Sunday, August 10, 2008

Marib Dam

The Republic of Yemen is located on the southwestern coast of the Arabian peninsula, the region once possessed by the ancient southern Arabian kingdoms that occupied the mouths of large wadis (valleys) between mountains and desert. The first-millennium-b.c. kingdom of Saba sprang up in the dry delta of the Wadi Dhana that divides the Balak Hills. In the eighth century b.c., at the height of their prosperity, the Sabaeans had established colonies along both sea and land trade routes to Israel, and they dominated the region. Their capital, Marib, among the wealthiest cities of ancient Arabia, stood 107 miles (172 kilometers) east of Sana’a, the capital of modern Yemen. It is generally agreed that artificial irrigation was practiced near ancient Marib as early as the middle of the third millennium b.c. About 2,000 years later a dam was built to harness the biannual floods and systematic irrigation was introduced. Some scholars believe that the Marib Dam was the “greatest technical structure of antiquity.”

Around 685 b.c., under King Karib’il Watar, Saba enlarged its borders. Territories were conquered in the southwest of the peninsula; Ausan in the south was defeated and Sabaean rule extended northwest as far as Nagran. In the second half of the sixth century b.c., two kings successively built the Marib Dam near the mouth of the Wadi Dhana, the largest water course from the Yemeni uplands. By impounding water during the two rainy seasons, the dam provided irrigation for some 25 square miles (65 square kilometers) of fields and gardens. Replenished and enriched by sedimentary deposits, this agricultural land supported a population estimated to be about 30,000.

The first dam was a simple earth structure, 1,900 feet (580 meters) long and probably only 13 feet (4 meters) high, built between rocks on the south side of the wadi and a rock shelf on the north. Its location a little downstream of the wadi’s narrowest point permitted space for a natural spillway and sluices. Around 500 b.c. a second 23-foot-high (7-meter) earth dam was built. It was triangular in section; both faces sloped at 45 degrees and the upstream side was faced with stone set in mortar. The final form of the Marib Dam was not built by the Sabaeans.

Late in the second century b.c. the Himyarites, a tribe from the extreme southwest of Arabia, established their capital at Dhafar and gradually absorbed the Sabaean kingdom, gaining control of South Arabia. They undertook the next major reconstruction of the Marib Dam, building a new 46-foot-high (14-meter), 2,350-foot-long (720-meter), stone-faced earthen wall, incorporating sophisticated hydraulic systems. It was nearly 200 feet (60 meters) thick at the base, built on a stone foundation, and created a lake that was probably 1.5 square miles (4 square kilometers) in area. At each end of the wall there were sluices, constructed with what has been described as the “finest ancient masonry … in Arabia,” through which water was channeled to extensive irrigation networks on both sides of the valley floor. The southern sluice system had a 10-foot-wide (3.5-meter) spillway about 23 feet (7 meters) below the top of the dam. The northern system included a spillway and a massive channel outlet between the spillway and the earth wall. It carried water via a 3,300-foot-long (1-kilometer), 40-foot-wide (12-meter) stone-lined earthen conduit, rectangular in cross section, to a distribution point that fed 12 irrigation canals. The discharge flowing into the conduit was controlled by a pair of gates; it also passed through a large settling basin.

When the Romans began to trade with India directly via the sea routes, the South Arabian economic monopoly was broken. The overland route declined, and social structure began to disintegrate. The Himyarite dynasty was toppled by an Ethiopian invasion in a.d. 335, reestablished toward the end of the fourth century, and again overthrown by the Ethiopians in 525. The Himyarites were absorbed into the wider South Arabian population.

The Marib Dam was regularly breached, usually by overtopping, during the extreme floods that occurred about once in fifty years. Just as regularly—for example, in a.d. 450 and 542—substantial repair work was undertaken. But when it was overtopped in 575, it was not repaired. Its final destruction was later recorded in the Koran (632–650), attributed to the judgment of Allah: “But they turned aside, so We sent upon them a torrent of which the rush could not be withstood, and in place of their two gardens We gave to them two gardens yielding bitter fruit. …” There is also a Yemeni proverb, “The Marib dam was destroyed by a mouse.’ Archeologists and engineers attribute its collapse to lack of adequate, regular maintenance or to the gradual failure of the foundation. Whatever the case, deprived of their water supply, the lifeblood of their crops and gardens, thousands of people from Marib returned to the nomadic life or migrated northward. The collapse of the dam expedited Bedouin insurgence from the Najd, and Islam was introduced around 630.

In December 1986 a new 125-foot-high (38-meter) earth dam was officially inaugurated. It closes off the Wadi Dhana a little under 2 miles (3 kilometers) upstream of the old dam site. Like its ancient predecessor, it was designed to impound water for irrigating the Marib plains; a 12-square-mile (30-square-kilometer) lake with almost a capacity of 437 million cubic yards (400 million cubic meters) has transformed 45,000 acres (18,000 hectares) of desert into productive farmland.

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