Perennial floods in Assam

As the monsoons sweeps through Northeast India every year Assam gets flooded. Different NGOs and government agencies are looking for the solution to floods, but I believe we should be more focused on the root causes of the annual flood so that it can be solved at the base level.

If you see the geographical location of Assam, it is located over the foothills of the Himalayas. From Arunachal to Nagaland, it is surrounded by mountains of elevation up to 5000 ft. So, in the peak monsoon season, the rainwater is bound to flow towards the valley, nothing can be done on that. Flowing water through tributaries to the mighty Brahmaputra is normal, but the problem arises when the carrying capacity of river water overflows. Here the second point comes, the water carrying capacity of the rivers which is controlled by river length, width and depth. It is apparent that the fast-flowing rivers of Assam are young in age and they entered Assam just after a short length from their source. At the peak of Monsoon time, the rainwater is carried by the river along with the sediment which caused shallowing of the river and it is the primary source of rising of the river beds at the valleys; this is what that is happening in Assam year after year. The average depth of the Brahmaputra is only 40-50 metres which is receding day by day due to more sediment supply by the tributaries. This is not the case of other States as the geographical location and young mountains surrounding the valley accelerate the sedimentation issues in Assam. The third most important point is the elevation difference between Upper Assam and Lower Assam. If we analyse the elevation of Dhemaji to Goalapara we will see the only difference of 10-15 metres along with little depressions like in Kaziranga. Because of the difference, the gradient of the river to carry the sediment is low and it results in the deposition of sediment in the river bed. The lower the depth of the river is more prone to flood.

The fourth obvious cause is climate change and rainfall data in North East India. Many of us are unaware of the relation of these three factors -- flood, rainfall, and climate change. Global warming causes the melting of glaciers in the Himalayas which is the source of the Brahmaputra and many tributaries of it. If we compare it to the rainfall data, we would see that the rainfall is about 120 inches in the eastern side of Assam and around 70 inches in the western side of Assam which is way higher than the average rainfall of India that is 12-25 inches. At the peak monsoon season, the rainfall is generally higher, along with that the melting of glaciers added more volume of water which exceeds the carrying capacity of the river. Then the river bed rises to the danger level, overflowing of water ultimately resulted in the flood. Another important point is the quality of soil in Assam. Technically, it is newer as a comparison to southern and central Indian soil as the sediment of Assam are the products of the Himalayas which is the youngest mountain in the world. The capacity to resist water-flow in the sediment of Assam is low resulting in the multiple breakdowns of the embankments. These are the reasons behind the source of a larger volume of water which cannot be carried by the rivers of Assam eventually cause the flood.

Along with these the most common problem is the role of the people of Assam. The river Brahmaputra is a boon as well as curse for the state. Being in a geographically and geologically flood-prone state our role should be towards plantation of trees, cleaning of gully banks instead of deforestation and throwing waste material to the gullies and water bodies. It is common that we talk about flood only during the monsoon, in other times very less of us think about the disaster. We need a pragmatic solution. So, it's time we changed our mindset towards flood and understand its real root causes which will definitely help us to understand the larger picture behind the most flood-prone state in India.

Gaorav Baruah,

Geological Survey of India

Contact no. 7002963350