Many developing countries continue to construct dams to secure domestic water supply for communities, agricultural irrigation and hydroelectric power generation. Such multifunctional dams can also be operated to provide flood protection to downstream communities, as well as being sensitive to downstream ecological flow requirements (e.g. providing flow pulses to support fish spawning). Hydroelectric power is a key energy source, often critical to provide energy for drinking-water pumps, with additional growth potential evident in Latin America, Africa and Asia (Campuzano et al. 2014; UNEP 2016e). In addition, the use of reservoirs for pumped hydro-energy storage systems is increasingly being used to offset the fluctuating nature of other renewable sources of energy (Rehman, Al-Hadhrami and Alam 2015; Barbour et al. 2016) On the other hand, efforts to employ run-of-the-river hydroelectric power technologies have shown promise in the Amazon region in supplying electricity to rural communities, exhibiting fewer environmental impacts than traditional dams (Sánchez, Torres and Kalid 2015).
In recent years, dam construction in industrialized countries has slowed considerably. Many older dams are being decommissioned for economic (e.g. high dam operation and maintenance costs) and environmental reasons (e.g. effects on migratory fish, downstream ecosystems and sediment patterns) (O’Connor, Duda and Grant 2015; UNEP 2016e). Dam density nevertheless remains highest in industrialized countries (Figure 9.20).
More than 1,270 dams have now been constructed across Africa for irrigation, hydroelectric power production and domestic water supply purposes, although only about 20 per cent of the potential to generate hydroelectric power is currently being utilized and lack of resources to properly maintain dams has resulted in reduced power generating capacity in some places. Increased dam construction in some locations (e.g. Ghana, Benin, Burkina Faso) has caused water contamination, irregular flows, methane emissions and degraded ecosystems, including increased sedimentation, and invasive aquatic plant and animal species (Zarfl et al. 2014; UNEP 2016d). Increased sediment trapping associated with reservoirs has been linked to subsidence in deltaic areas and reduced soil fertility, for example the White Volta delta (Boateng, Bray and Hooke 2012; Anthony 2015). Recent construction of large dams has generated significant controversy in many areas, including Africa (e.g. Grand Ethiopian Renaissance Dam; Yihdego, Khalil and Salem 2017), Turkey, the Balkans and the Mekong basin.
Drought is a major risk for hydroelectric power generation. Brazil experienced one of its most debilitating droughts ever in 2015, resulting in decreased reservoir water levels and flows, with many hydroelectric power facilities nearing zero capacity, and causing water shortages to several major Brazilian cities, including São Paulo (Poindexter 2015). The Brazilian example demonstrates the need to foresee conditions that must be dealt with by engineering systems while striving to find an even balance between guaranteeing water supplies and minimizing social or environmental costs.