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Floods: Occurrence and Distribution

 

Floods are among Earth's most common – and most destructive – natural hazards. A flood occurs when water inundates land that’s normally dry. It is an overflow of water from lakes, rivers or oceans that submerges nearby land. Water levels in lakes and rivers fluctuate dramatically due to seasonal shifts in inflow water volume and the rate of evaporation. Any region where rain falls is vulnerable to floods – although rain is not the only impetus. There are few places on Earth where people need not be concerned about flooding. Most floods take hours or even days to develop, giving residents ample time to prepare or evacuate. Others generate quickly and with little warning. Such floods, which are called flash floods, can be extremely dangerous, instantly turning a babbling brook into a thundering wall of water and sweeping everything in its path downstream.

The most common cause of flooding is the incapability of watercourses to drain away water during an unusually heavy rainfall. Floods however, are not always caused by heavy rainfall. They can result from other natural or man-made phenomena. For instance, inundation in coastal areas can be caused by a storm surge associated with a tropical cyclone, a tsunami or a high tide, particularly when the rivers are flowing at a higher than normal river level. Inundation of normally dry areas can be caused by dam failure, triggered for example by an earthquake.

Other factors which may contribute to flooding include:

·      Rivers or streams overflowing their banks.

·      Encroachment of encroaches land areas by sea-water during high tides. 

·      Excessive rain (coupled with poor drainage system)

·      A ruptured dam or levee

·      Ground cover and topography

·      Rapid ice melting in the mountains

·     An unfortunately placed beaver dam

Flooding can also occur due to an earthquake triggered landslide blocking the channel, or when such a temporary dam is swept away due to increasing water level. Coastal flooding occurs when a large storm or tsunami causes the sea to surge inland.

Disaster experts classify floods according to the likelihood of their temporal occurrence. A hundred-year flood, for example, is an extremely large, destructive event that would theoretically be expected to happen only once every century. But this is a theoretical number. In reality, this classification means there is a one-percent chance that such a flood could happen in any given year. Over recent decades, possibly due to global climate change, hundred-year floods have been occurring worldwide with frightening regularity. Highly advanced computer modeling of the terrain allows authorities engaged in disaster management to predict with amazing accuracy where floods will occur and how severe they are likely to be. Many governments mandate that residents of flood-prone areas purchase flood insurance and build flood-resistant structures. Massive efforts to mitigate and redirect inevitable floods have resulted in some of the most ambitious engineering efforts ever seen. These include building extensive levee system and massive dikes and dams.

IMPACT OF FLOODING:

Moving water has remarkable destructive power. When a river overflows its banks or the sea drives inland, structures poorly equipped to withstand the water’s strength are no match. Bridges, houses, trees, and cars can be swept away. The erosive force of moving water can drag and carry away the material constituting the foundation of a building, causing it to crack and tumble. In the US, even with advanced disaster management systems in place, floods cause an annual damage to property and infrastructure worth about $6 billion, and kill on average 140 people. Coastal flooding alone caused some $3 trillion damage worldwide in 2007. The world’s worst floods have occurred in Yellow River valley of China, where millions of people have perished in floods during the last century. India’s vulnerability to severe flooding during monsoons is demonstrated year after year, with the season invariably ending in significant loss of life and property. One research study for the period 1978-2006 based on official data reports that there were 2,443 flood events that led to the death of nearly 45,000 people and caused economic losses worth US $16 billion.

The same story is playing out this year too. Residents of five States are currently struggling to cope with the effects of intense rainfall. Floods in Bihar have claimed over 350 lives and rendered more than 12 million people homeless. Many of those lucky to have been rescued owe it to the National Disaster Response Force. Most flood destruction is attributable to humans' desire to live near picturesque coastlines and in river valleys. Aggravating the problem is a tendency for developers to backfill and build on wetlands that would otherwise act as natural flood buffers.

THE AFTEREFFECTS OF FLOODS: When floodwaters recede, affected areas are often blanketed in silt and mud. The water and landscape can be contaminated with hazardous materials, such as sharp debris, pesticides, fuel, and untreated sewage. Potentially dangerous mold blooms can quickly grow over water-soaked materials. Residents of flooded areas can be left without power and clean drinking water. Contamination of soil and groundwater leads to outbreaks of deadly waterborne diseases like typhoid, hepatitis A, and cholera.

Flooding, particularly in river floodplains, brings with it some good things too. The famous fertile floodplains of the Mississippi Valley in the American Midwest, the Nile River valley in Egypt, and the Tigris-Euphrates in the Middle East have supported agriculture for millennia because annual flooding brings with it millions of tons of nutrient-rich silt.

DISTRIBUTION OF FLOODS:

On a global scale, river floodplains and coastal areas are the most susceptible to flooding. However, it is possible for flooding to occur in other areas too, particularly those that witness unusually long periods of heavy rainfall. Bangladesh is the most flood prone area in the world. This country is a low lying riverine country located between the foothills of the Himalayas and the Indian Ocean. It is vulnerable due to a long monsoon season which causes heavy rainfall.

Global Flood Hazard Distribution

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The dataset used for this map is taken from the Dartmouth Flood Observatory’s global listing of extreme flood events compiled from various sources for the 19-year period from 1985 to 2003. Some degree of flooding is evident in more than one-third of the world’s land areas.

The eastern coasts of all continents are more susceptible to flooding than the western coasts. This id due to the fact that all tropical cyclones move in the oceans from east to west, striking the eastern coasts. The foothill areas of all major mountain belts are also the most vulnerable to floods.

Floods in India: The peculiar nature of India’s climate, dominated by monsoons, causes situations where drought and floods may affect different pockets at the same time of year. The main reasons for floods in India are:

1.    Heavy concentrated rainfall

2.    Cyclone and strong winds, and

3.    Inadequate drainage

Indiscriminate deforestation in catchment areas and upper reaches leads to soil erosion. This in turn causes silting of river courses downstream. A thinned soil cover also results in reduction of infiltration and consequent increase in runoff of large volumes of water. Overgrazing, especially in the foothills, leaves the soil without cover and therefore vulnerable to erosion. Unscientific farming practices like shifting cultivation result in loss of vegetation cover and consequent soil erosion. 

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Map of India showing the major rivers and the flood-prone areas. 1 = flood-prone areas; 2 = major rainstorm zones after Dhar and Nandargi (1995); 3 = percentage of annual rainfall contributed by southwest monsoon; 4 = major hydro-geomorphic zones mentioned in the text (I–IV).

Among the severely affected areas of the country are the Brahmaputra valleys, north Bihar (Kosi River and north Gangetic plain) and lower West Bengal. Apart from these, floods affect large areas in the following belts:

  1. The lower courses of rivers in the north Indian plains which get silted and change their courses. Such situations are prevalent in the states of Punjab, Haryana, Himachal Pradesh, Delhi, Rajasthan, Uttar Pradesh, Bihar and West Bengal 
  2. Inadequate drainage in parts of Haryana and Punjab is the main cause of inundation. The tributaries of the Indus – the Jhelum, Satluj, Beas, Ravi and Chenab – cause floods in Jammu and Kashmir, Punjab, Haryana, western Uttar Pradesh and Himachal Pradesh.
  3. Certain areas in central India and the peninsula get flooded by the Narmada, Tapti, Chambal, Godavari, Krishna, Cauveri and Pennar.
  4. Large tracts along the east coast get flooded due to cyclonic storms.

The total area affected by floods in India is between 7.5 million hectares and 10 million heactares. Eastern Uttar Pradesh and northern Bihar are the worst affected regions of the country.

 

REFERENCES:

Global Flood Hazard Frequency and Distribution, v1 (1985 – 2003).  http://sedac.ciesin.columbia.edu/data/set/ndh-flood-hazard-frequency-distribution

Flood-Prone Areas of India: Reasons, Distribution, Consequences and Measures. P. Tiwari. http://www.geographynotes.com/articles/flood-prone-areas-of-india-reasons-distribution-consequences-and-measures/950

O.N. Dhar and S. Nandargi (19945 Zones of severe rainstorm activity over India – some refinements. International Journal of Climatology, 15(7), 811-819. DOI: 10.1002/joc.3370150708.

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