The proposed Tarbela Action Plan is based on computer simulations of sediment flows. These simulations were designed to: a) determine whether flushing was technically feasible and could be used to estimate storage capacity that could be sustained in the long run and; b) to analyze reservoir survey results and predict future sedimentation. Based on the simulations, three phased components of the action plan are proposed:
Reservoir Operating Strategy: Raise the minimum reservoir level to 1,365 feet by the year 1998 and by 4 feet each year thereafter. Second, limit the draw down period to a maximum of 15 days. This would ensure security of power tunnel intakes for the next 10 years, long enough to complete construction of the underwater rockfill dike, and minimize the inevitable reduction in live storage.
Underwater Dike: Construct a rockfill underwater dike to protect the intakes of tunnels 1 – 4 from inundation by sediments.The dike would require some 8 Mcm of rockfill, have a crest level of 1380 feet, with an overspill section at 1340 feet.
Flushing Bypass: Construct a low-level high-capacity bypass to flush sediments.This should be on the left abutment, between the main and auxiliary spillways. Flushing should be carried out over a 30-day period.
The implementation of this plan would ensure long term and sustainable storage with only a small annual reduction in capacity.The estimated retention at 6 MAF is exactly what Kalabagh is designed to hold.However, flushing would reduce energy benefits because reservoir levels would need to be held down in June and July. On the other hand, the long-term energy producing potential of Ghazi Barotha clearly depends on Tarbela not silting up. Abstracting from social and environmental considerations, purely financial and economic cost comparisons also unequivocally favor Tarbela rehabilitation over Kalabagh.
To recap,Kalabagh dam is not the clear winner it is projected to be. First, its viability is premised on water availability figures that are highly questionable .Second, the land constraint precludes substantive increases in cultivable area, additional water notwithstanding. Third, crop yield increases based on additional water do not account for the aggravated water logging and salinity that would result; furthermore, higher doses of water are associated with high input use, which degrades both soil, and water quality. Using existing water more efficiently is clearly a better option on both environmental and equity grounds. Fourth, hydel energy is not unequivocally cheaper, given the growing propensity to factor in displacement and environmental costs. Also, borrowing costs are likely to be higher as donors have indicated a clear preference for thermal power projects. Fifth, Kalabagh would further exacerbate ecosystem degradation, adding to mangrove and species loss and impoverishing communities, which depend on the ecosystem’s resources.Also, as an instrument of flood control Kalabagh is poorly supported by the historical evidence.In view of these facts, the option of implementing a sedimentation management project on Tarbela appears a clear winner on all grounds – financial, economic, social and environmental.
Please contact Sarah Siddiq (firstname.lastname@example.org), Research and PEP Coordinator, for more details.