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1 Background
For all river basins globally, the reservoir capacity is declining by around 5% compared to the installed capacity (Wisser et al., 2013). A study, for 285 dams, was made by the Italian national Committee for Large Dams (ITCOLD) in 2009. It indicates that the loss of storage in 53% of investigated hydroelectric reservoirs in Italy has an average of 47% due to sedimentation. The other dams have a loss in storage less than 5% (Bizzini et al., 2009).
Reservoir sedimentation creates environmental challenges. For instance, reduction of river bed slope upstream of the reservoir (low velocity), influencing water quality due to contaminated sediments stored in the reservoir, reducing resting habitat for fish due to fine sediment deposition, bed degradation downstream of the dam and coarsening the riverbed which may not be suitable for spawning (Spreafico, 2007). Moreover, the reservoir sedimentation upturns the probability of flood inundation area at the upstream reach of reservoir tail due to bed aggradation (Kantoush et al., 2010).
Detaining of sediment due to dammed rivers may generate unfavourable morphological changes downstream of the dam (e.g. bed degradation, bank erosion and scour formation) and contributes also to coastal erosion (Guertault et al., 2014). Around 53% of global sediment fluxes in regulated basins are potentially trapped in reservoirs. This may influence the downstream morphological behaviour and coastal area that rely on riverine sediment supply (Kondolf et al., 2014).
Therefore, sedimentation creates a long-term economic loss and needs to be mitigated (Liu et al., 2004). It is vital to include or improve the sediment management and update reservoir operations rules. To realize which factors can improve passing the sediment to the downstream reach of the dam. This knowledge will help to increase the probable life span of a reservoir. Moreover, it will lead to measures that can be taken against reservoir sedimentation, water shortage and erosion of coast and river banks. These measures are called sediment management practices.
1.1 Sediment management
In the recent reservoir design and implementation, the cumulative reservoir sedimentation, during the assessed economic period of the reservoir, is estimated and added to the reservoir storage within the dead storage.There are some additional measures which could be considered to mitigate the sedimentation such as sluicing, dredging, flushing, etc (Fruchard and Camenen, 2012).
Flushing is almost dredging for free. The cost inherent to flushing of 1000 ton of sediment is negligible, compared to the dredging cost for the same amount. There are two types of flushing:
- Hard Flushing: It is a drawdown of reservoir water level to a minimum. It is very effective in terms of flushing sediment, but it may generate high flow velocities inducing bed or bank erosion and uncontrolled sediment concentration which may directly harm the fish habitats downstream of the dam.
- Environmentally friendly flushing: It is achieved by a limited drawdown of the reservoir water level. The level is determined based on sediment concentration allowed in water to mitigate or avoid the impacts to the downstream ecosystem (Baran and Nasielski, 2011).
Danelli and Peviani (2012) recommended the use of real-time reservoir operation to get a better understanding of sediment transport through the dam. This will be the approach in this study. To investigate the sediment flushing using two different gate operation patterns, Funagira Dam in Japan will serve as a case study.
1.1 The study Area
The Funagira Dam is located in the Tenryuu River in Japan. Within the 1.5 km downstream of the dam the left bank suffers from erosion and further downstream point bars at both banks are growing as shown in Fig. 1.The observed erosion and sedimentation pattern is likely to be caused by a variety of bed shear stress and an eddy formation induced by the gate operation during floods.
In the past, the central gates were opened first and the other gates were opened next, following a pyramid shape operation when the discharge increases. Recently the gate operation has been modified to an equal opening shape operation in which the opening heights are increased equally, following discharge increase.
The dam has nine spillway gates (20m wide and 16 m height) with a crest level of 42.0 m AMSL (Above Mean Sea Level). The dam also has a hydropower plant containing three turbines with minimum and maximum operation water level of 54.8 m and 57.0 m AMSL, respectively.
While releasing the flood peak and flushing the sediment, the dam operator maintains the water level at 50.6 m AMSL in the reservoir, which is below the minimum operational level. During that time sediment is expected to be transported to the river corridor downstream of the dam. The pyramid shape operation is expected to be the reason behind the undesirable morphological changes downstream of the dam.
In this study we will focus on the period of the flood peak when the turbines are off and the gates are operated. And investigate the performance of both gates operations related to sediment transport.
