Dam Failures and Incidents

I. Reasons Dams Fail

Dams can fail for one or a combination of the following reasons:

1. Overtopping caused by water spilling over the top of a dam. Overtopping of a dam is often a precursor of dam failure. National statistics show that overtopping due to inadequate spillway design, debris blockage of spillways, or settlement of the dam crest account for approximately 34% of all U.S. dam failures.

2. Foundation Defects, including settlement and slope instability, cause about 30% of all dam failures.

3. Cracking caused by movements like the natural settling of a dam.

4. Inadequate maintenance and upkeep.

5. Piping is when seepage through a dam is not properly filtered and soil particles continue to progress and form sink holes in the dam. [See an animation of a piping failure.] Another 20% of U.S. dam failures have been caused by piping (internal erosion caused by seepage). Seepage often occurs around hydraulic structures, such as pipes and spillways; through animal burrows; around roots of woody vegetation; and through cracks in dams, dam appurtenances, and dam foundations.

II. History of dam failures around the world

Here are some cases of dam failures around the world

1. Malpasset arch dam failure in France in 1959 (421 deaths)

2. Vaiont dam overtopping incident in Italy in 1963 (2600 deaths)

The causes:

Instability of reservoir slopes causing a landslip & 125m high wave over the dam

Lessons learnt:

Measure pore water pressures & movements at depth as well as at the surface

3. Dale Dyke dam breach in 1864 ( 244 deaths )

The causes:

Internal erosion possibly caused by hydraulic fracture of the core

Lessons learnt :

Designs include wider cores, use of cohesive & compacted fill and placing pipes in tunnels through natural ground

Slide Failure at Dam – Association of State Dam Safety Officials (ASDSO)