Tipaimukh Dam : The Impact Assessment
In my previous posting I complained about India hiding the data. I am proved wrong. The data on Tipaimukh is there – open for everyone on the internet. Now I hardly find a ground to accuse Government of hiding anything. It looks like the Environment Impact Assessment is open from 2007. It can be accessed here.
At the same time, I found Bangladesh also did impact assessment of Tipaimukh dam way back in 1993. The report was prepared as a part of Flood Control Project (part 6) and covers upside and downside of the project in details. As the earlier one, this is also open in the Internet for years. It’s me to get the blame as I didn’t search enough before I conclude anything.
Since the debated effects are mostly in Bangladesh side, we can go over what the impact assessment says. It was prepared by Sara Bennett and Mujib Huq as part of SNC Lavalin International & Northwest Hydraulic Consultants Ltd. Let’s go inside it.
In summary, it mentions –
“Operation of the proposed Tipaimukh Dam/Cachar Plain Project on the Barak in India would moderate flows along the Kushiyara River and upper Surma River, decreasing monsoon flood levels and substantially increasing dry season flows. Impacts during reservoir filling could be even more significant. Ramifications for biophysical and socioeconomic environmental components include changes in monsoon cropping, reduced infrastructure and homestead flood damage, slower post-monsoon drainage, increased dry season in-channel fisheries habitat and improved migration access in the pre- and post-monsoon seasons, and so on.”
The same was best dealt with in section 8 of the plan. Section 8.3.2 deals with the impacts which is detailed in 8.7.2. It says –
“The Tipaimukh Dam/Cachar Plain Project on the Barak in India will substantially alter discharges of the Barak where it enters Bangladesh at Amalshid. Available information suggests that monsoon peak flows would decrease by about 30% (from 5250 to 3500 m3 s-1). Winter flows would double or triple, increasing 100 to 200% (from between 170 and 250 to 500 m3 s-1). Of the monsoon peak decrease, monsoon flow in the Surma and Kushiyara Rivers would decrease by 800 m3 s-1. Surma-Kushiyara and Surma-Sarigoyain floodplain discharges would decrease by the remainder of 1150 m3 s-1.
The Surma and Kushiyara Rivers along their entire lengths, and part of their tributaries, are also affected. At Fenchuganj on the Kushiyara, for example, model monsoon peak flows decreased by about 20% (from 2900 to 2400 m3 s-1). Peak levels decreased by 1 m (Regional Plan Figure 21A). Model winter flows increased by about 80% (from 250 to 450 m3 s-1). Levels increased by almost 2 m.
Further downstream in the Kalni-Kushiyara, model water levels increased by as much as 0.3 m in the monsoon and 1.5 m in the winter and pre-monsoon periods, as a result of sediment deposition. The affected reach extends as far as Ajmiriganj. By Bhairab Bazar, model flows and levels are almost unchanged from current conditions. In a simulation based on a drier year than 1991, however, model winter flows might increase significantly; simulated discharge hydrographs show that Bhairab Bazar winter flows are highly variable due to tidal effects.
Similar but somewhat smaller changes occur on the Surma (Regional Plan Figure 21B). At Kanaighat, model monsoon levels decrease by 0.5 m, while winter levels increase by 1 m or more. At Sukdevpur model water levels are almost unchanged from current conditions. …
The potential Tipaimukh Dam/Cachar Plain Project, which the model indicates would decrease upper Surma and upper Kushiyara monsoon peak levels by 1.5 m and increase winter discharges by 100 to 200%, for conditions similar to those in 1991.Expected sediment deposition in the Kalni and lower Baulai, which the model indicates would increase pre-monsoon and post-monsoon water levels by as much 1.5 m.”
Under 8.5 it mentions –
“Projects which would be significantly affected by dam implementation are Upper Surma-Kushiyara, Surma Right Bank, Surma-Kushiyara-Baulai Basin, and Kushiyara-Bijna Interbasin“
It’s good to add that all the above mentioned projects were for controlling the flood. They will be irrelevant if flood is prevented upstream. The potential hazards are all mentioned in 8.8.1 and 8.8.2. The first of these is the Earthquake and next one is the any other dam failure condition.
“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 1918 event, return period of 30 to 50 years) is between 40 and 60%; of magnitude 8.7 (similar to the 1897 event, the largest on record, return period of 300 to 1000 years) is perhaps 2 to 5%, assuming the events are random and can be described with a simple binomial probability model. … The Tipaimukh reservoir is huge (15,000 Mm3) 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 characterized 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 10 Mm3 and a ponded area of 100 km2, the depth of flooding would be an average of 1.0 m above the normal flood level. … we show modelled flood waves for a test case of a instantaneous failure, 50 m wide extending to 100 m below the crest of the dam. Discharge and water level hydrographs are presented for three locations (Figure 11): 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 can be seen from this that substantial attenuation of the flood wave would occur upstream of Amalshid and that the flood wave at Amalshid is a long-duration event.”
The other impact is mentioned in section 9.2.1. It notes –
“Dredging of the Kalni and Baulai appears to mitigate the effect of higher discharges in the post-monsoon season due to Tipaimukh Dam. Actual impacts would have differed if other upstream dam/irrigation scenarios had been adopted in the simulations.”
Now if one moves to the Indian EIA document, the entire 3.9 section would be found to deal with these issues. It covers almost all possible conditions. They have to be the same since it is Science.
If anyone is interested, he can go through those docuements and provide valuable feedbacks. I am not an expert and would not like to be one in short term. However I read both of them and I was satisfied with the mitigation effort and planning published in EIA and EMP documents. Whether all of them will be implemented – is another question needs to be dealt with.