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Oman is a semi-arid country with few permanent surface-water resources. The Wadi Dayqah dam project, which started impounding in August 2009, will provide a much needed additional supply of up to 35 million m 3 of water per annum. This paper describes the design and construction of the concrete gravity main dam, which at 75 m high is the tallest in the country. A year after construction started a tropical cyclone hit the region resulting in flood flows well in excess of the 10,000-year design flood. This prompted a major design modification to allow almost double the discharge over the central spillway section. Nevertheless the project remained on schedule.