In view of the rainy season that the country is going through, the Cahora Bassa Hydroelectric Plant (HCB) has drawn up some questions and respective answers that people most want to know in this rainy season, and MZNews publishes part of these questions in full below.

 It is known that this is the peak of the rainy season and it is raining quite frequently downstream of the dam. What is HCB doing to mitigate flooding in the downstream districts?

HCB interacts constantly with ARA-Centro-IP, the entity responsible for managing the Zambezi Water Resources in Mozambique, which shares information on a daily basis about the hydrological situation at all the rainfall and hydrometric level monitoring points.

In this sense, if any hydrometric station is on alert, and by monitoring short-term weather forecasts for the entire Zambezi Basin and forecasting effluent from the upstream dams (Kariba and Kafue Gorge Lower), HCB makes the necessary adjustments to their effluent in order to minimize the effects on the lower Zambezi, without, however, compromising the safety of the dam. Currently, HCB continues to closely monitor the evolution of the hydrological situation in the Zambezi basin in Mozambique, with particular emphasis on the lower Zambezi.

The rainy season starts in October, so why does HCB carry out its discharges at that time when there is also rain?

The HCB is discharged on two occasions. The first is when the reservoir has high storage levels at the start of the rainy season. In this case, discharges are made to create the capacity to cope with possible inflows at the peak of the rainy season (January to March).

In compliance with the Dam and Reservoir Operating Rules, the reservoir level should be 320.8 m on December 31st of each year, which corresponds to the storage of 75% of the reservoir's useful capacity.

The other occasion when spillways are opened is when, during the peak of the rainy season, flood flows occur that cannot be retained by the dam and when the rate of rise of the water level is greater than 15 cm per day. Note that all hydrological management operations are provided for in the Cahora Bassa dam's operating rules.

Why aren't they discharged in the dry season so that the population has water?

The operation of the Cahora Bassa dam is based on its own regulations and all the relevant variables are assessed in order to open or close the spillways in both the wet and dry seasons. Thus, having analyzed the energy production plan and the inflow forecasts, under normal conditions no discharges are made during the dry season, for the following reasons:

The natural hydrological regime of the river downstream should not be significantly altered due to the existence of the dam, and it should be ensured that in the dry season it has low flows and in the rainy season it has appropriate flows, whenever possible;

During the dry season, the water entering Cahora Bassa is usually less than the water used for production. At this time, the HCB plant uses the water stored in the reservoir to produce energy;

In addition, during the dry season we don't have exact information about what the next rainy season will be like, in terms of abundance or scarcity of water, which means that we avoid wasting water during this period.

Note that the rainy season under analysis is communicated by the competent meteorological entities with forecasts of possible rainfall levels. It would therefore be unwise to discharge water from the reservoir, which is useful for the following hydrological year, if inflows are low.

 What guarantees does HCB have that the discharges will not create flooding?

HCB has in-depth knowledge of the behavior of the Zambezi River and knows exactly what levels of discharges could possibly cause "floods" downstream. On the other hand, it is important to emphasize the knowledge of the main riverbed and the respective floodplains of the river, places that, under normal conditions, should not be occupied. In these cases, it is possible to predict up to what levels of discharges can actually cause such "floods" or occupy the entire riverbed.

In addition, HCB has established solid cooperation with the water resource management entities in the Zambezi river basin and, particularly in Mozambique, with the Administração Regional de Águas do Centro (ARA-Centro, IP). Thus, all discharges carried out by HCB are preceded by consultation with this entity.

However, its discharges, combined with other factors, can lead to flooding, which is why the authorities and the population are warned in advance to stay away from the river banks.

It is important to note that the existence of the Cahora Bassa dam attenuates the occurrence of floods coming from upstream, i.e. the floods in the Zambezi valley would be worse if the Cahora Bassa dam did not exist.

If the Kariba dam discharges considerably, is the Cahora Bassa dam prepared to handle large volumes of water?

HCB receives official and regular information from the managing body of the Kariba dam, the Zambezi River Authority (ZRA), which indicates whether or not the dam is discharging additional water, in addition to the water turbined for power generation.

However, based on official information on the volumes flowing into Cahora Bassa forecast for each rainy season, HCB draws up a discharge plan with a view to creating the appropriate capacity for the reservoir. It is also important to note that HCB maintains permanent communication with the ZRA.

What is the relationship between Cahora Bassa and Kariba?

The existing relationship between Cahora Bassa and Kariba is one of sharing hydrological and hydro-energy management information on a daily, monthly and occasional basis, whenever the hydrological situation so requires, within the framework of the Joint Technical Committee of Zambezi River Dam Operators (JOTC), which also includes the water resource management entities of Zimbabwe (ZINWA), Zambia (WARMA) and Mozambique (ARA-Centro, IP).

This committee meets ordinarily twice a year and extraordinarily whenever there is a relevant and urgent matter to deal with. Information is shared through bulletins containing hydrological information on the basin and the operation of the dams.

What contingency plans are in place if the Kariba dam collapses?

To address this aspect, the Internal Emergency Plan (IEP) for the Cahora Bassa Dam is currently being drawn up, in line with Decree 33/2017 of the Council of Ministers of July 19. The basis for drawing up this IEP will be the results of the recently[1]completed "Dam Break Analysis for the Zambezi River" study by consultant Mott MacDonald (2020). This study analyzed the various failure scenarios of all the major existing and planned dams in the entire Zambezi basin, including Kariba.

It has identified the most catastrophic scenarios and assessed their consequences. The IEP will provide HCB with concrete tools for action and intervention in the event of such a situation occurring.

How is it being coordinated so that discharges from the HCB don't coincide with any high flows from the Revúbuè, as happened in the relatively recent past?

Management of the Zambezi River basin, from Zumbo to the delta, including in addition to the main river, the tributaries, both upstream and downstream of Cahora Bassa, is carried out by ARA-Centro, IP, with the collaboration of HCB.

Communication between the HCB and ARA[1]Centro, IP is permanent and at all levels, with an infallible daily exchange of data. This synchronization between ARA-Centro, IP and HCB allows for joint analysis of discharge plans and assessment of their impacts.

At the same time, ARA-Centro, IP has established warning levels, both for rainfall and hydrometric levels, which help in making decisions about how much can be discharged and when, without undesirable consequences downstream.

Do upstream and downstream populations have information about the discharge plan?

The discharge plan, once approved by HCB and validated by ARA-Centro, IP, is shared with local governments and other stakeholders in the Zambezi Basin in Mozambique 4 just before the intensification of the rainy season, during the meeting of the Zambezi Basin Committee of which all these stakeholders are part.

On the other hand, during the rainy season, depending on when the HCB's discharge plan is updated, it is made public by ARA-Centro, IP issuing a communiqué or warning about the possible impacts of discharges on downstream hydrometric levels and the precautionary measures to be taken by populations and entities (public and private) downstream. In turn, local governments disseminate the communiqués and warnings at local level.

Therefore, it is important to inform that in August 2022, the first notice was issued to communicate about the discharges of the HCB, as of October 1, 2022, and on September 13, 2022, the second notice was issued, warning the populations to harvest their crops and consequently move away from the banks along the Zambezi River. In 2023, for the start of HCB discharges, ARA-Centro, IP issued another warning on January 30, and on February 15, following the recording of some rainfall in the Lower Zambezi, it issued another warning about the expected consequences and precautionary measures to be taken by the population.

The unloading process could potentially be disadvantageous for the operation of the turbines. How is the company preparing for possible constraints?

Frequent discharges (which occur every year) do not have a significant effect on the performance of generator sets. However, discharges do increase the drop in restitution and, as a result, the drop decreases and can affect the efficiency of generator sets.

With 8 spillways open and the plant operating with 5 generator sets, the efficiency of the generator sets can drop to around 90%, compared to 95.9% in normal operation. However, this maximum discharge condition is only possible in the event of an exceptional natural flood or due to upstream dam failure. (HCB)