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Brahmaputra Board mooted construction of multipurpose dams

By Ajit Patowary

GUWAHATI, June 25 - �The key to a long term, effective and reasonably permanent solution of the flood problem of the Brahmaputra valley lies in constructing some large storage reservoirs. This is clearly established by the study and analysis of the hydrometeorological, topographical and geological data... No other measure, be it embankments, afforestation or watershed management would independently contribute as much to the reduction of the flood problem of the valley.�

The above is quoted from Chapter XII of the Master Plan of Brahmaputra Basin, Part-I, Main Stem. The chapter deals with the issue of storage dams. The Master Plan was prepared by the Brahmaputra Board and it was published in 1986.

It further said the various benefits of flood control, hydro-power generation, irrigation, navigation etc., make multipurpose dams economically viable.

However, this recommendation of the Brahmaputra was not taken into consideration by the Union Government. It is alleged that after the Brahmaputra Board prepared many of the multipurpose dam projects, the Board was made to hand over the projects to the Power Ministry to turn them solely into hydel power projects.

And now, an impression is sought to be given by some quarters that dredging of the Brahmaputra bed would be able to help in solving the flood problem in the Brahmaputra Valley. The Master Plan suggested that for controlling silt in the Brahmaputra, soil conservation measures alone are not to be taken up in the catchment areas, but together with these measures, as a measure to trap the silt on the silt intensive rivers, storage reservoirs have to be created.

The Board said in the Master Plan of the Brahmaputra Basin that the reservoirs on the Siang (Dihang) and the major tributaries like the Subansiri, the Dibang, the Lohit and the Jia Bhoroli would contribute substantially to reduction of flood peaks, flood frequency and flood stages in the main river.

It further said that turbulent tributaries like the Pagladiya, the Puthimari, the Burhi Dihing, the Disang, the Dikhu, the Jhanzi, the Dhansiri etc., would require to be provided with storage to control flooding in their own valleys, it says.

It says the master plan for flood control caters to create storage reservoirs of 0.466 m ha m of the dams on the south bank tributaries up to and including Noa Dihing (Category-A), while it has made provision of creation of storage reservoirs of 3.081 m ha m for other tributaries (Category-B) and for the Siang (Dihang) (Category-C) it has provided for creation of storage reservoir of 3.55 m ha m. Thus a creation of a total provision of 7.097 m ha m was recommended by the master plan.

But the above amount of water sought to be live-stored constitutes only 14. 4 per cent of the average annual yield of the Brahmaputra of its 49.43 m ha m at Pandu. However, the live storage capacity of the Dihang reservoir alone is 3.55 m ha m, which is 50.02 per cent of the total proposed storage capacity in the Brahmaputra Basin. Studies show that on building Dihang dam a flood peak of 73,000 cumec at Pandu would come down to 43,000 cumec, resulting in lowering of the high flood level over a metre. This is indicative of great relief which the Siang (Dihang) dam can bestow by moderating flood in the entire valley as the dam site is located above the upper end of the valley, says the Master Plan for the Brahmaputra main stem.

While calling for steps to tackle the adverse impact of the reservoirs on the ecology while formulating the storage dam projects, it maintains that construction of a dam reduces the dominant discharge and the fluctuations in discharge and silt charge which would have favourable impact on the river regime, says the Master Plan.

Explaining the silt arresting capacities of the storage reservoirs, it states further that with the construction of the Dihang dam the annual average silt load of 9,000 metres per hectare (ha m) would be arrested with loss of reservoir capacity only 1.1 m ha m in 100 years, while the Subansiri reservoir will trap an annual silt load of 2400 ha m with a loss of capacity of only 0.28 m ha m of the reservoir in 100 years. The average annual sediment load at Bechamara point of the river is 10,910 ha m and at Pandu point it is 16,200 ha m, says the Master Plan.

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