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 Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي (Re: Asskouri)
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http://www.eawag.ch/services/pr/dokumente06/20060323/Me...-Review-20060323.pdf
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Independent Review
of the Environmental Impact Assessment
for the Merowe Dam Project
(Nile River, Sudan)
Cristian Teodoru, Alfred Wüest, Bernhard Wehrli
March 23rd 2006
Eawag
Seestrasse 79
6047 Kastanienbaum
Switzerland
Phone +41 (0)41 349 21 11
Fax +41 (0)41 349 21 68
www.eawag.ch
Table of contents
1. Executive summary...................................................................................4
1.1 General results......................................................................................................6
1.2 Specific results......................................................................................................6
1.3 Critical issues........................................................................................................8
2. General project description.....................................................................9
2.1 Nile catchment and climate....................................................................................9
2.2 Nile hydrology.......................................................................................................11
2.3 Nile control measures...........................................................................................13
2.4 Merowe Dam – technical details..........................................................................16
3. Environmental issues - lessons from Aswan High Dam......................18
3.1 Reservoir-induced seismicity...............................................................................19
3.2 Water losses...........................................................................................................20
3.3 Water quality.........................................................................................................20
3.4 Sedimentation........................................................................................................23
3.5 Biogeochemical cycles...........................................................................................26
3.5.1 Nitrogen...........................................................................................................26
3.5.2 Phosphorus.......................................................................................................30
3.5.3 Organic matter and silicate..............................................................................32
3.5.4 Primary production..........................................................................................34
3.5.5 Greenhouse gas................................................................................................35
3.5.6 Dissolved phosphorus balance.........................................................................37
3.6 Aquatic ecology.....................................................................................................38
3.7 Water-born diseases..............................................................................................43
4. Assessment of the Merowe Dam............................................................44
4.1 Seismicity...............................................................................................................44
4.2 Hydrology..............................................................................................................45
4.3 Water losses...........................................................................................................48
4.4 Reservoir water balance.......................................................................................50
4.5 Water quality.........................................................................................................52
4.6 Sediment balance..................................................................................................53
4.7 Biogeochemical cycles...........................................................................................56
4.7.1 Primary production..........................................................................................56
4.7.2 Greenhouse gas emissions...............................................................................57
4.7.3 Phosphorus balance..........................................................................................59
4.7.4 Increased salt content.......................................................................................60
4.8 Aquatic ecology.....................................................................................................61
4.9 Health-related impacts.........................................................................................64
5. Summary of the environmental impacts..............................................66
5.1 Hydrology and water balance..............................................................................66
5.2 Sedimentary aspects..............................................................................................69
5.3 Water quality and geochemistry..........................................................................70
5.4 Ecology and health-related impacts....................................................................73
6. The Lahmeyer report.............................................................................75
6.1 International standards........................................................................................75
6.2 Important deficiencies..........................................................................................75
6.2.1 Sedimentation..................................................................................................76
6.2.2 Hydrology........................................................................................................77
6.2.3 Irrigation..........................................................................................................77
6.2.4 Water quality....................................................................................................78
6.2.5 Greenhouse gas................................................................................................79
6.2.6 Fishes...............................................................................................................79
6.3 Recommendations for mitigating negative impacts...........................................80
6.3.1 Recommendation on reservoir level operation................................................80
6.3.2 Recommendation on sedimentation.................................................................81
6.3.3 Recommendations on water quality.................................................................81
References....................................................................................................83
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1. Executive summary
The Merowe Dam, presently under construction 800 km downstream of Khartoum on the Nile River in Sudan, will submerge the fourth cataract of the Nile and form a 200 km long artificial lake. With a surface area of 800 km2, the lake will inundate 55 km2 of irrigated land and 11 km2 of farmland used for flood recession agriculture. Merowe represents the current largest hydropower project in Africa. The main purpose of the 67 m high Merowe Dam is hydropower production. The capacity of 1’250 MW will be almost twice the current hydropower capacity in Sudan. The project includes an irrigation component but there are still uncertainties as to whether it will be implemented.
The total cost of the Merowe Dam Project is estimated to reach $1.2 billion. Beside the Sudanese Government, the project is financially supported by the China Export Import Bank, the Arab Fund for Economic and Social Development, and the Development Funds of Saudi Arabia, Kuwait, Abu Dhabi, and the Sultanate of Oman. The dam and the transmission lines are mainly being constructed by Chinese companies. Sudanese contractors are involved in building the dam and the resettlement sites. Several European companies are participating in the project: Lahmeyer International (Germany) manages the construction of the project; Alstom (France) is supplying electro-mechanic equipment; and ABB (Switzerland) is building transmission substations.
At the planning stage of dam constructions on major rivers, a full consideration of the environmental impacts is required according to international standards. The project participants are therefore required to prepare or contract an environmental impact assessment report (EIAR) in accordance with specific guidelines that address three major topics:
• Social issues - consequences of people resettlement from future flooded area;
• Archeological issues - resulting from destruction or submerging important archeological sites or places of high cultural value;
• Environmental issues - effect of large scale hydrological alteration of the natural river system with major impacts on the environment and water quality.
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In April 2002, Lahmeyer International prepared the EIAR for the Merowe Dam Project. The document focuses on the complex resettlement issues involving about 7’500 families. Among the environmental impacts it discusses are the hydrological changes, the erosion of the river bed and its banks, greenhouse gas emissions and changes in the aquatic ecosystem. The 150 page report was far from meeting European or international standards, such as the guidelines of the World Commission on Dams (WCD, 2000). No serious attempt was made to use the vast scientific knowledge base on environmental effects of large dams, although four decades of research on the Aswan High Dam (Lake Nasser in Egypt, Lake Nubia in Sudan) have revealed a dramatic sediment accumulation in the upper part of the reservoir, problematic water quality issues and detrimental downstream effects such as river bed erosion or water level fluctuations.
This independent review of the Lahmeyer EIAR (2002) was motivated by the mission of Eawag to use our competence in the assessment of surface water systems and their management in relevant contexts. In addition, Eawag has an intrinsic interest as man-made alterations of aquatic system are part of its core business. International Rivers Network (IRN) encouraged Eawag to carry out this review, and provided inputs by sharing documents and other information. IRN did not influence the contents nor the topics addressed in this review in any way. In preparing this review we worked towards achieving three objectives:
• to review the relevant literature concerning the environmental effects of the Aswan High Dam as a suitable reference system for large dam projects on the main reach of the Nile River,
• to identify and quantify possible environmental changes induced by the Merowe Dam,
• to provide a constructive critique of the Merowe EIAR including recommendations for further study and for developing mitigation measures.
The expertise of the authors covers the fields of aquatic physics, chemistry and sedimentology. Additionally, we obtained input from other specialists. The review was deliberately focused on natural science issues, where the authors follow an active
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research agenda (Friedl and Wüest, 2002; Friedl et al., 2004; Teodoru and Wehrli, 2005; Bratrich et al., 2004). Health aspects were covered only marginally and socioeconomic topics such as the resettlements and economic valuation were not addressed as they are outside our field of competence. Our report was written for the experts in Sudan, for the project parties and the interested stakeholders. With this case study we hope to intensify the scientific exchange and debate concerning environmental impact assessments for large dam projects.
1.1 General results
The following conclusions can be drawn for EIAR of large dam projects in general.
• The scientific analysis of environmental effects of river impoundments is vast and growing fast. The ISI database lists more than 200 publications under key words “Nile” and “dam”. Relevant scientific results should be used explicitly in preparing an EIAR. The past experience with existing dams in the same river system proved particularly valuable for predicting the impact of a new dam.
• The practice of “peer review” as it is used for improving scientific publications could well add credibility to an EIAR, particularly if the original report is prepared by a company with close ties to the project.
1.2 Specific results
Our analysis has identified the following topics of major concern regarding the Merowe Dam project
• The Merowe dam will act as a major sediment sink for the suspended load of the Nile River. Because the reservoir is much smaller in volume compared to the Aswan High Dam Reservoir, it is likely to lose more than 30% of its capacity over the next 50 years. A concept of management for the 130 Mio. tons of sediment
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accumulating every year in the Merowe Reservoir is lacking at present and deserves a high priority for a sustainable hydropower generation.
• Since about 90 % of the suspended load of the Nile water will be retained in the Merowe Reservoir, the outflow will have a large carrying capacity for particles and produce erosion of the river bed and the side banks. The Lahmeyer EIAR recommends monitoring of river cross sections to plan countermeasures for the cities and settlements downstream. Because bed and bank erosion is well documented after closure of the Aswan High Dam, geomorphological studies should be started immediately to identify key areas of concern.
• The Merowe Reservoir will become stratified during the hot season and settling algae can produce anoxic conditions in the bottom waters close to the dam. This will reduce the available habitat for fish species and increase the emission rates for the greenhouses gas methane and carbon dioxide.
• The total mass of organic matter contributing to greenhouse gas emissions will be an order of magnitude larger than estimated by the EIAR. In addition to the primary production within the reservoir, the suspended load of the Nile River will also carry organic material, which can be degraded to carbon dioxide and methane in the reservoir sediments at rates on the order of 200’000 - 300’000 tons of carbon per year.
• The effects of disrupting the river continuum on aquatic biodiversity have not been addressed adequately. The available species lists in the EIAR are inadequate and incomplete. Several species have migratory life cycles and spend time in both the tributaries and the main river. Such life cycles of important fish species should be analyzed in detail before a general assessment of the impacts of a large dam project on biodiversity can be made. Together with the Aswan High Dam, the Merowe Dam will genetically isolate an important reach of the Nile River.
• The dam and the hydropower station are designed for peak operation during a few hours per day. The resulting hydropeaking downstream is expected to produce water level fluctuations of about 4 m per day. Such intense fluctuations will have detrimental effects on aquatic ecosystems because the riparian zone of a river provides crucial habitat for aquatic life. The EIAR considers mainly economic
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effects such as upgrading of necessary pumping stations and ferry landing sites, but neglects the effects on daily life of the riparian population. A retention dam at the outlet of the power station could mitigate such negative side effects.
• The design of the dam allows for water abstraction for irrigation. No planning details are available in the EIAR as the decision for or against irrigation was postponed. An overview by the World Commission on Dams (WCD, 2000) revealed a high failure rate for irrigation schemes in arid areas. Open planning and communication of the goals and the implementation of irrigation schemes at Merowe is a key factor for their success, and should be included in the EIAR.
1.3 Critical issues
In summary, the EIAR for the Merowe Dam Project provided a detailed overview of the technical, hydraulic and hydrologic framework, and discussed issues of resettlement and ecological and economical side effects. The EIAR failed
• to base its assessment on the available scientific literature,
• to develop a plausible sediment management concept,
• to critically assess the ecological functioning of the reservoir ecosystem including its greenhouse gas production and the effects on fish biodiversity,
• to offer strategies for mitigating the downstream effects of hydropeaking.
We hope that the following review can partially close these gaps and provide some concepts for improving future EIA reports.
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2. General project description
2.1 Nile catchment and climate
Formed by the confluence of the three main tributaries, White Nile, Blue Nile and Atbara River, the Nile River flows over 6’700 km from the south glaciated highlands through alluvial plains and desert sands into the eastern Mediterranean (Figure 1). Stretching over 35 degrees latitude of the north-eastern African quadrant, the Nile River basin represents one-third of the entire African continent. With an area of almost 3x106 km2 extending over different geographical, topographical and climatological regions, the basin spans over nine African countries: Tanzania, Uganda, Rwanda, Burundi, Zaire, Kenya, Ethiopia, Sudan and Egypt (Figure 1).
The hydrographical and hydrological characteristics vary greatly over the basin with abundant rainfall in the headwaters and arid conditions in Sudan and Egypt. Therefore, although the watershed is large, the portion contributing to stream flow is almost half of the entire basin (only 1.6x106 km2) due to the fact that north of 18 °N latitude, rainfall is almost zero. Precipitation increases towards the headwaters to about 1’200 to 1’600 mm yr-1 on the Ethiopian Plateau and in the region of the Equatorial lakes: Victoria, Albert, Kayoga, and Edward (Mohamed et al., 2005). The seasonal pattern of rainfall follows the Inter-Tropical Convergence Zone (ITCZ), where the dry northeast winds meet the wet southwest winds and are forced upward causing water vapor to condense. The ITCZ follows the area of most intense solar heating and warmest surface temperature and reaches the northerly position of Ethiopian Plateau by late July. The southward shift of the ITCZ results in the retreat of the rainy season towards the central part of the basin after October. Therefore, the monthly distribution of precipitation over the basin shows one single but long wet season over the Ethiopian Plateau and two rainy seasons over the Equatorial Lakes Plateau (Mohamed et al., 2005).
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Figure 1. Map of the Nile River Basin
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2.2 Nile hydrology
The Nile River system has two main sources of water: (i) the White Nile with its source on the Equatorial Lake Plateau region around Lake Victoria and (ii) the Blue Nile and the Atbara River having their headwaters on the Ethiopian Plateau. The White Nile starts with its small tributary Kagera River, entering Lake Victoria near the border between Uganda and Tanzania. The river travels north crossing the equatorial region, receiving water from numerous streams and lakes. After leaving the lake area, the White Nile enters southern Sudan through rocky gorges and then flows through a large swamp area (the Sudd region) where it is joined by the Sobat from the east and the Bahr el-Ghazal, from the west. In the Sudd region a huge quantity of water evaporates or is transpired by aquatic vegetation. Only a small part of the Bahr el-Ghazal flow ever reaches the White Nile, as most of its water disappears in the swamps. Further down, the White Nile travels a mild slope north to the confluence with the Blue Nile at Khartoum.
The Blue Nile originates from Lake Tana on the Ethiopian Plateau, a region of high summer rainfall at about 1’800 m above sea level (a.s.l.). Originating also from the Ethiopian Plateau, the Atbara River joins the main course of the Nile about 300 km north of Khartoum. From here, no significant tributary contributes to the hydrologic regime of Nile River. The altitude profile of the Nile from Lake Victoria (1’135 m a.s.l.) to the Mediterranean Sea is shown in Figure 2.
Figure 2. Altitude profile of the Nile River (after Said, 1993).
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The present average contributions from each of the three main tributaries to the entire Nile River flow are schematically represented in Figure 3. Runoff from the Ethiopian Plateau via the Blue Nile and Atbara accounts for roughly 70 % of the annual water discharge, whereas the White Nile contributes about 30 % (Roskar, 2000). Although the contribution of the White Nile to the total annual flow is rather small, the White Nile is most important because of its continuous flow during the dry season when its discharge is large compared to that of the Blue Nile.
Subject to seasonal variations, about 80 % of the total annual discharge of the River Nile occurs during the summer rainy season (July to October) mainly with the Blue Nile and the Atbara River (Woodward et al., 2001). Atbara River runs dry at times of the year while the White Nile maintains the flow in the Nile over the entire year. Without the discharge of the upper White Nile the Nile River would probably run dry in May. The annual flow distribution and suspended sediment budget is shown in Figure 3.
Figure 3. The water and suspended sediment budget of the present Nile basin (after Woodward et al., 2001)
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River Gauging stations Catchment area Mean annual flow
[109 m2] [km3 yr-1]
Nile Aswan 3060 84.1
Atbara Atbara 180 11.1
Blue Nile Khartoum 330 48.3
White Nile Khartoum 1730 26.0
Table 1. Catchment area and the mean annual flows over the period 1901 to 1995 for the Nile River (after Mohamed et al., 2005).
More than 95 % of the mean annual suspended sediment load of the Nile River upstream of the Aswan High Dam (120x106 t yr-1, Woodward et al. 2001) comes with the Blue Nile (72 %) and the Atbara River (25 %) whereas the White Nile contributes only 3 % of the total load.
Apart from seasonal variations, the total annual discharge of the Nile River is subject to intense annual variations with the highest annual flows of 154 km3 yr-1 recorded in 1878 (Abu Zeid, 1987) and 120 km3 yr-1 measured in 1984 (Woodward et al., 2001). The lowest annual flow on record was observed in 1913 with only 42 km3 yr-1 (Abu Zeid, 1987). The mean annual flow of the Nile River and the three tributaries calculated over the period 1901 - 1995 (Mohamed et al., 2005) is shown in Table 1. A number of hydrological changes affected the Nile regime over the last century as a result of river control measures.
2.3 Nile control measures
Summarized by El-Hinnawi (1980), the Nile River control measures started in the early 19th century when a series of barrages were built to transform the old recession irrigation system to perennial irrigation so that instead of growing one crop per year, two or three corps could be grown on the same land. The Delta Barrage was built just below Cairo to control the Rosetta and Damietta branches of the Nile together with the Zift a Barrage on the Damietta branch. Constructed in 1902 and enlarged in 1938, a dam at Assiut was built
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to provide perennial irrigation in central Egypt. In 1908, the Esna Barrage followed by Nag-Hammady in 1930 was constructed on the Nile River to improve the water supply for irrigation schemes in Upper Egypt.
The first Aswan Dam was completed in 1902 to store Nile water when the river was at its annual high level. Heightened several times latter on, at full storage the reservoir extended up to Wadi Halfa. At the end of each storage period the sluices were opened to drain most of the lake and remobilize the accumulated sediment. The new Aswan High Dam, designed to be never drained, caused a revolution in the Egyptian irrigation system. Practically, all the fertile sediment from the Ethiopian Plateau was deposited in the reservoir. Although the missing suspended particles created certain disadvantages, the extra stored water and the reduction of silting in the irrigation channels allowed the perennial irrigation as well as a significant increase in irrigated area.
About 45 km south of Kharthoum, the Jebel Aulia reservoir was constructed in 1937 to hold back part of the White Nile flow during rich discharge of the Blue Nile. Since the Nile valley upstream of Jebel Aulia is very flat and open, a large quantity of water is being lost due to evaporation and seepage.
The Owen Falls Dam (Figure 1) completed in 1954 was the first control work on the upper White Nile. With a primary goal of producing hydroelectric power, the dam controls the outflow of Lake Victoria and therefore created the largest reservoir in the world.
In 1999, after a year-long debate, the Ugandan Parliament approved the construction of the Bujagali hydropower dam as a private hydroelectric power plant project in Uganda. The project is one of several hydropower plants planned to be scattered along the upper reaches of the White Nile including Owen Falls, Busowoko, Ralangala, Raruma, Ayago North, Ayago South and Murchison Falls. Situated 1’100 m a.s.l. at Bujagali Falls, about 8 km north of Lake Victoria, construction of the Bujagali plant was due to begin in January 2003, but was initially delayed after vocal protests by environmentalists and
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residents of the area. In February 2005, the Ugandan Government announced that the project will go ahead. With a volume of up to 750’000 m3 and a vertical drop of 30 m, the reservoir water will feed four turbines with a total installed capacity of 250 MW.
On the Blue Nile, the Sennar Dam (Figure 1) completed in 1925 serves the needs of Sudan by providing the basis for the Sudan’s agriculture economy. Its main function is to store water for the Gezira irrigation scheme during the flood season of the Blue Nile.
Further upstream, near the Sudan-Ethiopian border, the Roseires Dam (Figure 1) was completed in 1966 with the two primary purposes of increasing the storage capacity of the Blue Nile water and producing hydropower. Far above Roseires, below the Blue Nile Gorge, Lake Tanna (Ethiopia) was considered for many years as a good place for a storage reservoir to hold back a large proportion of the Blue Nile flood. Ethiopian interests around the lake, including historical and religious sites, prevented the realization of the project until today.
A large hydropower and irrigation project is currently under construction on the Tezeke River (a tributary of the Blue Nile) in the Tigray Region of northern Ethiopia. Scheduled for completion by the end of 2006, the Tezeke Dam with a height of 185 m will be 10 meters higher than the highly controversial Three Gorges Dam in China.
In summer 2005, Ethiopia signed an agreement with an Italian construction company to build a 460 MW hydroelectricity dam across the Beless River, a tributary of the Blue Nile in the north-western part of the country. With costs of over 690 million dollars, this project would represent the largest hydropower dam in the country. Including irrigation and drinking water projects for the autonomous Benishangul-Gumuz Region bordering Sudan, the dam is expected to be completed in about three years.
Currently under construction, the Merowe Dam on the Nile River, about 800 km north of the capital Khartoum (Sudan) is the largest contemporary hydropower project in Africa.
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Expected to be completed by 2008, the main purpose of the dam will be hydropower production.
2.4 Merowe Dam – technical details
Located on the Nile River in Sudan, about 800 km downstream of Khartoum and about 500 km upstream of Lake Nubia, the Merowe (or Hamadab) Project has originally been conceived as a multi-purpose reservoir dam for irrigation and hydropower production. In April 2002, the irrigation component (two irrigation intakes on the right and left bank of 150 m3 s-1 each) was still studied at pre-feasibility level although the two irrigation intakes have been incorporated in the dam structure design.
The dam is designed to have a length of about 9 km and a crest height of up to 67 m. It will consist of concrete-faced rockfill dams on each river bank, an earth-rock dam with a clay core in the left river channel and a live water section in the right river channel (sluices, spillway and power intake dam with turbine housings). The powerhouse will be equipped with ten 125 MW Francis turbines. Once finished, the dam will create a reservoir with a volume of 12.4 km3, representing about 20 % of the Nile's annual flow and a surface area of 800 km2 stretched over 200 km river length. At the maximum storage level at 300 m a.s.l. and a maximum water depth of 57 m, the reservoir will submerge about 66 km2 of irrigated and flood recession land. The reported average residence time is 0.2 years for an annual inflow of 84 km3 yr-1 (Table 2).
The planners expect an annual electricity generation of 5.5 TWh, corresponding to an average load of 625 MW, or 50 % of the rated load. To utilize the extra generation capacity, the Sudanese power grid will be upgraded and extended as part of the project. It is planned to build about 500 km of new aerial transmission lines across the Bayudah desert to Atbara, continuing to Omdurman/Khartoum, as well as about 1’000 km lines eastwards to Port Sudan and westwards along the Nile, connecting to Merowe, Dabba and Dongola.
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Special features of the project described by EIAR (2002) are listed below:
o The hydropower plant will be operated with a minimum water release flow of 600 m3 s-1 for 20 h per day and a maximum flow of 3’000 m3 s-1 for 4 h per day. This operation mode results in daily water level fluctuations in the river downstream with a maximum amplitude of 4.9 m from January to March.
o The dam design incorporates special sluices and particular operation rules to reduce reservoir sedimentation and related capacity losses over a 50 yr period to 17 % of the original active capacity (83 % will still remain active).
o The dam structure will be equipped with a grout curtain preventing water losses to the downstream groundwater.
Value Unit
Reservoir area 800 [km2]
Length (river) 200 [km]
Max. water level 57 [m]
Annual inflow 84 [km3yr-1]
Storage volume 12.4 [km3]
Evaporation 2.4 [km3 yr-1]
Retention time 0.2 [yr]
Max. daily water release flow 3000 [m3 s-1]
Min. daily water release flow 600 [m3 s-1]
Average suspended solids 1.7 [g l-1]
Table 2. Merowe Dam: reservoir characteristics described by EIAR (2002)
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3. Environmental issues - lessons from Aswan High Dam
It is important to recall that environmental effects of a river development program are usually complex and always cause changes with positive and negative implications for the environment. River impoundments generally cause a series of multifaceted ecological alterations. Elimination of the annual natural flooding, water storage, increased surface area, increased evaporation and infiltration to adjacent aquifers, reservoir sedimentation and downstream erosion, increased residence time and thermal stratification, high in-situ primary production, changing nutrient transport, greenhouse gas emissions and water-born diseases are a few typical consequences (Rosenberg et al., 1995; Rosenberg et al., 1997; Friedl and Wüest, 2002).
A reliable assessment of environmental effects of river damming requires adequate basic information. Unfortunately, such information was only partly available in the EIAR (2002) for the Merowe Dam Project as prepared by Lahmeyer International. In such a case of a limited database at the river reach of interest it is a useful strategy to review the existing literature on existing dams within the same river system. In the following we therefore review the available literature on the large impoundment 500 km downstream, the Aswan High Dam Reservoir. The review of the environmental impacts of the Aswan High Dam serves as a reference system for identifying and evaluating the magnitude of the potential impacts of the Merowe Dam on water quality and Nile River ecology.
The Aswan High Dam (AHD), the world second largest artificial lake by volume after Bratsk in Russia has been the subject of controversial discussions during the design, construction and after completion in 1965. With a design capacity of 162 km3 at a maximum water level of 182 m a.s.l., its main purposes are electricity production and water storage. With an area of 6’000 km2, the lake behind the AHD extends about 500 km south from Aswan with about 300 km in Egypt (Lake Nasser) and 200 km in Sudan (Lake Nubia). The total capacity of the reservoir consists of the dead storage of 31.6 km3 (85 to 147 m a.s.l. of lake water level), the active storage of 90.7 km3 (147 to 174 m) and
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the emergency storage for flood protection of 41 km3 between 175 and 182 m a.s.l. (Shalash, 1982).
3.1 Reservoir-induced seismicity
On 14 November 1981, six years after the AHD Reservoir reached its maximum level of 72 m (1975), a first earthquake occurred near the edge of the lake, about 65 km to the south-west of the dam on the Kalabsha fault at a depth of about 20 km (Kebeasy et al., 1991; Abu-Zeid et al., 1995). With 5.5 degree on the Richter scale, the local magnitude was felt as far as 900 km south at Khartoum and causing some damage at Aswan. As the event occurred at a significant distance from the reservoir and at considerable time after impounding (16 years), it was not clear whether it was a reservoir-induced earthquake.
Subject of many studies, the reservoir behind the AHD was considered not to contribute to seismic activity as the depth of seismicity was larger than 15 km and the penetration of water to affect water pressure at this depth was hypothetical (Meade, 1991). However, the theory of induced earthquake mechanisms of Simpson (1976) suggests that the first event should appear at some distance from the deepest part of the reservoir and may occur some time after impounding when the effects of increased pore-pressure overcome the effect of loading. Also, the existence of long-continuity aftershock sequence as in the case of Aswan with a frequency of 0 to 10 events per month between 1982 and 1998 was considered a feature of reservoir-induced seismicity (Selim et al., 2002).
The statistical investigation of reservoir-induced seismicity in AHD has shown a strong correlation with the water level fluctuations. The seismicity was observed active during periods of decreasing water levels (Selim et al., 2002). These correlations supported the conclusion of reservoir-induced seismicity at AHD.
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3.2 Water losses
As an effect of increased water surface area exposed to arid climate conditions, the evaporation from the lake behind the AHD, based on isotope analyses, was estimated to vary between 18 and 21 % (19 % in average) of the total river input (Aly et al., 1993). A later review of previous literature data established a large range for evaporation from Lake Nasser between 1.7 m yr-1 and 2.9 m yr-1 (Sadek et al., 1997). Based on water balance, energy budget and modelling techniques, narrower range of 2.1 m yr-1 to 2.6 m yr-1, with an average of 2.35 m yr-1, was calculated by Sadek et al., (1997). In a 2002 technical report, based on the available data at the Nile Forecasting Center in Cairo, it was estimated that the annual evaporation from the AHD Reservoir varied between 12 and 12.6 km3 yr-1 which correspond to an evaporation rate of 2.0 to 2.1 m yr-1 (Roskar, 2000). Compared to the reservoir volume of 162 km3, the evaporation represents about 8 % per year but more than 15 % of the river inflow of 84 km3 yr-1.
In addition to the water loss via evaporation, the seepage of the AHD to the lateral groundwater aquifers was calculated to reach 1 km3 yr-1 representing 1.2 % of the river inflow (Aly et al., 1993).
3.3 Water quality
Measurements between 1980 and 1990 in the AHD revealed annual water level fluctuations between 2 and 18 m with an average of 6 m (Rashid, 1995). Besides direct effect on vegetation abundance and distribution (Ali et al., 1995) and fish ecology, large seasonal fluctuations of the lake level affects the settling of the population around the lake shore.
A general effect of water storage due to river impoundment in arid and semi-arid areas is the onset of thermal stratification of the water column. Surface temperatures of 27 to 30 °C were measured in Lake Nasser from May to August 1976 and 1977 whereas
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temperatures of 20 to 22 °C were recorded at 16 m depth (Rashid, 1995). During November to February 1976 and 1977, no differences in temperature were observed between the surface and deep water (Rashid, 1995).
The AHD Reservoir exhibits a distinct stratification pattern which varies from the main channel to the side bays, locally known as khores. Measurements in the AHD Reservoir showed that the thermal stratification of the water column starts usually in May extending from north to south to almost the entire water body. At the beginning of the flood period in late July, the thermal stratification is usually vanished in the southern reaches of the reservoir whereas the northern sectors remain stratified until late October when the seasonal cooling leads to deep convective mixing (Abu-Zeid, 1987).
Dissolved oxygen concentration in the water column reflects closely the main phases of thermal stratification. The oxygen profile decreased with depth. For example, from March to June 1976/1977, dissolved oxygen concentrations above 8.5 mg l-1 (with high values up to 14 mg l-1 near the surface) characterized the uppermost 10 m of the northern sector of AHD but were as low as 2 mg l-1 near the bottom (Abu-Zaid, 1987; Rashid, 1995). In July/August, the dissolved oxygen concentration ranged between 9.8 to 0.0 mg l-1 in the upper 15 m. A stable stratification of the lake water column was therefore, evident with an oxygen minimum between 10 and 25 m. During 1978 an oxygenated layer was limited to the top 10 m in July and August for the northern AHD whereas in October-November, stratification started disappearing with the oxic layer extending to 40-50 m below surface (Abu-Zaid, 1987). In December, the lake water becomes usually saturated or near-saturated with oxygen from the surface down to the bottom (Rashid 1995). Also, in the northern part of Lake Nubia, dissolved oxygen was absent near the bottom during August 1976 in the side bay whereas in the riverine section of the lake, the concentration was about 8 mg l-1 all the time (Rashid, 1995). In Lake Nasser (Egypt) oxygen-free conditions at the reservoir bottom were extending progressively towards the dam over a general stratification period of between 5 and 8 months (Entz, 1980a). The flood affected the stratification only in the southern part of the lake and therefore, the mixing process was often incomplete (Rashid, 1995).
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Measurements of dissolved oxygen in the water column along the main body of AHD during 1982 and 1984 revealed a wide spatial and seasonal variation in concentrations (Ahmed et al., 1989; Mohammed at al., 1989). At 10 km south of the dam the values ranged from a minimum of 4.3 mg l-1 in autumn 1982 to a maximum of 9.0 mg l-1 in winter 1982. At 245 km south of the dam, the highest values of 8.1 were recorded in the summer 1982 and 1983 respectively (Figure 4). The saturation of water with oxygen in the AHD was associated with the high level of algal photosynthetic activity (Mohammed et al., 1989). The vertical distribution at 10 km south of the AHD showed oxygen supersaturation only in April and June 1982 and May 1983 at times of intensive phytoplankton development (Ahmed et al., 1989). The oxygen saturation in the reservoir remained below saturation in 1982 and 1984 with a lowest level of 28 % during summer when the thermal stratification was well established (Mohammed et al., 1989).
Salinity, defined as the total content of all dissolved ions per volume of water, is controlled by the net accumulation of salt from all sources minus the losses through outflow and mineral precipitation. Several indications of an increased salt content of the Nile water between Aswan and Cairo (Egypt) were observed during 1963-1971, after closing the AHD (Hilal and Rasheed, 1976). The longitudinal series and an earlier comparison of seasonal changes at Aswan and Cairo showed that the observed increase in salt content was mainly attributed to increases in the ions Na+ and Cl-. Conductivity values in Lake Nasser were higher than expected form the simple seasonal mixture of Blue and White Nile water (Talling, 1980). Monitoring of Lake Nubia has shown a gradual annual increase in average total dissolved solids concentration from: 153 mg l-1 in 1978 to 156 mg l-1 in 1979, 158 mg l-1 in 1980, 162 mg l-1 in 1981 and 163 mg l-1 in 1982 (ILEC, 2005). Constantly, about 30 % higher values were measured during the dry season months compared to the flooding and after-flooding season. This was explained by the increased contribution of the White Nile water with relative higher total dissolved solids compared to the Blue Nile and the dilution effect during the flood periods. However, the linear increase in total dissolved concentration between 1978 and 1982 was attributed to the water losses by evaporation.
22
Water quality measurements at the AHD indicate that the salinity varies from 5 to 20 %, depending on the reservoir surface and the seasons (Abu-Zeid, 1987). A salt balance model based on measurements since 1913 showed that the evaporation in the Aswan Reservoir resulted in a 10-15 % increase in total dissolved solids of the water released from the dam (Abu-Zeid, 1987). This increase in salt content per se is currently not a serious concern – critical is the loss of water.
3.4 Sedimentation
It has been shown that, prior the AHD construction and operation in 1964, more than 90 % of the Nile sediment load was carried to the Mediterranean Sea (Shalash, 1982). Since the operation of the AHD in 1968, the sediment balance has been drastically modified. Based on 100 years long records at various locations upstream and downstream of AHD, the total suspended solids (TSS) inflow concentration was determined as 1.7 g l-1 (kg m-3) corresponding to an average load of 142x106 t TSS yr-1 (Shalash, 1982). Outflow measurements and long-term predictions were used to calculate an average deposition in the AHD of up to 136x106 t TSS yr-1, representing a total retention of 96 % of the incoming load (Shalash, 1982).
Using an average sediment density of 1.56 g cm-3 (Shalash, 1982) and corrected for compaction (dry weight density of 2.6 g cm-3 and a porosity of 40 %), the amount of annually retained sediment of 136x106 t TSS yr-1 corresponds to an accumulated volume of 87x106 m3 yr-1 (Shalash, 1982). A comparable sediment volume of 119x106 m3 yr-1 was measured to be annually deposited in the AHD Reservoir based on sedimentation data over a 5 years study interval between 1987 and 1992 (Eldardir, 1994). At this accumulation rate the reservoir dead storage capacity of 31.6 km3 will be lost in ~ 360 years, close to the preliminary calculated design life time of 450 years (Shalash, 1982). This new results imply that after the 41 years since the AHD closure in 1964, the reservoir has lost ~ 11 % of its dead storage capacity (~ 0.3 % annually).
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Several studies on the AHD Reservoir have shown that the major part of the sediment load is deposited close to the inflow of the reservoir in Lake Nubia where a new delta is forming.
Figure 4. Seasonal local variations in dissolved oxygen concentration in Lake Nasser (Egypt) from spring 1982 to winter 1983/1984 (after Ahmed et al., 1989)
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After less than 30 years, this new delta accumulated a 40 m thick fan about 200 km long and 12 km wide (Eldardir, 1994). The prediction based on sedimentary aspects and hydraulic factors anticipate that the new delta will appear on the lake surface with almost complete closure of the reservoir within the next 50 years (Eldardir, 1994). Therefore, dredging of this sediment may be soon required.
Studies from the eighteenth century, and confirmed by more recent ones, have shown that during the natural hydrologic regime of the Nile River the annual deposition rate on the river bed and the often cultivated side banks was around 1 mm yr-1 accounting for 7 % of the annual average TSS load (Shalash, 1982). About 24x106 t yr-1 nutrient-rich sediments were deposited mainly on the Egyptian flood plains before the AHD construction. At present only 2.1x106 t yr-1 are left in the Nile water to be deposited on Egyptian soils (Balba, 1979). The low sediment content (25 and 40 mg l-1) in the water downstream of the dam combined with more bank exposure due to low water levels, accelerated the Nile channel degradation. Field measurements over a period of 15 years after the AHD construction showed rates of river bed degradation between 2 and 5 cm yr-1 depending on the rate of decrease in water levels (Abu Zeid, 1987). Similar results were found by Kotob and Mottoleb (1981) after the first 12 years of the AHD operation, when annual bed degradation rates as high as 3 cm yr-1 were measured (Table 3).
In addition to the bed degradation, bank erosion was also observed along the river channel which was partially caused by local efforts for river regulation before closing the AHD (Abu-Zeid, 1897).
Observations since 1898 at the Nile delta and the littoral zone indicate active coastal erosion processes along the Mediterranean shore. Explained by recent hydrological changes in the Nile River regime, and the missing supply of suspended solids, the costal erosion was recently associated with a general subsidence (Frihy, 1998; Elraey et al., 1995; Stanley, 1996; Stanley and Wingerath, 1996; Stanley, 2000).
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Location downstream from AHD Distance from AHD Max. drop in river bed Max. drop in water level
[km] [cm] [cm]
Aswan Dam 6.5 12 58
Esna Barrage 165 25 76
Naga Hammadi Barrage 359 25 75
Assuit Barrage 539 2 55
Table 3. Maximum drop in river bed and water level downstream AHD between 1964 and 1978 (after Abu-Zeid, 1987).
Studies by Stanley and Wingerath (1996) have shown that clay-sized material (< 2 μm) is the major fraction transported from the lake behind AHD to the river below. Based on kaolinite tracer analyses, this material was found to be of eolian origin due to erosion of lake-margins and river banks.
3.5 Biogeochemical cycles
3.5.1 Nitrogen
Available data on carbon and nutrient cycles in the AHD Reservoir are quite inconsistent and cover only short temporal and spatial scales, which hamper balancing these biogeochemical cycles with reasonable accuracy.
The nutrient concentrations in AHD were reported to be higher in the southern part of the reservoir. Ahmed et al., (1989) found a general decrease of NO3 – N concentration towards the dam (Figure 5) when studying the Lake Nasser (Egypt) between 1982 and 1994. Exceptions form this trend were recorded during the summer 1982/1983 when high values up to 400 μg NO3 l-1 were measured 100 km south of the dam (Figure 5). On the S-N transect, the average concentrations were 61, 136, 284 and 289 μg NO3 l-1 for spring, summer, autumn and winter 1982, respectively whereas 261 and 286 μg NO3 l-1, respectively, were measured during the summer and winter 1983.
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The vertical distribution at a sampling site 10 km in front of the dam showed low nitrogen concentrations for early spring and late summer. The lowest value of 2 μg N l-1 and 8 μg N l-1 was observed in September 1982 in the surface layers (Ahmed et al. 1989). The reduction of N concentrations in the trophogenic zone down to 8 m in August and September 1982 was attributed to high rates of phytoplankton growth (Ahmed et al., 1989; Mohammed et al., 1989, Figure 6).
Figure 5. Seasonal local variations in nitrate - nitrogen concentration in Lake Nasser (Egypt) from spring 1982 to winter 1983/1984 (after Ahmed et al., 1989)
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Figure 6. Depth-time distribution of NO3-N and chlorophyll a concentration integrated over the water column down to a depth of 3 and 20 m in AHD at a station 10 km south of the dam (after Mohammed et al., 1989).
A close correlation between nitrate concentration and chlorophyll was found by Mohammed et al. (1989) (Figure 6). Low N-concentrations were postulated to limit the primary production for at least some algal genera or species (Mohammed et al., 1989).
A similar drop of nitrate-nitrogen down to 20 μg N l-1 limiting the growth of algae species was also reported for the Blue Nile during the maximum growth of the diatom Melosira (Rzoska and Talling, 1966).
Measurements carried out in during February 1970 in Lake Nasser close to the AHD showed irregular variations of nitrate concentrations from the surface to the bottom, ranging from a minimum of 280 μg N l-1 at 20 m depth to a maximum of 950 μg N l-1 at 10 m (Saad, 1980; Figure 7). Small amounts of nitrite were detected in Lake Nasser with
28
the vertical distribution fluctuating between a minimum of 20 μg Nl-1 at 50 m depth and a maximum of 42 μg NO2 l-1 at 30 m (Saad, 1980; Figure 7). Average concentrations over the entire water column were 670 μg N l-1 for nitrate and 30 μg N l-1 for nitrite, respectively.
Figure 7. Vertical distribution of nitrate, nitrite, phosphate (reactive and total), silicate and dissolved organic matter (DOM) in Lake Nasser measured during February 1979 (after Saad, 1980)
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Measurements of nitrite along the Nile River below the AHD form Aswan City to the Nile Delta (Figure 8) showed rather constant concentrations of 30 μg N l-1 between Aswan and Luxor followed by to a total depletion at the next stations and a slight increase up to 6 μg N l-1 at Rosetta. The dynamics of nitrate and nitrite indicate active processes of nitrification of ammonia from sewage input and denitrification of nitrate in suboxic zones in the stratified reservoir. Quite low nitrogen concentrations in the surface waters are a strong indication for primary production, while increasing nitrate concentrations in bottom waters are caused by mineralization processes (Saad, 1980).
The quality of available data allows only to calculate tentative scenarios of nitrogen uptake and release: If we assume that the decrease from 500 to about 60 μg N l-1 observed during the winters 1982/1983 and 1983/1984 (Figure 5) in the S-N transect in Lake Nasser (km 245 - km 10) was due to nitrogen uptake by mainly by phytoplankton and macrophytes, the annual biological nitrogen consumption in Lake Nasser would correspond to about 71’000 t N yr-1. With a molar ratio of 106 C:16 N, the equivalent primary production rate required to fix annually 71’000 t N yr-1 is 67 g C m-2 yr-1. This rate is found to be much lower than a minimum of 270 g C m-2 yr-1 characterizing the eutrophic systems. Also, if the concentration increase along the flow path by about 440 μg N l-1 in summer 1982 by 220 μg N l-1 one year later (Figure 5) was due to the mineralization of the organic matter, an average mineralization flux of 50’000 t N yr-1 or 70 % of the total nitrogen consumption could be estimated. In summary, the observed nitrogen dynamics points to a rather low primary production.
3.5.2 Phosphorus
Little is known about the phosphorus budget in the AHD. In general the PO4 concentration was described to have a spatial and temporal variability with higher concentrations of between 120 and 160 μg P l-1 reported for the southern part of the reservoir (Lake Nubia – Sudan) compared to 30 and 160 μg P l-1 for the northern part of in Lake Nasser (Rashid, 1995). The values were highest in August and November and lowest in February and increased with depth (Rashid, 1995). In February 1970,
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measurements in the Lake Nasser at a site close to the AHD showed PO4 values fluctuating between a minimum of 10 μg Pl-1 at 50 m depth, and a maximum of 90 μg P l-1 at 10 m (Saad, 1980; Figure 7). Total phosphorus profiles also showed considerable irregular variations between 60 μg P l-1 and 175 μg P l-1 (Saad, 1980). In general, the values of reactive phosphate found in most samples of Lake Nasser were much lower than those of non-reactive phosphate illustrating mineral origin of total phosphorus. Therefore, high concentration of reactive phosphate was attributed to the decomposition of organic matter and the release of absorbed phosphate. The average concentration of the reactive phosphate of 35 μg P l-1 was about 2.5 times lower than total phosphorus (Kanawy, 1974).
Some phosphate measurements along the Nile River below the AHD from Aswan City to the Nile Delta are shown in Figure 8. The values of reactive phosphate in the Nile water below the AHD ranged between a minimum of 4 μg Pl-1 at Suhag to a maximum of 40 μg P l-1 at Assyut. Reactive phosphate was depleted at the intermediate station of Luxor where the non-reactive phosphate contributed 100 % to the total phosphorus. During October 1988 and March 1990, a limnological study was conducted in Lake Nasser, Aswan Reservoir (a small water body between the old Aswan Dam and the AHD, 7 km to the south) and the Nile River north of the old Aswan Dam (Ali et al., 1995). The chemical parameters of the water bodies and hydro-soil samples are summarized in Table 4 and Table 5.
Lake Nasser Aswan Reservoir Nile River
Max. Min. Mean Max. Min. Mean Max. Min. Mean
Temp. (°C) 30.0 16.0 25.8 24.5 15.0 20.8 27.0 17.0 22.5
D.O. [mg l-1] 17.0 4.3 8.55 12.3 4.0 9.06 13.4 4.0 7.89
S.R.P. [μg P l-1] 391 0.0 52 260 0.0 39 114 0.0 42
NO3 [μg P l-1] 409 0.0 115 890 0.0 280 615 0.02 221
NO2 [μg P l-1] 9 0.0 3 64 0.0 15 18 0.0 6
Table 4. Maximum, minimum and mean values of temperature, dissolved oxygen (D.O.), soluble reactive phosphate (S.R.P.), nitrate and nitrite of Lake Nasser, Aswan Reservoir and River Nile (after Ali et al., 1995).
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3.5.3 Organic matter and silicate
A vertical distribution of the silicate and organic matter content in Lake Nasser is shown in Figure 7 and concentrations in the Nile River below the AHD are shown in Figure 8. The upper 40 m depth are characterized by a general concentration of 11.5 mg SiO2 l-1 with an increase up to 13 mg SiO2 l-1 at 50 m depth and a decrease to a minimum 10.2 mg SiO2 l-1 at 60 m depth. The values at 70 and 80 m amounted 12.3 and 11.5 mg SiO2 l-1, respectively. Similar to the Lake Mariut (Aleem and Samaan, 1969), vertical distribution of silicate was postulated to be influenced by the physicochemical conditions of the lake rather than by diatom consumption (Saad, 1980). Dissolved organic matter (DOM) content in Lake Nasser was found to increase from a minimum of 1.56 mg l-1 at the surface to a maximum of 10.6 mg l-1 at 30 m depth attributed mainly to the decomposition of the phytoplankton in the water column (Saad, 1980; Figure7). In general, a constant concentration of about 8 mg l-1 was measured below 40 m (Saad, 1980). It should be noticed, however, that the irregular trend in vertical distribution of nutrients in February was due to the absence of a clear thermal stratification as a result of cooling-induced mixing of the lake water during winter (Saad, 1980).
Dissolved organic matter and silicate, along the Nile River behind the AHD measured at Aswan, Luxor, Suhag, Assyut, Cairo, Rosetta and Damietta, (see Figure 1) are shown in Figure 8. The gradual increase from 2.8 mg l-1 at Aswan to 4.2 mg l-1 measured at Rosetta (Figure 8) was attributed to regional conditions such as phytoplankton abundance and surface runoff as well as sewage and industrial pollution (Saad, 1980). Low values were ascribed to coincide with the decrease in the autochthonous and allochthonous supply of organic matter as well as the increase in decomposition rate. The silicate content showed a gradual decrease along the Nile River from Aswan towards the outlets, from a maximum of 11.2 mg SiO2 l-1 at Aswan to a minimum of 3.2 mg SiO2 l-1 at Damietta (Saad, 1980). Inverse correlated to the chlorosity content, the decrease in the dissolved silicate towards the delta was attributed mainly to the uptake by diatoms and therefore, assumed that the diatom population gradually increases along the Nile below the AHD (Saad, 1980).
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Figure 8. Distribution of DOM, silicate, phosphate, nitrite and nitrate along the Nile River below the AHD to the Nile Delta (after Saad, 1980)
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Organic Matter PO4
[mg g-1] [μg P g-1]
Lake Nasser 19.3 0.58
Berba East 18.8 0.50
Kalabsha a 39.1 1.03
Kalabsha b 19.4 0.57
Turgumi 7.5 0.23
Amada 11.7 0.55
Aswan Reservoir 106 4.50
Awad Island 124.5 2.48
Nag Tongar 87.5 6.53
River Nile 51.9 0.28
West Bank 51.9 0.28
Table 5. Chemical composition of hydrosoils from River Nile, Aswan Reservoir and Lake Naser (after Ali et al., 1995).
3.5.4 Primary production
Large geomorphological and hydrodynamic differences in the extent of thermal stratification and the depth of photic zone were considered the main reasons for different ecosystem characteristics of the main channel of the AHD Reservoir compared to side bays (Abu-Zeid, 1987). In general, rates of biological production in the AHD Reservoir were estimated as high as 8-15 g O2 m-2 day-1 from diurnal changes in open water measurements in some side bays (Abu-Zeid, 1987). Similar, high rates of gross primary production between 5.23 and 13.2 g C m-2 day-1 were measured in March 1970 whereas, higher rates of between 10.7 and 16.4 g C m-2 day-1 were recorded in 1979 when the biologically activity zone of the lake extended down to about 4 m (Latif, 1984). However, an average of 10 g C m-2 day-1 or 3’650 g C m-2 yr-1 calculated from the above values represents an extremely high rate beyond the highest values measured in the lakes.
In order to estimate a realistic rate of primary production for the entire reservoir, some assumptions were made. The primary production corresponding to an average phosphorus concentration of 50 μg P l-1 (Table 4), typically varies between 150 and 250 g C m-2 yr-1. It can be considered that the reservoir consists of 5 % side bays (300 km2)
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where the production reaches high rates up to 3’600 g C m-2 yr-1, and the rest of 95 % main channel (5’700 km2) with an average production of 200 g C m-2 yr-1. According to this scenario, a weighted average primary production of 370 g C m-2 yr-1 can be calculated. Even suffering with large degree of uncertainty, above calculated primary production for the AHD varying between 200 and about 400 g m-2 yr-1 may represent a better estimate.
3.5.5 Greenhouse gas
Since the beginning of the 19th century, the anthropogenic emission of greenhouse gases to the atmosphere is considered to be responsible for a significant increase in radiative forcing leading to a global warming process. The current scientific concern is that under present rates of economic and population growth, the global mean temperature will rise by 3 °C by the end of this century accompanied by an increase of the global precipitation levels by 15 % (Harrison et al., 1989). Higher temperatures will lead to increased evaporation rates, and increased global precipitation will alter the river runoff depending on the regional climate and hydrology. Harrison and Whittington (2001) estimated the impact of potential climate change on hydropower production. They concluded that a temperature increase by 4.7 °C for the Nile catchment could result in a 22 % increase in precipitation whereas the hydropower production may decrease by 20 % due to more extreme hydrological events.
Carbon dioxide (CO2) as the major anthropogenic greenhouse gas increases by approximately 0.4 % in the atmosphere (Siegenthaler and Sarmiento, 1993). Its anthropogenic sources are primarily fossil fuel combustion and biomass burning processes. Methane (CH4) is the second most important greenhouse gas with sources in natural wetlands, irrigated rice paddies, cattle and artificial reservoirs. On a global scale, the CO2 and CH4 fluxes from man-made reservoirs contribute an estimated 4 and 20 %, respectively, to the total anthropogenic emissions (St. Louis et al., 2000). No references regarding the greenhouse gas emissions from the AHD reservoir were found in the literature.
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The greenhouse gas emission from reservoirs is a complex process involving at least four emission pathways: ebullition, diffusive flux, storage and flux through aquatic vegetation (Bastviken at al., 2004). Measurements of CH4 and CO2 emissions from hydroelectric reservoirs in northern Quebec with ages between 1 and 13 years indicated that processes in the water column such as oxidation and vertical advection of gases had a large effect on the C gas emission rates to the atmosphere. The emissions are not related to the type of the flooded ecosystem (Duchemin et al., 1995).
It was estimated that 70 % and 90 %, respectively, of the global CO2 and CH4 emissions from reservoirs of 3’500 mg m-2 d-1 CO2 and 300 mg m-2 d-1 CH4 originate from tropical reservoirs (St. Louis et al., 2000) although they account for only 40 % of the total reservoir area of 500’000 km2 (Galy-Lacaux et al., 1999). Even as upper boundaries, these fluxes may help to estimate the magnitude of the greenhouse gas emissions from the AHD. Therefore, with a surface area of 6’000 km2, the reservoir could release on the order of 7’660x103 t CO2 and 650x103 t CH4 per year.
A more realistic prediction on the greenhouse gas emission from the AHD can be obtained using the average primary production. With a rate of 370 g C m-2 yr-1 and a total surface area of 6’000 km2, the total in-situ carbon fixation within the reservoir would correspond annually to 2’200x103 t C yr-1. If we consider that 20 % of the organic carbon production is accumulated in the sediment of the reservoir (up to 440x103 t C yr-1) whereas the rest is degraded within the water column up to 20 % and 60 % is washed out of the system. Additionally, due to high sedimentation rate, half of the accumulated load may be buried in the sediment, therefore partially lost from the system, and the other half is available for anaerobic decomposition or oxidation. Following this scenario, out of a total 2’200x103 t C yr-1 produced in-situ within the reservoir, up to 220x103 t C yr-1 is annually available to be converted in the sediment into greenhouse gas. Decomposition of organic matter in the water column may contribute with up to 440x103 t C yr-1.
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3.5.6 Dissolved phosphorus balance
No literature data could be found on the nutrient inflow in the AHD. As they are important for predicting the range of the primary production in the new Merowe Reservoir, the phosphorus concentration can be roughly estimated from mass balance calculation. Considering that for an annual estimation the reservoir is a conservative system, the input must be balanced by the net sedimentation and the output.
For a primary production rate of 370 g C m-2 yr-1 or an equivalent P flux of almost 9 g P m-2 yr-1, the phosphorus uptake would represent 54x109 g P yr-1. Following the same scenario as for the greenhouse gas estimation, 20 % of the P uptake as 10.8x109 g P yr-1 can be considered to accumulate at the lake sediment, where half is retained by sedimentation and the other half is released back into the water column. Therefore, the net sedimentary P retention will be 5.4x109 g P yr-1.
It has been ascribed that prior the AHD construction, the Nile flood delivered annually to the Mediterranean coastal about 7.2 to 11.2x103 t (3.2x103 t in dissolved form and 4-8x103 t on sediment) of biologically-available phosphorus and 6.7x103 t inorganic nitrogen (Nixon, 2003). Low post-AHD discharges due to high nutrient retention in extremely productive lake behind the dam were estimated to be 0.03x103 t P yr-1 and 0.2x103 t N yr-1 (Nixon, 2003). Therefore, up to 3.2x103 t yr-1 P can be considered to represent the net P retention in the sediment of the AHD. This value is comparable with above estimated retention of 5.4x109 g P yr-1.
The outflow P concentration can be calculated from the average value reported for the small lake behind the AHD of 39 μg P l-1 (Table 4 – “Aswan Reservoir”). If the annual discharge at the AHD is 84 km3 yr-1, the output load would represent 3.3x109 g P yr-1. However, the output load can be actually much higher.
The mass balance can be approximated by the following equation:
Pinput – Pnet_retention – Poutput = 0
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Figure 9. Phosphorus mass balance for the AHD expressed in 109 g P yr-1.
Where: Pinput represent the inflow P load; Pnet_acc is the net P retention in the sediment; and Poutput is the P output load (Figure 9).
If all parameters are considered in the equation, the balance indicates an input load of 6.5x109 g P yr-1. For an inflow of 84 km3 yr-1, the incoming P concentration would correspond to ~ 77 μg P l-1.
3.6 Aquatic ecology
According to Latif (1984), the phytoplankton in the AHD Reservoir consists largely of:
(i) blue-green algae constituting 95 % in August 1976 and 81 % in October 1979 of the total phytoplankton;
(ii) (ii) diatoms with up to 66 % in November 1988 but only 2 %, respectively in August 1988;
(iii) (iii) green algae with up to 8 % in November 1988 and 3 % respectively in August 1988.
In the AHD the diatoms were dominant in cold winter months whereas the heat tolerant blue-green algae were found to be dominant in summer (Ahmed et al., 1989; Mohammed
38
et al., 1989). The general conclusion was that the diatom population is replaced by the green-algae as the temperature gradually rises, which in turn are succeeded by blue-green algae. Diatoms and blue-greens were the most dominant groups although their distribution and abundance were affected by thermal stratification and floods (Mohammed et al., 1989).
The undesirable blue-green algae species Cyanophyta, typical an impoundment organism, was found predominant in the northern sector of the AHD (Abu-Zeid, 1987). Capable of nitrogen fixation and representing a dead end in the food chain, they cause taste and odor problems (Abu-Zeid, 1987).
The zooplankton in Lake Nasser is described to belong to the common group of Copepoda, Cladocera and Rotifera (Rashid, 1995). In the main channel and side bays, Copepoda is the dominant zooplankton with 72 and 75 %, respectively, followed by Rotifera with 17% in main channel and 10 % in side bays and Cladocera with 11 % in the main channel and 15 % in side bays (Rashid, 1995).
The chironomid (benthos) fauna is rich in the shallow lacustrine areas of the lake. In the semi-riverine section their abundance decreases, and in the riverine section, the chironomids are replaced by ephemeropteran (mayflies) larvae whereas the bottom oligochaetas are replaced by bivalves (Rashid, 1995). The abundance of all mussels in Lake Nasser was massively reduced during the fist year of the reservoir formation as a result of oxygen-free bottom conditions. Even under low oxygen conditions, an increase in the biomass of oligochaetes was observed after 1973. This was possible due to their ability to withstand oxygen deficiency for several weeks or months, being able to utilize the high organic matter content of the sediment (Entz, 1980a).
The fish population of the AHD Reservoir is dominated by 9 families. A list of the families and the main species is given in Table 6. The fish abundance and distribution in AHD is described to vary among the different sectors of the reservoir and side-bays. Many factors play an important role in the fish population and density as the migration of
39
certain type of fish is dependent on the arrival of the turbid flood water, the preference of riverine or semi-riverine conditions, reproduction habitats and spawning or food and feeding habits. The fish food items in the AHD Reservoir are periphyton, phytoplankton and zooplankton, insects larvae (chironomids), gastropods, bivalves, juvenile fishes and fresh water shrimps (Rashid, 1995).
Family Species
1 Cichlidae Tilapia nilotica
Tilapia galilaea
Tilapia zilli
Oreochromis aureus
Astatotilapia sp.
Hemichromis letournexi
2 Centropomidae Lates niloticus
3 Characinidae Alestes nurse
Alestes baremose
Alestes dentex
Citorrhinus spp.
Distichodus spp.
Hydrocynus forskahlii
Hydrocynus lineatus
Hydrocynus brevis
4 Cyprinidae Barbus spp.
Barbus bynni
Labeo niloticus
Labeo coubie
Labeo horie
5 Bagridae Bagrus bayad
Bagrus docmac
6 Clariidae Heterobranchus bidorsalis
Clarias lazera
7 Schilbeidae Eutrophius niloticus
Schilbe mystus
Schilbe uranoscopus
8 Synodontidae Synodontis schall
Synodontis serratus
9 Mormyridae Mormyrus kannume
Mormyrus caschive
Mormyrus anguilloides
Petrocephalus bane
Table 6. The list of main fish families and species in the AHD Reservoir
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According to their feeding habits, the fish species in the AHD can be classified into four categories (Rashid, 1995) or more:
(a) Periphyton feeders: Labeo spp, Oreochromis niloticus, Oreochromis aureus and Sarotherodon galilaeus.
(b) Omnivores: Barbus spp. and schilbeides.
(c) Molluscivores: Synodontis spp and mormyrids
(d) Piscivores: Lates spp., Hydrocynus spp., Bagrus spp., Clarias spp. and Heterobranchus spp.
(e) Plankton feeders: Alestes spp.
(f) Macrophyte feeders: Tilapia zilii
Although fish may sometime change their feeding habits according to food availability, changes in the food web will certainly trigger major changes in the fish population, densities and distribution and species composition.
Different characteristics of reproduction and spawning behavior of each fish species and their ability to adapt to new created conditions is an important aspect controlling the fish dynamics and species composition. For example, species as Oreochromis niloticus, Sarotherodon galilaeus,, Hydrocynus forskahlii and Alestes nurse are spawning fractionally in most of the years whereas the other species spawn once or twice per year (Rashid, 1995). Even characterized by low fecundity and low mortality rate because of the parental care of up to 3 mm in diameter eggs (Latif and Rashid, 1972 and 1983), the fractional spawner Oreochromis niloticus is the most predominant species in fish landings in AHD contributing up to 70 % of the catch (Rashid, 1995). Its excellent growth of up to 55 cm, its preference for shallow near-shore waters and mouth-breeding habits has used to explain its perfect adaptation. Reaching a large size, Lates niloticus is another important contributor to the fish landing in the AHD ensuring his linger by producing several millions pelagic eggs of about 600 μm in diameter (Rashid, 1995).
Spawning of some cyprinids and characins species which live mainly in Lake Nubia (Sudan) is induced by the flood. The fishes move upstream beyond the Second Cataract
41
where the area of the reservoir is much narrower and the early flashes of the flood probably trigger their spawning process (Rashid, 1995).
Since 1965, the fishery in the eastern Mediterranean is suffering a great decline. The impoundment of the nutrient-rich floodwater at the AHD has been postulated as a main cause of this decline. The lack of nutrient-rich Nile sediment, deposited in the AHD Reservoir, has been ascribed to affect the sardine fishery and crustacean population in the eastern Mediterranean by 60 % (George, 1972). In contrast, an increase in fish population by a factor of 5 to 6 has been reported in Lake Nasser few years after the AHD construction (El-Hinnawi, 1980; George, 1972). The total fish landing in AHD of 34’000 t in 1981 was followed by a decrease to about 15’700 t in 1989. In 1990 the fish landing increase to 22’000 t, reaching in 1991 a total of 30’800 t. However, after the initial “bloom”, the fishing yield was predicted to stabilize as the environmental conditions of the lake are expected to reach a steady-state. Further, an increase in population of predator species and other factors (increase in fishery activities) were used to predict a remarkable decline in fishing yields in the next years (George, 1972).
The most important species in the fish landings in AHD are cichlidae with Oreochromis niloticus and Sarotherodon galilaeus forming about 90 % of the total fish landings (George, 1972; Rashid, 1995). Cyprinids Labeo nilotica and L. horie rank second and together with Barbus bunni formed 6 % (Rashid, 1995). The catfish Bagrus spp. and the large species Clarius lazera is the next rank contributor. The characins Alestes baremose, Alestes dentex and the tiger fish Hydrocynus spp., centropomids Lates niloticus, synodontids and schilbeids, close the list of predominant species (George, 1972; Rashid, 1995). It has been shown that seasonality plays an important role in fish landing with the period of March to April, which coincides with the peak spawning of Tilapia in Lake Nasser, being characterized by the highest fish landings.
Water stratification in the reservoir created by the Roseires Dam on the Blue Nile caused heavy fish mortality in 1967, when oxygen depletion affected temporally the entire
42
reservoir water body (El-Hinnawi, 1980). Also, the Nile oyster (Etheria eliptica) has been smothered by enormous quantities of silt deposited in the upper reaches of the lake.
3.7 Water-born diseases
The creation of AHD combined with changes in the downstream irrigation to a perennial system has caused an increased in the incidence of schistosomiasis (El-Hinnawi, 1980). Intermediate host of bilharziasis snails (Bulinus sp.) were reported to appear in the shallow littoral zones of AHD Reservoir in great number at the end of 1974 (Entz, 1980a). In 1942, Anopheles gambiae introduced malaria from Sudan in the area of the actual AHD resulting in about 100’000 deaths of which 10’000 occurred in Upper Egypt (George, 1972). Progressive inundation of the cultivated river valley with rich soils triggered a massive development of chironomid swarms (lake flies) in Lake Nasser within the first 10 years of its existence (Entz, 1980b). While the relatively calm water of the reservoir favors the spread of some diseases, the rapid water flow through dam sluices encouraged the breeding of a black fly (Simulium) carrying a human disease known as river blindness – onchocerciasis (George, 1972). Culex pipiens, the vector for filariasis was reported to be present along with the latter reported Phlebotomus spp., vector of the disease kalazar – leishmaniasis (George, 1972).
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4. Assessment of the Merowe Dam
In the following section, the experience from the AHD is extrapolated to the Merowe Dam by taking into account the similarities as well as the differences of the two reservoirs on the same river system. The environmental alterations deriving from the future dam construction are identified and the extent of the ecological impacts quantified and discussed in detail below.
4.1 Seismicity
Summarized by Adams (1983), the effects of man-made reservoirs to local seismicity have been scientifically recognized since middle of 1940 (Lake Mead in Colorado) with a wide public attention of induced seismicity by reservoir construction following the filling of the Lake Kremasta in Greece. Large earthquakes as in China (at Xinfengjiang 1962), Central Africa (at Kariba in 1963) and India at Konya in 1967 were reservoir-induced. The Konya earthquake had a magnitude of 6.5 on the Richter scale and caused more than 200 deaths. Analyses of more than 100 reservoirs, which presumably induced earthquakes, suggested that a minimum depth and volume were necessary for induction. A depth over 100 m was by far the most important factor for a seismic effect.
Based on the moderate size of the reservoir (maximum water depth 57 m), no reservoir-induced seismicity has been anticipated by the EIAR (2002) for the Merowe project although the project design adopted a Maximum Credible Earthquake of 6 on the Richter scale. However, the study of some reservoirs located in northern New Mexico and Brazil revealed that seismic activity was triggered even around small reservoirs with water depths of less than 50 m (Coelho, 1987; El-Hussain and Carpenter, 1990). Seismic activities extending up to some 20 km from the reservoir site suggest that geological conditions such as the existence of active faults and pre-reservoir groundwater elevation are very important for reservoir-induced seismicity (Coelho, 1987; El-Hussain and Carpenter, 1990).
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Even there are still many discussions concerning the processes that triggers seismic activities in man-made reservoirs, there are two basic mechanisms, to which most of the scientists agree: (i) the additional stress on the underlying formations caused by filling of the reservoir which is more related to the volume of the water in the reservoir rather than water levels; and (ii) the increase in pore water pressure along faults which depend upon the water levels in the reservoir above pre-reservoir groundwater levels rather than the volume of water (Abu Zeid, 1995). It is generally accepted that water weight or pressure cannot cause earthquakes in areas not subjected to previous seismic activity. In all reported cases of reservoir-induced seismicity, there were existing historic active faults in the area of the reservoir but only three documented chases have recorded seismicity greater than a 6 on the Richter scale (Abu Zeid, 1995).
The EIAR (2002) describes the Merowe area lying in an inter-plate region relatively stable with the tectonic situation “…quite complex but without significant implications for the project”. Their conclusion is based on the fact that the generally north-west to south-east faults of Tectonic Rift System do not extend to the area of the project. From the experience of AHD and some other cases, reservoir-induced seismicity may occur at the Merowe Reservoir within the first few years after the reservoir filling even though the maximum depth of the reservoir will not exceed 57 m. However, it is unlikely that the magnitude of a possible earthquake at Merowe will go beyond the Maximum Credible Earthquake design of 6 on the Richter scale. Therefore, reservoir-induced seismicity may not represent a major hazard for the Merowe Dam project. However, an updated seismic hazard assessment should be carried out before the dam construction to determine if all required measurements for the safety of the structure have been taken into account.
4.2 Hydrology
As in the other cases of control measures along the entire Nile, damming the river at Merowe will alter the hydrological regime eliminating the downstream annual flooding which flushed and cleansed the river once a year. The natural hydrological regime of the
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Nile River is characterized by wide annual and seasonal variation. A factor of almost four was found between the lowest flow of 42 km3 yr-1 recorded during the year 1913 and the highest runoff of 154 km3 yr-1 measured during 1978 (Abu Zeid, 1987; Aly et al., 1993). Seasonal variation in the flow regime as presented by EIAR (2002) show also large differences between the dry periods when the flow does not exceed 900 m3 s-1 and the flood periods between July and October with an average runoff of 7’400 m3 s-1 (annual peaks up to 10’000 m3 s-1, EIAR, 2002).
For the Merowe Reservoir, EIAR (2002) predicted an annual inflow of 84 km3 yr-1 corresponding to an average flow rate of 2’660 m3 s-1. The same flow of 84.7 km3 yr-1 was calculated by Roskar (2000) for the period 1900 to 1990 for the Aswan Reservoir. In 2004 at the 7th German-Arab Business Forum in Berlin, Egon Failer, director of the Hydropower and Water Resources Division from Lahmeyer International GmbH presented an annual flow of 65 km3 yr-1 (Failer, 2004). According to this author, the river inflow would vary between 700 m3 s-1 to a maximum of 5’600 m3 s-1 during flood periods. An average value of 2’055 m3 s-1 was predicted by Failer (2004) based on “long-term measurements over 50 years”.
Some hydrological parameters according to EIAR (2002) are presented in Table 6. Some parameters as the residence time, evaporation and reservoir filling rate were re-evaluated and the results are discussed below.
The reservoir volume represents 12.4 km3 with the seasonal storage capacity described as 8.3 km3. The daily dam operating rules are:
(i) on-peak production for 4 h at 3’000 m3 s-1 representing 43.2x106 m3 day-1 and;
(ii) off-peak production for 20 h at 600 m3 s-1 corresponding to 43.2x106 m3 day-1.
This represents an annual turbinated volume of 31.5 km3 yr-1. With this operation rules, the storage volume of the reservoir can be safely filled within 4 weeks during the wet season (average flood discharge of 6’400 m3 s-1) even if the power plant is running at the maximum capacity of 3’000 m3 s-1.
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Merowe Reservoir
Maximum reservoir area [km2] 800
Reservoir area at low stand [km2] 350
Agricultural area [km2] 66
Storage volume [km3] 12.4
Permanent volume [km3] 4.1
Volume change [km3] 8.3
Reservoir length [km] 200
Maximum depth [m] 57
Hydraulic head [m] 51
Average depth [m] 15.5
Water level change [m] 15
Residence time [yr] 0.15
Power generators [MW] 1250
Water balance [km3 yr-1]
Average annual inflow [m3 s-1] 2664 84.0
Average runoff flood (4 months) [m3 s-1] 6400 67.2
Average runoff dry season (8 months) [m3 s-1] 800 16.8
Max. runoff flood [m3 s-1] 7400 77.8
Max. runoff dry season [m3 s-1] 900 18.9
Average daily release [m3 s-1] 1000 31.5
Max. release flow (4 h day-1) [m3 s-1] 3000 15.8
Min. release flow (20 h day-1) [m3 s-1] 600 15.8
Average irrigation [m3 s-1] 233 7.3
Max. irrigation flow (8 months) [m3 s-1] 300 6.3
Min. irrigation flow (4 months) [m3 s-1] 100 1.0
Max. theoretical turbine flow [m3 s-1] 2498 31.5
Average evaporation [mm day-1] 6 1.75
Regime at low flow
Aver. runoff dry season (8 months) [m3 s-1] 800
Minimum turbinate [m3 s-1] 600
Reservoir filling rate [m3 s-1] 200
Max. time to fill empty reservoir [day] 480
Regime at high flow
Aver. runoff flood (4 months) [m3 s-1] 6400
Peak capacity [m3 s-1] 3000
Reservoir filling rate [m3 s-1] 3400
Min. time to fill empty reservoir [day] 28
Table 6. Our calculated parameters concerning Merowe Reservoir and its hydrology
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The stored volume can then feed an additional continuous runoff of 400 m3 s-1 during 8 months. If used only during four peak hours (1/6 of the day), the additional peak discharge is 2’400 m3 s-1. This means that the reservoir can operate as designed with 3’000 m3 s-1 during 4 peak hours and with 600 m3 s-1 during the rest of the day (20 hours). Critical questions regarding the hydrological regime and the dam operation rules target some subjects as:
(i) how often do very dry years occur and what are their discharge characteristics?
(ii) (ii) what is the probability that the reservoir cannot provide the 4 hours of peak power?
According to Roskar (2000), the trend of runoff has shown an average of 100.6 km3 yr-1 for the period 1871-1900 with 82.9, 84.4 and 87.1 km3 yr-1 being the averages for the subsequent 30-year time periods. The frequency analysis with the assumption that the natural flow will have the same periodic behavior in the future as during the past 128 years indicates a high probability that the flow will not be lower than 80 km3 yr-1 in the future and might slightly increase up to 95 km3 yr-1 around 2125 (Roskar, 2000).
4.3 Water losses
An experiment at the AHD employing increased upstream water consumption showed that it is possible to secure water supply for Egypt in the range of the current irrigation demand (55.6 km3 yr-1 based on the 1959 Nile Water Agreement) even if the upstream water consumption in Sudan would increase from the current 18.5 km3 yr-1 to 25 km3 yr-1 (Roskar, 2000).
The annual volume lost due to evaporation within the reservoir was considered by EIAR (2002) to be as high as 2.4 km3 yr-1 (Table at Page 2-1) or even smaller down to 1.9 km3 yr-1 (Page 4-5). An evaporation rate of 3 m yr-1 (8.22 mm day-1) as reported by EIAR (2002) may represent a slightly higher estimate compared with the most recent values of
48
2.08 m yr-1 to 2.3 m yr-1 calculated by Sadek et al. (1997) for Lake Nasser. Using these new data the evaporation may result in an annual water loss from the Merowe Reservoir of around 1.75 km3 yr-1 (±20 % error). Comparable with 1.9 km3 yr-1as calculated by EIAR (2002), this annual evaporation from the reservoir at the maximum level represents 14 % yr-1 of the total storage volume. Therefore, with a surface area more than 85 % smaller compared to the AHD, impounding the Nile River at Merowe will result in a water loss by evaporation of up to 2 % of the annual river inflow of 84 km3 yr-1. The annual precipitation in the area characterized by an average of 50 mm yr-1 would represent an additional input of 0.04 km3 yr-1. Corresponding to about 2 % of the evaporation, precipitation can be neglected in the total water balance.
The possible water use for irrigation is described in the EIAR (2002) as following: 2x150 m3 s-1 for 8 months and 2x50 m3 s-1 during 4 months. This represents 6.3 km3 yr-1 and 1.05 km3 yr-1, respectively, or a total of 7.4 km3 yr-1. Therefore, the water abstraction for agricultural use would reach a high value of up to 60 % yr-1 of the reservoir volume or 9 % of the annual river inflow. Except for the Multaga irrigation scheme for which the net irrigated area is described as 5’600 ha, no additional information is given by the EIAR (2002) concerning the total proposed irrigation scheme. At the end of January 2006, Yang Zhong, the Deputy Managing Director of the contractor firm, stated that “with the dam’s bulky reservoir, more than 60’000 hectares of farmland could be irrigated through the long sluices, benefiting more than 3 million Sudanese” (Xinhua, 2006).
Neglected by the EIAR (2002), another possible water loss due to increasing water level in the lake may come from infiltration to adjacent aquifers. The lateral seepage in Lake Nasser (Egypt) was calculated to reach annually 1 km3 accounting for 0.6 % yr-1 of the lake content (Aly et al., 1993). Extrapolating the estimated percentage to the Merowe Reservoir, the lateral seepage may contribute to the total water loss with 0.07 km3 yr-1. Even being as twice as high as the contribution from annual precipitation, the seepage is rather small for the total water balance. According to our calculations, the total water losses from the Merove Reservoir including evaporation, irrigation and seepage of 9.2 km3 yr-1 represents 11 % of the annual Nile River inflow.
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4.4 Reservoir water balance
Based on previous calculated parameters, a zero-order water balance is computed below and the results are schematically shown in Figure 10. The inputs into the reservoir are represented by:
QIN - average inflow of 84 km3 yr-1;
P - annual precipitation which contribute insignificantly with only 0.04 km3 yr-1 for the maximum reservoir area of 800 km2.
The outputs are:
QT - the annual turbinated flow of 31.5 km3 yr-1;
E - annual evaporation of 1.75 km3 yr-1 at the maximum surface area of 800 km2;
I - the water abstraction for irrigation of 7.4 km3 yr-1;
S - lateral seepage of only 0.07 km3 yr-1;
According to the Nile Water Agreement of 1959, Sudan has the right to use 18.5 km3 yr-1 from an average flow of 84 km3 yr-1 and must ensure annually a minimum release of 55.5 km3 yr-1 downstream to Egypt, while 10 km3 is assumed as annual loss via evaporation (Roskar, 2000). Therefore, during the dam operation, additionally to the turbinated flow of 31.5 km3 yr-1, a supplementary flow (QS) of minimum 24 km3 yr-1 must be released downstream.
During the impounding period, ensuring a minimum release downstream of 55.5 km3 yr-1, and neglecting the evaporation and irrigation needs, the reservoir can gain annually 28.5 km3, reaching its maximum storage capacity of 12.4 km3 in 159 days (5 months).
84 km3 yr-1 – 55.5 km3 yr-1 = 28.5 km3 yr-1
The length of the impounding period depends mainly on the starting time. Generally, the dam operation policy is to diminish the impounding phase as much as possible in order to
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produce electric power as soon as possible. Therefore, in order to establish the optimum period, a simulation of the reservoir impounding for different starting dates must be performed. The absence of sufficient hydrological data restrains us to run such simulation. It is known that about 80 % of the total annual discharge of the River Nile occurs during the summer rainy season from July to October. Therefore, during the 4 months period, if no significant water for irrigation is required, the reservoir can fill up to 95 % of its total storage capacity.
After impounding, if all the parameters are considered, the changes in the water volume (ΔV) over a period of time (ΔT) are dictated by the following equation:
ΔV/ΔT = QIN + P – QT – I – E – S – QS
According to our calculation, even at full energy production and securing a minimum downstream flow of 55.5 km3 yr-1, the reservoir can store annually up to 19 km3, representing more that 150 % of its capacity. This implies that the reservoir can operate at the maximum capacity even for a smaller water inflow as low as 65 km3 yr-1. This allow, on a regular base, for additional upstream water storage, increased irrigation scheme or mitigate drought periods.
Figure 10. The water balance for the Merowe Reservoir
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4.5 Water quality
The EIAR (2002) states that due to the seasonal water level variations, the area of the reservoir will fluctuate between a maximum of 800 km2 at the highest level and 350 km2 at low stand. Therefore, water level fluctuations may expose during the dry season up to 450 km2 of reservoir slops to soil-forming conditions which oxidizes and rapidly degrades previous formed lacustrian organic matter (Beuning et al., 1997). Consequently, the operation schedule will have direct influences on the texture and sediment composition. It is known that both sediment composition and water conditions play an important role in controlling the production and distribution of aquatic plants. The submerged aquatic plants exploit nutrient sources from the surrounding water and sediment. Therefore, nutrient availability from these sources will influence directly the submerged plant community and abundance on the disturbed littoral zone. Overall, this may have a substantial environmental impact on the aquatic life
A common effect of water storage due to river impoundment in arid and semi-arid areas is the onset of thermal stratification of the reservoir water column. Thermal stratification in natural lakes depend on external driving forces as hydro-meteorological conditions, location, wind induced surface forces, etc and internal properties such as lake morphometry (surface, shape depth), light absorption and the theoretical water residence time, function of reservoir volume and flow. The variations in the water level are important for the mixing of the lake water column and the distribution throughout the reservoir of the water during the seasonal flood. The extent of the flood and the penetration-depth distribution will determine the general pattern of thermal stratification.
Stratification of the water column is also expected in the Merowe Reservoir. An empirical dependence of reservoir stratification on residence time (RT) expressing the temperature difference (ΔT0-30) between the surface and the deeper layers at 30 m was found by Straskraba and Mauersberg (198 for several reservoirs in Czech Republic. This is approximated by the equation:
ΔT0-30 = 20 (1-exp (-0.0126 * RT IV-IX))
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| العنوان |
الكاتب |
Date |
| عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 03-30-06, 02:39 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | على محمد على بشير | 03-30-06, 02:57 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Tragie Mustafa | 03-30-06, 03:18 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 03-30-06, 04:12 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | على محمد على بشير | 03-30-06, 04:23 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | على محمد على بشير | 03-30-06, 04:27 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | كبسيبة | 03-30-06, 03:56 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | abdalla BABIKER | 03-30-06, 05:08 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | قلقو | 03-30-06, 05:57 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Hani Arabi Mohamed | 03-30-06, 06:21 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | محمد مختار جعفر | 03-30-06, 07:42 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 03-30-06, 10:15 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | الأمين عثمان صديق محمد | 03-30-06, 10:29 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 03-30-06, 12:16 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | كبسيبة | 03-30-06, 01:30 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 03-30-06, 01:55 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Mannan | 03-30-06, 06:09 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Elmoiz Abunura | 03-30-06, 11:07 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | حسن الجيلى سعيد | 03-31-06, 03:02 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 03-31-06, 03:56 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | wadaldeem | 03-31-06, 11:36 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 03-31-06, 04:18 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Mannan | 04-01-06, 02:21 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 04-01-06, 03:13 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | wadaldeem | 04-01-06, 08:09 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | عبد الوهاب المحسى | 04-01-06, 09:49 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | عبد الوهاب المحسى | 04-01-06, 10:11 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Tragie Mustafa | 04-01-06, 01:48 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 04-02-06, 03:29 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | عبد الوهاب المحسى | 04-02-06, 06:16 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | معتصم الطاهر | 04-02-06, 06:45 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 04-02-06, 04:43 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | عبد الوهاب المحسى | 04-02-06, 06:21 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | عبد الوهاب المحسى | 04-02-06, 06:23 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | عبد الوهاب المحسى | 04-02-06, 06:41 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 04-03-06, 02:06 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | banadieha | 04-03-06, 02:44 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | banadieha | 04-03-06, 04:02 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | عادل فيصل راسخ | 04-03-06, 06:29 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 04-03-06, 09:15 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | مهيرة | 04-03-06, 09:58 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | عبد الوهاب المحسى | 04-03-06, 03:08 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 04-04-06, 03:28 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 04-04-06, 09:34 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | عبد الوهاب المحسى | 04-06-06, 05:04 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | adil amin | 04-07-06, 10:06 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 04-15-06, 05:57 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 04-16-06, 05:16 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 04-17-06, 04:56 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | عبد الوهاب المحسى | 04-20-06, 03:14 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 04-21-06, 09:58 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 04-22-06, 09:43 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 04-23-06, 12:40 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 04-24-06, 12:49 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Nazar Yousif | 05-05-06, 05:04 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | adil amin | 05-05-06, 10:05 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | الأمين عثمان صديق محمد | 05-05-06, 05:08 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 05-06-06, 12:34 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 05-05-06, 06:19 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Suad I. Ahmed | 05-06-06, 01:34 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 05-07-06, 11:05 AM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 05-09-06, 04:28 PM |
| Re: عاجل : شبكة الانهار الدوليه تطالب بوقف العمل في خزان مروي | Asskouri | 05-22-06, 05:42 AM |
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عبد الوهاب المحسى
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انستقرام
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