Civil Engineering December 2021 | Vol 29 No 11

T he Neckartal Dam and Phase 1 Bulk Water Supply, inaugurated on 13 March 2020, is a symbol of hope for a Namibia that wishes to move forward from a series of drought hardships faced over the past few years. By implementing this environmentally sustainable project, success has been made to secure water supply and assis- tance in providing for the country’s food independence. The project is part of vital new infrastructure planning that will deliver water to the Karas Region in the arid southern region of Namibia. It will provide irrigation for about 1 960 ha of land, promoting agriculture and employ- ment in the sub-economic area of the country. The project may be extended in future with a second phase, which will increase the irrigatable land to approxi- mately 5 000 ha. PROJECT OVERVIEW The dam, which is the largest in Namibia, is located 40 km west of the small town of Keetmanshoop. It is the first phase of the Neckartal Irrigation Scheme which will improve the region’s agricultural development. The Fish River’s water volume, im- pounded by the dam, through deliberate releases, flows downstream for 13 km to reach an abstraction weir. From there, water is pumped from the adjacent pump station (capable of pumping at 2.1 m 3 /s) to a holding dam in an approximately 9 km- long steel pipeline of 1.1 m diameter, from where it is routed to the irrigation areas for agricultural use as needed. The dam was built using the roller compacted concrete (RCC) construction technique and is 78.5 m high with a crest length of 518 m. The dam has a storage volume of 857 million m 3 extending over a 39 km 2 surface area at its full supply level. Over 1.1 million manhours were utilised to construct the project. SPILLWAY CONSTRUCTION The continuous uniform double curved shape for the dam spillway (an Ogee spillway) inherently posed some difficul- ties for constructability. Innovation was required to decrease the construction time while maintaining the accuracy of the constructed profile and avoiding honeycombing and blowhole formation on the finished Ogee surface. For the main dam, conventional con- struction methods needed reconsideration to reduce the time of construction of the Ogee crest. Eventually, construction was executed using controlled permeability formwork. This technique reduced the construction duration of the Ogee crest by a factor of more than two when compared to the conventional construction tech- niques used for the construction of the abstraction weir’s Ogee crest. 24/7 CONSTRUCTION PROGRAMME Construction of the project continued 24 hours a day, all year round, except for a short break between each Christmas and New Year’s Day. This required that special thought be given to the seasonal vari- ability during the design of the project. For the duration of the wet season, large cofferdams had to be constructed, diverting the rushing waters of the Fish River to enable construction and allow access without flooding of the site. Two large culverts, 4.71 m wide × 5.8 m high at the inlet side, were constructed transversely through the dam wall. These multipurpose culverts were used for vehicles to travel between the up- and downstream sides of the dam during the dry season and for routing river flow during the wet season. In Keetmanshoop and in the sur- rounding areas, the project created a total of 5 500 jobs. 3 000 workers were hired directly and 2 500 indirectly, with 65% of these workers coming from the Karas Region.  SANRAL SA I CE 2021 Nat i onal Awards Category: I nternat i onal FINALIST International category (Sponsored by SKI Civil and Structural Engineers) KEY PLAYERS Client: Namibia’s Ministry of Agriculture, Water and Land Reform Consultant: Knight Piésold Contractor: Salini-Impregilo Construction of Neckartal Dam and Phase 1 Bulk Water Supply Civil Engineering  December 2021 41 A 600 m conveyor belt, traversing 95 m down the side of the gorge, transported fresh RCC to the site The 78.5 m high RCC gravity arch Neckartal Dam under construction