Dam failure is defined by the International Commission on Large Dams (ICOLD) as the ‘collapse or movement of part of a dam or its foundation, so that the dam cannot retain water.
In general, a failure results in the release of large quantities of water, posing serious risks for the people or property downstream. The most common cause of failure of earth and rock-fill dams is overtopping (31% as primary cause and 18% as secondary cause). This is followed by internal erosion in the body of the dam (15% as primary cause and 13% as secondary cause) and in the foundation (12% as primary cause and 5% as secondary cause). With masonry dams, the most common cause is overtopping (43%) followed by internal erosion in the foundation (29%).
Where other works were the seat of the failure, the most common cause was inadequate spillway capacity (22% as primary cause and 30% as secondary cause). Foundation problems are the most common cause of failure in concrete dams, with internal erosion and insufficient shear strength of the foundation each accounting for 21% of failures.
- The construction of large dams has led to the displacement of some 40 to 80 million people worldwide.
- The aggregate storage capacity of large dams, based on dam design, is about 6 000 k m3. Assuming that half the design storage is achieved in reality, the aggregate real storage capacity of large dams globally is similar to total freshwater withdrawals estimated at around 3 800 k m3.
- An estimated 0.5–1% of the total fresh water storage capacity of existing dams is lost each year to sedimentation in both large and small reservoirs worldwide.42 This means that 25% of the world’s existing fresh water storage capacity may be lost in the next 25 to 50 years in the absence of measures to control sedimentation. This loss would mostly be in developing countries and regions, which have higher sedimentation rates.
- Riverine ecosystems impacts
At least 20% of the world ’s more than 9 000 fresh water fish species have become extinct, threatened or endangered in recent years. The watersheds of the world are the habitat of 40% of the world’s fish species, and provide many ecosystem functions ranging from nutrient recycling and water purification to soil replenishment and flood control.
- Social consequences of large dams
The construction of large dams has led to the displacement of some 40 to 80 million people worldwide.
Ecosystem transformations do not only occur in the upper, lower and mid-reaches of watersheds, they also impact on river estuaries, which are frequently complex ecosystems. Closing the mouths of major rivers, salt intrusion, destruction of mangroves and loss of wetlands are among the many issues at stake.