Southern Africa’s largest hydroelectric scheme tackles debris problem

Southern Africa’s largest hydroelectric scheme the Cahora Bassa Dam , located in Mozambique, has recently partnered with British-based Bolina Booms to provide log screen booms to maintain the hydropower plant.

The plant, operated by Hidroélectrica de Cahora Bassa, provides the majority of the energy to Mozambique’s national grid, which makes maintenance absolutely essential in order to prevent disruptions to national energy supplies.

The water impounded behind the 170-metre-high reinforced concrete dam stretches an incredible 280km upstream and is subject to severe fluctuations in levels due to rainfall and other weather conditions.

Debris problems at Cahora Bassa Dam

As a result, the Southern Africa’s largest hydroelectric scheme had been experiencing ongoing operational problems with waterborne debris overwhelming the existing competitor boom which had been in place for a number of years and could not hold the debris back.

Debris booms are able to prevent materials such as plastic, invasive plant life or wood (such as logs or branches) from accumulating and blocking the dam, as well as preventing damage to the dam itself. Damage to the dam can occur as a result of heavy floating objects being carried at speed on the surface of the lake impacting the dam’s structure and consequently weakening it. The effect of this is a reduction in the dam’s longevity and an increase in frequency with which the dam will have to be maintained, incurring long-term increased costs for the dam’s administrators, Hidroeléctrica de Cahora Bassa (HCB).
Furthermore, obstructions to the dam’s sluice gate as a result of debris accumulation will reduce its capacity to channel water, in turn reducing the dam’s efficiency, which will eventually necessitate cleaning and removal of the obstruction and cause disruption to the dam’s supply and operation – further increasing maintenance costs.

If debris accumulates in a dam’s sluice gate to the degree that clearing is required, it is a manual task for a team of people, which carries with it an inherent degree of risk, given its location. Reducing the amount of debris that is able to reach the sluice in the first place reduces the frequency of maintenance, and thus the risk that workers are exposed to.
In order to overcome these issues and to increase the reliability of the installation, Bolina utilised its LSB600 Log Screen Boom, developed from its highly successful Traffic Control Booms tested to over 60-tonnes. It incorporates a subsurface mesh joined together to provide a seamless barrier for logs and debris, with full articulation provided between units.
The unique tubular backbone of this boom system and the Bolina lug connections make the boom immensely strong without stressing the floats, which are fully demountable with the booms designed for continuous motion and heavy loads.

A total of 380 metres of boom were shipped from the United Kingdom in containers to Cahora Bassa, together with a gate assembly designed by the Bolina team. The gate was included to allow the passage of maintenance boats either side of the boom and is the largest that Bolina had constructed to-date.

The work was supervised by Bolina field engineers but undertaken by local workers employed by the power company using readily available hand tools. The construction of the boom on site was completed and was then simply floated into place and secured in position, utilising the end connections from the previous boom.

The highly effective boom captures and diverts much more debris than the previous solution. This has both increased the operational efficiency of the plant and reduced the need for the maintenance team to clear debris from around the intakes.