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7/23/2019 river nile erosion.pdf http://slidepdf.com/reader/full/river-nile-erosionpdf 1/4 Urbanization and the processes of erosion and sedimentation in the River Nile Mostafa M. Soliman Abstract.  The processes of erosion and sedimentation were in a dynamic state along the River Nile and its small islands during flood tim es. However after building the High Dam of Aswan, the situation changed and these islands appear to be in a stable position. This paper is mainly concerned with studying the urbanization process on the Islands south of Cairo. Effets des processus d urbanisation sur l érosion et la sédimentation dans le Nil Résumé.  Le processus d érosion et de sédimentation se trouvait en état dynamique lors des périodes de clue tout au long du Nil et de ses petites iles. Actuellement, en conséquence de la construction du Barrage d Assouan, la situation  s est  transformée et les iles paraissent se trouver maintenant en position stable. Cette communication se rapporte surtout à l étude des processus d urbanisation de ces iles au sud du Caire. INTRODUCTION Effects of man on erosion and sedimentation in rivers result from changing the natural water-way by building dams and irrigation structures at several locations on the rivers. These effects are discussed for a small reach of the River Nile in the vicinity of the city of Cairo. The River Nile is one of the longest and oldestrivers in the world. Many historians have said that the Nile is the gift of Egypt. Most of the Egyptian wealth depends to a great extent on this great water resource. Fertile lands and many important Egyptian cities are located on both sides of the main waterway. Therefore it is very important to treat and control the river to overcome the effects of erosion and silting processes at the rural and urban areas along the river. This is clearly demonstrated along the aforementioned reach which divides Cairo City in.two parts. Located within this reach are several islands which are utilized either for urban facilities, parks or cultiv able land. Since the completion of the High Aswan Dam, no sediment has been released downstream from the dam and the river discharge is also very different from before. The River Nile is moving towards a new equilibrium, and thus there is an urgent need to deal with the problems of degradation, bank stability, river control for navigation and protection of the urban areas on the islands. GENERAL DESCRIPTION The Nile is a meandering river. The short reach considered here forms a bend with a large radius of curvature (F ig.l). There are several islands located in this reach, of which some are rural areas while others are urban and form part of Cairo City. They were covered by water during floods before the construction of the High Dam. The flood water which carried a large quantity of silt and fine materials, deposited part of these materials on the islands and along the banks of the river. However during summer the water carries no silt, and large quantities of the deposited sediments are

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Urbanization and the processes of erosion and sedimentation in the River

Ni le

Mostafa M. Sol iman

Abstract.  The processes of erosion and sedim entation were in a dyna mic state along the River

Nile and its small islands during flood tim es. How ever after building the High Dam of A swan,

the si tuation changed and these islands appear to be in a stable posit ion.

This paper is ma inly c oncerned with stud ying the urban ization process on the Islands south

of Cairo.

Effets des processus d urbanisation sur l érosion et la sédimentation dans le Nil

Résumé.  Le processus d érosion et de sédim entation se trouvait en état dyn am ique lors

des périodes de clue to ut au long du Nil et de ses peti tes i les. Actu elleme nt, en conséquence de

la con struc tion du Barrage d Asso uan, la si tuation

  s est

  transformée et les iles paraissent se trouver

maintenant en pos i t ion s table .

Ce tte com m unic ation se rap po rte su rtou t à l étud e des processus d urbanisa t ion de ces i les

au sud du Caire.

INTRODUCTION

Effects of man on erosion and sedimentation in rivers result from changing the natural

water-way by building dams and irrigation structures at several locations on the rivers.

These effects are discussed for a small reach of the River Nile in the vicinity of the

city of Cairo.

The River Nile is one of the longest and old estriv ers in the world. Many historians

have said tha t th e Nile is the gift of Egy pt. Most of the Egyptian wea lth depends to

a great exten t on this great water resource. Fertile lands and many impo rtant Egyptian

cities are located on b oth sides of the main w ate rw ay . Therefore it is very im portant

to treat and control the river to overcome the effects of erosion and silting processes

at the rural and urban areas along the river. This is clearly demonstrated along the

aforementioned reach which divides Cairo City in.two parts. Located within this

reach are several islands w hich are utilized either for urban facilities, parks or cultiv

able land.

Since the completion of the High Aswan Dam, no sediment has been released

downstream from the dam and the river discharge is also very different from before.

The River Nile is moving towards a new equilibrium, and thus there is an urgent need

to deal with the problems of degrada tion, bank stability, river con trol for navigation

and protection of the urban areas on the islands.

GENERAL DESCRIPTION

The N ile is a meandering river. The short reach considered here forms a bend w ith a

large radius of curvature (F ig .l ). There are several islands located in this reach, of

wh ich some are rural areas while othe rs are urban and form part of Cairo City. They

were covered by water during floods before th e construction of the High Dam . The

flood water which carried a large quantity of silt and fine materials, deposited part

of these mate rials on the islands and along the banks of the river. However during

summer the water carries no silt, and large quantities of the deposited sediments are

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124 Mostafa M. Soliman

FIG UR E 1. Nile reach near Cairo.

eroded from the banks of the islands. These processes created fairly stable conditions

along the river. However after the con struc tion of the High Dam (which is an effect

of man) many changes occurred along the m ain water course. No sediment is released

downstream from the dam and the discharge is regulated to satisfy power, navigation

and irrigation requirem ents . Since the released water is nearly clear, degrad ation

below the Dam is expected and several control works are proposed to redu ce the

degradation prob lem s. River con trol nea r big cities is required to overcom e th e

instability due to m eandering.

THEORY

The expected erosion along the River Nile will be mainly due to mean dering. Many

authors believe that the main action of degradation or meandering is due to second

ary currents, which are created n ot only in curved parts (Soliman and T inney , 1968)

but also in straight reaches.

Meandering causes lateral movement due to the existence of secondary currents

in the vortices in river bend s. Neill (19 71 ) presented the following d ifferential equa

tion for the channel shift in confined meanders:

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Urba nization and the processes of erosion and sed imen tation 125

de _ 1

  àA

àt L At

(1 )

where

e

  is the average transverse distance of channel shift in a specified tim e;

A

  is the area of erosion on left and right banks within a meander wave length;

L

  is the curved length .

Secondary currents due to bed irregularity may cause sediment to move in straight

reaches. Rao and Se etharamiah (1 967 ) gave a value for the upward co mp onen t of the

secondary velocity as follows:

F = 0 . 3 1 5 ( F ) 0 -

9

5 / 0 . 4 5

C

-0 .1

where

Fis the upward component of the secondary velocity in cgs units;

V

  s  the mean velocity of flow;

2)

S e c . 2 — 2

Sec. 4 _4

FIG UR E 2. Cross sec t ions a long the reach.

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126 Mostafa M. Soliman

/ is the slope of the channel;

C   is the concentration of solids by weight which equals  SG/Q   where  S  y

s

hw> Is

  ar

>d

being the specific w eights of solids and water respectively,

 G

 is the volumetric rate

of sediment, and  Q   is the mean channel discharge.

OBSERVATIONS

Figure 1 shows the river bend near Cairo City, while Fig.2 shows a series of cross

sections along the reach. The following observations are no ted :

(a) There is very little change of wa ter surface elevation after the High Dam

con struction . This may be due to the backw ater effect of the Delta Barrage some

25 km north of Cairo.

(b) The bed level increased slightly in certain loca tions .

(c) The cross-section shapes are more uniform within the straight reach near

the main island (Fig.2).

(d) The suspended sediment concen tration is rather higher than expected

but much less than that before construction of the dam.

ANALYSIS

From the very limited observations in this reach, there is little evidence of river bed

degradation, although silting problems occurred near Dahab Island and south of

it on the river ban ks. Silting in this zone may be attrib ute d to the deceleration of

the flow because of the backwater effect of the Delta Barrage.

The deepest bed line (shown do tted at Fi g.l) has changed little since the construc

tion of the High Dam . In spite of this encouraging result against degrad ation or

meandering problems, yet more data collection and analysis are needed.

River control is also recommended in this area to weaken the secondary currents

created by the river bends shown in Fig.2.

From the above analysis the author believes that the islands south of Cairo could

be used for urb an settlement w ithou t muc h fear of erosion, assuming that necessary

revetments are constructed around the island.

CONCLUSION

The co nstru ction of High Dam across the Nile is one of the effects of ma n on the

hydrolog ical cycle, river erosion and th us on urban environm ents. This is clearly

dem ons trated in this paper for a certain reach of the River Nile near Cairo. A few

islands south of Cairo could be used for urban facilities after some river control work

has taken place.

REFERENCES

Neill, C. R. (1971 ) River bed transport related to m eander m igration rates.  Proc. Amer. Soc.

civ.

  Engrs J.

 Waterways Harbours

 and CoastalEngngDiv.

Nov., 783-786.

Rao,

 B.

 V. and Seetharamiah (1967) Development and role of secondary currents in alluvial

streams.  Tw elfth Congress of the International Association for Hydraulic Research

pp.

 4 7 2 -4 7 8 .

Soliman, Mostafa M. and Tinney, E. R. (1968 ) Flow around 180° bends in open rectangular

channels.

 Proc.  Am er. Soc. civ. Engrs J. Hydraul Div.

July, 893-907.