Development of Nile River islands between Old Aswan Dam and new Esna barrages

Peer review under responsibility of National Water Research Center.

Abstract

This paper investigates the development of Nile River islands in the first reach which extends between Aswan and new Esna barrages. A wide range of data compiled and used in this investigation which include produced maps in 1939, 1982 and 2003, and hydrological data. Compiled data drew a complete picture for up-to-date information on morphological changes since 1939. The analysis indicated that islands length; thus, area has reduced since 1939. Also, islands tend to reshape by elongating. The reduction in area is attributed in part to the merging of islands in either bank and adjustment of the river to the new flow conditions after the operation of High Aswan Dam. El-Mansouria Island which is the largest island in area was focused on. Recent human interferences accelerated the merging of island in the west bank. Consequently, river morphology has changed around the islands.

Keywords:

• Aswan
• New Esna barrages
• Nile River
• Morphology
• Islands width
• Lengths

1 Introduction

Alluvial islands formation is a natural process influenced by the hydrology and sediment transport capacity of the system. The study of island formation is fundamental in understanding the morphological process of rivers. In Egypt, the first reach which extends for 169.085 km from downstream of Old Aswan Dam till upstream of the new Esna barrages has drawn special attention (Fig. 1) since it has the highest flow rate from Aswan to the Mediterranean, and large number of islands. Significant research has been carried out to investigate the process of islands and channel formation. Leopold and Wolman (1957) discussed types of islands. The fluvial island was formed as a result of the deposition of the coarse material from the suspended load. The island formed as a result of the less competence of the river to move the bed load, so it has collected to form the early stage of the island after that it received more deposits to extend in length and width. Torab (1990) studied the effect of High Aswan Dam on the morphology of Damietta branch. Based on the analysis of aerial photos, Torab found that the branch width has declined by 27% between 1956 and 1985. In addition, he documented the morphological changes in islands. He attributed these changes to the change in flow conditions after the construction of High Aswan Dam.

Fig. 1 Nile River in Egypt.

Osterkamp (1998) described several scenarios in which islands could disappear. Perimeter sediment deposition could eliminate an island by several methods. The first method is by preferential in-filling of one of the side channels that effectively raises the bed level in one branch (the minor channel) but not the other, and thereby shifts the flow into a single path. The second method is by sedimentation around the whole perimeter of the island until it eventually coalesces with other nearby islands or the floodplain, again forcing the flow into a single path. A third method of island elimination is by the flow preferentially incising one of the side channels and leaving the other branch ‘high and dry’. This is common downstream of dams after peak flows have been reduced. If a low flow regime persists for long enough, vegetation may accumulate between an island and its floodplain. The meandering nature of a river can cause it to laterally migrate and abandon one of the branches around an island. Floods can eliminate an island by two methods. The first is by simply increasing the flows to levels high enough that the entire island is eroded away. The second one is by changing the main direction of the flow during flood, thereby altering the angle of attack from the water and gradually wearing away the island by abrasion. Ham and Church (2002), studied bar growth and islands development during the twentieth century in a gravel reach of Fraser River in the Province of British Columbia in Canada. They found that below-average flood between 1977 and 2002 caused vegetation growth which got matured on elevated bar surfaces, thus, creating new island area. In addition, bed material influx to the reach was being increasingly deposited along or adjacent to high bar or island surfaces rather than within main flow channel. As a result, islands had grown in size and narrowing and deepening channel took place. The authors did not quantify the relation between the hydrological characteristics and islands growth.

This study describes the hydrological and morphological conditions of the first reach and focuses on the geometrical characteristics of islands between Aswan and new Esna barrages. Historical data collected between 1982 and 2003 are used to investigate change in islands morphology with time.

2 Data collection and procedures

Large set of data were compiled and used in the analysis which is grouped in two categories. The first category is the data deducted from the historical maps available at Nile Research Institute. These data are the hydrographic maps of Nile River bed produced in 1982 and 2003. The hydrographic maps of 1982 were produced by Kenting Earth Sciences LTD in 1982 at a scale 1:5000 for Nile Research Institute. While Hydrographic data of 2003 was produced by Nile Research Institute for the Ministry of Water Resources and Irrigation at scale 1:5000. In addition, inland topographic maps produced in 1939 were used in counting islands number and their geometrical properties. Islands numbers, locations and geometrical characteristics like length, width, shape and area were deducted from these maps and summarized in Table 1. The islands were defined according to the high water level marks which were defined in the maps. That is, submerged islands were not included in the analysis.

Table 1 The further details regarding island geometry.

No.	Name	Distance D/S of OAD (km)	Length in 1939 (L39) (m)	Width in 1939 (W39) (m)	L39/W39	Length in 1982 (L82) (m)	Width in 1982 (W82) (m)	L82/W82	Area in 1982 (feddan)	Length in 2003 (L03) (m)	Width in 2003 (W03) (m)	L03/W03	Area in 2003 (feddan)	Remarks
1	Aswan	6.1	1350	400	3.38	2525	500	5.05	142.96	1980	440.00	4.50	143.89	These two islands are within the city of Aswan. They are developed for recreational purposes
2	Gaziret El Nabatat	6.6	600	150	4.00	790	170	4.65	14.3	790	170.00	4.65	3.40
3	Bahrif	15.75	3450	675	5.11									Merged in east bank. Maps of 1939 showed presence of earth dam at the north part of the island connecting it to the east bank
4	El kubania 1	21.4	2025	350	5.79	3000	740	4.05	307.48	675.29	244.46	2.76	25.49	Merged in 1982, split in 2003. According to maps of 1982 and 2003, Gaziret Elkubania1 was originally two islands are close to merge together to form one island. The islands has elongated since 1982. A small island (Elkubania2) tends to merge with Elkubania at the upstream end. The two islands are very close to merge with the west bank
5	El kubania 2	21.77	300	70	4.29					3339.92	420.49	7.94	196.13
6	El Tawaysa (El Harbyab)	37.9	2100	475	4.42	3625	825	4.39	462.73	3362.59	803.60	4.18	334.91	These groups of islands tend to merge and they are located near the west bank. Balula1 and Balula2 are newly formed island located between Balula3 and Eltawaysa. The main course of the river shifted from the west to the east of the islands. The islands tend to merge with west bank. Balula 1 and 2 are more recent and submerged during high flow
7	Ballula1	41.3	2400	425	5.65	1350	400	3.38	94.8	1356.78	373.26	3.63	65.06
8	Ballula2	43.17				850	275	3.09	53					Submerged in 2003
9	El arab1 (El Mostagadda)	46.54	1075	253	4.25					1011.92	359.77	2.81	51.12	Merged in east bank in 1982
10	El arab2 (El Mostagadda)	47.25	1875	575	3.26
11	El-mansouria	50.73	5375	1175	4.57	6250	1250	5.00	1189.9	5825	1027.48	5.67	1044.77	The island has the largest area in the first reach. The island is located near the west bank. Since the island is located in the inner curve, the island tends to merge with the west bank
12	I1	51.6	375	75	5.00									Disappeared
13	Muneiha	55.82	2125	750	2.83									Merged in east bank
14	Fareseya (Aqulit)	66.59	1950	425	4.59	3550	550	6.45	421.34	3040.8	538.64	5.65	288.28	Survey of 2003 indicates the tendency to merge of the island with the east bank. The island area has reduced since 1982
15	I2	81.61	1125	350	3.21									Submerged in 1982 and 2003 in west bank
16	I3	82.49	825	150	5.50									Submerged in 1982 and 2003 in west bank
17	Besawi	83.44	2750	575	4.78	2025	710	2.85	208.54	1961.35	507.12	3.87	175.54	The I2, I3 appear during low flow and tend to merge together in 2003 and located near the west bank. A roadway culvert existed in 1939 at the north of the Besaw and did not exist in 2003
18	Gaziret Salwa	88								1390	330.00	4.21	72.80	New Island Gaziret Elkarabla tends to merge with the west bank. Its length has decrease since 1982 from 1505 m to 1390 m in 2003. Gaziret Salwa did not exist before Aswan High Dam in 1939
19	Karabla	90.35	1275	725	1.76	1525	500	3.05	144.2	1420.94	591.86	2.40	126.36
20	Gaziret El Itab	97.86	1750	175	10.00	1550	650	2.38	159.63	2452.21	307.15	7.98	125.49	This island is located near the west bank. The western channel is getting narrower
21	I4	99.5	975	125	7.80									Merged in eletab
22	I5	101	175	50	3.50	600	200	3.00	22.45	473	173.00	2.73	14.34	This island is close to the east bank
23	El malikia (El Radisia)	103.06	3125	875	3.57									Merged in east bank
24	Elhessania	106	750	200	3.75									Merged in Elmalikia
25	Abou Ghallab	107.78				1690	525	3.22	182.46	1283.98	423.54	3.03	78.15	These islands are, Abou Ghallab (km 108), Qandala (km 108.94), sheikh Mahmoud (km 110.25), and Elfuza (km 112.41). Elfuza is close to the east bank and tends to merge with the east bank
26	Qanadla	108.75	2025	125	16.20	1330	150	8.87	35.86	1207.05	131.76	9.16	25.40
27	Shaikh Mahmoud	109.4	2950	675	4.37	2100	500	4.20	183.96	1826.98	501.68	3.64	138.89
28	El Fuza	110.5	2250	375	6.00	3425	525	6.52	288.64	3166.94	464.79	6.81	205.81
29	I7	123.3	2250	375	6.00	1425	350	4.07	89.79	1507.25	223.53	6.74	52.74
30	ElDomareya (Elkalh Sharq)	123.8	3800	1125	3.38									Merged in east bank
31	El subha (El kalh)	127.19	3125	550	5.68	3600	840	4.29	491.1	3234.22	762.08	4.24	377.77	A roadway culvert exists at the west bank of the island. The island tends to merge with the west bank
32	I12	133.24				1370	190	7.21	53.67	1339.99	139.70	9.59	30.06	New Island near the east bank
33	El-Hagz	139.06	7750	1550	5.00									Merged in east bank
34	I14	148.89	875	75	11.67
35	Upstream El Riqiyya	152.79				575	225	2.56	27.8	672.46	249.75	2.69	23.98	The island is very close to the west bank
36	Zarnikh (El-Hela)	159.5	2575	500	5.15	2850	500	5.70	267.35	2684.99	432.42	6.21	200.86	The island length increase from 2775 m in 1934 to 2685 m in 2003. Also, its width increased from 375 m in 1934 to 432 m in 2003
Sum			65,350						4841.96				3801.24

The second set of data is the hydrological data set presents water level and flow rate downstream Aswan Dam. The flow rate presents the conditions downstream Aswan Dam after and before the operation of High Aswan Dam. Water levels at various staff gauges along the reach between Aswan and new Esna barrages were compiled.

3 Hydrology of the 1st river reach

The operation of High Aswan Dam has its influence on flow rate and flow level downstream Aswan Dam throughout the year. Historical records (Fig. 2) show remarkable reduction in the maximum flow rate after High Aswan Dam. Although, Nile River is regulated by High Aswan Dam throughout the year, the analysis of hydrological data set shows slight increase in the released flow from High Aswan Dam during the period between 1995 and 2008 (Fig. 3). The variation on flow discharge throughout the water year which extends between August and july of the following year is remarkable. During the period of maximum flow rate, discharge can reach up to 4.5 time the minimum flow rate (Fig. 4). The fluctuation in the flow rate is reflected on the water level downstream Aswan Dam (Fig. 4). Flow frequency analysis indicates that exceedance probability of 250 million m³/day is 5% of 160 million m³/day is 50% (Fig. 5). The suspended sediment transport, which normally contributes the majority of sediment transport, has been reduced after the construction of High Aswan Dam from 3800 ppm to 129 ppm during the flood period (Fig. 6). The reduction in suspended sediment has its implications on the river morphology as will be discussed later. On conclusion, the flow rate had dropped sharply after the operation of High Aswan Dam and suspended sediment transport as well. Between 1982and 2003, a little change took place in flow rate.

Fig. 2 Flow rate before and after the construction of High Aswan Dam.

Fig. 3 Daily discharge downstream Old Aswan Dam between 1995 and 2008.

Fig. 4 Maximum and minimum discharges and water level downstream Old Aswan Dam.

Fig. 5 Flow frequency analysis.

Fig. 6 Suspended sediment concentration before and after operation of High Aswan Dam NRI (1980).

4 Status of islands between Aswan and new Esna barrages

The recent hydrographic survey of the Nile River carried out in 2003 indicated that the total number of islands between Aswan and new Esna barrages is 23 (Fig. 7). These islands were identified according to the maximum water level as mentioned previously. Analysis of data indicates a reduction in the island area from 4841.96 feddan in 1982 to 3811.24 feddan in 2003. That is, the reduction of area is 27% over nearly 20 years period, further details regarding island geometry is summarized in Table 1. From the analysis of data in Table 1, it can be concluded that area and number of islands has decreased. The reduction of the number of islands can be attributed in part to the merging of islands with the east or west bank. In some cases, islands were merged together into one island. Roadway culvert and earth dams which are manmade structure have their impacts in accelerating the aggradation process and the merge of islands with banks.

Fig. 7 Nile River islands between Aswan and new Esna barrages in 2003.

Based on Figs. 4 through 6, the number and the area of island has reduced after the operation of High Aswan Dam and regulating the flow rate downstream the dam and reducing greatly rate of suspended sediment transport. The low rate of the suspended sediment transport increased the flow potential to erode islands banks. Period of low and high flow after the operation of High Aswan Dam (Fig. 4) has its impact also in merging islands with either right or left bank. During low flow period, flow abundance anabranch and flow mainly in the main branch. Thus, allowing growth of vegetation on the anabranch. During the high flow rate period, vegetation traps bed load and increases bed level in the anabranch. This period of high and low flow may result in merging of islands in banks (Table 1).

5 Human factors influencing islands

5.1 Effect of backwater curve on islands

In order to determine the influence of the backwater curve of new Esna barrage on the island development, a one-dimensional numerical model was applied to predict water level upstream new Esna barrage. The model, HEC-RAS 4.1 is developed by the US Corps of Engineering and applies the standard step method to predict the backwater curve. Two scenarios were used in the analysis, the first scenario was during low flow period which is 60 million m³/day, and the second scenario was during the maximum discharge which is 275 million m³/day. The model results were calibrated against manning roughness for the low and high flow conditions as shown in Fig. 8 which shows the predicted water surface profile and the measured water surface at specific gauge stations. Manning roughness was adjusted to obtain water surface which matches closely the observed gauges readings at the specific discharges. The low and high water level at islands were deducted from the model results. Thus, islands geometrical characteristics were obtained. From the model results, it can be concluded that the backwater curve extends its effectiveness to 40 km in case of the low flow and 34 km in case of high flow. Two islands lie within the influence of the backwater curve of new Esna barrages. These are El-Riqya Island which is located at km 152.790 and El-Hela Island which is located at km 159.500 downstream of the Old Aswan Dam. The first one did not undergo major changes. However, area of the former one is sharply increased since year 1939. In 2003, the island length has increased and its maximum width as well. Its area has reduced by 33% since 1982. The influence of backwater can be regarded as it reduces the mobility of the flow; thus, increases the aggradation and built up of islands. As the reach undergo low sediment transport rate, the backwater has little effect on islands development.

Fig. 8 Water surface profile for maximum and minimum discharges between Old Aswan Dam and new Esna barrages.

5.2 The manmade interference

There are five islands underwent human interference through construction of roadway culvert or roadway dam as found from topographic maps and recent version of aerial photographs. These islands are El-Mansouria, Fareseya (Aqulit), Karabla, El-Fuza, and El subha (El-kalh). The roadway culvert accelerated the rate of the island merging in banks and causes changes in the river morphology in the vicinity of the island. Roadway culvert acts as a dam by creating backwater curve in the upstream (Raslan, 2009). The influence of manmade structure on Gaziret El-Mansouria will be focused on.

5.3 Islands after operation of High Aswan Dam

The operation of High Aswan Dam resulted in a drop in the annual flow rate downstream High Aswan Dam. The maximum flow rate dropped from 700 million m³/day during the flood period to 280 million m³/day after operation of High Aswan dam (Fig. 2). This was not the only consequence of the High Aswan Dam, the suspended load has decreased and hence the total load. The decrease in suspended sediment concentration has resulted in the degradation of the river reach after the operation of HAD as reported by Shalash et al. (1996). The main features in the Aswan new Esna barrages reach is merging of islands in banks. Thus, reduction of islands in number and area took place after the operation of High Aswan Dam (Table 2). The total land area of islands declined from 20.34 km² (4841.96 feddan) to 16.01 km² (3801.24 feddan) between 1982 and 2003. In order to characterize the geometrical properties of islands two parameters were calculated:

1.	Aspect ratio is a measure of the length/width,(1) $\text{Aspect}\text{ratio}(R_{\text{A}})=\frac{\text{Length}}{\text{Width}}$(1)

2.	Elongation factor is a measure for the shape of the islands and it indicates islands shape with respect to its area. It can be calculated as follows:(2) $\text{Elongation}\text{factor}(K)=\frac{\pi L^2}{4A}$(2)

where L is the islands length, and A is the island area.

Table 2 Geometric characteristics of islands in the first reach between 1939 and 2003.

Year	Length (m)	Aspect ratio (RA) Length/width	Degree of elongation (k)	Area (km^2)	Number of islands
1939	65,350	5.31	N/A	N/A	37
1982	46,005	4.48	5.02	20.34	21
2003	46,003.66	5.01	6.172	16.01	23

It can be concluded that the reach is still under active process of erosion and sedimentation.

Islands may merge in banks and eroded with the result that the total area of islands are reduced by 27% since 1982. The lack of sediment after the operation of High Aswan Dam resulted in the degradation between Aswan and new Esna barrages. Islands also tend to elongate because of the reduction of its width. In the meantime the waterway width has increased (Fig. 9). Fig. 10 shows the relation between Islands width and total cross section width in 1939, 1982 and 2003. When considering Figs. 9 and 10, it can be affirmed that erosion took place for islands banks since the width of flow has increased and the ratio of islands width to cross section width has decreased as in Fig. 10. As a result, islands Elongation has increased between 1982 and 2003 as shown in Fig. 11.

Fig. 9 Total waterway width at islands.

Fig. 10 Islands width/total cross section width.

Fig. 11 Islands elongation.

6 El-Mansouria Island

Among the well-known islands in the first reach is El-Mansouria Island for its large area and its fertile land. The Island is located from km 47.350 to km 53.450 downstream of the Old Aswan Dam at the inner curve of the Nile River course. Table 3 summarizes the geometrical characteristics of the island before the operation of the High Aswan Dam and its current status. A roadway culvert near its downstream end had changed its shape and characteristics. It affected the aggradation and degradation process around the island. The flow capacity has reduced in the western channel. The elevation of the thalweg line in 2006 which follows the deepest point in the western channel has increased since 1982 (Fig. 12). On the other hand, flow rate has increased in the eastern channel causing degradation near the downstream end of the island as can be seen from comparing the thalweg line in 1982 and 2003 (Fig. 13).

Fig. 12 Thalweg line in 1982 and 2003 in the western channel of El-Mansouria Island.

Fig. 13 Thalweg line in 1982 and 2003 in the eastern channel El-Mansouria Island.

Table 3 Geometrical properties change of El-Mansouria Island.

Year	Length (m)	Width (m)	Length/width	Area (feddan)
1939	5375	1175	4.57	925.65
1982	6250	1250	5	1189.9
2003	5825	1027.48	5.67	1044.77

Between year 1939 and 1982 sedimentation took place at the upstream for 621 m in the upstream direction. The left channel width reduced from 256 m in 1939 to 136 m in 1982. While the right channel reduced from 850 m to 587 m. At the downstream end, since the river course tends to curve to the right, deposition took place at the inner curve. The width of the island has increased near the downstream from 1006 m in 1939 to 1272 m in 1982 (Fig. 14). The most significant changes which happened between 1982 and 2003 were the erosion at the upstream part of the island and thus, the reduction in the island length and width.

Fig. 14 Development of El-Mansouria Island.

7 Effect of wind blown sand on aggradation

Attia and Raslan (2010) studied the effect of wind-blown sand on aggradation on Nile River between Aswan Dam and new Esna barrage. The authors utilized equations proposed by Bagnold (1938, 1943). These equations were based on measurements in the Egyptian Western Desert and wind tunnel. For sand with a mean diameter of 0.25 mm, the sand discharge in metric ton per meter width per hour was:(3) $g_s=5.2\times {10}^{-4}{(u-u_1)}^3$(3) where u is the wind velocity in centimeters per second at 1 m height, u₁ is the threshold velocity at 1 m height which was found equal to 400 cm/s. The equation indicates that only wind speed greater than 4 m/s will lead to the movement of sand. Table 4 shows the quantity of sediment discharge corresponding to different wind speeds. Western Bank of the Nile River in the Governorate of Aswan is largely exposed to the wind-blown sand of the western desert. It was estimated that 39 km of western bank of the Nile River within the Governorate of Aswan had no vegetation covers or not more than 100 m of vegetation covers. That is, river west bank was in direct contact with desert sand. The wind atlas provided essential information which can be utilized to investigate the potential of desert sand that may be dumped annually in the Nile River (Fig. 15). Although, the wind speed in the domain of Aswan Governorate is not the highest as indicated in the map since it ranges between 4 and 5 m/s (14.4–18 km/h), the west bank of the Nile River in the governorate lacks enough vegetation cover and the right bank is in direct contact with desert sand as indicated previously. According to Attia and Raslan (2010), comparing wind speed obtained from the wind atlas of Egypt with values of sand discharge in Table 4, it can be concluded that during a wind speed of 5 m/s (18 km/h) up to 520 t/m/h of sand may be dumped on the agriculture land near Aswan. The final figure may be determined depending on the wind duration and vegetation cover.

Fig. 15 Egypt wind speed (m/s).

Table 4 Sand discharges related to different wind speed.

Wind speed (m/s)	Sand discharge (t/m/h)	Wind speed (m/s)	Sand discharge (t/m/h)
4.17	2.407407	6.39	7089.102
4.44	45.65158	6.67	9860.741
4.72	195.8916	6.94	13,274.36
5.00	520	7.22	17,396.82
5.28	1084.849	7.5	22,295
5.56	1957.311	7.78	28,035.78
5.83	3204.259	8.06	34,686.02
6.11	4892.565	8.33	42,312.59

8 Conclusions

The reach between Aswan and new Esna barrages was investigated in terms of islands evolvement and morphology changes. Data sets present conditions before the operation of High Aswan Dam, after operation of the dam and present situation were utilized in the analysis. The data analysis indicated that islands areas have decreased after the operation of High Aswan Dam. Several islands were merged with either the right or the left bank. Islands areas are in decline either due to land erosion or merging with banks. Human interference contributes to changes in islands development.

Acknowledgments

The authors would like to thank Nile Research Institute for providing the hydrological data required for conducting the present research sincere efforts are greatly appreciated.

Notes

Peer review under responsibility of National Water Research Center.