Landslide hazard assessment and risk reduction for the development in the urban area of Dessie town (Wollo, Ethiopia)

This paper presents the results of a geo-environmental study carried out in the Dessie town area (Wollo), one of main urban centers of Ethiopia (200,000 inhabitants, where landslides heavily interact with a disorderly, chaotic and expanding human settlement. Consequently, the structural vulnerability of private and public buildings and infrastructures, the absence of public awareness in urban planning and deregulated private behavior and, possibly, the effects of climate change, induce high risk conditions for life, buildings, infrastructures and economic activities. Dessie town is located between 2500 and 2900 m a.s.l. in a small tectonic basin making part of the eastern margin of the northern Ethiopian Plateau. The basin bottom is filled with colluvial, fluvial and lacustrine deposits overlaying a basalt bedrock, deeply dissected by the drainage network of the Borkena River headwaters. Detailed field survey and large scale (1:5.000) geomorphological mapping, supported by the analysis of multi-temporal aerial photos, allowed to point-out the geological features of the bedrock and near-surface deposits and the morphodinamic role of the present-day geomorphic processes, including landslides of different typology, extension and state of activity. The general geomorphic evolution of the area results in: i) fast-moving landslides on steep basalt slopes; ii) debris slides on the talus belt; iii) rotational slides and reactivation and enlargement of deep-reaching, dormant landslide bodies along the Borkena River banks. On the base of a detailed landslide inventory a GIS-based methodology, involving geological/geomorphological field survey, site analysis, macroarea analysis and susceptibility analysis was applied. Site analysis lead to the definition of discriminating parameters (lithological and morphometric conditions necessary but not sufficient to trigger a landslide of a given type) and predisposing factors (conditions that worsen slope stability but are not sufficient to trigger a landslide of a given type in the absence of discriminating parameters). The different predisposing factors were subdivided into classes, whose intervals were established by descriptive, statistical analysis of landslide inventory data. A numerical index, based on the frequency of landslide occurrence, quantified the contribution of each class to slope instability in each Homogeneous Territorial Units (HTU), characterized by distinctive landslide susceptibility levels. At each of the above landslide scenarios was attribute a landslide hazard level (H1, H2, H3 and H4) with reference to the different landslide types affecting the area, in analogy to the standard terminology used in shared scientific literature and institutional protocols. By intersecting the landslide hazard maps with a urban vulnerability map, based on the distribution of buildings and infrastructures, on their civil importance and structural resistance, on the density and persistence of people occupation, a landslide risk map with different risk levels (R1, R2, R3 and R4) was derived. The landslide risk map and the related report will constitute the starting point for a possible plan of risk mitigation regarding the urban area of Dessie. This project could provide operative tools for: i) supporting urban planning and settlement regulation; ii) adapting environmental restoration; iii) improving hydraulic and slope stabilization works.