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A methodology for hydrological post-flash flood field investigations

Gaume Eric

Post event survey and investigation is one way to gain experience on natural hazards. The importance of the systematisation and standardisation of such investigations and re-analysis is progressively recognised in all the geophysical sciences as shown by the growing number of scientific papers and programs on the subject.
Large research efforts have been made on the analysis and modelling of the meteorological aspects of the flash-flood triggering storms (c.f. the proceedings of Plinius conferences on Mediterranean storms) or on landslides and concentrated flows. In comparison, the analysis of the dynamics of the runoff processes during flash-floods is still at its infancy.
Most of the existing reports on flash-floods are restricted to measured point rainfall intensities and some peak discharge estimates, generally for gauged river cross-sections. But recent works have demonstrated that additional valuable data can be gathered after major flood events even on ungauged watersheds. These data, mainly peak discharge estimates based on flood marks and sometimes on films and partial time sequences of floods based on witnesses' interviews, can be used in combination with rainfall estimates to analyse the dynamics of the rainfall-runoff processes on the affected watersheds. This opens new perspectives: with the help of the Radar rainfall estimations it is possible to analyse the flash-floods wherever they occur and not only on well gauged watersheds at the appropriate time and space scales.
A first attempt to formalize a post-flood field investigation procedure will be presented. The proposed approach has been developed within the European FLOODSITE research project. The method has been tested at the national scale for the purpose of the Gard 2002 post flood investigation: 18 hydrologists from 8 different institutions were involved. The European research project HYDRATE (2006-2008) will give the opportunity to test it on further case studies with international teams.