Reduce sediment production (watershed management)
Upstream sediment trapping
13.20 Mm³ (original)
The 93-M Hvammur hydropower project has integrated in the design measures to avoid, minimise and mitigate sediment-related issues. The operational regime will minimise ice erosion in the river banks, control erosion and upstream reservoirs will minimise over 90 per cent of the sediment load and mechanical dredging will contribute to extend the life of the reservoir and enhance the neighbouring farmlands.
The Hvammur hydropower plant is a run-of-river project on the Þjórsá river basin in the south of Iceland. Landsvirkjun, the national power company, is developing the project which is still in the preparation phase. The Hvammur dam construction will start at the end of 2018. Figure 1 shows the location of the project as well as other existing and planned projects on the Þjórsá river basin.
The Hvammur dam will create an intake pond called Hagalón with a total capacity of 13,2 Mm3 to serve the 93-MW hydropower plant that would generate 720 GWh annually.
The zoned earth-rockfill dam will have a height of 120 m. The operational regime is expected to be normally full, thus, the maximum and minimum operating level will be 116 and 114 masl respectively. At full level, the reservoir surface will cover 4 km2. The flooded area is seen in the simulation in figure 2. The spillways at elevation of 107 masl will have a discharge capacity of 3,800 m3/s. The power plant will host two Kaplan turbines of 46.5 MW each designed to operate with a gross head of 32 m and a design power discharge of 350 m3/s.
The Þjórsá is the longest river in Iceland. The river originates in the Hofsjökull glacier at 2,000 masl and runs Southwest through the highlands of Iceland. Due to the ice erosion, the river carries high sediment concentrations and sediment load. The specific sediment yield in the basin is 160 t/km2 per year.
The river basin is characterised by an average air temperature of 2.2 ⁰C and an annual average precipitation of 975 mm. The mean annual inflow is 10,344 Mm3 and the coefficient of variation is 0.1 which means that the inflow does not fluctuate far each year. The glaciated area covers 14 per cent of the basin and it supports a steady flow during the spring and summer.
Hvammur dam will be located in the lowlands as seen in figure 1, where the wind erosion dominates. The weathering of the soil pack is severe due to the lack of vegetation. Sand is the main material of the river bed with a D50 grain size of 4 mm.
The tributary area to the dam comprises 7,300 km2 of which only 470 km2 remain unregulated. As result, the suspended sediment concentration at Hvammur site has been considerably reduced making possible for the salmon to populate the area.
The Þjórsá river basin is highly regulated with multiple dams which trap most of the sediments transported through the river flow. In addition, Landsvirkjun has carried out active erosion control throughout several years. These upstream projects and measures will prevent 90 per cent of the coarse sediment to reach Hagalón reservoir.
Therefore, the expected annual suspended sediment load at the dam site is 75,000 tons and it is expected that the Hvammur dam will likely trap 100 per cent of sediments above grain size 0.06 mm. The sediment rating curve for suspended load is defined by the formula Qs=0.000005*Q^2.57 (sediment in kg/s and discharge m3/s). Being the dry bulk density of deposits 1.5 t/m3, the estimated figure is 50,000 m3 annually which the dead storage capacity (6.7 Mm3) of the reservoir will be able to offset in about 134 years.
However, sediment eroded from the river banks, the river bed and airborne sediment will also contribute to the sedimentation in the reservoir.
Erosion and sedimentation issues have been monitored since 1968 in the Þjórsá river basin. This information has been fundamental to assess the potential sedimentation issues at Hagalón reservoir and to decide on the sediment management strategies to implement.
Mechanical excavation will be carried out at the reservoir delta where most sediments carried on the river flow will settle. Plain suction dredge will be used and the sediments will be deposited on the west bank to enhance the land to farm.
Besides the monitoring, Landsvirkjun also conducted research on ice-related erosion. One of the findings showed that full-level operating regime would reduce the risk of ice-induced erosion of the reservoir banks. A set surface level will allow an ice cover to form easily and the water to flow beneath it. Therefore, the Hvammur dam is planned to operate at normally full level regime. In addition, small dykes have been constructed to reduce the bank erosion caused by the fluctuation from river regulation.
To address the wind erosion, Landsvirkjun has taken part together with the soil conservation authority in watershed management activities for revegetation.
Both regular sediment and water elevations monitoring has been ongoing for years and will continue, both upstream and downstream.
After construction of the dam, bathymetry measurements will be made regularly but annual bathymetric surveys are not considered necessary.
The sampling of delta sediment and bottomset bed is already planned. Other measurements include grain size and concentration of released sediment in the downstream area.