The melting snow is causing floods in the plains of north Bihar state of India.. But higher temperature is also causing more rain and snowfall in the glacial resources. The flood in coming years will be severe because the glaciers are melting and at the same time the rainfall has increased many folds.
Flood hazard has long been recognized as one of the most recurring , wide spread and disastrous natural hazards in the densely populated regions of South Asia. In many parts of the Indian subcontinent, flooding reaches catastrophic proportions during the summer monsoon season. For centuries, monsoon floods in the Ganga and the Brahmaputra Basins have brought countless disasters to the inhabitants who have historically occupied the banks of these rivers. Some scientists have attributed the demise of the Harappan (Indus) civilization to a series of large floods on the Indus River. Catastrophes of out standing proportions have occurred in the past and there appears to be no end to the multicentury-old scourge of floods and associated problems.
The interface between humans and hydrologic features across earth’s surface has helped shape human culture. From the earliest agricultural, complex societies established along some of the great rivers of the world to the bustling seaports of today, humans have gained from the myriad advantages of living in proximity to water. Fertile soil, ease of transportation, and availability of resources (both materials and energy) have allowed for the development of complex material and intellectual cultures. The relationship between water and humans also brings a great deal of risk. Flooding is one of these risks. The impact of floods on humans has been evident from ‘Genesis’ to tonight’s evening news.
Definition of floods:
Streams are linear water features that flow under the impetus of gravity. The amount of water contained in a stream is usually regulated by contributions of groundwater and surface runoff to the stream channel (Knighton,1998). Much of the time water in a stream flows within the confines of its channel. When inputs of water increase sufficiently, stream discharge leaves the stream channel and covers all or part of the adjacent flood plain (Jennings and Gruntfest, 2003). Since the flood plain surface is usually a virtually flat surface and near the elevation of the stream channel, water can easily spread over the flood plain once water exceeds the elevation of the stream’s banks.
Flooding is created by the delivery of larger than normal amounts of runoff into stream channel. Periods of above-average precipitation lead to floods.
In India floods are the most common feature since the dawn of civilization. At Mohen jo Daro flood control structures existed as early as 2700 to 3000 BC. These structures as well as storm water drainage works there show that heavy rains and consequent floods have been occurring in this country even in those prehistoric times. Floods have been occurring almost regularly each and every year in different parts of the country. In India, generally, floods occur during the southwest monsoon season. Heavy rainfall has been the main cause of floods in India for any river basin including plains of north Bihar which are some of the most susceptible areas in India. A recent review by Kale (1997) indicates that the plains of the north Bihar have recorded the highest number of floods during the last 30 years. The total area affected by floods has also increased during these years.
The Himalayan rivers are fed by the melting snows and glaciers of the great Himalayan range during spring and summer, and also from rains during monsoon. They carry significant flows during the dry weather due to snowmelt and minimum flows during winter.
Frequent overbank spilling of north Bihar Rivers is essentially an interplay of meteorological conditions in the region and hydrological and morphological characteristics of the rivers. The plains of north Bihar are characterized by monsoon rainfall and average annual rainfall ranges between 120 and 200 cm. The foothills above the plains experience even higher rainfall (greater than 200 cm.). Moreover, the distribution of rainfall, both in space and time, is extremely uneven which makes the individual floods unpredictable. Further, the mountain-fed rivers of the plains have mixed contributions from snowmelt runoff and monsoonal rainfall. Even during lean periods (summer), runoff contribution due to snowmelt is quite significant (Sinha and Jain, 1998). The monsoon follows the peak summer months and the cumulative effect of monsoon rainfall and snow melt runoff results in sudden increase in the river discharge with respect to lean discharge of most of the north Bihar rivers (almost 40-50 times). The shallow alluvial channels between the narrow banks and embankments cannot effectively carry this sudden jump in discharge consequently resulting in breaches and spilling of banks.
Recent trends are showing that Himalayan glaciers are melting due to impact of global warming. These melting may increase the flow intensity of river water followed by heavy precipitation, flowing through north Bihar plains. Major threat is from Ganga, Kosi and Gandak which are “mountain fed” rivers and are characterized by large catchment areas. Kosi and Gandak should be given more importance as they directly flow into north Bihar plains from the mountain.
The Himalayas have the largest concentrations of glaciers outside the polar region. They feed numerous mountain lakes in Nepal and Bhutan as well as seven Asian rivers: the Ganges, Indus, Brahmaputra, Mekong, Thanlwin, Yangtze, and Yellow Rivers. Tributaries of the Kosi River encircle Mt Everest from all sides and are fed by the world's highest glaciers. The Gandak river basin is reported to contain 1025 glaciers and 338 lakes. These contribute substantially to the lean season flows of the river. The short-term result of glacial melting in the Himalayas has been flooding and landslides, which claim approximately 400 lives each year in Nepal. However, scientists fear that in future decades the water level in these rivers could decline sharply, leading to severe water shortages and threatening an agricultural region that feeds over one billion people.
As the climate warms, Himalayan glaciers are melting more rapidly with each passing year, and that means first floods and then droughts for people of north Bihar in India.
The WWF issued a new report documenting the rate of retreat of Himalayan glaciers. It shows that the world's highest glaciers are receding at an average rate of 10 to 15 meters (33 to 49 feet) per year, a rate that is accelerating as global warming increases.
In India, the Gangotri glacier, which supports one of India’s largest river basins, is receding at an average rate of 23 meters per year.
The melting snow is causing floods in the plains. But higher temperature is also causing more rain and snowfall in the glacial resources. The flood in coming years will be severe because the glaciers are melting and at the same time the rainfall has increased many folds.
A recent study by the Indian Space Research Organization, using satellite imaging to gauge the changes to 466 glaciers, has found more than a 20 percent reduction in size between 1962 and 2001, with bigger glaciers breaking into smaller pieces, each one retreating faster than its parent. A separate study found the Parbati glacier, one of the largest in the area, to be retreating by 52 meters a year during the 1990s. Another glacier that Dobhal has tracked, known as Dokriani, lost 20 percent of its size in three decades. Between 1991 and 1995, its snout inched back almost 17 meters each year.
Even the Himalayas have grown measurably warmer. A recent study found that mean air temperature in the northwestern Himalayan range had risen by 2.2 degrees Celsius (4 degrees Fahrenheit) in the last two decades, a rate considerably higher than the rate of increase over the last 100 years.
The loss of these glaciers would have a tremendous impact on the ecosystem of the region. With the retreat of glaciers in the Himalayas, a number of glacial lakes have been created. A growing concern is the potential for Glacial Lake Outburst Floods—researchers estimate 20 glacial lakes in Nepal and 24 in Bhutan pose hazards to human populations.
Indian glaciers are among the least studied in the world, lacking the decades of data that scientists need to deduce trends. Nevertheless, the nascent research offers a snapshot of the consequences of global warming for this country and raises vital questions about how India will respond to them.
Jennings, S., Gruntfest, E. 2003. Floods, from the handbook of weather, climate, and water: Atmospheric chemistry, hydrology, and societal impacts. Wiley.
Kale, V.S. 1997. Flood studies in India: A brief review, Jour. Geol. Soc. India, v.49, pp. 359-370.
Knighton, David, 1998. Fluvial Forms and Processes: A new perspective. Arnold, London.
Sinha, R., Jain, V. 1998. Flood hazards of north Bihar rivers, Indo-Gangetic plains. Ed. Vishwas S. Kale, Flood Studies in India. Geol. Soc. India, Memoir 41, pp. 27-52.
Sandeep Chamling Rai, Trishna Gurung, et alia. "An Overview of Glaciers, Glacier Retreat and Subsequent Impacts in Nepal, India and China" (pdf). WWF Nepal Program.