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Systematic, palaeoflood and historical data for the improvement of flood risk estimation


Problems to be solved
During the last few years, Europe has been the scene of several severe floods of immense social impact. A statistical approach to the flood risk problem using gauge record data may lead to considerable losses of property and life. If the reliability of flood risk assessment is to be improved, there is a critical need to specifically extend the record of the instrumental period (systematic period) of extreme floods. This record is usually limited to only a few dozens of years and the largest floods are therefore under-rated in the data. Information on hydrological variability and extreme floods can be completed using either or both palaeo flood hydrology (new and developing branch of hydrology and geomorphology based on geologic indicators) and/or historical information (based on documents and chronicles). Long records of extreme floods are then applied successfully in the risk analysis, together with the more traditional empirical, statistical and deterministic methods to estimate the largest floods. These extreme floods are of most interest to the planners and engineers but rarely feature in the systematic period. This information is especially useful for floodplain management, design of hydraulic structures, flood hazard reduction, management of riparian ecosystems and flood emergency plans (i.e. sustainable floodplain development). The recent catastrophic floods in Europe warn of the critical need for Paleo-hydrologic data on floods over long-time scales. SPHERE aims at developing new scientific frameworks and technical tools integrating multidisciplinary approaches (geologic, historical, hydraulic, statistical and GIS) on extreme flood risk assessment. In particular, a complete catalogue of major past floods will be constructed for an area in France and one in Spain Scientific objectives and approach The main objectives of SPHERE include:
1) the use of palaeohydrological geological evidence (floods over the last 5000 years) and historical information (floods over the last 600 years) to develop a catalogue of past extreme floods corresponding to different regions, and to improve existing methodologies developed for palaeoflood reconstruction and adapting and modifying them to the particular European conditions, in terms of geographical, physiographical and climatic characteristics.
2) improving the understanding of hydraulic processes and numerical models by developing methods of treating historical and palaeoflood data to estimate peak discharges using 1D or 2D hydraulic models, according to the geometric and geomorphologic characteristics of the river or stream (alluvial or bedrock channels).
3) the development of statistical tools for Flood Frequency Analysis (FFA); these tools will serve to address problems related to the use of non-stationary and non-systematic data. Measurement of the interest in non-systematic data for FFA will be provided by Monte Carlo simulation and application to the different case studies using long palaeoflood and historical series. The efforts proposed should help identify new directions for the use of non-systematic information in FFA.
4) the design of a practical system for the analysis of systematic and non-systematic flood data at the regional and local level using GIS procedures including flood frequency analysis and hydrological modelling. This is measurable and verifiable in terms of the capacity of the system to produce maps of historical and palaeoflood distribution (for single events, specific time intervals, etc.), data analysis (frequency and magnitude distribution of extreme events through long-time spans) and statistical analysis using systematic and non-systematic data.
5) the development of flood risk decision making tools to be applied at different scales (from a single river reach to a drainage basin on a regional scale) that will be readily available to evaluate dam safety, spillway dimensions for high-magnitude low-frequency flooding, flood hazard mapping (especially in urban areas) etc. These tools will be based on the system obtained according to point 4. The measurability is given by the capacity of the system to include data regarding dams and flood protection structures within the basin showing their limitations for high magnitude flooding. In addition, the visual capacity of the system will be used in risk education and civil protection with regard to long-term flooding. The GIS will provide the appropriate tool to gain knowledge on the areas suffering large flood events over the last centuries.
6) the establishment of short- and long-term relationships between climate and floods in areas with different climatic features: evolution, trends and approximation to a return period. To diagnose, classify and compare different meteorological synoptic patterns responsible for rainfall and floods in Europe, to improve flood forecasting and storm tracking.
7) the preparation of a methodological guide for climate/flood data collection and the temporal analysis of the data. The design of general methodology for the hydrometeorological analysis of floods as an attempt to standardise the scientific-technical reports concerning these events. Expected impacts: Palaeoflood and historical flood data are presently used in several countries at the national scale to construct a catalogue of past flood events. For example, the U.S. Bureau of Reclamation has released a special program for reconstructing palaeofloods (using geological indicators), to evaluate dam safety and to aid in hydrologic dam safety decisions and in the management of reservoirs. The Hydrological Survey of Israel considers palaeoflood data to be essential for flood hazard estimation and even for that of water resources. China also approaches the evaluation of flood frequency and magnitude using historical flood data to both define the limits of flood hazards and to design engineering structures. Several projects on flood risk have been launched by the European Commission. These mostly involve hydrological forecasting using radar or hydro meteorological data from the instrumental record. None of these projects take into account the vast quantity of past flood information for the purpose of flood risk assessment. The project here presented represents a step-forward in that it sets out to quantify real events occurring at a given basin over several millennia and to apply this information in the flood frequency analysis.
In most of the studied drainage basins, the estimated discharges from palaeofloods and historical floods exceed those recorded by instrumental flood series, illustrating that an improved knowledge of extreme events can be gained from geological and documentary evidence. In NE Spain, the SPHERE project has shown that events during the last 400 years produced discharges upto 30% greater than the biggest gauged flood events of this century (the 1962 and 1971 events). Some of the floods witnessed in the summer of 2002 in Central Europe and in the south of France have been designated the largest since instrumental records began. However, some of these discharges were lower than those estimated using palaeoflood and/or historical flood evidence e.g. Gardon River, France. These results indicate that paleoflood studies can provide a better view on the magnitude of the largest floods that can affect a drainage basin. New methods on palaeoflood analysis have been developed including:
(1) a systematic procedure of collecting, storing and analysing slackwater flood deposits,
(2) advances in chronological constrains using multiple dating methods such as TL, OSL, radiocarbon dating (AMS) and Caesium-137.

Documentary flood data is available at almost any major city of Europe and it is a valuable source to understand the societal impacts of flooding. Detailed descriptions on water levels, economic losses, casualties and socio-economic disruption associated to different flood magnitudes can be used in flood management and education. In three studied rivers in Spain, over 1000 documents were used, whereas in France a similar number were available from the General State Sources. This information was compiled and stored in the SPHERE databases. The uncertainties of the historical flood discharge estimation is minimised by taking into account the local topography and infrastructures present near the river at the time of the flood, whereas in palaeoflood discharge estimations best estimations are obtained on bedrock or resistant river reaches.

The SPHERE Project has revealed the complementarity of palaeoflood and historical flood information. Past flood records in terms of time and discharge were completed using a combination of both types of data sources. In terms of risk planning the traditional method of using only hydrological data or hydrometeorological calculations should be modified. This information has been shown to be insufficient in the absence of long-term gauge records. As part of the SPHERE project, new user-friendly software (FRESH) run in a windows environment has been developed for the calculation of return periods incorporating palaeoflood and historical flood data. New mathematical procedures on FFA (Flood Frequency Analysis) and high return period quantiles using upper bounded probability distribution functions have been explored for high-risk infrastructures (dam safety, nuclear power plants, etc) based on a combination of palaeoflood, documentary and systematic data together with upper flood limits based on the envelope curve and PMF (Probable Maximum Flood) estimation. The reliability of the high return period quantiles and the PMF estimates can be computed using Monte Carlo simulations. In the flood series derived from palaeoflood and historical data, periods of increases in flood magnitude and/or frequency are observed over the last 3000 years (e.g. late Bronze Age and 16-17th centuries AD).

This indicates that climatic variability clearly effects flood magnitude and frequency. These past flood-climate relationships can be used for a better understanding of the potential effects of the present Greenhouse Global Warming. Non-stationarity of the flood record is not surprising, given that it may also be evident within instrumental series of 50 years length. Furthermore, non-stationarity is predicted in current scenarios of future climate change. Tools should be developed that allow quantification of the non-stationarity of a flood series and new procedures in flood frequency analysis of mixed populations. Flooding was produced by distinct circulation patterns (CPs). At basins with Mediterranean fluvial regime, CPs are defined by local parameters of the basin (e.g. Ardeche, Llobregat), whereas in basins with an Atlantic regime (e.g. Isere) CPs producing floods can be identified from both National (France) and European CPs classifications. The physical evidence of the water level reached during a past flood event provides clear and tangible evidence of flood risk in a given area and, therefore, should be used as an invaluable education tool in risk education. Data integration and analysis were performed by digital databases supported by a standardised and compatible database management system, namely the SPHERE GIS. About 6000 past flood records were stored.

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Participants (12)