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Kyoto University (2017)

Integrated Hydro-geomorphological Approach to Flash Flood Risk Assessment and Mitigation Strategies in Wadi System

Mohammed, Abdel-Fattah Sayed Soliman

Titre : Integrated Hydro-geomorphological Approach to Flash Flood Risk Assessment and Mitigation Strategies in Wadi System

Auteur : Mohammed, Abdel-Fattah Sayed Soliman

Université de soutenance : Kyoto University

Grade : Doctor of Philosophy (PhD) 2017

Résumé partiel
The generation and processes of wadi flash floods are very complex and are not well understood. Therefore, the flash floods modeling, risk assessment, mitigation and management in the arid region are hindered by the unique characteristics of wadi system, lack of powerful hydrological models and data deficiency. In this thesis, we developed an integrated approach was developed based on geomorphometric analysis, hydrological modeling, risk assessment, mitigation and management. The Qena, Abadi, (The Eastern Desert of Egypt), Samail and Aday wadis (north coast of Oman) were selected to apply and validate the developed approaches and methods. The challenges related with the arid wadi flash floods were highlighted and the previous wadi flash floods studies were critically reviewed. To investigate the relationship between variations in geomorphometric and rainfall characteristics and the responses of wadi flash floods Qena Wadi was employed and was divided into 14 sub-basins with areas ranging from 315 to 1,488 km2. The distributed Hydrological River Basin Environment Assessment Model (Hydro-BEAM) was used to obtain a good representation of the spatial variability of the rainfall and geomorphology in the basin. Thirty-eight geomorphometric parameters representing the topographic, scale, shape and drainage characteristics of the basins were considered and extracted using geographic information system (GIS) techniques. A series of flash flood events from 1994, 2010, 2013, and 2014, in addition to synthetic virtual storms with different durations and intensities, were selected for the application of this study. The results exhibit strong correlations between scale and topographic parameters and the hydrological indices of the wadi flash floods, while the shape and drainage network metrics have smaller impacts. The total rainfall amount and duration significantly affect the relationship between the hydrologic response of the wadi and its geomorphometry. For most of the parameters, we found that the impact of the wadi geomorphometry on the hydrologic response increases with increasing rainfall intensity. Flash flood simulation and risk assessment is essential to construct efficient warning, mitigation and management of the increasing threat of flash floods in wadi systems. The Rainfall Runoff Inundation (RRI) distributed model and sensitivity analysis for its parameters were applied and implemented in Wadi Samail as a case study for the extremist flash floods events in Oman (1,000 years return period). The most severe tropical cyclone in the recent history in Oman, Gonu-2007 and Phet-2010 were used to calibrate and validate the hydrological model. The results and the statistical analysis indicating that RRI model could efficiently simulate the extreme flash flood events in the arid wadi system. Where for model calibration, Nash-Sutcliffe efficiency (NSE) and Percent bias (PBIAS) indices equal to 0.93 and -14.3 respectively and for model validation NSE and PBIAS equal to 0.86 and -12.0 respectively. Further improvements for RRI model is recommended to include the transmission losses and groundwater processes for better representation of the wadi system. Flash flood susceptibility has been assessed using the significant geomorphometric parameters and flash flood index.

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