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Neckartal’s agricultural potential still on ice

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Neckartal is a curved gravity dam in the Fish River of southern Namibia. Image credit: Leon Louw for WhyAfrica

Neckartal’s agricultural potential still on ice

Neckartal, nicknamed the Desert Dragon, is a curved gravity dam in the Fish River of southern Namibia. I took an off-road detour to visit the dam about 40km from Keetmanshoop in the //Karas Region on the fifth day of the recent WhyAfrica Southern Africa Road Trip.

By Leon Louw founder and editor of WhyAfrica

The Neckartal dam and Phase 1 bulk water supply was inaugurated by the then Vice President of Namibia Dr Nangolo Mbumba, on 13 March 2020.

The dam was designed by South African based civil engineers Knight Piésold Consulting and constructed by Italian headquartered Salini-Impregilo. The project is located about 40km west of the small town of Keetmanshoop and is the first phase of the Neckartal Irrigation Scheme (NIS) which the Namibian government hopes will improve the region’s agricultural development, especially for cultivating products such as lucerne, grapes and dates.

The dam is part of vital infrastructure planning to provide water and irrigation to the arid southern region of Namibia. “The irrigation scheme covers about 1,960 hectares of land and will promote agriculture and employment 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 approximately 5,000 hectares,” Mbumba said at the project’s inauguration.

When I visited the dam as part of WhyAfricas Road trip, the irrigation scheme was yet to be commissioned. According to the Namibian Ministry of Water, Agriculture and Forestry, there are several disputes about land acquisition and the delay in starting up the irrigation scheme is because the government has not been able to secure the necessary land needed to make the scheme a reality.

Once the scheme gets underway it would irrigate agriculturally developed land, aligned to the government’s Green Scheme Policy, approximately 21km away. “From a downstream abstraction weir, pumped water is piped to a reservoir above the irrigation area, then gravity fed to farms. Controlled release of the dam water to the weir fills the reservoir and, simultaneously, generates hydropower,” says David Stables, Principal Project Leader for the Neckartal Project at Knight Piésold Consulting.

Three dams in a parched landscape 

Neckartal is in an extreme arid region of Namibia, and to see so much water in this part of the country, is impressive. Neckartal is the largest dam in Namibia with the volume exceeding that of the second largest dam in the country, Hardap, by a factor of three.

But Neckartal is not the only big dam in the southern parts of Namibia. In fact, Namibia’s largest three dams are all located in the southern regions of //Karas and Hardap. After Neckartal and Hardap (also in the Fish River), the Naute Dam is about 50km south-west of Keetmanshoop and provides the town with potable drinking water. The source of Naute dam’s water is the Löwen River, a tributary of the Fish River. The Naute Dam was built between 1970 and 1972 by South African company Concor and was officially commissioned in September 1972.

Standing on Neckartal’s dam wall is almost surreal. The water mass is surrounded by an expanse of parched land, in my experience only eclipsed by the extreme aridity of the Richtersveld desert lands and some parts of the Fish River Canyon, claimed to be the second largest canyon in the world after the Grand Canyon in the USA. When the sun beats down and the dry air almost suffocates you, it is not hard to understand why the dam was named the Desert Dragon.

The birth of a desert dragon  

The Neckartal dam overflowed for the first time in January 2021, amidst a global pandemic. The structure across the Fish River has a crest length of 520m and a height of 80m and can store 850 million cubic metres (Mm³) of water. Over one million cubic metres of reinforced cement concrete (RCC) was needed to complete the dam wall.

In a water scarce region of Africa with limited capacity for large dams, Neckartal is huge. It is the eighth largest dam in Southern Africa and the reservoir stretches more than 36km upstream. The dam has a shoreline of almost 290km.

Neckartal is in the Fish River, a major tributary to the Orange River. The outflow of the Fish River joins the Orange River (the longest river in South Africa) at the border with South Africa south of Neckartal and about 100km from the Atlantic Ocean to the west.

The Necartal dam has a shoreline of almost 290km. Image credit: Leon Louw for WhyAfrica

Environmental considerations

The Fish River is an ephemeral system, in other words the river only flows during periods of rainfall in the catchment. According to the environmental report the local fauna and flora have adapted to these harsh conditions and the lack of water limits the number of animals that can be sustained by the Fish River.

The impoundment of Neckartal Dam has generated a local sustainable water source, thereby creating a much larger habitat for plants and animals. According to locals in the area animal and birdlife have already increased significantly in the surrounding area since impoundment started.

During the design phase of the project, alternative options for installation of the 9 km-long pipeline between the pump station and the holding dam, above and below ground, were investigated. In the end, the decision was made, in combination with environmental requirements, that the pipeline be buried below the natural ground level, to limit the obstruction of animal migration and to ensure that the infrastructure is less intrusive to the natural surroundings.

Innovative construction technology

According to a report by Knight Piésold Consulting the Neckartal project consists of a 78.5m high RCC gravity arch main dam; a 10m high RCC abstraction weir (both structures have uncontrolled Ogee crests); a 2.1 square metre (m2) per second pump station; a 9 km-long pipeline of diameter 1.1m; and an HDPE-lined embankment holding dam with a water storage capacity of 90,000m2.

Projects of this magnitude need innovative construction technology to be executed successfully and even more so for the Neckartal Dam due to its desolate location in the arid climate of southern Namibia.

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 variability 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, 4.71m wide by 5.8m 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.

To overcome the long concrete haulage distance of more than three kilometres, a 600m-long conveyor belt, traversing 95m down the side of the gorge, was used to quickly transport fresh RCC from the concrete batch plant situated high on top of the left bank down to where construction was taking place.

Throughout the peak of summer, high temperatures of more than 40 degrees Celsius required that thought be given to the placement of concrete to avoid excessive heat during the hydration process of concrete curing.

A special finite element model was designed to estimate concrete temperatures during the placement of the RCC, whilst the cast of Conventional Vibrated Concrete (CVC) was only allowed during the cooler hours of the night. All these design ingenuities contributed to the successful completion of the project.

The continuous uniform double curved shape for the dam spillway (an Ogee spillway) inherently posed some difficulties for the construction team. Innovation was required to decrease the construction time whilst maintaining the accuracy of the constructed profile and avoiding honeycombing and blowhole formation on the finished Ogee surface.

For the main dam, conventional construction methods (using sliding formwork or guide rails) 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 techniques used for the construction of the abstraction weir’s Ogee crest.

To aid the monitoring capabilities of the site engineers’ supervision (monitoring, and quantifying different construction activities), unmanned aerial vehicles (UAVs) were introduced. UAVs were utilised for photographic surveys to develop accurate three-dimensional models that were used to monitor the construction progress of the project.

In addition, accurate surveys, project monitoring, material quantity measurements, building information modelling integration and the sharing of insights around the construction site were carried out with the aid of the UAVs. The project was one of the first dam construction projects in the Southern African Development Community (SADC) region to apply this technology.

Neckartal Dam is the most significant concrete structure built in the last decade in Southern Africa. It is one of the largest concrete structures constructed in southern Africa in recent years, if not the largest concrete structure of recent times. It is primarily constructed of concrete with very few other construction materials.

Source: Knight Piésold

Leon Louw is the founder and editor of WhyAfrica. He specialises in the extraction and responsible utilisation of natural resources, the primary sector of African economies and Africa’s political economy. 

WhyAfrica reports about, and publishes newsletters, magazines and research reports about natural resources and the primary sectors of African economies, and the infrastructure, equipment and engineering methods needed to extract and utilise these resources in an efficient, responsible, sustainable, ethic and environmentally friendly way, so that it will benefit the people of Africa.

Furthermore, WhyAfrica promotes Africa as an investment and travel destination, analyses the continent’s business environment and investment opportunities, and reports on how the political economy of African countries affects its development.         

WhyAfrica provides you with business intelligence that matters. Africa is our business, and we want it to be yours too. To subscribe to WhyAfrica’s free newsletter or digital magazine, and for more news on Africa, visit the website at www.whyafrica.co.za or send a direct message. WhyAfrica launched its first ever digital magazine in November 2021.

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