Ash produced as consequence of explosive volcanic eruptions can cause multiple hazards both close to the volcano and at great distances. Ash fallout can produce a wide range of impacts on exposed assets (edifices, roads, lifelines), while airborne ash jeopardizes air travel safety. The attention on the impacts of ash dispersal on air traffic increased after the severe consequences of the 2010 Eyjafjallajökull eruption (Iceland), which caused an unprecedented closure of the European North Atlantic airspace. Thus, the need to predict the dispersal of volcanic ash became more important for both civil-defense authorities and airline stakeholders (companies, service providers). For this reason, tephra transport and dispersal models, which allow predicting tephra sedimentation and dispersal caused by a modelled eruptive scenario, became crucial in volcanic hazard and risk assessment. These models have been used for multiple purposes: short-term operational forecast of volcanic ash dispersion; reconstruction of past eruptions in order to constrain key eruption parameters; and generation of probabilistic hazard maps for hazard assessment of tephra dispersion and fallout. This chapter reviews the applications to long-term hazard assessment of volcanic ash dispersal and deposition for multiple eruptive scenarios at some of the most active volcanoes in the world. We present the improvements achieved during the last two decades focusing on recent developments of tephra dispersal hazard assessment, and we discuss some relevant aspects that must be improved in the future in order to optimize long-term hazard assessment strategies.
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