Abstract
The Unazuki Reservoir is located on the Kurobe River, which is influenced by a catchment with one of the highest sediment yields in Japan. Due to a sufficiently available discharge during flood events, annual sediment flushing with full water-level drawdown (i.e., free-flow sediment flushing) is conducted to preserve the effective storage capacity of the reservoir. Nevertheless, the upstream half of the reservoir (i.e., study segment) suffers from the excessive deposition of coarser sediments. Remobilization of these coarser materials and their transportation further downstream of the reservoir is a priority of reservoir owners for sustainable reservoir functions, such as flood-risk management and hydroelectric energy generation. In this paper, an already conducted sediment-flushing operation in the Unazuki Reservoir is simulated, and its effects on sediment scouring from the study segment of the reservoir together with changes in bed morphodynamics are presented. A fully 3D numerical model using the finite volume approach in combination with a wetting/drying algorithm was utilized to reproduce the hydrodynamics and bed changes using the available onsite data. Afterwards, the effects of discharge adjustment on the morphological bed changes and flushing efficiency were analysed in the study segment using an additional single-discharge pulse supplied from upstream reservoirs. Simulation results showed that an approximately 75% increase in the average discharge during the free-flow stage changed the dominant morphological process from deposition into an erosive mode in the study segment. If the increase in discharge reaches up to 100%, the flushed volume of sediments from the target segment can increase 2.9 times compared with the initiation of the erosive mode.
Highlights
Sediment deposition occurs in reservoirs as a consequence of the construction of dams on rivers [1]
Afterwards, the influence of operational discharge adjustment scenarios using additional discharge pulses during the free-flow phase was numerically modelled in the study segment to investigate how it affects bed morphology and whether it can increase the total volume of flushed sediment (TVFS) and flushing efficiency (FE), which is defined as the ratio of the flushed sediment volume to the water-used volume
In the study segment in the Unazuki Reservoir, a portion of the flushed sediment load released from the upstream Dashidaira Reservoir in addition to the flood-borne sediments will settle
Summary
Sediment deposition occurs in reservoirs as a consequence of the construction of dams on rivers [1]. Dam owners were interested in assessing potential approaches (e.g., adjusting the flushing discharge) for mitigating this problem To achieve this goal, the bed changes index (BCI) was introduced to represent the dominant morphological processes (i.e., erosion and deposition) quantitatively in different subareas in the study segment. Afterwards, the influence of operational discharge adjustment scenarios using additional discharge pulses during the free-flow phase was numerically modelled in the study segment to investigate how it affects bed morphology and whether it can increase the total volume of flushed sediment (TVFS) and flushing efficiency (FE), which is defined as the ratio of the flushed sediment volume to the water-used volume. It should be noted that the additional discharge can be supplied from upstream reservoirs, and its onsite test was performed tentatively
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