Wednesday, June 6, 2007

Dam and Water Management

Dam Construction
Summarised extracts from a full article, see resources below, where end notes, references and bibliography are given.
by: Foundation for Science Technology and Civilisation.
In his `History of Dams,' Norman Smith, began his chapter devoted to Muslim dams,(endnote 1) by stating that:
`Historians of civil engineering have almost totally ignored the Muslim period, and in particular historians of dam building, such as there have been, either make no reference to Moslem work at all or, even worse, claim that during Umayyad and Abbasid times dam building, irrigation and other engineering activities suffered sharp decline and eventual extinction. Such view is both unjust and untrue.'(endnote 2)
Similar point is raised by Pacey, who notes that it is often said that hydraulic engineering `made little progress under the Muslim,' and that the latter's achievements hardly evolved beyond the Greek or Roman's. Pacey corrects this view, pointing out that the Islamic civilisation adapted ancient techniques `to serve the needs of a new age,' and that the Muslims extended the application of mechanical and hydraulic technology enormously.(endnote 3) To explain the reasons behind the belittling Muslim achievements as observed by Smith, Pacey and others(endnote 4) is a mammoth a task which requires people versed in political, religious, and historical matters.
Dams and Construction Techniques
The Muslims built many dams in a rich variety of structures and forms. The majority of the earliest Muslim dams were completed in Arabia itself; and full information on their height, length, and ratios between height and length is given by Schnitter. He also specifies that with the exception of the Qusaybah dam near Medina, a 30 m high-205 m long structure, which was slightly curved in plan, the alignment of all others were straight.(endnote 5) About half such dams were provided with a flood overflow at one end, and often with a downstream training wall to guide the spilled water to a safe distance from the dam's foot. Schnitter also observes that about a third of such very early dams (7th-8th century) are still intact.(endnote 6) In Iraq, in the vicinity of Baghdad, a considerable number of dams were built during the Abbasid Khalifate.(endnote 7) Most such dams are on the Tigris, but a few are on water diversions, further illustration of high engineering skills. In Iran can be found the Kebar dam, dating from the 13th century, the oldest arched dam known to have survived.(endnote 8) The dam has a core of rubble masonry set in mortar, the mortar made from lime crushed with the ash of a local desert plant, the addition of ash making the lime hydraulic. This resulted in a strong, hard and impervious mortar, ideal for dams, the very reason for such dam's long life, and the absence of cracks in it. Much earlier than this dam, in today's Afghanistan, were three dams completed by King Mahmoud of Ghaznah (998-1030) near his capital city. One named after him, was located 100 km SW of Kabul, and was 32m high, and 220m long.(endnote 9)
Dam construction in Muslim Spain was prolific. In the city of Cordoba, on the river Guadalquivir, can be found what is probably the oldest surviving Islamic dam in the country.(endnote 10) According to the twelfth- century geographer al-Idrisi it was built of Qibtiyya stone and incorporated marble pillars.(endnote 11) The dam follows a zig-zag course across the river, a shape which indicates that the builders were aiming at a long crest in order to increase its overflow capacity. Remains of the dam can still be seen today, a few feet above the river bed, although in its prime, it was probably about seven or eight feet above high- water level and eight feet thick.(endnote 12)
Techniques used by Muslim masons and engineers reached great heights of ingenuity. On the river Turia, still in Spain, as an instance, modern measurements have shown that the eight canals have between them a total capacity slightly less than that of the river, thus raising the possibility that the Muslims were able to gauge a river and then design their dams and canals to match.(endnote 13) Smith elaborates on such skills.(endnote 14) Muslim engineers used sophisticated land surveying methods to locate their dams in the most suitable sites, and also to lay out very complex canal systems. For such, they used astrolabes and also trigonometric calculations.(endnote 15)
Around Baghdad water was diverted into the Nahwran Canal which supplied water for irrigation, whilst improvements were made to existing, old systems.(endnote 16) Dams were built of carefully cut stone blocks, joined together by iron dowels, whilst the holes in which the dowels fitted were filled by pouring in molten lead.(endnote 17) An impressive structure of masonry is Hill's impression of the dam at Marib in Yemen, with its carefully cut and fitted blocks using lead dowels in their joints.(endnote 18) It was also fourteen metres high and 600 metres long, with elaborate waterworks including sluices, spillways, a settling tank and distribution tank. So strong a structure, it survived for about ten centuries until lack of financial and technical means made it impossible to maintain.(endnote 19)
Back in Spain, according to Scott, the masonry of the reservoirs was of the finest description, and the cement used was harder than stone itself.(endnote 20) Contingencies were provided for in such manner that no overflow occurred, and no damage resulted even during the worst flooding. Evidence of Muslim engineering `genius' is the fact that these dams needed hardly any repair in a thousand years.(endnote 21) The eight dams on the Turia River at first sight seem to have an exaggerated amount of weight placed on their foundations, the masonry of each dam going some fifteen feet into the river bed, and further support provided by the addition of rows of wooden piles. Such solid foundations were justified by the river's erratic behaviour, which in times of flooding reaches a flow that is a hundred time greater than normal, the structure having to resist the battering of water, stones, rocks and trees.(endnote 22) These dams, now over ten century old, still continue to meet the irrigation needs of Valencia, requiring no addition to the system.(endnote 23)
On the River Segura, the Muslims built a dam in order to irrigate vast lands in the Murcia region.(endnote 24) Because of the nature of the terrain, not just the location, but the design and construction had to be absolutely perfect, too. The height of the dam was only 25 feet, yet its base thickness was 150 and l25 feet, which may seem excessive. Such thickness was necessary to meet the softness and weakness of the river's bed to prevent it from sliding along. The water flowing over the crest initially fell vertically through a height of 13-17 feet on to a level platform, running the length of the dam. This served to dissipate the energy of the water spilling over the crest. The over-flow then ran to the foot of the dam over flat or gently sloping sections of the face. In this way the whole dam acted as a spillway and the energy gained by the water in falling 25 feet was dissipated en route. Thus the risk of undermining the downstream foundations was greatly reduced. Like with other dams, rubble masonry and mortar were used for the interior, and the whole was finished with large masonry blocks.(endnote 25)
The Destruction of Muslim Dams
Like with much else regarding Islamic civilization, once the transfer was accomplished, destruction followed. Muslim dams did not escape in their vast majority the onslaught against Islam. In 1220, the armies of Jenghis Khan devastated the whole eastern parts of the Muslim land. The destruction of al-Jurjaniyah dam south of the Aral Sea diverted the River Oxus from its course and deprived the Aral Sea of water, causing it to nearly dry out centuries later.(endnote 51) A hundred and sixty three years later, in 1383, it was Timur's hordes, which this time completed the work of their predecessors. The Tartars laid the land waste, Zaranj the capital of the province of Seistan, suffering terrible fate; its dams and all its irrigation works completely laid waste. A similar fate befell the Band-I-Rustam, and the region of Bust.(endnote 52) Today, hardly anything survives in those lands once the seats of great civil engineering accomplishments.
by: FSTC Limited, Sun 30 December, 2001

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