Natural Hazards and Earth System Science www.nat-hazards-earth-syst-sci.net 1561-8633 1684-9981 9 1 2009 10.5194/nhess-9-145-2009 http://www.nat-hazards-earth-syst-sci.net/9/145/2009/ http://www.nat-hazards-earth-syst-sci.net/9/145/2009/nhess-9-145-2009.html http://www.nat-hazards-earth-syst-sci.net/9/145/2009/nhess-9-145-2009.pdf 145 159 2009-02-17 Improving risk assessment by defining consistent and reliable system scenarios B. Mazzorana bruno.mazzorana@provincia.bz.it J. Hübl S. Fuchs Department of Hydraulic Engineering, Autonomous Province of Bolzano, Bolzano, Italy Institute of Mountain Risk Engineering, University of Natural Resources and Applied Life Sciences, Vienna, Austria During the entire procedure of risk assessment for hydrologic hazards, the selection of consistent and reliable scenarios, constructed in a strictly systematic way, is fundamental for the quality and reproducibility of the results. However, subjective assumptions on relevant impact variables such as sediment transport intensity on the system loading side and weak point response mechanisms repeatedly cause biases in the results, and consequently affect transparency and required quality standards. Furthermore, the system response of mitigation measures to extreme event loadings represents another key variable in hazard assessment, as well as the integral risk management including intervention planning. Formative Scenario Analysis, as a supplement to conventional risk assessment methods, is a technique to construct well-defined sets of assumptions to gain insight into a specific case and the potential system behaviour. By two case studies, carried out (1) to analyse sediment transport dynamics in a torrent section equipped with control measures, and (2) to identify hazards induced by woody debris transport at hydraulic weak points, the applicability of the Formative Scenario Analysis technique is presented. 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