Dam failures are catastrophic events characterized by the sudden and uncontrolled release of impounded water, leading to immense damage and loss of life downstream. These incidents are comparatively rare but can have devastating consequences, often resulting from a combination of structural issues, natural events, and operational deficiencies. Historically, earthen embankment dams are the most commonly involved in dam break incidents, especially during flood events.
Here are some notable case studies of dam failures:
The 1979 Machchhu Dam Failure (Morbi Disaster)
The Machchhu Dam failure, also known as the Morbi disaster, occurred on August 11, 1979, in Gujarat, India.
- Location and Date: The Machchu-2 dam was situated on the Machhu River near Morbi town (now in Morbi district), Gujarat, India. The failure happened on August 11, 1979, after a week of intense monsoon rains.
- Dam Structure and Purpose: The Machchhu II dam was constructed in 1972 as a composite structure, featuring a masonry spillway in the river section and earthen embankments on both sides. It was primarily intended for an irrigation scheme. The dam stood 22.56 meters (74 ft) above the river bed, had a total crest length of 3742 meters for the earth dam, and a storage capacity of 1.1 × 10⁸ m³. Its spillway was designed to handle 5,663 cubic meters per second (m³/s).
- Causes of Failure: The failure was primarily caused by excessive rainfall and massive flooding that led to the disintegration of the four-kilometer-long earthen walls of the Machchhu-2 dam. The actual observed flood flow following the intense rainfall reached 16,307 m³/s, which was almost three times what the dam was designed for. Despite attempts to open all spillway gates, three gates were not operational, further reducing its effectiveness. This led to the overtopping of the earthen embankments, which rapidly eroded and breached.
- Consequences: Within 20 minutes of the collapse, a wall of water 12 to 30 feet (3.7 to 9.1 m) high inundated the low-lying areas of Morbi industrial town, located 5 km below the dam. Estimates of the number of people killed vary widely, ranging from 1,800 to 25,000 people. The flood also caused significant economic loss, damaging farmland and decreasing crop productivity.
- Human Stories: Survivors recounted scrambling for rooftops and higher ground. Over a hundred people died when the Vajepar Ram Mandir, where they sought shelter, was submerged. Women were forced to drop their babies into the furious surge to save themselves, and many lost loved ones in an instant.
- Official Narrative vs. Reality: The official cause was initially termed an "Act of God". However, the book No One Had A Tongue To Speak by Tom Wooten and Utpal Sandesara debunks this claim, pointing to structural and communication failures that contributed to and exacerbated the disaster. An independent judicial Commission of Inquiry was set up to investigate, but it was allegedly "wound up" by the then Chief Minister before completing its work, as the investigation focused on flawed design practices in the irrigation department.
- Reconstruction and Lessons Learned: The Machchhu-2 dam was rebuilt in the late 1980s. During reconstruction, the capacity of the spillway was increased fourfold, fixed at about 21,000 m³/s, to better accommodate extreme flood events. The disaster underscored the critical need for adequate spillway design and maintenance, reliable gate operation systems, and effective early warning and communication systems for downstream populations.
Banqiao Reservoir Dam Failure (China, 1975)
- Fatalities: This incident is considered one of the deadliest dam failures in history, causing more casualties than any other. It resulted in an estimated 171,000 deaths and displaced 11 million people.
- Causes: The dam failure was caused by extreme rainfall from Typhoon Nina, which exceeded the dam's design capacity. The spillways were unable to handle the massive inflow, leading to overtopping and breach, which then triggered a series of downstream dam failures.
Niedów Dam Failure (Poland, 2010)
The Niedów Dam in Poland failed on August 7, 2010, due to extreme rainfall.
- Location and Date: The Niedów dam, an earth type dam, was located on the Witka river in southwest Poland. The failure occurred on August 7, 2010, after exceptionally high rainfall between August 6 and 8.
- Dam Structure and Purpose: Constructed in 1962, the dam had a central concrete water release structure with movable gates and two earth embankments on its left and right sides. The earth dam body was compacted sand, with an upstream slope shielded by a double layer of concrete slabs sealed with bituminous material. The dam crest was 5 meters wide and served as a road. Its primary function was water supply for a coal power station and nearby settlements, not flood mitigation.
- Causes of Failure: Extreme rainfall, reaching 150-250 mm over two days in the Witka catchment, led to a flood event with a return period of over 100 years. The intense rainfall and already saturated ground caused flash floods, resulting in the dam's overtopping. The left embankment began to overtop around 17:00, with water causing erosion around lighting foundations and washing out the downstream slope. The concrete slabs on the dam crest disintegrated as they lost support. A crucial moment was the collapse of the left training wall flanking the central concrete dam, leading to a rapid outburst. The right dam also overtopped about 15 minutes later, following a similar, but less dynamic, breaching process.
- Consequences: The failure resulted in the rapid release of nearly 8.5 million m³ of water, causing substantial material losses downstream. The total volume of water released due to the dam failure was 22 million m³ . The flood peak from the dam break significantly impacted the Lusatian Neisse river downstream.
- Insights: The presence of reinforced elements like concrete plates on the upstream dam slope and the asphalt road on the crest significantly affected the breaching dynamics, making the process different and longer than for uniform earth embankments. Prediction formulas for dam breaches were found to be inadequate for this specific case, often overestimating peak outflows due to these unique structural features.
Other Notable Dam Failures and Incidents:
- Teton Dam Failure (USA, 1976): This earthen embankment dam failed due to internal erosion (piping), causing catastrophic flooding downstream. Geological problems, unsuitable bedrock, seismic activity, and caves were identified as contributing factors.
- Johnstown Flood (USA, 1889): Caused by the failure of the South Fork Dam, it killed over 2,200 people. The failure was blamed on poor maintenance by owners, who lowered the dam crest, and was followed by exceptionally heavy rainfall.
- Mill River Dam (USA, 1874): Failed due to lax regulations and cost-cutting, leading to an insufficient design. This event spurred increased regulation of dam construction.
- St. Francis Dam (USA, 1928): Its failure was caused by the geological instability of the east canyon wall.
- Vajont Dam (Italy, 1963): While the dam structure itself did not collapse, its failure is notable. Filling the reservoir caused a geological failure in the valley wall, leading to a massive landslide into the lake that created a wave overtopping the dam and killing over 1,900 people downstream.
- US Dam Failures of the 1970s: This decade saw several significant failures, including the Buffalo Creek Flood (1972), Canyon Lake Dam (1972), Laurel Run Dam (1977), and Kelly Barnes Dam (1977). These incidents prompted President Jimmy Carter to direct inspections of non-federal high-hazard dams, leading to the establishment of dam safety programs in most US states.
In general, dam failures are complex, often resulting from a cascade of mistakes rather than a single error. Key causes include overtopping due to inadequate spillway design or extreme floods, piping/seepage from internal erosion, foundation defects, structural failures, and inadequate maintenance.
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