Kariba Dam

 

 by Er.E.Santosh Kumar & Er.S.Padma Bharath

PRESENTATION ON KARIBA DAM

 

The Dam was designed by French engineer and inventor  Andre Coyne et Bellier Bellier,, a specialistdesigned in “Arch over Dams”, he personally 70 dams in fourteen countries, Kariba Dam  being one of them.

 

The Kariba Dam is a double curvature concrete arch dam located at 16°31′18″S 16°31′18″S 28°45′41″E in the Kariba Gorge of the 28°45′41″E Zambezi River Basin between Zambia and Zimbabwe.

 

The dam was built by the British who colonized and ruled Rhodesia which whi ch is now known as Zambia and Zimbabwe. Construction cost is about US 480 million dollars.

The Kariba Dam is now owned and operated by the Zambezi River Authority , wh whic ich h is  jointly and equally owned by Zimbabwe and Zambia.

 

The arch dam was constructed  between 1956 and 1959 and supplies  water to two underground hydropower plants located on the north bank in Zambia and on the south bank in Zimbabwe. The north bank power station was commissioned in 1960 and the south bank power station in 1976.

Sir Duncan with Her Majesty Queen Elizabeth the Queen Mother at the official opening of the kariba dam.

 

The name Kariba (Kariva  –  meaning trap) refers to a rock that comes out of o f the water At the entrance to the gorge close cl ose to the dam wal wall. l. The rock is said to be the home of the Nyaminyami , the river God .

 

Re Settlement: Settlement:

Due to the building of the dam wall , tribes of about 57,000 Tonga people living along the

Zambezi valley upstream of it in both Northern Rhodesia (no (now w Zamb Zambia) ia) and Southern Rhodesia (now Zimbabwe) were forced out of Kariba and now inhabit other parts of Zambia and Zimbabwe.

 

420 kilometers downstream from Victoria Falls, the Kariba Dam is the one of the largest man-made reservoir in the world.  At a height of 128m and with a crest length of of 617m, the dam has has the capacity of holding 181 billion cubic meters of water.

 

The Kariba dam creates Lake Kariba  which is 280km long at full level, 32km across at its widest, 5,400 square kilometers surface area and has a catchment area of 6,63,000 square kilometers.

 

Fishery & Tourism:

Lake Kariba Tiger fish catch.

Kariba was designed as a single purpose hydropower project, project, but as it turned out  both fishery and tourism became tourism  became important benefits. Fishery and fish farming have become one of the most important secondary benefits Fishery and of the Kariba project.

 

Construction of the dam has led to the areas preservation of wilderness in National parks along the lake shore. shore. This has helped grow a tourist industry in the area boosting the local economy.

 

Raftingthe onVictoria the Zambezi  below Falls River,

 

Technical Data: Catchment Catch ment Area Area is 6, 63,000 63,000 sq.km. Surface area is 5400 sq.km. Total storage is 180 180.6 .6 CUB CUBIC IC KM Live Storage is 64.8 CUBIC KM Height of dam dam - 128me 128meters ters Crest length - 617me 617metres tres Crest Thickn Thickness ess - 13 metres. metres. Base thickn thickness ess - 24metr 24metres. es.  Volume of concrete - 1.036 million cubic metres.

 

Spillway: Caterpillar gates, 6 no. of gates each 8.8m wide and 9 metres height. Discharge capacity – 9000M3/s. Kariba was designed for the safe passage of a 1 in 10000-year flood.

 

Tailrace outlets

 

South Zimbabwe

South Power station

North Zambia

North power Station

 

Hydel Power Generation: Generation: Power from the reservoir is generated through two underground power stations located on the North (left) bank in Zambia and on the South (right) bank in Zimbabwe . There are twelve francis type turbines, six on either side of the Zambezi River. More specifically 6 x 125 MW (750 MW) on the South bank and 6 x 180 MW (1,080 MW) (four old and two recently commissioned) on the North bank. The Kariba Dam supplies supplies 1,626 megawatts (2,181,000 hp) of electricity to parts of bo both th Zambia Zam bia(th (thee Cop Copper per belt) belt) and Zimbabw Zimbabwee and generat generates es 6,400 6,400 gigawa gigawatt-h tt-hour ours(2 s(23,00 3,000 0 TJ) per annum.

 

Floods During Construction Period: Period:

On the Christmas Eve 1955 an unprecedented flood stormed down the George, washing away the foundations of the dam and in November 1956, heavy rainfall a month before they were due, causing more flooding and impeding work on the dam. On 20th February 1959, a platform in a shaft to the power station collapsed killing 17 workers and by this time the Dam construction has cost the lives of over 87 people.

 

Operation Noah: Noah:  As the reservoir began to fill, animals got trapped on newly formed islands  Which led to ‘Operation ‘Operation Noah’ Noah’ which was the rescue of these animals from 1958 to 1961, captured and removed around 6,000 large animals and numerous small ones threatened  by the lake's rising waters.

 

Expedition leader Rupert Fothergill (L) along with other rescue workers saving an antelope from the flooding waters caused by the Kariba dam.

Baby saddlebill stork rescued from the flooding  waters of the Kariba Kariba dam playing playing with dog.

Game wardens bagging a porcupine during the rescue of animals from the flooding waters of the Kariba dam.  

Seismic Activity : The filling of what was then the world's largest man made reservoir was followed by considerable earthquake activity. The weight of Kariba’s reservoir translates into a mass of 180 billion metric tons. The reservoir is located in a tectonically active area, area, at the southern end of the African Rift Valley.

Since its construction and filling in the early 1960s, Kariba has caused  caused  numerous earthquakes in the area, 20 of them larger than magnitude 5 on the Richter scale. Project documents did not discuss the possibility of reservoir-induced seismicity and the need to take this into account in the design of the dam, dam, so the seismic activity’s affect on the dam’s safety is unknown.

 

Lake Kariba region was considered a seismic prior to the impoundment in early 1959. Figure 2, extracted from the Archer and Allen (1979) catalog . of earthquakes for the Kariba region, illustrates that, shortly after impoundment, seismicity increased dramatically. As witnessed at other reservoirs associated with RIS, the figure displays the apparent correlation between the changes in the reservoir's water level and the increase in regional seismicit seismicity  y .

 

 ADVANTAGES OF THE KARIBA DAM  POWER: Electricity prices prices decreased by up to 30% between 1961-1977. 1961-1977. TOURISM: The Kariba Dam created one of the worlds largest artificial lakes  behind the concrete wall.plus Thisthe created such things as fishing,  water sports, swimming, dam opportunities itself attractedfor attention. EMPLOYMENT: Opportunities for employment from tourism and recreation. FISH: A successful fishing scheme was set up for commercial comm ercial and sport fishing.  WILDLIFE: Operation Noah - By 1959 the operation had rescued 6000 animals from the negative dam impacts.

 

 DISADVANTAGES OF THE KARIBA DAM  DISPLACEMENT: were displaced 27,000 Southern lived on the Zambian Northern 57,000 side andpeople the remaining 27,000and on resettled, the Zimbabwean bank of the river river (Gillett et al, 2002). 2002). Not all of the popul population ation were were not informed informed about the dam dam prior to early construction. TONGA: 30,000 of these were a part of the Tonga tribes. Social and cultural heritage  was lost and minimum compensation was provided. FISH: The dam blocked the migration m igration of the native fish species, this separated spawning habitats from rearing habitats. RIVER CHANGES: The Zambezi River experienced irreversible morphological changes c hanges especially in the delta and wetlands. SEDIMENT BUILD UP: The lower Zambezi River shows continuous aggradations (deposition of sediment) and sediment fining. ENVIRONMENT: There was no Environmental Impact Assessment Assessment (EIA) carried out prior to construction.

 

INCREASED EARTHQUAKE ACTIVITY: construction LakeofKariba caused increased earthquake activity. TheKariba faulteddam rift valley is in theand middle Zambia and Zimbabwe and the lake re-activated some of the faults as 3 seismographs allowed the location of epicenters for 159 of 2000 tremors recorded during three years while the lake filling.

 

Problems and the Kariba Dam Rehabilitation Project

 

On 3 October 2014 the BBC reported that: “The Kariba Dam is in a dangerous state. Opened in 1959, it was built on a seemingly solid bed of basalt. But, in the past 50 years, the torrents from the spillway have eroded that bedrock, carving a vast crater that has undercut the dam’s foundations. Engineers are now warning that without urgent repairs, the whole dam will collapse. If that happened, a tsunami-like wall of water would rip through the Zambezi valley , reaching the Mozambique border within eight hours. The torrent would overwhelm Mozambique’s Cahora Bassa Dam and knock out 40% of southern Africa’sRiver hydroelectric Along with wildlife in the valley, the Zambezi Authoritycapacity. (ZRA) estimates thatthe thedevastation lives of 3.5 of million people are at risk.”

 

Spillway torrents have excavated a massive cavern in the Zambezi river bed, now 10 times bigger and deeper than the original design dimensions, that threatens the stability of the  wall foundations.

 

It is necessary to reduce the turbulence from water, discharged at 8,000 tonnes a second at times, into the plunge pool below the dam by enlarging the cavern, and allowing the turbulence resulting from the spillway discharge to be dissipated in a less damaging way.

 

 Apart from the need to rehabilitate the plunge pool, there is also a need to rehabilitate the six sluice gates that make up the spillway. spillw ay. The work needed within the sluices is associated with the refurbishment of the concrete surface of all sluices which have been distorted over the years due to an advanced alkali-Silicate reaction(ASR).  Without functional sluices the reservoir level cannot effectively be maintained to take into account the flood regime of the Zambezi River. Without the ability to release  water from the reservoir, there is a danger of the reservoir being too full prior to a flood Event, and the subsequent flood event causing over topping of the dam wall which could lead to dam failure.

 

Funding for Rehabilitation of the Dam: Dam: The European Uniongrant (EU)to isZambia the largest — with a Kariba 64 million Euro (USD100million) for thefinancier project, making the Dam Rehabilitation Project one of the largest EU funded projects in Africa. The other financiers are the World the World Bank ($75 million loan), the African the African Development Bank ($36 Bank ($36 million grant to Zimbabwe and a $39 million loan to Zambia), and Sweden ($25 million grant) with the balance being paid by Zambia and Zimbabwe, through their jointly owned Zambezi River Authority (ZRA) (ZRA)— —operators of the dam. The dam repair has raised hopes for people in Kariba and Siavonga, the two towns overlooking overlooking Kariba with with the promis e of jobs jobs an d increased increased business busine ss will opportunities that the influxDam, of people intopromise the area for theand duration of the project  bring. The rehabilitation of the world’s largest man-made man-made reservoir will prevent both from turning into ghost towns.

 

French firm Razel-BEC International Development Development vice  vice Yamba president Mr. Ericengineering Thouvenel (left), Zambian Secretary to The Treasury Mr.Fredson and head of delegation of the European Union to Zambia Mr. Alessandro Mariani (right) sign the $294 million contract for rehabilitation at the Kariba Dam Wall. Wall . The deal deal will allow allow Razel-Bec Razel-Bec to reshape the plunge pool beneath pool  beneath Kariba and make repairs to the dam's spillway . ZRA officials said they expect work on the plunge pool to be complete by 2021, and work on the spillway to be finished in 2022.

 

The project will be carried out under the auspices of the Zambezi River Authority, a  bi-national organization managing the Zambezi River on behalf of the governments of Zimbabwe and Zambia. The rehabilitation contract includes reshaping of the plunge pool and protection of its fault zone using reinforced concrete mattress and refurbishment of the spillway to improve the operation and reduced the risk of the upstream spillway control facility. Reshaping the plunge pool will lead to an increase in efficiency and an improved capacity to dissipate the energy generated by the spilling events — those moments when the spill gates are opened to release water. The aim of the Kariba Dam Rehabilitation Works is to improve the stability of the plunge pool through reshaping its profile. profile. This will limit the preferential erosion towards the foundations of the dam along zones of weak rock . The project also aims to rehabilitate the six sluice gates of the spillway, enabling the ongoing use of the spillway function to manage the reservoir levels.

 

Generally, the rehabilitation of the plunge pool will pool  will include a number of activities activities:: (i) The constructi construction on of a cofferdam cofferdam just downs downstream tream of of the plunge plunge pool pool,, whic block off the plunge pool from the downstream river. (ii) The pumping/dewatering pumping/dewatering of the plunge pool. (iii) The excavation of the plunge pool. (iv) The deposition of excavated rock material in the existing quarry on the north ban (v) The reshaping of the excavated plunge pool into terraced steps excavation and pumping will be carried out simultaneously. While excavations are being carried out on one of the plunge pool steps, the pumps wil willl keep lowering the water level underneath. The objective is to be able to excavate continuously even when switching from one step to the next one situated below it. An estimated 295,000 m³ of rock will be carefully excavated due to the excavation depth below the current Tail Water Level L evel (TWL). The reshaping of the plunge pool into terraced steps will reduce dynamic pressures in the pool and reduce flow recirculation towards dam toe. As a result, it is estimated that the power density will be reduced from 25 kW/m3 to 7.5 kW/m3. Trial blasts will be carried out, whereby increasing charges

of explosives will be fired and the impacts of the vibrations on the surrounding sensitive structures will be measured.  

In order to arrive at the above process the engineering team undertook; Multi-beam Bathymetric Survey of the Pool; Plunge Pool Geotechnical Investigations; and Plunge Pool Hydraulic Modeling.

 

Multi-beam bathymetric photo of the Kariba Dam plunge pool.

 

The reshaping of the plunge pool Calls for the construction of a temporary  cofferdam or barrage. This will enable the blasting and excavation of 300,000 cubic metres of rock from the

downstream face and north and south  bank sides of the pool in the dry season.  

The Zambezi River Authority (ZRA) The ZRA operates under the Zambezi River Authority Act, and is mandated to Harness and manage the Zambezi River waters for socio-economic development and to Maintain the Kariba Dam complex including any future dams or infrastructure on the river forming a common border between Zambia and Zimbabwe. The ZRA will manage the Kariba Dam Rehabilitation Project together  With other Consultants, Engineers and Construction entities as necessary necessary and Appointed for the project.

 

References:  Wikipedia, Case study prepared for World commission on dams, zaraho.org.zm, zaraho.org.zm, zimfieldguide.com, zimfieldguide.com, sundaymail.co.zw , internationalrivers.org internationalrivers.org..