Reservoir Sedimentation in Arid regions:
Challenges, field investigation and Sediment
Transport Modeling
Mohamed Saber (Corresponding author) Yusuke Ogiso,
Sameh Kantoush, Tetsuya Sumi Tahani Alharrasi
Disaster Prevention Research Institute, Kyoto University (Gokasho, Uji, Kyoto 611-0011, Japan)
E-mail: mohamedmd.saber.3u@kyoto-u.ac.jp
Abstract
Globally, sedimentation is still the most serious technical problems in reservoirs management, but in arid regions, the challenge became much more serious, with increasing extreme flash floods. However, several previous studies have been done concerning developing the hydrological models for Wadi flash floods simulation, assessment of sedimentation impacts associated with extreme flash floods is still missing in wadi system. There are several reasons showing the critical impacts of sedimentation at wadi basins, such as: i) the lack of previous studies especially in regard of their impacts on reservoir and infiltration, ii) the deterioration of dam ability to store water influencing on its functions for floods protection, and iii) decreasing the infiltration and consequently affecting the groundwater recharge. The aforementioned problems are expected to be more serious and destructive with the climate change impacts. Therefore, assessment the adverse impacts of sedimentation at wadi basins is desperately crucial to bring forth a secured integrated water and sediment management. The current challenges for the sedimentation in wadi system were discussed, and some efforts in order to understand the sediment dynamics and impacts on the reservoirs were introduced throughout numerical modeling and detailed field investigations. The present paper proposes the possible future management strategies to overcome the current problems in arid regions.
Keywords: Reservoir sedimentation, Arid regions, Sediment dynamics, field survey, Sediment management.
1 Introduction
Reservoir sedimentation is a global challenge, especially in arid regions when the monitoring and information are missing as well as the lack of effective management techniques. In arid and semi-arid regions, a large proportion of sediment yield and erosion are formed due to flash floods (Walling, D.; Kleo, A. 1979). However, a little attention paid to sedimentation impacts associated with flash floods in such regions, especially in the Arab regions (the most hyper aridity conditions). Sedimentation process is controlled by many factors including geological and topographical features of the basins, land uses and soil types variability, intensity and frequency of extreme storms, and climate change and human impacts.
Previous researches and studies have been concerning about Wadi flash floods modeling, forecasting and management (Saber, 2010, Saber et al. 2013, Abdel-Fattah et al. 2017), but addressing of sedimentation issues under the impact of climate changes are still missing in wadi basins. Flash floods in arid regions are associated with very high sedimentations. Bedload discharge is several orders of magnitude higher in arid ephemerals than in humid (Laronne & Reid, 1993). Also, suspended sediment concentration is evidently high in hyper-arid, arid and especially in semi-arid environments (Alexandrov, Laronne, & Reid, 2003). In humid environments suspended sediment concentrations are often lower than 1% and in most cases even lower than tenths or hundredths of a percent, whereas in drylands suspended sediment concentration may reach 10–40% (Cohen & Laronne, 2005). The long interval between floods in dryland channels allows sufficient time for weathering and eolian processes to take place, enabling fine sediment to form on hillslopes and the silt–clay component of the total load is supplied from hillslopes. As the interval between floods increases, so does this finer fraction of suspended sediment concentration increase (Lekach & Schick, 1982).
Therefore, in this study, the current challenges of sedimentation, approaches and management strategies are presented. The main objective is to answer the questions: How can we understand the current circumstances and impacts of reservoir sedimentation?, What are the current and available sedimentation management techniques? how can we overcome the current challenges for long-term sustainable management in arid regions.
2 Reservoir sedimentation challenges
There are several problems of sedimentation based on our field investigations and survey in wadi system (Fig. 1) including: 1) decreasing reservoir storage capacity (Fig. 1a), 2) affecting the groundwater recharges (sediment clogging) (Fig. 1b&c), 3) increasing the disaster risk of flash floods, and 4) downstream environmental impacts. Due to the infrequent occurrence of flash floods in the past, the decision makers and planners did not take in their consideration the sedimentation issues. Therefore, currently, the flash floods
become more frequent and devastating with huge sedimentation yield causes a real challenge of sediments in most of the reservoirs.
Fig. 1 (a) Reservoirs are full by sediments in some wadi in Oman, and Sediment clogging at upstream (b), and downstream (c).
3 Approach and methods
In order to come up with a sustainable long-term management of sediments and water. We propose the following integrated approach (Fig. 2) considering the climate change impacts, hydrological modeling of water and sediment, assessment of the sedimentation impacts on the reservoir capacity and infiltration processes. This requires a comprehensive and an integrated study including developing models and monitoring along with field investigations.
Fig. 2 An integrated Approach for sustainable management of water and sediment conceding the climate change.
4 Results and ongoing research
First, the hydrological models have been already developed for flash floods forecasting, and currently, developing sediment transport models are still in progress. Second, conducting detailed field investigations and sediment monitoring are the most important work for model validation and deep understanding for the physical process of sedimentation and its impacts. Two field investigations (Fig. 3) on Dec 2017 and Sept, 2018 were conducted at Wadi Mijlas and Samail in Oman. Some measures have been conducted including (wadi channel leveling, sedimentation Pedon analysis, detecting of flash floods marks, drone survey, infiltration tests, and field questionnaire about flash floods).
In order to simulate the sediment transport at the study reservoir, the model TELEMAC-SISYPHE is used. The model was calibrated in the flash floods of 2017 based on the deposition thickness and distribution conducted in the field survey (Fig. 4a). The sediment concentration calibrated value was found about 34.45 (g/l), which show the best correlaation between the measured and simulated sediemnt thickness (Fig.4 d). Then, the same parameter setting was used to siumlate the other flash floods seimentaion. The spatial sediment erossion and depositons is also depicted as shwoin in in Fig. 4. C)
Fig. 3 Field survey at Wadi Mijlas, Oman (Dec. 2017, Sept. 2018).
Fig. 4) Measure sediment Bars in the field at Wadi Mijlas, Oman, b) DEM developed from Drone as input for Telemac Sisyphe, c) Simulation results of bed evolution FF 2017, d) Simulated and measured sediment layer thickness of 2017 flash floods (cm)
Using satellite and radar high resolution data is still being processing to assess the sedimentation changes and also numerical models to estimate the sediment transport, then all these outcomes will be compared and validated by the field investigation findings. Such integrated approach, is important to understand deeply the sedimentation impacts on the reservoirs as well as infiltration processes in arid regions.
5 Conclusion
In this paper, the current challenges of sedimentation in arid regions were discussed and reported. A comprehensive and integrated approach to understand the sediment dynamics coupled with water flow under the impacts of climate changes was proposed. The sediment transport modeling using TELEMAC-SISYPHE was successfully applied and calibrated at wadi reservoir. The project is still running and several parts are still under progress. The expected final outcomes could be the way forward to a long-term sustainable management of sedimentation in wadi system based on deep understanding for sedimentation process during the flash floods.
Acknowledgment
This paper is based on the achievements of the international collaborative research program of DPRI, Kyoto University, “Project 2018”
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