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.

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.

Dam failures are always catastrophic, especially on the downstream side

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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.

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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.
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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Dam failures are always catastrophic, especially on the downstream side, resulting in a colossal loss of life and property. Analyze the various causes of dam failures. Give two examples of large dam failures.