The media hype about an imminent Kariba Dam failure

Thu, 13 Apr 2017 09:33:31 +0000

 

By Ronald Lwamba

Kariba Dam operators have been advised to open only three spillways in an event of a flood, to alternate the gates that they open, and not to open Gate 6 at all.

What kind of evacuation plan would be needed should the dam fail in some way, and is that plan in place? However, it is standard practice to have an Emergency Action Plan in place, these days, detailing actions to be taken, the area to be inundated, identification of key personnel, emergency services, drills, etc. I know there is one for Itezhitezhi Dam because I was involved in its preparation.

The turbines at the power plant are quiet efficient in the use of the water resource. The new units have a guaranteed efficiency of 93% at an operating head of 89m and this is despite the myth about poor Chinese machines having been installed.

The maximum efficiency is 95.99%. The old units have a guaranteed efficiency of 92% at an operating head of 88m. The guaranteed efficiency is 94.18%. Apart for reasons of scheduled maintenance and other repair works the generating units’ availability is good. (search for performance curves)

The depletion of the reservoir is a boon to the rehabilitation works because it is unlikely that spilling of water will occur as work progresses as there will be no excess water as the inflow will go into the depleted storage space, thereby allowing the rehabilitation works to go on uninterrupted i.e. work on the floodgates and openings and also the reshaping of the plunge pool on the Zambezi River bed.

In addition, the hydrology of the Zambezi River and specifically the upper catchment and mid-reaches flowing into the Kariba reservoir are heavily influenced by the Barotse floodplains which  attenuates or weakens the flow and results in a lag time of about three months between peak rainfall and peak runoff. So there will be advance warning from rainfall monitoring in the upper catchment, allowing for time to respond.

This, coupled with the large size of the reservoir, means that Kariba has the ability to absorb large flood events and prevent a sudden rise in water levels and therefore it is highly unlikely for a flood event to take anyone by surprise. It is also possible to modify the spillway gate operation rules to hold the reservoir level down in advance of a flood, but this is most unlikely to be necessary.

With the current rainy season the challenge is how much of the runoff should be used to generate electricity and how much should be stored taking into account the capricious moods of the weather in the Zambezi River catchment which is unpredictable. The Barotse flood plain is a huge chunk of mainly bush land cut by tributaries that deposit water into Africa’s fourth-biggest river on its way to the gigantic Lake Kariba, some 185-billion tonnes i.e. 185 billion cubic metres of water in extent. So far, the rainy season has been reasonable.

Although the risk of spilling from the dam has reduced due to the current low water levels in the reservoir after some dry years caused by the El Niño, which has since been replaced by El Niña and therefore, it is highly unlikely that the rehabilitation works will be interrupted due to spilling of water, there is also the increased generation capacity which has resulted from the power rehabilitation project, which has increased the generating capacity from 600MW in the Kariba North Bank Power Station to 720MW and the 360MW Kariba North Bank Extension, which can take up any excess water instead of spilling. The Zimbabwe South Bank has six turbines producing a total of 666MW. China’s SinoHydro is installing a further 300MW capacity at a cost of $533m funded by China Bank, which is likely to be completed in the near future. It is important to note that most of the water was spilled in the first twenty years of operation mainly because for the most part there was only one power station on the Zimbabwean side, which resulted in the excessive erosion of the plunge pool.

The cost of the rehabilitation works is $295m rescue operation  and is being funded by the European Development Bank ($100m), African Development Bank ($75m), World Bank ($75m) and Swedish government ($47m). The rehabilitation programme is guided by the so-called independent Panel of Experts (PoE), the role of which is to provide an independent review and advice to the Zambezi River Authority (ZRA).

“The PoE has met on several occasions, most recently with a joint meeting of the ZRA, the financing partners and the various engineers,” reports the World Bank spokesman.

“They continue to provide advice to ZRA and the financing partners and have endorsed the proposed approach to the rehabilitation works.”

The region’s peak rainfall months occur from March to May. However, unseasonably heavy falls, causing flooding in some parts of the Zambezi River catchment, have already been reported.

“The Zambezi River upstream from the Victoria Falls experiences a rainy season from late November to early April, and a dry season the rest of the year,” notes the World Bank.

“However, the river’s flood season is February to May with a peak in April.”

As already alluded to, he discounts fears that the current floods upstream could fill the dam up suddenly and that this could be a danger.

On completion of the south bank upgrade in 2019, Kariba’s total capacity will be about 2000 MW.

There have been concerns about the long term functioning of the floodgates mainly due to the affects of ASR.  There have also been problems with concrete carbonation leading to the exposure of reinforcement in those areas adjacent to the gates.

The upstream stop beam guides that allow for temporary dewatering of the sluices so that work can take place on the gates need to be refurbished and strengthened. There are also plans to install an emergency gate that would come into operation in the event of a floodgate failure.

A team of French engineers regularly inspects the dam, and the problems have been known about for some time.  Experts say that the structure is still essentially sound, particularly as Kariba is an arch dam and that there is very little that can be done about the ASR except to understand what future expansion is to be expected. Gates can be made adjustable to compensate for concrete distortion. And the scour problem is controllable and doesn’t threaten the dam’s stability. However, any artificial enlargement of the scour hole should be done with care.

Kariba’s designers, the French firm Coyne et Bellier (now known as Tractebel Engineering)  have been assigned to regular checkup missions, monitoring and technical assistance. From  1963 and 2010, project management has been performed by Bernard Goguel, an internationally renowned expert on dam surveillance. In 2010, a new contract was signed. Tractebel Engineering now advises the Zambezi River Authority on matters pertaining to safety, maintenance and satisfactory operation of the dam. It includes engineering studies and works, supervision of the spillway rehabilitation, construction of an emergency gate, and sustainability of the plunge pool.

One report suggests that financing the dam wall’s rehabilitation is tricky because while Zambia is entitled to financial assistance from the international donor community, Zimbabwe is not. But it is impossible to repair only one side of the dam. The Zimbabwean government, the report continues, is currently limited in its access to concessional financing until settlement of outstanding arrears to multilateral development banks is done according to a World Bank report.

The solution that has been suggested which is extremely complicated and difficult to follow is by making a loan of the funding package to the Zambian government, a nation in good financial standing despite what the opposition says (my words).

Zambia will in turn provide the Zambezi River Authority with a loan for the project implementation, which will be repaid through its water tariffs charged to the Zambia Electricity Supply Commission and the Zimbabwe Power Corporation.

Kariba’s floodgates are designed to release the magnitude of floodwater that occurs once in 10,000 years. “So if one gate is out of action for repair and the spillway capacity reduced, it is most unlikely to result in any problem. “Even with one gate out of action the spillway capacity would probably still be enough to pass the flood event. So the risk is very small,” Mr Bates says.

The World Bank largely concurs. “The objective is to stabilise the plunge pool and prevent further scouring, particularly along the weak fault/seam zone towards the dam foundations,” its report notes. “A 3 dimension model was used to assess how the plunge pool scouring progresses and excavation would affect the stress field of the dam foundation.”

The World Bank assessment cautions, however, that the scouring is only controllable if no more than three, nonadjacent gates are opened. This will limit the spillway discharge capacity during large floods and means lowering the maximum safe reservoir level, which in turn would reduce the dam’s power generation capacity.

A set of new stop beams with an emergency opening and closing roller gate will be installed.

A hydropower specialist says the project will take several years to complete “partly because only one gate can be worked on at a time, but also because the plunge pool excavation work cannot be done if the lake level is high and the dam is spilling.

In recent announcements, officials in Zimbabwe and Zambia have been at pains to reassure the public that, while the situation is serious, it presents no immediate danger. A rehabilitation programme has been drawn up by local and international engineers; funding has been raised and is in the bank.

The second objective of the project is to rehabilitate the six sluice gates of the spillway, enabling the ongoing use of the spillway function to safely manage the reservoir levels. The spillway is located in the arch dam. It is made of six sluices equipped with downstream gates yielding a total capacity of 9000 m3/s. A reservoir rule curve is imposed to create a buffer volume of 23.2 km3 in order to safely pass the 10 000 year return period flood.

To date, the sluices can be inspected or repaired in the dry when closed by a set of stop beams in still water (downstream gates closed). Considering the distorting effects of the concrete swelling on the geometry of the spillway (Alkali-Aggregate Reaction or Alkali-Silica Reaction (ASR) as it is referred to in some literature) and the ageing of the hydro-mechanical equipment (now operated for 50 years), planned and unplanned maintenance operations in the dry should become more frequent if no rehabilitation is undertaken. In addition the upstream grooves are in poor condition and need refurbishing.

To address the need for maintenance and to be able to close the sluices in any circumstance (including cases where a downstream gate is jammed partially or fully opened), the ZRA plans to equip the spillway with an emergency gate. This new gate will be operated by a new gantry and will slide into the rehabilitated upstream grooves.

The new emergency gate shall close by its own weight against full water flow if one of the downstream floodgate is jammed in a semi-opened position. It is handled by a gantry and lowered or lifted as a whole. It slides into the refurbished upstream grooves.

The new gantry located on the dam crest will be able to lift or to lower the gate as a whole. It is mounted on rails and transfers the emergency gate from one sluice to another. Installing the new emergency gate involves rebuilding the grooves, the sills, the lintels, the surrounding concrete and partly the top of piers. The refurbishment works within the sluices (grooves, sill, and lintel) will be done after dewatering the sluices to perform the works in dry condition.

The rehabilitation measures are not expected to have any significant adverse environmental or social impacts, with any potential impacts likely to be temporary in nature. The major environmental impact was the formation of the lake which forced thousands of people to be resettled in less productive areas some sixty years ago. That, as they say, is water under the bridge.

 

The author has worked as a Town Engineer for the then Municipal Council of Livingstone and Zesco, initially as a Resident Engineer for Itezhitezhi rising to the post of Senior Manager, Civil Engineering where, among other things, he was in charge of the preparation of feasibility studies for hydropower projects. He was also a Part-time Lecturer in Renewable Energy at UNZA.  He holds a Bachelor of Engineering Degree (Civil) from the University of Zambia (1974), Post Graduate Diplomas in Water Resources Development (University of Roorkee, India) and Hydropower Development (University of Trondheim, Norway) and a Master of Engineering in Water Resources Development (University of Roorkee, India). rbclwamba@gamail.com

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