Flood

Flood

By: Staff Reporter
Floods refer to the overflowing of a river or water body beyond its normal confines. They may take the shape of flash floods, surface floods, rapid onset or slow onset floods, depending on the manner of their occurrence.
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A simplistic definition of flood may be the overflowing of the normal confines of a stream or other water body. The events leading to it may be complex, caused by a combination of heavy rainfall and rivers/water body dynamics, which can happen at any time of the year.

The US National Oceanic and Atmospheric Administration (NOAA) defines floods as “An overflow of water onto normally dry land. The inundation of a normally dry area caused by rising water in an existing waterway, such as a river, stream, or drainage ditch.” According to the standard definition of floods, introduced by the Australian government in 2011, floods are “the covering of normally dry land by water that has escaped or been released from the normal confines of: any lake, or any river, creek or other natural watercourse, whether or not altered or modified; or any reservoir, canal, or dam.

Floods generally develop over a period of days, when excess rainwater above the carrying capacity of the channel spreads over the land next to it (‘floodplain’). However, they can also happen very quickly with heavy rain inundating the basin in a short period. These ‘flash floods’ occur with little or no warning and can lead to huge loss of human lives.

Coastal areas are also at risk from sea flooding, when storms and cyclones cause storm surges, bring in walls of seawater onto the land. The worst cases of flooding may occur if there is a combination of storms, ‘spring tides’ and low atmospheric pressure.

Floodwater can seriously disrupt public and personal transport by cutting off roads and railway lines, as well as communication links when telephone lines are damaged. Floods disrupt normal drainage systems in cities, and sewage spills are common, which represents a serious health hazard, along with standing water and wet materials in living spaces. Bacteria, mould and viruses, cause disease, trigger allergic reactions, and continue to damage materials long after a flood.

Although floods can distribute large amounts of water and suspended sediment over vast areas, restocking valuable soil nutrients to agricultural lands; in the present scenario, with poor soil management practices and rapid urbanisation, floods only contribute to loss of livelihoods. It erodes large tracts, ruins crops, destroys agricultural land / buildings and drown livestock and humans.

Types of floods

Floods may be caused due to various reasons. On the basis of their manner of occurrence, floods are classified into various sub types. Flash floods occur within a very short time (2-6 hours) as a result of heavy rainfall, when a levee or dam has breached, or after a sudden release of water by a debris or ice jam, as in glacial lake outburst phenomenon. Sometimes, intense rainfall from slow moving thunderclouds can also cause floods. Flash floods are the most destructive since there is usually no warning, thus no preparedness, taking people by surprise. Rapid onset floods are similar to flash floods, but take a little more time to develop and last for just a day or two. The large amount of water dispersed may become difficult to manage. However, these do not surprise people the way flash floods do. Slow onset floods result when slow rise of water in channels and water bodies lasts for days and weeks and spreads over many kilometres. These generally occur in flood plains. Coastal floods affect coastal areas or areas close to the sea shore when a heavy storms, combined with high tides can cause sea levels to rise above the normal, forcing sea water into the land and hence, causing coastal flooding. Surface floods also known as pluvial floods occur on dry lands when heavy rainfall creates a flood event independent of an overflowing water body.

Floods are primarily caused by rains that exceed the capacity of the drainage system. Heavy rain over a very short period can result in floods since the massive amount of water cannot percolate into the soil. At other times, moderate rain spread over a long period can also result in floods. Also, rivers overflowing their banks when there is more water upstream than usual, can result in floods. Strong winds in coastal areas can carry seawater on to coastal lands and cause flooding. The situation worsens if the winds carry rain. At times, a tsunami can cause sea water to flow inland into coastal habitations. Dam or embankment breach can cause floods when man-made blocks mounted to hold water, give way. At times, excess water intentionally released from the dam to prevent it from bursting—especially during heavy monsoons, can also cause floods. Moreover, ice that is accumulated in winter remains frozen until summer after which it suddenly melts with the rise in temperature. This snow-melt causes massive movement of water into places that are usually dry and can be termed as a snow-melt flood.

Urban flooding

Until recently, any pre-emptive strategy always focussed on riverine floods and flooding of rural tracts. Although urban flooding has occurred in the past, it is only now that it is recognised as a phenomenon that can debilitate vast populations and destroy infrastructure. According to the National Disaster Management Authority, (NDMA) “urban flooding is significantly different from rural flooding as urbanisation leads to developed catchments which increases the flood peaks from 1.8 to 8 times and flood volumes by up to six times”. Encroachments over natural watersheds, and drainage channels in major Indian cities has resulted in several major floods, such as those in Ahmedabad in 2001, Delhi in 2002 and 2003, Chennai in 2004, Mumbai in 2005, Surat in 2006, Kolkata in 2007, Jamshedpur in 2008, Delhi in 2009 and Guwahati and Delhi in 2010, besides the 2014 Srinagar, and 2015 Chennai floods.

The Mumbai floods of 2005 were an eye-opener, and saw NDMA release the urban flood guidelines in 2008. However, failure to tackle the root cause of such disasters has resulted in repeated debacles.

Flood prone regions of India

In India, an average of 40 million hectares or 12 per cent of the total land is flood prone, as per the India Disaster Knowledge Network, of the SAARC Disaster Management Centre. Every year, about 8 million hectares in India is affected by floods—several hectares of crops lost, along with a few hundred lives, with millions rendered homeless. The most flood prone region of the nation is the basin of the Himalayan rivers covering parts of Punjab, Haryana, Himachal Pradesh, Delhi, Rajasthan, Uttar Pradesh, Bihar and West Bengal (Fig. 1). The Kosi and the Damodar are the main rivers causing floods here. Another flood prone region is the north-western river basin covering the states of Jammu and Kashmir, parts of Punjab, Haryana, Western Uttar Pradesh and Himachal Pradesh. Rivers such as Jhelum, Sutlej, Beas, Ravi and Chenab are flood prone. The Central and Peninsular river basins of Narmada, Tapi, Chambal and Mahanadi are flood prone too. Heavy floods often occur in the Godavari, Krishna, Pennar and Cauvery.

Analysis of flood affected states in 2015

Assam: According to the Assam State Disaster Management Authority (ASDMA), over 12000 villages in 17 districts of the State were affected by several waves of floods caused by the Brahmaputra, Jia Bharali, Dhansiri and Kopuli in early September 2015. The floods affected 12,68,770 people and inundated 92,820 hectares of cropland.

One of the most flood-prone regions of India, Assam suffers from regular floods every year owing to its unique topographical and geographical situation. The flood prone area of the State as assessed by the Rastriya Barh Ayog (RBA) is 31.05 lakh hectares against the total area of the State which stands at 78.523 lakh hectares. This is about 9.40 per cent of the total flood prone area of the country.

Fig. 1: India’s flood prone areas
Fig. 1: India’s flood prone areas

In the post-independence period, Assam has faced major floods in 1954, 1962, 1972, 1977, 1984, 1988, 1998, 2002, 2004 and 2012. Almost every year three to four waves of flooding ravage Assam. Average annual loss due to floods in Assam is to the tune of INR 200 crore. As the Water Resources Division of the Government of Assam points out, the two major rivers of Assam—the Brahmaputra and the Barak and the 50 tributaries that feed them, leave the State vulnerable to floods every year. Additionally, the Assam valley gets affected by cloudbursts and heavy rains in the upper reaches of its rivers in Meghalaya and Arunachal Pradesh, as in 2004 and 2014, respectively.

Maharashtra: Heavy rains and ensuing floods in north-eastern areas of Maharashtra left at least five people dead in the city of Nagpur in 2015, according to National Oceanic Atmospheric Administration (NOAA) reports on August 14, 2015. Roof top rescue by IAF helicopters saved 1,500 people, particularly those marooned in Pipli village.

More than two lakh hectares, about 6.5 per cent, of land area in Maharashtra is prone to floods and Paturtaluka in Akola district has the largest flood prone area in the State. Nanded and Nashik are frequently affected by floods in the monsoons as are the basins of Tapi, Wardha and occasionally the Pen-Ganga.

Maharashtra has a long history of floods. The 1996 flood in the State destroyed 2,899 lakh hectares of land, killing 198 people. In September, 2005, the flood claimed approximately 1,200 lives and affected 20 million people. Much of Mumbai’s drainage system collapsed and as the flood waters subsided. There was a continued risk from water-borne diseases, which had caused an estimated 150 deaths in the weeks following the flooding.

Manipur: More than six out of Manipur’s nine districts were flooded following heavy rainfall in the last week of July, 2015. All 12 houses of the village of Joumul in Chandel district were buried and waters of all major rivers of the State rose to an alarming level according to media reports. The European Commission’s Humanitarian aid and Civil Protection department (ECHO) termed the floods as the worst in 200 years. Thousands of hectares of paddy fields and a large number of fish farms were washed away, with the Sugnu-Serou Lamkhai road being rendered unusable.

Most flood prone areas of Manipur are those where dams are built. Imphal, Thoubal, Bishnupur, Chandel and Churachandpur are frequently flooded. Almost two third population of Manipur is concentrated in the Manipur Valley (constituting 8.2 per cent of the State’s land), with rivers from the hills flowing into the valley, which makes it even more disaster prone—leading to flash floods almost annually.

In the last two decades, beginning from September 1997, breaches of embankments took place at four different places in Manipur, which damaged 4965 houses. In September 1999, incessant rainfall between August and September affected southern parts of the valley, damaging 7,300 houses and 15,300 hectares of paddy fields. In August 2002, severe flood occurred which caused embankment failures at 59 places. About 10,000 houses and 20,000 hectares of paddy fields were affected.

Gujarat: A wave of thunderstorms hit Gujarat on June 21, 2015 and killed dozens of people and several Asiatic lions in Gir. Water logging affected crops in more than two lakhs hectares. Further rains in July killed at least 72 people and over 81,000 cattle. The worst affected districts were Banaskantha, Patan, Rajkot and Kutch causing three deaths in each district.

About 1.39 million hectares of Gujarat’s land (about 7 per cent) is prone to flooding. Owing to its geo-climatic, geological and physical features, Gujarat is vulnerable to all-major natural hazards. All major rivers in the State pass through a wide stretch of the very flat terrain before reaching the sea. These flat lowlands, where the cities of Ahmedabad, Surat and Bharuch are also located, are prone to flooding.

In 2005, Gujarat flood was caused by heavy monsoon rains in June affecting many parts of the State. The death toll stood at 123 while more than 250,000 people were evacuated.

Rajasthan: Flood affected the State in late July 2015, resulting in 28 deaths. The districts of Jalore, Jhalawar, Baran, Sirohi, Barmer and Dungarpur were the worst affected. World Meteorological Organisation (WMO) reports say that Bikaner received 363.5 mm of rain in 24 hours on August 2, 2015.

Though most parts of Rajasthan receive scanty rainfall, the State has a history of floods and inundations, mostly along the basins of rivers like Luni and Chambal. Major portions of Bharatpur districts falling under the basin of River Banganga, and the basins of River Ghaggar in Sriganganagar are prone to floods. Barmer district is the most affected area in Rajasthan. A total of 3.26 million hectares (about 10 per cent) of the State is flood prone.

In 1981 heavy rainfall caused flooding in Jaipur, Tonk, Nagaur and Sawai Madhopur and caused extensive damage to property and life. Barmer received 720 mm of rainfall in 1990 and in 1994, 600 mm rain fell over the district. In 2006, Barmer was deluged with 750 mm of rainfall in the last week of August which is five times the district’s average annual rainfall. Over 800,000 of Barmer’s two million people were reportedly affected due to the floods.

Odisha: With three rivers—Subarnarekha, Budhabalang and Baitarni overflowing during July 28-30, 2015, flooding affected 240,000 people across 282 villages in five districts of Keonjhar, Jajpur, Mayurbhanj, Balasore and Bhadrak. In August, flooding was reported in seven districts affecting over 480,000 people across 644 villages.

The 482 km long of coastline of Odisha exposes the State to flood, cyclones and storm surges. About 9 per cent, or 1.40 million hectares of the State’s land is prone to flooding. Rivers such as the Mahanadi, Subarnarekha, Brahmani, Baitarni, Rushikulya, Vansadhara and their many tributaries are prone to floods.

A devastating flood in 2006 affected 18912 villages and 67.39 lakh people and destroyed 4.90 lakh hectare of crop. 105 persons lost their lives. Floods in 2007, 2008 and 2009 affected 5, 19 and 15 districts, respectively out of 30 districts resulting in heavy losses to life and property.

West Bengal: Cyclone Komen hit West Bengal on July 30, 2015 and brought heavy rainfall and flooding across many southern districts of the State. Nearly 10,000 villages in 12 districts were affected. Local authorities said that 200,000 people had to be evacuated and housed in 1,537 relief camps. As many as 38,000 homes were fully damaged and over 200,000 homes were left partially damaged.

Approximately 55.8 per cent of West Bengal is susceptible to floods. The State has 37,660 sq km of flood prone area spread over 111 blocks where the total geographical area of the state is 88,752 sq km. Furthermore, major flood producing rivers beyond the state’s jurisdictional limits, viz, Teesta, Torsa, Joldhaka, Kaljani etc. from Sikkim and Bhutan are mainly responsible for disastrous flash floods in North Bengal.

In 1991, flash floods damaged 35,000 houses. In 2000, flash floods triggered incessant torrential storms with 1262 fatalities besides dislocating thousands of people.

Heavy rains in 2005 caused floods in many parts of Bengal. About 3,000 coastal villages were inundated and 60,000 huts washed away. Similarly, in 2006, heavy rainfall caused 50 deaths and 30,000 mud houses were destroyed. In 2007, heavy rain from tropical depression in the Bay of Bengal caused flooding leading to 51 deaths and affecting 3.2 million people.

Tamil Nadu: Flash floods in Virudhunagar district killed six persons in May, 2015. Another flash flood hit the Sathuragiri Hills, near Watrap, about 80 km south west of Madurai city, sweeping away eight persons who had gathered to worship at the Sundara Mahalingam Temple. But a month long spell of rain—the worst in a hundred years, starting November 4, 2015, left Chennai totally paralysed, with several crores worth of property destroyed.

From the flood hazard map of India (Fig. 1), it is seen that no area in Tamil Nadu’s 32 districts falls in the risk zone. In Chennai Metropolitan Area, there are only few areas along the rivers and canals and low-lying areas, that are susceptible to flooding/inundation during heavy storms.

A total of 347 people died due to floods in Tamil Nadu in 2015 and economic losses were calculated at INR 200 billion. For the month of November, Chennai reported 1,024 mm (40.31 inches) of rain, more than 300 per cent of the normal rainfall expected for the entire month. December continued this wet pattern as more than 300 mm (12 inches) of rain fell in Chennai on the first day of the month.

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