Probe International
May 1, 2008

Overtopping: 1/3 of all dam failures globally

Overtopping occurs when the level of a reservoir exceeds the capacity or height of the dam. This can be caused by an inadequate or dysfunctional spillway or by settlement of the dam crest (for an explanation of settlement, see foundation defects). Overtopping occurs when water levels rise rapidly and without adequate warning (for example, due to flashfloods, heavy rains, a landslide in the reservoir that creates a tsunami, or if a dam upstream collapses). The result can compromise the structural integrity of the dam or it can quickly erode the land on either side of the dam, in effect disengaging the dam from its river slope embankments. In earthen dams the main cause of failure is erosion caused by overtopping or a failed spillway. (Example of failure due to overtopping: Banqiao Dam)

Foundation defects: 1/3 of all dam failures

Defects can occur in the foundation supporting the dam. For example, the mere weight of a dam structure has an impact on the ground underneath. If this weight is not properly taken into account in the engineering of the dam, the ground underneath can settle unequally and compromise the foundation (imagine the leaning Tower of Pisa). Similarly, dams built on slopes must be properly engineered to avoid issues with instability or landslides. Any event causing the movement of a foundation, such as an earthquake, can also compromise a dam’s foundation. The main cause of concrete dam failure is a problem with the foundation. High uplift pressures and uncontrolled foundation seepage can also compromise the dam’s foundation. (Example of catastrophic dam failure due to foundation defects: St. Francis Dam)

Piping and seepage: 1/5 of all dam failures.

Embankment dams – which are generally semi-permeable – can be compromised when too much water seeps or leaks through the structure. Dam failure can occur when the structure becomes weakened from internal erosion, an effect referred to as piping. This can occur along hydraulic structures,spillways, conduits, or cracks. Such seepage or leakage can even be caused by an animal burrowing in and around earthen dams. (Example of dam failure due to piping and seepage: Kelly Barnes Dam)

Other reasons

Dams which are improperly maintained or built with inadequate materials or unsound design can result in structural weaknesses that lead to catastrophic dam failure. (Dam failure due to improper maintenance and structural weakness: Val di Stava Dam)

Prepared by Probe International, May 2008
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Sources: Hawker, Patrick, 2000. A Review of the Role of Dams and Flood Management, Thematic Review IV.4: An
Assessment of Flood Control and Management Options, www.dams.org/
“Notable Dam Failures,” Washington State Department of Ecology, 2007.
www.ecy.wa.gov/PROGRAMS/wr/dams/failure.html [PDF]
“Why Dams Fail?” Federal Emergency Management Agency, 2006.

www.fema.gov/hazard/damfailure/why.shtm [PDF]
“Operations, Monitoring and Decommissioning of Dams Final Version,” World Commission on Dams, November
2000, www.dams.org