A compilation of data on European flash floods

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Summary

Flash floods are one of the most significant natural hazards in Europe, causing serious risk to life and destruction of buildings and infrastructure. This type of flood, often affecting ungauged watersheds, remains nevertheless a poorly documented phenomenon. To address the gap in available information, and particularly to assess the possible ranges for peak discharges on watersheds with area smaller than 500 km2 and to describe the geography of the hazard across Europe, an intensive data compilation has been carried out for seven European hydrometeorological regions. This inventory is the first step towards an atlas of extreme flash floods in Europe. It contains over 550 documented events. This paper aims at presenting the data compilation strategy, the content of the elaborated data base and some preliminary data analysis results. The initial observations show that the most extreme flash floods are greater in magnitude in the Mediterranean countries than in the inner continental countries and that there is a strong seasonality to flash flood occurrence revealing different climatic forcing mechanisms in each region.

Introduction

Flash floods are one of the most significant natural hazards and cause serious loss of life and economic damage. The average annual economic loss due to natural hazards over the world has been estimated at €40 billion (Münich Re, 2003). This can be compared to the total economic damages estimated at €1.2 billion for the Gard 2002 single flash flood event (Huet et al., 2003) and €3.3 billion for the Aude 1999 flash flood (Lefrou et al., 2000). In Europe, lowland floods are rarely associated with fatalities except in cases of levee failures; in contrast flash floods often result in loss of life. The most striking examples are the Lynmouth flood in the UK in 1952 – 34 deaths (Dobbie and Wolf, 1953), the Barcelona flood in Spain in 1962 – over 400 deaths (López Bustos, 1964), the Piedmont region floods in Italy in 1968 and 1994 – respectively, 72 and 69 deaths (Ferro, 2005, Guzzetti et al., 2005) and the Aude flood in France in 1999 – 35 victims (Gaume et al., 2004).

Despite being a serious natural hazard that affects countries throughout Europe, flash floods remain a poorly understood and documented natural phenomenon. The occurrence of extreme events in ungauged watersheds generally means that there is no measured discharge information or formal records of the magnitude of the event. Furthermore, data on previous flash floods is scattered among local authorities where flooding has occurred and various companies and research units that may have unpublished technical reports. These sources of information are often difficult to obtain and are generally in the national language of the country where the flood occurred. Existing inventories of past floods (Barredo, 2007; Herschy, 2005; Costa, 1987a, Unesco, 1976, Pardé, 1961) contain only few data on flash floods occurred in Europe and lack quantitative information on the meteo-hydrological and hydraulic characteristics of the events. The lack of centralised national and international databases for storing meteo-hydrological, hydraulic and socio-economic data relating to past flash flood events within Europe hinders the development of understanding of their magnitude and occurrence (Creutin and Borga, 2003).

To address the gap in available information, and particularly to assess the possible ranges for peak discharges on small watersheds and to describe the geography of the hazard across Europe, an intensive data compilation has been carried out initially for the seven European regions listed below:

  • Catalonia region, Spain, Mediterranean region,

  • Cevennes-Vivarais, France, Mediterranean region,

  • Italian Alps and Ligury, Alpine Mediterranean and Mediterranean region,

  • Slovakia, Inland Continental region,

  • Greece, Mediterranean region,

  • Romania, Inland Continental region,

  • Austria, Alpine Inland Continental region.

This inventory is the first step towards an atlas of extreme flash floods over Europe. This paper presents the data collection strategy, the data set and some first interpretations. This work has been carried out for Hydrate, which is a currently ongoing European Commission funded project that is aiming to improve techniques for flash flood forecasting. Such inventories can never be perfectly consistent and comprehensive. The very first analyses provided hereafter nevertheless illustrate their usefulness for understanding magnitude, occurrence, and geographical distribution of hydrological extremes.

Section snippets

Data collection strategy

The initial aim of the data compilation was to develop a catalogue for each region that included the most extreme flash flood events between 1946 and 2007. In this research, extreme flood events induced by severe stationary storms have been considered as flash floods. This relatively broad definition includes almost all the past events reported as flash floods in Europe, except dam break floods. The duration and spatial extension of the area affected by such floods depend on the causative storm

The data catalogue

The data catalogue is composed of a series of filled data templates. Their content is presented in Table 1. Each record in the inventory provides data on the flood characteristics at a particular cross-section. For any flood event there may be several records, each with details of the peak discharge at a different cross-sections and, often, different sub-catchments. The records contain fields with basic geographic information such as the name of the river, the name of the cross-section and the

Comparison of envelope curves

Envelope curves provide an effective graphical summary of previous floods in a given region. They have been widely used in past publications on extreme floods (Castellarin, 2007, Herschy, 2005, Bayazit and Onoz, 2004, Stanescu, 2000, Kadoya, 1992, Anselmo, 1985, Mimikou, 1984, Crippen and Bue, 1977, Marchetti, 1953, Jarvis, 1924) and have the advantage of being relatively unaffected by the data compilation density because they are determined by the maximum values of a sample. Generally, the

Conclusions

Flash flood data from seven hydrometeorological regions in Europe have been collated. The resulting data base is provided at http://www.Hydrate.tesaf.unipd.it/ (European flash flood data center page). The data set is a step towards a European Flash Flood database and atlas of extreme events. Events that occurred between 1946 and 2007 on watersheds of an area less than 500 km2 were collated. Analysis of the data sets shows that flash floods in each region occur in watersheds of various sizes less

Acknowledgements

The research presented in this paper has been carried out as part of the European Commission funded project, Hydrate, Sixth Framework Programme, Contract No. 037024.

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