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Unified Concept of Freeboard in Hydraulic Design and in Flood Mapping

Author(s): Kukas Hunzinger; Martin Jaggi; Jurg Speerli; Heinz Weiss; Benno Zarn; Jean-Pierre Jordan; Olivier Overney

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Abstract: In Switzerland, different approaches are used to calculate the freeboard and its consideration in hydraulic design and in flood mapping. Engineers must rely on their experience and on state-of the art rules, as no unified concept at the national or regional level has been established. The desire for a unified and systematic concept of the freeboard is widely shared by professional engineers in the whole country. The Swiss Commission for Flood Protection (KOHS) has leaded a discussion in different workshops with experts from scientific institutions, from regional and federal water authorities and from engineering companies. A literature review identified the approach in neighbouring countries (Austria, Germany, Italy). Interviews with Swiss experts showed no homogenous and consistent practice in the determination of freeboard and on how it is taken into account in hydraulic design and flood mapping. Two concepts are competing with different requirements in terms of calculation, complexity and level of detail. Concept A: freeboard as a variable resulting from an error propagation calculation depending on uncertainties in the river bed level and in the water depth, or on uncertainties due to backwater or wave effects, and to floating materials. Concept B: freeboard as a constant value depending on the type of water course and correcting factors according to the hydraulic situation (dams, bridges, etc. ). Advantages and consequences of both concepts have been discussed, and the consideration of freeboard in flood mapping has raised a highly challenging discussion. Although it is unanimously accepted that freeboard does not account for hydrologic or constructive uncertainties, freeboard can take a wide range of value in hydraulic design. In flood mapping, there is no agreement on whether freeboard should be accounted for, and, if it should, what are the assumptions for its calculation. With its recommendation on freeboard, the KOHS wishes that the freeboard will be taken into account in hydraulic design on a unified and coherent basis in Switzerland.


Year: 2012

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