Okukubi Dam

The Okukubi Dam (億首ダム) is a trapezoidal cemented, sand and gravel (CSG) dam on the Okukubi River in Kin, Okinawa Prefecture, Japan. Construction began in 2009 and its reservoir was filled by 31 March 2013.[2][3] It is the first trapezoidal CSG-type to be constructed as a main dam in the world.[4][5] Its purpose is to provide flood control and water for municipal use along with the irrigation of 70 ha (173 acres) of land.[6]

Okukubi Dam
Downstream face of the dam in July 2011
Official name億首ダム
CountryJapan
LocationKin, Okinawa
Coordinates26°27′54″N 127°55′54″E
StatusOperational
Construction began2009
Opening dateMarch 2013
Construction cost¥3 billion ($38.2 million USD)
Owner(s)Development and Construction Department, Okinawa General Bureau
Dam and spillways
ImpoundsOkukubi River
Height39 m (128 ft)
Length462 m (1,516 ft)
Elevation at crest29 m (95 ft)
Dam volume339,000 m3 (443,395 cu yd)
Spillway typeLabyrinth weir
Spillway capacity430 m3/s (15,185 cu ft/s)
Reservoir
Total capacity8,560,000 m3 (6,940 acre⋅ft)
Active capacity7,860,000 m3 (6,372 acre⋅ft)
Catchment area14.6 km2 (6 sq mi)
Surface area0.61 km2 (0 sq mi)[1]

Background
Officials laying the dam's cornerstone

Construction on the dam had originally begun in 1978. At that time, it was designed as a concrete gravity dam. Construction was halted soon after it began though.[7] The dam was redesigned over the next few decades and after the new project was approved, preliminary construction for the dam began in late 2003. Principal construction began on March 24, 2009 and there was a groundbreaking ceremony on June 5, 2009.[2] To divert the river, a cofferdam did not need to be constructed. The existing Kinmu Dam 100 m (328 ft) upstream was modified to divert the river on the left bank of the river.[8][9] In May 2010, the river was diverted and by August of the same year, over 1,042,000 m3 (1,362,885 cu yd) of rock and stone had been excavated from the site.[10] On February 26, 2011, a ceremony was held to bury the cornerstone of the dam prior to beginning the fill of the dam's body. It was attended by Japanese and U.S. officials (due to the dam's location within the U.S. Marine Central Training Area).[11] It will cost ¥3 billion (US$38.2 million) and it was completed in March 2013.[2][12][13]

Design
Upstream side of the dam

The dam is a 39 m-tall (128 ft) and 462 m (1,516 ft) long trapezoidal CSG-type with a structural volume of 339,000 m3 (443,395 cu yd). The cemented, sand and gravel (CSG) design is a mixture of a concrete gravity dam and an embankment dam. Essentially, it is a sand and gravel embankment coated in concrete. This design is cheaper because it uses less fill and concrete when compared to a gravity or embankment dam. Its wide base provides the required strength while the concrete coating seals the embankment and adds to its durability.[4] The dam's crest lies at an elevation of 29 m (95 ft) and sits at the head of a 14.6 km2 (6 sq mi) catchment. The dam's main spillway is a labyrinth weir-type over its main crest. It has a design discharge capacity of 430 m3/s (15,185 cu ft/s). The dam is equipped with an emergency spillway as well, which will have a capacity of 160 m3/s (5,650 cu ft/s).[14] It impounds a reservoir with a capacity of 8,560,000 m3 (6,940 acre⋅ft), of which 7,860,000 m3 (6,372 acre⋅ft) is active (or "useful") capacity. The reservoir's surface area is 0.61 km2 (0.24 sq mi).[15]

References
  1. "Okukubi Dam Overview" (in Japanese). TAISEI Corporation. Retrieved 24 July 2011.
  2. "Construction progress" (in Japanese). TAISEI Corporation. Retrieved 24 July 2011.
  3. "okukubi Dam flood high water reached!". Sarutra. 1 April 2013. Retrieved 2 August 2013.
  4. "Okukubi Dam is the world's first trapezoidal CSG dam". North Dam Construction Office. Retrieved 24 July 2011.
  5. (Organised by Spanish National Committee on Large Dams -SPANCOLD), Edited by L. Berga (2003). Roller compacted concrete dams : proceedings of the IV International Symposium on Roller Compacted Concrete (RCC) Dams, 17-19 November 2003, Madrid, Spain. Rotterdam [u.a.]: Balkema. p. 27. ISBN 90-5809-564-9. {{cite book}}: |first= has generic name (help)
  6. "Purpose". North Dam Construction Office. Retrieved 24 July 2011.
  7. "Dams in Okinawa: Okukubi" (in Japanese). PCPulap. Retrieved 24 July 2011.
  8. "Schedule" (in Japanese). North Dam Construction Office. Archived from the original on 27 September 2011. Retrieved 24 July 2011.
  9. "River Diversion" (in Japanese). North Dam Construction Office. Retrieved 24 July 2011.
  10. "Okukubi Dam" (in Japanese). CMED. Retrieved 24 July 2011.
  11. Baez, Dengrier M. (4 March 2011). "Ceremony marks completion of Dam". Marines.mil. Retrieved 24 July 2011.
  12. "Managing a wild horse with a rotten rope: A contemporary history of Okinawa". The Ryukyu-Okinawa History and Culture Website. Retrieved 24 July 2011.
  13. "億首 （ おくくび ） ダム" (in Japanese). The Association of Certified Chief Managing Engineer Dam Construction. Retrieved 7 February 2013.
  14. "The design of the spillway of the dam neck billion (adoption of the labyrinth weir)" (PDF) (in Japanese). North Dam Construction Office. Retrieved 24 July 2011.
  15. "Key Features of Dams in Okinawa". North Dam Construction Office. Archived from the original on 27 September 2011. Retrieved 24 July 2011.
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