When the Brahmaputra flooded

Climate change effects are likely to continue to cause intense flood and drought in Assam

Written by Nayan Sharma | Published: July 17, 2012 1:16:09 am

Climate change effects are likely to continue to cause intense flood and drought in Assam

The recent unprecedented floods,accompanied by the erosion of the Brahmaputra,has brought the influencing factors of flood and erosion processes in Assam into focus. First,recent research indicates the likely impact of climate change,with the temperature projected to increase in the Brahmaputra basin in coming decades. This may trigger melting of glaciers in the Himalayas.

Thus,parameters directly dependent on temperature,like potential evapotranspiration,are also assumed to be on the rise. This will have a severe impact on the hydrology of the Brahmaputra river basin. Annual precipitation is not projected to change significantly,but seasonal amounts are likely to trigger intense rainfall events. Different climate change indicators,like the length of the longest dry periods,indicate more frequent and prolonged droughts. Increases in the number of consecutive dry days and in the maximum five-day precipitation amount in the region of the Tibetan Plateau for the monsoon season,as well as large temperature trends,suggest that Assam is highly sensitive to future changes in climate. Climate change effects are,therefore,likely to cause intense floods and droughts in Assam.

Second,the increased sediment supply from watershed areas due to deforestation has exceeded the sediment-carrying capacity of the river network,resulting in continuous stream bed rise,intensification of braiding and bank erosion,culminating in significant loss of the discharge-carrying capacity of the river channels. Out of an annual runoff of 537.2 km3 for the Brahmaputra system,429.76 km3 volume of water flows during the four monsoon months. Due to constant silt deposition of the main-stem Brahmaputra and its tributaries,the channel cross-sections over the years are becoming insufficient to safely carry the huge flow volume of 429.76 km3,and the excess water that the channel cannot carry spills the bank or breaches the old embankments by over-topping or heavy seepage. This results in widespread flooding.

Third,the annual specific yield of 3 cusec/ sq mile of the Brahmaputra is the highest in the world,while the average valley width is only about 40-50 km. The Brahmaputra itself occupies about 10 km. Thus,the spill of excess flow from the silted river channels inundates large areas in no time. Even the 4,000-plus wetlands in the Brahmaputra valley are degrading due to wanton encroachments and siltation and are not much use in flood absorption. From a technical standpoint,the only way to prevent flooding in such a scenario is by creating numerous flood detention storage reservoirs at suitable sites,so that the excess flow spilling from silted rivers is held back in the reservoirs till the monsoon is over.

Fourth,the loss of land due to stream bank erosion of the entire Brahmaputra river system is probably one of the highest in the world. These eroded bank materials,coupled with the huge sediment volume contributed by the catchment area of about 5,40,000 sq km,is much more than the sediment-carrying capacity of the Brahmaputra river channel. Notably,the annual sediment volume carried by the Brahmaputra is around 800 million tonnes,one of the highest in the world.

Considering the extent of the problem at hand,it would be naïve to believe that a simplistic approach hinging on a single measure can make a dent. The solution has to be a well-considered package of measures based on strong scientific analyses and supported by prudent pilot studies. A massive soil conservation campaign is needed to effectively reduce the sediment volume of the Brahmaputra,along with the creation of flood detention reservoirs,river training for erosion control,land reclamation and channelling using the new RCC Jack Jetty Field system,which can be supplemented by corrective dredging operations on a selective basis.

China’s river management approach for the Yangtze and Yellow Rivers may be of interest. The technology of water and sediment regulating based on the integrated water and sediment management concept was adopted to manage the Yellow River. Further,extensive soil conservation efforts are being undertaken through use of terracing,strip farming,sediment retention dams,planting of trees and grass barriers.

The writer is head of department of water resources development & management,IIT Roorkee

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