FROM A PROBLEM TOWARDS POTENTIAL IMPROVING RURAL ROAD DESIGN IN DRYLANDS, A MODELLING CASE STUDY FOR A CATCHMENT IN THE TIGRAY REGION, ETHIOPIA.
Summary
The Ethiopian government aims at the improvement of rural road systems in the Tigray region, situated in the Ethiopian Highlands. Road systems currently cause alterations in local hydrology which result in erosion related problems. Farmer communities living along the road are affected and the road authorities invest a substantial amount of their budget in road maintenance. There is a high potential for improved rural road design, including a further integration of the concept of road water harvesting into road system design. Improved rural road design would reduce the current observed problems and enable an extra water source for agricultural purposes to promote regional food security.
This research was of an explorative character and aimed at an improved understanding of the combined effect of road alignment and culvert design on local hydrology and erosion patterns at a catchment scale. A model was developed in the PCRaster environment to facilitate the execution of a scenario study focussing on a road system in the Tigray region, taking into account the low data availability and quality. A set of 28 road system scenarios was formulated, based on three different road alignments and different techniques for culvert positioning. The study aimed at a further development of concepts on improved road alignment and culvert allocation. The formulated scenarios were evaluated through the application of a Multi Criteria Analysis which was developed to address three objectives of rural road performance: Erosion, costs and the potential of water harvesting practises.
The most important contribution of this research towards improved rural road design is the newly developed method for road design analysis on a catchment scale, the integration of both process-based modelling and a post-hoc analysis by a MCA has a potential for further investigation and eventual extension when the encountered model shortcomings are addressed. It enables an integration of a range of aspects with a different character towards a balanced design which includes a certain degree of flexibility by the adjustable criteria weighting system applied in the MCA. The model results revealed that the model is able to identify the general impacts of a road system on local hydrology and erosion. The impact of road alignment on the formulated indicators is mainly related to the size of the upstream area and was supported by applied statistics (one-way ANOVA and Tukey test). An estimation on the enhanced food production enabled by integrating a water harvesting aspect into road design, showed at least a doubling of the current cultivated irrigated area in the region and is able to produce a substantial amount of the annual food demand. The modelling process revealed an event related to culvert positioning, which might impede the actual differences between culvert scenarios. Discharge is being cross-drained forth and back caused by an increased number of cross-draining opportunities for the road system scenarios with a higher number of installed culverts or culvert scenarios being align with natural discharge patterns. An adequate evaluation of road system performance and the potential formulation of improved rural road design concepts, requires a reformulation of more adequate indicators and a better integration of the erosion objective.