Scientists in Egypt are particularly interested in the sustainable management of water and land resources. Global climate change will have a dramatic impact on the Egyptian water and land resources as well as its coastline and agriculture. Egypt is likely to become one of the most vulnerable countries in the world in the next several decades. Many climate scenarios predict that climate change will severely affect rainfall in the Nile basin and the flow of the Nile River in general and the High Aswan Dam Reservoir (HADR) in particular. Global warming and the higher temperatures will lead to higher evaporation rates, which, in turn, will result in less water availability at the HADR. Egypt’s Ministry of Water Resources and Irrigation predicts that the evaporation losses will, compared to the mean annual evaporation rates for the last 30 years, be approximately 3% to 10% higher by the year 2100. Since the construction of the High Aswan Dam fifty years ago, high sediment loads are a tremendous problem. 6.6 Billion Cubic Meter (km 3) of sediments were deposited in the HADR during this period. The sediment has raised the lakebed level as well as the water level and caused a larger surface area. These developments have decreased the storage capacity of HADR and have increased the evaporation rate. The presented paper investigates the impact of lowering the lakebed by removing sediments from the HADR with a distinct emphasis on evaporation losses. A digital elevation model for the HADR was developed to describe the hydrological characteristics and to assess the consequences of removing sediment deposits. The results show that the removal of sediments will reduce evaporation losses by about 1.1 km 3 projected for 2100, which represents 6.5% of the total projected evaporation losses.
Dams and their reservoirs play an essential role in people’s lives and the development of countries. The purposes of dams are manifold: They are in use for water supply or irrigation, hydropower as well as for flood control, navigation, water quality, sediment control … These multipurpose dams provide domestic and economic benefits to the people in industrial, developing, and rural countries [
The accumulation of sediments in reservoirs can lead to several problems, such as the loss of storage capacity, which, in turn, reduces the functional efficiency of the reservoir in the long run and increases spillway flows and risk of flooding in downstream waterways. Furthermore, sediments may clog reservoir intakes and outlet structures and scour hydraulic machines. An increase of sedimentation load is likely to cause the loss or impairment of fish, macro-inver- tebrates, and other aquatic organisms, to reduce water quality due to turbidity in the water, to lower dissolved oxygen levels, and to lead to higher water temperatures [
In the period between 1964 and 1971, Egypt constructed the High Aswan Dam (HAD) to overcome the great variability between water supply from the Nile River and Egyptian needs of water quantity and seasonal availability. The Nile River presents the main source of Egyptian water supply. The HAD forms a large man-made reservoir called High Aswan Dam Reservoir (HADR). The reservoir is divided into Lake Nasser in Egypt (approximately 350 km long) and Lake Nubia (approximately 150 km long) to the south in Sudan. The HADR is one of the biggest artificial lakes in Africa and the world. Worldwide, it is the third largest artificial reservoir in terms of storage capacity and the second largest artificial reservoir in terms of surface area [
The Nile’s average annual inflow in the HADR of 84 km3 contains approximately 134 million tons of suspended matter, mostly silt. The suspended matter is heavily loaded with inorganic clay, silt, sand, as well as other organic debris (detritus). 98% of the annual sediment load is moved during flood season [
Deposition in the reservoir is governed by a number of factors: The most important one is the sudden decrease of flow velocity as soon as the river reaches the open area of Lake Nubia [
The life time span of the HADR has been predicted several times before and after operations began at the HAD. In 1964, Russian engineers estimated that the life time span of the dead zone would be 500 years. The German company Hochtief, in 1970, expected that the dead zone will be filled in 750 years, and, in the same year, the American Building Authority calculated the life time span of the dead zone to 1000 years [
With its limited water resources, Egypt will be significantly affected by climate change [
Recent researches focused on the HADR and investigated options for the reduction of the high evaporation losses. Since the construction of the HAD, many studies have offered methods for estimating evaporation losses. In contrast, only few recent projects have discussed about tools for reducing them. [
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Using an up-to-date digital elevation model (DEM) of the HADR, [
Further on, the accumulation of sediment reduces the water volume which, shortens the life time span of the HAD [
The HADR is located upstream of the HAD, 17 km south of the old Aswan Dam. The HADR is approximately 500 km long and 3.5 km wide at its widest point. It covers a surface area of 6514 km2 at an altitude of 182 m AMSL and has a storage capacity of 162 km3 of water. This is about twice the average annual yield of the Nile at Aswan, which is estimated at approximately 84 km3 [
The morphology of the HADR is characterized by the presence of numerous side extensions (embayments), locally known as khors, which are considered the key feature of the reservoir. There are over 100 khors, 48 of which are located on the eastern side of the reservoir [
There are basically four storage zones in the HADR. The dead storage zone is the storage below the level of 147 m AMSL and a storage capacity of 31 km3. This zone is reserved to be filled with sediment; as discussed above, this process may take up to 500 years. The live storage zone between 147 m and 175 m AMSL is the active storage, with a storage capacity of 90 km3. The zone between 175 m and 182 m AMSL is the flood storage, with a storage capacity of 41 km3. This
zone is divided into two zones: the flood control zone between 175 m and 178 m AMSL, which is used to contain water from the annual flood and which needs to be emptied at the beginning of the water year (August 1), and the surcharge storage zone between 178 m and 182 m AMSL, where excess water can spill over the top of the dam in a spillway. The maximum upper limit is 183 m AMSL [
Between 1973 and 1999, the MWRI conducted bathymetric surveys for only a few cross sections because it was very difficult to measure the reservoir’s morphology and sedimentation processes in the HADR with traditional survey methods. However, since 1999, the MWRI has used a hydro-acoustic system with a Differential Global Positioning System (DGPS) and Echo Sounder to collect data on depth and location as an alternate method of mapping the bottom of the reservoir. This technology provides bathymetric data in a format that can be used to create digital maps. In general, survey data has been collected along cross sections of the reservoir, as shown in
The bathymetric survey of the HADR has been conducted on a regular basis by the MWRI. Due to the analysis of these cross sections, it became clear that the sedimentation rate was extremely high in Lake Nubia across the border to Sudan. Therefore, the MWRI has usually conducted comprehensive surveys for Lake Nubia in its entirety and for the sections in Egypt's part of Lake Nasser that are characterized by high erosion or sedimentation rates, as shown in
In addition to a small percentage of small-size gravel, the main river’s sediment load includes sand, silt, and clay. During a bathymetric survey, the MWRI also monitors the soil at the bed of the HADR. These samples have been collected to determine the properties of the deposits in these cross-sections. The samples have been collected from eastern and western parts of the cross section as well as from the middle. Sieve analyses and sedimentation analyses were applied to the samples to determine their characteristics and distributions. There are three main categorizes: clay and silt, sand, and gravel.
This study uses the data that have been generated by the MWRI since 2007, to analyze and identify the characteristic features of the soil at the bottom of the HADR. The average distribution of clay and silt, sand, and gravel were stored in data files. A point shape file was created including the soil classification data for each section along the HADR using a GIS-system.
mean sea level. The data was checked to eliminate wrong altitudes or coordinates outside the HADR. The data was created as a point shape file. The surveyed points reached up to 300,000 points for one mission. In this paper, the data collected for the missions executed in 1999, 2003, 2006, 2007, 2008, 2009, and 2010 was used to generate point shape files, as shown in
[
The HADR Bed Soil Sediment Categories Database (HADRBSDB) was created to record the sediment categories of the bed soil data of the HADR, which has been collected by the MWRI since 2007.
section to the Daka cross section, which is located 478.5 km upstream of the HAD, and they reached almost 100% at the southern end of the reservoir.
The DEMs of Lake Nubia for the years 1999, 2003, 2006, 2007, 2009, and 2010 were compared to monitor changes at the bottom of the HADR due to sediment and erosion events.
In the period from 1964 to 1977, successive sediment events occurred, particularly in Lake Nubia, whereas the El-Madik section at 130 km in Lake Nasser experienced minor erosion problems. There were minor sediment problems in Lake Nasser, particularly between the Sara and Apreem sections from 325 km to 331 km, while the reach between Korosko and El-Madik from 182 km and 130 km did not experience any major changes. In the period from 1964 to 1977, the Madik Amka section at 368 km experienced the largest increase of sediment
deposition of about 43 m. The Second Cataract section at 357 km experienced the overall largest increase in the amount of sediment, which reached over 60 m. In the period from 1977 to 1999, the largest sediment layers were found between the Samanh and Abd El kadir sections from 403 km to 352 km. The bed level increase gives a value of 20 m, which is equivalent to approximately one meter of sediment deposition per year. During the past decade, the Second Cataract section was characterized by a sediment deposition of over 12 m and an annual sedimentation rate of 1.2 meter.
The sections between Second Cataract and Dabarosa (from 357 km to 338 km) experienced high sedimentation as well. In the other direction, the sections north of Second Cataract all the way up to Diwaishat at 431 km, experienced major erosion problems. The highest erosion rate was measured at Amka section (364 km) up to approximately 80 cm per year. Therefore, the area between Second Cataract section and Dabarosa section was isolated to study the changes in the lakebed during the past decade.
section width (12.4 km) at Section Second Cataract, which follows the narrow sections upstream, which have a maximum width of 3.7 km. The accumulated sediments are decreasing gradually to Section Dabrosa, where the width of cross sections decreases slightly.
From 1964 to 2010, the total erosion volume was approximately 126 Mm3 while the total volume of sediment reached over 6.6 km3. Approximately 5.5 km3 of sediments were deposited in Lake Nubia in the past five decades, whereas the total change in terms of sedimentation in Lake Nasser was approximately 1.1 km3. Changes to the HADR’s total water volume were usually positive due to the small volumes caused by erosion in different years, which account for only 2% of the changes to the bed level of the HADR. The total annual mean sediment volume deposited in the reservoir was approximately 140 Mm3 whereas the total annual mean volume that was removed from the bed by erosion was approximately 4 Mm3. The sections between Second Cataract and Dabarosa have shown the majority of these extensive changes in sediment volume during the last 15 to 20 years.
The deposits accumulated at major sections in Lake Nubia from 1977 to 2010 due to successive sediment and erosion events are shown in
Halfa Dighaim experienced successive major sediment events. The sediment deposits increased from Section Daka to Section Second Cataract and then decreased to Section Dabarosa. The largest amount of accumulated sediments with 800 Mm3, which represents 17% of the total sediments deposited in Lake Nubia till 2010, were deposited at Section Second Cataract. Furthermore, the 2.8 km3 of sediment deposited between Sections Amka and Dabrosa, a distance of approximately 26 km, account for approximately 60% of the total sediments in Lake Nubia and 50% of the total sediments in the HADR during the period from 1977 to 2010.
The effects of removing sediment deposits from the HADR and lowering the lakebed were investigated. The water level and the surface area before and after the potential removal of the sediments were computed using new equations developed for both cases, as shown in
area is approximately 270 km2, which accounts for over 6% of the surface area of the HADR. Based on a mean annual evaporation rate of 2700 mm, the maximum water savings could be up to approximately one km3 under current climatic conditions. By 2100, the maximum reduction with regard to annual evaporation losses could reach approximately 1.1 km3, based on the results of ECHAM5 and HadCM3 models.
The paper shows that the Nileʼs average annual inflow contains approximately 134 million tons of suspended particles such as clay, silt, sand, and organic debris. Most of these sediment particles were deposited south of HaIfa in Lake Nubia, where a new delta had formed. Sedimentation processes in the reservoir are governed by a number of factors. The most important one is the sudden decrease in flow velocity as soon as the inflow reaches the open area of Lake Nubia. 6.6 km3 of sediments as total amount of deposits in the HADR during the past five decades, 5.5 km3 were deposited in Lake Nubia and 1.1 km3 in Lake Nasser. 2.8 km3 of sediments were deposited in the reach between the Amka to Dabrosa sections, which are located approximately 26 km apart. This amount represents approximately 60% of the total sediment load in Lake Nubia, which, in turn, represents 50% of the total sediment deposits in the HADR during the period from 1977 to 2010. In contrast, in Lake Nasser between Korosko and El-Madik, there were only minor changes of the bottom level during the past two decades. Erosion in the bed of HADR is very limited; 126 Mm3 was the total volume removed from the bed by erosion during the past five decades. The HADR is characterized by primarily silt and clay sediments in Lake Nasser. Deposition of sandy sediments increased in Lake Nubia and reached almost 100% at southern end of the HADR.
The paper recommends the removal of 6.6 km3 sediment deposits from the HADR to reduce its surface area by 355 km2 and, consequently, to reduce evaporation losses by approximately 1.1 km3 by 2100. Removing the deposits and lowering the lakebed would not only reduce evaporation losses, but it would also prevent the formation of blockages in the waterway of the Nile in general and at new delta emerging at the southern tip of Lake Nubia. These valuable sediments could be used to reclaim and develop new arable land in Egypt and to protect the socially and economically significant areas in the Nile delta from the rising sea level. Although Lake Nubia is comprehensively and regularly surveyed, there is not enough data on Lake Nasser, in part because neither Egypt nor Sudan has enough resources to survey this large area on a regular basis. A comprehensive bathymetric survey of the entire reservoir, including its khors, is needed to update digital elevation models of the HADR and to build new mathematical models that can be used to compute its hydrological characteristics (water level, surface area, water volume). A feasibility study of the most appropriate methods for removing sediment deposits from HADR needs to be conducted as well. Furthermore, a feasibility study that evaluates the possibilities for using sediment from the HADR has to be conducted, and a pilot project needs to be completed to evaluate the characteristics of sediment for its potential use in agricultural land reclamation and to identify how these sediments affect crop yields.
We are indebted to Nile water sector in the Ministry of Water Resources and Irrigation in Egypt for data access. We greatly acknowledge the review of the English text by Dr. Micha Edlich in the Writing Center for Academic English at Leuphana University of Lüneburg, Germany.
Elba, E., Urban, B., Ettmer, B. and Farghaly, D. (2017) Mitigating the Impact of Climate Change by Re- ducing Evaporation Losses: Sediment Removal from the High Aswan Dam Reservoir. American Journal of Climate Change, 6, 230-246. https://doi.org/10.4236/ajcc.2017.62012