Four fallacies centring on Tipaimukh Dam

Dr M. Monirul Qader Mirza

Bangladesh and India share basins of 54 international rivers. The waters of these rivers play an important economic role for agriculture, urban and rural water supplies and navigation sectors. Among the shared rivers between the two countries, until recently, the Ganges was the most debated because of withdrawals of its waters at Farakka Barrage point and 90 other points above of it.

Thus far, Brahmaputra and the Meghna/Barak have remained virgin in terms of water impoundments and withdrawal/transfer. However, this is no more the case. India's recent decision to go ahead with the plan of building a dam at Tipaimukh on the Barak River has ignited heated debates on its merit in India and Bangladesh.

Fallacy 1. Bangladesh consented to build the Tipaimukh Dam Project

Immediately after the independence of Bangladesh, the two countries established the Indo-Bangladesh Joint Rivers Commission (JRC) to foster cooperation on water resources development and sharing benefits. In the very first meeting of the JRC, India informed Bangladesh that it had identified a project for the construction of a storage reservoir on the Barak River for flood control. The Bangladesh side urged Indian officials to take conditions in Bangladesh into consideration.

The 2nd through 6th meetings, project outline, at the constitution of joint committee of engineers (JCE), flood problems of Sylhet and Cachar regions and hydrological studies on the Barak Dam project were discussed. In the 7th meeting of the JRC, India informed the commission that a dam site at Bhubandhar was identified.

However, Bangladeshi officials exclaimed that the proposed dam site would have minimal flood moderation potential and therefore, further investigation was recommended. In the Minutes of the 14th meeting of the JRC, the following statement regarding the Barak Dam project was made: "With regard to the flood problem of Sylhet, Cachar and adjoining areas, the Commission should jointly examine the scope of the Indian scheme of storage dam on Barak River at Tipaimukh and study expeditiously the potential flood control and other benefits in Bangladesh and report the progress to the Commission at its next meeting."

From the record of discussions of the JRC, the following conclusions can be drawn: (1) a dam at the Tipaimukh site was not recommended by the Joint Committee of Engineers constituted by the JRC; (2) the proposed dam at Tipaimukh is absolutely an Indian proposal. It was identified by the Indian Central Water Commission in 1974; (3) records do not demonstrate that Bangladesh ever formally consented to build a dam at Tipaimukh; and (4) it was never jointly studied and no report submitted to the 15th meeting of the JRC or in any subsequent meetings. Salient features of the Tipaimukh Dam latter appeared in the 'Indian Proposal to Augment the Ganges Water at Farakka' and its updated version submitted to Bangladesh in 1978 and 1983, respectively, and later in the 'Indo-Bangladesh Task Force Report on Flood Management 1990'. The logic of the inclusion of this project into the augmentation proposal of the Ganges waters was not known, because, in the official discussions, diversion of any water from the Meghna/Barak basin to the Ganges never came up.


Fallacy 2: Tipaimukh will reduce extreme flooding in Bangladesh

This is not as simple as stated by India in the 'Indo-Bangladesh Task Force Report on Flood Management 1990.' "The storage will moderate the floods in the Cachar district of Assam and Sylhet and Dhaka districts of Bangladesh and provide these areas much awaited relief from floods besides reducing the chances of obstructing the drainage of the main Padma (Ganga) in the event of synchronization of the flood flows and backing up of Meghna as it happened in 1988" (p.14).

Bangladesh countered the Indian claim with, "The updated Indian proposal provides gated spillway for all the dams and claim benefits "to a significant measure" from Dihang dam 'to a large extent" from the Subansiri dam and "to a vast areas in India and Bangladesh" from the Tipaimukh dam for flood moderation." The above Indian claims are not substantiated by facts and figures" (p.26).

Bangladesh further stated, "The Indian proposal of constructing a dam at Tipaimukh will also not help in flood moderation in Bangladesh as the Meghna basin within Bangladesh is a saucer shaped land mass consisting of a large number of beels and haors. The soil moisture deficiency is comparatively less than in other areas of Bangladesh. The average pre-monsoon precipitation is above 900 mm and after meeting the soil moisture deficiency it partly goes to depression storages in the different beels and haors in the area. Sometimes pre-monsoon precipitation also causes early flash floods." (p.26).

"The intensity and duration of monsoon rainfall is of high magnitude in the Sylhet area. The average being of the order of 4000 mm. Above 70-80% of the monsoon precipitation goes to direct runoff which causes heavy flood almost every year in the area. The estimated runoff depth is of the order of 3m. As such the Indian proposal of constructing dam at Tipaimukh will not help flood moderation in Bangladesh as the monsoon precipitation in Sylhet basin is enough to cause floods in Bangladesh." (p.26).

On the contrary, India countered Bangladesh's argument with, "The Tipaimukh reservoir having 9.00 milliard cum. as its live storage will be very effective in absorbing high floods in the valley and protecting the Cacher and Sylhet areas from the flood-ravages. The Tipaimukh reservoir will moderate the floods generated by heavy precipitation over a ctachment of 12,758 Sq.km which otherwise would continue to ravage the downstream areas in an uncontrolled manner. Areas of both the countries will get considerable relief from the flood menace after the Tipaimukh reservoir comes up." (p.61). The limitation of this 9 millard cubic meters of live storage for flood moderation and hydro-power generation is explained below.

Initially, the main purpose of such dam construction in the Meghna/Barak basin was 'flood control'. However, over the years, it purposes were shifted to 'hydropower generation' and 'irrigation'. A multi-purpose dam has to encounter many problems to optimize its operation to serve all the purposes (could be opposing) efficiently.

For example, in order to mitigate downstream flood problems, the storage behind the Tipaimukh dam should be maintained at the lowest level possible at the beginning of the monsoon. If this is the case, then power generation from the dam will have to be compromised. To maximize power generation, water levels (therefore the storage volume) must be maintained at the highest possible levels. This will have limited impact on flood moderation. Therefore, experts are concerned over the stated flood control benefit of the Tipaimukh dam in extreme conditions, as its main purpose is to generate electricity.

Moreover, there are also other concerns. After a detention from a heavy rainfall event, generated runoff water should be released through the spillway as quickly as possible, within the capacity of the downstream channel to maintain safety of the dam itself. Firsthand examples exist in our neighborhood. In 2000, the South-Western region of Bangladesh was suddenly engulfed by waters arriving from neighboring India. The source of the flood waters was the water released from a string of dams in West Bengal, where the capacity of the dams was exceeded due to the sudden onrush of runoff from the catchment areas. Similar incidents were experienced in other parts of the world as well.

"During the El Niño of 1983, climate and hydrologic forecasts failed to predict abnormally heavy spring runoff in the Rocky Mountains. Dam operators along the Colorado River maintained high water storage levels, failing to prepare for the potential of the flooding. By the time operators began to react, water was bypassing the dams via their spillways and wreaking havoc throughout the system. Ultimately, the Glen Canyon dam in Arizona was heavily impacted with flood flows eroding large volumes of rock from within the canyon walls that support the dam. Fortunately, the flooding peaked and control was regained before the dam was breached" (http://science.jrank.org, 2009). As the North-East region of India is very prone to rainfall with long rainy seasons, such events will likely happen in the near future and bring about havoc in the downstream areas of the Barak/Meghna basin in India and Bangladesh.

The reservoir simulation study downstream of the Tipaimukh shows that it would withstand 100-years' of floods. It should be noted that large floods are often designated as a "one-hundred-year flood" but a 100-year flood does not mean that such a flood occurs once every 100 years. Instead it signifies that there is a one in one-hundred (or 1%) chance of such a flood occurring in a given year, with two exceptions: Sonaimukh and Badarpurghat would be 0.75m and 0.65m above the danger levels, respectively.

However, the data used for the calculation the 100-year flood for the Tipaimukh Dam is significantly dated. In its letter of clearance, dated October 24, 2008, the Ministry of Environment and Forests (MEF), Government of India asked the concerned authority to update the peak flood estimation for the project. The MEF stated, "Peak flood estimation by frequency method, utilizing a short series of 16 years (1966-1981), when the observed peak is 9100 cumec will lead to a significant increase 1 in 100 year flood estimated as 4931 cumec only. Flood frequency study has to be carried out utilizing the data from 1966 to 2007 and got vetted by CWC." Although the MEF urged NEEPCO to update flood analysis, recently it has released the environmental impact assessment (EIA) report for general public without any revisions.

Considering a 100-year flood from a short record of only 16 years is an unusual hydrologic practice. Without update of the hydrological assessment and related design parameters, the project will introduce significant risks for the population living in the downstream areas. Any increase in volume of 100-year flood means higher flood depths in the downstream areas of the Dam in India and Bangladesh.

The probable maximum flood (PMF) considered as the 'design flood' for the Dam is also questionable. Design of the spillway and safety of the dam are related to the PMF. The Tipaimukh Dam falls under the large category as defined by the Indian Standard IS: 11223-1985 "Guidelines for Fixing Spillway Capacity" because of its size (>60 million cubic meters) for which the inflow design discharge should be the PMF.

The PMF is the flood that may be expected from the most severe combination of critical meteorological and hydrologic conditions that are reasonably possible in a particular catchment and it is related to the probable maximum precipitation (PMP). Two other methods, standard project flood (SPF) and flood of a specific return period, also have to be considered. The maximum of the three methods is selected as 'the design flood'. The SPF is computed from the Standard Project Storm (SPS) over the watershed considered and may be taken as the largest storm observed in the region of the watershed. For the Tipaimukh Dam, the PMF has not been calculated separately as per guideline of the World Meteorological Organization (WMO).

Instead, the severest storm that occurred in the Barak Basin on from 10-12 June, 1929 has been adopted as the SPS and the PMP value is determined by increasing the SPS by 10% arbitrarily. Eventually the standard project flood (SPF) was calculated from the SPS and the PMF was determined to be 16,964 cubic meters. It should be noted that the PMF usually 60-150% higher than the SPF. There are examples (Hutong Dam on the Lohit River in Arunachal) in India where the PMF values have been calculated to be 30-40% higher than the SPF. Considering methodological aspects and examples of other dams in India, it can be concluded that the 'design flood' of the Tipaimukh Dam is underestimated.

The flood cushion area in the reservoir is 40,500 ha m and has been allocated from EL 172.5 to FRL (Full Reservoir Level) 175m. The area kept for holding flood water is inadequate. As stated before, this is usually a compromise for a multipurpose dam when hydropower generation is the main priority. For a design flood of 16,964 cubic meters, it will take only 6.6 hours to fill the allocated area. Then the spillway and other routes have to be opened to flush incoming flood waters out.

The impact of future climate change in the 'design flood' of the Tipaimikh Dam has not been considered. The Intergovernmental Panel on Climate Change (IPCC) in its Fourth Assessment Reported projected a significant increase in temperature and precipitation globally with regional variations. It also projected increases in heavy rainfall storms.

All global climate models are generally in agreement on increases in precipitation in the Ganges, Brahmaputra and Meghna basins. Note that climate change impacts will be felt in the economic and engineering life of this project. Therefore, any further increase of the PMF value due to climate change will put this project at increased risks of failure.

Flooding as a result of a dam break, poses great concerns for the engineers associated with the design of the dams. A dam break could occur from: spillway design failure, overtopping due to extreme rainfall, seismic activity, sliding of a mountain into the dam and debris flow. For the Tipaimukh dam, extreme rainfall and seismic activity are very important.

As stated above, the design discharge of the dam is underestimated; there is a high possibility of overtopping. On the other hand, the Tipaimukh project area is located in a zone that is vulnerable to high seismic activity (Figure 1) which can cause structural failure of the dam. Some extreme earthquakes occurred in the region are: Cachar earthquake 1869 (M>7.8), Cachar earthquake 1984 (M=5.8); Shillong plateau earthquake 1897 (M=8.7); and 1950 Assam-Tibet earthquake (M=8.7). Failure could occur due to large flood waves generated by earthquakes-induced landslides. Once a dam is overtopped and a breach is formed, it grows rapidly due to the spillage of water from the reservoir.

Critical values are the rate at which the dam is breached and the size of the opening. Breach widths 2 to 3 times the dam height have been reported. Failure time varies between a few minutes to several hours. Debris flow as result of excessive rainfall could also cause failure of the Dam. For example, Only one extreme 24-hour 540 mm rainfall event added almost 300,000 cubic meters of silt to the reservoir of the Kulekhani Hydropower project in Nepal. Government of Nepal borrowed $40 million or one third of the project cost to repair the project.

Regarding the Tipaimukh Dam, the Indian Ministry of Environment and Forests cautioned, "The project is provided with a spillway capacity much lower than the design flood, because of routing effect. But, the earth and rockfill dam has to be safe against over topping. The dam break flood has been estimated to submerge all densely populated villages and large towns like Silchar going under 3 to 10m deep water." The flood water will quickly cross the border, and will endanger the lives of millions of civilians in Bangladesh.

The calculation of water yield volume for the dam is also problematic. The average and 90% (will occur 9 out of 10 years) of dependable yields are 12.5 and 8.1 billion cubic meters, respectively. They were calculated based on a very limited number of recorded years which does not fall under any climatological analysis standard. Lengths of the station records varied from 2-8 years.

The density of the rain gauge stations in the basin is also well below the World Meteorological Organization's standard. Short meteorological records cannot catch high and low variability of rainfalls. Therefore, they cannot be used for water yield calculation and its risk analysis. For the Tipaimukh dam, no risk analysis regarding inflow of water to the reservoir has been carried out. There are further concerns.

The Barak basin in India falls within the meteorological sub-division # 4 (Nagaland, Manipur, Mizoram and Tripura) of the India Meteorological Department (IMD). Average rainfall in this sub-division is calculated at 1,970 mm using available records from 1871-2006. The following figure of rainfall records demonstrates three important features/implications: (1) rainfall in the Barak basin is characterized by high variability (12%) with extreme high and low rainfall events. In the available record of 136 years, annual rainfall fell below the long-term average for 74 years or 54% of the time. Therefore, it is highly unlikely that the Barak dam will be able to receive enough water every year to fill its storage.

In failing to do so, hydropower generation will be highly challenged. (2) if the dam is not filled up enough in the monsoon, water supply in a regular flooding season will substantially decrease in Bangladesh. This will cause significant impacts on the ecosystem that was developed thorough the geologic time. (3) long-term rainfall records show a decreasing trend in this sub-division placing the project at a risk of attaining the desired goal of power generation and economic viability of the project.

Fallacy 3: Agriculture in Bangladesh will benefit from the Tipaimukh Dam

This idea was concretely iterated in the 'Indo-Bangladesh Task Force Report on Flood Management 1990' in the context of a barrage at Fulertal. In the Report, the Indian argument was "The regulated tail water discharge of Tipaimukh dam project of about 405 cumec along with the contribution from intermediate catchment up to Fulertal is proposed to be utilized for irrigation by construction of a barrage near Fulertal and canal system on both banks. An area of about 30000 ha in adjacent Tripura is also planned to be brought under irrigation. Thus, around 3.1 lakh ha area in India can be brought under irrigation through Tipaimukh project."

It further stated ""…There are good possibilities of serving additional adjacent areas in Bangladesh also but these need to be investigated and studied." In the dry season, the contribution from the intermediate catchment is insignificant. Therefore, we can assume that 405 cubic meters of water will be available for irrigation purposes. The key question is whether this amount of water is sufficient to irrigate 3.1 lakh hectares (7.66 lakh acres) as indicated in the 1990 report.

The following table displays water requirement (@ 2 meter) for rice irrigation plus land preparation. The estimate of two meter water requirement (includes water application efficiency) is based on published Indian literature. The calculation shows that there won't be enough water available only to irrigate rice crop in India. Therefore, any irrigation benefit for Bangladesh with a barrage at Fulertal is highly questionable.

However, if there is no barrage at Fulertal, we assume that 405 cubic meters of water will flow down to Bangladesh. A possible change in hydrograph of Barak at Amalshid is seen from Figure 3. The Meghna basin in Bangladesh is full of depressions (many of them below sea level), a consistent addition flow of water in the late monsoon through to summer can cause water logging in the basin. This may put boro rice cultivation into jeopardy.

Fallacy 4: The Project will enhance environment and ecological qualities

In the 1990 'Indo-Bangladesh Task Force Report on Flood Management' it was stated "…no adverse effect in the area. The project would on the other hand enhance the environment and ecological qualities." A large number of articles in electronic and print media as well as the NEEPCO's EIA Report have already highlighted on the environmental impacts of the project in India. The impacts on Bangladesh have not been investigated by the concerned authority although it was agreed in the 14th meeting of the JRC. The most critical issue for Bangladesh is the impact on 'environmental flow requirement' (EFR).

It is flows that are essential within a stream and dependable wetlands to maintain its natural resources at desired or specified levels and is calculated based on "low" and "high" flow months. In the Surma and Kushiyara rivers, the EFR cannot be met in the high flow months due to reduction in flow. On the other hand, without the barrage, the flow to be available in the dry months will be significantly higher than now. With a barrage, the situation will be just reverse. Environmental setting of the delicate haor basins cannot be seen from a seasonal perspective. In one season sudden reduction of flow and in the other season increase in flow (Figure 3) would create a complete chaos for the environment and the ecosystem.

Concluding Remarks: The Tipaimukh Dam is a special project in the context of linkages between water supplies that is required from upstream to maintain delicate ecosystems in the downstream floodplains in Bangladesh. Ecosystems do not follow simple arithmetic, it is maintained in complex ways and our knowledge on details of the complexities is still limited. The hydro-meteorological analysis for the Tipaimukh was conducted on a very short record and its update is a must. The environmental impact analysis should consider the entire Barak/Meghna basin as a single unit.

(The writer is a water resources engineer and an Adjunct Professor with the Department of Physical and Environmental Sciences, University of Toronto at Scarborough, Canada.)