Multi-scale hydrometeorological observation and modelling for flash flood understanding

Abstract

Abstract. This paper presents a coupled observation and modelling strategy aiming at improving the understanding of processes triggering flash floods. This strategy is illustrated for the Mediterranean area using two French catchments (Gard and Ardèche) larger than 2000 km 2 . The approach is based on the monitoring of nested spatial scales: (1) the hillslope scale, where processes influencing the runoff generation and its concentration can be tackled; (2) the small to medium catchment scale (1-100 km 2 ), where the impact of the network structure and of the spatial variability of rainfall, landscape and initial soil moisture can be quantified; (3) the larger scale (100-1000 km 2 ), where the river routing and flooding processes become important. These observations are part of the HyMeX (HYdrological cycle in the Mediterranean EXperiment) enhanced observation period (EOP), which will last 4 years (2012)(2013)(2014)(2015). In terms of hydrological modelling, the objective is to set up regional-scale models, while addressing small and generally ungauged catchments, which represent the scale of interest for flood risk assessment. Topdown and bottom-up approaches are combined and the models are used as "hypothesis testing" tools by coupling model development with data analyses in order to incrementally evaluate the validity of model hypotheses. The paper first presents the rationale behind the experimental set-up and the instrumentation itself. Second, we discuss the associated modelling strategy. Results illustrate the potential of the approach in advancing our understanding of flash flood processes on various scales.

Advances in flash floods understanding and modelling derived from the FloodScale project in South-East France

“…Recent examples in France include the events in Nîmes (1988), Vaison-laRomaine (1992), Aude (1999), Gard (2002Gard ( , 2005, Draguignan (2010), Alpes-Maritimes (2015) or the series of events that affected the south-east of France in 2014. Flash floods often occur over very short time and spatial scales [1][2] with a sudden onset and a rapid rising time. These events can affect large territories, but their impact may be very local in catchments that are generally ungauged.…”

“…To progress in flash flood understanding and modelling, it is necessary to progress on two fundamental questions in hydrology [7]: 1/ the change of scale problem or how to transfer knowledge acquired at a given scale to another scale; 2/ the prediction in ungauged basin (PUB) problem, in order to assess the risk everywhere over a given region, requiring models able to provide reliable prediction at various scales (from a few km 2 to 1000 km 2 ). To go into that direction, Kirchner [8] advocated for field experiments, specifically designed to advance the science of hydrology and address the change RI VFDOH SUREOHP LQ RUGHU ³WR JHW WKH ULJKW DQVZHU IRU WKH ULJKW UHDVRQV´ 7KH VWUDWHJ\ LV EDVHG RQ H[SHULPHQWV RQ nested catchments, allowing the sampling of spatial heterogeneity at all scales.…”

“…To go into that direction, Kirchner [8] advocated for field experiments, specifically designed to advance the science of hydrology and address the change RI VFDOH SUREOHP LQ RUGHU ³WR JHW WKH ULJKW DQVZHU IRU WKH ULJKW UHDVRQV´ 7KH VWUDWHJ\ LV EDVHG RQ H[SHULPHQWV RQ nested catchments, allowing the sampling of spatial heterogeneity at all scales. These recommendations formed the basis of the experimental and modelling strategies set up in the framework of the FloodScale project [2], aiming at increasing flash flood understanding and simulation.…”

Multivariate density model comparison for multi-site flood-risk rainfall in the French Mediterranean area

“…In the French Mediterranean region, streamflow simulation accuracy and dynamics can be significantly enhanced when exploiting information from rainfall at higher spatial resolution (Lobligeois et al 2014;Patil et al 2014;Braud et al 2014). Therefore, analyses to characterize the spatial variability of flood-risk rainfall will contribute to the understanding of flash flood processes.…”

“…They can be triggered by a combination of three factors: the moisture generated by the Mediterranean Sea, upper-level cold troughs coming from the North and the complex orography in the region (the Alps, the Pyrenees and the Massif Central Mountains in the South of France) (Delrieu et al 2005). Such heavy rainfall might cause flash floods that can be defined as a sudden rise of the water level (in a few hours or less) together with a significant peak discharge (Braud et al 2014). Flash floods can potentially cause fatalities and important material damage and are known as the main natural hazard in the Mediterranean area (Borga et al 2011).…”