Erosion and solid matter transport in inland waters with reference to the Nile basin

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1 Erosion and solid matter transport in inland waters with reference to the Nile basin M. Salah E. Shalash Abstract. The River Nile is one of the most remarkable geographical features of North Africa. Its catchment covers an area of km 2, extends from latitude 4 S to latitude N, and experiences a great variety of climate. The vegetation within the Nile basin is varied, it includes alpine flora in the higher parts, dense tropical forest, tall elephant grass, thin savanna forest, thick vegetation of tropical swamps, thorny forest and scanty vegetation of desert country. Also, there are dense crops grown on irrigated lands. In general, the Nile basin may be divided into four main subbasins: (1) The White Nile whose head waters rise south of the equator. Its runoff is 29 per cent of the total Nile runoff and its water is clear. (2) The Atbara River which rises in north Ethiopia. This is a flashy river and is dry for half the year. Its runoff is muddy and constitutes 14 per cent of the total Nile runoff. (3) The Blue Nile which also rises in north Ethiopia. Its runoff equals 57 per cent of the total runoff of the Nile. The flow is muddy during the rainy season. (4) The Main Nile which flows northwards to the sea. There is no additional runoff apart from a few desert streams which carry mud during winter rainstorms only. Therefore, within the Nile basin there are only two main catchment areas; the Blue Nile and Atbara River, which erode and supply the Main Nile with suspended sediment. The average annual suspended sediment load measured in the Main Nile is 134 million tons. The total sediment derived from rainstorms over the Eastern Desert of Egypt amounts to 1 million tons. As there are no direct measurements of the sediment load for the individual basins, it is difficult to estimate the eroded land in each basin separately. Mechanical analyses show that there is some difference between the sediment derived from the south and that coming from the north. The rate of annual soil erosion in the Blue Nile and Atbara River basins is about 0.1 mm, and less than that within the main river basin. Erosion et transport solide des eaux continentales dans le cas plus particulieur du bassin du Nil Résumé. Le Nil constitue une entité géographique remarquable de l'afrique Septentrionale et son bassin couvre une superficie de km 2. Ce bassin s'étend entre les latitudes 4 S et N, et il englobe une grande variété de climats. La végétation du bassin du Nil est diversifiée: flore alpine dans les secteurs les plus élevés, forêt tropicale dense, prairies à grandes graminées type Pennisetum, savane forestière claire, végétation dense des zones marécageuses tropicales, forêt à épineux, végétation très clairsemée des zones désertiques. En outre, il existe des zones de cultures denses sur les terrains irrigués. D'une façon générale, le bassin du Nil se divise en quatre sousbassins principaux: le Nil Blanc, l'atbara, le Nil Bleu et le Nil principal. Il n'existe en fait que deux grands bassins versants, le Nil Bleu et l'atbara, qui provoquent une érosion et alimentent le Nil proprement dit en éléments en suspension. Le transport solide annuel moyen en suspension mesuré dans le Nil proprement dit est de 134 millions de tonnes. Les transports solides totaux au cours des orages d'hiver qui s'abattent sur le Désert Oriental d'egypte s'élèvent à 1 million de tonnes. Faute de mesures directes des transports solides dans les deux bassins, il est difficile d'estimer les pertes en terre respectives de chacun d'entre eux. Les analyses granulométriques montrent qu'il existe des différences entre les éléments solides provenant du sud et du nord. Le taux d'ablation annuelle du sol dans les bassins du Nil Bleu et de l'atbara se situe au voisinage de 0.1 mm, et il est inférieur à cette valeur dans le bassin du fleuve principal. 278

INTRODUCTION Erosion and solid matter transport in the Nile basin 279 Water and climate are the most important controls of erosion, transportation and deposition of sediment.

2 INTRODUCTION Erosion and solid matter transport in the Nile basin 279 Water and climate are the most important controls of erosion, transportation and deposition of sediment. Runoff entrains sediment from the soils over and through which it flows and is thus an agent of erosion. The transported load consists of material in solution and suspension, of colloidal particles, and of material rolled and pushed along the floor of the river channel. The rolled material is commonly designated the tractional load and is affected by traction. The dissolved load is largely acquired by solution of material in the soil and rock over and through which the water flows. The colloidal load is acquired by contact of water with soil and rock. Suspended load is also acquired by abrasion, impact, grinding or hydraulic action and is transported by turbulence, whereby upward directed side currents prevent deposition of sediment. Transportation of material in suspension, over long distances, depends upon the dimensions, specific gravities and shape of the particles and upon the velocities and turbulence of the main flow. The rate of soil erosion and sediment transportation are controlled by so many factors that generalizations are very difficult. Important parameters are the physical, chemical and mineral character of the soil and rocks, the structure of the rocks, climate, plant growth and slope or relief. A brief account of the Nile basin and the variables which affect soil erosion in its subbasins is given below. THE NILE BASIN AND ITS SUBBASINS AFFECTED BY SOIL EROSION AND SEDIMENT TRANSPORTATION The Nile basin is one of the most important geographical features within northern Africa. The basin covers an area of km 2, which is roughly about one tenth of the area of the African continent. The river is the longest in the world, and its basin extends from latitude 4 S to latitude N and includes greater variety than that of any other river. Its source is at an altitude greater than 5120 m above m.s.l. in central Africa while its mouth is in the Mediterranean Sea. Because the basin extends over such a range of latitude and altitude it is subject to a great variety of climate. Also, the vegetation varies widely along the river basin. The vegetation included alpine flora on the higher parts which are covered with snow, dense tropical forest, tall elephant grass, thin savanna forest, thick vegetation of tropical swamps, thorny forest and finally desert which occupies the northern part of the basin, where intensive crops are grown on the narrow strip of irrigated land along the main Nile valley. In general, the Nile basin may be divided into four main subbasins with respect to climate, topography, vegetation and sedimentation, (see Fig. 1). ( 1) The White Nile whose head waters rise south of the equator and whose average runoff equals 29 per cent of the total Nile runoff. The White Nile water is clear throughout the year. All the tributaries of the White Nile contribute clear water except for one main tributary called the Sobat River. The area of the Sobat basin is km 2. During rainstorms some of the surface sous of the basin are eroded and contribute as much as ppm of suspended sediment to the flow. This amount of sediment deposits in the swamps at its junction with the Main White Nile. (2) The Atbara River basin which rises in northern Ethiopia (Fig. 1). This river is flashy and is dry for almost half the year. The Atbara River contributes 14 per cent of the total runoff of the Nile and also contributes suspended load to the Main Nile during the flood season between August and October. (3) The Blue Nile basin; its head waters rise on the Ethiopian Plateau (see Fig. 1) and it contributes an average annual runoff equal to 57 per cent of the total Nile annual runoff. This river also contributes a considerable amount of suspended sediment

3 M 6 O / T R RA /V A /v S A ^.ALcXAMtmAt THE NILE 3c*/# 1:ia>.o«eooo BAS/N Qlrvr^p E G Y P T nyrtnnxr O FIGURE 1. The Nile basin

4 Erosion and solid matter transport in the Nile basin 281 to the Nile during the flood season. (4) The Main Nile which carries the flow and the sediment northwards to the sea. In this part of the basin there is no addition of runoff apart from some springs and desert streams which carry sediment only during winter rainstorms. CLIMATE OF THE BASIN Within the White Nile basin which extends from the Lake Plateau to southern Sudan, the climate varies with altitude and exposure, and rain falls for nearly the whole year. The rainfall curve has two maxima; one in April and the other in October. The magnitude of the average rainfall varies between 800 and 2000 mm. The mean air temperature does not vary much throughout the year. On the Ethiopian Plateau, in the Blue Nile and Atbara^River basins, the climate, as in all mountainous tropical areas, varies with latitude, altitude and slope. There is a well marked rainy season from June to September, with maximum rainfall occurring at the end of July. The average rainfall varies between 200 mm near the mouth of the Atbara River to 2000 mm at its source. The main source of the Nile flood is the rainfall on the Ethiopian Plateau. Within the Main Nile catchment the rainfall is almost nil, except in the northern strip near the Mediterranean Sea where rain falls during the winter. SURFACE SOIL OF THE BASIN The description of surface soils is limited to the two main subbasins of the Blue Nile and Atbara River since they are the main sources of the sediment load in the Main Nile. The surface soils of the Ethiopian Plateau are derived from the in situ weathering of the volcanic rocks. The reddish colour of the soil in the undulating area of the plateau is a highly characteristic feature of the landscape. Two samples of soil gave the following mechanical analyses [in per cent] : Particle size grouping Stones and \ gravel Coarse sand Fine sand Silt Clay Sample Sample The Main Nile from the confluence of the Atbara River to the sea acts primarily as a carrier of water and sediment. The fertile land on either side of the River Nile in this reach is the result of the suspended load in the flood flow being deposited as alluvial soil. This silt phenomenon attracted the attention of soil scientists and hydrologists in very early times. They estimated, in their early studies of the sediment in the Nile, that the average rate of deposition amounted to 1.0 mm/year along the narrow strip of the Nile Valley beside the river. Recent studies confirmed this figure and the general acceptance was that the mean annual rate of sediment deposition was equal to 0.90 mm. This figure sharply decreased to zero during the construction of the High Dam at Aswan and the suspended load carried by the flood flow of the Nile is now deposited in the reservoir at an average rate of 132 million tons per year. SUSPENDED SEDIMENT MEASUREMENTS Measurements of the concentration of suspended sediment in the Nile flow have been 10

5 282 M.Salah E. Shalash TABLE 1 Period August end September end October end Sediment concentration [ppm] Total load [million tons] Total million tons carried out since 1929 by the Hydrological Department of Egypt. The sediment concentration was measured at several gauging stations along the Main Nile from north of Atbara to Cairo, but for the purpose of this study the first gauging station was at Kajnarty, (see Fig. 1) 399 km upstream of the Aswan Dam. Measurements of sediment concentration in the Blue Nile and the Atbara River also showed the same results as the Kajnarty station. Several methods have been used to determine the distribution of the sediment concentration across the section and for estimation of load quantities. Table 1 shows the average sediment concentration and total load at Kajnarty for the period during the flood period of August, September and October. The mean annual total discharge passing Kajnarty for the period 1912^1957 during the months August, September and October is 53.6 billion m 3. The Blue Nile contributes 71.5 per cent of the total average flood runoff and the Atbara River contributes 18.7 per cent of the flood. The rest of the flow is contributed by the clear water of the White Nile. MECHANICAL ANALYSIS OF SEDIMENT The mechanical analysis of the suspended sediment based on the international specifications is summarized as follows: Coarse sand Fine sand Silt Clay 0.2 mm mm mm <0.002 mm Table 2 shows the mechanical analyses of the suspended load passing Kajnarty during the flood season in DESERT WADIS There are several desert wadis on the eastern bank of the northern reaches of the Main Nile. Their catchment areas vary between 400 and km 2. These natural wadis function only when thunderstorms occur during winter and their flow carries fine suspended sediment to the Nile. These occurrences are remarkable as the flow in the Main Nile at this time is fairly clear. The sediment concentrations during these

6 TABLE 2 Erosion and solid matter transport in the Nile basin 283 Date August September October Sediment concentration [ppm] Coarse sand Fine sand Silt Clay storms were measured in the Nile whenever they occurred and values ranged between 3000 and 5000 ppm, but they lasted only for a few weeks. The grain sizes of this sediment are finer than that carried during the normal flood and consist of silt and clay. ESTIMATION OF SOIL EROSION It is easy to come to conclusions concerning the total amount of erosion occurring within the basin. Such estimates can be easily obtained from the suspended sediment load carried by the river, but it is extremely difficult to estimate the erosion rates occurring in different areas of the basin. From the sediment measurements mentioned previously the total mean annual sediment load for the subbasins of the Blue Nile and the Atbara, has been calculated as 128 million tons. Of this figure the Blue Nile contributes 75 per cent and the Atbara River contributes 21 per cent. It is known that soil erosion is mainly associated with heavy rain or thunder storms within the basin, but rainfall of this type never totally covers such large basins as the Atbara or Blue Nile. No doubt, there is a relation between the amount of erosion in a basin and its drainage density. Investigations are at present in progress and, as a rough estimate, it would seem that only 10^50 per cent of the total basin area is liable to erosion. Until this relation is confirmed, estimates of soil erosion refer to the total area. In the Atbara basin the mean annual erosion is equal to 0.24 mm, and in the Blue Nile area the annual erosion is equal to 0.2 mm. CONCLUSION The mechanical analysis of the deposited sediment within the main Nile Valley confirms its origin in the eroded surface soil of the Atbara and Blue Nile subbasins. Also the annual rate of deposition is slightly greater than the annual rate of erosion.

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