Appropriate model use for predicting elevations and inundation extent for extreme flood events

Flood risk assessment is generally studied using flood simulation models; however, flood risk managers often simplify the computational process; this is called a “simplification strategy”. This study investigates the appropriateness of the “simplification strategy” when used as a flood risk assessment tool for areas prone to flash flooding. The 2004 Boscastle, UK, flash flood was selected as a case study. Three different model structures were considered in this study, including: (1) a shock-capturing model, (2) a regular ADI-type flood model and (3) a diffusion wave model, i.e. a zero-inertia approach. The key findings from this paper strongly suggest that applying the “simplification strategy” is only appropriate for flood simulations with a mild slope and over relatively smooth terrains, whereas in areas susceptible to flash flooding (i.e. steep catchments), following this strategy can lead to significantly erroneous predictions of the main parameters—particularly the peak water levels and the inundation extent. For flood risk assessment of urban areas, where the emergence of flash flooding is possible, it is shown to be necessary to incorporate shock-capturing algorithms in the solution procedure, since these algorithms prevent the formation of spurious oscillations and provide a more realistic simulation of the flood levels

Flash floods Observations and analysis of hydro-meteorological controls Preface

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Surveying flash flood response: gauging the ungauged extremes

The monitoring of flash flood events gives us the unique opportunity to observe how catchments respond when most of the surface and subsurface hydrologic flow paths are active. These events often reveal aspects of hydrological behaviour that either were unexpected on the basis of weaker responses or highlight anticipated but previously unobserved behaviour (Archer et al., 2007; Delrieu et al., 2005). Characterizing the response of a catchment during flash flood events, thus, may provide new and valuable insight into the rate-limiting processes for extreme flood response and their dependency on catchment properties and flood severity. Flash flood events, however, are difficult to monitor because they develop at space and time scales that conventional measurement networks of rain and river discharges are not able to sample effectively (Creutin and Borga, 2003). As these events are locally rare, they are also difficult to capture during classical field-based experimentation, designed to last a few months over a given region, or on experimental catchments with drainage areas of a few km2. This explains why the investigation of flash flood events is by necessity event-based and opportunistic as opposed to driven by observations from carefully designed field campaigns. Post-event surveys play therefore a critical role in gathering essential observations concerning flash floods. Traditionally, indirect peak discharge estimates and collection of rainfall maxima have been used to document these events, as well as to provide an answer to the questions that are invariably asked after a major flood: Why did such a major flood occur? and How frequently might such a flood be expected to occur? Collectively, these studies contributed to the establishment of regional peak discharges envelope curves and to the development of more understanding of regional behaviour of extreme floods. However, focus on peak discharges and point rainfall maxima alone provides limited insight into the hydrological controls of flash flood response. Flash flood monitoring requires rainfall estimates at small spatial scales (1 km or finer) and short time scales (15-30 minutes, and even less in urban areas). These requirements are generally met by weather radar networks. This is shown schematically in Figure 1, which reports typical monitoring scales of weather radar systems and raingauge networks together with the time and space scales of a number of flash flood generating storms observed in Europe in the last 15 years (Borga, 2007). Rapidly increasing availability of good quality weather radar observations is greatly expanding our ability to measure and monitor the rainfall distribution at the space and time scales which characterise the flash flood events (Borga et al., 2007). These technical advances have the potential to enhance the information content of post-event surveys. Realising this potential calls for the development of a methodology for flash flood response survey which goes beyond the collection of indirect peak discharge estimates by focusing on three concepts, which are revised in this short commentary

Value of distributed post-flood maximum discharge estimation for the evaluation of a regional flash-flood modelling approach

-In September 2002 a flash flood killed 23 human lives and generated 1.2 billion Euros of damages in less than 24 hours over an area of 20 000 km2 located in the south of France. The Gard river basin was hit by a storm that locally received more than 600 mm in one day. This storm triggered catastrophic flash floods on many upstream tributaries as well as the most important flood ever reported of the major rivers (Gard, Ceze and Vidourle). The distributed prediction of such extreme events remains an open question due to scarcity of observations and the unknown individual hydrological behaviour of very small basins. Due to the high spatial and temporal variability of rainfall and of the physiographic conditions, physically-based distributed hydrological models offer perspective for the simulation of such flash-floods at a regional scale, and more specifically in small ungauged basins which are recognized as the most vulnerable. However, the evaluation of model performance for such events remains largely open. Traditional stream gauge networks provide estimate at only a few locations, and these values are very uncertain for large discharges. Indeed, stage discharge relationship are in general extrapolated far beyond gauged values -if they are not destroyed- for extreme floods. Alternative sources of data are therefore necessary for model evaluation. In this paper, such an alternative solution is presented and illustrated using data from the Gard September 2002 event. It consists in using a post flood field survey data set made of estimation of maximum peak discharge and time of peak. Such estimations are conducted at the regional scales for catchments of various sizes (for a few to about 100 km2) in areas affected by different rainfall amounts in order to sample a large range of hydrological responses. These estimations are derived from hydrological investigation using interviews of witnesses and river cross-sections surveys