ICOLD bulletin 109
SUMMARY AND CONCLUSION
In 1995, for the world as a whole, there were very approximately :
-10 000 dams more than 30 m high,
-20 000 dams 15 m to 30 m high
-more than 100 000 dams 10 m to 15 m high, most impounding a few hundred thousand cubic metres of water, but 10 000 – 15 000 storing more than one million cubic metres and classifying as ‘large’ dams.
Dams in the range 10-30 m mostly lie on catchments of 1-50 km² area and have ungated spillways of 20-500 m3/s capacity, although some are very large structures whose reservoirs are reckoned in tens or hundreds of million cubic metres with spillway capacities in excess of 10000 m3/s.
Around ten per cent of 10-30 m dams are masonry or concrete, the remainder mostly being earth embankments relying on clay for watertightness.
While 75 per cent of dams more than 60 m high are in the industrialised countries, 75 per cent of those below 30 m have been built in the developing countries with little money or machinery (labour-intensive embankments and masonry) whose dispositions vary in different regions.
IMPROVING SAFETY AND EFFICIENCY AT EXISTING DAMS
More than half of all deaths from dam failures have been caused by floods. destroying dams less than 30 m high, and for dams existing today, this will remain the major risk in the coming years. The developing countries having 75 per cent of all< 30 m dams in densely populated areas, with high specific discharges, account for more than 90 per cent of this risk.
The risk could be reduced by a factor of ten at little cost within a few years :
-by using simple methods to identify the few per cent of dams where most of the risk is concentrated,
-by very quickly improving safety without seeking perfection,
-by developing warning systems notifying downstream populations of risks associated with exceptional floods (whether or not they cause failure).
It is also possible, at low cost, to increase storage at many existing dams and optimise their utilisation, and the potential gain is considerable.
Analysis of earlier failures and their consequences should prompt an in-depth review of many accepted ideas, standard criteria and conventional design methods.
Future dams in the 10-30 m range will mostly be embankments, construction becoming steadily more mechanised in the developing countries; but changing economics and technology, cheaper cement, and the need to consider larger floods should increase the proportion of masonry, concrete and RCC dams in the coming decades.
Considerable progress is possible in the cost, safety and performance; in, for example, embankment dam drainage and protection, gravity dam design, spillway design and equipment, flood management to minimise downstream hazard, warning systems, etc.
Lastly, many < 30 m dams represent a modest investment but may have considerable environmental impact. By investing a little more, they could and should be systematically designed as a powerful means of improving the landscape by blending into it rather than changing its character.