Journey back through time to help manage river floods
Statistically, there is little likelihood of anybody experiencing a major river flood whose average recurrence interval is one hundred or one thousand years. Predicting and designing of such events involves going back in time, three or four centuries, by scrutinising records of severe flooding. Joint researches by Cemagref hydrologist Michel Lang and historian Denis Coeur have reconstructed the history of three French rivers, the Guiers, Isere and Ardeche, a unique picture which has been incorporated in the European SPHERE project involving French, German, Canadian, Spanish and Israeli teams.
esearch on three rivers
The search began with the Guiers, the historical boundary between France and Savoy. Using a qualitative recension of river floods, a list of the ten largest events recorded over the last 300 years was drawn up. Attention then turned to the Isere, to test the method on a river which had been extensively developed and controlled, and to see if the past could yield insights into the future. Four centuries of data was used. Contemporary eyewitness accounts are not ignored, and can be consulted on the Internet at http://www.lyon.cemagref.fr/hh/base-in/base_in_anglais/isere1859/presentation.htm where there is an illustrated account of the worst river flood, in 1859. The geography of the flood plain made it impossible to calculate river flow from water levels, because the flood plain is too wide and its contours insufficiently documented before the 19th century. Abundant information is available for the Ardeche river. Two sites were given special attention near the gorges, at Vallon Pont d’Arc and Saint Martin d’Ardeche. River flow has been recalculated from 1644 to the present. The largest river flood has been reconstituted with a 50% margin of error, which is comparable to today’s level of accuracy on high streamflows.
Historical records yield information on the damage caused by a river flood, how the emergency was managed and, sometimes, more technical data on water levels or river geometry. Palaeohydrologists may be called in to take a close look at the sediments and organic material in the flood plain and date the deposits. In the Ardeche area, they inspected caves to look for precious particulates loaded with information and count how many times the water level rose higher than the cave. Working from the historical shape of the river bed and a hydraulic model, the researchers built up streamflow data series and assessed the magnitude of each event. With these long series in hand, it was possible to compare different methods of estimating rare river floods and validate, on the Ardeche, the Gradex method used in France for assessing the safety of large dams.
A combination of hydrologist and historian is vital in addressing flooding so that the riparian population retain the memory of the risks involved. On the Saone river at Lyon, the researchers intend to make historical information available in the form of a column showing the present water level and the levels reached by the most extreme floods, together with the main vagaries of the river. This is in line with the new legislation on risk, making it a duty to mark flood levels on public buildings.
The historical data collected may also help in finding strategies for managing rare events, by optimising emergency plans and setting extreme flood levels more accurately. Methodological guidelines for technicians are being drafted jointly with the European countries taking part in the SPHERE programme.
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