A few days ago a short article was published by the Lusakatimes. The level of interest has encouraged me to expand on my thoughts on this issue. Loadshedding has become normal in Zambia for some years now. Zambians have become used to it, and anyway, what can the average person do about it? The answer here is to apply some thought! SERIOUS THOUGHT. Aside from the frivolous suggestions that have been proposed such as filling the lake up with buckets or urinating in it to raise the level!
To be relevant, a basic understanding of how the current system operates is important, and once that has been achieved solutions start seeming to be obvious.
Several people commented on the idea of my last article, and in response to those comments I would like to clarify that the diversion I suggested should not be full time. As there is no large scale storage capacity beyond the point of abstraction on the Kafue river, and Kariba has massive, presently unused, storage capacity, it can profitably be used to increase overall water storage when there are exceptionally large floods! The 70% mentioned by Kasolo Lubembas comment will still be maintained by the power generation at the Kafue Gorge.
The new project at Kafue Gorge Lower will not place any extra demands on the flow of the river at that point as it is what is known as a run-of-the- river facility. The same idea as Batoka Gorge project now under consideration. These systems just use an additional drop of exactly the same water! This suggestion is not to divert the main flow, but only to use the storage capacity of Lake Kariba to capture excessive flows such as when there is flooding in very wet years.
As a fish farmer, water management and an understanding of environmental flows and the effects of diversions on aquatic life are subjects I am familiar with. The simple fact is that the days of natural flows are long gone, on both the Middle Zambezi and the Kafue rivers, as their flows are already now manipulated for power production.
Interestingly, most people seem to think a canal would have to be constructed ALL THE WAY to Kariba. This is not so. A very short one, just from the Kafue river to an area lower than the river is all that is required. After that the water will make its own way into the lake. This would probably be only 15 Km or so, but I leave that to the engineers to determine precisely.
How much would it cost?
Obviously cutting a canal (or a tunnel) through rock is an expensive business. But as a country with vast mining experience, we have the technology and the human resources readily available to complete this task. And many of those miners on the Copperbelt are currently out of work. Can we not use their skills here? Additionally, a canal like this could also be used for other purposes. This could offset much of the cost of construction and make it a very profitable investment. Water could be used for irrigation in one of the driest areas in Zambia, guaranteeing successful crops (two crops a year!). Possible irrigation areas are shown below, and it would work on gravity flow, saving Zesco power that is in such short supply, and saving costs charges for pumping water as well!
Another comment by Spaka like lilo mentioned Kariba may not be able to cope with the extra water.
But currently nearly US$ 300 million is being spent on refurbishing Kariba! Nevertheless, let us look at this problem. For those that are aware of the history of Kariba, you will know that when Kariba was designed it was planned with FOUR spill gates. Two more were added as an afterthought, due to exceptional river flows. Opening them ALL AT ONCE is the reason the erosion of the plunge pool has happened. Kariba was not designed for this.
In any case, there is a SERIOUS fundamental flaw in the design of the dam. That flaw is that the overflow control system and the dam wall are combined into one and the same structure!
Any sensible engineer would have separated these functions such as in the Unites States on the Hoover Dam, shown below, and most other dams in the world. On these, the spillway is completely divorced from the dam itself. Having a dam that eats away at its own foundations is really not a very good idea!
A separate spillway has numerous advantages, as the recent crises on the Oroville dam have demonstrated very visibly. If that dam had a spillway on the actual dam wall, a catastrophe would have been inevitable.
So is there an intelligent alternative for Kariba?
A simple glance at the map shows that there is indeed a viable alternative. An alternate spillway that can be used in an emergency is easily possible at very little cost. The terrain is highly suited to its construction.
With the spillway function completely removed from dam wall area, there will be no more erosion of the dam wall foundations – ever. The current repairs are only a temporary fix, as they will again be subjected to erosion, and that will necessitate repairs costing us millions again at some time in the future. This will be a PERMANENT SOLUTION.
Another problem with the Kariba dam wall that is not that widely known is that the wall has actually moved since it was built! This is due to a geological formation in the foundation rocks on the south side of the dam that contains mica. This acts as a lubricant and facilitates the slipping of the rock strata, hence the wall movement. The engineers are aware of this problem, and many millions have been spent trying to stop it. Tunnels and drainage shafts have been bored in the rock, and the top of the south bank has been covered with concrete so prevent water seeping into the rock.
Water is the problem. When it is dry, there is no movement. But when it gets wet, the slippage commences again. Of course it is not possible to stop rainfall, but most of the water causing the problem is not rainfall. It is the spray that is caused by spilling water from the gates!!!! And the profiling of the plunge pool will do nothing to stop that!
So what will an alternate spillway look like and what will it cost? Water Engineers are very familiar with these. They are known as passive systems that let the water overflow automatically when the dam reaches a preset level. One of the main reasons we have load shedding today is because, to operate Kariba safely, the amount of incoming water must be assessed and water spilled out to make space for more. In 2011, a whole years supply of water was intentionally let out of the dam! That was a mistake and the loadshedding we have had since can be directly blamed on this. So an alternate spillway will solve this problem too, and make sure Lake Kariba is always as full of water as it can possibly be.
By Adrian Piers