Abstract
Abstract. Planning and verification of hydraulic infrastructures require a design estimate of hydrologic variables, usually provided by frequency analysis, and neglecting hydrologic uncertainty. However, when hydrologic uncertainty is accounted for, the design flood value for a specific return period is no longer a unique value, but is represented by a distribution of values. As a consequence, the design flood is no longer univocally defined, making the design process undetermined. The Uncertainty Compliant Design Flood Estimation (UNCODE) procedure is a novel approach that, starting from a range of possible design flood estimates obtained in uncertain conditions, converges to a single design value. This is obtained through a cost–benefit criterion with additional constraints that is numerically solved in a simulation framework. This paper contributes to promoting a practical use of the UNCODE procedure without resorting to numerical computation. A modified procedure is proposed by 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. Simple equations to compute the correction term are provided for a number of probability distributions commonly used to represent the flood frequency curve. The UNCODE procedure, when coupled with this simple correction factor, 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 in the sample length reduces the correction factor, and thus the construction costs, while still keeping the same safety level.
Highlights
The flood frequency curve is commonly used to derive the design flood as the quantile QT corresponding to a fixed return period T
The Uncertainty Compliant Design Flood Estimation (UNCODE) procedure is a novel approach that, starting from a range of possible design flood estimates obtained in uncertain conditions, converges to a single design value
The UNCODE approach to flood frequency analysis provides a solution to quantify the design flood estimate when considering the uncertainty of the distribution quantile; application of the full UNCODE procedure may be cumbersome and computationally demanding for the practitioner
Summary
The flood frequency curve is commonly used to derive the design flood as the quantile QT corresponding to a fixed return period T. Botto et al (2014) further demonstrated that the optimal design flood obtained from the cost–benefit analysis with linear cost and damage functions is equivalent to the design flood QT obtained from the standard frequency analysis, provided that uncertainty is not accounted for and the ratio between d and c equals the return period T. This result can be shown by setting to 0 the derivative of CTOT with respect to Q∗, in order to find the minimum of Eq (1); this leads to the Hydrol. Other than a useful practical tool for design purposes, the analysis reported in this note provides a method to quantify the “value” of newly available hydrological information or the effect of data scarcity on Q∗T due to uncertainty
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