Dam Synchronisation and Flood Releases - agriwaterpedia.info

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.
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