The Great Man-Made River


Geography and Climate

Changes in climate over the centuries have shaped the landscape of Libya. Early writers recorded that much of North Africa was a savannah and there are ruins of inland cities and traces of dried-up lakes in the Fezzan. Marshes ere known to exist at Kufra 1,700 years ago, but available history of North Africa confirms the gradual formation of desert during the last 7,000 years.

Climatic observations during the last 7,000 years have indicated a gradual world-wide trend towards hotter and drier conditions. Over the past 400 years records have shown an apparent 150-year cycle of minor changes, giving 50 years of hotter weather followed by 50 years of variable and erratic weather and 50 years of colder climatic conditions. It is propounded that the world is currently passing from the variable to the colder climatic condition. The Great Jamahiriya (Libya) covers an area of about 1,800,000 square kilometers spanning three climatic zones: the Mediterranean, the semi-desert, and the vast desert zone of the northern Sahara with its sprinkling of oases.

The present population of about 3.5 million is growing and lives mainly in the Mediterranean coastal zone, with a large proportion in its cities of Tripoli and Benghazi.

The fertile lands of the Jeffara Plain in the north west of the country, Jabal al Akhdar in the north east, and the coastal plain east of Sirt, all support a flourishing agriculture which is dependent upon rainfall. To the south, separated by a strip of semi-desert, the desert is encroaching ever nearer the Gulf of Sirt. In the semi-desert, which serves primarily as pasture, rainfall is slight and irregular, and the natural balance of plant life is fragile. As more and more livestock feeds within this diminishing area, the plants disappear or wither under the twin stresses of overgrazing and lack of water; every year the desert claims a few more metres as the sands creep in. Low rainfall and a high evaporation rate characterize the desert zone.

Extremes of temperature and lack of vegetation have resulted in erosion of the soil, leaving rock, sand and dust. Records of rainfall distribution show 500 millimeters falling annually on Jabal al Akhdar reducing to 150 millimeters in the coastal region around Benghazi. 200-350 millimeters fall annually along the Jabal Nefussa and the western coast. Along the coast of the Gulf of Sirt, the annual rainfall decreases rapidly with distance inland, and south of Jabal Nefussa and Jabal al Akhdar it similarly diminishes until only a few millimeters are recorded annually at Sarir in the east and Sabha in the west. Rain usually come between October and March, but occasionally fall in April and May. December and January are the wettest months; in the summer, drought is the norm. The mean annual humidity at the coast is high, between 70% and 80%, and prevailing winds are north-easterly in north west Libya and north-westerly in the north east of the country. In spring and autumn, strong southerly winds – Ghiblis – blow from the desert, filling the air with sand and dust and raising the temperature to about 50 C. These strong winds are a major erosion factor in the desert, transporting sand from one place to another.

High temperatures experienced in the coastal zones reduce with altitude in the hills. In summer, coastal temperature near sea level can exceed 43C, but in winter these can fall below zero. On rare occasions, snow has been recorded on the hills and in the mountains. Inland, the temperature rises as the rainfall and humidity decrease and maximum shade temperatures of 57C and 55C have been recorded at Azizia and Brak respectively.

Mean annual humidity inland varies between 30% and 40%. The difference between daytime and night-time temperatures inland is extreme; a variation of up to 37C has been recorded. The resultant expansion and contraction shatters the exposed rock, which is then eroded and swept away in the wind. It is over this terrain that the waters from under the desert will be conveyed; traversing 1,200 km. of desert plains in the east of the country and 700 km. of rocky plateaux, wadis & escarpments in the west . The Great Man-Made River Project is considered a new concept by which the unremitting desert will be exploited for its fresh water resources, allowing vast areas of production. In the east of the country, the great fertile lands to be cultivated and realising self-sufficiency in local food Man-Made River will carry water to Sirt and Benghazi from wellfields which will exploit proven reserves of high quality water at Sarir and Tazerbo, with a future expansion to a wellfield at Kufra.

The ground falls steadily along the route of this pipeline, from Tazerbo at an elevation of 270 metres above sea level to a low point in line with Jalu and Maradah, where it is just above sea level. From this low point it rises gently across a limestone plain until it reaches an elevation of 95 metres at a location south of Ajdabiya, where a holding reservoir is sited. From Ajdabiya one branch of the pipeline continues north, over undulating rocky ground crossed by wadis, to Benghazi; a second line turns west to Sirt over sands, old beach deposits and numerous wadis draining into the sea from the raised coastline. Great care is being taken to minimise disturbance where the pipeline passes through towns and villages. Any agricultural land disturbed by the installation of the pipeline will be carefully reinstated. In the west of the country, water from beneath the Fezzan region will be conveyed to the Jeffara plain. Wellfields will be developed in the gravel plains of the Sarir Qattusah and in other areas to the north and east of Jabal Fezzan.

Extension of the existing wellfield at Wadi Aril is also being considered. From Sarir Qattusah the pipeline will run to Wadi Aril. North of Wadi Aril the terrain rises steeply to the Hamadah al Hamra plateau, and it will be necessary to pump the water over the Qargaf Arch escarpment, which forms a natural barrier between the Hamadah plateau and the Murzuk basin. From a high point near Jabal Sawda, the pipeline descends as it continues north, and then enters the gently rising Hamadah al Hamra gravel plain, where underlying limestone deposits and volcanic sheets of lava are frequently dissected by wadis draining northeast to the sea.
The pipeline runs through the eastern part of the Hamadah al Hamra plateau for about 300 km. The surface changes slowly from a gravel plain in the south to thin sand cover in the north where the wadis become more pronounced with major wadis at Suf al Jin, Zamzam and Bani Walid. Agriculture and grazing in this semi-desert is limited to the wadi floors. North of Wadi Suf al Jin, the land rises more steeply to Jabal Nefussa with increasing vegetation cover and improved grazing land. The pipeline route is tortuous as it passes north, crossing ever more deeply incised wadis. These crossings have been specially designed to protect the pipeline from the force of flash floods which are known to sweep down the wadis following intense rain in the hills. Jabal Nefussa rises to 400 metres above the Jeffara plain to the north. To cross this, it is planned to drive a tunnel near Tarhunah with a hydroelectric power plant incorporated at the base of the north escarpment, to generate electricity as the water descends the escarpment to the Jeffara plain. The plain is highly cultivated and the route, which is planned to minimise disturbance to farmland and populated areas, ends near Suq al Ahad.
The task of the Great Man-Made River Project is, therefore, to bring water from under the desert, over thousands of kilometres of the harshest desert plains, mountains, sabkhas and wadis to serve the people, their agriculture and industry.

Formation of Underground Water

For centuries the vast deserts of southern Libya formed a barrier crossed only by caravan trade routes which followed established tracks from oasis to oasis. From 1953, these vast and largely unknown areas were progressively investigated in the search for new oilfields.

This led not only to the discovery of large oil reservoirs but also great quantities of fresh water. The following brief review of the geology of Libya describes the nature and locations of the rock formation and aquifers beneath the desert. The oldest rocks in Libya lie at its southern boundaries, in certain areas of the Tibesti, Hasawnah and Awenat mountains. The metamorphic rocks are the oldest of all; radiometric dating indicates that they are more than 2,800 million years old. During the Paleozoic era, Libya was subjected to strong tectonic movements which, from about 600 million years ago, caused significant igneous activities through the weak zones of the earth’s crust, giving rise to the Qargaf Arch.

The presence of these igneous rocks and the associated tectonic movement influenced the process of erosion and resulted in the formation, during the Paleozoic era and the first period of the Mesozoic time, of various marine and continental fluvio-aeolian deposits. A study of the fossils in these rocks shows that the country had been covered by vegetation and by the sea on more than one occasion during this period. At the end of the Mesozoic era, and as a result of the tectonic movements which probably caused the formation of the Red Sea, the Atlas Mountains and the Alps, uplifts occurred in Libya, giving rise to Jabal Nefussa and Jabal Al Akhdar.

Associated downward movements to the Sirt basin during the Tertiary era allowed the Mediterranean Sea water to flow southward to the foot of the Tibesti mountains. During that geological time, the Mediterranean Sea frequently varied in level and as a result various sedimentary deposits were accumulated and formed. However, the sedimentation formed during the Quaternary era and was limited to continental fluvio-aeolian deposits.

The continental sandstones related to the above geological times represents the most important aquifers and the tectonic movements were the main cause of the topographical uplifts which resulted in the huge underground basins. During the last fifty million years of the process of erosion and sedimentation, other areas located on lines of weakness of the earth’s crust, near Jabal Nefussa, Hassawnah, Aghai and Awenat, were subjected to significant volcanic activity. The Mediterranean Sea established its present boundaries at the end of the Tertiary era.

During the ice ages in northern Europe, the climate of North Africa became temperate and there was considerable rainfall. The excess rainfall infiltrated into the ground and was trapped in the porous rocks between impermeable layers, forming reservoirs of underground fresh water, particularly in the thick sandstone sequences. Radio-carbon dating shows that the majority of this fresh water is between 38,000 and 14,000 years old although there are some lenses of water dating from 7,000 years ago. All these ages coincide with known rainy periods. Four major underground basins have been located during exploratory drilling for oil in Libya. In the south east, the Kufra basin covers an area of 350,000 square km. and has an estimated groundwater storage capacity of 20,000 cubic kilometres in the Libyan sector, distributed through an aquifer layer over 2,000 metres deep.

The basin extends from beyond the southern border of the country to north of Kufra, then swings north-east over the border with Egypt. South of Tazerbo, the basin connects with the Sirt basin which underlies the Sarir Calanscio gravel plain to the Mediterranean. The fresh water aquifer in the Sirt basin is some 600 metres deep and it is estimated to hold over 10,000 cubic kilometres of water. South of Jabal Fezzan, the Murzuk basin extends from the Qargaf Arch in the north to beyond the south western borders of Libya. The total area of this basin is estimated to be 450,000 square kilometres, with an upper aquifer thickness of around 800 metres and an estimated storage capacity of 4,800 cubic kilometres. North of Jabal Fezzan, the Hamadah and Jufrah basins extend from the Qargaf Arch and Jabal Sawda to the coast. The massive Paleozoic aquifer underlies both basins. The water that is trapped within these basins is slowly moving from the high mountains in the south, where intermittent rain provides some recharge, towards the coast where it discharges naturally at springs, oases or sabkhas, or, through man’s intervention, at wells. The pressure of the water in the basins is sufficient to cause it to rise to within a few metres of the surface if a well is sunk.

This artesian condition occurs naturally at springs and oases. In subkas, the water reaches ground level over large areas and becomes heavily contaminated with salts through concentration by evaporation. However, the quality of the water located in the Murzuk basin is very good, the percentage of soluble salts being approximately 300 parts per million, and in the Kufra basin it is excellent, the percentage of soluble salts being approximately 250 parts per million. In the Sirt and Hamadah basins the water quality reduces as it approaches the coast, mainly because the rock in which it lies changes from continental sandstones to marine limestones which can have a very high salt content. In some low-lying areas, or sabkhas, the water reaches the surface where it evaporates, leaving the water near the surface brackish and unsuitable for use by man. The philosophy of the Great Man-Made River Project is to recover the water from deep wells further inland before it drains to the sabkhas and is lost through evaporation. The wellfields for the Great Man-Made River Project are being constructed 400 to 700 kilometres inland to tap the better quality water available there.

They are spread over large areas where the aquifers come close to the surface. The wellfields are designed to extract the water from the rocks at a rate which will not excessively lower the level of the water, so that pumping costs can be kept to a minimum and wellfields will remain productive over a longer period. The Great Man-Made River Project will initially take two million cubic metres of water a day from the eastern wellfields and one million cubic metres a day from the western wellfields.

This rate of extraction provides several hundred years of potential production. A small amount of recharge of the basins will occur from the heavy but intermittent rains in the southern uplands.

Engineering

Extraction of the water known to lie below the desert has been contemplated for many years. In 1974, Libya took the first steps towards exploitation of this valuable resource when studies were commenced which were to develop into the implementation of the Great Man-Made River project. This, the world’s biggest and most far-sighted civil engineering project of its kind, will deliver large quantities of water over immense distances, from deep in the desert to the agricultural coastal areas.

It has been demonstrated that its utilisation in the desert areas overlying these water resources would be uneconomical. The project development is planned in 5 phases. The first phase, the largest, has been completed and consists principally of a system that extracts and carrys 2 million cubic metres of water daily the coastal region where the majority of the population lives. However, the system is designed to be expanded to carry 3.68 million cubic metres of water daily in the future, utilising a total of about 1,900 kilometres of pre-stressed concrete cylinder pipe, ranging between 1.6 metres in diameter for wellfield networks and 4.0 metres in diameter for the main conveyance pipeline, laid and buried in a six to seven metre deep trench.

The wellfield at Tazerbo is the start of the phase 1 conveyance system. Here, a field of 108 production wells ( and a number of piezometric observation wells ) is yielding 1 million cubic metres per day at a flow rate of 120 litres/second per well utlising 98 wells only, the remaining wells are available on stand-by. The wells are connected to three parallel collector pipelines, spaced ten kilometres apart, with each well spaced at about 1.3 kilometres. The wells are connected to the collector pipelines by means of short lateral pipelines equipped with the necessary valves, limit switches, high pressure transmitters, flow meters and other safety devices. The collector pipelines are connected to a larger spine collector pipeline, which, in turn, conveys water to an off-line steel header tank at Tazerbo, of 170,000 cubic metres capacity.

The function of the header tank is to balance the hydraulic pressures, allowing the system to adjust to variations in water flow and prevent water loss through the overflow facility. From this header tank, the main conveyance system is routed northward to two similar header tanks at Sarir. Somewhere of the system a chlorination station is installed to inhibit biological growth on the inside of the pipe. At the end of this system, pressure control valves are installed to reduce the excess water pressure ( an additional head of about 80 metres ) arising from the difference in level between the Tazerbo and Sarir wellfields after allowances have been made for friction losses in the 256 kilometre length of the section. Pressure control valves are installed in the main conveyance pipeline close to the two header tanks at Sarir, each with a capacity of 170,000 cubic metres and having a similar balancing function to the header tank at Tazerbo. The second wellfield, located at Sarir, 256 kilometres north of Tazerbo, consists of 126 production wells in addition to a number of piezometric observation wells.

The production wells are connected to three east-west parallel collector pipelines, spaced ten kilometres apart, with the wells spaced at about 1.3 kilometres on these pipelines. This wellfield is producing 1 million cubic metres of water per day at a flow rate of 102 litres/second per well, utlising only 90% of the total number of wells, with the remaining 10% available as stand-by. Water is being collected in 1.6 metres’ diameter collector pipelines connected at their eastern ends to a larger collector pipeline of 2.8 metres’ diameter, running to the two header tanks which are joined to the main Sarir-Sirt conveyance pipeline of 4 metres’ diameter. The entire system is cross-connected to allow the output from the two wellfields to flow into both header tanks, or into only one of them when the other header tank needs maintenance. Provision is also made for future connection of the existing Sarir North and South agricultural wellfields to the system if required. From Sarir two parallel pipelines of 4 metres’ diameter convey the water to the Ajdabiya holding reservoir, 380 kilometres to the north.Wellfield modelling studies were used to determine the number of wells, their depth and water level, the type of pumping equipment required and the quantities of water which can be extracted without jeopardising production of the wells.

The wells at both Tazerbo and Sarir fields are about 450 metres deep, and submersible pumps are used within the wells at a depth of about 145 metres. At each wellhead, a control unit is installed to receive, and respond to, operating signals from the permanent communication and control system. The control unit will start or close down the pump, thereby protecting it against any extraordinary or emergency condition during operation.From the Sarir area, the daily flow of 2 million cubic metres of water is conveyed northward through the twin parallel pipelines to Ajdabiya holding reservoir, which has a capacity of 4 million cubic metres. Sarir is 150 metres above sea level and the water can therefore flow by gravity to a point near Jalu which is a few metres above sea level and to the higher ground at Ajdabiya holding reservoir, 90 metres above sea level. The minimum pressure rating of the pipe in this sector of the pipeline is 14 bars (140 tonnes/square metre ).The holding reservoir is a circular earth embankment, nine metres high, and three kilometres in circumference and about 900 metres in diameter. To limit seepage losses, the inside surface of the holding reservoir is provided with an impervious membrane lining, protected by two layers of soft sand and gravel. From the holding reservoir, the water flows through two pipelines running along the coastal belt, west to Sirt and north to Benghazi respectively. The pipeline leading west to Sirt is four metres in diameter and is designed to convey a water flow of 820,000 cubic metres a day by gravity. A maximum flow of 2.3 million cubic metres a day in the future can be achieved by using three pumping satations. The northern pipeline heading to Benghazi delivers a maximum gravity flow of 1,180,000 cubic metres a day and a maximum flow of 2.5 million cubic metres a day with two pumping satations appropriately located. Turnouts have been located along both pipelines to deliver water to agricultural, municipal and industrial users as defined by studies already carried out. Each pipeline discharges into a circular earth embankment end reservoir, with a storage capacity of 6.8 million cubic metres at Sirt and 4.7 million cubic metres at Benghazi.

These reservoirs are designed to balance fluctuations in supply and demand. The choice of pipe for the conveyance lines involved studies to select the most appropriate meterial and method of installation.
Pre-stressed concrete pipes four metres in diameter were found to be more cost-effective by a significant margin than other alternatives considered, since most of the raw material required for their production are available in the country. It was also established that pipe buried in trenches six to seven metres deep was the best method of protecting the pipe from hazards, including temperature variations and other environmental factors.A pre-stressed concrete cylinder pipe consists of a thin steel cylinder embedded in a concrete core, wrapped with high tensile steel wire, and then coated with a cement mortar to protect it from climatic conditions and normal corrosion.

This cement mortar encourages a chemical reaction which limits corrosion by steel oxidation. This is considered an effective remedy in areas where the soil is non-aggressive. The aggressiveness, or otherwise, of a soil depends upon the presence and concentration of chloride and sulphate salts within the soil and on the availability of groundwater to assist their attack.

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