A data-based comparison of flood frequency analysis methods used in France

Abstract

Abstract. Flood frequency analysis (FFA) aims at estimating quantiles with large return periods for an extreme discharge variable. Many FFA implementations are used in operational practice in France. These implementations range from the estimation of a pre-specified distribution to continuous simulation approaches using a rainfall simulator coupled with a rainfall-runoff model. This diversity of approaches raises questions regarding the limits of each implementation and calls for a nation-wide comparison of their predictive performances.This paper presents the results of a national comparison of the main FFA implementations used in France. More accurately, eight implementations are considered, corresponding to the local, regional and local-regional estimation of Gumbel and Generalized Extreme Value (GEV) distributions, as well as the local and regional versions of a continuous simulation approach. A data-based comparison framework is applied to these eight competitors to evaluate their predictive performances in terms of reliability and stability, using daily flow data from more than 1000 gauging stations in France.Results from this comparative exercise suggest that two implementations dominate their competitors in terms of predictive performances, namely the local version of the continuous simulation approach and the local-regional estimation of a GEV distribution. More specific conclusions include the following: (i) the Gumbel distribution is not suitable for Mediterranean catchments, since this distribution demonstrably leads to an underestimation of flood quantiles; (ii) the local estimation of a GEV distribution is not recommended, because the difficulty in estimating the shape parameter results in frequent predictive failures; (iii) all the purely regional implementations evaluated in this study displayed a quite poor reliability, suggesting that prediction in completely ungauged catchments remains a challenge.

Uncertainties of flood frequency estimation approaches based on continuous simulation using data resampling

“…The quantile 9 estimates are considered more accurate when using the SHYREG method. Indeed, the conclusions of the 10 ANR Extraflo project do not recommend using a Gumbel distribution to estimate quantiles of extreme floods 11 (Kochanek et al, 2014). 12…”

“…On one hand this study showed that RFFA 15 approaches are preferable to FFA approaches (Carreau et al, 2013;Kochanek et al, 2014). On the other 16 hand, methods based on the exploitation of rainfall data are preferable for estimating extreme floods 17 (Kochanek et al, 2014). Indeed, during extreme rainfall events, the soil becomes saturated and the behaviour 18 of flood levels mainly depends on the duration and intensity of maximum rainfall.…”

“…Models 5 differ in the type of rainfall generator and/or rainfall-runoff model used (Blazkova and The SHYREG method and other more classical FFA or RFFA methods were evaluated in a national French 12 research project (national Extraflo project, https://extraflo.cemagref.fr) in which different training/testing 13 decompositions on a set of more than 1,000 French gauging stations are used and reliability and stability 14 indexes are proposed to assess the results (Renard et al, 2013). On one hand this study showed that RFFA 15 approaches are preferable to FFA approaches (Carreau et al, 2013;Kochanek et al, 2014). On the other 16 hand, methods based on the exploitation of rainfall data are preferable for estimating extreme floods 17 (Kochanek et al, 2014).…”

“…Compared to Gumbel distribution, which is considered stable but not accurate, the SHYREG method has 10 shown its qualities in terms of accuracy (Arnaud et al, 2016; Kochanek et al, 2014). 11…”

Comparison of Flood Frequency Analysis Methods for Ungauged Catchments in France

“…Most of the time, operational FFAs are implemented by fitting a statistical distribution to local data in gauged sites or by estimating flood quantiles through empirical relationships called "rational" approaches [44]. Recent studies implemented a data-based comparison of diverse FFA methods used in France [9] and concluded that methods using regional data or exploiting the rainfall information should be preferred to purely at-site statistical data [18,45]. Nevertheless, the transfer of this kind of FFA method to ungauged catchments has remained limited with only one regionalisation method tested and the performance was insufficient to draw general conclusions [45].…”