Dam Synchronisation and Flood Releases - agriwaterpedia.info

DAM SYNCHRONISATION AND FLOOD RELEASES IN THE ZAMBEZI RIVER BASIN: ANNEX 4 OF FINAL REPORT
5 Investments in Cahora Bassa reservoir
5.1 Introduction to the investments
There are plans for an extension of Cahora Bassa North Bank, as an extension of the existing
hydropower station. This new power station is not intended for continuous power generation,
but to be used to generate extra power during peak demands, which are a better economic and
financial value and help to reduce the stresses on the system during these demands.
As part of the development, an additional spillway is deemed necessary. The additional
hydropower generation requires the storage to be maintained at a higher level than at present in
order to secure continuous supply of water to the turbines at an adequate head (interview HCB,
29 April 2010). The current flood rule curve required lowering the lake before the high-flow
period. The designed flat flood rule curve will be at a constant level of 326 m.a.s.l., while the
current flood rule cure is between 320 and 329 m.a.s.l., dependent on the season. This flat rule
curve will reduce storage capacity required for dam protection during floods which the additional
spillway will take care of. The additional spillway thus makes it possible to increase hydropower
production. However, it will also impact downstream flood flows.
5.2 Situation assessment: stakeholder views
HCB has strongly recommended that strong conclusions on the extra spillway be left for
determination by studies that are currently in progress for the final evaluation on the extension
of Cahora Bassa North Bank. This chapter was therefore based on available literature up to 2003
and did not have the benefit of results from the current studies. Thus, the proposals made here
will need to be reevaluated in the light of information and data currently under production.
5.3 Description of investment
In Phase 1 of the feasibility study for the North-bank Extension, an additional bottom spillway
was designed for Cahora Bassa, which had a design discharge of 3,600 m 3 /s. During phase 2 of
this feasibility study, it was decided not to pursue the additional spillway up to a feasibility study
level, but rather to make a preliminary design, adjusting it to a discharge of 5,000 m 3 /s. This is
about 20% of the inflow design flood of 23,400 m 3 /s and is in addition to the existing spillway.
(Li-EDF-KP, 2001).
An overview provided by HCB mentioned that an update of the feasibility study for the Cahora
Bassa North Bank Power Station was conducted by Hidrotécnica Portuguesa with a final report
produced in April 2002. In a paper by Salvador Fernandes et al. (2003) results of the study were
described, for three design capacities: 2200 m 3 /s, 3600 m 3 /s and 4400 m 3 /s. This study showed
that a design capacity of 2200 m 3 /s was economically most beneficial. However, this discharge
capacity is insufficient to accommodate the design flood and a Design Flood Rule Curve would
still be required.
The different designs are presented in Table 5.1.
46