The year 2010 witnessed a series of unprecedented floods not just in India but globally. From floods in Himachal Pradesh (July 2010) and Leh (August 2010), floods occurred in several parts of Karnatka, Tamil Nadu, Andhra Pradesh and south Orissa during November-December 2010. Globally, severe floods in east China (May 2010), Rio Lorogo, Brazil (June 2010), Pakistan (August 2010) and Queensland, Australia (December 2010) hit headlines. The year 2011 kicked off with yet another flood in Rio de Janeiro, Brazil (January), as muddy waters continue to ravage Queensland – pointing out very clearly that even developed countries are not quite free from flood risk.
Notwithstanding the justification, we in India with a legacy of floods, need to rethink strategies of flood management. Most floods are caused by excessive rainfall spanning a very short time, cloudbursts or cyclones in coastal regions. Barring sudden cloudbursts resulting in floods, as in Leh and Pakistan, flooding due to excessive rainfall can be predicted – if proper monitoring of water gauging stations and communication systems is in place. However, it is pertinent to understand that flood control strategies in most river basins in India are primarily embankment based. Such man made structures have influenced the natural flow regime of rivers and modified the flood intensity, frequency and pattern. The construction of barrages and other interventions has further aggravated the problem. Many of the Himalayan rivers are highly sediment charged and the rising riverbed and reduction in carrying capacity due to extensive sediment deposition in upstream reaches of a barrage has been a major problem with them. The engineering assumption that jacketing the river would increase the velocity leading to scouring has instead resulted in silting of river beds and increased water logging and soil salinity in the adjoining floodplains. The construction of protective levees and dykes besides the large sediment flux from the Himalayan catchments has further complicated the flooding problem in these rivers. In many cases, large areas have been inundated due to breaches in embankments coupled with rapid shifting of rivers. Unplanned roads and bunds have also resulted in severe drainage congestion and channel disconnectivity, increasing the inundation period significantly.
Despite an astronomical increase in the expenditure on flood control in India, the recurrence of floods as well as damage due to them has only exacerbated. Floods pose a constant threat to engineering structures and public utilities with their repair/restoration consuming significant chunks of flood relief and public money. There are also issues of poor planning and non cognizance of river processes in designing these structures.
Floods are basically natural processes. They become hazardous when, first, people start living or working in areas where they occur and second, when human intervention or land use affects natural river processes. Risk analysis is an important component in understanding the impact from hazardous processes. The role of the scientists working on floods is to identify the factors and alert planners and decision makers to these causes. Strategies can then be formulated to mitigate the flood risks.
Typical steps of flood risk analysis may include identification of the flood prone reaches of a river; determination of the probability of a flood of a given magnitude; observation of precursor events; flood forecasting and development of a reliable warning system. India has a sound scientific knowledge base to follow the first four steps. The lacuna lies in following a standard protocol for issuing flood prediction or warning.
The first step towards a solution is recognising that something is wrong with our flood management strategy. The time has now come to move from flood control to flood management, which emphasises the process based understanding of floods and favours a non structural approach to reduce flood risk. Flood hazard zoning and floodplain regulations are two options which aim to minimise flood damages and the cost of flood protection. It is a compromise through regulatory mechanisms – between indiscriminate use and complete abandonment of floodplains. This approach does not discount the role of physical barriers, reservoirs and channelisation works in protecting lives and property. However, we have to recognise that the floodplains belong to the rivers. Any encroachment that reduces the channel cross section and carrying capacity in a reach of the river would increase the flood risk in the total span of the river. A more acceptable approach would be a combination of minimal physical barriers and floodplain regulations so as to maintain the ‘naturalness’ of the river system as far as possible.
Today, lying at the core of river management is an emphasis on minimising damage to floodplain ecosystems and improving the river health. A recent initiative by the Ministry of Environment and Forests (MoEF), Government of India is the Ganga River Basin Management Plan (GRBMP). It is being prepared by a consortium of Indian Institutes of Technology (IITs) and focuses on the ecology based approach – identification of flood risk zones is an important component of this initiative.
Accurate flood hazard mapping is needed for flood hazard zoning. It can be based on fixed distance from river or bank, past floods or floods of a particular frequency e.g. 100 year flood and area inundated by largest flood recorded. High resolution, repetitive remote sensing images provide quick means to map flood hazard zones. These can then be combined with flood frequency analysis and inundation modelling to assign the flood magnitude associated with each zone or even delineate areas of a particular flood magnitude. Based on this, a relationship between regulatory flood depth and readily measurable stream and/or drainage basin characteristics can be developed.
In many parts of India large populations live close to the river. Where regulatory floodway and floodway fringe areas are occupied, floodplain regulations may require relocation. A National Flood Insurance Programme for people living in flood prone areas should be taken up. Such a programme could provide insurance cover for flood damage and would discourage people from living near flooding rivers.
A formal audit of the impact of engineering structures in terms of benefits accrued and degradation of natural equilibrium and ecosystem is yet to be taken up for any river system in India. Nevertheless, there is enough information to suggest that present systems have been unsuccessful in reducing flood risk and thus alternative methods must be explored. Flood management now and in the future must focus on a strategy of ‘living with the floods’ using an ecology based approach.