Ilhan Özgen

Upscaling the shallow water model with a novel roughness formulation

This study presents a novel roughness formulation to conceptually account for microtopography and compares it to four existing roughness models from literature. The aim is to increase the grid size for computational efficiency, while capturing subgrid scale effects with the roughness formulation to prevent the loss in accuracy associated with coarse grids. All roughness approaches are implemented in the Hydroinformatics Modeling System and compared with results of a high resolution shallow water model in three test cases: rainfall-runoff on an inclined plane with sine-wave shaped microtopography, flow over an inclined plane with random microtopography and rainfall-runoff in a small natural catchment. Although the high resolution results can not be reproduced exactly by the coarse grid model, e.g. local details of flow processes can not be resolved, overall good agreement between the upscaled models and the high resolution model has been achieved. It is concluded that the accuracy increases with the number of calibration parameters available, however the calibration process becomes more difficult. Using coarser grids results in significant speedup in comparison with the high resolution simulation. In the presented test cases the speedup varies from 20 up to 2520, depending on the size and complexity of the test case and the difference in cell sizes. The proposed roughness formulation generally shows the best agreement with the reference solution, compared to the other models investigated in this study.

Modeling of Flash Floods in Wadi Systems Using a Robust Shallow Water Model—Case Study El Gouna, Egypt

Flash floods as a result of extreme rainfall events occur in different regions all over the world and often cause losses of human life and severe damages to settlements and the environment. The Red Sea region of Egypt is affected by flash floods almost every year leading to losses of human life and infrastructure damages. The lack of data and the highly random occurrence of these extreme events make the management of flash floods very difficult. In the future, the frequency and intensity of extreme rainfalls might increase due to climate change and measures to reduce the risk of flooding have to be realized urgently. To investigate flash floods in Wadi Bili, a shallow water model of the region of El Gouna was established. In preparatory work, sensitivity studies were carried out for an idealized catchment. The inflow from upstream was found to be the most important parameter. The model of El Gouna was set up using a DEM and the flash flood event of 9 March 2014 was simulated by implementing the measured data of rainfall and runoff. The simulation results include the distribution of water depths and flow velocities in the computational domain at different time steps. The results are plausible in terms of flooding areas and ranges of water levels and flow velocities. Later on, the model will be applied to investigate different scenarios of structural measures to protect the city of El Gouna against flash floods, e.g., dams, canals, basins, and local measures for buildings.