Revision summary
Common causes are overtopping in extreme rain, piping, foundation failure, design error, earthquakes, and gates that do not work. Banqiao in China in 1975 failed in typhoon rain; deaths were in the tens of thousands in a cascade of breaches. Machhu II at Morbi in 1979 failed in monsoon inflow; official deaths were in the low thousands. Teton 1976 and Panshet–Khadakwasla 1961 are other documented large failures. Downstream loss is huge because the reservoir empties as a sudden wave onto towns that grew below the dam.
Model answer
Introduction
A large dam holds a reservoir against gravity. When the structure, the foundation, or the spillway fails, that water becomes a flood with almost no warning on the downstream side. The loss of life and property is therefore out of proportion to a normal river flood.
Body
Causes of dam failures
- Overtopping: the inflow from extreme rain or a failed upstream dam exceeds spillway capacity; water flows over an earth or rockfill crest and erodes it. This is among the most common causes worldwide.
- Piping and seepage: internal erosion through the embankment or along conduits opens a tunnel; the dam collapses inward.
- Foundation failure: weak, karst, or poorly grouted rock; settlement or sliding of the base.
- Structural and design faults: inadequate freeboard, poorly designed spillways, or materials that do not match the design.
- Seismic loading: shaking, liquefaction, or seiche in the reservoir; also induced seismicity in some large reservoirs.
- Operation and maintenance: gates that do not open, silted spillways, missing instrumentation, and flood forecasts that never reach the operator.
- Upstream cascade: one breach loads the next dam, as in some Chinese 1975 failures.
- Age and conflict damage are rarer Indian peacetime causes but matter in old masonry dams.
Two large failures
- Banqiao Dam, Henan, China, August 1975: Typhoon Nina dropped extreme rain; Banqiao and other dams in the Shimantan cascade overtopped and breached. Downstream plains flooded for many kilometres. Deaths ran into tens of thousands (official and later estimates differ; the event is among the deadliest dam disasters on record).
- Machhu II Dam, Morbi, Gujarat, 11 August 1979: an earthen dam on the Machhu river failed in extreme monsoon inflow; the wall of water hit Morbi town. Official deaths were in the low thousands; unofficial counts were higher. The cause combined hydrological load, embankment design, and insufficient spillway capacity.
(Other well-documented cases used in the same literature include Teton Dam, Idaho, United States, 1976, a piping/foundation failure, and the Panshet–Khadakwasla floods at Pune in 1961.)
Why downstream loss is colossal
- Stored potential energy is released in minutes, not days.
- Settlements below a dam often grew after the dam, in the false safety of a regulated river.
- Warning time is short; silt and debris raise the killing power of the wave.
Flow diagram
flowchart TD C[Causes] --> O[Overtopping] C --> P[Piping seepage] C --> F[Foundation seismic] C --> M[Poor operation] O --> B[Breach] P --> B B --> D[Downstream flood] D --> L[Life and property loss]
Conclusion
Dam failures come from overtopping, piping, foundations, design, seismic load and poor operation. Banqiao 1975 and Machhu II 1979 show how a breached reservoir becomes a night-time flood. Instrumentation, spillway capacity and downstream evacuation plans are the only honest mitigation.
Quick related
Students also ask
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What is oil pollution? What are its impacts on the marine ecosystem? In what way is oil pollution particularly harmful for a country like India?
Next question in the 2023 paper (Q8). View answer →
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Is overtopping only a monsoon problem?
It is most often a flood problem, but a blocked spillway or a cascade breach can overtop even when the local forecast was ordinary.
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Why name foreign dams in an India paper?
Banqiao is the largest documented death toll. Machhu II is the large Indian monsoon-embankment case. Together they cover hydrology and design.
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