Dr Debabrata Roy Laifungbam
and Dr Soibam Ibotombi
NewAge, May 23, 2009
THE scenario and consequences of a Tipaimukh Dam-break has not been thoroughly studied. NEEPCO has yet to complete a basic scientifically-sound environment impact assessment even though it is geared up to start construction after having opened international bidding for engineering, procurement and construction. Such a study has to be conducted by international as well as national dam-safety experts as the impacts of a dam-break will have both severe upstream and downstream effects.
However, the downstream effects of a Tipaimukh Dam-break have been studied by the government of Bangladesh since 1992-94. In the Flood Action Plan 6 as part of the North Eastern Regional Water Management Plan of Bangladesh, the scenario of a dam failure at the Tipaimukh Dam project was investigated by international hydraulic and environmental experts in the context of a comprehensive flood action plan for Sylhet district.
India is also planning a major Cachar plain irrigation project downstream of the dam. Bangladesh already knew a fact that we in Manipur do not know still. Surprisingly, for the people of Manipur, the Tipaimukh project is not the only project at the drawing board on the Barak River. This means that water released from the dam reservoir will be further diverted for the irrigation project planned in Cachar district, contrary to NEEPCO’s recent claims.
FAP 6 had a future-without-plan component that looks at a dam-break scenario with minimally adequate project description available through the Joint Rivers Commission (Indo-Bangladesh). Bangladesh has pending issues with the government of India with regard to the dam that includes the effects of flow regulation. Regulation of the Barak’s flow by the Tipaimukh Dam would provide India with the opportunity to irrigate the Cachar Plain; this India proposes to do.
Since the Cachar plain irrigation plan involves the loss of water, it is a matter of great concern to Bangladesh, particularly its northeastern region as no statement is available as to how much water India intends to take from this scheme. For the purposes of the FAP 6 study it was assumed that the total depth of irrigation water to be applied is 1 M and that the water is diverted on a continuous basis during the six dry months (November through April).
According to the Bangladesh study, the risk that the Tipaimukh Dam poses for Bangladesh is extremely significant for the Meghna river system (including the Surma and Kushiyara rivers of Sylhet). The study recognises that the region is known to be vulnerable to earthquakes. These events, though relatively rare, are extreme in intensity, and can reverse existing morphologic trends and even induce re-configuration of the drainage system.
The likelihood that during 1991-2015 the region would experience an earthquake of magnitude 7.6 (similar to the July 8, 1918 event with its epicentre at Srimangal of magnitude 7.6, return period of 30 to 50 years) is between 40 and 60 per cent; of magnitude 8.7 (similar to the June 12, 1897 event with its epicentre at the Shillong plateau with magnitude 8.7, the largest on record, return period of 300 to 1,000 years) is perhaps 2 to 5 per cent, assuming the events are random and can be described with a simple binomial probability model.
On past evidence, river channels and sedimentation patterns in the northeast region may be subject to major disruptions following a severe seismic event. During past earthquakes, instances of ground liquefaction, landslide, rapid subsidence, collapse of river banks, and changes to river courses have been documented (District Gazetteer, 1917). The effects of earthquakes along the Brahmaputra river were described in 1899:
‘Strong ground shaking triggers liquefaction of river cross-sections in a few seconds; underwater slopes slide towards the stream axis, the bottom of the river heaves, and the banks become lowered; water immediately starts to rise and overflows the banks and adjacent zones where infilling of the channels takes place. Natural sills form, causing temporary lakes to develop; channels gradually re-open by scouring where currents are strong enough, and consequently water levels decrease.
‘Where channels remain blocked, streams desert their old channels to form new ones; and in subsequent years, the huge amounts of sediment poured into the river as a result of the earthquake gradually moves downstream. Sediment transport is higher than previously and siltation conditions are therefore modified.’
Earthquakes are believed to have also induced landslide and slope failures in headwater catchments in the Shillong plateau, which could greatly increase the amount of sediment supplied to the region for long periods of time. Joglekar (1971) described apparent impacts of major earthquakes on the upper Brahmaputra in Assam, India. After the severe earthquakes of 1947 and 1950, the bed level near Dibrugarh rose substantially. Between 1947 and 1951, low water levels rose by as much as three to four metres; thereafter they were steady.
THIS risk is, however, a significant issue relating to future environmental management of the northeast region water system of Bangladesh.
A dam-break is a catastrophic failure of a dam which results in the sudden draining of the reservoir and a severe flood wave that causes destruction and in many cases death downstream. While such failures are rare and are not planned they have happened to dams, large and small, from time to time. The International Commission on Large Dams has identified 164 major dam failures in the period from 1900 to 1965.
With respect to the safety of the proposed Tipaimukh Dam, hydraulic and environmental specialists opine that well-designed and constructed rockfill dams are perhaps the safest type for large heights (Tipaimukh would be among the largest of such dams in the world), but local circumstances may be much more important in this respect than dam type.
Two examples illustrate the types of failures that have been reported. The most famous, the Teton Dam in the United States was a 90m high earth-fill dam which failed in 1.25 hours. The flood wave which was released had a peak discharge of 65,000m3 s-1 at the dam and a height of 20m high in the downstream canyon. The Huaccoto Dam in Peru was 170m high, similar to the Tipaimukh Dam; it failed over 48 hours due to a natural landslide in the reservoir.
Generally, a flood wave travels downstream at a rate in the order of 10km hr-1 although velocities as high as 30km hr-1 have been reported near failure sites. From these wave velocities, it would appear that the initial flood wave could travel the 200km distance from Tipaimukh Dam site to the eastern limit of Bangladesh within 24 hours having a height of perhaps 5m. Peak flooding would occur some 24 to 48 hours later. High inflows would persist for ten days or longer and the flooded area would likely take several weeks to drain.
The Tipaimukh reservoir is huge (15,000Mm3) compared with experience reported in the literature. In the event of a significant unplanned discharge, the river system in Bangladesh would respond (drain) rather slowly, as characterised by the outflow rate relative to the floodplain storage volume), such that most of the water released would remain ponded over the northeast region for some time. Assuming a release volume of 10Mm3 and a ponded area of 100km2, the depth of flooding would be an average of 1.0 m above the normal flood level.
There will be first an imperative need for Bangladesh and India to cooperate in formulating and implementing risk management measures if the Tipaimukh Dam as presently designed should be constructed. A wide range of risk management measures are normally undertaken, including regular inspections by independent engineering teams, instrumentation and plans for warning downstream populations of deteriorating conditions of a dam, evacuation plans, and so on. As and when India’s plans proceed, there will be a clear need for Bangladesh to avail itself of expert technical assistance from dam safety specialists experienced with very large dam/reservoir systems and trans-border risk management.
For illustrative purposes only, the Bangladesh study modelled flood waves for a test case of an instantaneous failure, 50m wide extending to 100m below the crest of the dam. Discharge and water level hydrographs were presented for three locations: at the exit from the mountain valley (km 80), at Silchar (in the middle of the Cachar plain, km 140) and at Amalshid (km 200).
It was forecast that substantial attenuation of the flood wave would occur upstream of Amalshid and that the flood wave at Amalshid would be a long-duration event. Depending on the breech geometry and peak discharge, the flood peak would occur at Amalshid approximately 2 to 3 days after the dam break had occurred and flooding would continue for ten days or more. The flood levels at Amalshid would rise to approximately 25m PWD (peak water discharge), which is at approximately 8m above the floodplain level. This flood level depends on the boundary assumptions made and could vary depending on floodplain conveyance.
Socioeconomic aspects: ‘An electric bulb from every tree’
AS PER the technical report of NEEPCO (1998), the dam will have a firm generation of 401.25MW only implying that 401.25MW of power only will be generated regularly, and this is the best scenario. And again as per the past Central Government formula, the government of Manipur will get only 12 per cent of 401.25MW, i.e. 40-43MW free (sharing with Mizoram where 90 per cent is claimed by Manipur state but this is subject to the government of India set norms which has changed from time to time; it has been revised since).
In order to get this 40-43MW of power, the state will be losing around 293.56Km2 under submergence of reservoir water which includes 4760ha of gardens, 2053ha of rice cultivable land, 178.21Km2 of total 7251.36Km2 of forest land besides affecting a numbers of villages (15+90). Let us introspect as well as retrospect the case of the Tipaimukh dam in comparison with the Loktak hydroelectric project and analyse the possible implications in the next 50 years hence especially for the natural resources that will be deprived of the state.
When the Loktak project was initiated in the late 1960s – the tall claims made by the authority/government were: thousands of hectares of cultivable land will be generated by draining water of Loktak lake to Leimatak river, price of 1 unit of power will be only 5 paise, the installed capacity of 105MW is 10 times more than the power what the state requires and there will be no power problem for the next 50 years or so, etc.
Now, it is over 20 years of commissioning of the project – thousands of cultivable land have been submerged under the lake (reservoir) water contrary to what they claimed, 50-70 paise was price of 1 unit of power at the time of commission, power supply is at its worst nowadays and likely to worsen, which every citizen knows; and rehabilitation and compensation issues are yet to be settled at the Gauhati High Court.
And besides, a range of grave environmental and ecological problems especially of the Loktak Lake threatens this internationally important wetland’s very existence along with the Keibul Lamjao National Park, with ecological damage to the entire Imphal Valley and the catchment areas. The State gets about 6-10MW of free power intermittently from the Loktak Hydro Project. The question is whether it is sufficient to compensate the economic, natural resources and environmental loss which the State bears presently?
Now let us examine the possible implications of the Tipaimukh project in a similar manner. As pointed out above, the 293.56Km2 of submerged area consists of 5760ha and 2053ha of garden and cultivable lands respectively. These figures, the authors believe, are far underestimated because at present, less than 50 per cent of arable and cultivable lands in the Barak river beds are utilized due to thin population of the region, which will be possibly utilized in the next 50 years due to population increase. So approximately a total of about 15,626ha (11520ha + 4106ha) of cultivable land will be lost.
Again, although 178.21Km2 of the total forest area will be permanently submerged under water, practically the natural resources of a much larger forest area will be unavailable permanently to the State. Net present value levy for forest land conversion to non-forestry use as per the Supreme Court directives would also make the project economically unviable, as claimed by NEEPCO on January 28, 2006 (Tipaimukh Multipurpose project tariff increases by 67 paisa/unit on this account of NPV) in its submission to the Supreme Court’s expert committee.
Compensatory afforestation programmes will take over large tracts of other categories of forested lands besides Reserve Forests as well, but most of these programmes will never be implemented. After completion of the project, the project authority will claim that depletion of forest and other natural resources in the nearby catchment area will increase siltation in the reservoir leading to the reduction in storage capacity of the reservoir. This, in turn, will reduce the generation capability of the power plant and so on.
The same was true in the case Loktak project where the lake water level is to be maintained as a reservoir in order to generate electricity, submerging thousands of cultivable land contrary to what the authority claimed in the beginning. So the question that can be raised is whether it will be a wise policy, in the long run, to surrender such a huge natural resources just for 40-43MW of free power. This is a huge question, no doubt, to ponder upon.
Discussion and conclusion
Structural and tectonic setting, plate kinematics and interaction as well as seismic potential of Manipur state and the serious implications for the entire region’s existing geomorphologic trends and even induce re-configuration of the drainage system amount to scientific and technical objections to the construction of a huge dam of the magnitude proposed in case of the Tipaimukh dam. Because, such a outmoded design dam may have the potential risk of a great disaster, killing hundreds and thousands of lives, and causing generational incalculable losses to future economic options, livelihoods and cultures.
So, the government must rethink about the construction of such a huge dam. Instead, it is advisable to construct relatively smaller projects with improved modern designs in order to scale down the magnitude of possible disaster since earthquake prediction and prevention is beyond human capability. It would be wiser and economically more sustainable to consider smaller dams or run-of-the-river schemes with an objective to reduce human induced disaster, and save the river.
Construction of smaller projects not only will tone down the magnitude of the possible human induced disaster but also will provide balanced sustainable development avenues for various regions of the State as well as minimise the environmental and ecological instability. In the meantime, the government of Manipur also should reassess all the power projects especially in terms of its operational efficiency and potentiality instead of simply waiting for a mere 40-43MW free power from Tipaimukh project which could last as long as 20-25 years.
For instance, the expected maximum head (difference between reservoir water level and power generation unit) is about 160m in the case of Tipaimukh project while in the case of Loktak Lake the head is about 269m which is approximately 100m more than that of the proposed Tipaimukh project. But such a tremendous head is wasted just to generate a variable 40-80MW of power only. This is nothing but sheer wasting of huge natural resources by severely underutilizing the immense potential. So, the potential of the installed Loktak project should be fully harnessed by Manipur after reassessing and renovating with an objective of enhancing its efficacy and benefit to the State while the project exists.
In conclusion, let us not waste and surrender our huge natural resources just for 40-43MW of power, and let us introspect, learn through mistakes of the past and rectify ourselves than repeating it. Because a wrong decision of ours will cost heavily on our future generation who will, otherwise, never forgive us. Let us remember popular Native American proverb which says, ‘The frog does not drink up the pond in which he lives.’