river nile erosion.pdf
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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.