Social – and not – civil engineering will help floodplain societies to benefit from floods, while reducing their vulnerability to them.
Recent years have seen growing media attention directed to the floods in the lowlands of Ganga-Brahmaputra basin – the home of one-tenth of humanity. The misery and insecurity of this large, and predominantly poor population, in the aftermath of heavy monsoons has rightly drawn sympathetic attention of the world. Looking beyond immediate relief measures, the challenge is to attain an understanding of the hydro-ecology of these floods, to study their devastation, as well as their benefits. In this task, however, environmentalists have often taken a simplistic yet alluring stand, blaming hill deforestation while ignoring other factors at work. There is need for a holistic ecological analysis of the hydrological processes of floods and the socio-biological adaptations that have evolved. There must be a more scientific and objective basis for the environmental analysis of the mega-disasters in this large basin.
A flood is defined as the subservience of land not normally inundated. Surely, one cannot describe as flood the regular inundation of lowlands from high tides. One should also not describe as flood, the inundation of floodplains during the monsoon downpour, when the rivers of the basin are normally expected to carry a volume of water several hundred times greater than, in the dry months. The behaviour of the monsoon has never been uniform, with spatial and temporal variations in the rainfall an integral part of its normal behaviour. Hydro-ecological analysis of floods must distinguish between this normal pattern of behaviour of nature from the collective, yet subjective, expectations of human beings who are increasingly colonising the floodplain and bearing the resultant risks
Matter dislodged from steep and unstable Himalayan slopes, forested or otherwise, together with water, is what moves downwards along the drainage corridors of the Himalaya. Floods in the mountains are linked to this displacement of matter. In the high Himal, the failure in the lip of glacial moraines allow the icy lake formed behind it to burst out in a violent flood. These are known as glacial lake outburst floods (GLOP). In the mid-hills, landslides regularly block rivers and then collapse to release devastating flash floods into the lower reaches. This phenomenon has periodically occurred in the Himalaya and is known among the Nepali hill people as bishyari. The water level goes down, often accompanied with fish kills, and if it is daytime and the villagers are alerted, they head quickly for high ground. Unlike in the plains, there is no lead time to prepare for catastrophe. Bishyaris and GLOF release tons of debris and sediment, which ultimately find their way to the plains and raise the level of the river-beds. They constitute one more reason for the flood prone nature of the Ganga-Brahmaputra basin.