Complete immunity from floods in Brahmaputra valley is a utopian concept and techno-economically not viable. Even in the most advanced countries, complete immunity has not been achieved despite colossal investment for flood protection in many of their rivers that are much smaller and less complex than the Brahmaputra. Therefore, flood control measures are ideally aimed to reduce the frequency and magnitude of floods, to provide reasonable protection and to mitigate flood damages. These measures can be–i. Engineering structural measures and ii. Administrative and non-structural measures.
An Engineering measure is aimed either to reduce flood volume or to confine the floodwater within the river channel. Both the measures aim at avoiding the spill of floodwater into the inhabited areas. These are discussed below-
Reduction of Flood volume: This can be achieved through–i. diversions, ii. detention basins or iii. storage reservoirs.
In the case of the Brahmaputra, the diversion of water required is of the order of 15000-20000 m3/sec to have any impact on its floods. Because of the narrow and thickly populated valley area, the possibility of such a huge artificial channel through the already flooded valley is not advised. The alternative is the intervention of floodwater in the Himalayas and diverting it along the hills which is an even more difficult proposition, is impractical and plagued with adversity.
The Brahmaputra valley is very flat. The construction of a detention reservoir in the valley itself with adequate storage volume to have an impact on flood in the valley is estimated to bring more area under submergence rather than its reduction. Therefore, the construction of storage reservoirs in the hills is the only option available to reduce flood volume in the Brahmaputra river.
All experts in Government or otherwise agree on this aspect. The Master Plan of Brahmaputra river finalized in 1986 by Brahmaputra Board strongly recommends for construction of large storage reservoirs in some of the major tributaries including on the Siang (the main stem of the Brahmaputra) for the reduction of flood volume. In fact, this is the most successful and preferred method of flood mitigation across the world.
For this reason, studies and investigations towards the construction of such reservoirs were taken up by the Government around 1975. Based on exhaustive investigations, the Detailed Project Reports (DPR) of storage projects on Siang and Subansiri – the two largest contributors of Brahmaputra were prepared initially in 1983-84 by Central Government through the Brahmaputra Board. Flood storage provided in these 2 projects were estimated at 8500 Billion Cubic Meter (BCM) and 2700 BCM respectively.
As per the recommendations of the Brahmaputra Master Plan, investigations on other major tributaries for DPR preparation of Reservoir Projects were taken up from 1983-84 onwards with an aim to create more storage reservoirs. It may be mentioned here that such reservoir projects were not found viable on economic consideration of flood benefits alone as such benefits were mostly intangible. However, it was realized that along with flood mitigation, these storage projects would generate a huge amount of electricity at a very competitive price which made these projects techno-economically attractive.
As per the study carried out in the Master Plan of Brahmaputra, the lowering of flood levels estimated due to these projects would be substantial. The Siang project alone would reduce the flood height of the Brahmaputra by 1.41m at Dibrugarh and 1.15 m at Guwahati. Similarly, reduction of flood level by four dams (Siang, Subansiri, Lohit, and Burhi-Dehing) at Pandu would be 1.63m. These figures are for a very large and rare flood that is likely to occur once in a 100 years period. Therefore, the reductions in flood height would be much better for a flood of lesser magnitude.
What do these reductions in flood level mean in terms of relief? The Brahmaputra valley is very flat and even a 30 cm reduction in flood height may stop water spread by a km in many locations. Thus, the relief would be huge for a reduction of 1m or above in flood level. However, for various reasons, none of these projects investigated by the Government has been taken up so far for execution as planned initially.
Later developments- Moderation of flood through Storage reservoirs:
The initially planned mega storage project on the Siang river at Rotung was ruled out later in view of strong objections by host state due to the expected submergence of important towns of Along, Pangin, Kambang, Yingkiong, etc besides a number of villages. Similarly, the planned storage project on the Subansiririver at Gerukamukh was also ruled out due to objections for the expected submergence of Daporizo town, Tamen town, and several villages.
A Run of River scheme is under active consideration on the Siang at downstream of Rotung whereas a Run of River scheme is already under implementation on the Subansiri river at Gerukamukh itself. All rivers/Projects of significance in terms of storage for mitigation of floods in Brahmaputra valley were gradually handed over to private developers as per the decision of Central Government and concerned State Governments. Unfortunately, almost all these projects have been planned and DPR prepared keeping the generation of power as the main objective and as run of the river schemes. A run of the river scheme has no flood moderation aspect. Rather, it distorts downstream flow adversely if used as a peaking power plant.
Many people would like to argue that a series of lower height dams may deliver similar benefits. In this regard, it may be mentioned that a number of dams in a river in a cascade may result in the generation of electricity close to the generation of an alternative single large dam but total storage put together would be only a small fraction of a single large reservoir. Therefore, the benefits of flood moderation originally anticipated from these storage projects would not be forthcoming if developed in the manner they are being planned now.
Moderation of flood through Storage reservoirs – options left
The Siang river contributes 33% of flow at Pandu and is critical to flood control in the Brahmaputra basin. Although the original storage site in the Siang river has been ruled out due to objections of the state concerned and no alternative sites have similar storages, there is still scope for a reservoir of substantial volume on the Siang river in the upstream of Boleng town.
A reservoir at this site has the potential to moderate floods in the valley. Similarly, there are also possibilities to create reservoirs with good storages in the upper region of the Subansiri and Kamala River (a major tributary of Subansiri) to offset the loss of storage in the present run of the river proposal under implementation at Gerukamukh as compared to the original reservoir proposal. There is a need to review the project proposals of Lohit, Dibang and Jia-Bhoreli rivers for increasing storage capacity in these major tributaries of Brahmaputra.
The projects in these rivers are either already framed or are in the process of framing by private developers, mostly as Run of the River schemes which will have little or no bearing on flood control. The creation of storage reservoirs with flood cushions in the remaining lesser tributaries of the Brahmaputra will also moderate flood in the flood plains of these tributaries in the valley and improve the overall flood situation. Further, the integration of reservoir operations of all present and future reservoir projects in tributaries will also result in better management of floods in the Brahmaputra valley.
From the experience of the Ranganadi project, inter-basin transfer of water for generation of power in a run of the river scheme, especially in Himalayan rivers, has been found to have far-reaching disastrous impacts. In such projects, most of the floodwater and silt is released to the original river whereas clean water during most periods of the year is diverted for the generation of hydropower.
Therefore, bedload propagation is deprived in the original river during the long silt free period of flow. It may be mentioned that bedload propagation is high in a river during its silt free period which makes the river bed deeper. The absence of silt-free flow causes a rapid rise of the river bed, decreases channel capacity and causes floods in the countryside as has happened in Ranganadi. Thus, this kind of projects are to be discouraged at the proposal stage itself as hydropower at the cost of people’s misery is not desirable.
From this discussion, it can be concluded that the most potent weapon for mitigation of flood in Brahmaputra valley remains totally unutilized today for various reasons. For this to happen, a strong political will along with a highly attractive ‘Relief and Rehabilitation Plan’ and alternative livelihood to project affected people with an emphasis on conservation of their cultural and political identity are needed in tandem.
Dhruba Jyoti Borgohain is a retired chief engineer of the Brahmaputra Board. He can be reached at: email@example.com