Review of applied-statistical methods for flood-frequency analysis in Europe

Flood frequency analysis is used for establishing a relationship between flood magnitude and frequency of occurrence (return period) and for estimating the design-flood at a given location of interest. The approach can be implemented locally (At-Site Flood Frequency Analysis, SFFA); or regionally (Regional Flood Frequency Analysis, RFFA), which is used to limit unreliable extrapolation when available data record lengths are short as compared to the recurrence interval of interest, or for predicting the flooding potential at locations where no observed data are available. Both SFFA and RFFA are mature disciplines and consolidated methodologies are available for many European regions. As a result, different European countries, and sometimes even different regions within a country, have adopted different methodologies, which are often selected on the basis of traditional approaches or restricted due to limitation of available data. The main objective of the COST Action ES0901 European procedure for flood frequency estimation (FloodFreq, http://www.cost-floodfreq.eu/), which started in 2010, is to undertake a pan-European comparison and evaluation of methods for flood frequency estimation under the various climatologic and geographic conditions found in Europe, and different levels of data availability, as required by European Flood Directive (2007/60/EC). In particular, Working Group 2 (WG2) is focusing on an assessment of statistical methods for flood frequency estimation. In the first phase of WG2, state-of-the-art methods were collected from all member countries of the WGs, and presented in a report form. In this report, the description of applied frequency analysis methods is presented. The report also include a catalogue of flood data availability/unavailability across Europe together with relevant information (e.g., catchment descriptors, climatological [see above] and hydrological characteristics, indications on frequency distribution recommended for use in flood frequency studies) are collected and presented. Finally, this report presents some preliminary outcomes of analyses that aim to identify in an L moment-based framework the most suitable parent distributions for representing the frequency regime of annual maximum flood across Europe.