Title

Flash floods: Understanding the runoff generation processes and the use of satellite-rainfall in hydrologic simulations

Date of Completion

January 2010

Keywords

Engineering, Civil|Engineering, Environmental

Degree

Ph.D.

Abstract

Flash floods pose a threat to human lives and economies worldwide. The tremendous societal and economical impact of this natural hazard necessitates the development of accurate warning systems that can help to mitigate the flash flood risk. The inadequate understanding of the hydrologic processes leading to flash floods and the lack of observations associated with flash flood inducing storms are the main reasons that hamper the development of effective monitoring systems. The objectives of this thesis are to (a) advance our understanding on the physical mechanisms and controls of runoff generation during flash floods and (b) investigate the use of space-based precipitation observations as a way of improving our current monitoring strategies over remote and complex environments. The study takes place over the alpine region of northeastern Italy, an area that suffers from frequent flash floods. The backbone of this research is the integration of detailed ground and remote sensing observations with a physics-based distributed hydrologic model for simulating a series of major flash flood events. Results show that runoff generation mechanisms during flash floods follow a similar pattern with intense type of floods. An interesting and counterintuitive finding is that initial soil moisture conditions can play an important role in flash flood evolution and magnitude for the case of basins with high soil moisture capacity. It is shown that the error in rainfall derived from remote sensing magnifies as it propagates through the nonlinear rainfall-to-runoff transformation and exhibits a definite dependence on basin scale. Results also suggest that the rainfall-to-runoff error magnification is greater for drier soil moisture conditions. The hydrologic simulations based on satellite-rainfall forcing revealed some potential, but the predicted hydrographs are generally associated with large uncertainties that depend, among other factors, on the relationship between satellite product's resolution and the scale of application. Overall, the findings suggest that the current state of satellite-based flood prediction suffers from the inability of satellite precipitation observations to accurately estimate the magnitude of high rainfall rates and that improvement of current precipitation products is needed to allow us to invest towards that direction. ^