Sedimentation issues began in the Asahi reservoir in 1990, following major typhoons in 1989 and 1990. A sediment bypass tunnel, in operation since 1998, has successfully bypassed sediment-laden water downstream and stabilised the volume of the Asahi reservoir.
The 1,206 MW Okuyoshino hydropower station is a pure pumped storage power plant that shifts water between the Asahi lower reservoir and the Seto upper reservoir. The complex was completed in 1978, but the power station was not commissioned until 1980. Owned and operated by the Kansai Electric Power Company (KEPCO), Okuyoshino hydropower plant is located in the Shingu River system in the Nara Prefecture, in southeast Japan, as shown in figure 1.
The Okuyoshino pumped storage power station benefits from a hydraulic head of 505 m, which is the difference in height between the two reservoirs. The six Francis turbines of 201 MW each are reversible units that serve to both pump and turbine water with a maximum discharge of 288 m3/s.
The Asahi dam, an 86.1 m arch dam, created a reservoir with 15.47 Mm3 of gross storage capacity, of which 12.63 Mm3 was the original active storage capacity used for pumping water to the upper reservoir. The Asahi reservoir operates at the normal water level of 462 masl and the minimum level of 430 masl. The Seto reservoir, created by a 110.5 m rock-fill dam, offers 16.85 Mm3 of storage volume for power generation. The Seto reservoir is located in an upper valley with a small catchment area of 2.9 km2.
Due to turbidity caused by sediment inflow arising from landslides and logging upstream, a bypass tunnel was constructed between 1992 and 1998. The Okuyoshino pumped storage power station layout is shown in figure 2.
Hydrology and sediment
The plant is located in one of Japan’s rainiest areas, with an annual precipitation rate of over 2,000 mm. As shown in the annual hydrograph, there is heavy rain around the month of June, followed by the typhoon season in September. In September 1990, a maximum discharge of 662 m3/s was recorded.
The 39.2 km2 catchment upstream of the Asahi dam produces an average annual flow of 81.3 Mm3 per year, with an annual coefficient of variation of 0.44. The annual hydrograph does not contain a typical seasonal hydrograph, while the multi-year hydrograph contains some apparent periodicity, as shown in figures 3 and 4.
The mean total annual sediment load transported by the river at this location is 94,400 m3/year, consisting of 72,600 m3/yr of suspended load and 21,800 m3/yr of bed load. The mean grain size of the bed load is 30 mm, with an estimated maximum size of 100 mm.
The reservoir sedimentation challenges at the Asahi dam are prolonged turbidity and decreasing reservoir storage capacity. By 2016, the reservoir capacity had reduced by about 6.5 per cent. The Seto reservoir, on the other hand, does not experience any sediment-related issues, since the inflow from the catchment is relatively small compared with the water pumped from the Asahi reservoir.
The 1989 and 1990 typhoons caused major mountainside collapses, resulting in large-scale flooding with a very high sediment load. In addition, changes in the watershed caused by logging activities increased the erosion rate and therefore the sediment yield. The mean annual accumulated sediment volume exceeded expectations and increased sharply to 85,000 m3/yr. As shown in figure 5, the sediment aggradation between 1989 and 1995 is four times greater than the sediment accumulated between 1980 to 1988. By 1997, the reservoir volume had reduced by about 4 per cent.
When floods occur, the water flowing into the reservoir becomes turbid and remains so for long periods of time. This first became a significant concern in 1990, when high turbidity lasted for more than 200 days because of an increase in collapse areas triggered by the large-scale typhoons of 1989 and 1990. As a result, water releases from the Asahi reservoir remained turbid for extended periods, with a negative impact on the local fishing industry. Figure 6 shows the number of days of turbidity persistence and the ratio of the collapse area.
Sediment management strategies
To solve the sediment management problems at Okuyoshino, a sediment bypass tunnel was constructed between 1992 and 1998. The sediment bypassing facility comprises a weir, an intake, a bypass tunnel and an outlet. Figure 7 shows the weir and the tunnel inlet. The steel weir has a height of 13.5 m and a crest length of 45 m. The intake structure of dimensions 14.5 m by 3.8 m is made of steel-lined reinforced concrete and has one gate. The bypass tunnel is 2,350 m long with a 3.8 m by 3.8 m hood cross-section. The outlet structure is also made of reinforced concrete and is 15 m long.
The tunnel slope is about 1:35 with a discharge capacity of 140 m3/s and a peak discharge of about 200 m3/s, defined by a one-year return period flood. To bypass sediment, the bypass tunnel is operated on average 40 days each year. The inlet to the tunnel is lined with steel and the tunnel floor consists of high-strength concrete. The sediment bypass tunnel plan, cross-section and profile are shown in the figure 8.
Over the first four years, the sediment bypass tunnel was used 16 times each year to bypass about 40 per cent of the annual run-off. The average operation frequency, however, is 13 times per year.
The advantages of the sediment bypass tunnel over other sediment management techniques are that it can be applied to existing dams and that it does not involve reservoir drawdown, and therefore no storage capacity loss. Another advantage is the benefit to the downstream environment. On the other hand, the main drawbacks are the construction cost and the maintenance cost, due to the abrasion of the tunnel invert.
After significant typhoons, the main problem is abrasion along the invert. The average abrasion depth can be up to 200 mm; hence, the maintenance needs of the tunnel inlet and invert can be significant. The repairs are carried out during non-flood season and take place approximately every two years.
Real-time monitoring is essential for efficient bypassing. Basic monitoring covers flow discharge and sediment concentration. Besides the monitoring of sedimentation, monitoring of turbidity, eutrophication, and aquatic organisms has been also carried out at the reservoir and in the downstream reach.
The implementation of the bypass tunnel has been deemed successful, with an estimated reduction in annual sediment deposition in the reservoir of about 80 per cent. An additional benefit is that the bedload that is passed downstream, enhancing aquatic conditions by replenishing the river bed downstream with coarse sediment. The river profile has recovered and aquatic species have been conserved. The problem relating to turbidity downstream has also been solved because highly turbid flow is passed downstream through the bypass tunnel during flood conditions.
As shown in the longitudinal profile (see figure 9) of the Asahi reservoir, storage volume loss has stabilised since the sediment bypass tunnel entered into operation in 1998. However, storage capacity lost prior to the commissioning of the bypass tunnel has not been recovered.