A transitional stage in the current regime in the Suez Canal

Transcription

1 672 Comment A transitional stage in the current regime in the Suez Canal In their remarks Hassan and El-Sabh (1975) confine themselves to a seasonal comparison, utilizing only five monthly sections of one year, In this way, they exclude the experience gained from previous observations of the seasonal changes in the canal. Morcos (1967; Morcos and Messieh 1973) approach was to study the seasonal cycle of 1966, and particularly the sections of September 1966, in the light of seasonal observations made during the preceding years, in order to detect any change after the construction of the Aswan High Dam. This is well illustrated in Fig. 1 which shows the drastic difference between the September 1964 and September 1966 sections, from which southward and northward movements, respectively, are inferred. However, Hassan and El-Sabh argue that Morcos section was made on 28 and 29 September 1966 and that observations made earlier in the same month would show the normal conditions of southward movement. Their section, made in the first half of September 1966, is the basis of their argument; but the weakness of this section is the fact that the 160-km length of the canal was covered in 15 days (3-17 September 1966), compared with two days in Morcos section. Unfortunately no direct measurements of current were made in the canal during September Currents can only be inferred from salinity distribution in that month, taking into consideration calculations of the water and salt balance in Sep- tember 1966, seasonal variation in salinity distribution in the preceding years, and previous direct measurements of currents and water level in the canal between 1933 and 1937 and in A discussion of the main arguments follows. Between 1933 and 1937, the scientists of the Suez Canal Company conducted a most extensive quantitative study of the water regime of the canal by registering the mean water level and current using tidal gauges and two continuous recording cur- rent meters at two points, south and north of Great Bitter Lake. Baussan (1938) concluded that between the northward current in winter and the southward current in late summer, there is a transitional period during which the current is only reversed in the northern canal. (For simplicity, the portion of the Suez Canal north of the Bitter Lakes will be designated as the northern canal and that south of the Bitter Lakes as the southern canal.) Thus, water flows from the Mediterranean to the Bitter Lakes, while the current in the southern canal continues to be northward from the Red Sea to the Bitter Lakes. Examples given by Baussan were July-August 1935, June, July, August 1936, and June In these months the waters flowed from both ends of the canal toward the Bitter Lakes. This is to be expected, since compensation must be made for the high evaporation during that season from the Bitter Lakes (85.5% of the total area of the whole canal). Such a conclusion is confirmed by the recent estimation of evaporation from the Bitter Lakes by Miller and Munns (1974, p. 306) who wrote that the seemingly contradictory evidences of net inflows into the Canal from both the northern and southern ends of the Canal in the same period, together with extra volumes of Nile water entering at Lake Timsah, are reconciled with the excessive evaporation potential of the Bitter Lakes region. Thus, Hassan and El-Sabh s observation and main argument that Mediterranean waters penetrated in early September 1966 into the northern canal does not necessarily imply that the current in the whole canal (i.e. the net flow) was southward. During September 1964, 19,291 X 10 mr of water were discharged into the Mediterranean from the Damietta Branch of the Nile which opens 60 km west of Port Said. In September 1966, the mouth of the Damietta Branch was closed by an earth dam and there was no further direct flow of Nile water from this branch to the

2

3 674 Comment alone amounted to 16 x 10 m3 month-l from May to September and 22 X lo6 m3 month-l from October to December. It is believed that this amount has increased due to the increase in the area of cultivated land, especially after the erection of the Aswan High Dam. Although this amount is very small compared with the discharge from Damietta Branch (19,291 X 10 m3 month-l in September 1966) into the Mediterranean, it has a direct impact on the salinity of the canal. Assuming that the volume of the northern canal is X log m3 (80 km long and 1,800 m2 in cross section), and that the salinity before dilution is 42%,, the salinity resulting from the dilution by X log m3 month-l of drainage water of l%, salinity will be X log m3 X S1 = X 10g m3 X 42%, X log m3 X lp/,, Sl = 37.4%0. Thus, the amount of drainage water in only 1 month could decrease the salinity of the canal to a value less than that observed by El-Sabh ( 38.43s0). If we assume that the canal was filled with Mediterranean water of 39%0, the salinity would drop to 34.1%0. The above discussion indicates that the drop of salinity in the northern part of the canal cannot be taken solely as evidence of a southward current in the canal, although it does not of course exclude the possibility. In late September 1966, the northern canal was filled with water of a salinity ranging between 43s0 and 39s0, due to a northward flow of highly saline water from the Great Bitter Lake (GBL) diluted by drainage water. Assuming that the salinity of GBL water is 46%, and that of the drainage water 1%0, the mixed water in the northern canal would attain a salinity of 40.8z0, which is very close to the salinity observed. Figure 2 illustrates the position of the GBL water mass and the point of maximum salinity at 6-m depth along the canal in April and September, as noted from all the available observations from 1924 to From 1933 to 1937, currents in the canal were continuously measured by two IDRAC continuous-recording current meters at two points of the canal, south and north of GBL. These records and the five sections from April 1933 to April 1935 (illustrated in Fig. 2) show that a strong net northward current in Aprils is associated with a small (and less saline) GBL water mass and that the relatively weak net southward current in Septembers coincides with a large (and more saline) water mass. The GBL water mass, as well as the point of maximum salinity, is shifted to the north in April and to the south in September. Likewise, the velocity of current can be inferred from the position, volume, and salinity of the GBL water mass in the salinity sections made in the years before and after the sections, as shown in Fig. 2. The shift to the north on 29 September 1966 is clearly exceptional. In early September 1966, the water mass shows an intermediate position, occupying the Bitter Lakes Basin with no penetration into the southern or northern canals. Thus, the water mass during that entire September is basically different from that of the preceding Sep- tembers with respect to its position in the canal. However, it retains its other important characteristics, namely its large volume and high salinity which are due to the sluggish current over the salt bed, as previously indicated (Morcos 1967). This water mass thus extends a greater distance along the northern canal in September 1966 than it did in May or December For this reason, comparison of the two cases to draw a conclusion regarding the direction of the current, as attempted by Hassan and El-Sabh, would be misleading. The only safe conclusion to be drawn from such a comparison is that the current in September 1966 is weaker than in May or December On the other hand, this clarifies my approach, which depends on comparing conditions in a particular month (say September) over a number of years, as illustrated by Fig. 2. Since the water mass in these months retains the same characteristics

4 Comment 675 SUEZ 1 SEPTEMBER 1931 APRIL 1933 SEPTEMBER MAY 1934 SEPTEMBER 1 APRIL 1935 APRIL 1954 SEPTEMBER APRIL 1964 SEPTEMBER 1964 APRIL 1966 SEPTEMBER 1966 APRIL 1967 I I I I I I I I I I km Fig. 2. Displacement of the Great Bitter Lake water mass and the point of maximum salinity along the Suez Canal at 6-m depth in April and September during the five sets of monthly cruises 1924, , , 1964, and The distance along the Suez Canal measured in kilometers south of Port Said Lighthouse appears on the abscissa. The GBL water mass in each month is represented by a shadowed area enclosed between the position of the southern and northern 44% isohalines at 6-m depth. The more saline water mass of September is heavily shadowed. Two positions of the points of maximum salinity are joined by two dotted lines, one for the northerly shifted points of April and one for the southerly shifted points of September (after Morcos and Gerges 1974). (greater volume and higher salinity) any Morcos (1960a) studies of tidal curdisplacement in the position of the water rents in the promontory of Kabrit, between mass becomes meaningful in the interpre- the two basins of the Bitter Lakes, indicate tation of the direction of the current (not that the resultant currents are much weaker meaningless, as suggested by Hassan and than the resultant currents in the southern El-Sabh). canal. They show less seasonal variabiliy

5 Comment in magnitude and direction and may even give evidence of an opposite direction to the stronger resultant currents in the southern canal. (For simplicity, the resultant current is taken as the difference between ebb and flow currents divided by 2.) As mentioned before, the GBL water mass increases in volume and salinity in summer, due to high evaporation and slow currents over the salt bed (Morcos 1960b, 1967). Such an increase may become even greater as a result of the water s extended contact with the salt bed in transitional periods when water flows from both ends of the canal toward the Bitter Lakes. The trapped and swelling water mass gradually pushes its boundaries northward and southward. The LBL is an integral part of the basin of the Bitter Lakes and its connection with the GBL has a larger cross section than does the canal proper. In the absence of a strong northward current the LBL is filled by the GBL water mass, due to the natural process of expansion in volume and increase of salinity of this water mass. This is enhanced by diffusion and turbulence and should not be taken as evidence of a southward current in the canal as is done by Hassan and El-Sabh. A similar distribution with the LBL filled with the GBL water mass was observed in August and November 1931, July, August, October, and November 1933, June and July 1935, and June These are months of transitional characteristics or even weak northward currents. In this sense, figure 7 in Morcos and Messieh (1973) was constructed to illustrate the similarity in salinity distribution in the southern canal between El- Sabh s section in early September 1966 and Morcos section in June There is definitely no southward current in the canal in June 1955 and the similar distribution of salinity in early September 1966 should not be taken as evidence of a southward current. Noting that the LBL has a mean cross section five times greater than that of the Suez Canal proper, Hassan and El-Sabh concluded that the GBL water excursion in the 15-km LBL is equivalent to 5 x 15 = 75 Table 1. The monthly average of maximum velocities of currents (cm s-l) in 1949 at two points along the Suez Canal. (Average of two daily observations at maximum flood current (IV) and maximum ebb current (S) and resultant current, R = N - S/2.) Month Shallufa (km 146.1) Gineifa (km 134.0) N S R N S R Jan Jun Jtdl Sep km in the canal proper. Such a conclusion, not justified dynamically, is invalid. Penetration into the southern canal requires much greater energy than that required to fill LBL, the waters of which can simply be regarded as the shallow shelf water of GBL. Moreover, the observations made in September 1964 and the preceding Septembers showed the GBL water mass not only filling LBL but also penetrating the whole southern canal down to Suez Bay. Table 1 gives the monthly average of maximum velocities of currents at two points in the Suez Canal in 1949 (after Morcos 1960a). Values are given for only 4 months. January and September values are, respectively, the maximum northward and maximum southward resultant currents over 1 year. June values represent the minimum northward resultant current before it changed direction in July, when the minimum southward resultant current was observed. The results show that in June and July the resultant currents are very sluggish, and that between the two main seasons there is a transitional period where no appreciable currents occur in the canal. This period is interesting when calculating the water balance in the Bitter Lakes, since the uncertainty in the estimate of the contribution of currents to the water balance will be at a minimum. Table 2 shows the total volume of the water mass of GBL >44%0, 8~ in log m3, and the total salt content of this water mass, ZwS in log kg, as well as average salinities in June and July The estimation was made by summation of the real volume-and total salt

6 Comment 677 Table 2. Changes in the volume, total salt content, and average salinity in the GBL water mass from June to July Month 1955 mogm3 CzGIOgkg S%. June July Increase content-between each two successive isohalines within the water mass bounded by the southern and northern 44F0 isohalines. Table 2 demonstrates that the change from the northward regime to the southward regime is accompanied not only by an increase of salinity but also by an increase in the volume of the water mass, which produces a significant increase in the total salt content (2.333 * log kg month-l). This fact was overlooked by Hassan and El-Sabh in their attempt to calculate the salt budget in the canal, since, with the reversal of currents, part of the salt flowing into the Bitter Lakes from the north adds to the salinity increase due to evaporation, while the water carrying this salt from the north increases the volume of the more saline GBL water mass. Reliable information on the slope of the water surface along the canal could only be gained from observations of mean sea level (M.S.L.) at selected points along the canal especially in the region of the Bitter Lakes, in addition to observations at the two ends of the canal. In 1935, 1936, and 1937, three tide gauges were installed in addition to the two permanent tide gauges at Port Said and Port Tawfiq (Suez) : Deversoir (km 98), at the northern end of Great Bitter Lake; Kabrit (km 121), between Great and Little Bitter Lakes; Gineifa (km 134), at the southern end of Little Bitter Lake. Figure 3 shows the slope of the water surface along the canal in the 6 months between June and November The data for these three years show that in most cases the Bitter Lakes occupy an intermediate height between the Red Sea and the Mediterranean when the current is predominantly northward in win- I Km0 50 I 150 Fig. 3. The slope of the water surface along the Suez Canal in each month from June to November 1935 (after Morcos and Gerges 1974). Intermediate points are Deversoir (km 98 ), Kabrit ( km 121), and Gineifa (km 134). ter or predominantly southward in September. However, in some transitional months the Bitter Lakes are either higher or lower than the M.S.L. at both ends of the canal. According to Baussan (1938) the observations give the following patterns of circulation: 1. From winter to early summer the current flows from the Red Sea to the Mediterranean, as indicated by the slope in June This is followed by a reversal of current in the northern canal, with waters flowing from the Mediterranean to the Bitter Lakes while the current in the southern canal continues to be northward from the Red Sea to the Bitter Lakes (e.g. July- JUI A%! -sep act NW

7 678 Comment August 1935, J une, July, August 1936, June 1937). In these months the waters flow from both ends of the canal toward the Bitter Lakes. 3. This is followed by a predominant current established between the Mediterranean and Red Sea (e.g. September 1935, 1936, and 1937). 4. The current in the southern canal regains its original direction from the Red Sea to the Bitter Lakes, while the waters of the Mediterranean continue to flow from the Mediterranean to the Bitter Lakes (e.g. October 1935 and 1936). 5. Finally, the current is reversed between the Mediterranean and the Bitter Lakes and regains its initial northward direction along the whole canal (November 1935 and 1936). In the various parts of the Suez Canal, currents change direction over periods of hours and days due to tidal and nontidal factors, but the main criterion is the net outflow from the canal which has a seasonal character. Quantitatively, the current regime in the Suez Canal is defined by the net outflow from the canal. This net outflow can be taken as the resultant expression of the complex system of currents in the canal. Continuous recording of currents from 1933 to 1935 by IDRAC recording current meters at two points in the southern and northern canals demonstrated quantitatively a net northward outflow into the Mediterranean from the Red Sea during most of the year and a net southward outflow into the Red Sea from the Mediterranean during late summer. For example, in 1935, there was a net northward outflow of 5,250 x lo6 m3 into the Mediterranean in the first period and a net southward outflow of 150 X lo6 m3 into the Red Sea in the second period (C.U.C.M.S. 1937). Passing through the Great Bitter Lakes, this latter outflow causes a sharp increase in the salinity in Suez Bay during August and September every year. Morcos and Messieh (1973) showed that such an in- crease in salinity did not take place at any time during Therefore, it can be safely stated that the net southward outflow from the canal into the Red Sea, hitherto always observed in late summer, did not occur in late summer Finally, Hassan and El-Sabh agree that an unusual phenomenon occurred in the current regime in the Suez Canal during 1966, and that (p. 671) the degree of influence of the stoppage of the Nile flood can be established only by repeated observations over several years. In their view, this unusual phenomenon is only a decrease in the magnitude of the reversal, but according to the evidence given above it was a change in the seasonal pattern of the current regime. My conclusion is that the northward current regime during winter and early summer 1966 was followed by a transitional period with very weak currents flowing from both ends to the Bitter Lakes. By mid-september the current had regained its northward direction without being reversed to the south. S&m Division of Marine Sciences UNESCO, Paris France References A. Morcosl BAUSSAN, J Variations annuelles de niveau le long du Canal de Suez. Rev. Geogr. Phys. Geol. Dyn. 11: COMPAGNIE UNIVERSELLE DU CANAL MARITIME DE SUEZ Debit du Canal Maritime. Comm. Consult. Int. Trav., Reun : and appendix 17. EL-SABH, M. I Seasonal hydrographic variations in the Suez Canal after the completion of the Aswan High Dam. Kiel. Meeresforsch. 25 : HASSAN, E. M., AND M. I. EL-SABH Circulation and salinity distribution in the southern part of the Suez Canal. Limnol. Oceanogr. 20 : Former address: Department of Oceanography, Faculty of Science, University of Alexandria, Alexandria, Egypt.

8 Comment 679 MILLER, A. R., AND R. G. MUNNS The Bitter Lake salt barrier, p In L Ockanographie physique de la Mer Rouge, IAPSO-UNESCO-SCOR Symp., Paris, CNEXO Publ. Ser. Actes Colloq. 2. I~IORCOS, S. A. 1960a. The tidal currents in the southern part of the Suez Canal. Publ. Int. Assoc. Sci. Hydrol. Comm. Surface Waters 51: b. Die Verteilung des Salzhalts im Suez Canal. Kiel. Meeresforsch. 16: Effect of the Aswan High Dam on the current regime in the Suez Canal. Nature ( Lond. ) 214 : AND M. A. GERGES Circulation and mean sea level in the Suez Canal, p In L Oceanographie physique de la Mer Rouge, IAPSO-UNESCO-SCOR Symp., Paris, CNEXO Publ. Ser. Actes Colloq. 2. AND S. N. MESSIEH Circulation and salinity distribution in the southern part of the Suez Canal. Limnol. Oceanogr. 18: