Dam Synchronisation and Flood Releases - agriwaterpedia.info

DAM SYNCHRONISATION AND FLOOD RELEASES IN THE ZAMBEZI RIVER BASIN: ANNEX 4 OF FINAL REPORT
The pumping scheme near Mponda is not likely to happen in the near future. When the power
interconnector between Malawi and Mozambique is realized, it should seriously be considered to
rely on power supply from Mozambique, rather than drawing down Lake Malawi water levels
beyond the natural levels. Both options for Kholombidzo Reservoir, High and Low, will need
further study before decisive conclusions can be made. There will be a contribution to flood
control and a considerable contribution to hydropower production, which has to be weighed
against environmental impacts.
Investments in multipurpose dams on the Zambezi and Kafue Rivers are being planned and
implemented; existing dams are being adapted and new dams are being planned. Hydropower
turbines that are being added at existing dams will not only contribute to power generation, but
will also increase the capacity to generate hydropower if reservoir water levels are reduced before
forecasted floods, as more water is able to pass through the turbines rather than spillways. Such
projects are planned for Kariba, Itezhi-Tezhi and Cahora Bassa dams. The newly planned dams
on the Zambezi and Kafue Rivers are designed for power generation and are or have been
debated from an environmental perspective. The large power deficit in the region makes decision
processes on new dams progress faster. Financial viability of these dams will dictate their mode
of operation, which will most likely not be optimal from a flood control perspective, despite the
fact that the construction of every new dam adds to flood control. It is recommended to
consider the Hydropower Sustainability Assessment Protocol (IHA, 2010) for further design and
development of operating rules.
As for new multipurpose dams on tributaries, stakeholders in Zimbabwe, Zambia and
Mozambique indicated that new dams in their own country for flood control purposes only, have
low priority. In Zambia and Mozambique in particular, the usefulness of dams for irrigation
purposes is emphasized. The identified dams for multipurpose use on tributaries generally have
little storage capacity in comparison to the flows coming from the Zambezi main stem but they
will contribute in making floods more predictable. A considerable number of dams need to be
developed, to have a significant impact. From a technical perspective, several dam sites were
identified that could contribute to flood protection. In Zimbabwe, there is a list of technically
suitable dam sites available which could be developed. Of these dams, the Gwayi-Shangani dam
(635 Mm 3 ) and the Kudu dam (1550 Mm 3 ) on the Sanyati can have some positive impact in
respect to flood control. Technically suitable dam sites have been found on the Luia and the
Revubué rivers. However, it is pointed out that to develop dams which will contribute to
economic activity, the technical suitability of a dam site is less of a determining factor, and socioeconomic,
institutional, financial, political and environmental considerations often outweigh
physical site advantages. From an environmental viewpoint, the identified dam sites on the
Luangwa and Lusemfwa are strongly not recommended. Dam sites on the Sanyati (Kudu) will
affect spawning of Tiger fish. For dam sites of tributaries downstream of Cahora Bassa, the
impact on sediment is of major importance.
Final conclusions and recommendations on the investments at Cahora Bassa will have to await
the results of on-going studies commissioned by HCB which are due for completion in 2011.
The benefits of the spillway in combination with the flat rule curve are for power generation and
the environment. Both for the ecosystem of Cahora Bassa reservoir as for the downstream
ecosystem, the adjusted flat rule curve will be beneficial. Flooding can increase downstream, but
the impact depends on the operation of the dam and the coordination with floods at tributaries
from downstream. This flooding impact will need attention when evaluating different options of
spillway and operating rule curve. Coordination with tributaries downstream require a good flow
forecasting system.
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