Roseires Dam, a concrete buttress dam located in the Blue Nile, was commissioned in 1966 to supply irrigation and domestic water, and generate power. By 2017 water for irrigation had yet to be supplied and construction to facilitate the supply was ongoing. The hydropower plant generates 1,800 GWh of energy annually. The hydroelectric plant contains Kaplan turbines with a total installed capacity of 280 MW, a gross head of 45 m and a design discharge of 840m3/s.
The original maximum height of the dam was 68 m, which increased to 78 m in 2013. The dam contains five 3 m x 5 m low level sluice gates designed to pass floods and sluice sediment. The sluice gates are located towards the right of the power intakes at the lowest level of the river cross section. Irrigation intakes are located on the left and right banks. The irrigation intake on the right bank is currently being prepared to supply irrigation water. The left bank irrigation intake has previously not been used to supply irrigation water. Figure 1 presents an elevation of Roseires Dam looking downstream.
The dam contains a gated ogre spillway with a discharge capacity of 694 m3/s. In addition, the dam was designed with five low level outlets with a discharge capacity of 5,208 m3/s to pass floods and sluice sediment through the reservoir. In 2012, the dam height was increased to adapt to sedimentation near the power intake and this as well increased the discharge capacity of the low-level sluice gates to 7,500 m3/s. Likewise the reservoir volume more than doubled to 7,400 million m3.
Hydrology and sediment
The Blue Nile originates from the Ethiopian highlands with a catchment area of 254,230 km2 upstream of Roseires Dam (see Figure 2, “Rosaries dam”). The average annual rainfall of 800 mm produces an annual average flow of 50 km3/s with an annual coefficient of variation equaling 0.19.
The sediment load of 146 million tons per year is largely produced during the flood season between July and October. The Blue Nile sediment concentration varies from year to year with concentrations as high as 2.6 percent by weight at Roseires Dam during the 1988 flood. The sediment concentration also varies throughout the flood season, starting with traces in mid-June and gradually increasing until it reaches its maximum in mid-July. It then decreases again to insignificance at the beginning of November. The suspended sediment accounts for approximately 90 percent of the total sediment load in the Blue Nile and consists mainly of silt and clay. The bed load consists of sandy material. The specific sediment yield is 480 t / km2yr.
The original Roseires reservoir volume in 1967 was 3.024 km3. By 2017, the approximate reservoir volume had reduced by 30percent, estimated as 1.996 km3. Whereas storage loss has been a significant problem, sediment blockage at the power intakes in 1976 resulted in a prolonged blackout. A bathymetric survey was consequently executed in 1976 to determine the extent of the sedimentation problem (see Figure 3). Since then, bathymetric surveys have been done periodically in 1981, 1985, 1992 and 2015.
The dam was designed with the intent to sluice sediment on an annual basis. This is done by lowering the water surface elevation in the reservoir during the flood season with the minimum level being maintained at 467 masl between June 15 and September 15, and the reservoir refilled with water by November. At the end of the sluicing period, the gates are closed and the water surface elevation of the reservoir increases to store water with the maximum operating level at 490 masl (the maximum operating level prior to dam heightening was 480 masl). Figure 4 shows the variation of the water level within the reservoir over the year in 1975.
Although sluicing was implemented annually, the amount of sediment routed through the reservoir was insignificant. Compared with high flows in the Nile River, on the order of 8,000 m3/s, the discharge capacity of the low-level outlet (5,208 m3/s – original design) are relatively high. However, the placement of low-level outlets distant from the power inlets does not allow them to clear sediment upstream of the power inlets. Therefore, significant amounts of sediment accumulate at the hydropower inlets which severely impacts power generation. To manage this effect, the accumulated sediment at the inlets is removed by mechanical dredging. Figure 5 illustrate the elevation of deposited sediment and distance in meters from Roseires Dam.
An adaptive measure was also implemented to increase the height of the dam in, thereby increasing the reservoir volume. Roseires Dam was heightened by 10 m to raise its storage capacity from 3 to 7.4 billion m3 and its power generation by 50 percent. Heightening was completed in 2012. The impacts of dam heightening on reservoir sedimentation is under study.