Existing and proposed hydropower dams are not properly evaluated for the risks of natural hydrological variability, which is extremely high in the Zambezi river, not to mention the risks posed by climate change, says US State of Wisconsin Department of Safety and Professional Services hydrologist Dr Richard Beilfuss.
“The result could be that uneconomic dams underperform in the face of more extreme drought and more dangerous dams that have not been designed to handle increasingly damaging floods,” he notes.
Overall, Africa’s fourth-largest river will experience worse droughts and more extreme floods. Dams currently being proposed and built will be negatively affected, yet energy planning in the basin does not include the necessary steps to address these significant hydrological uncertainties, he warns.
The basin exhibits the worst potential effects of climate change among 11 major sub-Saharan African river basins. It will also experience the most substantial reduction in rainfall and runoff, according to the International Panel on Climate Change. Multiple studies estimate that rainfall across the basin will decrease by 10% to 15%.
“The basin is likely to experience significant warming and higher evaporation rates in the next century. Because large reservoirs evaporate more water than natural rivers, big dams could worsen local water deficits, resulting in less water for hydropower. Currently, more than 11% of the Zambezi’s mean annual flow is lost to evapora- tion from large hydropower dam reservoirs, which increases the risk of shortfalls in power generation and significantly impacts on downstream ecosystem functions,” he explains.
The designs for two of the larger dam projects proposed for the Zambezi, the Batoka Gorge and Mphanda Nkuwa dams, are based on historical hydrological records and have not been evalu- ated for the risks associated with reduced mean annual flows and more extreme flood and drought cycles. Under future climate scenarios, these hydropower stations, which are based on records of flows, are unlikely to deliver the expected services, says Beilfuss.
The occurrence of more frequent extreme floods threatens the stability and safe operation of large dams. Extreme flooding events, a natural feature of the Zambezi river system, have become more costly downstream since the construction of large dams. If dams are ‘underdesigned’ for larger floods, the result could be serious safety risks to millions of people living in the basin.
The Zambezi river is already highly modified by large hydropower dams, which have profoundly altered the hydrological conditions that are most important for downstream livelihoods and the preservation of biodiversity. The ecological goods and services provided by the river, which are key to enabling societies to adapt to climate change, are under grave threat.
Large-dam hydropower poses economic and adaptation risks. Africa has been referred to as the continent most at risk of being nega- tively affected by climate change. Successful adaptation will require new ways of thinking about water resources. We need to act now to protect our rivers as sources of livelihoods and food security,” says International Rivers Africa programme director Rudo Sanyanga.
“The region’s energy planners and governments must acknowledge these hydrological risks and take steps to improve planning and manage- ment of large dams in the basin. Existing and future dams should, at least, undergo a thorough analysis of climate risks,” notes Beilfuss.
Source Article from http://www.engineeringnews.co.za/article/proposed-zambezi-hydropower-dams-risks-2012-10-12
Proposed Zambezi hydropower dams pose some risks, expert warns
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