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Global changes in climate have been widely documented but the relationships between these changes and floods are not easy to establish. Hydroclimatology offers an important tool in order to improve our knowledge about the flood producing mechanisms, which can be used to explain historical and palaeohydrological events. Storm cells and mesoscale systems develop into small floods, mainly at tributary streams. But a synoptic scale relates better to Guadiana River floods, dominant in winter. At a higher level, macroscale atmospheric configuration can also explain flood generation at the Guadiana River and, particularly, the North Atlantic Oscillation (NAO) that is one of the most important recurrent patterns of atmospheric circulation. There is no reliable evidence, however relating NAO and historical floods in Europe, especially for central European countries. This situation changes in the Iberian Peninsula, specially in the Southwest that shows a good relationship between winter floods and a negative NAO phase. Recent Guadiana River floods (XX century) could be related to these phenomena, but there´s not and index who covers previous events. Guadiana palaeoflood records compiled using slackwater deposits are dated by 14C radiometric methods and associated, when it is possible to a historical flood event. Results shows event clusters during cooling phases and it is likely that they reflect moments of climatic variability., Ministerio de Ciencia y Tecnología (España), Depto. de Geodinámica, Estratigrafía y Paleontología, Fac. de Ciencias Geológicas, TRUE, pub

Climate change implies changes in the frequency and magnitude of flood events. The influence of climate variability on flooding was evaluated by an analysis of sedimentary (palaeofloods) and documentary archives. A 500-year palaeoflood record at Montlleó River (657 km2 in catchment area), eastern Spain, revealed up to 31 palaeofloods with a range of discharges of 20–950 m3 s−1, and with at least five floods exceeding 740–950 m3 s−1. This information contrasts with the available gauged flood registers (since year 1971) with an annual maximum daily discharge of 129 m3 s−1. Our palaeoflood dataset indicates flood cluster episodes at (1) 1570–1620, (2) 1775–1795, (3) 1850–1890, and (4) 1920–1969. Flood rich periods 1 and 3 corresponded to cooler than usual (about 0.3 °C and 0.2 °C) climate oscillations, whereas 2 and 4 were characterised by higher inter-annual climatic variability (floods and droughts). This high inter-annual rainfall variability increased over the last 150 years, leading to a reduction of annual maximum flow. Flood quantiles (>50 years) calculated from palaeoflood+gauged data showed 30%–40% higher peak discharges than those using only instrumental records, whereas when increasing the catchment area (1500 km2) the discharge estimation variance decreased to ~15%. The results reflect the higher sensitivity of small catchments to changes on flood magnitude and frequency due to climate variability whereas a larger catchment buffers the response due to the limited extent of convective storms. Our findings show that extended flood records provide robust knowledge about hazardous flooding that can assist in the prioritization of low-regret actions for flood-risk adaptation to climate change.

This paper provides a new methodological approach to analyse secular patterns of flooding (magnitude and frequency) from sedimentary evidence (palaeofloods), taking into account changes in channel geometry, and their links to historical environmental changes and the inherent social and demographic evolution within the catchment. A case study analysis was focused in Rambla de la Viuda (drainage area of 1500 km2) whose stream flow is related to extreme rainfalls. A 500 years sedimentary archive was reconstructed from eight stratigraphic profiles comprising continuous sequences of slackwater flood deposits interbedded with episodic colluvial and edaphic horizons. Discharge estimates associated to sedimentary flood evidences were obtained from one-dimensional hydraulic modelling. The stratigraphic units were sampled to characterise their geochemical and paleobotanical (phytoliths) contents. Palaeoflood chronology was obtained from radiocarbon and luminescence (OSL) dating, supported by documentary data (written historical documents). A high frequency and high magnitude palaeoflood period took place during the 15th-middle 16th century, which seem to correlate in time with general wetter conditions. Three short-term environment stability conditions (land use and climatic) also made possible the development of three paleosols. The lowest flood magnitude and discharges in the sedimentary record was found between the mid-17th to mid-18th centuries, under prevailing drier environmental conditions. Episodic high magnitude flooding took place at late 18th century, correlating in time with palaeovegetation and geochemical evidences of important changes on land use (deforestation and grazing). Poorer developed soils were found at upper stratigraphic sequences (19th century) characterised by thick units of colluvium deposits, usually culminating sequences of short-lived continuous slackwater flood units. Despite of the potential human influence (land-use) on soil hydrology, the lo

Mediterranean fluvial hydrology is characterised by decadal-to-multi-centennial length wet and dry episodes with abrupt transitions related to changes in atmospheric circulation. Since the mid-1990s site-based flood chronologies from slackwater deposits in bedrock rivers and regionally aggregated flood histories from alluvial deposits have developed increasingly higher resolution chronological frameworks, although regional coverage is still uneven. This paper analyses the spatial and temporal distribution of extreme Holocene hydrological events recorded in fluvial stratigraphy in the Iberian Peninsula (Spain and Portugal), southern France, southern Italy, Northern Africa (Morocco and Tunisia) and eastern Mediterranean (Greece, Crete, Turkey, Cyprus and Israel). This study constitutes the most comprehensive investigation of Holocene river flooding ever undertaken in the Mediterranean and is based on the analysis of 515 14C and 53 OSL dates. It reveals that flood periods in different regions cluster into distinct time intervals, although region-wide flooding episodes can be identified at 7400–7150, 4800–4600, 4100–3700, 3300–3200, 2850–2750, 2300–2100, 1700–1600, 1500–1400, 950–800, ca. 300, 200–100 cal. BP. Periods with more frequent floods in the western Iberian region coincide with transitions to cool and wetter conditions and persistent negative NAO mode. In Northern Africa increased flood frequency coincides with periods of generally drier climate,while in the eastern Mediterranean there is a higher incidence of extreme flood events under wetter conditions. Our meta-data analysis identifies an out-of-phase pattern of extreme events across the Mediterranean over multi-centennial timescales,which is particularly evident between the western Iberian and eastern Mediterranean regions. This centennial-to-multi-centennial see-saw pattern in flooding indicates that bipolar hydroclimatic conditions existed in the Mediterranean during the Holocene.

Mediterranean fluvial hydrology is characterised by decadal-to-multi-centennial length wet and dry episodes with abrupt transitions related to changes in atmospheric circulation. Since the mid-1990s site-based flood chronologies from slackwater deposits in bedrock rivers and regionally aggregated flood histories from alluvial deposits have developed increasingly higher resolution chronological frameworks, although regional coverage is still uneven. This paper analyses the spatial and temporal distribution of extreme Holocene hydrological events recorded in fluvial stratigraphy in the Iberian Peninsula (Spain and Portugal), southern France, southern Italy, Northern Africa (Morocco and Tunisia) and eastern Mediterranean (Greece, Crete, Turkey, Cyprus and Israel). This study constitutes the most comprehensive investigation of Holocene river flooding ever undertaken in the Mediterranean and is based on the analysis of 515 14C and 53 OSL dates. It reveals that flood periods in different regions cluster into distinct time intervals, although region-wide flooding episodes can be identified at 7400–7150, 4800–4600, 4100–3700, 3300–3200, 2850–2750, 2300–2100, 1700–1600, 1500–1400, 950–800, ca. 300, 200–100 cal. BP. Periods with more frequent floods in the western Iberian region coincide with transitions to cool and wetter conditions and persistent negative NAO mode. In Northern Africa increased flood frequency coincides with periods of generally drier climate,while in the eastern Mediterranean there is a higher incidence of extreme flood events under wetter conditions. Our meta-data analysis identifies an out-of-phase pattern of extreme events across the Mediterranean over multi-centennial timescales,which is particularly evident between the western Iberian and eastern Mediterranean regions. This centennial-to-multi-centennial see-saw pattern in flooding indicates that bipolar hydroclimatic conditions existed in the Mediterranean during the Holocene.

Optically Stimulated Luminescence (OSL) of young palaeoflood sediments): The challenge of accurately estimating the deposition age of incompletely-bleached samples in luminescence dating has motivated developments in both measurement and analysis methods over the last few years. In this study, we compare the behaviour of CAM and IEU models when applied to dose distributions from a sequence of eight recent (40-1000 yr) flash flood deposits, potentially affected by incomplete bleaching. These dose distributions were obtained from quartz single grains and quartz small multigrain aliquots (~30 grains). Comparison was made between the different measuring and analysing techniques applied. The most suitable method was assessed by comparison of these results with those available from independent age control. It is shown that the measurement of multigrain aliquots in combination with the minimum age model IEU on OSL dose distributions is appropriate for the accurate dating of young flash flood deposits.

Fluvial records contain evidence of past hydrological changes in terms of water/sediment fluxes and extreme hydrological events (floods), which can be linked to Earth's climate variability. Sedimentological records of hydrological extremes can be complemented with historical documentary information and botanical records. In recent years, fluvial and palaeoflood records supported with excellent chronological data have become available for many rivers worldwide (Gregory et al., 2006), but still regional and global palaeohydrological reconstructions and syntheses have yet to be fully developed. Related to the interpretation of alluvial records, a major recent achievement has been the development of new combined analysis of large radiometrically dated fluvial databases, which allow fluvial activity periods to be more precisely defined and to be characterized in terms of forcing hydrological conditions (Macklin et al., 2006). The approach also serves for comparison of records of fluvial activity in different countries, regions and sites with a range of climate proxies and, for the more recent record, land-use change indicators. The meta-analyses demonstrate the value of the growing spatial coverage and increasing chronological precision of fluvial archives for reconstructing past hydrological events, as well as for understanding river response to environmental/climate change in the past and the future. This special issue brings together a range of papers that reconstruct regional fluvial chronologies of extreme events based on multiple proxies and documentary information, which evaluate the relationship between river hydrology, climate and atmospheric circulation variability. The research presented in this volume was drawn together within the framework of the INQUA Project 1220 on Hydrological EXtreme Events in Changing Climate (HEX Events), and the 1st Workshop on HEX Events held at Utrecht University on November 29th–30th, 2012.

Mediterranean fluvial hydrology is characterised by decadal-to-multi-centennial length wet and dry episodes with abrupt transitions related to changes in atmospheric circulation. Since the mid-1990s site-based flood chronologies from slackwater deposits in bedrock rivers and regionally aggregated flood histories from alluvial deposits have developed increasingly higher resolution chronological frameworks, although regional coverage is still uneven. This paper analyses the spatial and temporal distribution of extreme Holocene hydrological events recorded in fluvial stratigraphy in the Iberian Peninsula (Spain and Portugal), southern France, southern Italy, Northern Africa (Morocco and Tunisia) and eastern Mediterranean (Greece, Crete, Turkey, Cyprus and Israel). This study constitutes the most comprehensive investigation of Holocene river flooding ever undertaken in the Mediterranean and is based on the analysis of 515 14C and 53 OSL dates. It reveals that flood periods in different regions cluster into distinct time intervals, although region-wide flooding episodes can be identified at 7400–7150, 4800–4600, 4100–3700, 3300–3200, 2850–2750, 2300–2100, 1700–1600, 1500–1400, 950–800, ca. 300, 200–100 cal. BP. Periods with more frequent floods in the western Iberian region coincide with transitions to cool and wetter conditions and persistent negative NAO mode. In Northern Africa increased flood frequency coincides with periods of generally drier climate, while in the eastern Mediterranean there is a higher incidence of extreme flood events under wetter conditions. Our meta-data analysis identifies an out-of-phase pattern of extreme events across the Mediterranean over multi-centennial timescales, which is particularly evident between the western Iberian and eastern Mediterranean regions. This centennial-to-multi-centennial see-saw pattern in flooding indicates that bipolar hydroclimatic conditions existed in the Mediterranean during the Holocene.

We present an annual reconstruction of extreme rainfall events interpreted from detrital layers and turbidites interbedded within a varved sediment record since the 14th century in Montcortés Lake (NE Spain, 1027ma.s.l.). Clastic microfacies intercalated within the biochemical calcite varves were characterized and their depositional dynamics interpreted using high-resolution geochemical and sedimentological analyses. Annual number of detrital layers was compared against instrumental records of extreme daily rainfalls providing minimum rainfall thresholds and return periods associated to the identified types of clastic microfacies. Non-continuous detrital layers were deposited during rainfall events higher than 80mm (>2-year return period) while graded detrital layers and turbidites were associated with higher magnitude rainfall events (>90mm and >4-year return period). The frequency distribution of extreme hydro-meteorological events is not stationary and its pattern coincides with historical floods from the nearby Segre River. High frequency of heavy rainfalls occurred during the periods AD 1347-1400 and AD 1844-1894. A lower frequency of heavy rainfall was found during the periods AD 1441-1508, 1547-1592, 1656-1712, 1765-1822 and 1917-2012. The 20th century stands out as the longest interval within the studied period of very low number of extreme rainfall events. Variability in extreme rainfall events prior to the 20th century is in phase with solar activity, suggesting a mechanistic link in mid-latitude atmospheric circulation patterns that ceased during the 20th century. © 2014 Elsevier Ltd.

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A long-term flood record from the Buffels River, the largest ephemeral river of NW South Africa (9250 km2), was reconstructed based on interpretation of palaeoflood, documentary and instrumental rainfall data. Palaeoflood data were obtained at three study reaches, with preserved sedimentary evidence indicating at least 25 large floods during the last 700 yr. Geochronological control for the palaeoflood record was provided by radiocarbon and optically stimulated luminescence (OSL) dating. Annual resolution was obtained since the 19th century using the overlapping documentary and instrumental records. Large floods coincided in the past within three main hydroclimatic settings: (1) periods of regular large flood occurrence (1 large flood/~30 yr) under wetter and cooler prevailing climatic conditions (AD 1600–1800), (2) decreasing occurrence of large floods (1 large flood/~100 yr) during warmer conditions (e.g., AD 1425–1600 and after 1925), and (3) periods of high frequency of large floods (~ 4–5 large floods in 20–30 yr) coinciding with wetter conditions of decadal duration, namely at AD 1390–1425, 1800–1825 and 1915–1925. These decadal-scale periods of the highest flood frequency seem to correspond in time with changes in atmospheric circulation patterns, as inferred when comparing their onset and distribution with temperature proxies in southern Africa.

[EN]: Flood response to Global Warming is one of the major uncertainties on the latest IPCC (Trenberth et al., 2007). The lack of long instrumental records makes difficult to provide robust links between extreme events and climate variability. In this paper, we describe the methodological procedures used on scientific research for reconstructing past floods in the context of variability. Evidences from past floods based on sedimentary and geomorphic records (palaeofloods) show a higher frequency of floods during the initial and final stages of cold periods (e.g. Little Ice Age 1550-1850 A.D.). During the instrumental period (1910 to the present), Iberian Atlantic rivers show a decrease on frequency for ordinary flood events, and in Mediterranean rivers data point out to an increase in the hydrological variability with higher frequency of flash floods., [ES]: Entre las principales incertidumbres del Informe del Panel Intergubernamental sobre Cambio Climático (IPCC) (Trenberth et al., 2007) se encuentra la predicción de crecidas extremas en respuesta al Calentamiento Global. La falta de registros instrumentales dificulta en gran medida la posibilidad de diagnosticar la magnitud y frecuencia con la que ocurrirán los eventos extremos en relación con la variabilidad climática. Las evidencias de crecidas pasadas basadas en indicadores geológicos (sedimentarios y geomorfológicos) permiten ampliar los registros de eventos extremos (paleocrecidas) a los últimos milenios, y comprender su respuesta en relación al clima. En este trabajo se pretende describir el procedimiento metodológico utilizado en la literatura científica para la reconstrucción de eventos extremos obtenidos del pasado reciente basado en el estudio de procesos geológicos externos. Los registros sedimentariosobtenidos en diferentes ríos peninsulares muestran que las paleocrecidas son más frecuentes durante los estadios iniciales y finales de periodos fríos (e.g. Pequeña Edad del Hielo 1550-1850 A.D.). En el periodo instrumental (1910 hasta la actualidad), los ríos Atlánticos de la Península Ibérica han experimentado una disminución de la frecuencia de las crecidas ordinarias, y, en las cuencas Mediterráneas los datos existentes apuntan al aumento en la irregularidad hidrológica y la generación de crecidas relámpago.

Palaeoflood chronologies from seven Spanish river basins and floodplain aggradation chronologies from thirteen rivers are analysed. These fluvial records were divided in to two sub-sets, namely Atlantic (10 ka record) and Mediterranean (3 ka record) river basins, which represent distinct modern hydroclimatic conditions. In Atlantic basins floods result from intense, widespread rainfalls associated with Atlantic frontal systems transported by westerly airflow. Mediterranean river flooding is related to heavy rainfall induced by mesoscale convective systems (MCSs) during autumn months. Evidence from radiocarbon dates in slackwater flood deposits shows six periods of flood clusters at 10,750–10,240; 9550–9130; 4820–4440; 2865–2350; 960–790; and 520–290 cal BP. Despite the different flood-producing weather conditions in Atlantic and Mediterranean rivers, the radiocarbon sample clusters overlap and indicate changes in large-scale atmospheric circulation and climatic conditions in the Iberian Peninsula. Comparison with proxy records of mean temperature for the Northern Hemisphere demonstrates a relationship between the period of slackwater flood deposition and cold climatic phases (e.g. the 2650 yr BP climatic event or AD 1590–1650 period of the Little Ice Age). Radiocarbon dates from aggraded floodplain sediments were clustered at 2710–2320, 2000–1830, and 910–500 cal BP. The first cluster period is in phase with the timing of slackwater deposition, whereas the third (910–500 cal BP) occurs in between two periods of increased flood frequency as indicated by the palaeoflood and documentary flood records. It is argued that the 910–500 cal BP floodplain aggradation period reflects the first post-Roman evidence of environmental change related to generalised land-use changes at the catchment scale, which produced high sediment load transported to overbank areas during high flows.

Global changes in climate have been widely documented but the relationships between these changes and floods are not easy to establish. Hydroclimatology offers an important tool in order to improve our knowledge about the flood producing mechanisms, which can be used to explain historical and palaeohydrological events. Storm cells and mesoscale systems develop into small floods, mainly at tributary streams. But a synoptic scale relates better to Guadiana River floods, dominant in winter. At a higher level, macroscale atmospheric configuration can also explain flood generation at the Guadiana River and, particularly, the North Atlantic Oscillation (NAO) that is one of the most important recurrent patterns of atmospheric circulation. There is no reliable evidence, however relating NAO and historical floods in Europe, especially for central European countries. This situation changes in the Iberian Peninsula, specially in the Southwest that shows a good relationship between winter floods and a negative NAO phase. Recent Guadiana River floods (XX century) could be related to these phenomena, but there´s not and index who covers previous events. Guadiana palaeoflood records compiled using slackwater deposits are dated by 14C radiometric methods and associated, when it is possible to a historical flood event. Results shows event clusters during cooling phases and it is likely that they reflect moments of climatic variability.

[EN]: Spanish regulations on dam safety recommend the estimation of a return period of 1000 years for the Design Flood and of 10,000 for the Safety Check Flood. The criteria used for the calculations are based on Flood Frequency Analysis (FFA) from gauge record data or on the Probable Maximum Flood (PMF) which is the hypothetical maximum of rainfall turned into runoff. The short gauge records affords little support for the FFA and for the hypothetical PMF, which in addition to the absence of a associated probability, limits the utility of these indices for risk-based dam safety decisions. Palaeoflood techniques are a means of directly assessing the probability of extreme floods and testing the validity of the PMF-based models. In this paper, palaeoflood record was applied to a case study in the Guadalentín river, upstream of the Valdeinfierno reservoir (372 km2) with a spillway capacity of 550 m3s-1 (design flood). In addition, available historical flood data was collected. Palaeoflood data and gauge record, were combined for the FFA, using existing methods for the fitting of distribution functions, such as the maximum likelihood. Palaeoflood analysis provides a discharge of 2350 m3s-1 for the design flood (1000-yr return period), and 3450 m3s-1 for the safety check flood (10,000 yr-return period). The discharge estimated by the PMF is 5786 m3s-1, showing an overestimation of this empirical method. Some recommendations for the calculation of the safety check flood and design flood are provided which can improve existing legislation on dam safety., [ES]: En el Reglamento Técnico sobre Seguridad de Presas y Embalses se recomienda, en función de la categoría de la presa considerada, el uso de las Avenidas de Proyecto con periodo de retorno de 1.000 años para la Avenida de Diseño, y de hasta 10.000 para la Avenida Extrema. Su estimación a partir de las exiguas series de datos foronómicos o meteorológicos del registro sistemático (ordinario) carece del rigor científico que requiere la envergadura de estos proyectos. En este trabajo se presentan los resultados del análisis de paleocrecidas en el río Guadalentín aguas arriba del embalse de Valdeinfierno (372 km2) con un aliviadero de 550 m3s-1, y su aplicación a la determinación de los cuantiles requeridos para la determinación de las avenidas de diseño y máxima. Los depósitos analizados corresponden a arenas y limos acumulados durante las crecidas en zonas de remanso (slackwater flood sediments). En el análisis de frecuencia de crecidas se han combinado los caudales estimados para estos depósitos con los datos de aforo utilizando el método de máxima verosimilitud para ajustar una distribución Gumbel. En este análisis se obtiene un caudal de 2350 m3s-1 para la avenida de diseño (1.000 años de periodo de retorno), y 3450 m3s-1 para la avenida máxima (10.000 años de periodo de retorno). Los caudales asociados a crecidas con periodos de retorno elevado se han comparado con los resultantes del cálculo de la avenida máxima probable (PMF), que es de 5786 m3s-1.

The catastrophic floods recently occurring in Europe warn of the critical need forhydrologic data on floods over long-time scales. Palaeoflood techniques provideinformation on hydrologic variability and extreme floods over long-time intervals(100 to 10,000 yr) and may be used in combination with historical flood data (last1,000 yr) and the gauge record (last 30–50 yr). In this paper, advantages anduncertainties related to the reconstruction of palaeofloods in different geomorphologicalsettings and historical floods using different documentary sources are described.Systematic and non-systematic data can be combined in the flood frequency analysisusing different methods for the adjustment of distribution functions. Technical toolsintegrating multidisciplinary approaches (geologic, historical, hydraulic and statistical)on extreme flood risk assessment are discussed. A discussion on the potential theoreticalbases for solving the problem of dealing with non-systematic and non-stationary data ispresented. This methodology is being developed using new methodological approachesapplied to European countries as a part of a European Commission funded project (SPHERE).

[EN]: This paper is focused on the combination of historical, palaeoflood and instrumental records applied to estimate magnitude and frequency of floods in a high-gradient, small catchment in the Spanish Central Pyrenees: the Montardit basin, 15,2 km2. Historical data corresponding to past floods occurring in the Montardit drainage basin during the 20th Century were compiled through interviews with inhabitants of villages located in the basin. The largest floods of the stream were recorded in November 1907, in August 1924 and in 1944. The results of the methods applied in this study suggest that for high-frequency, low-magnitude events there is a good correlation between peak discharges yielded by palaeoflood reconstruction (7 to 46 m3 s-1) and those estimated through rainfall-runoff modelling (8 to 46 m3 s-1). In contrast, high magnitude events show a higher historical frequency than that provided by rainfallrunoff methods., [ES]: En este estudio se analiza la magnitud y frecuencia de las crecidas en una pequeña cuenca (15,2 Km2) de los Pirineos Centrales españoles (cuenca de Montardit) mediante el uso combinado de diferentes métodos basados en evidencias geológicas, históricas y datos instrumentales. De acuerdo con los datos históricos las mayores crecidas registradas en la cuenca durante el siglo XX se produjeron en 1907, 1924 y 1944. Los caudales estimados m e d i a n t e técnicas de reconstrucción de paleocrecidas (7 a 46 m3 s- 1) y métodos hidrometeorológicos (8 a 46 m3 s- 1) son similares para eventos considerados de elevada frecuencia y baja magnitud. Por el contrario, las crecidas de elevada magnitud muestran una frecuencia histórica mayor que la predicha por los métodos basados en modelos hidrometeorológicos .

[ES] En este estudio se analiza la magnitud y frecuencia de las crecidas en una pequeña cuenca (15,2 Km2) de los Pirineos Centrales españoles (cuenca de Montardit) mediante el uso combinado de diferentes métodos basados en evidencias geológicas, históricas y datos instrumentales. De acuerdo con los datos históricos las mayores crecidas registradas en la cuenca durante el siglo XX se produjeron en 1907, 1924 y 1944. Los caudales estimados mediante técnicas de reconstrucción de paleocrecidas (7 a 46 m3 s- 1) y métodos hidrometeorológicos (8 a 46 m3 s- 1) son similares para eventos considerados de elevada frecuencia y baja magnitud. Por el contrario, las crecidas de elevada magnitud muestran una frecuencia histórica mayor que la predicha por los métodos basados en modelos hidrometeorológicos., [EN] This paper is focused on the combination of historical, palaeoflood and instrumental records applied to estimate magnitude and frequency of floods in a high-gradient, small catchment in the Spanish Central Pyrenees: the Montardit basin, 15,2 km2. Historical data corresponding to past floods occurring in the Montardit drainage basin during the 20th Century were compiled through interviews with inhabitants of villages located in the basin. The largest floods of the stream were recorded in November 1907, in August 1924 and in 1944. The results of the methods applied in this study suggest that for high-frequency, low-magnitude events there is a good correlation between peak discharges yielded by palaeoflood reconstruction (7 to 46 m3 s-1) and those estimated through rainfall-runoff modelling (8 to 46 m3 s-1). In contrast, high magnitude events show a higher historical frequency than that provided by rainfallrunoff methods.

This paper reports a comparative study of the use of palaeohydrological and hydrological techniques to estimate magnitude and frequency of floods applied to two high-gradient, small catchments located in the Spanish Central Pyrenees: the Arás basin, 18.6 km2; the Montardit basin, 15.2 km2. Palaeohydrological (palaeoflood) techniques, based on palaeostage indicators and the application of both Manning's equation and critical flow equation, were used to estimate discharge values corresponding to seven cross-sections of the Montardit stream. This method yielded discharges ranging from 7 to 112 m3 s−1. Palaeoflood and historical flood analyses indicated a greater frequency of high magnitude events for both the Arás and Montardit basins than suggested by the application of two different methodologies based on precipitation data (the rational and the unit hydrograph methods). The 1996 Arás basin flood was modelled using the unit hydrograph method, with most reliable results (217 m3 s−1) obtained using a semidistributed basin model. For small, ungauged mountain basins, palaeoflood reconstruction provides an excellent tool to obtain past flood discharges on which to base the calibration of parameters (e.g. runoff threshold) used in rainfall–runoff methods. It is proposed that the combined results of long-term flood record analysis (based on palaeoflood and historical data) and of standard hydrometeorological methods will aid in the assessment of flood risks for application to the design of emergency measures and floodplain management in ungauged mountain streams.