Among all the disasters that occur in India, floods are the most commonly occurring natural disasters due to the irregularities of the Indian monsoon. Flood is most prevalent and costliest natural disaster in the world which devastates both life and economy at a large extent. Any flow whichis relatively high and which overflows the natural or artificial banks in any reach of the river is calledflood.
About 75% of the annual rainfall in India is concentrated in 3 - 4 months of the monsoon season.Flood, an excess of water, can be caused by heavy rainfall followed by inadequate capacity of rivers tohold the water within their banks. According to National Flood Commission, about 40 million hectaresof land area is prone to flood in the country.
On an average, the area affected by floods annually isabout 8 million hectares, out of which the cropped area affected is about 3.7 million hectares. India witnesses flood due to excessive rain which then results in overflow of rivers,lakes and dams, which adds to cause large amounts of damage to people's lives and property.
Fig. Kerala floods during the year 2018
Fig. Uttarakhand Floods during the year 2013
Causes of Floods
Inadequate capacity of the rivers to contain within their banks the high flows broughtdown from the upper catchment areas following heavy rainfall, leads to flooding. The tendency tooccupy the flood plains has been a serious concern over the years. Because of the varying rainfalldistribution, many times, areas which are not traditionally prone to floods also experience severeinundation.
Areas with poor drainage facilities get flooded by accumulation of water from heavyrainfall. Excess irrigation water applied to command areas and increase in ground water levels due toseepage from canals and irrigated fields also are factors that accumulate the problem of water logging.The problem is exacerbated by factors such as silting of the riverbeds, reduction in the carryingcapacity of river channels, erosion of beds and banks leading to changes in river courses, obstructionsto flow due to landslides, synchronization of floods in the main and tributary rivers and retardation dueto tidal effects.
Floods and landslides result not just from rainfall but also from other factors such asland cover and topography. Indiscriminate development and encroachment of flood plain areas,improper planning and construction of roads, railway lines, etc., are also responsible for increase inflood damages.
Regions in the country prone to floods
India can be broadly divided into the following four regions for a study of flood hazard.In addition the Andaman and Nicobar Islands and Lakshadweep have peculiar characteristics, whichresult in drainage congestion, flooding and erosion in coastal areas.
1. The Brahmaputra River Region This region consists of the rivers Brahmaputra and Barak and their tributaries, andcovers the states of Assam, Arunachal Pradesh, Meghalaya, Mizoram, Manipur, TripuraNagaland, Sikkim and the northern parts of West Bengal. The catchments of these rivers receivevery heavy rainfall which occurs mostly during the months of May – June to September. As aresult floods in this region are severe and quite frequent.
Further the hills, where these riversoriginate, are fragile and susceptible to erosion and thereby cause exceptionally high siltdischarge in the rivers. In addition, the region is subjected to severe and frequent earthquakes,which causes numerous landslides in the hills and upset the regime of the rivers.The predominant problems in this region are cloud bursts followed by flash floods, soilerosion in the watershed and bank erosion along the rivers, flooding caused by the spilling ofrivers over their banks, drainage congestion and the tendency of some of the rivers to changetheir courses. The plain areas of the region suffer from the inundation caused by spilling of theBrahmaputra.
2. The Ganga River Region The river Ganga has many tributaries, the important ones being Yamuna, Sone, Ghaghra,Raphti, Gandak, Burhi Gandak, Bagmati, Kamla Balan, Adhwara group of rivers, Kosi and theMahananda. It covers states of Uttarakhand, Uttar Pradesh, Jharkand, Bihar, south and centralparts of West Bengal, Punjab, parts of Haryana, Himachal Pradesh, Rajasthan, Madhya Pradeshand Delhi.
The rainfall increases from west to east and from south to north. The flood problem ismostly confined to the areas on the northern bank of the river Ganga. Most of the damage iscaused by the northern tributaries of the Ganga. In the north – western parts of the region andsouthern parts of West Bengal, there is a problem of drainage congestion. The flooding anderosion problem is serious in in the states of Uttar Pradesh, Bihar and West Bengal. The problemof flooding and drainage congestion is getting accentuated due to large scale encroachment offlood plains flood plains of the rivers for habitation and various development activities.
iii. The North – West River Region The main rivers in this region are the Indus, Sutlej, Beas, Ravi, Chenab and Jhulem.These rivers are the tributaries of the Indus. They carry quite substantial discharges during themonsoon and also large volumes of sediment. They change their courses frequently and leavebehind vast tracts of sandy waste.
This region covers the states of Jammu and Kashmir, Punjaband parts of Himachal Pradesh, Haryana and Rajasthan. Compared to the Ganga andBrahmaputra regions, the flood problem is relatively less in this region. The major problem isthat of inadequate surface drainage which causes inundation and water logging over vast areas.Indiscriminate use of water for irrigation and development of low lying areas and depressionshas created problem of drainage congestion and water logging.
4. The Central India and Deccan Region Important rivers in this region are Narmada, Tapi, Mahanadi, Godavari, Krishna andCauvery. These rivers are mostly well defined and stable courses. They have adequatecapacities within the natural banks to carry the flood discharge except in delta area. The lowerreaches of the important rivers on the east coast have been embanked, thus largely eliminatingthe flood problem.
However the embankments need to be raised and strengthened to lateststandards to continue to provide protection against against floods and erosion. This regioncovers the states of Andhra Pradesh, Telangana , Karnataka, Tamil Nadu, Kerala , Orissa,Maharashtra, Gujarat and parts of Madhya Pradesh. The region does not have serious floodproblem except that some of the rivers in Orissa state namely Mahanadi, Brahmini, Baitarniand Subarnarekha are prone to floods every year.
The delta and coastal areas of the states onthe east coast periodically face flood and drainage problems in the wake of monsoondepression and cyclonic storms. The problem is accentuated when the floods synchronize withhigh tide. The rivers Tapi and Narmada, are occasionally in high floods affecting areas in thelower reaches in Gujarat.
Damages caused by Floods
The floods not only result in loss of precious human lives, cattle and damage to publicand private property but create a sense of insecurity and fear in the minds of people living in the floodplains. Floods cause extremely large numbers of fatalities in every country, but due to India's extremelyhigh population density and often under enforced development standards, a large amount of damagesand many deaths could be happen. The after effects of floods such as the agony of survivors, spread ofepidemics, non availability of drinking water, essential commodities and medicines, loss of dwellingsetc. make floods the most feared among the natural disasters.
Benefits
Floods can also bring many benefits, such as recharging ground water, making soil morefertile and increasing nutrients in some soils. Flood waters provide much needed water resources in aridand semi arid regions where precipitation can be very unevenly distributed throughout the year andkills pests in the farming land. Fresh water floods particularly play an important role in maintainingecosystems in river corridors and are a key factor in maintaining floodplain biodiversity. Flooding canspread nutrients to lakes and rivers, which can lead to increased biomass and improved fisheries for afew years.
Floods in India over a Decade
India has faced many floods during the past years. Here is the list of some worst floodswhich were occurred in last decade.
- i. Kerala floods in 2018
- Gujarat floods in 2017
- Chennai floods in 2015
- Jammu and Kashmir floods in 2014
- Uttarakhand floods in 2013
- Bihar floods 2008
Kerala Floods 2018
Introduction
Kerala state has an average annual precipitation of about 3000 mm. The rainfall in thestate is controlled by the South-West and North-East monsoons. About 90% of the rainfall occursduring six monsoon months. The high intensity storms prevailing during the monsoon months result inheavy discharges in all rivers and causes severe floods.
Kerala experienced an abnormally high rainfall from 1 st June 2018 to 19 th August2018.
This resulted in severe flooding in 13 out of 14 districts in Kerala state. As per IndianMeteorological Department (IMD) data, Kerala received 2346.6 mm of rainfall from 1 st June 2018 to19 th August 2018 and expected rainfall was 1649.5 mm. This rainfall was about 42 % above the normal.
The rainfall over Kerala during June , July and 1 st to 19 th August was 15% ,18% and 164% respectively,above normal. Water was released from several dams due to heavy rainfall in their catchments. Thewater levels in several reservoirs were almost near their Full Reservoir Level (FSL) due to continuousrainfall from 1 st June. As per annual records of IMD, it has been found that the rainfall depths recordedduring 15 th – 17 th August 2018 were comparable to the severe storm that occurred in the year 1924.
Effects
The storm of 15 th - 17 th August 2018 was spread over the entire Kerala with eye centredat Peermade , a place between Periyar and Pamba sub basins . The storm was so severe that the gates of35 dams were opened to release the flood runoff. All 5 overflow gates of the Idukki dam were openedfor the first time in 26 years. Heavy rains in Wayanad and Idukki caused severe landslides and left thehilly districts isolated.
On August 15 th , Kochi International Airport , India's fourth busiest in terms ofinternational traffic , and the busiest in state , suspended all operations until August 26 , followingflooding of its runway. As per the reports in media, the flooding has affected hundreds of villages,destroyed several roads and thousands of homes have been damaged. The Kerala State DisasterManagement Authority placed the state on a red alert as a result of the intense flooding. A number ofwater treatment plants were forced to cease pumping water, resulting in poor access to clean andpotable water, especially in northern districts of the state.