The media hype about an imminent Kariba Dam failure 

Thu, 06 Apr 2017 09:51:02 +0000

Ronald Lwamba

The 1990-93 seasons which were very dry were also experienced at Itezhitezhi Dam. In 1991 water could not be released through the main spillway for power generation at Kafue Gorge, as was the practice, because the water level had fallen below the sill of the spillway and water had to be released through the use of a low level outlet. I conducted a site visit to a ZNBC TV crew, which came to Itezhitezhi Dam to witness and film the drought. Unfortunately I missed the TV clip in which I featured when it aired on national television.

I would gladly give my right arm to see this clip which must be somewhere in the archives. he Itezhitezhi Dam stores water for the Kafue Gorge hydroelectric power plant located 400 km downstream.

The dam could not be constructed at Kafue Gorge itself because the topography here is flat (the Kafue Flats) which would have resulted in a very shallow reservoir which could have resulted in high evaporation losses. It is a rock-filled type with an impermeable core of clayey laterite, and reaches a maximum height of 55 m.

Because water is required continuously at Kafue Gorge for power generation the spilling of water at Itezhitezhi Dam is also released continuously. The picture has since changed with the installation of a power station because the same water that generates electricity at Itezhitezhi Dam can be used to generate electricity at Kafue gorge, and the continuous spilling of water for power generation at Kafue Gorge is no longer necessary. However, the spillway is still important in the evacuation of excess flood water.

Every reservoir has a certain capacity to store water. If the reservoir is full and flood waters enter the same, the reservoir level will go up and may eventually result in over-topping of the dam. To avoid this situation, the flood has to be passed to the downstream side and this is done by providing a spillway which draws water from the top of the reservoir. A spillway can be a part of the dam or separate from it. For a dam made of earth and rock fill like the Itezhitezhi Dam this would be disaster, if over-topping occurs, as the earth and rock fill materials would simply be washed away unlike a concrete dam such as the Kariba Dam which may survive an over-topping. This is why there is an additional emergency spillway at Itezhitezhi that comes into play if the capacity of the main spillway is exceeded.

There is a main concrete spillway with three ports, with radial gates for both normal regulation of the river’s flow and passing of overflow exceeding the reservoir’s capacity. The chute is a concrete lined channel. The method of energy dissipation at Itezhitezhi Dam is different from that at Kariba Dam. Here the method of energy dissipation is through a flip bucket, also called a ski-jump dissipater, which throws the jet at a sufficient distance away from the spillway where a large scour hole may be produced. Initially, the jet impact causes the channel bottom to scour and erode. The scour hole is then enlarged by a ball-mill motion of the eroded rock pieces in the scour hole.

A small amount of the energy of the jet is dissipated by the internal turbulence and the shearing action of the surrounding air as it travels in the air. However, most of the energy of the jet is dissipated in the plunge pool.

After hearing about the excessive erosion at the plunge pool in 1991 at Kariba Dam, and because the spillway at Itezhi tezhi Dam was not in use because of low reservoir levels I decided to examine the extent of the plunge pool formation at Itezhitezhi.

I found that it had eroded up to 20 m deep, which was not very bad considering that the spillway had been in constant use, 24/7, for about 15 years. The other advantage is that the spillway is located on the reservoir rim away from the main dam embankment unlike the spillway at Kariba Dam which is through the main dam wall and the formation of an excessively eroded plunge pool cannot, therefore, compromise the structural integrity of the main dam. Despite these two problems having been known for years, there was a sudden media frenzy that alleged that the dam could collapse within three years as if these problems have just been discovered now. There has been much hype about an imminent Kariba Dam failure. The Zambezi River Authority’s communications manager, Elizabeth Karonga, says politicians were trying to convey to outside funding agencies the urgent need for major rehabilitation of the dam.  “ZRA regrets the misinterpretation on the facts about the Kariba Dam wall impending collapse, which caused alarm among stakeholders and the public in general,” said ZRA communications manager Elizabeth Karonga. While the situation at the Kariba Dam wall is cause for grave concern, the engineers on the ground have the situation under control. This does not translate to an immediate calamity. However, all urgency is expected in order to avert any such catastrophe as dam failure.”

Details of the dam wall’s weakness filtered out into the tightly knit Kariba and Siavonga township communities last year, setting off alarm bells. In response, statements were issued by officials, local bodies and politicians — for the most part wildly contradictory, creating much of the alarm and confusion. According to a Kariba businessman these alarmist public pronouncement about an impending dam failure were aimed at “rattling the cages of international funders and it worked. It got their attention and after that money for repairs came quite quickly, all $300m.”

A Kariba Dam wall collapse would release 185 km3 (185,000,000,000 m3) water when full, threaten the lives of 3.5 million people living on the Zambezi River floodplain between the hydro scheme on the Zambia-Zimbabwe border and the Mozambique coast and some have even suggested that it may even flood Lusaka, which I cannot verify, and economic damage to the region will be several billion dollars and will include the washing away of the Cahora Bassa Dam.

Around 1.7 GW of electricity generating capacity would be lost across the region. It would be a repeat on a massive scale of the 1959 collapse of the Malpasset dam in the South of France, which was also the work of the Kariba Dam designer André Coyne, see my earlier article on “The History of Hydropower Development in Zambia- The Construction of Kariba Dam”.

Fears of collapse have been exacerbated by recent reports that climate change would lead to more extreme flooding in the Zambezi catchment area. A nightmare scenario includes all six floodgates having to be opened at the same time for the first time in more than three decades, thus accelerating scour and triggering a foundation failure. Alternatively, the gates could jam due to ASR expansion, leading to overtopping and possible dam wall collapse.

The design has six spillway gates that release water at the rate of 9000m3/s when the reservoir levels go above the maximum flow stipulated by a “rule curve.” At most times the spillway gates are opened in advance of predicted floods for dam safety reasons.

ZRA has engaged consultants who have proposed a number of options to address the problem. The preferred option being referred to as reshaping the plunge pool is one that will excavate and widen the hole in order to dissipate the energy from the spillways, thus removing 295,000 m3 of rock. The bigger plunge pool will reduce the excessive turbulence that scours the hole.

This work will involve building a cofferdam 18-20m downstream of the plunge pool, dewatering and blasting the pool area in order to widen it.

Generally, the rehabilitation of the plunge pool will include a number of activities: (i) The construction of a cofferdam just downstream of the plunge pool, which will block off the plunge pool from the downstream river. (ii) The 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 bank. (v) The reshaping of the excavated plunge pool into terraced steps. The primary aim of reshaping the plunge pool profile is to improve the stability of the plunge pool, limiting preferential erosion towards the foundations of the dam, along zones of weak rock. In order to arrive at the solutions described below the engineering team undertook; Multi-beam Bathymetric Survey of the Pool; Plunge Pool Geotechnical Investigations; and Plunge Pool Hydraulic Modelling.

Excavation and pumping will be carried out simultaneously. While excavations are being carried out on one of the plunge pool steps, the pumps will 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 (TWL). The reshaping of the plunge pool into terraced steps will reduce dynamic pressures in the pool and reduce flow recirculation towards the 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.

The choice of explosives to be used will be considered very carefully. While ammo-nitrate fuel oil (ANFO) is commonly used as it is inexpensive and has sufficient strength, its water sensitivity is high. It is therefore recommended that surface bulk emulsions are used. Blasting will take place for approximately six months The Project Documents states that after 50 years of operation serving the southern African region, the Kariba Dam now requires a series of rehabilitation works for its continued safe operation. A failure to invest in the timely rehabilitation of the dam will result in the gradual degradation of key safety features associated with the structure to a level that is not acceptable in accordance to international standards.

Questions about the dam’s safety in regards to the plunge pool issue include:

• Will the vibrations resulting from plunge-pool blasting damage the dam wall or compromise its stability?
• How effective will the new plunge pool be in limiting future damage to the 128-metre wall?
• What do experts believe is the remaining life span of the dam?
• And how will the current situation affect flood management?

Alan Bates, a UK-based hydropower specialist with wide experience on African dam projects  and a frequent visitor to Kariba, says although he has no knowledge of the dam’s current predicament, the plan to expand the plunge pool makes sense in terms of general engineering practice. “The plunge pool will be made larger deliberately as this will allow the turbulence resulting from the spillway discharge to be dissipated in a less damaging way,” he says.

“Think of it as sitting in your bath tub and splashing — you get water all over the floor and walls. If you did the same in a large swimming pool you would dissipate the same energy but without all the damage.”

It has always been known, Mr Bates says, that a plunge pool would develop downstream of the dam but the very narrow scour hole that has resulted over the years is concentrating the dissipation of energy, leading to erosion in undesirable directions (such as towards the dam).

“By excavating a much larger plunge pool in the downstream direction, the erosive power of the turbulence is reduced and unplanned erosion should be minimised or even eliminated,” Mr Bates says: “It is normal design practice these days to excavate the plunge pool as part of the dam construction contract in order to prevent this sort of problem occurring. Both Kiambere and Masinga dams in Kenya, on which I have worked, had plunge pools downstream of the spillway excavated in advance and included a sloped downstream face to the plunge pool excavation.” This is a clear testimony that lessons have been learnt from the Kariba Dam experience.

Filling up the plunge pool with concrete is not an option, he maintains, “because this would only mean the spillway discharges would wash out all the concrete the first time the spillway gates are opened. Concrete is weaker than granite and the hole would soon reappear”.

 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