Analysing the role of geomorphic features in Kosi flood – An article in Geospatial World

Kosi is known as the sorrow of Bihar due to its frequent and often disastrous floods. The major geomorphic units of flood-inundated area in Kosi fan (terrain of Kosi river in India) are sand bars in the form of inter-connected drainage networks, an ox-bow lake and its meandering cut-off and imprint.

Canals have been constructed to prevent floods by utilising excess volume of water for irrigation purposes. The main canal that originated from the barrage aligns roughly in the east-west direction in the upper part of the Kosi fan. It has many distributaries that flow from north to south. The alignment direction of distributaries, canal and island/sand bar is similar, both sloping from north to south.

Open source satellite imagery of the flood period was used to study the spatial pattern of flood, canal network and drainage pattern. All the thematic layers were superimposed on flood inundation map to understand its significance during the 2008 flood in Kosi mega fan.

An assessment of satellite imagery revealed that most of the drainages are flanked by canals. The alignment of canal network has confined the flood water in a narrow track and forced it to flow into the inter canal regime in existing drainage. The canal network has proved to be a strong barrier for flood water by controlling its lateral spread.

During inundation, drainage changes into canal and canal works as an embankment where inter-canal or inter-drainage and canal space is very narrow. The results of this investigation will help to identify inundation areas for site specific breaching of embankment of Kosi river. 

The article in conclusion states that canal is a man-made route for free flowing of water towards down slope. During the Kosi flood in 2008, drainage turned into canals and they worked as embankments along both side of drainages, where drainages are flanked by canals. Thus, canal network checks lateral spreading of flood water and saves life and property in adjacent areas. But areas located inside the canal-drainage-canal set up faced high water level during flood because the canal checked its lateral flows. A braided river without canal spreads water over wide areas and there is less possibility of rise in water level during a flood. 

The article can be viewed at Geospatial World here