The Kariba Arch

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Source: Wikipedia

Lake Kariba is Africa’s biggest inland water reservoir stretching over 280 kilometes. Placed strategically in Kariba gorge of the Zambezi river basin, all this water is held up by a wall which is 128 meters in height and 579 meters long. The dam is a concrete
arch dam which is famous as one of the largest man made water boundary.
The hydroelectric station which runs on the feeds from the dams generates enormous 6,700,000,000 kilowatt-hours of electricity annually. The Structure stands tall since 1959 even when the retrofitting and repair of the structure has been due by a decade.
According to a july 2012 report, engineers have confirmed that the plunge pool below the dam has deepened beyond expectation and has now eroded to a depth of more than 81 meter into the rock substrata. The grave concern deepens because at this depth water is released after going through the dam’
s spillway. This continued erosion and its direction towards the dam wall will result in the undercutting of the 128 meter high wall.
It has been estimated that if proper measures are not taken then the wall will collapse in three years. This catastrophe will affect 3.5 million people in all of the southern African region, mainly in Mozambique and Malawi
The cause of failure and design of such a magnificent arch surely intrigues a civil engineer’s mind.
The structure of the dam as in the figure 1 implies its concept of transferring of water load onto the side valleys or abutments. The volume of concrete used in arch dam is quite less in comparison to a Gravity dam or Buttress dams.
The creation of plunge pool in Mega dam is very necessary and tactical way to deal with erosion. Also, the flow channel provided after initial impact is such that by depth moderation the jet is brought under control and at the same time managing the energy level of the jet for production of electricity. These pools are created as second dams mainly to avoid direct erosion by vertical water jet of the river bed.
A concrete apron is sometimes provided for free falling jets to safely land without creating turbulence.
The high amount of Dynamic uplift pressure which develops at the impact sight also is reason to the deepening of the pool. This happens mainly because all the energy dissipation takes place at the delivery point of the spill way.
Hence Design of the plunge pool is of great importance.

Design of flow in a Plunge Pool

A three staged design is prepared to dissipate the energy on impact and also to prevent scouring. This involves the most important shearing region.

The pressure on the floor slab makes it the most important and crucial member of the plunge pool. Maximum uplift is caused when there is instantaneous maximum pressure occurs on the downward side of the slab and minimum pressure on the upward side.

The cost of retrofitting and repair of the Kariba dam will cost approximately 250 million US dollars, a great pressure is on Zimbamwe government to raise this fund as the African Development bank has speeded up talks with World Bank and the European Union.
In normal view, this failure is a classic example of magnificent art put to waste by ignoring repairs and securing the impact floor from time to time which should have been a protocol after years of service from the structure.

The assessment of many such dams in the Alpine gorges concludes on these few measures which can be taken to delay the failure and hence buy time to provide long term retrofitting.

• The water coming through the spillway should be dispersed before it hits the apron and the pool.
• A dynamic air water cushion can be provided at the impact area.
• The region built in the plunge pool for shear dissipation of the jet can be elongated.
• The Tractive force which is the force acting on the perimeter region is of great importance and non-uniform distribution of this force causes erosion at the perimeters.
• Scouring over the years have caused expansion of the pool, hence to avoid this the mixing region of jet into the normal flow of river should be done after careful energy management

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Source: international rivers organisation

It is still haunted by the lack of coordination and planning it showed during the february 2014 Tokwe-Mukosi floods which has already alarmed Zambia to bring order to this issue which will affect two major downstream countries.
A total failure on behalf of Zimbabwe was observed when the disaster relief was nothing less than misery for half the victims who still wander for shelter like basic amenities.

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