Technical note: Design flood under hydrological uncertainty

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Botto, Anna ; Ganora, Daniele ; Claps, Pierluigi ; Laio, Francesco (2017)

Planning and verification of hydraulic infrastructures demands for a design estimate of hydrologic variables, usually provided by frequency analysis, neglecting hydrologic uncertainty. However, when hydrologic uncertainty is accounted for, the design flood value is no longer a deterministic value, but should be treated as a random variable itself. As a consequence, the design flood is no longer univocally defined, making the design process undetermined. <br><br> Both et al. (2014), with the development of the Uncertainty Compliant Design Flood Estimation (UNCODE) procedure, have shown that it is possible to fix the ambiguity in the selection of the design flood under uncertainty by considering an additional constraint based on a cost-benefit criterion. This paper contributes with an easy-to-use framework to implement the UNCODE procedure without resorting to numerical computation, but using a correction coefficient that modifies the standard (i.e., uncertainty-free) design value on the basis of sample length and return period only. The procedure is robust and parsimonious, as it does not require additional parameters with respect to the traditional uncertainty-free analysis. <br><br> Simple equations to compute the correction term to the standard estimate are provided for a number of probability distributions commonly used to represent the flood frequency curve. This new design tool provides a robust way to manage the hydrologic uncertainty and to go beyond the use of traditional safety factors. With all the other parameters being equal, an increase of the sample length reduces the correction factor, and thus the construction costs, still keeping the same safety level. This improvement is shown to be more effective when short samples are extended.
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