Long-term flood records for flood risk management - NIVA

STRIVER TECHNICAL BRIEF Strategy and methodology for improved IWRM - An integrated interdisciplinary assessment in four twinning river basins TB No. 5

Long-term flood records for flood risk management Palaeo-flood records, historical data and systematic discharge records provide evidence of the largest floods occurring on rivers, as well as their frequency and magnitude. The possibility of using long-term, nonsystematic records opens new possibilities for flood risk estimation, which can be used on flood risk assessment and management.

The STRIVER Technical Brief series summarizes results from the EC FP6-funded STRIVER project aimed at an applied research audience and water managers

Long-term flood records for flood risk management Benito, G., Botero, B., Centro de Ciencias Ambientales (CSIC) Lana-Renault, N., García-Ruiz, J. M., Vicente-Serrano, S., Instituto Pirenaico de Ecología (CSIC) Beguería, S., Estación Experimental Aula Dei (CSIC)

Abstract Reliable estimation of flood magnitude/frequency relationship is on the basis of flood hazard estimation, and it is the basis of risk assessment and management. Data on flood magnitude and frequency is usually obtained from systematic river level and discharge records, which very seldom cover time periods larger than a century. However, past flood information can also be obtained from non-systematic sources such as historical records based on documents and chronicles and palaeohydrology records (a developing branch of hydrology and geomorphology based on geologic indicators). The use of non-systematic data can dramatically extend the length of the data series used for hazard estimation, and provides useful information on the most extreme flood events ever occurred in a given river. Recent development of extreme events analysis techniques has allowed incorporating non-systematic data, resulting in much more precise estimation of the flood hazard.

References This STRIVER Technical Brief is based on the following research report: Benito G. and Botero B. Long-term flood records of the Tagus River: Methodological guidelines, flood risk analysis and climate change perspectives. In: N. Lana-Renault, S. Beguería, S. Vicente-Serrano, J.M. García Ruiz, An organised ”new data” report for each basin, providing complete datasets, and an analysis of the data generated. Deliverable 3.1. – STRIVER Project. Recommended reading: Baker, V. R. 1989. Magnitude and frequency of palaeofloods, In: K. Beven and P.Carling (Eds.) Floods, their hydrological, sedimentological and geomorphological implications. John Wiley, Chichester, pp. 171-183. Benito, G., Sopeña, A., Sánchez, Y., Machado, M.J., and Pérez González, A. 2003. Palaeoflood Record of the Tagus River (Central Spain) during the Late Pleistocene and Holocene. Quaternary Science Reviews 22: 1737- 1756. Francés, F. 2004. Flood frequency análisis using systematic and non-systematic information. In: G. Benito, V. R. Thorndycraft (Eds.), Systematic, palaeoflood and historical data for the improvement of flood risk estimation. CSIC, Madrid, 55-70. Lang M., Fernandez Bono J.F., Recking A., Naulet R., Grau Gimeno P. 2004. Methodological guide for paleoflood and historical peak discharge estimation. In: G. Benito and V. Thorndycraft (Eds.) Systematic, Palaeoflood and Historical Data for the Improvement of Flood Risk Estimation: Methodological Guidelines. CSIC Madrid, Spain, pp.43-53. Naulet, R., Lang, M., Ouarda, T. B. M. J., Coeur, D., Bobee, B., Recking, A. and Moussay, D. 2005. Flood frequency analysis on the Ardeche River using French documentary sources from the last two centuries. Journal of Hydrology 313, 58-78. Ouarda, T. B. M. J., Rasmussen, P. F., Bobée, B. and Bernier, J. 1998. Use of historical information in hydrologic frequency analysis. Water Sciences Journal/Revue des Sciences de l’Eau 11: 41-49.

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Throughout Europe, national legislation on flood risk assessment is based on flood frequency analysis to estimate discharges associated with different return periods (50, 100, 500 years). The usual procedure involves extrapolation from gauged hydrological records documenting typically 30 to 40 year of observed (normally small) floods to the estimation of the quantiles of very rare, large floods. This conventional method of addressing flood risk assessment can be improved by including information of past floods, which should be accomplished using clear systematic procedures and methodologies. Long records of extreme floods are then applied successfully in risk analysis together with the more traditional empirical, statistical and deterministic methods to estimate the largest floods. These extreme floods are the ones planners and engineers are most interested in but are very rare in the observational record.

Reconstructing past flood events Information about floods in the preinstrumental period can be retrieved from geological evidences (palaeoflood hydrology) or from documentary evidence (historical hydrology). The combination of paleoflood and historical flood data with systematic data series has the potential to reduce the uncertainty associated with the standard method, and provides a more holistic approach that can strengthen our understanding of past floods.

Flood data from systematic gauge records Each European country manages its own hydrological gauge station network, which provides information on the water level and river discharge. During large floods conventional stream gauge stations have great difficulty in accurately recording extreme floods. They may be inundated, damaged by the water or even totally destroyed producing gaps in the gauged flood record of the largest floods. Therefore, in many cases, data of the largest floods within systematic gauge records are actually post-flood indirect estimates. Extrapolation of the rating curve, from low stage-discharge to high stage-discharge, is then a fundamental step for discharge estimation and can add substantial error. Error on flood discharge estimates are generally considered to be in the range of ±10% to ±100%, depending on the quality of the rating curve and its extrapolation to large floods.


View of slackwater flood sediments of the Tagus River near Talavera.

Palaeoflood hydrology is the reconstruction of the magnitude and frequency of recent, past, or ancient floods using geological evidence. The most commonly used evidence are slackwater flood deposits that indicate the minimum water surface elevation of past flood events and can be used to estimate the flood magnitude and frequency. If there is good preservation of the flood deposits at a particular site, the number of flood events represented in the particular sediment profile can be identified. This task requires a detailed

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stratigraphical description with a major emphasis on interpreting the breaks and contacts indicative of individual flood events. In order to obtain an accurate understanding of flood frequency, the slackwater palaeoflood deposits must be dated. Dating the sedimentary flood units also enables the identification of human recorded events in cases where historical, instrumental and palaeofloods temporal overlap. Radiocarbon dating and optically Stimulated Luminescence (OSL) are standard dating methods employed in palaeohydrologic research. Information about documentary (historical) floods can be derived both from narrative written sources, in the form of annals or chronicles; official economic records that containing data on the income and expenditure of a community; newspapers, frequently containing information about the causes and the course of the floods; pictorial documentation; epigraphic sources that may contain a brief description of a flooding or show the peak flood water level. An added value of documentary records is obtained from description and quantification of flood impacts on past societies, including economic losses, recovery strategies and flood management.

Epigraphic mark of the Tagus River flood occurred on January 8th, 1856

Palaeoflood and historical discharge estimation The water levels associated with the different floods (sediment height or epigraphic marks) can be converted into discharge values using common hydraulic models (Lang et al., 2004). The maximum discharge is obtained by a heuristic process using the proper hydraulic model by comparing the observed water levels and the simulated ones. In the majority of cases these models assume one-dimensional flow based on (1) slope-conveyance, (2) slopearea, (3) step-backwater, and (4) critical-depth methods.

Fact box The largest palaeoflood archive of the Tagus River was reconstructed in the middle course (at Puente del Arzobispo near Talavera, 35,000 km2; at Alcántara, 51,958 km2). Slackwater flood deposits provided an excellent record of more than 80 large flood episodes during the last 10,000 years. The palaeoflood record shows that extreme floods were not randomly spaced in time but tended to cluster during specific periods. The historical database of the Tagus River contains 355 records of documented historical flood events from AD 849 to 1979. Even the most incomplete series, mainly corresponding to periods before the 14th Century, provide valuable information on periods of concentrated floods.


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Statistical analysis using nonsystematic data

Palaeofloods, floods and longterm variations of the climate system

A basic assumption is that all floods exceeding a certain magnitude have been observed and recorded in the historical or palaeoflood registers. Hence, non-systematic data are treated as censored data, i.e. a series of all the events above a given threshold. Traditional methods of parameter estimation have been modified in order to combine systematic block maxima data and censored data. This methods allow fitting the data to a given probability distribution function (pdf). Some widely used pdfs include the Gumbel and the Generalised Extreme Value distributions, or the Gamma and the Pearson III.

A better understanding on how future climatic variations might influence flood magnitude and frequency is possible through the analysis of the palaeoflood record. Palaeoflood and historical flood reconstructions have revealed connections between floods and major climate shifts in the past and may provide clues about the effects of climate change scenarios on future flood frequency.

Fact box Floods in the Tagus River are highly related to persistent rainfall which may remain for several weeks due to the passage of series of Atlantic cold fronts over the Iberian Peninsula in winter. A strong correlation is generally observed between negative winter (DJF) NAO index (Luternacher et al., 2002) and floods above 1500 m3s-1 in Talavera.

Figure 1. Relationship between winter (DJF) NAO index and reconstructed

discharge of historical floods in the Tagus River in

Talavera since 1400 AD.

Scenarios and predictions of future variations of the NAO index are currently being generated with the use of climate simulation models (GCMs). In both cases, if the NAO index increases or if it remains at the levels of past decades, we can expect a clear downward trend of extraordinary floods in the Tagus River and the rest of Atlantic basins of the Iberian Peninsula.


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The STRIVER Policy and Technical Brief series translate the results from the project into practical and useful information for policy makers and water managers. The Briefs are also available online: www.striver.no About STRIVER STRIVER- Strategy and methodology for improved IWRM - An integrated interdisciplinary assessment in four twinning river basins is a three year EC funded project 2006-2009 under the 6th framework programme (FP6) coordinated jointly by Bioforsk and NIVA. The point of departure for STRIVER is the lack of clear methodologies and problems in operationalisation of Integrated Water Resource Management (IWRM) as pointed out by both the scientific and management communities.13 partners from 9 countries participate as contractual partners in addition to an external advisory board. Title of project:

Strategy and methodology for improved IWRM - An integrated interdisciplinary assessment in four twinning river basins (STRIVER) Instrument: SUSTDEV-2005-3.II.3.6: Twinning European/third countries river basins. Contract number: 037141 Start date of project: July 2006 Duration: 36 months

Project funded by the European Commission within the Sixth Framework Programme (2002-2006) Disclaimer

The information provided and the opinions given in this publication are not necessarily those of the authors or the EC. The authors and publisher assume no liability for any loss resulting from the use of this report. Editors: Per Stålnacke and Udaya Sekhar Nagothu (Bioforsk) Launch date: 17 November 2008


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