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After a decade, sedimentation reduced half of the total storage capacity of Dashidaira causing a functional disturbance of the intake facility and a reduction in effective water storage. A combination of sluicing (added in 2001) and flushing techniques has stabilised the reservoir volume. The combined flushing and sluicing strategy is implemented once a year in synchronisation with the downstream Unazuki Dam in three steps: the rapid drawdown of the reservoir, flushing and sluicing, and filling the reservoir once more.

Project description

The Kurobe River contains a cascade of five dams, with the Unazuki Dam the most downstream and Dashidaira Dam located just upstream of it (see Figure 1). Sediment is managed annually by passing it through Dashidaira and Unazuki in sequence.

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The Dashidaira Dam, a 76.7 m high concrete gravity dam, was commissioned in 1985 with an installed capacity of 124 MW, a head of 211.5 m, and a design discharge of 74 m3/s. The riverbed elevation at the dam is 282 masl, with the spillway invert at 325 masl and the maximum operating level at 343 masl. The reservoir length is 2.9 km and the average river slope is 0.02.

The project was originally designed to flush sediment on an annual basis through low level outlets with an invert elevation of 300 masl. The three sets of 5 m x 5 m low-level outlets consist of slide gates upstream, roller gates in the middle, and radial gates downstream.

Hydrology and sediment

The catchment area upstream of the Dashidaira Dam is 461.2 km2, with a mean annual precipitation of 3,800 mm, a mean annual flow volume equal to 1,839.03 Mm3, and an annual coefficient of variation equaling 0.51. See Figure 2 for the rate of dam inflow over a year in 2016. The dam and reservoir were designed for an annual sediment load of 250,000 m3.

The actual sediment load is not public information. For operational purposes the bedload is estimated using the Ashida-Michiue equation and the suspended sediment load with the Lane-Kalinske formula. The wash load is determined by using a rating curve as shown in Figure 3. The bedload consists principally of cobbles of 20 mm average diameter.

Sedimentation challenges

The first concern regarding sediment entailed an increased riverbed level (see Figure 4) that resulted in a functional disturbance of the intake facility and a reduction in effective water storage. Downstream riverbed degradation and coastal erosion were later identified as significant concerns.

Although the dam was originally designed to flush sediment when it was commissioned in 1985, the first flushing event occurred 9 years after commissioning in 1994. The original total reservoir volume was 9.01 Mm3, which reduced to the current 4.57 Mm3. The total volume in 2017 was 50.7 per cent of the original capacity. With sediment management having stabilised the reservoir volume, the original live storage volume of 1.66 Mm3 reduced to 1.37 Mm3, i.e. it is retained at 82.5 per cent of the original live storage volume.

Sediment management

The sediment management at Dashidaira is coordinated with passing sediment through the downstream Unazuki Dam in sequence. The methods used to manage sediment consist of combined sluicing and flushing. The project was originally designed to facilitate flushing. Sluicing was introduced in 2001 as an additional sediment management technique.

Sediment management is accomplished in three stages (see Figure 5). The first consists of drawing down the water surface elevation in both reservoirs. The second stage involves the combination of sluicing and flushing of sediment. Lastly, the third stage entails filling the reservoirs with water again. In the Dashidaira Dam, the water surface elevation is drawn down to 305 masl during one day. In other words, the drawdown rate is about 72 m/day. The flushing and sluicing operation takes place over a period of one day per year. See Figure 6 for more details regarding the operating rules for sediment passage and removal.

Since the first flushing event in 1994, the net reservoir storage has maintained a stable condition. The amount of water used during the annual flushing operation is 0.78 per cent of the mean annual river flow volume, resulting in an average suspended sediment concentration of 13,000 mg/l and a maximum value on the order of 48,000 mg/l. The sediment concentration during the sluicing period is about 12,000 mg/l. The sluicing operation is deemed to be about 50 per cent effective and flushing 94 per cent effective.

When the dam was originally designed, it was estimated that about 9 x 104 m3 would be flushed per event with an expected variation of 104 m3 to 17 x 104 m3 per flushing event. The actual amount of sediment flushed, and eventually passed through as a result of sluicing and flushing, is much greater than the originally estimated values as can be seen in Figure 7. Figure 8 presents a comparison of the longitudinal and cross sectional profiles of sediment between 1994 (when flushing commenced) and 2016. It is noted that the sedimentation in the reservoir has substantially been stabilised since 1994.

Monitoring

The sediment management activities at the Dashidaira Dam are carefully monitored throughout the year. This includes four bathymetric surveys per year and water quality, bed load material, and aquatic organisms monitoring as shown in Figure 9.

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