The effect of the High Aswan Dam on the hydrological regime of the River Nile

Transcription

1 The nfluence of man on the hydrologcal regme wth specal reference to representatve and expermental basns L'nfluence de l'homme sur le régme hydrologque avec référence partculère aux études sur les bassns représentatfs et expérmentaux (Proceedngs of the Helsnk Symposum, une 1980; Actes du Colloque d'helsnk, jun 1980): AHS-ASHPubl.no. 10. The effect of the Hgh Aswan Dam on the hydrologcal regme of the Rver Nle S. SHALASH Research nsttute of Hgh Dam Sde Effects, Caro, Egypt Abstract. Results from the many theoretcal studes undertaken snce 1954 to determne the magntude and rate of degradaton, and the effect of the degradaton on hydraulc structures on the Nle below the Hgh Aswan Dam, have vared wdely. n ths paper four methods are used to fnd the actual degradaton, and drop n water levels and bed levels between 1964 and 1978, to assess the effect of the Hgh Aswan Dam on the hydrologcal regme and the hydraulc structures. Effet du grand barrage d'assouan sur le régme hydrologque du NU Résumé. De nombreuses études théorques ont été entreprses depus 1954 pour détermner l'ampleur et la vtesse de dégradaton ans que les effets de la dégradaton sur les structures hydraulques du Nl à l'aval du grand barrage d*assouan et les résultats de ces études varent largement. Dans cette communcaton quatre méthodes ont été utlsées pour détermner la dégradaton actuelle ans que la basse des nveaux de l'eau et du nveau du Ut entre 1964 et 1968, pour détermner l'effet du grand barrage d'assouan sur le régme hydrologque et les aménagements hydraulques. NTRODUCTON Before the constructon of any dam across ts channel the Rver Nle was n a state of equlbrum. The natural hydrologcal regme of the Nle has two man features - a low clear flow and a flood flow loaded wth suspended matter whch lasts for four months every year. n order to ntroduce perennal rrgaton for crop producton, the Nle from Aswan to the Delta Barrage (see Fg. 1) was partally controlled by means of medum head dams and a seres of low head regulators. These hydraulc structures were desgned n such a way so as not to dsturb the natural hydrologcal regme of the rver. Ths state contnued untl 1960^1969 when the Hgh Aswan Dam was constructed. Long ago, durng the Napoleonc era, French scentsts estmated that the deposted suspended matter annually on the rver bed was equvalent to 1.0 mm. Recent studes by the Hydrologcal Survey of Egypt confrmed ths fgure and found that the thckness equals 0.9 mm (equvalent to 1.2 mllon tons). The mean annual weght of suspended load carred by the rver channel was 124 x 10 6 tons. The man problem before the dam was bult was the deposton of suspended matter n waterways throughout Egypt. Snce the constructon of the dam, 98 per cent of the total suspended load carred by the flood, s beng deposted upstream from the dam. As a result of ths change n the rver's regme, degradaton phenomena began to appear downstream. The nternatonal Commttee of Consultants, whch had been called to examne the Hgh Aswan Dam project, drew attenton to the degradaton problem and ts effect on the major hydraulc structures downstream from the dam n ther report n Ever snce the degradaton problem was frst mentoned n 1954, and even before the start of constructon of the Hgh Aswan Dam, many scentsts have tred to solve ths problem. Unfortunately, results have not been consstent. Some experts even gave a tmetable for the degradaton process. After 14 years of operaton of the Hgh Aswan Dam, however, feld methods of 244

2 - As»*?? Djr> H4! 0 _^ "5 Effect of the Hgh Aswan Dam on the Nle 245 M Dr &&;4V 4A *M-_$ DfTj\ T S s4 v^ 8,AX&& m S cj SAB 1 a. \ str *:*«=,s<=pt. c & for., Pumps ->7 P. 4PunpSéej!PV, ' t_ ^^t' 528 FGURE 1. 4swdn Djm Correnfs of The aoxo'v Bovnct-'ry Scfaecn syp* 4 As Sadr! Prncpal rrgaton works on the Nle. "* 2-4^ t* S V ^:% * *-? 5 tëcstrrya- S- actual degradaton showed the stuaton was qute dfferent from that thought prevously. NLE CHANNEL The Rver Nle, along the 946 km of the Aswan to the Delta Barrage reach, flows n a northerly drecton along a well developed valley (Fg. 1). The old Aswan Dam s the

3 246 S. Shalash TABLE 1. Name Dstance [km] Year of constructon Foundaton Head [m] Capa [km Hgh Aswan Dam Aswan Dam Esna Barrage Naga-Hammad Barrage Assut Barrage Delta Barrage 6.5 S N 59 N 544 N 946 N Rocky H southern boundary and the Delta Barrage s the northern boundary of ths reach. Three low head barrages have been constructed wthn ths reach. Bref descrptons are gven n Table 1. Three types of rver channel can be dstngushed between Aswan and the Delta Barrage and are classfed as: channel n the valley sdes; channel through flood plans, and channel on one sde of the valley, manly the rght sde. The dstnctve features of the three types of rver channel are that the channel cross sectons have relatvely low shape factors, varyng between.0 and 4.0, and that the degree of channel snuosty s small and vares between 1.04 and 1. HYDROLOGCAL REGME BEFORE AND AFTER THE HGH ASWAN DAM Flow The natural hydrologcal regme of the Nle has two seasons the flood whch begns n early August and lasts untl md-november, and the low flow season whch lasts from md-november to the end of uly. Average dscharges vary between and m s; mnmum dscharges vary from 400 to 500 m s. The maxmum daly dscharge was recorded n 1878 and equalled m s and the mnmum daly dscharge was 50 m s recorded n The annual average total runoff s gven n the Nle Basn Seres of volumes (Hurst et al, ). The flow at the Aswan Dam equals km. The mean monthly dscharges measured downstream of the dam are shown n Table 2 for the recorded perods. The rato between maxmum and mnmum natural flow was 15 to 1. The perod of constructon of the Hgh Aswan Dam lasted from 1960 to 1969, but partal control of the flood flow started n Full control of the released dscharges took place n Table 2 shows the monthly dscharges along the rver for the recorded perod After constructon of the Aswan Dam, the rato of maxmum to mnmum dscharges was 10:1, but after the Hgh Aswan Dam the rato became 2:1. Sedment load Durng the flood season, large quanttes of fne sedment accompanes the flow and s carred manly n suspenson. The annual average total sedment load whch passed Aswan equals 124 x 10 6 tons. Monthly records of sedment concentraton and weght durng the recorded perod are shown n Table. The maxmum daly sedment concentraton was 4000 ppm and the mnmum concentraton was 25 ppm,.e. the rato was 160:1. From mechancal analyss the sedment matter was found to consst of per cent fne sand, 5 per cent slt, mm n dameter. The specfc weght of sedment n water equals tm. After the constructon of the Hgh Aswan Dam, 97 per cent of the total sedment load was deposted upstream from the dam. The concentraton of suspended matter

4 Effect of the Hgh Aswan Dam on the Nle 247 ro n n M-) *»< U-> tn»-l.-4 o> «n * - < o m 00 * > 10 *-«E O c on < X n o r-«o X a 5 W <; H 5 0 <=, S <-> Q 2 ^2 s^» <*-, o <N r-- «vo t- C»-< r-1 ^ 1 S M 00 S O- Os Q -.- w -) CO <

5 248 S. Shalash n downstream flow vares between 25 and 40 ppm durng the year. Table shows mean monthly sedment concentratons. Clay dsappeared from the sedment load whch conssted of 5 per cent slt and 65 per cent very fne sand, wth average dameter equal to 0.0 mm. DEGRADATON Before the constructon of the Hgh Aswan Dam, a layer of suspended matter used to be deposted annually on top of the rver bed. After buldng the hgh dam, a large lake was created upstream and the flow dscharged was almost free from suspended load. Ths flow, downstream of the dam, started to regan ts natural equlbrum through degradaton phenomena. Theoretcal approach Ten years before regulaton took place, many scentsts tred to fnd an answer to three man questons: What was the rate of degradaton? What was the ultmate value n the drop of bed and water levels? How long would t be before the equlbrum state was reached? Usng avalable flow and sedment data, some theoretcal and emprcal equatons, as well as takng nto consderaton the exstng control ponts between Aswan Dam and the Delta Barrage (namely Esna, Naga-Hammad and Assut barrages) and usng ther own experence, they came to the followng conclusons. Fathy (1956) used the tractve force equaton and hs own experence about ultmate rver slope and thought the rver bed would drop 54.2 m n 86.0 years. Mostafa (1957) used another theoretcal approach and stated that the drop n rver 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 Hammad Barrage, 7.0 m by the year 2006, and Assut Barrage, 6.5 m by the year 206. The Swedsh consultants VBB (1960), stated that the maxmum degradaton below the hydraulc structures would be.5 m wth no menton of rate or tme. n a study on sedment dscharge n the Rver Nle, Shalash (1965) gave an emprcal formula for sedment whch could be carred by Nle flow and estmated the rate of degradaton as a few centmetres per year, and the ultmate drop n bed and water levels to vary between 1.0 and 2.0 m whch would be reached between 100 and 00 years. Smons (1965), estmated the ultmate degradaton downstream of the dams as m. Hydroprojects USSR (1975) n a study of the drop n bed and water levels downstream of the dams estmated the ultmate degradaton to vary between.5 and 11.0 m and the tme to vary between 100 and 700 years. Feld methods Four methods have been used n feld studes by the Research nsttute of Saad-El-Aal Sde Effects, Caro, to fnd the exact amount of degradaton, drop n bed levels, and decrease n water levels: (1) calculaton of bed degradaton by feld sedment measurements; (2) calculaton of water level drop from feld dscharge measurements; () calculaton of water level decrease from feld slope measurements ; (4) calculaton of bed change from feld cross sectons. Hydrologcal statons were erected downstream of the control ponts and several staff gauges were constructed along the rver channel. Measurements of suspended load were taken along the Nle, especally below the dams, and cross sectons across the channel were taken perodcally. From these feld measurements, taken over 14 years, degradaton of the rver bed and the decrease n the water levels downstream of

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7 250 S. Shalash TABLE 4. Comparson between feld methods ( ) Locaton downstream from the Dam Max. drop n rverbed [cm] S.S. method G.H.D. method S.W.S. method Max. drop n water level [cm] G.H.D. Aswan Dam Esna Barrage Naga-Hammad Barrage Assut Barrage the dams were observed to vary between 50 and 20 mm per year. The rate of decrease n water and bed levels was found to be hgher at the begnnng of the operaton of the Hgh Dam than now (Fg. 2). A comparson (Shalash, 1974) of the dfferent feld methods s shown n Table 4. Besde bed degradaton, bank eroson was also observed along the rver channel. Ths phenomenon s not only due to effects of the Hgh Aswan Dam but also to local rver tranng devces whch were constructed on the banks before the dam. CONCLUSONS (1) Man's nfluence on the natural hydrologcal regme of rvers could be avoded or mnmzed by proper selecton of the type of hydraulc structure. (2) Theoretcal approaches are not the proper way to fnd out ether the magntude of man's nfluence or predctng the damage caused. () When a bg hydraulc structure s to be bult, ts sde effects must be consdered so that a complete pcture of the project s avalable for the decson makers. REFERENCES Fathy, A. (1956) Some consderatons on the degradaton problem n the Aswan Hgh Dam Scheme. Hurst, H. E.etal. ( ) The Nle Basn, vols 1-10: Government Prntng Offce, Caro, Egypt. Hydroprojects USSR (1975) Comprehensve scheme for utlzaton of the Rver Nle. Mostafa, G. M. (1957) Rver bed degradaton below large capacty dams. Shalash, S. (1965) A study of degradaton of the Nle bed after Saad-El-Aal. Shalash, S. (1974) Facts about degradaton n the Rver Nle. Smons, D. B. (1965) Evaluaton, degradaton and related hydraulc problems downstream from the Aswan Dam. WB (1960) Report on hydroelectrc projects n Egypt.