Dr C.P. Rajendran
The writer is an adjunct professor at the National Institute of Advanced Studies, Bengaluru, and author of a forthcoming book, Earthquakes of the Indian Subcontinent.
The Mullaperiyar dam is located in Kerala but is operated by Tamil Nadu. Tamil Nadu, a water-deficient state, has argued that water in the dam should be maintained at the highest possible level. However, we don’t have reliable data collected using modern methods to indicate the dam’s stability. If it breaks, Kerala will bear the brunt.
The Mullaperiyar dam was built 126 years ago, but safety concerns surrounding it have refused to die down. The dam is located on the Periyar river in Kerala but is operated by Tamil Nadu. And the controversies of this gravity dam have become an emotive issue in the respective states and a cow to be milked for politicians, depending on side on which they find themselves in the state legislative assemblies. A lease indenture initially signed by the dewan of Travancore and the State Secretary of the Madras Presidency in 1886, and renewed thoughtlessly in 1970, provided the rights of all waters in Mullaperiyar and its catchment to Tamil Nadu for 999, years with some annual taxes for water and electricity payable to Kerala. Although the validity of this agreement became a matter of dispute later between these states, Tamil Nadu refused to back down from the original terms. The argument also raises questions about the authority of the Union government – to pass valid orders respecting the powers of the states. The case in question is about sharing the watershed of a dam located in one state but used exclusively by another.
The water from the Mullaperiyar dam was diverted east by Tamil Nadu through a tunnel, to augment the flow of the Vaigai river, and dam its course locally, creating a source of irrigation for land in Theni, Dindigul, Madurai, Sivagangai and Ramanathapuram districts. Thus, the beneficiary of the deal, Tamil Nadu, a water-deficient state, wants to continue the status quo and keep the water level in Mullaperiyar at the maximum limit of 142 feet – against Kerala’s demand to maintain it at 136 feet. Kerala believes that it is on the bad end of this deal – a sitting duck in the event of a dam burst and a potential downstream disaster. The dam was indeed built in 1895, before the advent of sophisticated engineering and construction practices. Although the safety concerns were raised way back in 1961, following heavy rains, it became a live issue when the water level hit 142 feet on November 21, 2014 for the first time in 35 years. Since then, the discussion in Kerala on the Mullaperiyar issue has followed the ups and downs of the weather system, reaching a crescendo when the state receives intense rainfall. More pertinently, the evidence vis-à-vis the dam has been taking backseat. Although not related to an interstate river, resolving the safety issues of Mullaperiyar dam is complicated for because it must be handled by two states with different water-management priorities.
This year, the safety issue came alive again after Kerala experienced floods; to further complicate the matter, a UN-sponsored report that reviewed the threats posed by ageing dams around the world included mulled eventually decommissioning these structures. It noted that the Mullaperiyar dam is situated in a seismically active area and has major structural flaws. About 3.5 million people live downstream of the dam. Factors such as earthquakes, landslides, changes in groundwater level and siltation, etc., affect the structural integrity of dams. Dams deform because of internal and external loads – some are elastic (not permanent) and some are non-elastic (permanent). These loads are not constant and change over time. Deformation and seepage are functions of these loads. Deformation effects can also vary depending on the construction materials used. The weight of an engineered structure and the hydrostatic pressure generated by the water can augment the existing forces and cause movements. The hydrostatic pressure of a reservoir can also cause a permanent horizontal deformation perpendicular to the dam’s centreline. The impact of earthquakes on dams is not straightforward. The function of dams in earthquake-prone areas is known to have been impacted by landslide-caused tsunamis, sediment fills and/or soil liquefaction – rather than outright structural collapse. Again, the location of the dams, in relation to the geometry of the fault and whether the tectonic environment encourages vertical or horizontal movement, add to the uncertainties.
The Kerala government, for its part, had constituted two technical committees and used their feedback to argue that the dam is indeed structurally weak. The conclusions of both the committees’ studies are based on finite element models, a numerical formulation that divides a large system into smaller parts called finite elements. The structural representation of the dam was divided into smaller meshes. Numerically represented inputs of the pressure exerted either by earthquakes or by impounded levels of water on these grids were used to compute how much these elements would participate in the overall deformation. Partial differential equations used to model these elements are then assembled into a larger body of equations to model the entire system. These numerical models helped provide the first approximations of dam behaviour against the vibrations during an earthquake or merely the impact of water pressure at different filling heights. One report, by IIT Roorkee, concludes that if an earthquake of magnitude 6.5 occurs within a 16 km of the dam, the structure will suffer structural problems even when the water level is at 136 feet. A committee led by R.N. Iyengar, a former professor at the Indian Institute of Science, Bengaluru, also concluded that the dam would become structurally weak if the water level reached the maximum – even without any earthquake-generated vibrations
The Iyengar report explains how different inputs were used to simulate the numerical models for scenarios of moderate earthquakes and non-earthquake situations. Since there are no examples of ground accelerations from moderate local earthquakes (magnitude >6), their study used the estimates of ground accelerations of the 1991 Uttarkashi and 1967 Koyna earthquakes to constrain the vertical motion. The IIT Roorkee report also used comparable numerical methods to arrive at similar conclusions. Despite these reports, an empowered committee constituted by the Supreme Court banked on studies by Anna University to conclude that the reservoir was safe. However, the details of the methods used in this report are not available, to judge their reliability. The committee assumes that there are no fault structures under the dam site and uses the argument that no gravity dams have ever collapsed due to an earthquake. But a counter-argument is that there are few global examples of gravity dams of similar vintage, and the assumption of lack of active faults needs to be validated.
Thus, the Supreme Court judgment in May 2014 allowed Tamil Nadu to raise the water level to 142 feet, after taking some steps to strengthen the dam. As such, the Mullaperiyar issue is not just an inter-state water dispute but a difference of opinion on the safety of the dam. Experts have noted that the court may not have much role in resolving these issues, which can be better addressed by the intervention of independent experts. The fact of the matter is that we have very little dam deformation data from Mullaperiyar. None of the stakeholders (including Kerala and Tamil Nadu) have bothered to monitor dam deformation using advanced techniques like satellite-based measurements. In the last 20 years, we should have amassed a vast database, and experts from both India and outside should have reviewed it. Conventional surveying can also monitor the motion of points on a structure relative to stable monuments. Techniques of digital photogrammetry depict a fast tool for 3-D deformation modelling.
Eventually, it is in the best interest of both states – both progressive and capable of thinking ahead of their times – to create an independent group of experts to check existing technical reports and to collect and analyse dam deformation data, to decide on the dam’s future stability prospects. It will be a long-drawn programme by design, but it should done, based on information and sound science.