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DAM SYNCHRONISATION AND FLOOD RELEASES IN THE ZAMBEZI RIVER BASIN: ANNEX 4 OF FINAL REPORT Comments: Strong conclusions and recommendations need to wait for results of on-going feasibility studies on the extension of Cahora Bassa North Bank, expected in 2011. If the flat rule curve is introduced, Cahora Bassa may be in even more need for a good flow forecasting system, see Annex 3. A reaction to the new operating rule curve will also draw from conclusions in Annex 2. 112
DAM SYNCHRONISATION AND FLOOD RELEASES IN THE ZAMBEZI RIVER BASIN: ANNEX 4 OF FINAL REPORT Intervention Sheet # PROVIDE ADEQUATE BOTTOM OUTLETS IN NEW DAMS 4.5 Timeframe: Short term, medium term, long term Budget range: < 0.5 MUSD for development of guidelines Linkages: Annex 4, Chapter 8 Concept: ZAMCOM, when formally established, has the mandate to advise riparian states, after which the member states decide and enforce decisions. Bottom outlets have different functions, among which is sediment management. Purpose: Justification: Actions/ Responsibilities: Benefits/ beneficiaries: Means of implementation: Specific assumptions/ risks: Comments: Understanding of ministries of water in member states of the function of bottom outlets can contribute to design decisions that will affect the river basin for a long time. Bottom outlets are difficult to change after construction of the dam and therefore need careful consideration at design stage. Sizing of bottom outlets has much downstream impacts, impacts on the lifespan of the dam and cost impacts. Development of guidelines or combining of guidelines to ascertain that environmental, safety, lifespan and cost impacts of sizing of bottom outlets are sufficiently taken into account during dam designs. Assessment of correctness of size of bottom outlets for larger dams Bottom outlets are often required for other reasons than sediment management: preventing siltation of the dam, reducing turbidity of the released water to downstream users of the water, being able to maintain minimum environmental flow releases, being able to lower the water level fast in the event of an emergency for dam safety purposes. Dependent on Actions to be taken: seminars or design manuals, design approvals. Engineering councils in riparian states, designers of dams and owners of the dams; SADC Secretariat could support development of guidelines. Ministries of Water Member states Ecosystem, downstream water users, residents in floodprone areas downstream In considering the use of bottom outlets for sediment management downstream it is important to understand that; flushing can have detrimental environmental impacts both in the reservoir and downstream. Further, flushing whereby the dam is emptied at the beginning of the flood season, is only an option for small and medium sized dams that do not generate hydropower. For large reservoirs flushing of sediment is also not an option as sediment has already settled upstream. Flushing of sediment will in any case require very experienced dam operators. Also releasing water from bottom outlets requires experienced dam operators, to carefully manage the required mixing of water from different levels of the dam, to prevent direct release of cold, nutrient-rich (hypolimnion) waters from the bottom of the reservoir downstream. The costs of bottom outlets increases considerably with the maximum flow to be accommodated. Capital costs of bottom outlets also depend on the type of dam and the head of water. Larger gates create other forces on the dam wall and may need extra protection measures against erosion downstream. This intervention sheet also advises on new dam structures that are discussed in intervention sheets 4.2 and 4.3. 113
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