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Modeling of flash floods in small catchments

Project head
Prof. Dr.-Ing. R. Hinkelmann
Scientific assistant
Franziska Tügel M.Sc.
Project period
Oktober 2016 - Oktober 2019

Motivation

This research is about different methods and applications of flash flood modeling. The simulation of flash floods is an important tool to analyze flow processes during and directly after heavy rainfall events and to investigate possible mitigation measures as well as to develop early warning systems. Two-dimensional shallow water models can simulate flow processes more detailed than classical hydrological models and have a stronger conceptual and mathematical ground. They can be used for rainfall-runoff modelling in small catchments or in combination with hydrological models to simulate only the last part of the flow from a valley to a city.

The first objective is to carry out a case study for the region of El Gouna. Therefore, a shallow water model was set up with the Hydroinformatics Modeling System (hms). By implementing a digital elevation model the topography of the region of El Gouna is taken into account. During the flash flood event on 9 March 2014, data of rainfall and runoff were measured and are published in the doctoral thesis of Hadidi (2016). This event was simulated with hms. The model will be applied to investigate different scenarios of structural measures to protect the city of El Gouna against flooding. A second real case will be investigated in the same way. Further investigations could concern (1) transport processes of sediments and obstacles (e.g. trees), which are carried away due to the flood and can cause further damages, (2) water-harvesting methods to find solutions on how to store and use the fresh water supplied by heavy rainfalls and (3) a combination of one-dimensional and two-dimensional shallow water models.

Study Area

City of El Gouna and discharge measurment at wadi oulet
Location of El Gouna and discharge measurement at wadi oulet
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The Eastern Desert of Egypt is characterized by lots of wadi systems coming from the mountains and draining into the river Nile or the Red Sea. Rainfall in this region is rare but sometimes very heavy and can generate flash floods in the wadi systems. Many cities and settlements are located inside the catchments and deltas of the wadis. Just a few areas are protected by dams, drainage canals or embankments. In most cases rainfall data in the mountains are only available through satellites, there might be one or a few rainfall measurement stations in plane parts of the catchment close to the Red Sea or the Nile. Most of the wadis are ungauged, thus almost no runoff measurements are available. During 8 and 9 March 2014, heavy rainfalls caused flash floods in different wadis. For the event in El Gouna, measurements of rainfall and runoff have been carried out and are published in Hadidi (2016). El Gouna is a touristic town characterized by many lagoons and is located at the Red Sea coast about 20 km North from Hurghada. The main wadi affecting the city is wadi Bili, whose catchment area has an extension of approximately 880 km² coming from the Red Sea Hills and draining into the Red Sea.

Simulation Results

The flash flood coming from wadi Bili to El Gouna was simulated by using the DEM AW3D30 ©Jaxa to consider the topography and by implementing the hydrograph measured by Hadidi (2016) as boundary condition at the wadi oulet. By using the Green-Ampt model, infiltration can be considered. First results show the propagation of the flood in terms of flow velocities and water depths.

Simulated flow velocities in m/s after 11 hours of simulation, plotted on a map of El Gouna (©Google 2017).
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Simulated water depths (m) after 11 hours of simulation, plotted on a map of El Gouna (©Google 2017).
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Simulated water depth after 11 hours plotted on a map of El Gouna (©Google 2017) with consideration of infiltration (sandy clay loam with an initial water content of 4 %)
Lupe

Animation of Flow Velocities (m/s) and Water Depths (m)

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