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Modelling, Design and Construction Monitoring of Neckartal Dam, Namibia

Author: G.L. Coetzee, S.J. van Vuuren
Conference: AFRICA 2019
Date: April 2-4, 2019

Neckartal is the largest dam in Namibia, with a full supply volume of 853 x 106m3, exceeding the volume of what was previously the largest dam, Hardap, by a factor of three. Projects of this magnitude need innovative construction technology to be implemented successfully. In the case of Neckartal, a special challenge was its isolated location in the arid climate of southern Namibia, with irregular, high peak runoffs. During the design of the dam, two physical models were built. Results from the physical model testing were used to improve the safety of the spillway and reduce potential scour erosion downstream. Throughout the construction of the dam, innovative techniques were used, which are described in this paper.

In the early 20th century, German colonialists identified the site of the Neckartal dam, in the arid southern Karas region of Namibia. The Neckartal Dam and Phase 1 Bulk Water Supply Project is on the Fish river, about 41 km west of Keetmanshoop and some 22 km north of Seeheim. The dam has a catchment area of 45 365 km2 and a mean annual runoff (MAR) of 397 x 106m3/year, categorizing the storage volume of the dam as 2.14 of the MAR. At the time of Namibia's independence in 1990, planning of the dam was initiated, although a provisional design has been undertaken in the 1960s. The Namibian Ministry of Agriculture, Water and Forestry (MAWF), also referred to as the client, decided to implement the project with the aim of improving employment and aiding the long-term sustainable economic development of the Karas region. The project envisages elevating the agricultural development of the region with 1960 ha of irrigable farmland to be developed during Phase 1, which may be further expanded to 5000 ha.

The final design of the structure consisted of a 65 .5 m-high, curved, stepped, gravity, roller compacted concrete (RCC) wall with an uncontrolled ogee spillway, consisting of a lower spillway section and a higher spillway raised by 2.4 m. There is a multi-level intake structure with eight DN1600 and two DN3000 intakes. Other features of the dam include a spillway chute on the right bank to prevent flood erosion of the downstream foundation, two internal galleries, a control room and outlet works, together with the machine hall and sleeve valve house. With a recommended design discharge (RDD) of 9060 m3/s and a safety evaluation flood (SEF) of 21 480 m3/s, the spillway length is significant and necessitates the widest possible spill area to reduce the unit discharge rate to an acceptable criterion of less than 30 m3/s/m.

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