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The influence of man on the hydrological regime with special reference to representative and experimental basins — L'influence de l'homme sur le régime hydrologique avec référence particulière aux études sur les bassins représentatifs et expérimentaux (Proceedings of the Helsinki Symposium, June 1980; Actes du Colloque d'Helsinki, juin 1980): IAHS-AISHPubl.no. 130.

The effect of the High on the hydrological regime of the River

S. SHALASH Research Institute of High Dam Side Effects, Cairo,

Abstract. Results from the many theoretical studies undertaken since 1954 to determine the magnitude and rate of degradation, and the effect of the degradation on hydraulic structures on the Nile below the High , have varied widely. In this paper four methods are used to find the actual degradation, and drop in water levels and bed levels between 1964 and 1978, to assess the effect of the High Aswan Dam on the hydrological regime and the hydraulic structures.

Effet du grand barrage d'Assouan sur le régime hydrologique du NU Résumé. De nombreuses études théoriques ont été entreprises depuis 1954 pour déterminer l'ampleur et la vitesse de dégradation ainsi que les effets de la dégradation sur les structures hydrauliques du Nil à l'aval du grand barrage d*Assouan et les résultats de ces études varient largement. Dans cette communication quatre méthodes ont été utilisées pour déterminer la dégradation actuelle ainsi que la baisse des niveaux de l'eau et du niveau du Ut entre 1964 et 1968, pour déterminer l'effet du grand barrage d'Assouan sur le régime hydrologique et les aménagements hydrauliques.

INTRODUCTION Before the construction of any dam across its channel the River Nile was in a state of equilibrium. The natural hydrological regime of the Nile has two main features - a low clear flow and a flow loaded with suspended matter which lasts for four months every year. In order to introduce perennial for crop production, the Nile from Aswan to the Delta Barrage (see Fig. 1) was partially controlled by means of medium head and a series of low head regulators. These hydraulic structures were designed in such a way so as not to disturb the natural hydrological regime of the river. This state continued until 1960^1969 when the High Aswan Dam was constructed. Long ago, during the Napoleonic era, French scientists estimated that the deposited suspended matter annually on the river bed was equivalent to 1.0 mm. Recent studies by the Hydrological Survey of Egypt confirmed this figure and found that the thick­ ness equals 0.9 mm (equivalent to 1.2 million tons). The mean annual weight of suspended load carried by the river channel was 124 x 106 tons. The main problem before the dam was built was the deposition of suspended matter in waterways throughout Egypt. Since the construction of the dam, 98 per cent of the total suspended load carried by the flood, is being deposited upstream from the dam. As a result of this change in the river's regime, degradation phenomena began to appear downstream. The International Committee of Consultants, which had been called to examine the High Aswan Dam project, drew attention to the degradation problem and its effect on the major hydraulic structures downstream from the dam in their report in 1954. Ever since the degradation problem was first mentioned in 1954, and even before the start of construction of the High Aswan Dam, many scientists have tried to solve this problem. Unfortunately, results have not been consistent. Some experts even gave a timetable for the degradation process. After 14 years of operation of the High Aswan Dam, however, field methods of 244 Effect of the High Aswan Dam on the Nile 245

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NILE CHANNEL The River Nile, along the 946 km of the Aswan to the Delta Barrage reach, flows in a northerly direction along a well developed valley (Fig. 1). The old Aswan Dam is the 246 S. Shalash TABLE 1. Name Distance Year of Foundation Head Capa [km] construction [m] [km High Aswan Dam 6.5 S 1969 Sandy HI 164 Aswan Dam 0 1902 Rocky 40 5.5 Esna Barrage 167 N 1908 Sandy 5.0 - Naga-Hammadi Barrage 359 N 1930 Sandy 5.0 - 544 N 1902 Sandy 5.0 - Delta Barrage 946 N 1936 Sandy 4.0 -

southern boundary and the Delta Barrage is the northern boundary of this reach. Three low head barrages have been constructed within this reach. Brief descriptions are given in Table 1. Three types of river channel can be distinguished between Aswan and the Delta Barrage and are classified as: channel in the valley sides; channel through flood plains, and channel on one side of the valley, mainly the right side. The distinctive features of the three types of river channel are that the channel cross sections have relatively low shape factors, varying between 3.0 and 4.0, and that the degree of channel sinuosity is small and varies between 1.04 and 1.3

HYDROLOGICAL REGIME BEFORE AND AFTER THE HIGH ASWAN DAM Flow The natural hydrological regime of the Nile has two seasons — the flood which begins in early August and lasts until mid-November, and the low flow season which lasts from mid-November to the end of July. Average discharges vary between 10 000 and 11 000 m3/s; minimum discharges vary from 400 to 500 m3/s. The maximum daily discharge was recorded in 1878 and equalled 13 200 m3/s and the minimum daily dis­ charge was 350 m3/s recorded in 1914. The annual average total runoff is given in the Series of volumes (Hurst et al, 1931—1960). The flow at the Aswan Dam equals 84 000 km3. The mean monthly discharges measured downstream of the dam are shown in Table 2 for the recorded periods. The ratio between maximum and minimum natural flow was 15 to 1. The period of construction of the High Aswan Dam lasted from 1960 to 1969, but partial control of the flood flow started in 1964. Full control of the released dis­ charges took place in 1968. Table 2 shows the monthly discharges along the river for the recorded period 1968-1977. After construction of the Aswan Dam, the ratio of maximum to minimum dis­ charges was 10:1, but after the High Aswan Dam the ratio became 2:1.

Sediment load During the flood season, large quantities of fine sediment accompanies the flow and is carried mainly in suspension. The annual average total sediment load which passed Aswan equals 124 x 106 tons. Monthly records of sediment concentration and weight during the recorded period are shown in Table 3. The maximum daily sediment concentration was 4000 ppm and the minimum concentration was 25 ppm, i.e. the ratio was 160:1. From mechanical analysis the sediment matter was found to consist of 25—40 per cent fine sand, 35 per cent , 0.15—0.33 mm in diameter. The specific weight of sediment in water equals 1.5—1.6 t/m3. After the construction of the High Aswan Dam, 97 per cent of the total sediment load was deposited upstream from the dam. The concentration of suspended matter Effect of the High Aswan Dam on the Nile 247

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DEGRADATION Before the construction of the High Aswan Dam, a layer of suspended matter used to be deposited annually on top of the river bed. After building the high dam, a large lake was created upstream and the flow discharged was almost free from suspended load. This flow, downstream of the dam, started to regain its natural equilibrium through degradation phenomena.

Theoretical approach Ten years before regulation took place, many scientists tried to find an answer to three main questions: What was the rate of degradation? What was the ultimate value in the drop of bed and water levels? How long would it be before the equilibrium state was reached? Using available flow and sediment data, some theoretical and empirical equations, as well as taking into consideration the existing control points between Aswan Dam and the Delta Barrage (namely Esna, Naga-Hammadi and Assiut barrages) and using their own experience, they came to the following conclusions. Fathy (1956) used the tractive force equation and his own experience about ultimate river slope and thought the river bed would drop 54.2 m in 86.0 years. Mostafa (1957) used another theoretical approach and stated that the drop in river bed would be : downstream of the Aswan Dam, 8.50 m by the year 1986; Esna Barrage, 9.0 m by the year 1991 ; Naga Hammadi Barrage, 7.0 m by the year 2006, and Assiut Barrage, 6.5 m by the year 2036. The Swedish consultants VBB (1960), stated that the maximum degradation below the hydraulic structures would be 3.5 m with no mention of rate or time. In a study on sediment discharge in the River Nile, Shalash (1965) gave an empirical formula for sediment which could be carried by Nile flow and estimated the rate of degradation as a few centimetres per year, and the ultimate drop in bed and water levels to vary between 1.0 and 2.0 m which would be reached between 100 and 300 years. Simons (1965), estimated the ultimate degradation downstream of the dams as 3.0-4.0 m. Hydroprojects USSR (1975) in a study of the drop in bed and water levels down­ stream of the dams estimated the ultimate degradation to vary between 3.5 and 11.0 m and the time to vary between 100 and 700 years.

Field methods Four methods have been used in field studies by the Research Institute of Saad-El-Aali Side Effects, Cairo, to find the exact amount of degradation, drop in bed levels, and decrease in water levels: (1) calculation of bed degradation by field sediment measure­ ments; (2) calculation of water level drop from field discharge measurements; (3) calculation of water level decrease from field slope measurements ; (4) calculation of bed change from field cross sections. Hydrological stations were erected downstream of the control points and several staff gauges were constructed along the river channel. Measurements of suspended load were taken along the Nile, especially below the dams, and cross sections across the channel were taken periodically. From these field measurements, taken over 14 years, degradation of the river bed and the decrease in the water levels downstream of Effect of the High Aswan Dam on the Nile 249

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/Sdo 250 S. Shalash TABLE 4. Comparison between field methods (1964-1978) Location downstream Max. drop in riverbed [cm] Max. drop in from the Dam water level S.S. G.H.D. S.W.S. [cm] method method method G.H.D. Aswan Dam 12 21 22 58 Esna Barrage 25 29 41 76 Naga-Hammadi Barrage 25 23 24 75 Assiut Barrage 8 3 27 55 the dams were observed to vary between 50 and 20 mm per year. The rate of decrease in water and bed levels was found to be higher at the beginning of the operation of the High Dam than now (Fig. 2). A comparison (Shalash, 1974) of the different field methods is shown in Table 4. Beside bed degradation, bank erosion was also observed along the river channel. This phenomenon is not only due to effects of the High Aswan Dam but also to local river training devices which were constructed on the banks before the dam.

CONCLUSIONS (1) Man's influence on the natural hydrological regime of rivers could be avoided or minimized by proper selection of the type of hydraulic structure. (2) Theoretical approaches are not the proper way to find out either the magnitude of man's influence or predicting the damage caused. (3) When a big hydraulic structure is to be built, its side effects must be considered so that a complete picture of the project is available for the decision makers.

REFERENCES Fathy, A. (1956) Some considerations on the degradation problem in the Aswan High Dam Scheme. Hurst, H. E.etal. (1931-1960) The Nile Basin, vols 1-10: Government Printing Office, Cairo, Egypt. Hydroprojects USSR (1975) Comprehensive scheme for utilization of the River Nile. Mostafa, G. M. (1957) River bed degradation below large capacity dams. Shalash, S. (1965) A study of degradation of the Nile bed after Saad-El-Aali. Shalash, S. (1974) Facts about degradation in the River Nile. Simons, D. B. (1965) Evaluation, degradation and related hydraulic problems downstream from the Aswan Dam. WB (1960) Report on hydroelectric projects in Egypt.