NILE RIVER MORPHOLOGY CHANGES DUE TO

THE

CONSTRUCTION OF HIGH ASWAN DAM IN EGYPT

BY M.B.A.SAAD

Head of the Planning Sector

Ministry of Water Resources and Irrigation

Egypt

2002

Table of Contents

ABSTRACT ..................................................................................................................................................3

1. INTRODUCTION............................................................................................................................4

2. NILE RIVER REGIME BEFORE AND AFTER THE CONSTRUCTION OF HAD ............................................................................................................6

2.A. CHANGES IN THE THALWEG MEANDER WAVELENGTH:................................................................................6 Table 1 - Approximate 1982 meander wavelength .........................................................................................6

2.B. CHANGES IN THE SIZE AND GRADATION OF BED MATERIAL.......................................................................7 2.C. CHANGES IN BED FORMS AND RESISTANCE TO FLOW .................................................................................7 2.D. RIVER BED DEGRADATION...........................................................................................................................7

3. HYDRAULIC PROPERTIES OF THE NILE RIVER AFTER HAD.........8 3.A. CHANGES IN THE WATER SURFACE SLOPE ..................................................................................................9 3.B CHANGES IS THE NILE RIVER DISCHARGES AND SEDIMENT LOAD .................................................................11

3.b.1 Water Discharge ..................................................................................................................................11 3.b.2. Suspended Sediment............................................................................................................................11

4. CHANGES IN THE LENGTH AND STABILITY OF RIVER BANKS .11 4.A. LENGTH OF THE BANK ................................................................................................................................11 4.B. STABILITY OF THE BANK.............................................................................................................................12

5. SUMMARY AND CONCLUSIONS ..................................................................................13

6. REFERENCES: ...............................................................................................................................14

NILE RIVER MORPHOLOGY CHANGES DUE TO THE

CONSTRUCTION OF HIGH ASWAN DAM IN EGYPT

BY

M.B.A.SAAD

Head of the Planning Sector

Ministry of Water Resources and Irrigation

Egypt

Abstract The construction of the High Aswan Dam (HAD) has made a great contribution to the economic and social development of Egypt. The Nile River was partially closed in 1963 during the construction of HAD, and it was completely closed in 1968 after its completion. Prior to the construction of the HAD the average flow discharge measured at El-Gaafra station (41 km. downstream of HAD) varied along the whole year, and ranged between 930 m3/s during winter period and 9300 m3/s during flood season. As HAD was completed a complete control of the Nile River discharges was achieved, and the channel has no longer been subjected to high floods. The flow rate of water also became under full control, with a maximum value of about 2800 m3/s. The surplus flood water has been stored in Lake Nasser (about 500 km. long). Accordingly, the maximum monthly discharge has been reduced by 300%, while the minimum monthly discharge has been increased by 40%. Moreover, a substantial change in the sediment regime of the river downstream of the dam occurred, which in turn disturbed the stability of the hydraulic geometry of the river. Due to the reduction in the discharges, the water surface slope through the four reaches (between Aswan and Delta Barrage) decreased by only (2 % to 7 %) to become between 4.8 and 8.4 cm/km. Such small slopes produce relatively small velocities and consequently small rates of sediment transport. Before HAD the peak monthly concentration at of sediment load at El-Gaafra station was 3000 ppm (Hurst et at, 1959). Measurements of sediment concentration made after the construction of HAD at the same site showed that the maximum concentration was 65 ppm. The sediment load during flood times has been deposited in Lake Nasser upstream the HAD-and the clear water released downstream the dam resulted in degradation along the riverbed. As result of the changes occurred in the river flow discharge and its sediment discharge, the Nile River downsteram of the HAD develops its channel pattern and channel properties, which are the reflection of the water discharge and the sediment load that the river conveys. The aim of this paper is to present the changes in the flow and sediment regimes which occurred in the Nile River after the construction of HAD, and their effects on the channel pattern and channel properties downstream of the dam.

1. Introduction The River Nile is the second longest river in the world. The total length of the river and its tributaries amounts to some 37,205 km, and its mainstream being about 6,700 km long. The total area of the Nile basin is about 2.9 million km2 and the river traverses ten African countries, namely: Burundi, Rwanda, Tanzania, Zaire, Kenya, Uganda, Ethiopia, Eritrea, Sudan, and Egypt. The Nile has (two) main sources, namely, the Equatorial Lakes Plateau, and the Ethiopian Plateau, while other sources contribute negligible quantity of water to the basin stream. a)The Equatorial Lakes Plateau: it contains (three) basins, namely: the basin of lakes Victoria and Kyoga whose waters collect in Victoria Nile; the basin of lakes George and Edward and the basin of the Semiliki river which connects lake Edward to lake Albert and; the basin of lake Albert out of which the Albert Nile flows down to Limuli and the river is called Bahr El Gebel. b)The Ethiopian Plateau: it contains (three) basins, namely: the basin of Sobat River; the basin of the Blue Nile and; the basin of Atbara River. The annual average total of the normal Nile yield estimated at Aswan from both the Equatorial Plateau and the Ethiopian Plateau is 84 billion m3, of which 12 billion m3 from the Equatorial Plateau, and 72 billion m3 from the Ethiopian Plateau. There are (seven) dams and reservoirs built on the Nile and its tributaries for either annual storage or long-term storage. These dams are: Old Aswan Dam, Sennar Dam, on the Blue Nile, Gebel El-Aulia Dam, Owen Falls dam, Roseries Dam, on the Blue Nile, completed in 1965 (annual storage), Khashm El-Girba Dam, on Atbara River, completed in 1965 (annual storage), High Aswan Dam, on the Nile River, completed in 1968, (long-term storage). Beside the above mentioned dams for water storage, other control works such as barrages and weirs were constructed along the river course in Egypt for the purpose of raising the water levels upstream these barrages to feed the main canals. These barrages were built at: Esna, Naga Hammadi, Asyut, Delta Barrage, Zifta, and Edfina. Figure (1) shows a map of the Nile Basin and the locations of the main dams along the mainstream and its tributaries, and also the locations of the main barrages on the Nile River downstream HAD.

2. Nile River Regime Before And After The Construction of HAD 2.a. Changes in the Thalweg Meander Wavelength:

The overall channel wavelength pre-HAD obtained from the historical mapping is of the order of 1800 m. The hydrographic mapping made in 1982 (post HAD) shows that the thalweg is developing a meander pattern within the confines of the overall river channel. The meander wavelength of six reaches were measured from this mapping and are listed in Table (1). The length of each reach was about 20 km and they were chosen where well-defined meander pattern were easily distinguishable. Proceeding in the downstream direction, the wavelength initially decreased from a maximum of 4500 m, some distance upstream of Esna Barrage to a minimum of 2500 m downstream of Naga Hammadi Barrage, but then increased to 3300 m in the vicinity of El-Ekhsas gauging station. The decrease and then increase in the wavelength in the downstream direction follows the trend of the peak monthly discharges of the river, as shown in Table (2). Table 1 - Approximate 1982 meander wavelength

Reach Name

Reach Location

km

Meander Wavelength

m Idfu

110-130

4500

Shanhoria

244-264

4000

Girga

398-420

3000

Hawata

596-620

2500

Beni Mazar

727-750

2700

Geziret el Makatfiya

856-876

3300

Table 2 - Summary of peak monthly discharges of the River Nile

Peak Mean Monthly Discharge

(1969-1988)

Gauging Station

m3/s

106*m3/d

Aswan/El Gaafra

2560

221

Hanadi

2350

203

Samata

2110

182

Salam

1690

146

El Ekhsas

1710

148

2.b. Changes In The Size And Gradation Of Bed Material

The median diameter of the bed material has increased in the time period (1964-1978). The

average median diameter (D50) before HAD for the total length of the Nile River between Aswan

and Delta Barrage, was about 0.22 mm, which after HAD became courser and ranged between

0.23 and 0.42 mm. Similar analysis of the data verifies that the standard deviation of the bed

material has become more uniform. Furthermore, there is evidence of coarsening of the bed

material to behave as an armour coat inhibiting further degradation in many areas.

2.c. Changes In Bed Forms And Resistance To Flow

Resistance to flow is a function of grain size, bed form, bank roughness and other factors

resulting from man activities. At flow velocities between 0.4 m/s and 1.2 m/s the dunes will

develop, whilst ripples will form at velocities less than 0.33 m/s. Before HAD the bed of the Nile

River was covered with dunes. But due to the reduction in the discharges and velocities after

HAD, the energy of the river became insufficient to erase the existing dunes that being formed

before HAD. For this reason the dunes are still covering the channel bed but with superimposed

ripples caused by low discharges. Records of the river bed indicated that dune with amplitude of

1.0 to 1.5 m and spacing of about 30 m are relatively common. These dunes significantly affect

the flow velocity and the transport of bed material. The Chezy roughness coefficient computed

for some specified discharges showed that the values of Chezy (C) increased from (30 to 50)

m1/2/s.

2.d. River Bed Degradation

As mentioned before, the suspended sediment in the Nile River was reduced from 3000 ppm

during flood seasons to about 65 ppm after HAD, and the flow could be considered as a clear

water. Consequently, the flow discharge has been reduced from 9300 m3/s during the flood

season to a maximum value of about 2800 m3/s after HAD. As a result of these two reasons,

river bed degradation and bank erosion took place and caused lowering of the water level and a

decrease in the bed elevation downstream the existing barrages. Analysis of the available data

shows that river bed degradation took place between (1963 and 1978) and the conditions now are

almost stable. The variation in the water level was rapid between (1963 and 1973) with no

significant variation recorded between (1973 and 1990).

Table (3) shows the drop in water level downstream the Barrages from 1963 to 1990 for different

discharges between 930 m3/s and 2900 m3/s.

Table (3) Drop in water level downstream the barrage from (1963 to 1990).

Water Level Drop, cm

Discharge (m3/s)

Station

930

1040

1160

1390

1740

2310

2900

El-Gaafra

Esna

Naga-Hammadi

Assuit

80

73

84

60

76

75

88

63

73

77

90

62

72

77

85

57

60

73

75

56

50

47

-

-

37

-

-

-

3. Hydraulic Properties Of The Nile River After HAD This reduction in the discharges has significantly altered the hydraulic properties of the river

channel at the annual peak flow. Table (4) summarizes the Hanadi measurement station

hydraulic properties at the annual flood condition.

Table 4 - Summary of Hanadi hydraulic properties for pre-HAD and post-HAD mean

annual maximum monthly discharges

Condition

Discharge

m3/s

Hydraulic

Depth

m

Mean

Velocity

m/s

Area

m2

Top

width

m

Pre-HAD

8200

11.2

1.50

5500

487 Post-HAD

2350

5.9

0.84

2800

474

The pre-HAD properties were obtained from streamflow gauging made in 1958 and 1963. The

post-HAD properties were taken from 1988 measurements.

The Data in Table (4) indicate that for the post-HAD the annual maximum discharge, the

hydraulic depth, the mean velocity, and the cross sectional area are approximately 50 % lower

than the corresponding values for the pre-HAD. However the top width has reduced only by 3 %

because this gauging station is located about 12 km downstream Esna Barrage in a narrow and

stable section.

The post-HAD hydraulic properties of the river channel at the annual peak flow for the five

measurement stations are summarized in Table (5). It can be seen that the hydraulic depth, the

area, and the top width generally reduce in the downstream direction as does the discharge.

Table 5: Summary of post-HAD hydraulic properties for all gauging stations at the mean

annual maximum monthly discharges.

Station

Discharge

m3/s

Hydraulic

Depth m

Mean

Velocity m/s

Area

m2

Top

width m

El-Gaafra

2560

6.1

0.77

3300

545

Hanadi

2350

5.9

0.84

2800

474

Samata

2110

5.5

0.98

2200

393

Salam

1690

5.5

0.77

2200

403

El-Ekhsas

1710

4.8

0.86

2000

415

3.a. Changes In The Water Surface Slope

The overall slope of the water surface of the Nile River is affected by Esna, Naga Hammadi, and

Asyut Barrages. The slopes of the river reaches that are unaffected by the backwater of the

barrages (from just downstream of the barrage to a location upstream of the backwater zone of

the next downstream barrage) are given in Table (6) for the pre-HAD and post-HAD conditions

at the discharge of 2000 m3/s. The water surface slopes were obtained from the staff gauge daily

water level data cards. Starting at the upstream end of the reaches, where the discharges are

known, the progression of the chosen discharge down the reach for both ascendant and recession

limbs of the flood hydrograph was followed and the water levels were determined.

Table (6): Comparison of pre-and post-HAD water surface slopes at 2000 m3/s

River Reach

Water Surface Slope

m/km

Ratio of

Pre-HAD

(1963)

Post-HAD

(1988) HAD PreHAD Post

Aswan to Ramady

(95.5 km)

0.049

0.048

98

Esna to Samata

(130 km)

0.056

0.052

93

Naga Hammadi to

Megris(150 km)

0.071

0.067

94

Asyut to Leethy

(329 km)

0.086

0.083

97

From Table (6), it can be seen that the water surface slopes of the Nile River reaches outside the

backwater zones of the barrages have reduced very little as a result of HAD. The post-HAD

slopes are 93 % to 98 % of the pre-HAD values.

3.b Changes is the Nile River discharges and sediment load

The flows of the Nile River in Egypt are greatly influenced by the operation of HAD, as

mentioned before. Figure (2) shows the changes in discharge and sediment load at El-Gaafra

station before and after the construction of HAD.

3.b.1 Water Discharge

The regulating effect of HAD manifests itself hydrologically in three ways: peak discharges have

been decreased, minimum discharges have been increased, and the timing of the peak has been

altered. Prior to the HAD, the peak mean monthly discharge of about 8180 m3/s occurred in

September, and the minimum discharge of about 810 m3/s occurred in April. Now, due to the

operation of the dam, the peak discharge of about 2560 m3/s occurs in July, and the minimum

discharge of about 1300 m3/s occurs in January.

The change in the water level at El-Gaafra station was about 8 m for pre-HAD, and became 3 m

for post HAD. The corresponding values at El-Ekhasas are 7 m and 2 m respectively.

3.b.2. Suspended Sediment

The mean annual suspended sediment loads have been measured at the five gauging stations for

the conditions pre-HAD, during the construction of the dam, and post-HAD. For the pre-HAD

conditions the suspended loads decreased in the downstream direction from 129 million tons/year

(Mt/y) at El-Gaafra to 94 (Mt/y) at El-Ekhsas, which means that deposition of suspended

sediment occurred.

During 1964 the suspended load increased from 26 (Mt/y) at El-Gaafra to 52 (Mt/y) at El-Ekhsas.

This increase in sediment load in the downstream direction was mainly attributed to river bed

degradation and bank erosion. Over the final three years of the dam construction (1965 to 1967)

the suspended load decreased further. Nevertheless, the loads still increased in the downstream

direction form 4.2 (Mt/y) at El-Gaafra to 11.3 (Mt/y) at El-Ekhsas.

4. Changes In The Length And Stability Of River Banks 4.a. Length of the bank

Comparison of the old maps and field survey shows a significant reduction in the total length of

the river bank, from 2409 km in 1950 to 2048 in 1978, and a further reduction to 2035 km in

1988 as shown in Table (6) and Figure (9). This reduction was primary due to the blocking of

secondary channels behind islands which joined the main bank. Islands are defined as having

permanent vegetation distinct from sand bars. Out of a total of 150 islands from Aswan to Cairo

in 1950, 114 islands have joined the main bank by 1988.

Before HAD, the length of the bank undergoing erosion was roughly estimated to be more than

500 km. In 1981 field observations revealed that the total length of the eroded bank downstream

HAD was 351 km. This was reduced to 242 km in 1988. The reduction in the length of bank

undergoing erosion between the years 1981 and 1988 was primarily due to the increase in river

bank protective works such as stone revetments and spurs.

Table 7: Summary of Bank erosion and river works along the Nile River between 1981 and

1988

Length of Banks

Condition of Banks

1981

1988

1981

Reach

Main Bank

km

Island

km

Main Bank

km

Island

km

Eroded

km

Revetment

km

Spurs

No. Aswan-Esna

333

24

333

24

34

22

162

Esna-Nag Hammadi

386

43

386

35

80

42

345

Nag Hammadi-Asyut

371

49

371

39

107

52

565

Asyut-Roda

765

77

765

82

130

76

997

Total

1855

193

1855

180

351

192

2069

4.b. Stability of the bank

Stability of river banks is controlled by factors such as bank height, bank slope, soil properties,

and water level.

Bank height: May be caused by bed degradation, increase of the river velocity or the effect of

moving barges. Material deposited on top of the bank leads to the same results as bed scour.

Bank slope: Due to channel meandering, helical currents are generated and cause scour of the

bank under the water surface. As scour develops, banks cave and fail leading to lateral shifting of

the channel.

Soil Properties: Irrigation water applied to soil near a bank changes the properties of the soil.

Seepage water moves some soil particles and creates small passages in the soil and weakens its

structures.

Water Level: During the periods of low flow in the river (winter period), the groundwater starts

to seep to the river, while during the periods of high flow the river recharges the soil, thus

changing the soil properties.

5. Summary And Conclusions It can be seen that after the construction of the High Aswan Dam (HAD), significant changes in

the variables that reflect the geomorphology and hydraulic response of the Nile River channel

took place. These changes cannot be considered as side effects for the HAD project, simply

because the recorded variations did not affect either the efficiency of the HAD or the

performance of the Nile River as an alluvial stream. However, some of these changes can be

counted as a direct impact of the construction of the HAD, and should be monitored and treated

in such a way as to improve the overall efficiency of the river system. These changes can be

presented as follows:

a. The Thalweg Meander Wavelength: The meander wavelength has been decreased in

the downstream direction of the river, and then increased following the trend of the peak

monthly discharge of the river.

b. Size and Gradation of Bed Material: The median diameter D50 has increased from 0.22

mm to (0.23-0.42)mm due to sorting, and the bed material has become more uniform.

c. Form Roughness: The dune being formed before HAD are still existed but with

superimposed ripples caused by low discharges. The Chezy coefficient increased from 30

to 50 m1/2/s.

d. River Bed Degradation: It took place between (1963 and 1978) and the conditions now

are almost stable. The drop in the water level due to degradation was between 37 cm and

84 cm.

e. Water Discharge: It has been completely controlled by HAD and consequently the mean

annual maximum monthly discharge was reduced from 8200 m3/s to 2350 m3/s.

f. Hydraulic Depth: It was decreased by about 50 % due to the reduction in the discharge.

g. Mean Velocity: It was decreased by about 50 %, depending upon the reduction in the

discharge and water depth.

h. Cross Sectional Area: It was reduced by about 50 % due to the reduction in the

discharge.

k. Top Width: It was reduced at Hanadi gauging station by only 3%.

j. Water Surface Slope: The ratio of the post-HAD to the pre-HAD water surface slope are

in the range of 93 % to 98 %.

i. Suspended Sediment Load: The mean annual suspended Sediment was reduced from

129 million tons/year at El-Gaafra before HAD to 4.2 million tons/year after HAD.

l. Length of Banks: The total length of the river bank was reduced from 2409 km in the

year 1950 to 2035 in the year 1988.

6. References: 1. Abul-Atta, 1978, "Egypt and Nile after the construction of High Aswan Dam",

Ministry of Water Resources and Irrigation, Egypt.

2. River Nile Protection and Development, "River Regime of the Nile in Egypt, June

1992.

3. Nile Research Institute 1981, "River Erosion Catalogue", Report No. 86, Cairo,

Egypt.

4. Shalash, S. "The effect of the High Aswan Dam on the Hydrological Regime of

the River Nile", Proc. of IAHR, Helsinki, Pub. No. 130. June 1980.

5. River Nile Protection and Development "Fluvial Characteristics of the River

Nile", Working paper 200-10, WRC, Qanater, Egypt, Dec. 1990.

6. Simons, D.B. and E.V. Richardson, 1963,"A study of Variables Affecting Flow

Characteristics and Sediment Transport in Alluvial Channels", Proceedings,

Federal Interagency Sedimentation Conference, Jackson Miss., PP. 193-206.

7. River Nile Protection and Development "Results of Analysis of Water Samples

along Rosetta and Damietta Branches", 1990.

8. International Commission On Large Dams (ICOLD), 61 Executive Meeting and

Symposium, Nov. 1993.