Feedback from Uncertainties Propagation Research Projects Conducted in Different Hydraulic Fields: Outcomes for Engineering Projects and Nuclear Safety Assessment

Abstract

Hydraulic river models are applied for various purposes such as safety against flooding, navigation, or ecological rehabilitation. Much effort has been put into the development of sophisticated numerical model systems. These numerical models are based on a deterministic approach and the results are presented in terms of measurable quantities (water depths, flow velocities, etc.). However, the modeling of river processes involves numerous uncertainties associated both to the numerical structure of the model, to the knowledge of the physical parameters which force the system, and to the randomness inherent to the natural phenomena. As a consequence, dealing with uncertainties can be a difficult task for both practitioners (Iooss in Journal de la Société Française de Statistique 152(1):1–23, 2011, [1]) and new guidance (ASN in protection of basic nuclear installations against external flooding, 2013 [2]). In the context of nuclear safety, the Institute for Radioprotection and Nuclear Safety (IRSN) assesses studies conducted by the operators for different reference flood situations (local rain, small or large watershed flooding, sea levels, etc.), in agreement with the recommendation reported by the guide ASN n°. 13 (ASN in protection of basic nuclear installations against external flooding, 2013 [2]). The guide provides some recommendations to deal with uncertainties, by proposing a specific conservative approach to cover hydraulic modeling uncertainties. Especially, the most influencing parameter of the numerical model is identified and an unfavorable value is taken in order to cover a whole set of parameters. Depending on the situation, the influencing parameter might be the Strickler coefficients, levee behaviors, simplified topographic assumptions, etc. Obviously, identifying the most influencing parameter and giving it a penalizing value is challenging and usually questionable. In this context, IRSN conducted cooperative (Compagnie Nationale du Rhone, I-CiTy laboratory of Polytech’Nice, Atomic Energy Commission) research activities since 2011 in order to investigate feasibility and benefits of Uncertainties Analysis (UA) and Global Sensitivity Analysis (GSA) when applied to hydraulic modeling. A specific methodology, presented in Sect. 2, was tested by using the computational environment Promethee, which allows carrying out uncertainties propagation study. This methodology was applied with various numerical models and in different contexts (Sect. 3), as river flooding on the Rhône River (Nguyen et al. in La Houille Blanche 5:55–62, 2015 [3]) and on the Garonne River (in the context of the Garonne river test case), for the studying of local rainfall (Abily et al. Environ Model Softw 77:183–195, 2016 [4]) or for tsunami generation, in the framework of the ANR-research project TANDEM. The feedback issued from these previous studies is analyzed (technical problems, limitations, interesting results, etc.) in Sect. 4 and the perspectives and a discussion on how a probabilistic approach of uncertainties should improve the actual deterministic methodology for risk assessment (also for other engineering applications) is finally given.