Cracking Dams: Arch Dams, a

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Key Developments in the History of Arch Dams
The Roman Empire
Vallon de Baume It is rather astonishing that the Romans only sparcely applied the arch in dam constructions, a design otherwise so masterfully employed in their buildings and bridges. One of the few true arch dams built by the Romans was located in the Vallon de Baume, France in order to supply the nearby city with water. The dam was 12 m high and 18 m long and curved with a radius of about 14 m and a central angle of 73^o. It consisited of two masonry walls, 1.3 m wide on the upstream and 1.0 m on the downstream side, with an intermediate earth core 1.6 m wide.	The Vallon de Baume Dam sketch from Schnitter courtesy of A.A. Balkema (#)
Monte Novo Another true arch dam was found at Monte Novo, Portugal. Unfortuantely, it was recently submerged by a modern reservoir. As a result, it is undetermined whether or not the dam was built by the Romans. However, who else but the Romans could have built such a sophisticated structure? It was 5.7 m high and 52 m long, including the wing walls at both ends. The middle part was curved with a radius of 19 m and a central angle of about 90^o. The dam was built of blocks of shist laid horizontally in lime mortar. The key feature of the Monte Novo dam was that the curved part at both ends passed into the wing walls without any abutment blocks to absorb the horizontal arch thrusts. Apparently the builders of the Monte Novo dam did not entirely rely on their bold design because two buttresses were added to the downstream face. They also provided two outlets which released water to drive mills located downstream.	The Monte Novo Dam sketch from Schnitter courtesy of A.A. Balkema (#)
The Mystery of Dara Most interesting is, however, the dam built near Dara, Turkey. Procopius of Caesarea (AD 490-562) stated that Chryses of Alexandria, did not build this barrier in a straight line, but in the form of a crescent, in order that its arch, which was turned against the stream of the water, might be able to better resist its violence. The upper and lower parts of this barrier are pierced with apertures, so that, when the river suddenly rises in flood, it is forced to stop there and to flow no further with the entire weight of its stream, but passing in small quantities through the aperatures, it gradually diminishes in violence and power.... This statement shows that the Roman engineers understood the principle of arch dams and flood retention basins. However, recent preliminary surveys found no trace of the arch dam--only the remains of two gravity wing walls. Nevertheless, in the 70 m wide gap between the wing walls there might have been a slightly curved dam about 5 m high.
Mongolian Arch Dams in Iran Kebar and Kurit The Mongolians built the first arch dams since the Romans. The earliest one was built around 1300 near Kebar, Iran. The Kebar dam was 26 m high, 55 m long, and was curved with a radius of 35 m and a central angle of only 40^o. The arch did not abut against the canyon walls, instead it passed on both sides into straight wing walls like the Monte Novo dam.
The second Mongolian arch dam was built around 1350 in Kurit, Iran. The Kurit dam is especially remarkable for its extraordinary height of 60 m. After increasing the height by 4 m in 1850, the Kurit dam was to remain the world's highest dam until early in the 20th century. The dam was built in a very narrow canyon and its crest length reached only 44% of the height. It therefore remained erect, despite the fact that a large segment of lower half of the vertical downstream face broke away.	The Kurit Dam photo from Schnitter courtesy of A.A. Balkema (#)
Postmedieval Europe Elche The first true arch dam in Europe since the Roman times was built from 1632 to 1640 near Elche, Spain. The Elche dam's main arch was 75 m long at the crest and was curved to a radius of 62 m with a central angle of 70^o. The rather slim cross section of the dam was completely adequate according to modern computer analysis.	The Elche Dam photo from Schnitter courtesy of A.A. Balkema (#)
The Elche dam was designed by Joanes del Temple who adapted it masterfully to the peculiarites of the site with one flank consisting of a low ridge. He let the arch across the main gorge end in an artificial abutment block on the ridge, from where a long wing wall completed the structure. Also, the other end of the arch crest abutted not against rock, but against a wing wall directed upstream. Thus, in the Elche dam's uppermost part, the arch thrusts did not have much support and were therefore deflected downwards. This had been the case to an even greater extent at the Mongolian Kebar dam and the Monte Novo dam.
The Evolution of Modern Arch Dams Variable Radius In 1914, the construction of the first variable radius arch dam was completed at Salmon creek, Alaska. The shape of the dam became the standard for variable radius type arch dams. The upstream bulging served to off-set the undercutting by the stronger curved lower arches near the abutments while the pronounced downstream toe reduced the tensile stresses at the upstream heel. The full realization of the new design was made posible by building the dam entirely out of concrete.	The Salmon Creek Dam photo courtesy of Ram P. Sharma Morrison Knudsen Corp. (#)
The technical as well as economic advantages of this design were immediately recognized. It rapidly gained popularity in the western USA and elsewhere. Although it never replaced the regular arch dam completely, the variable radius arch design was to prove especially suitable for high and large dams.
Radical Designs A few dams have been experiments in themselves. The La Gage dam in France and the Tolla Dam in Corsica were both made exceedingly thin in order to provide a full-scale check on the validity of theoretical analyses when applied to dams approaching the ultimate degree of slenderness. The two dams are a complete success and have been shown to posses adequate margins of safety. Whether or not such thin dams are psychologically justifiable is another matter.	The Tolla and Le Gage Dams sketch from Thomas
The Double-Curved Arch Dam "When it became noised abroad that a dam 90 ft. high and but 8 ft. thick at the base was to be built only two miles above Ithaca, to form a lake of 60 acres area, many people immediately saw visions of a Johnstown flood, and protests began to appear in public prints." But the dam still holds strong in 1999. It is curved both along its crest and along the height. ref: Williams, 1904	The Ithaca Arch Dam photo courtesy of M.V. Polaha (#)
The 1960's brought a revival of arch dam construction in the USA, particularly so after the adoption of the double-curvature design by the USBR for some of its most important structures like, for example, 143 m high Morrow point dam built in 1968. Furthermore, arch dams became established in such regions of great dam building activity as the Iberian Peninsula, Japan, and the former Soviet Union (the highest is the Inguri dam of 1980 in Georgia at 272 m and longest at 680 m and most concrete at 3,960,000 m3).	The Morrow Point Dam photo courtesy of Bureau of Reclamation (#)
At the same time, both the designs and construction techniques reached considerable uniformity. But in spite of the high perfection and productivity attained in their construction, arch dams lost ground in the last few decades. Similarly to the classical gravity dams, they were outranked by embankment dams, even in the field of very high structures.
Epilogue: A Tragic Display of Strength The great reserves of strength inherent in an arch dam were dramatically displayed in 1963 when the reservoir behind Vaiont Dam in Italy was virtually destroyed by a landslide. Vaiont, at that time the second highest dam in the world, was built across a narrow gorge on limestone foundations so that the crest 858 feet above the valley bottom was only 623 feet in length. Some large-scale instability in the mountainside above the reservoir had been observed earlier by the engineers during filling.	The Vaiont Landslide photo from Kingston University School of Civil Engineering (#)
They were allowed to proceed very slowly, and three years later, on Oct. 9, 1963, with filling still incomplete, about 314 million cubic yards of soil and rock slid down into the reservoir, sending a tremendous volume of water to a height of 853 feet on the opposite side of the valley. The flood overtopped the dam to a depth of 328 feet and surged down the valley, causing a major tragedy, the destruction of several villages with a large loss of life. Yet, only superficial damage was caused to the dam, which, at its crest, is about 11.2 feet thick.