Blanc SIMI 5-6 - Blanc - SIMI 5-6 - Environnement, Terre et Espace

Multi-scale hydrometeorological observation and modelling for flash flood understanding and simulation – FLOODSCALE

Better understand flash floods

Multi-scale observation and modelling of flash floods

Better understanding flash floods

The FloodScale project is a contribution to the HyMeX (Hydrological Cycle in the Mediterranean Experiment) international program. Its general objective is to improve the understanding and simulation of the hydrological processes leading to flash floods, which are one of the most destructive natural hazards, especially in the Mediterranean region. The spatial and temporal variability of rainfall, landscape characteristics and soil humidity are recognised as important influential factors in flash flood generation. However, their quantification and the role of their variabilities at the various scales remain open questions. Understanding flash floods is particularly challenging since it requires observations at very small space and time scales (1 km2, 5min), as well as over large areas. As an illustration, the September 2002 flash flood event in the Gard was localised on an area of about 5000 km2, but half of the casualties occurred in catchments smaller than 20 km2. Flash floods are very difficult to observe using operational rain gauge and discharge networks. Weather radar data proved to be particularly valuable for characterizing the spatial and temporal variability of rainfall, although the radar signal interpretation is more difficult in complex terrain and/or over urbanized areas, which are the most prone to such events. Gauging flash-flooding rivers with classical techniques also remains a challenge due to practical and security reasons.

FloodScale builds on the concepts of Hydrometeorological Observatory (HO) promoted in France since 2002 with the Cévennes-Vivarais Mediterranean Hydrometeorological Observatory (OHM-CV; www.ohmcv.fr). To progress in flash flood modelling two fundamental questions in hydrology need to be addressed (i) the change of scale problem or how to transfer knowledge acquired at a given scale to another scale; (ii) the prediction in ungauged basin (PUB) problem, in order to assess the risk at every point over a given region. FloodScale will address these questions using a combined multi-scale observation and modelling effort, relying on nested-catchments instrumentation covering the following scales: (i) the hillslope scale, where process understanding on runoff generation and concentration can be tackled; (ii) the small to medium catchment scale (1-100 km2) where the impact of network structure, rainfall, landscape, initial soil moisture variability can be quantified; (iii) the larger scale (100-1000 km2) where the river transfer and flooding processes become important. The potential of innovative observations (enhanced weather radar and disdrometer networks, gauging of river using remote sensing techniques, dense limnimeter networks, very high resolution remote sensing data, lidar Digital Elevation Models…) at various scales, complementing traditional measurements will be assessed within the project. FloodScale will also favour the combination of various measurements on the same hillslopes/catchments (soil moisture, infiltration tests, geophysics, geochemistry, geomorphological and vegetation surveys, river gauging..) in order to enhance our potential for understanding the active processes during and between floods.

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The FloodScale project is a contribution to the HyMeX (Hydrological Cycle in the Mediterranean Experiment) international program. Its general objective is to improve the understanding and simulation of the hydrological processes leading to flash floods, which is one of the most destructive natural hazards, especially in the Mediterranean region. To reach these goals, two fundamental questions in hydrology, the change of scale problem and the prediction in ungauged basins, need to be addressed. We propose a combined multi-scale observation and modelling effort, relying on nested-catchments instrumentation covering the following scales: (i) the hillslope scale, where process understanding on runoff generation and concentration can be tackled; (ii) the small to medium catchment scale (1-100 km2) where the impact of network structure, rainfall, landscape, initial soil moisture variability can be quantified; (iii) the larger scale (100-1000 km2) where transfer phenomena in rivers and flooding become important. The potential of innovative observations at various scales, complementing traditional measurements will also be assessed. FloodScale focuses on the Gard and Ardèche catchments of the Cévennes-Vivarais Hydrometeorological Observatory (OHM-CV). It builds on existing observation networks to propose (i) an harmonization of existing instrumentations; (ii) an enhancement of the measurements to better understand active hydrological processes during and between flash floods; (iii) innovative approaches for the documentation of ephemeral flash floods events; (iv) the evaluation of new data analysis and modelling approaches at various scales. The project is organised in tasks addressing increasing monitoring scales: hillslope (Task 2), small to medium catchment (Task 3), and larger scales (Task 4). Task 5 will integrate the results from the other tasks in a regional modelling. In Task 2, various hillslopes corresponding to typical Mediterranean landscapes will be instrumented according to the dominant hydrological processes. The objective of this task is to derive i) a perceptual model of the hillslope functioning according to runoff type, soil depth, slope, bedrock permeability; ii) an hillslope typology; iii) a modelling approach summarizing the hillslope functioning. Task 3 will rely on nested small catchments monitoring (1-100 km2, including Task 2 hillslopes) using distributed discharge or water height measurements to document the hillslope/network transition. The understanding of the small to medium catchment functioning will be based on data analysis -focusing on the connectivity between active areas-, hydro-landscape mapping and hydrological modelling -focusing on a proper accounting of lateral flow-. Task 4 addresses distributed observations of rainfall and discharges and their uncertainty for larger catchments. The value of combining enhanced high resolution radar observations will be evaluated for rainfall estimation. In terms of discharge, the focus will be on gauging rivers during floods. Rainfall-runoff data analysis and modelling will also be performed in order to understand the spatial and temporal scales of active hydrological processes. Task 5 will use all the data and modelling results obtained in the other tasks to derive integrated hydrological modelling at regional scale. As models will be preferentially used without calibration for hypothesis testing, adapted metrics for model evaluation will be proposed, based on the hydrological “signatures” derived from rainfall-runoff data analysis. The project gathers a multidisciplinary team with specialists in hydrology, hydrometeorology, hydraulics, statistics, pedology, geography, geophysics, geochemistry, field experiments, data analysis and modelling to enhance the emergence of new hydrological understanding.

Project coordination

ISABELLE BRAUD (CENTRE NATIONAL DU MACHINISME AGRICOLE, DU GENIE RURAL, DES EAUX ET DES FORETS - CEMAGREF ANTONY) – isabelle.braud@irstea.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

HHLY CENTRE NATIONAL DU MACHINISME AGRICOLE, DU GENIE RURAL, DES EAUX ET DES FORETS - CEMAGREF ANTONY
LTHE CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE RHONE-ALPES SECTEUR ALPES
HSM CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE LANGUEDOC-ROUSSILLON
ESPACE CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE COTE D'AZUR

Help of the ANR 740,000 euros
Beginning and duration of the scientific project: December 2011 - 48 Months

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