In an area where rain is scarce, wide-scale water storage is often necessary. Rainfall is not frequent in southern Arabia, but when it does come it is torrential. Bare slopes are scoured, soil eroded and the turbulent mixture pours into the low-lying flat lands as a short-lived flood. Without human intervention, the water and soil are lost, most of it flowing out into the ocean. One of the most elaborate methods of water retention devised in ancient Arabia was Marib Dam.
Eight kilometres from the city of Marib (in Yemen), the Wadi Dhanah (in the Shebe Valley; fed by a few weeks of rain twice per year) penetrates a narrow gap in the hills, reaching a velocity of up to 1000 cubic metres of water per second. Back into antiquity (at least 2000 BC and probably closer to 3000-2500 BC), simple earth dams were constructed (and probably annually renewed) in order to divert water into a canal network. The water thus trapped was used for irrigation, but the channels quickly silted up and even when the channels were unclogged, the silt had to be placed elsewhere. As a result, the sediments built up (to around 30 metres in some places) and new dams (as high upstream as possible) needed to be built. What is evident today is the last of those dams, complete with the remnants of an elaborate sluice system and its own series of canals.
The cities of south Arabia grew more proficient at masonry around 1000-500 BC, rivalling those of Egypt and Palestine. Some sources claim the dam was constructed in the 8th century BC, others insist the 7th and yet others suggest that prolonged planning (and perhaps a few failed attempts) were made between the 1st and 2nd millennia BC. A dense limestone was chosen for the dam and blocks weighing one or two tons were individually cemented together. The base was constructed of huge slabs of rock covered with loose volcanic stone. Originally, the work was accelerated by building the wall in sections, eventually placing a wedge-shaped block between the two segments. Later builders, though, were more efficient and effective; they incorporated the use of plaster and cement, which contained volcanic stone (which is abundant in the area) and made the cement more resistant to water. Due to this, the core was basically composed of cement and it was no longer consequential if inferior stone was used for the dam’s facing.
The dam was a success, as far as immediate human interests were concerned. Heavily fortified and economically vibrant, Marib was the capital of the Sheba Kingdom and remained important throughout the Sabean Dynasty (which ruled until the 6th century BC). With the dam, the local population were able to irrigate almost 1,600 hectares and support a population of some 30,000-50,000 people. It constantly required maintenance, which was not always possible, especially as Marib began to decline in the early centuries AD; there are several reports of ruptures between 500 and 600 AD, which were combated by Abyssinian occupiers. The last flood, which occurred in Muhammed’s lifetime (610 AD), has been described as “a breach too far.” While it lasted, the dam was 620 metres long, no less than 16 metres high and contained 150,000 cubic metres of water.
The Seventy Wonders of the Ancient World: the Ancient Monuments and How They Were Built, Thames & Hudson.