The multipurpose Unazuki Dam, constructed in 2001, is located on one of Japan’s most magnificent torrential rivers - the 85 km long Kurobe River, which originates from Mt. Washiba and flows into the Japan Sea. The tremendous rainfall received in the basin and the steep channel slope favoured hydropower development on the river. Unazuki Dam is the most downstream dam among the cascade of five dams along the river (see Figure 1). The Ministry of Land, Infrastructure, Transport and Tourism completed the 97 m high concrete gravity dam in 2001 mainly to protect the downstream alluvial fan from floods. Of the reservoir’s total capacity of 24.7 Mm3, the flood control capacity comprises 11.2 Mm3, allowing the dam to reduce a peak discharge of 6,900 m3/s entering the reservoir to a regulated discharge of 700 m3/s. The dam also generates electricity with an installed capacity of 20 MW at a head of 36 m with a design power discharge of 70 m3/s. It can also deliver up to 0.67 m3/s for public water supply.
Unazuki is the most downstream dam on the Kurobe River System and has a total catchment area of 617.5 km2. This catchment is very moist, with 3500 mm mean annual precipitation. The unregulated catchment between Unazuki and Dashidaira, the next dam upstream, is 156.3 km2, which is 25 percent of the total catchment area. The flow regime of Kurobe river through 2006 is illustrated in Figure 2.
Hydrology and sediment
The catchment’s steep topography, weak geology, high rate of rainfall and snowfall, plus approximately 7000 landslide sites in the upper reaches of Kurobe River, are all factors contributing to the high sediment yield produced by the catchment. The river has steep slopes, varying from 0.20 to 0.0125 as shown in Figure 3, and has incised the deep Kurobe River Gorge. The upstream riverbed is composed of gravels with an average diameter of 20 mm.
The upstream Dashidaira reservoir, constructed prior to Unazuki, has reached sediment equilibrium through flushing and sluicing operations. An average of 0.96 Mm3/year is released downstream into Unazuki reservoir, an annual load equivalent to 3.9 percent of the reservoir’s total capacity. Flushing and sluicing procedures at Unazuki and have greatly reduced the sedimentation rate. Despite this, the 2017 bathymetric survey revealed that the reservoir had lost approximately a third of its total capacity to sedimentation. If all the inflowing sediment were trapped the reservoir would be fully sedimented in 26 years. However, after 16 years of operation only 30 percent of the reservoir storage had been lost to sedimentation, as compared to over 60 percent without sediment management, and the rate of sediment accumulation is declining. Most of the sediment load smaller than 2 mm is released, but 90 percent of the inflowing sediments above 2 mm diameter are being trapped.
Before completion of Unazuki Dam there were many floods and sediment related disasters along Kurobe River, especially in 1969 and 1995. Downstream impacts of reservoir sedimentation were also identified including coastal erosion, riverbed degradation and related ecosystem effects as a result of sediment trapping by upstream dams.
Unazuki and Dashidaira Dams were the first in Japan to be constructed with large bottom outlets for the purpose of flushing sediments. Unazuki Dam has two sediment low level outlets (5 m W x 6 m H) plus two middle level drawdown outlet (5 m W x 4.7 m H). Schematics of Unazuki and Dashidaira Dams are presented in Figure 4. Although the rate of sediment trapping in Unazuki has been reduced, there continues to be a significant reduction in the active storage.
The current sediment management approach is aimed at maintaining sediment continuity along the river, trapping as little sediment as possible in the reservoirs. To this end, sequential sediment flushing and sluicing operations have been carried out annually at both Unazuki and Dashidaira Dams. Sediment management operations at the dam sites are coordinated at river system scale with the Kurobe River Sediment Flushing Evaluation Committee and the Kurobe River Sediment Management Council. This allows for monitoring of the natural flow regime along the river as well as sediment discharge impacts downstream. Adverse environmental impacts were created by flushing sediment through Dashidaira Dam in 1991 during the winter, the low flow period. Since then, sediment has been released by sluicing events scheduled to coincide with natural floods (June to August) to wash released sediment through the downstream channel during periods of naturally high sediment transport, thereby and minimize environmental impacts.
Both reservoirs are emptied and sediment released via bottom outlets during the first major flood event every year, when the inflow at Dashidaira Dam exceeds 300 m3/s, or 400 m3/s at Unazuki. In special cases, such as events with high sediment concentration, sediment release is initiated when the inflow exceeds 250 m3/s at Dashidaira Dam. A flushing discharge is maintained at 200 m3/s and 300 m3/s at Dashidaira and Unazuki respectively. Figure 5 illustrates the 3-step process of coordinated flushing between Unazuki and Dashidaira Dams:
Step 1: Simultaneous drawdown is conducted at both dams. At Dashidaira Dam the bottom outlets are opened, at Unazuki Dam the middle level outlet is opened but bottom outlets remain closed.
Step 2: Flushing or Sluicing*. The bottom outlets of Unazuki Dam are opened, and the previously opened middle level outlet (used for drawdown) is closed. Bottom outlet of Dashidaira Dam remains open.
Step 3: Refilling. All low-level outlets at both dams are closed, but high-level surface spillways remain open.
* The first event each year is classed as “flushing” because it releases a volume of previously deposited sediment, but subsequent sediment release events during the year are classed as “sluicing” because their primary focus is to pass inflowing floods and sediment downstream while minimizing deposition.
To minimize additional sediment deposition in the reservoir, sediment sluicing is triggered by flows larger than used for flushing, inflows greater than 480 m3/s at Dashidaira or 650 m3/s at Unazuki. Figures 6, 7 and 8 summarize the coordinated procedures implemented with regard to timing, gate operation, flow type, duration and more during sediment flushing and sluicing. The sequence of flushing and sluicing events including discharge values, volumes, and suspended sediment values are tabulated in Figure 9 for flushing and sluicing events from 2001 to 2007.
With this process Dashidaira has nearly reached a sediment balance, passing almost all inflowing sediment downstream. Unazuki has not reached equilibrium state, with about 30 percent of the inflowing sediment being trapped, while most of the sand and smaller sediment is released downstream. Sediment management operations of flushing and sluicing have therefore slowed down the rate of sedimentation of Unazuki reservoir, which would otherwise be fully filled after 26 years of operation.
Sediment management measures have also been used in the upstream watershed, including sediment trapping sediment before it reaches the reservoir by check dams, specifically – Sabo dams. This has been implemented in the Kurobe catchment since 1961, prior to the construction of either Dashidaira or Unazuki Dams. Sabo dams constructed in the the Baba, Nobose and Kokurobe tributaries have successfully reduced downstream sediment concentrations.
Environmental mitigation and monitoring
Sediment management activities at Unazuki includes bathymetric surveys after flood events, and monitoring of water quality, bed load material and aquatic organisms, as summarized in Figure 10. During sediment release operations, hourly measurements are made of water temperature, pH, Dissolved Oxygen (DO), turbidity, and Suspended Sediment concentration (SS) at three downstream stations. The longitudinal profiles of the reservoir before and after flushing operations (Figure 11) show that sedimentation appears to be moving towards a state of equilibrium and that the sediment that caused a delta formation is being redistributed towards the bottom outlet.
A major environmental problem associated with sediment release by flushing is the high suspended sediment concentrations that are generated downstream. Environmental monitoring locations include one at the delta, two immediately downstream of Unazuki and Dashidaira Dams, and one halfway between the Delta and Unazuki Dam. Active sand bars have formed downstream of Unazuki Dam, and the downstream supply of sediment has caused riverbed aggradation in some sections of the river that were previously deprived of sediment, creating rich aquatic habitats especially for both invertebrates and fish. Due to the environmental regulations for flushing sediment in the Kurobe River, since 1995 significant adverse environmental impacts associated with flushing have not been observed along the river downstream of the dam.
A higher level of control was demonstrated during the 2016 flushing event (the first sediment release event of the year) using real-time suspended sediment instrumentation installed below Unazuki Dam. Based on this real-time information, the bottom outlets were partially closed as soon as sediment concentrations at the station downstream of the dam reached 30,000 mg/l. Partial closure causes the water level to rise upstream of the gate, inducing sedimentation and lowering the downstream concentration. As soon as concentration decreased to 20,000 mg/l, the gates were again gradually opened until free flow was again established. This procedure was repeated several times as needed to maintain the concentration in the desired range. In contrast to uncontrolled flushing, which produces a large spike in suspended sediment concentration during the initial phase of the free flow period, this procedure spreads the sediment discharge over a longer time, allowing a larger volume of sediment to be released while still controlling maximum concentration levels.
At the end of every sediment release event, the river below the dam is flushed with clear water discharged from the dam’s high level spillway, to carry residual fine sediment downstream and clean the gravels on the river bed.
This case study provides an important example of minimization of the downstream impacts of flushing by using smaller and more frequent sediment release events, timed to coincide with naturally occurring high flows along the river. By reducing the sediment release during each individual event, and timing these events to coincide with large flows that effectively dilute and transport the sediment load downstream, environmental impacts were reduced to acceptable levels.