Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Flash flood modeling with a specific focus on arid regions and infiltration

Auteur : Tügel, Franziska

Université de soutenance : Technische Universität Berlin

Grade : Doktorin der Ingenieurwissenschaften
 Dr.-Ing. - 2023

Résumé
This thesis aims to study different techniques for and applications of flash flood modeling focusing on arid areas and infiltration. The main study area is the city of El Gouna, located in the Wadi Bili catchment at the Red Sea coast of Egypt. A combination of a calibrated hydrological model for the Wadi Bili catchment and a robust 2D shallow water model for El Gouna is set up and applied to investigate the impact of different rainfall scenarios on the flooding areas in and around El Gouna. The simulation results show a highly non-linear relation between rainfall and catchment response. Several structural mitigation measures in the upstream wadi catchment and directly around El Gouna are studied concerning their effectiveness. According to the simulation results, retention basins in the upstream wadi catchment that are designed to completely capture the event that was observed in March 2014 can successfully reduce and lag the downstream flood wave of the 100-year event but do not affect the peak discharge of the assumed worst-case scenario. In addition to reducing the flooding, such basins could retain and store the fresh water in the upstream catchment making it usable for Bedouins living in that area. The reduction of runoff by retention basins in the upstream wadi catchment has only a minor effect on the simulated flooding areas in El Gouna, as in the considered cases, the locally fallen rainfall contributes significantly more to the flooding in El Gouna than the flood wave from the wadi, except for the assumed worst-case scenario. In contrast, retention basins and drainage channels directly around El Gouna could significantly reduce water depths in the area under consideration, even in the assumed worst-case scenario. But one has to be aware that overloaded structural measures such as overflowing retention basins or canals could even increase flooding at other locations. The Green-Ampt approach, which is incorporated into the robust 2D shallow water model to account for infiltration, is used for the model domain of El Gouna, as the area mainly consists of natural surfaces with mostly sandy soil. Literature values for the Green-Ampt parameters of the given soil types lead to a strong overestimation of infiltration. Different rainfall-runoff experiments from the literature, for which observed runoff data is available, are studied to shed light on the applicability and limitations of literature values for the Green-Ampt parameters. Finally, different sources in terms of satellite images and community-based statements and pictures serve to improve the digital surface model of the study area and validate the simulation results for El Gouna. In future research, the consideration of infiltration and associated processes such as surface crusting or macropore infiltration should be improved in hydrodynamic models. In addition, the reduction of computational times, the availability and usage of high-resolution data, and the establishment of measuring devices for direct measurements should be further enhanced. The growing access to remote sensing and crowd-sourced data provides the potential to further investigate flash floods and better validate flash flood models, especially for ungauged regions. To study urban flash floods, hydrodynamic models should account for the drainage system and urban structures. In general, risk awareness and risk management for flash floods need further improvement.

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