The Sennar Dam’s design, with 80 low level sluice gates, has successfully sluiced the annual volume of sediment of the Blue Nile. When the operating policy was changed between 1981 and 1986, the rate of reservoir sedimentation increased almost tenfold. This was rectified by reinstating annual sluicing operations.
The Sennar Dam is located along the Blue Nile, about 300 km upstream of Khartoum and 270 km downstream of Roseires Dam and the Grand Ethiopian Renaissance Dam (GERD), due to be commissioned in 2022). The Blue Nile and the White Nile meet in Khartoum, Sudan to form the Nile River. These rivers and their tributaries contain several dams as indicated in Figure 1.
Large-scale gravity irrigation during the British colonial period in Sudan (1898- 1956) began in 1925 following the construction of Sennar Dam on the Blue Nile and progressively expanded thereafter. The main purpose of the Sennar reservoir is to store water to divert for irrigation schemes, while hydropower is supplementary.
The initial reservoir storage capacity was 930 Mm3 at a maximum water surface elevation of 421.7 m. The dam has 80 low-level sluice gates (2 m wide by 8 m high), capable of passing average flood flows, and 112 spillway gates to pass the peaks of extreme floods. The dam´s design is seen in Figure 2 and the cross section showing the main sluice gate is seen in Figure 3. In 1962, two turbines of 7.5 MW each, were installed in the dam for hydropower generation.
The main purpose of the dam was to store and divert water for irrigation in the adjacent Gezira scheme, and for securing drinking water supply during the dry season. The Gezira scheme has played an important role in Sudan’s economic development, serving as a major source of foreign exchange earnings and government revenue, contributing to national food security and providing a livelihood for the estimated 2.7 million people who live in the scheme’s command area.
The Blue Nile originates in the Ethiopian highlands with a catchment area of 311,548 km2, an annual average flow of 50 km3 and a sediment load of 146 million tons per year, mostly occurring during the flood season from July to October. The Blue Nile sediment concentration varies from year to year with concentrations as high as 2.6 percent by weight recorded at the Roseires Dam, upstream of the Sennar Dam, during the 1988 flood.
Figure 4 shows how 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. On the other hand, the peak of the river flow is in mid-August. 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 Sennar reservoir has been subject to storage loss due to sediment deposition as shown in the volume-elevation curve. The rate of sedimentation has been affected by the sediment management approaches that have been implemented since commissioning of the dam, as discussed further on. At this stage, the storage loss in 2009 was 60 percent.
The reported loss of storage due to sedimentation over the period 1925 to 1981 never exceeded 4.6 Mm3 per year (0.5 percent per year with respect to its original capacity), it increased to 54 Mm3 per year (5.8 percent per year of the original capacity) between 1981 and 1986; thereafter decreasing again. The storage loss between 1925 and 1981 (56 years) was 28 percent, and that between 1981 and 1986 (five years) was 29 percent. Between 1986 and 2009 (23 years) 3 percent of additional storage loss occurred. This aggradation evolution is featured in Figure 5. The reason for these changes in the rate of sedimentation is addressed in the sediment management strategies section.
In addition, new sedimentation problems occurred in the Gezira irrigation scheme, which receives water from the Sennar Dam. The reason for this is that crops other than cotton are now being cultivated in the scheme. The intensification of rotation and diversification of crops requires a supply of irrigation water throughout the year as opposed to only after the flood season (the requirement with cotton). The diversion of irrigation water to the Gezira scheme during the period of high sediment concentration has introduced irrigation canal sedimentation, which remains a major problem. The annual cost of maintaining the intakes of the main canals to the irrigation scheme clear of sediment alone amounts to USD 0.5 million, the cost of removing about 50,000 m3 of sediment. In addition, sediment is removed from 10,000 km of irrigation canals up to six times a year to maintain flows, with sediment piled up several meters high along all of the canal banks.
The dam was originally designed to facilitate sediment management. Following the construction of the Sennar Dam and up to 1962 (prior to the installation of the power turbines), the river could flow naturally through the reservoir without any impoundment during the flood period (July to September) by opening the 80 sluice gates; thereby sluicing sediment. This was possible because the scheme was originally constructed to exclusively irrigate cotton, which only requires supplementary irrigation after the rainy season. This resulted in a low rate of storage loss of only about 4.6 million m3 per year. Furthermore, the water diverted to the scheme contained very little sediment and no problems with sedimentation were experienced in the irrigation scheme canals.
The original operating rule was significantly changed during the period 1981 to 1986 by giving priority to irrigation water supply and not sluicing annually. During this period, as already indicated, the rate of sedimentation increased drastically to 54 million m3 per year. After 1986 annual sluicing was reinstated and the rate of sedimentation was substantially reduced again, resulting in low annual storage loss.
The operating rule of the dam changed in 1962 following the installation of the turbines. Whereas the original intent was to pass flood and sediment during the flood season by drawing down the water surface elevation as much as possible, it was necessary to maintain a certain minimum water level after 1962 to facilitate power generation. Figure 6 shows the average operating curve implemented at Sennar Dam. The minimum water level to be maintained in the reservoir was set at 417.5 masl. Despite setting a minimum water level, sediment is still successfully sluiced through the Sennar reservoir.
Figure 7 shows the sediment processes in the Sennar reservoir and illustrates how the sediment deposition is reduced during the sluicing period from the end of June to the end of August and the re-entrainment of sediment during the falling limb of the hydrograph in September.
Two factors played a role in the successful sediment management at Sennar Dam: good dam design and thoughtful development and implementation of operating strategies which minimise the impacts of sedimentation. The dam has enough low-level sluices to pass the annual volume of sediment in the Blue Nile during the flood season and the operating policy was crafted to allow such passage, minimizing the effects of reservoir and irrigation canal sedimentation.
When the operating policy was violated between 1981 and 1986, the rate of reservoir sedimentation increased almost tenfold. This was rectified by reinstating annual sluicing operations after 1986, which drastically reduced the rate of sedimentation.
Originally no sedimentation problems were experienced in the irrigation canals at the Gezira irrigation scheme. The reason for this was the scheme was originally intended to exclusively irrigate cotton, which only requires water once the flood season has passed, when the sediment load in the Blue Nile has decreased to insignificance.
The crop mix has since changed and the scheme now requires irrigation water throughout the year. The change in use requires diversion of irrigation water laden with sediment during the flood season, which results in the irrigation canals filling with sediment. Furthermore, by not irrigating 24 hours a day, water is stored overnight in the irrigation canals which encourages further sedimentation. This demands significant maintenance of the 10,000 km of irrigation canals, which require excavation of deposited sediment up to six times per year. Figure 8 illustrates de excavated sediment deposits along one of the canals.
In summary, the dam was correctly designed to pass sediment by sluicing. When sluicing operations are correctly implemented, the rate of reservoir sedimentation is minimal. When the irrigation scheme was operated as originally intended, i.e. to only irrigate cotton, no sedimentation problems were experienced in the irrigation canals. Since changing the crop mix, requiring all-year-round irrigation, the maintenance burden of the irrigation scheme has increased significantly.
Commissioning of the Grand Ethiopian Renaissance Dam (GERD) will drastically reduce the sediment load in the Blue Nile, which will undoubtedly alleviate future impacts of reservoir sedimentation and will likely reduce future maintenance requirements in the Gezira irrigation scheme.