Overview of Sediment Problems in Nile Basin

Abdalla A. Ahmed UNESCO Chair in Water Resources, P.O Box 1244, Khartoum, Sudan,

basharke@hotmail.com

V. Vanaker Flemish Counterpart of STWMC, Belgium, vanacker@geog.ucl.ac.be

Usama H. Ismael

Theme Researcher of DLFAC, UNESCO Chair in Water Resources, Sudan, uhamid@talk21.com

Al-Weshah R.

UNESCO Cairo Office, P.O. Box 11541, Cairo, Egypt, r.weshah@mail.unesco.org.eg Abstract Throughout geological times, natural processes of erosion, transport and deposition of sediment have shaped the landscape in fundamental ways. Erosion often causes severe damage to agricultural land by reducing the natural soil fertility and agronomic productivity. Besides, eroded soil is the largest pollutant of surface waters in the world. The largest source of sediment in the Nile Basin is located in the Ethiopian Highlands where 85% of the Nile water comes from. The soil that is eroded from the Ethiopian highlands creates serious problems in the operation and maintenance and sustainability of irrigation canals and large reservoirs constructed along the Nile (e. g Roseries, Sennar, Girba and Aswan High Dam). The large deposition of sediment in the reservoirs strongly reduces the lifetime of the reservoirs by reducing its water capacity. Besides, the clear water that is released from the reservoirs often induces erosion of the channel banks and channel bed (e. g. erosion downstream of Aswan High Dam). Furthermore, large sediment problems are also encountered in most catchments around the Victoria Lake. The large quantity of sediment that is eroded from the highlands and is transported by the rivers, often deposits in the flat area around the Victoria Lake. The problems associated with this deposition of sediment are varied. The sediment deposits reduce the flood carrying capacity of the stream channels, which often results in greater flood damage to adjacent properties. In this paper an overview of the distribution of sediment production, transport and deposition within the Nile Basin is highlighted. The sediment issue and the associated problems within

Fig (1) General Layout of the Nile

the Nile Bain will be discussed taking into consideration its socio-economic and environmental impacts. The sources of the sediment and the transport media will be explained. Moreover, the sediment deposition in quantity and distribution within the Nile Basin will be examined. Mitigation strategies and measures of sedimentation will be reported benefiting from the experience of the FRIEND/ NILE project in the Nile Basin. Keywords: Sediment production, watershed, Nile Basin, socio – economic, environment, FRIEND/ Nile Project, mitigation measures. Introduction Water is the vital resource to support all forms of life on earth. It is not evenly distributed over the world by season or location. Some parts of the world are prone to drought making water a scarce and precious commodity, while in other parts of the world it appears in raging torrents causing floods and loss of life and property. Water remains essential for mankind survival and the future development of the world. The population of the world continues to increase by 100 million people each year, so does the demand for water. Environmental pollution and water contamination make the situation very critical. The sediment is considered one of the main water pollution factors. The Nile River extends for some 6700 km through much of the northern Africa Fig (1). The main branches of the Nile River are: Blue Nile, White Nile and Atbara rivers. The White Nile originates from the equatorial lakes plateau, where the main lake is Victoria Lake. Fig (2) shows the longitudinal section of the Nile River from Lake Victoria to the drain in the Mediterranean Sea. Wu et al. (2005) stated that "although the Nile River south of Aswan is currently one of the least development of the major international rivers of the world, the river system offers numerous opportunities for development that would facilitate the management of Nile water". If the latter statement is true, what are the difficulties and constraints facing the achievement of such goals? On the other hand, how can we implement these projects taking into consideration the financial, environmental and ecological difficulties? In this paper one of the major problems regarding the sediment management within the Nile Basin is going to be examined. This issue has its implication on the Nile water resources utilization, starting from the watershed degradation, through the sediment deposition on the reservoir to the irrigation management difficulties in the canalization networks. The paper also gives an overview of the sediment problems in the Nile Basin and its impacts on the socio-economic and environment development. The source, transport and deposition

of the sediment throughout the Nile River are highlighted. Mitigation measures regarding the sediment management in Nile Basin are attempted. Four years of research work within the framework of the FRIEND/NILE project reveal a good experience on the behavior of the sediment in Nile River, which will be reflected in this paper. Water Flow Through The Nile Catchment The entire catchment of the Nile River comprises a catchment of 8.889.534 km2, and is draining 10 countries in North, Central and East Africa, Table (1). The catchment can be divided into three distinct regions according to the provenance of the water and its hydrological regime (Ahmed and El Daw, 2004) Fig (3). Fig (4) shows the schematic diagram of the natural flow of the Nile River according to the 1959 Agreement, and Fig (5) gives the hydrographs of different watersheds within the Nile River system. As the Nile River receives no effective inflow from the confluence of the Atbara River north to the Mediterranean, the three main watersheds are defined as follows: (i) the Eastern watershed draining the Ethiopian Highlands, (ii) the Central watershed draining the Sudd region and (iii) the Southern watershed draining the Equatorial Lakes plateau. The southern watershed is draining the area around the Equatorial Lakes, of which Lake Victoria is the largest one. The total estimated annual inflow entering Lake Victoria from stream flow and rainfall is 118 billion m3 while the evaporation is estimated to be 94.5 billion m3, leaving only 23.5 billion m3 to flow down the Nile River. About 86% of total water input falls as rain, and the average annual rainfall in the lake area ranges between 886 mm to 2609 mm. Evaporation losses account for 80% of the water leaving the lake.

Fig (2) Cross-section of the Nile River from Equatorial Source to Drain

Table (1) Nile Basin: Areas and Rainfall by Country

(Source: Ahmed and ElDaw, 2004)

Country Total area of the country

(km2)

Area of country within the basin

(km2)

As % of total area of basin

(%)

Average annual rainfall in the basin area (mm)

Min. Max. Mean Burundi 27 834 13 260 0.4 895 1 570 1 110 Rwanda 26 340 19 876 0.6 840 1 935 1 105

Tanzania 945 090 84 200 2.7 625 1 630 1 015 Kenya 580 370 46 229 1.5 505 1 790 1 260 Zaire 2 344 860 22 143 0.7 875 1 915 1 245

Uganda 235 880 231 366 7.4 395 2 060 1 140

Ethiopia 1 100 010 365 117 11.7 205 2 010 1 125

Eritrea 121 890 24 921 0.8 240 665 520

Sudan 2 505 810 1 978 506 63.6 0 1 610 500

Egypt 1 001 450 326 751 10.5 0 120 15 Nile

basin 8889 534 3 112 369 100.0 0 2 060 615

Fig (3) Nile River Basin Three Main Watersheds

Eastern Main Watershed

Central Watershed

Southern Watershed

The catchment area of Lake Victoria is divided between five African countries: Kenya, Tanzania, Uganda, Rwanda and Burundi. The main rivers flowing to the lake are Mara, Kagera, Mirongo, Grumeti, Mbalageti, Simiyu and Mori in Tanzania; Nzoia, Sio, Yala, Nyando, Kibos, Sondu-miriu, Kuja, Migori, Riaria and Mawa in Kenya and Kagera, Bukora, Katonga and Sio in Uganda. The Kagera, which drains from Burundi and Rwanda and part of Uganda, is the single largest river flowing into the lake. However, rivers entering the lake from Kenya, which contains the smallest portion of the lake, contribute over 37.6 % of surface water inflows. For the FRIEND/Nile project, the Tanzanian and Kenyan counterparts selected the Simiyu and Sondu rivers respectively as case-studies.

Fig (4) Schematic Diagram of the Nile River Natural Flow

10 10

Hydrograph of the Nile System

Fig (5) Nile River Hydrograph

As the Nile River receives no effective inflow from the confluence of the Atbara River north to the Mediterranean, the three main watersheds are defined as follows: (i) the Eastern watershed draining the Ethiopian Highlands, (ii) the Central watershed draining the Sudd region and (iii) the Southern watershed draining the Equatorial Lakes plateau. The southern watershed is draining the area around the Equatorial Lakes, of which Lake Victoria is the largest one. The total estimated annual inflow entering Lake Victoria from stream flow and rainfall is 118 billion m3 while the evaporation is estimated to be 94.5 billion m3, leaving only 23.5 billion m3 to flow down the Nile River. About 86% of total water input falls as rain, and the average annual rainfall in the lake area ranges between 886 mm to 2609 mm. Evaporation losses account for 80% of the water leaving the lake. The Nile River is the only surface outlet from the lake, with an outflow of annually 23.5 Billion m3. The waters originating from Lake Victoria provide hydropower through its only outlet, the Nile River, at Owen Falls in Uganda and other power plants downstream. The power from the two plants at Owen Falls provide 260 Mw, part of which is exported to Kenya. Hence, the water that is draining the Equatorial Lakes enters the swamp area (Sudd region) in Southern Sudan. In this region, several other rivers also contribute water and sediment to the Nile River. This region was recognized for the first time by Ahmed and El Daw (2004) to play an important role for future development in the Nile Basin, and was defined as the Central Watershed. In the Sudd region inside Sudan, evaporation losses are estimated to be around 33.9 billion m3, leaving 15 billion m3 to flow into the White Nile. The White Nile, which transports water and sediment from the Southern and the Central Watershed, is well regulated due to the relatively constant contribution of water to the Nile River from the Equatorial lakes and the swamps of the Sudd. This is in strong contrast to the rivers draining the Ethiopian Highlands: the Sobat, Atbara and Blue Nile. The Blue Nile (called Abay River in Ethiopia) starts from Tana Lake (the largest lake in Ethiopia). It is 84 km long, 66 km wide and 15 m depth. On a yearly basis, the Sobat River, the Blue Nile and the Atbara River contribute respectively 13.5 billion m3, 54 billion m3, and 12 billion m3. The flow of these rivers (Sobat, Atbara and Blue Nile) is highly concentrated in the period from July to October, where they contribute approximately 85% of the total Nile River flow. On the contrary, these rivers from the Ethiopian Highlands have very low flows in the period from January through June and the Atbara River is normally dry during that period. Further downstream of the confluence of the Nile River with the Atbara River, there is no effective inflow anymore. The losses due to evaporation are estimated to be about 7.0 billion m3 within Sudan downstream Malakal. And in the High Aswan Dam reservoir, the losses are estimated to be 10.5 billion m 3. Nile Watersheds Degradation and Sedimentation It is widely reported that land degradation occurs in the three watersheds areas in the Nile Basin, Fig (3). It is well known, the land degradation in watersheds starts in high altitude, where many factors particularly deforestation results in soil erosion. In Ethiopian Highlands as well as in the Equatorial Lakes usually the watersheds face growing stress from economic development, increasing human populations and often wasteful use of natural resources like wood cutting for more housing, mining, timber for energy needs, over grazing, …etc.

Watersheds degradation is believed to be the main cause of sedimentation in general and in the Nile Basin system in particular. The causes of sediment in the watershed can be summarized as follows:-

- Removal of forests or other vegetation sharply reduces water retention and increases erosions resulting in reduced water availability in dry seasons and more sedimentation downstream.

- Absence of trees provides bad effects on shrubs which lost shelters and some times die out under burning sun.

- Charges in river flow, sediment and pollutant loadings resulting from activities for inland degrade downstream ecosystem. Dams are the worst hit by sedimentation.

The Nile Basin water sheds are seriously suffering accelerated soil erosion. This means that valuable soil nutrients are lost from the land, where they are needed, deposited in the water system and ultimately the lakes e.g. Victoria Lake. Sediment Process The process of sedimentation usually happens in the following stages:-

i- Erosion ii- Entrainment (drawing of particles into fluid) iii- Transportation

iv- Compaction (deposition). The processes are highly complex. The detachment of particles in the erosion process occurs through the kinetic energy of raindrop impact, or by flowing water. Once a particle has been eroded it must entrain before it can be transported away. Both entrainment and transport depend heavily upon the weight, shape, size and forces exerted on the particles by the flow. Deposition occurs when the forces are diminished enough leading to a reduction or cessation of transport. Therefore, deposition is the counterpart of erosion, for example, when river flow enters a reservoir, its velocity and transport capacity are reduced and its sediment load is eventually deposited. The amount and rate of deposit are determined mainly by:-

- Detention storage time - The shape of the reservoir - The operating procedure of the reservoir

The depositional pattern usually starts with the coarser material depositing towards the reservoir headwater. The aggradations continues more and more until a delta is formed, as it happens in Roseires reservoir in the Blue Nile and Aswan High Dam reservoir in the Main Nile.

Fig (6) Total Annual Sediment Load (million tons) in the Nile River

Nile River Sediment Most of the sediment in the Nile flows from the Ethiopian Highlands through the Blue Nile and Atbara River. The White Nile and its tributaries lose most of its sediment load by spilling and deposition over flood plains, lakes and marshlands inside Sudan. Nearly all of the sediment (~ 90%) comes from the Blue Nile during the flood season (July - Oct.) .El Monshid et al (1997) estimated the sediment load of the Blue Nile at El Diem ( the entrance of the river to Sudan ) to be 140 million tons per year. At the same time their estimation of the sediment load at Aswan High Dam was 160 million tons taking into consideration the amount of sediment transported by Atbara River. This is compared to 150 Million tons for Mississippi River, 250 Million tons the Colorado River and 2000 Million tons for Yellow River in China. On the other hand there is no reliable means of bed load information in the Nile River. However, the bed load is believed to be negligible. It was estimated by Hurst et al, (1978) to be 25% of the total sediment load. The authors of this paper believe what has been reported by Hurst et al is exaggerated and far from the reality, the bed load may be less than 10 %. The coarser sand usually deposits in the upper portion of the Blue Nile near the Ethiopian/Sudan boarder, while the lighter sediment is carried by the flow downstream. The suspended sediment load distribution is 30% clay (<00.002 mm), 40% silt (0.002 – 0.02 mm) and 30% fire sand (0.02 – 0.2 mm). Therefore, it may be considered in general wash load. Fig (6) compares the average amount of sediment load in the different tributaries of the Nile Basin. Fig (7) shows the sediment concentration throughout the year at Aswan High Dam. It is clear that the peak of the sediment concentration falls within the period August-September. It is important to clarify and estimate the transport of sediment throughout the entire Nile Basin system in order to implement integrated sediment control and management system. Using different techniques and methods the FRIEND/Nile programme attempted to quantify and qualify the sediment in the Nile Basin for better and sustainable management. Figs (8,9) compare the discharge, the total amount of rainfall in the waters hed and the sediment concentration for the Blue Nile and Atbara River. The peak of the average discharge is (513 million m3 per day), comes after the peak of the sediment (5660 ppm) by about three weeks, while the peak of the total rainfall (2123 million m3 per day) comes one week before the peak of the discharge. This sequence of incidents from the technical point of view is quite sound, systematical and logical.

It is attributed to the fact that at the beginning of the rain falls the top soil in the catchment area is bare and loose and easy to be carried out by the runoff into the river channel. The time lag between these peaks depends totally on the rainfall intensity, duration and the condition of the catchment area. Sediment starts to increase from the early June until they reach their peak towards the end of August and beginning of September and hence decline towards the end of the rainy season after October. On the other hand, if we consider the water discharges measured at El Deim at the boarder of Sudan with Ethiopian (just after the Blue Nile leaves Ethiopian to Sudan), as runoffs of the Blue Nile catchment (watershed), the total average runoff coefficient is about 20% while the total average runoff coefficient for Atbara river is about 14%. Although it is difficult to verify these figures, however, our feeling they are a bit low, since the calculations are complex and may encounter many errors. Nevertheless, the Blue Nile runoff coefficient is higher than Atba ra River one. This can be attributed to several reasons:

• Atbara River catchment area is northern of the Blue Nile one; therefore, it is drier with less green cover.

• Higher sediment concentration in Atbara River compared to the Blue Nile.

• The Blue Nile contributes to the flow of the Nile system with 65% while Atbara River is contributing with only 16%..

0 . 0 0

1 0 . 0 0

2 0 . 0 0

3 0 . 0 0

4 0 . 0 0

5 0 . 0 0

6 0 . 0 0

J A N F E B . M A R . A P R . M A Y J U N . J U L A U G . S E P . O C T . N O V . D E C .

M o n t h sSe

dim

ent C

once

ntra

tion

(mlg

/litr

e)

Fig (7) Suspended Sediment Concentration in AHD

Fig (8) Comparison of rainfall, Discharge and Sediment

Fig (9) Comparison of Rainfall, Discharge and Sediment Yield in the

These low runoff coefficients are indicating how serious the problem is, and how difficult it is. The deforestation and the degradation of the Ethiopian Highlands have their clear negative impact on the whole system. In Sudan this, where these rivers run, situation is even worse. The problem has been reflected downstream in terms of sediment deposition in the reservoirs and the irrigation canalization networ ks, causing flood risks, crops damage, pumps intakes blockage, low production and hydropower generation difficulties, Fig(10). The rate of sedimentation in that period had never exceeded ½ % per year (4.6 million m3) with respect to the original capacity. Therefore, only 28% reduction in the reservoir capacity in that very long period of (56 years) occurred. On the contrary, the followed period (1981-1986) the sedimentation increased drastically with a rate of 80 million m3 per year (9½%) i.e. a reduction of 400 million m³ (43%) in only 5 years. However, Sennar dam in 61 years lost 71% of its original reservoir capacity (660 million m3). In addition to the increase of sediment load, in general, in the last two decades, however, the main reason behind the bad situation in the above period (1981-1986) is the change of the operational rules to satisfy the irrigation requirements, for the agricultural schemes, upstream and downstream the dam. Now Sennar reservoir is no longer used to store water, but to regulate the river flow and to generate hydropower from a limited capacity station (15Mw). Fig (11) shows the sediment volume and the content of the Roseires reservoir in the period (1964-1992). In the first ten years the drop in the capacity was 550 millions m3 with a rate of 55 Mm3 per year. In the second period (1976-1981) the reduction in the capacity was 100 Mm3 with a rate of 20 Mm3 per year. In the period (1981-1985) the reduction in the capacity was 120 Mm3 with a rate of 30 Mm3 per year. However, a drastic increase in the sedimentation rate occurred in the period (1985-1992) with a rate of 60 Mm3 per year and a reduction of 427 Mm3. This may be attributed to the drought period which took place during the eighties from the last century and the policy of the food security adopted in Sudan by increasing the irrigated area without taking any mitigation measures regarding the negative consequences on the reservoirs and the irrigation canal networks. Furthermore, the intensive irrigation during the period of the high sediment content (July-August). Plates (1, 2) show the magnitude of sediment and debris problem in front of Roseires dam.

Fig (11) Sediment Volume and Content of Roseires Dam

Fig( 10) Sediment Volume and Content of Sennar Dam

Sediment Socio -economic and Environment Impacts Sediment is socio-economic, environmental and geomorphologic resources, as well as, a tool of nature. However, changes in sediment quantity and quality can have a significant impact on a range of social, economic and environmental systems. The deposition of sediment in irrigation canals and its subsequent built-up of aquatic weeds results in losses in production of great magnitude. On the other hand, the cost of sedimentation includes loss of hydropower potential since method of sediment removal involves measures that lower the head and interfere with generator operation. The most serious effect, however, is the loss of agricultural production. For example in Sudan the sediment clearance from the irrigation canalization system costs more than 60% of the total cost of the operation and maintenance. Conclusion No doubt the sedimentation in the reservoir and the irrigation systems within the Nile Basin has environmental and socio-economic impacts. Therefore, suitable sedimentation management is a key for the sustainable water resources management. The total sediment load annually in the Nile Basin reaches Aswan High Dam ranges between 140 and 160 million tons. Conversely, changes in human activities within the catchment can have detrimental effects on both sediment quantity and quality. Sediment is socio-economic, environmental and geomorphologic resources, as well as a tool of nature. However, changes in sediment quantity and quality can have a significant impact on a range of social, economic and environmental systems. Neglecting to manage sediment in a sustainable way, either by a back of adequate sediment management strategies, or the cursory induction of sediment in generic policy and legislation, can result in costs to both society and environment. It is very important to evaluate environmental impacts involved in sediment management properly and mitigate them as much as possible. It can be concluded that sedimentation rate in the last decade (1990s) increased rapidly, indicating that huge and wide land degradation is

Plate (1) Delta Formation Upstream, Roseries Dam

The Data

Dam Wall

Plate (2) Debris in the Reservoir Upstream, Roseries Dam

Debris

Dam Wall

occurring in the catchment area of the Nile River system. Therefore, integrated sediment management is found to be the best policy to minimize the adverse impacts of the sedimentation within the entire Nile River Basin. Acknowledgments This paper was prepared based on the research activities of the FRIEND/Nile Project which is funded by the Flemish Government of Belgium through the Flanders-UNESCO Science Trust Fund cooperation and executed by UNESCO Cairo Office. The authors would like to express their great appreciation to the Flemish Government of Belgium, the Flemish experts and universities for their financial and technical support to the project. The authors are indebted to UNESCO Cairo Office, the FRIEND/Nile Project management team, overall coordinator, thematic coordinators, themes researchers and the implementing institutes in the Nile countries for the successful execution and smooth implementation of the project. Thanks are also due to UNESCO Offices in Nairobi, Dar Es Salaam and Addis Ababa for their efforts to facilitate the implementation of the FRIEND/Nile activities. References

El Monshid, B.E.F., El Awad, O.M.A. and Ahmed, S.E., 1977 " Environmental effect of the Blue Nile Sediment on reservoirs and Irrigation Canals" , Int. 5th Nile 2002 Conf. , Addis Ababa, Ethiopia . Hurst, Black and Simaika, 1978, "The Nile Basin" Vol. XI Egyptian Government Press, Cairo, Egypt. Ahmed, A.A. (Coordinator and Editor), 2004, "Sediment Transport and Watershed Management" Annual Report, FRIEND/ Nile P roject, Khartoum, Sudan. Ahmed A. A., El Daw A.K, 2004, ”An Overview on Cooperation of Trans-Boundary water: Case of the Nile River Basin, the 2nd Regional Arab water conf, Cairo, Egypt. Wu X., Whittington D. and Sadoff G., 2005 " Water Resources Management in the Nile Basin: The Economic Value of Cooperation ", Nile Basin Report.