A flash flood is a flood that follows the causative storm event in a short period of time. The term “flash” reflects a rapid response, with water levels in the drainage network reaching a crest within minutes to a few hours after the onset of the rain event, leaving extremely short time for warning.
As such, flash floods are characterised by high kinetic energy, resulting in a high threat to life and severe specific (per unit of area) damages to property and infrastructure. Furthermore, given appropriate morphological conditions, they trigger large erosion and sediment transport, and under appropriate topographic conditions, they lead to the formation of debris flows including woody debris as well as anthropogenic materials.
The two fold consequences of the space/time scales of occurrence of flash flood are that forecasting of flash-floods:
i)depends critically on meso-scale storm forecasting, with a specific attention to the processes leading to slow movement of the precipitation system;
ii)necessitates real time hydrological modelling, with a specific attention to the runoff generation processes over a wide range of scales and assessment of the soil moisture initial conditions. Without hydrological analysis, it may be impossible both to evaluate the flash flood potential of storms, particularly in the fringe of the flood/no flood threshold, and to assess the severity of the flash flood response.
Flash flood forecasting challenges traditional forecasting procedures because:
- the short lead time available, which implies both the integration of meteorological and hydrologic forecast, and the diagnosis of hydrological conditions most susceptible to flash flood triggering.
- the need to provide spatially distributed forecasts over river networks, rather than just at a few river sections.
- the ungauged basin problem due to the fact that the small basins prone to flash flood are rarely gauged and must be modelled without calibration.
Difficulties related to the ungauged basin problem are twice:
i) the enhanced uncertainty in flow modeling, and
ii) the characterisation of the severity of forecasted flows. In other words, what flow level creates damaging or hazardous conditions?
FLOODsite research have focused on developing
i) diagnostic methods, which aims to assess the potential for flash flood of catchments in a region, based on analysis of current soil moisture status, and
ii) methodologies of application of distributed hydrologic models which are particularly suitable for flash flood forecasting and warning in ungauged basins.