Eos,Vol. 84, No. 12, 25 March 2003
Claim of Largest Flood on Record Proves False
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A study of paleofloods in the Gardon River in southern France reveals the occurrence of past extreme floods that were larger than any observed historically. From 8 to 9 September 2002, during the course of the study, by a complete coincidence, an extreme flood claimed the lives of 21 people and caused millions of dollars worth of damage to the towns and villages along the river.This flood was larger in magnitude than any flood on record, according to gaged data since 1890. An autumn storm, which is typical of this region, struck with immense force.The rain cell migrated from the lower reaches of the basin on the evening of 8 September to the upper parts of the basin, producing 680 mm of rain in 20 hours.The flood’s peak discharge is preliminary estimated at 6000 m s- .This flood is now considered by the media and professionals to be “The largest flood on record.” However, our research proves otherwise. The Gardon River, located southeast of the Massif Central in the Cévennes region,is approximately 135 km long from its headwaters at 1500 m above sea level to its confluence with the Rhöne River at 6 m above sea level. In its lower eastern reaches, the Gardon River flows in a gorge incised into hard carbonate rocks. Paleoflood field research began a few months prior to the flood in that gorge,which lies downstream of Pont (bridge) St. Nicolas and upstream of the famous Pont du Gard. Caves at the wall of the gorge serve as natural stage gages and trap deposits of the largest floods. So far, the five highest flood events were identified in a cave more than 17 m above the normal base flow.Three of these five units were dated to AD 1400–1800 (420±40 BP to 250±40 BP) by using the radiocarbon method on charcoal found within the units.The
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Fig. 1. In this photograph of the study site at la Baume on the Gardon River, the dotted line marks the highest stage of the 2002 flood, which reached ~14 m above the riverbed. In the top left corner, the stratigraphy of the highest flood deposits within a cave is shown, with an arrow indicating the position of the cave at more than 17 m above the riverbed. September 2002 flood reached a stage of only 14 m above the normal base flow at this site, overtopping the other lower flood deposit sites along the river, but not reaching this 17-mhigh cave.Therefore, the extraordinary flood of September 2002 was not the largest by any means; similar, and even larger floods have occurred several times in the recent past. These results indicate that paleoflood studies can provide a good measure of the magnitude of the largest floods that can affect a drainage basin. Using a longer time scale than human collective memory, paleoflood studies can put in perspective the occurrences of the extreme floods that hit Europe and other parts of the world during the summer of 2002. Even if, in some areas, the floods reached magnitudes that were never experienced, witnessed, or documented historically, we should be cautious when claiming them to be the largest.
Acknowledgments
This study was funded by the European Union – FP5,(EVG1-1999-00039,SPHERE project: www. ccma.csic.es/dpts/suelos/hidro/sphere/home. html). The authors wish to thank François Bressand of the Flood Warning Office,the Office of Public Works (DDE), Nîmes, France, for the help lent during the work. —N.A. SHEFFER,Y. ENZEL, N.WALDMANN, and T. GRODEK, The Hebrew University of Jerusalem, Israel; and G. BENITO, Environmental Science Centre,
Consejo Superior de Investigaciones Científicas, Madrid, Spain
Catching the Wind: New Views of Aeolian Processes
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A joint meeting of the Fifth International Conference on Aeolian Research (ICAR-5) and the Global Change and Terrestrial Ecosystems-Soil Erosion Network (GCTE-SEN) brought together physical and environmental scientists interested in the latest challenges and discoveries in aeolian (wind as a geological process) research. The conference provided an opportunity for researchers focusing on both theoretical modeling and field experimentation aspects of aeolian processes to interact and learn the research needs and findings of their counterparts. Perhaps due to the meeting’s location in the heartland of the North American Great Plains, many presentations at ICAR-5 dealt with wind erosion of agricultural lands in the United States, Canada, and elsewhere. One hundred thirty scientists from 18 nations attended. The GCTE-SEN, a core project of the International Geosphere-Biosphere Programme (IGBP), has a dual thrust: (1) to design and undertake experimental and monitoring programs to provide a predictive understanding of the impacts of climate and land use on soil erosion, and (2) to refine and adapt current erosion models for use in global change studies from plot to regional scales. More information on the network is available at http://mwnta.nmw. ac.uk/GCTEFocus3/networks/erosion.htm. GCTE-SEN has sponsored meetings to evaluate, test, and compare water erosion models, but little has been done in regards to wind erosion and aeolian processes. The joint meeting that is the subject of this article was dedicated to the memory of Harold Dregne, an internationally renown expert on desertification. Dregne, a distinguished professor at Texas Tech University and ICASALS
(International Center for Arid and Semiarid Land Studies) director emeritus, authored 10 books and more than 170 scientific publications during his career. Dregne traveled throughout the world consulting for United Nations agencies and performing research for over 25 years on the causes and environmental impacts of desertification, until his untimely death earlier in 2002.
Processes, Instrumentation, Field Studies, and Modeling
One theme repeatedly addressed during ICAR-5 was an improved understanding of fundamental aeolian processes, especially the mechanisms by which sand motion at the Earth’s surface results in the emission of fine mineral dust aerosols. Data was presented suggesting that mineral aerosol emissions are more a function of surface roughness than sedimentary texture of the source area.Another fundamental theme of many presentations was the aerodynamics of the interaction