Some 715 GW of hydroelectric power are already installed worldwide, and in 2006, it supplied 20% of the global electricity demand and 88% of that from all renewable sources [4]. Large schemes of more than about 30 MW involve the construction of a convex dam across a deep river gorge whose sides and bottom must be geologically sound. In addition, a sufficiently large upstream area must exist for water storage (i. e., availability) and sufficient precipitation or glacial melt must be available to maintain this reservoir level. Viable large hydroelectric sites thus necessitate a special topography and geology, but are never­theless more numerous and powerful than geothermal ones as indicated by Table 1.1. Both renewable sources, however, are reliable and can accommodate the variations in power demanded by an industrialized economy. Water below a dam is drawn-off in large pipes (penstocks) to

drive vertically mounted turbines whose blades are protected from cavitation by a slightly rising outfall to downstream [10].

Formal legislation on carbon emissions [1] and the increasing costs of fossil fuels have been driving global construction programs for hydro­electricity. Suitable large-scale sites in the United Kingdom were fully developed during 1940-1950, and future opportunities will focus on small or microscale plants (< 20 MW) whose total potential is estimated at 3% of national consumption [5]. Redundant factories from the UK’s industrial revolution provide opportunities for microgeneration like the 50kW rated plant at Settle [6], but even after a copious rainfall the claim to supply 50 homes is optimistic. It is to be concluded that no large-scale hydro-sources are available now to compensate materially for the impending demise of the UK’s aging fossil and nuclear power stations. The situation [21] in the United States is that large and small-scale hydro-generation have remained largely unchanged over the past 10 years and that future renewable energy development will center on wind turbines [7].

Dams are sometimes breached by river spates or earthquakes despite the inclusion of such statistics in their design. For example environmental damage and a serious loss of life ensued from the failure of the Banqiao Dam [11] (China). Here there were 26,000 immediate fatalities and a further 145,000 from subsequent infections. No worse nuclear accident could be envisaged than that in 1986 of the RMBK reactor at Chernobyl which is designated 7 on the IAEA scale of 1 to 7. The 186 exposed settlements with a total population of some 116,000 were evacuated within 12-13 days. In the specific context of health issues, the International Chernobyl Project [13] of the IAEA reported

i. “Adverse health effects attributed to radiation have not been substantiated.”

ii. “There were many psychological problems of related anxiety and stress.”

iii. “No abnormalities in either thyroid stimulating hormone (TSH) or thyroid hormone (TH) were found in the children examined.”

The earlier Three Mile Island accident (1979) did not directly cause any on or off-site fatalities, though some occurred from remote road accidents due to the absence of an organized evacuation plan. Historic

catastrophic failures of large hydroelectric dams have thus caused far greater fatalities than the worst nuclear power plant accident, but their relative probabilities require of course quantification,[1] which must now account for the lessons learnt and practiced. Though all large dams are potential terrorist targets, the Ruhr-dam bombing raids in World War II demonstrate that success necessitates a scientifically sophisticated attack.