The Sunday News
PLANS are under way to rehabilitate the Kariba Dam spillway gates and plunge pool as concerns increase over its natural development in the future to avoid dam toe weakening.
Zambezi River Authority spokesperson, Ms Elizabeth Karonga said the company intended to carry out the reshaping of the plunge pool and the spillway refurbishment.
“There is a major concern over its natural development in the future, if an intense spillage episode were to occur in case of exceptional Zambezi floods. In order to control its future development and avoid dam toe weakening, the studies came to the conclusion that the best solution would consist of an enlargement of the plunge pool, mainly downstream but also on both banks.
This reshaping should indeed facilitate the evacuation of spillage flows downstream, and avoid the concentration of turbulence in a restricted and confined area,” said Ms Karonga.
According to a document produced by ZRA on the rehabilitation work to be done, the plunge pool reshaping works would involve enlargement of the plunge pool scoured in the riverbed, mainly downstream but also on both banks.
“Given the importance of these works and their surrounding structures (i.e. the dam and powerhouses), the excavation works must be perfectly controlled. Control can only be achieved in dry excavation. Therefore, the river will have to be dried out in the plunge pool area,” read part of the document.
Ms Karonga said an environmental impact assessment (EIA) had to be carried out first as the report would guide works to be done.
“Flood management during construction is a key feature of this project,” she said.
The document states that the three-month peak flood period of the Zambezi River usually takes place from February to April. The spillage season of the Kariba Dam usually takes place from January to August. Limited amounts of water are usually spilled every now and then in order to closely match the reservoir level rule from January to August.
“Because the works are located at the toe of the dam, they must be performed during the non-spillage period, which leaves a short time window. The compatibility of the works and the necessary spillage have been studied in the detailed design of the project to extend and ensure the non-spillage seasons –the works can be carried out in the plunge pool for seven months, after which time all materials and equipment will have to be dismantled to allow for the five months spillage period, before the works can be resumed for another seven months,” read part of the document.
Ms Karonga said the works would start with the rehabilitation and construction of the access road and site installations. She said the cofferdam would then be constructed, and the plunge pool water level would be lowered by pumping, after which the excavation works could start.
“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,” she said.
Ms Karonga said on completion of the plunge pool excavation works, some concreting works would be performed to reinforce the weak areas of the plunge pool. She said the cofferdam and access road to the plunge pool would be removed from the riverbed and the work site (including all installations and roads) would be restored.
Speaking on the importance of an EIA, Ms Karonga said it should inform the Governments of Zambia and Zimbabwe, the ZRA, national power utilities, interested and affected parties and other stakeholders about potential environmental and social impacts associated with rehabilitation works of the Kariba dam.
