Dam Safety Threat Factors for Mullaperiyar Dam

Risk /Threat Factors

• Age of the dam: The dam is 116 years old as of 2011. It has outlived its expected life span of 50 years. Strengthening the existing dam has its limitations and may not prove to be effective in the event of a disaster as seen in the Morvi Dam disaster of 1979.
• Elevation of the dam: The dam is situated approx. 3000 ft above msl and holds 15 tmc of water at full capacity (at 155 feet water level). The gravitational potential energy of the dam under these conditions is 3000 million joules. The height of the water column is 155 feet at full capacity and 1200 ft length at the crest. The damage that such a water mass can cause, in the event of a dam failure, is disastrous.
• Structural weaknesses: The dam was constructed using stone ruble masonry with lime mortar grouting following prevailing 19th century construction techniques that have now become archaic. Seepage and leaks from the dam have caused concern. Structural weaknesses of the foundation and weaknesses of the lime-surkhi ruble masonry core of the dam, need to be assessed.
• Earthquakes: The dam is situated in a seismically active zone. An earthquake measuring 4.5 on the Richter scale occurred on June 7, 1988 with maximum damage in Nedumkandam and Kallar (within 20 kms of the dam). Several earthquake tremors have occurred in the area in recent times. These could be reservoir-induced seismicity, requiring further studies according to experts. CESS (Centre for Earth Sciences, Thiruvananthapuram), IIT Roorkee and IISc (Indian Institute of Science) Bengaluru,  have reported that the dam cannot withstand earthquakes of magnitude above 6 point on the Richter scale.
• Floods and Landslides: Global warming and changing weather patterns leading to torrential rains and flooding can lead to overflow and collapse of the dam. Determination of the Probable Maximum Flood and Probable Maximum Precipitation parameters for the dam, considering current hydrological, meteorological and seismic data, is important to ensure dam safety.
• Safety maintenance and safety monitoring of the dam: Continuous safety monitoring is required to prevent disasters and to detect dam failure. Disaster Management schemes have to be in place to contain damages resulting from dam failure.

Only a formal risk assessment can identify all the threat factors and the probability of threat events happening. Similarly a disaster impact analysis would be required to determine the impact of a dam failure.

-Joseph Ponnoly

The Threat of Piping and Seepage at the Dam's Foundation

R.P.Kulkarni, in his article 'Irrigation Engineering in India' has surveyed the engineering history of masonry dams in India including Periyar Dam (height 45 m).  He mentions thus: " It may be noted that almost all these (masonry) dams have served well without any mishap but their sections are found to be inadequate according to the modern theories."

He points to the dangers of 'piping' or the process by which water percolating under a dam has sufficient pressure and velocity to erode the foundations and undermine the dam's structure.  This was noticed in the Coleroon dam built in 1836 by Arthur Cotton, another British Engineer.  He further says, "Similar failures of other Indian river dams eventually led to the first experimental investigations of seepage under masonry dams, and the work of Beresford and Clibborn in the 1890s was able to predict the failure in 1898 of the Narora weir on the river Ganges."

The question that arises is:
Have there been investigations and monitoring of  piping along with seepage under the Periyar dam's foundation so far?
-Joseph Ponnoly

Reference:
Kulkarni, R.P. Irrigation Engineering in India. Indian Journal of History of Science, 17(1); 28-45
http://www.new.dli.ernet.in/rawdataupload/upload/insa/INSA_1/20005af6_28.pdf