 5.2.2. Vegetation installation and treatment conditions 88 5.3. Flow measurement techniques 92 5.3.1. Flow measurement locations and the setup of ADV 92 5.3.2. Quantification of basic flow characteristics 93 5.4. Data analysis tools 95 5.4.1. Computation of average velocities (cm/s) 96 5.4.2. Computation of turbulence intensities (RMS) (cm/s) 97 5.4.3. Computation of Reynolds stress (Re) (N/m2) 97 5.4.4. Computation of Turbulent kinetic energy (TKE) (N/m2) 97 5.5. Results 98 5.5.1. The influence of vegetation on streamwise (Vu) and secondary (Vv, Vw) velocities 98 5.5.2. Influence of vegetation on turbulence intensities (RMS) (cm/s) 102 5.5.3. Influence of vegetation on turbulent kinetic energy (TKE) (N/m2) 104 5.5.4. Influence of vegetation on Reynolds stress () (N/m2) 104 5.6. Discussions 107 5.6.1. Influence of vegetation on 3D turbulent flow velocity 107 5.6.2. Influence of vegetation on turbulence intensities (RMS) (cm/s) 109 5.6.3. Influence of vegetation on turbulent kinetic energy (TKE) (N/m2) 109 5.6.4. Influence of vegetation on Reynolds stress (Re) (N/m2) 109 5.6.5. Applicability of results from a straight artificial channel on meandering natural rivers 110 5.7. Conclusions and recommendations 113 Chapter VI: General conclusions and recommendations 115 6.1. Which multipurpose plant species can stabilize gullies and riverbanks and at the same time offer attractive economic benefits to rural communities? 115 6.2. Which type of plant species are effective and suitable to rehabilitate gullies and 118 badlands within a short period? 118 6.2.1. Characteristics of plant species beneficial for badland rehabilitation…………….120 6.2.2. The role of exclosure of the badland site to facilitate the rehabilitation 121 6.3. Which type of vegetation and which plant characteristics are effective in reducing flow velocity of rivers near river banks and hence minimize bank erosion? 121 6.4. What is the best approach to select plant species to stabilize riverbanks and badlands interlinked with gullies and for sustainable degraded land restoration and management? 122 6.5. Recommendations for soil and water conservation practice 125 6.6. Scope for further research 125 References 127 Appendix 3.1. SW Ethiopia plant species database. 143 Appendix 3.2. Database with information on multipurpose trees, shrubs and grasses in SW Ethiopia preselected for gully and riverbank stabilization 146 Appendix 3.3. Score of each of the 40-plant species for six criteria groups based on the local communities’ evaluation. 148 Appendix 3.4. Score of each of the 40-plant species for six criteria groups based on the experts’evaluation. 149 Appendix 3.5. Pictures of the top six multipurpose plant species for gully stabilization based on mult-criteria-based decision analysis. 150 Appendix 3.6. Pictures of the top six multipurpose plant species for riverbank stabilization based on mult-criteria-based decision analysis. 151 Appendix 4. The cost and benefit of gully rehabilitation in SW Ethiopia. 152 Appendix 5.1. Velocity measuring coordinates 153 Appendix 5.2. The dates, time and rainfall amount during the flow experiment in the Gilgel Gibe catchment, SW Ethiopia 154nrpages: 155status: publishe

Multi-criteria-based plant species selection for gully and riverbank stabilization in a sub-humid tropical area

Copyright © 2017 John Wiley & Sons, Ltd. 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.status: publishe