General background

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Reliable estimates of expected extreme flood events are required for design and operation of vital infrastructure such as flood defences, bridges and culverts, and also for more general flood risk management and planning, e.g. emergency planning, flood risk mapping, and for defining flood insurance premiums. In practice, this information is obtained through the use of flood frequency estimation techniques based on the principle of analysing series of observed events to infer a probabilistic behaviour, which is then extrapolated to provide estimates of the likely magnitude of future extreme events (e.g. the magnitude of the flood expected to be exceeded on average once every 100-year). By nature, extreme flood events are seldom observed locally and hydrologists have little or no chance of gathering an adequate sample of catastrophes. This raises the question of how best to extrapolate to extreme events when no or only short series of recent events are available.

While the occurrence of extreme floods is a shared problem across Europe (and beyond), the lack of cross-boundary cooperation (national and regional) has lead to individual countries investing in research programmes to develop national procedures for flood frequency estimation. Examples of such programmes include the Joint Defra/EA Flood and coastal erosion risk management R&D programme in the UK, the research programme Risk Management of Extreme Flooding Events (RIMAX) in Germany, the Italian National grant: Characterisation of average and extreme flow in

ungauged basins by integrated use of data-based methods and hydrological modelling (CUBIST), the French research programme (2009-2012) for Extreme Rainfall and Flood Assessment (EXTRAFLO), and the Slovak National grant: Study of dependence and spatial properties of multivariate characteristics of hydrometeorological and hydrological extremes in the mountainous regions of Slovakia. As a result, no standardised European approach to flood frequency estimation exists. Where methods do exists they are often simple and their ability to accurately predict the effect of environmental change (e.g. urbanisation, land-use change, river training and climate change) is unknown. Also, the problem of consistent estimates of extreme floods for trans-boundaryrivers is rarely considered.

A COST Action developing a network of experts involved in nationally funded flood frequency estimation research projects provides an excellent opportunity for sharing knowledge and data, and to develop the next generation of hydrological tools providing solutions to common problems. FloodFreq will act as a European focal point for fragmented research into flood frequency estimation being undertaken at Member State level, and will constitute the first comparative study of different methods for flood frequency estimation on a European scale. The Action will also deliver a framework for assessing the impact of environmental change on flood frequency characteristics, and its outcome will lead to improved and spatially consistent European flood frequency estimation procedures.

It should be noted that the term ‘flood risk’ generally refer to the product of probability of occurrence times the consequences, where the term ‘flood frequency’ is an estimate of the probability and is the principle subject of this Action.

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