"taming the Kosi - a 'lag time' approach."

………………to be very precise on this context, the fundamental to handle the furious Kosi is to increase the ‘lag time’ in its entire catchment area, as rivers with a short lag time are more likely to flood.

Lag time is the time difference between maximum rainfall and peak flow.

The fluvio-geomorphological and meteorological changes and increasing human interventions in the form of:

• global climatic change and the resultant changes in the elements of weather / climate and their pattern
• alterations in the river basin morphology and its characteristic features
• removing woodland and forests, rapid urbanization, straightening river channels, artificially raising the height of river banks, and, so on and so forth, interferes in the factors and processes of the hydrological cycle.

Under normal circumstances the ‘precipitation input’ and ‘stream discharge’ equation goes like this:

The highlighted text in red background and font shows the area where humans negatively interfere into the system, thereby compelling the river to over flow its banks.

The ‘lag time’ can be increased by allowing the basin area to perform more of interception, percolation and storage than any type of flows and the ultimate discharge into the river channel.

Some of these following human activities can increase the ‘lag time’:

• planting more woodlands and forest.
• building dams to store and control discharge.
• diverting the flood water to other basin areas, say through river linking project.
  
• river channel dredging.
• extracting water for industrial and domestic uses.
• maintaining the wetlands as storage tanks.
• more number of percolation pits with the rapid urbanization process, etc.

In short, at macro (like the entire catchment / basin area) as well as micro level (water percolation pits) there need to be long term strategies and follow ups on flood management in the region.